RECONSTRUCTING AND CHARACTERISING THE HOLOCENE PALEOCLIMATE AND PALEOENVIRONMENT FROM SEDIMENTS IN NORTHERN VIETNAM [309692]
THESIS
RECONSTRUCTING AND CHARACTERISING THE HOLOCENE PALEOCLIMATE AND PALEOENVIRONMENT FROM SEDIMENTS IN NORTHERN VIETNAM
– A [anonimizat], 2019
ACKNOWLEDGEMENTS
Firstly, I [anonimizat]. [anonimizat], for his ideas and expert advice for my studying steps. Without his guidance helped me in all the time, I would have not completed this research.
Next, I am very grateful for the 4 years supporting from the Ministry of National Education and Scientific Research of Romania and the Ministry of Education and Training of Vietnam for me to do this Ph.D. program, the program of collaboration between the Ministry of Education and Training of Vietnam. And thanks to my home university (Vietnam National University) to give me a [anonimizat].
I especially thank Lect. dr. [anonimizat] X-ray diffraction samples in the laboratory ([anonimizat], University of Bucharest). Besides, I would like to thank all of my Vietnamese teachers in Vietnam and Prof. [anonimizat], Prof. [anonimizat], Dr. [anonimizat]. Luu Viet Dung who never cease to inspire and support me. I am also highly appreciated to Dr. Nguyen Tai Tue for all logistic supports during field sampling under project number QG.16.16 ([anonimizat] (VNU)).
Last but not the least, I [anonimizat], [anonimizat], [anonimizat], especially Ph.D. student: [anonimizat], for their constant support and their endless love during writing this thesis and my life in general.
TABLE OF CONTENTS
CHAPTER 1. GENERAL INTRODUCTION 1
1.1. The climate of the Holocene 1
1.2. Climate forcing factors during the Holocene 5
1.2.1. The change in the solar irradiance 5
1.2.2. Orbital forcing 6
1.2.3. Solar activity and solar forcing 6
1.2.4. Volcanic forcing 6
1.2.5. The role of people in the Holocene 8
1.3. Climate variability during Holocene in the world 8
1.3.1. The period between 9-8 cal. kyr BP 8
1.3.2. Classic “[anonimizat]” Rapid Climate Change 9
1.3.3. “[anonimizat]” RCC appearing at ~0.6 cal. kyr BP 11
1.4. The objective of thesis 12
CHAPTER 2. STUDY AREAS LOCATION AND STATE OF THE ART 13
2.1. Climate variability during Holocene in Northern Vietnam 13
2.1.1. Environment variability in the Northern Vietnam 14
2.1.2. Climate variability in Northern Vietnam 17
2.2. Red River Delta (Vietnam) 21
2.2.1. Geographical setting 23
2.2.2. Geological setting 24
2.2.3. Hydrology 26
2.2.4. Oceanography 26
2.2.5. Vegetation 28
2.3. [anonimizat] (Vietnam) 28
2.3.1. Geographical setting 29
2.3.2. Geological setting 30
2.3.3. Hydrology 30
2.3.4. Biodiversity 31
CHAPTER 3. METHODOLOGY 33
3.1. Overview of methods for reconstruction Holocene paleoclimates 36
3.1.1. Reconstructing Holocene climate records from lacustrine sediments 36
3.1.2. [anonimizat] 36
3.1.3. Reconstruction of paleoclimate changes inferred coral analysis 38
3.1.4. Reconstruction Holocene climate records from speleothems 39
3.2. Taking sediment core samples 40
3.2.1. Sediment core sampling in RRD 40
3.2.1. Sediment core sampling in Ao Tien, Ba Be National Park 41
3.3. Grain size analysis 42
3.4. X – ray diffraction analyses (XRD) 45
3.5. The grain density analysis 53
3.6. Organic matter analysis 53
3.7. Stable isotopes analysis 53
CHAPTER 4. THE LITHOLOGY, GEOCHEMICAL PROXIES, CLAY MINERAL CONTENT OF SEDIMENT 59
4.1. Red River Delta (Vietnam) 59
4.1.1. Lithological characteristics 59
4.1.2. Geochemical proxies (LOI, TOC, δ13C) 67
4.1.3. Clay mineral content 69
4.2. Babe Lake 70
4.2.1. Lithological characteristics 70
4.2.2. Geochemical proxies (OM, C/N ratio, δ13C) 73
CHAPTER 5. RECONSTRUCTION OF PALEOENVIRONMENT AND PALEOCLIMATE IN NORTHERN VIETNAM AS INFERRED FROM SEDIMENTALOGICAL AND GEOCHEMICAL DATA 77
5.1. Reconstruction of Paleoenvironmental and Paleoclimate in Ao Tien Lake 77
5.2. Recontruction of paleoenvironment and paleoclimate in the Red River Delta 80
5.2.1. Source of the organic matter 80
5.2.2. Source of clay minerals during the Holocene 82
5.2.3. Holocene evolution of the Red River Delta, Vietnam 83
5.2.4. Clay mineralogy indicating the monsoon along the Holocene 85
CHAPTER 6. CONCLUSIONS 90
REFERENCES 93
LIST OF FIGURES
Figure 1.1. The relationship between volcanic forcing and main atmospheric processes (Battarbee&Binney, 2009) 6
Figure 1.2. A record of volcanic sulfate concentrations in ppb derived from H2O4 measurements on the GISP2 ice core, Greenland (Zielinski, et al., 1994) 7
Figure 2.1. Northern Vietnam Overview 16
Figure 2.2. The sea level curve for the marginal part of the East Sea over the past 20,000 years (Tanabe, et al., 2003) 17
Figure 2.3. The correlation of various climate phases, the mean of sedimentation rate and sea level in the RRD along the Holocene (modified from Lieu (2006)) 19
Figure 2.4. The relationship between the various climatic phases and the pollen records of core VN (Li et al. 2006b) 19
Figure 2.5. The relationship between the various climatic phases and the pollen records of core GA (Li et al. 2006b) 20
Figure 2.6. Location and general physiographic map of the Red River Delta (Mathers et al. 1996) 21
Figure 2.7. Quaternary geology and topography of the Red River Delta, Vietnam (modified from Tanabe et al., (2006)) 22
Figure 2.8. Generalized geological map of the Red River Delta and surrounds (Mathers et al. 1996) 24
Figure 2.9. Simplified onshore Quaternary stratigraphic column of the RRD Basin (Tran&Nguyen, 1991) 25
Figure 2.10. Storms and tropical depressions on East Sea Vietnam 27
Figure 2.11. Map of Ba Be National Park, Vietam 28
Figure 2.12. Location of Ao Tien Lake, Ba Be National Park, Vietnam 29
Figure 2.13. Overview of Ao Tien Lake, Ba Be National Park, Bac Kan Province, Vietnam 29
Figure 2.14. Ba Be Lake and surrounding topography 30
Figure 2.15. Ecological landscape Ba Be National Park, Vietnam 31
Figure 3.1. The relationship between characteristic climates and deposition environments with climate proxies which were retained in sediment core 34
Figure 3.2. Flowchart illustrating the research process 35
Figure 3.3. Taking sediment core by drilling machine in the RRD, Vietnam 41
Figure 3.4. Taking sediment core by drilling machine in Ao Tien Lake 42
Figure 3.5. Preparation sample for grain size analysis 44
Figure 3.6. Preparation and measurement of the grain size analyses were completed at the Key Lab (Vietnam National University – VNU) 44
Figure 3.7. Samples were treated to remove organic matters, carbonate 45
Figure 3.8. The sediment samples were handled onto a glass substrate 45
Figure 3.9. Sample processing flow chart to prepare clay films and analysis clay minerals (Moore&Reynolds Jr, 1997) 46
Figure 3.10. X'Pert Panalytical Diffractometer in University of Bucharest, Romania 47
Figure 3.11. X'PERT PANalytical in University of Bucharest, Romania 50
Figure 3.12. Clay mineral identification flow diagram (Coastal and Marine Geology Program – USGS) 52
Figure 3.13. The processes of pretreatment and the treating sample for Stable isotopes analysis (Using Stable Isotope Mass Spectrometry (ISMS) – Nu Instruments systems) 53
Figure 3.14. Preparation sample for analysis: a – grind to fine powder; b – Eppendorf tube; c – treatment with HCl 54
Figure 3.15. Vibration mixer with 12 tubes in the laboratory 54
Figure 3.16. Stable Isotope Mass Spectrometry (ISMS) – Nu Instruments systems (Vietnam National University – VNU) 55
Figure 3.17. Gas chromatography system (Vietnam National University – VNU) 55
Figure 3.18. EuroVector EA3000 system (Vietnam National University – VNU) 56
Figure 3.19. The description of peaks associated between sample analysis and gas standard 56
Figure 3.20. Results of stability test of Stable Isotope Mass Spectrometry (ISMS) – Nu Instruments systems 57
Figure 3.21. The sample analysis program by The Nu Instruments Perspective 57
Figure 4.1. Photographs of typical sediment facies of the sediment core in the RRD, Vietnam 59
Figure 4.2. Tri-plot for textural analysis of all sediments in core sample RRD 60
Figure 4.3. Grain-size distribution in the sediment core of the RRD: (A), (B), (C) and D sediments 60
Figure 4.4. The bivariate relationship between: (a) the mean grain-size (µm) and sorting, (b) skewness and sorting, (c) skewness and kurtorsis 61
Figure 4.5. Grain-size distribution of the sediment core in RRD, Vietnam 62
Figure 4.6. Depth variation of sedimentary parameters (Mz,So, Sk, KG) in the core sample of the RRD 63
Figure 4.7. Sedimentary columns of the core sample in RRD. (A) Lithological characteristic of sediment core; (B) Mud content in sediment core; (C) Sedimentation rates were determined by linear interpolation between radiocarbon dating (14C) (Li, et al., 2006) 65
Figure 4.8. The variation of LOI (%), C/N ratios, TOC (%) and δ13C (‰) in sediment, Red River Delta with depth (m) 67
Figure 4.9. Relationship between TOC (%) and LOI (%) 68
Figure 4.10. Relationship between TOC (%) and TN (%) 69
Figure 4.11. Variation in clay mineral (smectite, kaolinite and illite) proportion (%) in the core sediment of RRD, Vietnam 70
Figure 4.12. Grain-size distribution and density of the core AT in Ao Tien, Vietnam 71
Figure 4.13. Triangle graph of grain-size composition shows that the sediment of AT core was mainly composed of sandy silt and silt 72
Figure 4.14. Depth variation of sedimentary parameters (Mz,So, Sk, KG) and grain size distribution in the core sample AT 73
Figure 4.15. The variation of LOI (%), OC (%), C/N ratios, δ15N and δ13C in the sediment core with depth (cm) 74
Figure 4.16. Relationship between LOI and density of sediment (a), LOI and OC content (b), LOI and C/N ratios (c), LOI and δ13C values 75
Figure 4.17. Relationship between OC content and Norg (a), OC content and C/N ratios (b), OC content and δ13C values 76
Figure 5.1. Bi-plot of δ13C and C/N ratios for sediment core in Ao Tien Lake. According to Lamb et al. (2006), C/N ratios of organic matter originated from phytoplankton, bacteria and algae typically generally <10, while originating from terrestrial vegetation, C3 vascular plant material normally >12 and the δ13C values of C3 plants varies between -33 and -23 ‰ 77
Figure 5.2. Bi-plot of δ13C and C/N ratios for sediment core in the sediment core (the RRD, Vietnam) 80
Figure 5.3. Variations in smectite/illite, kaolinite/illite and kaolinite/smectite ratios for the core from RRD. The clay/silt ratios, TOC/TN ratios and sea level are show for comparision 86
Figure 5.4. Comparison of the results from sediment core with other paleoclimate proxies. (a-d): Mean sediment grain sizes (Mz), TOC (%), C/N ratios and δ13C analyzed in sediment core; (e-f): Tropical pollen proportions (%) and total pollen concentrations in sediment core VN (Li, Saito, Matsumoto, Wang, Tanabe, et al., 2006); (g) Spelepthem δ18O record from Dongge Cave in southern China (Dykoski et al., 2005) 88
LIST OF TABLES
Table 2.1. Climate stations in the North of Vietnam (Sterling, et al., 2008) 18
Table 2.2. Features of the Red River (Milliman, 1995) 26
Table 2.3. The number of storms and tropical depressions 27
Table 2.4. Hydrological features in the Ba Be National Park 31
Table 3.1. The list of the sediment cores in RRD and Ao Tien Lake 40
Table 3.2. Size scale adopted in the GRADISTAT program, a modified Udden (1914), Wentworth (1922) and Friedman and Sanders (1978) (Blott&Pye, 2001) 43
Table 3.3. The identification of clay minerals base on peak of X-ray diffraction 51
Table 4.1. Sediment parameters in grain size distribution 64
GENERAL INTRODUCTION
1.1. The climate of the Holocene
The climate of Holocene has been an important role in the growth and development of modern society, but the knowledge about climate change during this time was limited. Decades of paleoclimate studies show the frequency of extreme climatic events along the last glacial maximum (LGM). Reconstruction of paleoclimate changes is important not only for understanding the character of the climatic features and human activities are impacting the climate system, but also providing a data systems support to run global climate models which in great detail and high accuracy (Berger, et al., 2012). The studies show the change in the spatial and seasonal incoming solar radiation by the quasi-cyclic variations in the Earth’s orbit and the major cause of global climate variability is solar radiation.
Holocene climate research has achieved a key role in providing wealthy data on variation of natural climate change since the LGM. Pioneering Holocene climate research based on investigation about macrofossils, megafossils, and peat stratigraphy of pine trees which preserved in peat (Scandinavia):
– Heinrich Dau (1829) became the first scientist to study of pine stumps and peat. He investigated some different types of peat bogs and the presence of pinewoods in peat, and the stratigraphic changes due to peat type and peat color. Moreover, he tried to explain the presence of pine megafossils as reflecting a phase during the hypothetical forest history of Denmark. However, he did not have a chance to test this hypothesis.
– Japetus Steenstrup (1841) proposed four periods in the history of the Danish forest. He also investigated the plant and animal preserved in peat bogs, which has been an important role in studying past environmental and climate changes. He was known as one of the first scientific studies of Holocene paleoecology.
– Continuous investigation on pine megafossils in Denmark, Christian Vaupell (1857) investigated that there had been changes not only in moisture but also in temperature during the Holocene. He provided the ecologic explanation for the occurrence of long-term forest plants in Denmark.
– Thomas F. Jamieson (1865) proposed the model of a Holocene climate optimum. He investigated several species that do not appear as far north as Scotland recently. This was similar to the result of Robert Lloyd Praeger, Waldemar Christopher Brøgger (Brøgger, 1901), etc…
– Base on the study plant macrofossils in the clays in southern Sweden and subsequently in Europe, Alfred Gabriel Nathorst (1870) suggested temperature difference as the main factor impacting the growth of plants and animals since the last Ice Age. Besides, he investigated the boreal and sub-boreal factors that had transmigrated due to historical continental climate, while, the Atlantic and sub-Atlantic factors had transmigrated due to historical ocean climate.
– Axel Blytt (1876) proposed an elaborate theory for the transmigration of the Norwegian flora and its difference floristic factors along historical ocean climate and continental climate.
– Rutger Sernander (1881) connected between the summer-temperature changes in Swedish and Blytt’s moisture changes, proposing the Blytt-Sernander four periods of post-glacial time. The Blytt-Sernander paradigm of climate change along the Holocene was conducted in his thesis at the University of Uppsala, Swedish (1894). This propose has been used widely in Scandinavia after accepting by International Geologic Congress in Stockholm (1910).
– During 20 years, a main conflict between the Andersson’s approach and Blytt-Sernander scheme (based exclusively on macrofossils). Lennart von Post (1946) showed that the history of post-glacial climate has been affected by both precipitation changes (Blytt-Sernander scheme) and temperature changes (Andersson’ approach). This proposal would have developed together with the improvement of pollen analysis.
– Samuelsson (1916) investigated that summer temperature was far from uniform which compared to the limit today, based on studying in the northern limit of hazel in Sweden. He suggested that the growing season could be longer along the Holocene. His idea was accepted by Iversen (1944), Hintikka (1963) and other scientists.
– Granlund (1932) investigated any shifts in moisture levels along the late Holocene. He also suggested Sernander’s Sub-Boreal period can be divided into least two “Sub-Atlantic” climate phases, the climate changed twice over to the Sub-Boreal type during Sernander’s Fimbulwinter and then again to the Sub-Atlantic. In 1946, Von Post abridged Granlund’s propose (1932) and reformation of the Blytt-Sernander paradigm.
– Fægri (1940) suggested the term Pre-Boreal which as a unit between the Younger Dryas and the Boreal during the late glacial.
Although the limitations of the various techniques available, the results of the study by Dau, Steenstrup, Andersson, Vaupell, Praeger, Jamieson, Blytt, Sernander, Granlund, and Brøgger were very remarkable. However, these results were largely pretermitted until the improvement of pollen analysis which as a tool for reconstructing past climates during the Holocene. Until the mid-1970s, the studies of peat stratigraphy in Scandinavia were inspirited by new techniques, which have been used for detecting changes in bog moisture (testate amoebae), evaluating peat humification and calculating chronologies (radiocarbon dating) (Aaby, 1976, Aaby&Tauber, 1975).
In 1916, Lennart von Post presented and suggested the potential applicability of technique which has been used to determine relative dating and reconstruct past vegetation and past climate. He introduced the term “regional parallelism”, meaning that the Holocene climatic optimum will be reflected in various ways across diverse taxa in various climatic regions and geographic regions. He also had the vision of pollen stratigraphy, it’s soon became the only approach used to detect past climate changes and to classify the Blytt – Sernander periods. In the 1920s-1940s, many publications conducted with pollen analysis studies in Europe, China, North America, South America, and New Zealand (Battarbee&Binney, 2009). Continuous studies on pollen from North America, New Zealand, Europe and China, investigating that pollen stratigraphies during the Holocene could be divided into three-fold. Also, he early has been used as a form of spectral decomposition and smoothing, suggesting two trends of long-term change and three cycles which according to the pollen of birch curve. Moreover, he showed that pollen analysis studies in several areas on Earth (North America, Chile, Japan…), to identify the various climate along the Holocene with the circulation systems of the Earth. In the 1940-1960s, the main advances in methods of pollen analysis were conducted by Johs. Iversen, Knut Fægri, and other scientists. In 1944, according to the study of fossil pollen which occurred in Holocene sediments in southern Scandinavia, Iversen suggested that summer and winter temperatures during the middle Holocene could be warmer than today. Iversen’s analysis was accepted by Walther et al. (2005). He investigated that the northern margin (in Scandinavian) was transformed in the past 50 years in response to climate changes recently. While there was evidence for the effects of human activity which resulted in changing land use conditions concerning climate change.
In the early 1950s, Willard F. Libby (1952) developed the process of radiocarbon dating, which given archaeology a new (absolute chronology). In 1966, Harry Godwin was an early application in Holocene research, following some publish by Donald Walker, Eric Willis, and others. In 1973, according to the dating program, Smith and Pilcher investigated that main pollen-zone boundaries and pollen-stratigraphic changes were not synchronous. This result was similar to the broader scales as North America or Europe (Battarbee&Binney, 2009). Szafer (1935) made a mapping pollen value in several areas (Poland) based on selected time intervals. Then, the same maps have been completed with the range of spatial scales, ranging from the single country scale to European continental. Iso-chrone map as a new mapping approach, which has also been conducted to illustrate the patterns of spatial variations (ages) due to the varied pollen types with consistent expansion or occurrence, thereby showing the patterns of range contraction and expansion (Birks, 1989, Davis, 1976, Moe, 1970). The connection between radiocarbon dating and pollen analysis based on pollen stratigraphic changes in spatial and temporal variation, which was an important role in Holocene climate research.
Until the early 1970s, Holocene researchers had collected a large amount of pollen-stratigraphic and other data (radiocarbon dating), which has been provided an independent chronology (Battarbee&Binney, 2009). Reconstruction of paleoclimate and paleoenvironment have been used these data, they were mainly qualitative and according to indicator species or a comparative approach (visually comparison of the fossil and modern assemblages).
The international Cooperative Holocene Mapping Project (COHMAP) started in 1977 for climate changes along the Holocene. Paleoclimate modelers were built with pollen data and it quickly achieved widespread popularity. Comparison between the results calculated which used to the Community Climate Model (CCM) for simulating past climates at 18000, 15000, 12000, 9000, 6000, 3000, and 0 years ago and the various paleoclimate data sources, pack-rat midden data, marine plankton data, and particularly terrestrial pollen data (Wright, 1993). COHMAP was showed a large improvement in Holocene climate research. This model has been used in the state of climate models which reproduced the past climate variability under the conditions of specified boundary layers. Moreover, this model also was described in detail for syntheses and compilations of paleoclimate proxy data which have been used to compare data modelling. It was understood the global climate system and reviewed the strong regional correlation between various constituents of the Earth’s climate system. Furthermore, it revealed remarkable of the circulation patterns of the Earth's climate process change over time.
Then, several researchers tried to combined the global scale of vegetation data at the level of biomes for 6000 radiocarbon years before the present time (BIOME 6000 project) and produced the comparison with other global syntheses of ice sheets, sea surface temperatures, lake level changes. According to COHMAP, and BIOME projects, climate researches were conducted enormous advances, in addition to enhance our understanding of climate in the past during the Holocene (Valdes, 2014).
Although Holocene climate research has been made advances by reconstructing the Holocene climate evolution over the past 200 years, it also has several problems that relate to data interpretation, dating, data collected, and limitations of our understanding (Battarbee&Binney, 2009).
Holocene climate variability between 2800-2000 years BP and 1500 year BP were determined and demonstrated in some areas in the world as the North Atlantic. Holocene climate changes as inferred from ice and marine core data. Besides, base on marine core data also shows some lines of evidence about past climates in other areas as West Africa (Tudhope, et al., 2001). Reconstruction of paleoclimatic changes from a detailed stable-isotope record for the ice core of Greenland which suggested that contrasts markedly with the amplitude of temperature variations and mean temperature over the late Pleistocene and Holocene environments (Dansgaard, et al., 1993). However, mean temperature trend variation from low values with a great fluctuation range to high values with a small fluctuation range on decadal to millennial time-scales. Thus, if using the result from Greenland as a basic establish for climate change during the Holocene, consequently limitation of evidence support to Holocene climate variability. However, along with the development of science, evidence about climate change in the world and the knowledge about external influences on the climate system, are very complicated.
1.2. Climate forcing factors during the Holocene
Climate is identified as a region’s weather patterns which include humidity, temperature, atmospheric pressure, and another meteorological variability over a long time. Thus, the climate is determined by the average weather in the location over long periods. Several external forcing factors impacted on the climate along the Holocene, including solar variability, orbital forcing, and volcanic forcing. In fact, all of these factors interacted with each other and they also impacted on the Earth’s climate system. Moreover, the role of human activities based on determining vegetation cover and land-use changes, which effected climate change along the Holocene.
1.2.1. The change in the solar irradiance
During the Holocene, solar irradiance has been changed due to different latitudes. In the late Holocene (about 6 cal. kyr BP), solar irradiance was more influential than the present time. In the early Holocene, The gradual influenced by external forcing drives the Earth's climate system and it caused the abrupt climate change and occurred similar in several areas of the world, however, these variations are not immediately apparent. In the Northern Hemisphere, for instance, the ice melts slowly and sea level rise steadily. Thus, sea level could middle Holocene sea-level after some millennials. Similarly, species evolve too slowly to adapt to global warming during Holocene. The processes of migration, competition, and evolution, are contributed to changing the surface of the Earth.
1.2.2. Orbital forcing
The main factors that affected climate change along the Holocene and related to three orbital parameters including obliquity, precession, and eccentricity. Although these parameters have low impacted on the total amount of solar radiation which released by the Sun, however, they are strongly influenced by solar radiation variations on different seasons and latitudes. Precessional changes due to perihelion at the time which occurred only in the summer of the northern hemisphere along the early Holocene. Therefore, daily insolation values at various latitudes (northern hemisphere) in the summer are higher than others (ranging from ~40 °W/m2 higher than present at 60 °N to 25 °W/m2 higher at the Equator). Besides, July insolation has slowly decreased over the 12 kyr before our time. While thermal anomalies were generally smaller during southern hemisphere summers and it normally located at lower latitudes (Mackay, et al., 2003).
1.2.3. Solar activity and solar forcing
Orbital forcing effects on incoming solar energy following attitudinally and seasonally. Therefore, it could be various responses to the forcing in the southern and northern hemispheres of the Earth which impacts on the Global climate system. Although the variation of solar irradiance could be impacted similar to all parts of the Earth, the amplification of solar radiation was not affected for all regions, caused by interactions and feedbacks in the atmosphere of the Earth (Rind, 2002).
1.2.4. Volcanic forcing
Figure .. The relationship between volcanic forcing and main atmospheric processes
(Battarbee&Binney, 2009)
Volcanic activities are the main cause of strong short-term cooling, caused by the injection of large amounts of gases (CO2, H2O, SO2, N2) and dust into the atmosphere perturbs reducing energy receipts at the surface of the Earth. The variation of atmospheric circulation made some temperature anomalies somewhere which means climate in somewhere was cool and others are warm. The climate over the mainland was warm during wintertime (in high altitude areas), caused mainly by volcano eruptions in the 20th century (Fig 1.1).
We could be investigated almost temperature variations caused by volcano eruptions after some years. Thus, volcano eruptions made climate change which occurs short-time during Holocene. However, volcano eruptions occur regularly in the past or they happen on a large scale at the same time, are affected by a long time (decades). Follow volcano eruptions during Holocene, a large heat release that led to a widespread of the decreasing in temperature. Therefore, mountain glaciers advance which made intensified atmospheric circulation and generally lowered temperatures (Battarbee&Binney, 2009). During the Holocene, volcano eruptions occur regularly in the period between 9500 and 11,500 yr BP (Lam, 2003). In the early Holocene, volcanic activity released 110 ppb sulfate to the stratosphere (in central Greenland) which can be many times higher than the amount of sulfate was released from the Tambora eruption in 1815 (the largest known historic eruption in the past century).
Figure .. A record of volcanic sulfate concentrations in ppb derived from H2O4 measurements on the GISP2 ice core, Greenland (Zielinski, et al., 1994)
Ice core provides wealthy information on volcanic eruptions. According to Zielinski et al. (1994) investigated a record of volcanic sulfate derived from the GISP2 ice core (Fig 1.2). Base on the results of Crowley (2000) about the volcanic aerosol, suggesting that the corresponding forcing for the 1000 years ago (Crowley, 2000). The study of volcanic ash layers in Siple Dome (Antarctica), Bay et al. (2004) investigated more evidence support to a causal connection between volcanism and millennial climate change (Battarbee&Binney, 2009). The high volcanic eruption during the main deglaciation of the early Holocene shows that ice-sheet unloading and/or sea-level rise was responsible for increased volcanism during this period. Volcanic eruption, solar variability, are imported role of the factors that influence climate change during Holocene in the last millennium. For example, abrupt cooling events over Europe and northern Alaska during the Maunder Minimum, caused by volcanic eruption occurred at the same time during this time (Nishimura&Nishino, 2003). Moreover, global warming due to CO2 concentration increases, deforestation occurs in the 1850s. Consequently, the climate was quite cool in the late 19th century.
1.2.5. The role of people in the Holocene
Along the Holocene, environment systems have been affected by the different economic development activities as building and agriculture. Impacts of human activities on creating and modifying the environmental systems which showed through the changing of land-use. these effects consist of feedbacks on the climate system, both direct and indirect, through changes in atmosphere gases (Battarbee&Binney, 2009).
Land-use changes arising from various economic development activities are also often implicated in the variation of environmental conditions as hydrologic and erosional regimes. They are not only changing the interaction between climatic and hydrologic variability but also play an important role in the creation, maintenance or destruction of habitats (biodiversity depends).
Impact of human activities on weather and climate associated with the appearance of mankind. It was related to the long-term fate of past cultures and civilizations. The nature of the social organization, its flexibility, and adaptive capacity are important in determining human in this society could be survivable and developing or extinct during Holocene (Battarbee&Binney, 2009).
Deltas are both one of the principal coastal landforms and important centers of human population where land-use changes arising from the human exploitation activities as agriculture, building,… According to a result by Yi et al. (2003), investigating that terrestrial plants in Asia along Holocene has been influenced by human activities during a millennial time scale (Li, et al., 2006, Yi, et al., 2003).
1.3. Climate variability during Holocene in the world
According to the results study base on the GISP2 Greenland ice core, Denton and Kalén (1973) investigated that glacier fluctuation records could be classified as 9-8, 6-5, 4.2-3.8, 3.5-2.5, 1.2-1, and 0.6-1.5 cal. kyr BP (Denton&Karlén, 1973, Martin, et al., 2007, Mayewski, et al., 2004).
1.3.1. The period between 9-8 cal. kyr BP
The feature of this period was severe climatic disruption and it was unique among rapid climate change (RCC) intervals along the Holocene, caused by this period occurred at a time when large Northern Hemisphere ice sheets were still at the moment. In the Northern Hemisphere, an abrupt climate event 8.2 kyr before present brought generally dry and cold climatic conditions (the “8k” event). The occurrence of ice rafting, strengthened atmospheric circulation over the Siberia and North Atlantic which showed considerably cooler temperatures than others (Mayewski, et al., 1997), and increased frequency of outbreaks of cold air from the northeast over the Aegean Sea (Rohling, et al., 2002). In the European Alps, the occurrence of glacier retreat suggested the impact of dry northerly winds in the Northern Hemisphere (Hormes, et al., 2001). In additional support to this hypothesis comes from northwestern North America and Scandinavia where mountain glacier advances occur and treeline limit was lower in Sweden (Mayewski, et al., 2004).
In the early Holocene, widespread aridity midway through humid conditions in low latitudes (deMenocal, et al., 2000, Martin, et al., 2007), Moreover, in this RCC interval, tropical Africa summer monsoons over the Arabian and the India subcontinent weakened (Gasse, 2000, Maley, 1982) and trade wind and rainfall fluctuated dramatically over the Caribbean with evidence as persistent drought appears in Africa, Haiti, Pakistan and the Amazon basin, the water level of Lake Titicaca fell sharply and increased precipitation in the Middle East (Mayewski, et al., 2004).
In the Southern Hemisphere, this period was characterized by weakness of polar atmospheric circulation over East Antarctica, decreased rates of snow accumulation (Steig, et al., 2000) and trending temperature changes in various areas between West and East Antarctica (Ciais, et al., 1994, Masson, et al., 2000), evidence as grounded ice in the Ross Sea retreat (Conway, et al., 1999), the warm sea surface temperatures (SSTs) in the western and eastern flanks of the southern Africa, an increasing of rainfall in Chile, caused by the intensification of southern mid-latitude westerlies (Mayewski, et al., 2004).
1.3.2. Classic “cool poles, dry tropics” Rapid Climate Change
Following the period between 9-8 cal. kyr BP, RCC interval occurs with varied intensity and geographic extent, but generally, it was characterized by low latitude aridity and high latitude cooling (Mayewski, et al., 2004). The most extensive of these reorganizations appeared from 6-5 to 3.5-2.5 cal. kyr BP, with less widespread events occurring at 4.2-3.8 cal. kyr BP and 1.2-1cal. kyr BP.
In the Northern Hemisphere, at least two of the reorganizations in the North Atlantic (6-5 cal. kyr BP and 3.5-2.5 cal. kyr BP) bracket and it features by alpine glacier advances (Denton&Karlén, 1973), ice rafting events (Martin, et al., 2007) and strengthened westerlies over Siberia, the North Atlantic (Meeker&Mayewski, 2002). In the first interval (from 6 to 5 cal. kyr BP), the treeline limit was extended and mountain glaciers advance in Scandinavia, but the situation opposite in the second interval (from 3.5 to 2.5 cal. kyr BP) (Martin, et al., 2007). Similarly, strengthened westerlies over central North America in the first interval, but westerly winds are weak in the second interval (Mayewski, et al., 2004). In otherwise, cooling event over the northeast Mediterranean was related to increasing of wintertime continental/polar air (Mayewski, et al., 2004).
At lower latitudes, the fist RCC interval marks by the humid period in tropical Africa that began from the end of the early Holocene to the middle Holocene. This period was characterized by an increasing trend of rainfall and acidification in the long term (Gasse, 2000), despite the fact that the climate became wetter in several areas (Florida, the Caribbean, Pakistan, etc) (Mayewski, et al., 2004). While, the decrease of rainfall over southern Tibet, northwest India (Enzel, et al., 1999) and the water level of Lake Titicaca drop during this interval. Trade wind over the Cariaco Basin and rainfall in Ecuador are relatively stable during RCC interval between 6 and 5 cal. kyr BP, but highly erratic from 3.5 to 2.5 cal. kyr BP. Although the climate became drier in the Amazon Basin, the Caribbean region and East Africa during the second RCC interval, it still was wet due to the stepwise decrease in Asian monsoon intensity (Dykoski, et al., 2005).
In the Southern Hemisphere, polar ice core records, mountain glacier advance (New Zealand), showing the intensification of the atmospheric circulation which caused by the increasing trend of summer insolation in the long term (Mayewski, et al., 2004). The climate overs South Georgia Island, SSTs of eastern South Africa, and southern Africa were cool. At the middle latitudes, the climate of Chile was drier during the first interval, but wetter during the second interval (van Geel, et al., 2000). According to the discontinuous lake sediment records from Antarctica, Ingolfsson et al. (1998) suggested conditions warmer than present based on the increased duration of southern summer insolation (Ingólfsson, et al., 1998).
Although evidence for the RCC interval between 4.2-3.8 and 1.2-1 cal. kyr BP appears in fewer of the record, however, they are characterized by the apparent synchrony and large spatial distributions. Those record also contains evidence which suggested global scale teleconnections as for the other intervals (Mayewski, et al., 2004). In the Northern Hemisphere, the westerly winds over the Siberia and North Atlantic are weak, the temperature fell in western North America during this RCC interval (Scuderi, 1993). Although other climatic disruptions, those records generally are synchronous, distributed in a wide range, signs, and intensities (Mayewski, et al., 2004). Some evidence as during the interval 4.2-3.8 cal. kyr BP, mountain glacier advance in North America, but retreat in Euro, while Scandinavian ice seems great unaffected. Moreover, North Atlantic Deep Water production was low from 4.2-3.8 cal. kyr BP, however, it increased during the 1.2-1 cal. kyr BP (Mayewski, et al., 2004).
At the latitudes, the climate was characterized by dry conditions in much of monsoonal Pakistan and tropical Africa (Gasse, 2000) during two RCC intervals. While the water level of Lake Titicaca fall, however, the climate of Haiti was generally wet. Trade wind strength over the Cariaco Basin was intensified (Haug, et al., 2001). Otherwise, mountain glaciers advance in Kenya (Karlén, et al., 1999) and aridity in Ecuador during the RCC interval from 1.2 to 1 cal. kyr BP.
In the Southern Hemisphere, the wind strength only changed in polar and temperatures in Taylor Dome, Antarctica have fluctuated. At the middle latitudes, the climate of Chile was dry during both of these RCC intervals. During the 4.2-3.8 cal. kyr BP, climate over South Georgia Island was warm. Similarly, the results from lake sediment records in the Victoria Land and Antarctic Peninsula (Hjort, et al., 1998, Ingólfsson, et al., 1998). Besides, mountain glaciers advance in New Zealand and the climate was cool and dry over eastern South Africa during the interval from 1.2-1 cal. kyr BP.
1.3.3. “Cool poles, wet tropics” RCC appearing at ~0.6 cal. kyr BP
During this RCC interval, the climate in both regions was cold and windy climatic conditions, however, the low latitude aridity that prevailed during earlier intervals does not generally characterize the tropics during this most recent interval. Moreover, some records are missing recent sections, caused by artifacts of sampling and anthropogenic influences (Mayewski, et al., 2004). Thus, this event in this study only from 6 to 1.5 cal. kyr BP.
In the Northern Hemisphere, strengthened westerlies wind over Siberia, the North Atlantic and mountain glacier advance, which showed the climate changes was the fastest and strongest during the Holocene in this interval (O'brien, et al., 1995), with exception of the short-lived about 8200 years ago (the “8k” event). At the low latitudes, the climate over the Cariaco Basin, Haiti, and Florida became arid (Haug, et al., 2001). Inversely, the climate in equatorial East Africa was humid conditions. Moreover, river discharge in Pakistan and Ecuador increased, indicating that both El Nino-southern Oscillation (ENSO) systems and Indian monsoon are affected (Mayewski, et al., 2004).
In the Southern Hemisphere, the climate over a large proportion of the Antarctica Peninsula became warmer, however, East Antartica was cold (Morgan, et al., 1997). Trade wind over the Amundsen Sea strengthens and East Antarctica (Kreutz, et al., 1997). The climate over South Georgia was cool, mountain glacier advance (New Zealand) and rainfall (Chile) increased during this interval. While the SSTs of Benguela are cool, climate over southern Africa remarks with climatic cooling and drying (Mayewski, et al., 2004).
1.4. The objective of thesis
Reconstructing and characterizing the environmental changes during Holocene in the North of Vietnam, supply base science for assessment and prediction climate change and environment, to enhance the effectiveness of programs to respond and adapt to climate change and extreme weather phenomena, which are increasingly complex movements.
The following tasks were completed:
– Taking core sample pieces, which located in Northern Vietnam with a case study in Red River Delta and Ao Tien Lake (Ba Be National Park).
– Calculating of textural parameters via mean grain-size (Mz), soring (So), skewness (Sk), kurtosis (KG) and sediment grain size distributions have been used to reconstruct the depositional environment of sediments.
– Clay minerals association (during X-ray diffraction analyses) in the sediment core were investigated and applied to connect with the weathering process in the past along the Holocene monsoon climate in the RRD, Vietnam.
– Reconstructions of Paleoenvironmental and Paleoclimate change by using stable isotopes analysis.
– The overall goal of this thesis is to investigate processes controlling sedimentation and rebuild the Holocene climate history in the RRD and Ao Tien Lake (Northern Vietnam).
The results are focused on two parts. Firstly, dealing with the lithology, geochemical, and clay mineral content of Holocene sediments in the case study, Northern Vietnam (Red River Delta and Ao Tien Lake), which was a comprehensive detailed categorization of sedimentary structures. Secondly, the reconstruction of the Paleoenvironment and Paleoclimate in Northern Vietnam as inferred from sedimentological and geochemical data along the Holocene.
STUDY AREAS LOCATION AND STATE OF THE ART
2.1. Climate variability during Holocene in Northern Vietnam
Study of the characteristic paleoclimate in Vietnam was accomplished by analyses pollen, diatoms, radiometric and conventional 14C ages (Funabiki, et al., 2007, Hori, et al., 2004, Tanabe, et al., 2003). Analyzing radiometric 14C and characteristics of Quaternary sedimentary facies in a sediment core of the Red River Delta (RRD) are invested in the transgression during the 9-6 cal. kyr BP which made the sea level was about 2-3 m high the present level and sea level was stable during the 6-4 cal. kyr BP, then sea level dropped, at first rapidly and later gradually, approached the present level at around 1 cal. kyr BP (Funabiki, et al., 2007, Tanabe, et al., 2003).
Analyzing characters in sediment Holocene of Red river mouth was assessed changing processes characterizing paleoenvironment based on comparing to climate directive in areas surrounding (China, Cambodia, East Sea) (Tanabe, et al., 2006). Analyzing pollen and radiometric 14C in sediment core showed characterizing climate in RRD during the last 5000 years, it included three cycles of warming and cooling: a cool and wet climate during 4.53-3.34 cal. kyr BP, 2.1-1.54 cal. kyr BP, and 0.62-0.13 cal. kyr BP; a warm and dry climate during 3.34-2.1 cal. kyr BP, 1.54-0.62 cal. kyr BP và at the present time (warm climate) (Li, et al., 2006). The result also showed to affecting by human activities in RRD by beginning appearance pollen with origin wet rice (at 3.34 cal. kyr BP). Studying characteristics of pollens and spores in sediment cores the RRD for the reconstruction of paleoclimate changes during 10.5-5.8 cal. kyr BP. In this period, the weather was characterized by warm and humid conditions, but, the climate was dry and cool during 8.2-8.4 cal. kyr BP.
Stable isotope ratio δ18O và δ13C of the primary carbonate components (formed by the chemical deposition processes) carbonate biochemical (formed from the growth of crustaceans and mollusks) which was stored stable, long term and shows a strong connection with variable cycles condition temperature, humidity, rainfall and characterized by climate in the last time. In Vietnam, some results of research on restoration of climate conditions in Vietnam have been made based on the stable isotope analysis method. The result of δ18O value in tree-ring samples was collected from old-growth F.hodginsii (Po Mu) at the Mu Cang Chai area, Yen Bai province, Vietnam shows δ18O significantly correlated with temperature, precipitation. Based on that, it is possible to calculate the PDSI drought index and the operation of ENSO in the last 300 years (Sano, et al., 2012). The result analysis of δ18O value in cave stalactites in Ninh Binh province (Vietnam) also shows characterized by climate during the last 5500 years (Lin, et al., 2006). Therefore, using stable isotopes method for reconstruction condition paleoclimate is a modern and reliable method higher than others used traditional directives (diatoms, pollen, facies,…) (Berger, et al., 2012, Stocker, et al., 2014).
2.1.1. Environment variability in the Northern Vietnam
Red River Delta is a vast triangular area, which located around the Red River basin area in Northern Vietnam. The region consists of 11 cities and provinces, namely, Ninh Binh, Ha Nam, Bac Ninh, Ha Noi, Hai Phong, Hai Duong, Thai Binh, Hung Yen, Nam Dinh, and Vinh Phuc. RRD also positioned on the western coast of the Gulf of Bac Bo, East Sea, Vietnam.
a. The Quaternary stratigraphy
Neogene sediments commonly underlie the Quaternary sediments which are comprised mainly of sand, gravels with lamination of clay and silt. The sediment thickens eastwards to attain a maximum thickness of about 200 m beneath parts of the coastline (Mathers&Zalasiewicz, 1999, Tanabe, et al., 2006). RRD to be a prograding coastal system, which formed mainly as a result of river sediment supply. During the LGM, the Red River incised valley which is positioned southwest of the present Red River channel (Tanabe, et al., 2003). The narrow, elongate valleys are oriented NW-SE (with about 20 km wide and more than 80 m deep) and they are filled with gravel, sand, and clay which made during low-stand, transgressive, and high stand deposits, respectively. While, a large proportion of the surrounding sediments are massive clay intercalated with peaty organic layers which formed in highstand deposits (Lam, 2003).
The Quaternary periods, comprising the Holocene and Pleistocene Epochs. The Pleistocene is divided into three periods, consisting of Le Chi (Q11 lc), Ha Noi (Q12-3.1 hn) and Vinh Phuc (Q13.2 vp) formations. The Holocene is divided into two periods: Hai Hung (Q21-2 hh) and Thai Binh (Q23 tb) formations (Tran&Nguyen, 1991).
The first sequence that formed in the early Pleistocene and corresponded with Le Chi formation. This formation is located west and northwest of the RRD and consisted mainly of coarse sediment deposited pebble (quartz, silica, marble), gravel, sand, silt, brown-gray clay. Distribution depth of this formation from 45-50 m to 65-70 m in the edges and the thickness varies from 5-10 m to 20-25 m and their origin was alluvial, proluvial. The sediment divides this formation into three layers: (1) the bottom layer includes cobble, pebble, gravel, coarse sand; (2) the middle layer includes predominantly middle-fine sand and silty sand; (3) the upper layer with mainly of fine sediment includes clay, silty clay, mixed with a little fine-grained sand that is light-gray, grayish-yellow (Lam, 2003).
The Ha Noi formation aged in the middle-late Pleistocene and formed from the fluvial and diluvial sediment. This formation distributed in edges of mounds, hills, and plains of Ba Vi, Soc Son (Hanoi). The sediment divides this formation into two layers: (1) the underlayer with mainly of coarse sediment deposited, their component from cobble, pebble, gravel, coarse sand in edges to coarse-middle sand in the center part. The sediment is characterized by poor sorting and low roundness; (2) the upper layer with mainly of fine sediment includes silty sand and sandy silt (Lam, 2003). The Ha Noi formation lies unconformably upon the Le Chi formation.
The Vinh Phuc formation was formed in late Pleistocene and the sedimentary origin were fluvial, fluviolacustrine, fluviomarine. This formation mainly of sand mixed with a little pebble, gravel in the bottom, silt sand, silty clay. The laterized sedimentary surface was mottled yellow-gray and brown-red. This formation has paleontological complex and pollen which characteristic of brackish, brackish-water environments (coastal estuaries areas). The sedimentary fluvio-marine origin of the Vinh Phuc formation only found them in drill hole at the coastal areas of Ninh Binh, Nam Dinh, Thai Binh provinces with mainly of silty clay that is blue-gray and green-gray (Lam, 2003).
The boundary between the last two geological epochs, the Pleistocene and the Holocene, is placed at the date 10,000 BP. A large portion of the laterized sedimentary surface of Vinh Phuc formation was yellow-gray and brown-red with some iron coatings caused by weather. This formation is covered by sedimentary of Hai Hung formation. The widespread of this surface has remarked and accepted as the Pleistocene-Holocene boundary. After the LGM, the sediments comprised of two formations, the Hai Hung and Thai Binh formations, in ascending order. (Lieu, 2006).
The origin of Hai Hung formation was fluvial, fluviolacustrine, coastal swamp, and marine facies types. According to the result of which suggested Hai Hung formation was formed in early-middle Holocene. The thickness of this formation varied from 2-5 m in edges to 15-20 m in the center of plain and reached 30-35 m in coastal (Lam, 2003).
Thai Binh formation was formed after the marine regression period (3000 years BP) and lies upon Hai Hung formation. The thickness of this formation varied from 1-2 m in the edges to 15-20 m in coastal (Lam, 2003). Thai Binh formation is subdivided into two parts. Their composition was mainly of fine-grained sand and silty clay and the origin was bog lake, fluviomarine and marine sediment which was formed in late Holocene.
b. Natural features in Northern Vietnam
Figure .. Northern Vietnam Overview
Northern Vietnam is positioned in a transition zone between subtropical and tropical ecosystems. The study area is a complex geological environment with the contact granite and limestone, high mountains and deltas, rugged terrain peaks and low plains, tropical and subtropical species. Because this area is located in the interaction zone between subtropical and tropical with biological influences of three biogeographic areas (East Sea, South China, and Indochina) (Sterling, et al., 2008). The north of Vietnam has a vast coastal zone running along the northwest side of the Gulf Tonkin (East Sea). The area study is characterized by lower topography in the northwest-southeast direction which could have a significant effect on the flow of the biggest rivers. The northern mountainous area composes of Ngan Son, Bac Son, Dong Trieu, Gam River and they are arc-shape mountain ranges which are gradually low toward the sea. Below these mountain ranges is the vast Red River Delta. Transition region from Northeast Mountain to the coastal area of the delta, from Vinh Phuc to Quang Ninh is hills with rounded slopes and peaks (altitude 200 – 300 m above sea level). They are the midland area of Northern Vietnam (Fig 1.3).
Northern Vietnam is outstandingly characterized by limestone terrain, consisting of a field on limestone (naturally eroded limestone), karst valley, cave, and underground rivers and streams.
c. Sea level evolution during Holocene
Figure .. The sea level curve for the marginal part of the East Sea over the past 20,000 years (Tanabe, et al., 2003)
The previously published data showed clearly the sea level curve for the marginal part of the delta plain over the past 20,000 years (Fig 1.4) (Doan&Boyd, 2001, Geyh, et al., 1979, Hanebuth, et al., 2000, Tran&Ngo, 2000). After the LGM, the sea level was approximately 120 m under the present sea level (PSL). Moreover, the sea level reached ~50, ~30, ~15, and ~5 m below the PSL at about 11, 10, 9, and 8 cal. kyr BP, respectively. The sea level rise began to decelerate between 10 and 9 cal. kyr BP. The sea level reached its present level between 8 and 7 cal. kyr BP. After attaining a high of 2-3 m above the PSL at 6-4 cal. kyr BP, then sea level reduced and reaching the PSL from 4 to 0 cal. kyr BP.
According to the result by Tanabe et al., (2003) the Holocene sea level was comprised of three phases: from 9 to 6 cal. kyr BP (phase I), sea level was enhanced from 15 m below the PSL to 3 m above the PSL (with a rate increase of 6 mm per year); Between 6 and 4 cal. kyr BP (phase II), sea level was stable; From 4 to 0 cal. kyr BP (phase III), sea level fell from 3 m above the PSL to the present levels with a mean rate of approximately 0.8 mm per year (Tanabe, et al., 2003).
2.1.2. Climate variability in Northern Vietnam
The climate of Vietnam is characterized by tropical monsoon climate and mean of humidity about 84 percent throughout the year. Northern Vietnam is situated in a tropical climate, however, it is impacted by northeast monsoon and differentiation in climate. Thus, this climatic region is characterized by instability in seasons and temperature. Northern Vietnam has four distinct seasons of spring, summer, autumn, and winter. The average rainfall is ranging from 1,127 mm (Nam Dinh) to 4,802 mm (Bac Quang, Ha Giang province). In the summer season (from May to October), the weather is hot, humid, and rainy until the presence of monsoons. The average temperature varied from 27-29 °C and the highest is from 31-33 °C. In the winter, the temperature falls (especially from December to January of the following year). The Vietnam weather in the northeast is general colder than the others.
Table .. Climate stations in the North of Vietnam (Sterling, et al., 2008)
Figure .. The correlation of various climate phases, the mean of sedimentation rate and sea level in the RRD along the Holocene (modified from Lieu (2006))
Figure .. The relationship between the various climatic phases and the pollen records of core VN (Li et al. 2006b)
The results of pollen, ASM-14C and facies study on two sediment cores (VN and GA) from the RRD in the subtropical zone of Asia (Li, et al., 2006): the duration from 10.47 to 5.34 cal. kyr BP, the presence of main low-land tropical or subtropical, suggesting that climate was warmer than at present. However, the climate was slightly cool between 9.31 and 8.54 cal. kyr BP; during the period 5.34-4.53 cal. kyr BP, The development of tropical or subtropical arboreal taxa showed that the climate became warmer. After 5 cal. kyr BP, three climate cycles were classified: a cool and wet climate stage during 4.53-3.34 cal. kyr BP, 2.1-1.54 cal. kyr BP (influence of wet environment) and 0.62-0.13 cal. kyr BP, a dry and warm climate stage during 3.34-2.10 cal. kyr BP, 1.54-0.62 cal, kyr BP and a warming trend at the moment. After about 3.34 cal. kyr BP, the increasing of Gramineae taxa and secondary forest in both cores (VN and GA) investigated the influence of human activities (cultivation of wet rice). Most notably, upland cultivated taxa increased obviously after 2.1 cal. kyr BP, showing human impact spreading up into this area (Li, et al., 2006) (Fig 1.6 and Fig 1.7).
Figure .. The relationship between the various climatic phases and the pollen records of core GA (Li et al. 2006b)
According to the result by Lieu (2006), investigating the influence of tectonic activities on the sedimentation rate in the RRD. In early Holocene (approximately 11-8 kyr BP), the sediment mostly had deposited in the SW of RRD with the sedimentation rates was 13.3 mm/year (which was larger than in another side) (Fig 1.5). The increasing sediment discharge in T2 because of the old valley in the late Pleistocene. In the middle of Holocene (approximately 8-4 kyr BP), the sedimentation rate in two profiles (T1 and T2) was both small either small, caused by the highstand of sea level. After 4 kyr BP, the subsidence of sediment in T1 was faster than T2 (higher sedimentation rate) (Fig 1.5) with the abundance of silt, clay sediments of in deltaic facies patterns (delta front, prodelta) and shallow marine facies. These data suggested that the block between Vinh Ninh and Chay River fault were the most subsidence in the late Holocene. Moreover, the sedimentary discharge to inland (YM, HN) is smaller at than in coastal areas in the late Holocene (TB1, TB2) which caused by the increasing of subsided rate by the sedimentary thickness of blocks continent to marine (Lieu, 2006).
2.2. Red River Delta (Vietnam)
The RRD is a crucial area for the economy of Vietnam. Heavy industry is located in several major cities including the capital Hanoi and Vietnam’s principal port Hai Phong. This is one of the most densely populated regions in the world with about 1,160 people/km2 (2003). In the delta region, Most of the people work in rice cultivation, however, it also includes other important economic activities as aquaculture, fisheries, land reclamation for agriculture, harbour construction,… The Red River was characterized by red colour, caused by a large quantity of silt rich iron oxide. Agriculture is important throughout the delta plain with paddy rice and jute as the principal crops (Mathers, et al., 1996).
Figure .. Location and general physiographic map of the Red River Delta
(Mathers et al. 1996)
The RRD, located on the western coastal zone of the Gulf of Tonkin (East Sea, Vietnam), is one of five largest deltas in Southeast Asia in term of delta plain area (approximately 160,000 km2), after the Chao Phraya, Irrawaddy and Mekong (Li, et al., 2006, Tanabe, et al., 2006). The dominance of catchment area covers parts of Northern Vietnam and its sediment and water discharge greatly influence the hydrological conditions in the Gulf of Tonkin, Vietnam (Fig 2.1).
The RRD was formed in Red river fault systems and has been minor since the late Miocene (Lee&Lawver, 1994, Tanabe, et al., 2003), which determined in the Neogene NW-SE-trending sedimentary basin. The basin of the RRD has filled with Neogene and Quaternary sediments to a thickness of exceeds 3 km (Mathers&Zalasiewicz, 1999). The Quaternary sediments are comprised mainly of sands, gravels with silt and clay laminations, which unconformable overlie the Neogene sediments. The Quaternary sediments have thickened seaward to a maximum thickness of 200 m beneath parts of coastline (Lam, 2003, Tanabe, et al., 2006).
Figure .. Quaternary geology and topography of the Red River Delta, Vietnam
(modified from Tanabe et al., (2006))
2.2.1. Geographical setting
The RRD plain has been divided into three subsystems (fluvial-dominated, tide-dominated, and wave-dominated) which based upon hydraulic processes and surface topography (Fig 2.2) (Mathers&Zalasiewicz, 1999). The fluvial-dominated subsystem, which located in the western part of the delta, where the fluvial flux is relatively stronger than others and is consisted of fluvial terraces, flood plain, meandering rivers, and meandering levee belts. The wave-dominated subsystem is characterized by alternating beach ridges and inter-ridge marshes, which spread in the southwestern area of the delta, where wave energy is high due to strong summer monsoon. Besides, the tide-dominated subsystem comprised of tidal flats, marshes, and tidal creeks/channels and is situated in the northeastern part of the delta, where Hainan Island shelters the coast from strong waves (Mathers&Zalasiewicz, 1999, Tanabe, et al., 2006).
The regional climate is characterized by a tropical monsoon climate with four seasons (spring, summer, autumn, winter) and humidity averaging from 84-100 % throughout the year (Lieu, 2006). While in the summer monsoon from May to October with heavy rainfall, hot and humid weather. In the dry season lasts from November to April of the following year, the climate is dry and cold. The average temperature is 29 – 30 °C the highest is 42 °C in summer (but it lasts a few days per month): May, June, July and the lowest is 9 °C (but it lasts a few days per season) between January and February. In general, the climate of the RRD is characterized by a two-season year, including summer (cool and dry climatic conditions) and winter (warm, wet climatic conditions) (Pruszak, et al., 2005, Van Maren, et al., 2004). Besides, the temperature depends on the elevation in the mountain area (Duong, 2009).
The near-shore behaviour of shallow-water waves is varied from direction southwest during the summer monsoon (June – September) to the northwest (December – March of the following year). The tidal is characterized by there is one time of high level and one time of low level (diurnal) with a range of 2.6-3.2 m. In the summer monsoon, tidal influences within the delta are restricted caused by the overwhelming effect of the high freshwater discharge. However, tidal effects are evident in all of the major distributaries almost as far inland as Hanoi in the dry season (Mathers, et al., 1996).
Based on the data collected by the General Department of Meteorology and Hydrology (Ministry of Natural Resources and Environment, Vietnam): during the period 1884 – 1989, in total 1,993 storms and tropical depressions hit Vietnam’ territory (approximately 5 tropical depressions/storms each year) , of which 30 % in the North (Lieu, 2006).
2.2.2. Geological setting
The delta plain is covered by mountainous areas, which consisted of Precambrian crystalline rocks as well as Palaeozoic to Mesozoic sedimentary rocks and these rocks are provided for the sediment deposited in the RRD. The structure of this area is confined in a straight narrow NW-SE aligned faulting (about 50-60 km wide and 500 km long). This major tectonic structure can also be traced southeastwards at depth beneath the Quaternary sediments of the delta plain. The NW-SE alignment of the Red River fault system regulates the distribution of the sedimentary basin, the drainage area, mountainous areas, and the straight course of the Red River (Tanabe, et al., 2003). In addition, research results by Nielsen, et al. (1999) suggest that the Red River fault system has been considered minor since the late Miocene (Lieu, 2006). The Red river basin is filled with more than 3 km of Neocene and Quaternary sediments with and the subsides with 0.04-0.12 mm each year (Mathers&Zalasiewicz, 1999) (Fig 2.3).
Figure 2.8. Generalized geological map of the Red River Delta and surrounds
(Mathers et al. 1996)
The Quaternary sediment in the Red river basin unconformably overly the Neogene deposits, which consists mainly of sands and gravels (Pleistocene and Holocene sequences). In the RRD, the sediment is thick from a few meters in Northwest to attain about 200 m beneath part of the coastal area (Mathers&Zalasiewicz, 1999, Pruszak, et al., 2005). In the marginal parts of the delta plain, the shallow water depths in the Gulf of Tonkin (< 50 m) indicate that much of the sequence is preserved offshore (Mathers&Zalasiewicz, 1999). The Quaternary sequence within the delta plain could be divided into five geological ages:late Holocene (Thai Binh formation), early to middle Holocene (Hai Hung formation), late Pleistocene (Vinh Phuc formation), middle to late Pleistocene (Hanoi formation) and early Pleistocene (Le Chi formation)(Nghi, et al., 1991, Tue, et al., 2019) (Fig 2.4).
Figure .. Simplified onshore Quaternary stratigraphic column of the RRD Basin
(Tran&Nguyen, 1991)
The simplified onshore Quaternary sequence of the RRD developed by Vietnamese workers which consisted of two main parts: low sea-level stand sediment (along the Pleistocene) and high sea-level stand sediments (along the Holocene) (Mathers&Zalasiewicz, 1999, Tran&Nguyen, 1991).
2.2.3. Hydrology
The catchment area of the Red River is about 169,000 km2, which originated in the mountainous of Yunnan Province (China), at an altitude above 2000 m and flows 1,200 km before it discharges into the Gulf of Tonkin (the Sea East, Vietnam). The annual water discharge is about 120 km3 of water and its total sediment discharge is about 130 million tons of suspended sediment each year (ranked 15th largest in the world) (Milliman, 1995, Milliman&Syvitski, 1992). The Red River distributes its flow through five branches with 25 % of the flow of the Red River draining into the sea via the Ba Lat mouth (Pruszak, et al., 2005). The water discharge of the RRD varies seasonally, caused by most of the drainage area is affected by the subtropical monsoon climate regime. The averages of year precipitation in the summer is about 1,600 mm (occupy 85-95 % of the total precipitation each year during the rainy season) and about 90 % of the sediment discharge occurs during the summer monsoon season each year (when the sediment concentration is near 12 kg/m3) (Mathers&Zalasiewicz, 1999) (Table 2.1).
Table .. Features of the Red River (Milliman, 1995)
2.2.4. Oceanography
The Red River delta coast belongs to the river-dominated coast. The northern part of the coast has a diurnal tidal regime with mean amplitude of 2.5-3.5 m. While the tide in the southern part is mixed with a diurnal dominance and the mean tidal range is 2-3 m (Duc, et al., 2007, Lieu, 2006). During the summer season (from April to September), tidal influences within the delta are restricted, caused by the overwhelming effect during high freshwater discharge, however, tidal effects are evident in both major distributaries almost as far inland as Hanoi during the dry season (from October to March of the following year) (Lieu, 2006, Mathers, et al., 1996, Mathers&Zalasiewicz, 1999).
Along the delta coast, the maximum and average wave heights are 5.6 and 0.73 m, respectively (Hori, et al., 2004). Waves usually have a dominant direction from the east – southeast during the wet season and from the east, northeast during the wet season (Duc, et al., 2007). In addition, according to the data given by the Vietnam Meteorological and Hydrological Administrator (VMHA), during the period from 1967 to 2017, there were 381 tropical depressions and storms, which impacted to Vietnam’s territory (approximated 8 storms and tropical depressions each year) and 107 of which (approximated 40 %) has entered the RRD (Table 2.3). The statistics of data showed an increase in the frequency and duration of storms and typhoons during the second half of the 20th century. The increasing of storms and typhoons played a major role as a factor in raising the annual average wave height, which impacted the geomorphology and sedimentology of the RRD (Lieu, 2006).
Table .. The number of storms and tropical depressions
Figure .. Storms and tropical depressions on East Sea Vietnam
2.2.5. Vegetation
The area of study has very diverse vegetation, caused by it is in the complex abiotic environment. This diversity is not only attributable to a large number of endemic species, but also to the fact that the Northern part of Vietnam is a meeting point of the floras of China, the Himalaya, and Malaysia. The highlands of the Hoang Lien Son ridge form the south-eastern part of the Sikang-Yunnan floristic province of the Holarctic floristic kingdom which Corresponded to a very important boundary area of the Holarctic and Paleotropic realms with numerous tropical, subtropical and temperate connections (Duong, 2009, Li, et al., 2006).
The Red River catchment area could be subdivided into the following altitudinal and vegetation zones: the tropical lowland (0-100 m); the tropical midland (100-700 m); the subtropical submontane belt (700-1600 m); the temperate submontane belt (1600-2400 m); the temperate montane belt (2400 m) (Duong, 2009).
2.3. Ao Tien Lake, Ba Be National Park (Vietnam)
Figure .. Map of Ba Be National Park, Vietam
Ba Be National Park was established in 1992, the area of 100.48 km2 covers Ba Be Lake, which located in Bac Kan Province, about 250 km from Hanoi. Ba Be Lake is the largest natural freshwater lake that covers more than 23,000 hectares boasting waterfalls, rivers, deep valleys, lakes, and caves set amid towering peaks (Fig 2.6).
Ao Tien located within Ba Be National Park (Bac Kan province) in northeastern Vietnam (Fig 2.6). Ao Tien Lake is the natural limestone mountain, formed by the collapsed of a limestone cave. The Ao Tien Lake has a surface area of 1.5 ha, with the average depth from 10-11 m, the maximum depth is 16 m (Fieldwork was conducted in May 2017) (Fig 2.7).
Figure .. Location of Ao Tien Lake, Ba Be National Park, Vietnam
2.3.1. Geographical setting
Figure .. Overview of Ao Tien Lake, Ba Be National Park, Bac Kan Province, Vietnam
The surrounding of Ao Tien is a limestone rock system with primaeval tropical forest. The regional climate is characterized by a tropical monsoon climate with two seasons. While in the summer monsoon from May to October with heavy rainfall, hot and humid weather. In the dry season that lasts from November to April of the following year, the climate is dry and cold (Ha, et al., 2017, Weide, 2012). The temperature of this area varied across a wide range between two seasons. The highest temperature in January and the lowest temperature in July. In the summer season, sediment from surrounding tended to increase because the increased rainfall and temperature changes lead to the raising of the deposition input at Ao Tien Lake. The source of water in Ao Tien Lake from water meteoric and groundwater exchanged with Ba Be Lake (Fig 2.8).
2.3.2. Geological setting
The surrounding Ba Be Lake is mainly a karst terrain with interspersed shale and igneous rocks. Ba Be Lake has lithology diversity (Limestone dominates the regional lithology) and tectonic unique which is resulted by long, complex geological conditions. Ba Be Lake was formed from the destruction of the South-East Asia continental mass at the end of the Cambria era (about 200 Ma) (Ha, et al., 2017), while the surrounding the ancient limestone mountains date back more than 450 million years (Fig 2.9).
Figure .. Ba Be Lake and surrounding topography
In this area has four-fault systems: NW-SE, NE-SW, sub-longitudinal, sub-latitudinal. Ba Be Lake is very different in comparison with other Karst lakes in the world with a special geological structure. It is a tectonic basin with the bed sediment formed by deposition which consisted mainly of clay. Therefore, the lake will not drain as the bottom and the water kept in long term storage.
2.3.3. Hydrology
Ba Be Lake is the largest natural freshwater lake in Vietnam with four rives and streams which connected to the lakes. Cho Leng River, Ta Han and Bo Lu streams, all in the south and southwest which pour water in the lake with a total catchment area of 420 km2. Lake outflow water in the Nang River through several subterranean caves and waterfalls then continues to flow in the Gam River.
In the rainy season, the water level up to 2-3 m in comparison with the previous state was noticed, caused by Gam River falls water into Ba Be Lake at this time. When the water level of Nang River decreases, water in Ba Be Lake continuous flows Nang River (Table 2.4).
Table .. Hydrological features in the Ba Be National Park
2.3.4. Biodiversity
Figure .. Ecological landscape Ba Be National Park, Vietnam
Ba Be National Park is rich in biodiversity and owns typically ecological features of lowland evergreen forests, lakes on mountains, and evergreen forests on limestone mountains. Ba Be National Park is home of 1,281 plant species, which includes 25 species that are listed in the Red Book of International Union for Conservation of Nature (IUCN). Moreover, many rare plants are listed in The Vietnam Red Book as well. Not only having a diverse flora, but Ba Be National Park also owns a rich fauna in terms of both quality and variety. Scientists found here 81 mammals, 27 reptiles, 17 amphibians, 322 species of birds, 106 fish species, and 553 species of insects and mites. Up to now, there are 66 rare and endemic species listed in the Red Book of both Vietnam and the world.
METHODOLOGY
The deposition environments could be retained as a proxy or proxies to environmental conditions and climate of the last. Examples of proxies include plants, marine sediments, lake sediments, swamp sediments, peat, coral sediment, glacier, and stalactites. In some case, with only a sample provide a wealth of information which was used to determine the essence of environmental conditions in the past. In a decade, using core sediment samples are widely used to reconstruct the character of climate and environment.
Proxies are material compositions that are preserved in environmental records. Those proxies are extracted, determined, and evaluated the information about climate and environmental conditions of the past with high reliability. Reconstruction of paleoclimate changes that is needed to determine proxies and their role in the reflection of climate conditions (representative factors). In the most case, these represent factors are indicated the variation of annual temperature or seasonal temperature. In general, the representative factors are reflected in spring temperature or summer temperature. For example, the main development factors of biological are reflected in seasonal conditions and the melting process of ice is reflected this summer is hotter than others.
Using sedimentary layers from peat swamps, seismic geomorphology and stratigraphy indicated water level variation of the lake which could be used to the reconstruction of paleoclimate. Other biological factors show environmental affection. The samples of aquatic organisms consist of algae… which adapted to the changing of chemical elements and temperature in the lake. Pollination plants provided provenance pollen which preserved in peat swamps. Those factors reflected deforestation, agricultural activities and also climate change. In these cases, we need to calibrate for climate aspects which provide the most important aspect of biological ripening.
Stable isotopes oxygen-18 (δ18O) and hydrogen (δD) in rainwater which interacted with air temperature. Therefore, stable isotopes were a representative factor for the reconstruction of paleoclimate in sediment core (ice, tree-ring, carbonate, stalactite, marine core sediment,…). Stable isotopes methods have been a popular tool for systems biology and abiotic.
All proxies only provide limited information about interpretation processes and a proxy cannot provid full and exact characteristics of climate change at the last time. Thus, factors should be used to apply and together for the reconstruction of paleoclimate in the current studies. There are several major factors and proxies often used independently and complemented each other which enhanced the accuracy of the resulting interpretation or new questions appeared in parameters differently.
Sediment core has resulted in the deposit sediment processes in the evolutionary history and development of the natural system on Earth. They include components with a close relationship and interaction of geological and biochemical processes (Fig 3.1).
The sedimentary layers were formed due to sediment deposition and accumulation processes in the sediment basin which have been affected by hydrodynamic processes, geochemical processes, and weathered processes. In this, climate parameters (temperature, precipitation, humidity,…) in conditions of the sedimentary deposition which directly affect by weathered, features of the hydrodynamic and geochemical environment. Sediment products are formed in weathered processes, sediment transport processes by flow and accumulation process,… They have preserved the stable isotopic components which are reflected climate features when they are formed. The stable isotopic components and characteristics of sediment could be used to interpret the deposit environment when sedimentation was made, climate features and the interaction of the sediment formation process in the evolution of sediment. Thus, for the reconstruction of paleoclimate changes by using stable isotope analysis, we need to use a historical approach, based on causality may indicate the relative of hydrodynamic processes, geochemical processes with paleoclimate characteristics at the time of sediment deposition.
Figure .. The relationship between characteristic climates and deposition environments with climate proxies which were retained in sediment core
The deposition has been affected by some factors as climates, hydrology, geology, geography, geomorphology, biology,… Therefore, the reconstruction of paleoenvironmental and paleoclimate changes in Northern Vietnam clearly shows the necessity for an integrated, interdisciplinary approach to understanding the relationship between characteristic environment, climate conditions and deposition environments due to a biochemical process.
To create an overview of the methods used in thesis, a flowchart illustrates the methods and in which order they are completed (Fig 3.2)
Figure .. Flowchart illustrating the research process
3.1. Overview of methods for reconstruction Holocene paleoclimates
3.1.1. Reconstructing Holocene climate records from lacustrine sediments
Reconstruction of paleoclimate inferred from analysis some climate proxies which were preserved in lacustrine sediments. This method very popular used in recently. Some previous studies recognized those lacustrine sediments could be provided wealthy information about climate and environment in the last periods (Leng&Marshall, 2004, Zhang, et al., 2011). The variation of proxies has been used to the reconstruction of paleoclimate changes inferred from lacustrine as annual sediments deposited (Voigt, et al., 2008), pollen and peat stratigraphy (Wick, et al., 2003), stable isotopes (Holmes, et al., 2016, Schwalb, 2003). In this, the most commonly used method is to use stable isotopes in recently years.
Stable isotopes (δ13C, δ18O, δD, δ15N) were analyzed in inorganic sediments (carbonate) and organic sediments which have been used to the reconstruction of the climate condition in the last time, especially in the Holocene with high reliability (Meyers, 1997). Analysis of stable isotopes (δ13C, δ18O) in carbonate sediment could provide information about the lacustrine temperature where carbonate sediments were formed. According to ostracode δ18O and δ13C values, Schwalb et al., (2003) investigated that these isotopic signatures could be allowed to trace hydrological changes which relate to climate warming (meltwater, changing of rainfall,… ) (Schwalb, 2003). In recent years, Zhang et al., (2011) was summarized and compare carbonate oxygen-isotope records (δ18O in calcite) from ten lakes (China). They investigated that the Holocene optimum occurred between 10.5 and 6.5 ka BP with the interval of maximum effective precipitation in the monsoon region (Zhang, et al., 2011).
3.1.2. Reconstruction of paleoclimate changes inferred tree-rings
The variation of tree-ring is proxy which provides important information about climate and time. In European, tree-ring has been used to study the paleoclimate as early as the late 18th century. Scientists were investigated that a narrower ring in the even more extreme winter times of 1708-1709 (Birks, et al., 2014, Luterbacher, et al., 2004). In North America, large potential of tree-ring as a proxy which was reconstructed the paleoenvironment. According to a study earlier by Andrew Douglass in the Southwestern United States, investigating that rings in his yellow pine record represented drought years in an area where growth was controlled by available moisture. His research was important contributed to the developing of optical dating methods and the reconstruction of paleoenvironment from tree-ring (Birks, et al., 2014).
Using tree-rings were widely applied in the last few decades that it is essentially impossible to keep up with the literature (Libby&Pandolfi, 1974, Libby, et al., 1976, Marion, 1981). Scientists have taken samples (trees) and produced chronologies in nearly all temperate-zone woody species. However, the tropics remain relatively unstudied, caused by the problem in confirming ring counts in tropical species and the poor survival of trees on tropical forest floors during the flood season (Birks, et al., 2014).
In traditional methods, the tree-rings width provides a wealth of information about climate. These tree-rings usually collected in areas were affected by extremely dry and hot conditions. An annual growth index value usually gets affected due to the changing of climate. However, in some cases trees have grown up in soil with groundwater, thus they are less affected by climate change and tree-rings width are similarly during the last time (Liu, et al., 2004). According to the ring width of juniper from Western North America, Holmes et al., (1986) reconstructed the historical droughts in the past form 500 to 700 years or extremely long (Birks, et al., 2014).
In the environment, temperature and humidity are important factors of weather. Trees have grown up in areas where occurred drought events and tree-rings have directly reflected these changes. The climatic signals are recorded in the sample which is originated from the limits of a tree’s distribution and growth tends is limited by only one factor (Birks, et al., 2014).
A different approach was developed from the 1970s and the stable carbon isotope composition of tree rings was conducted in some areas. They could provide palaeoclimate reconstructions with perfect annual resolution and statistically defined confidence limits (Liu, et al., 2004, Marion, 1981). In a short time, the variation of δ13C values in tree-rings have been used as indicators of the source of Carbon dioxide emission (CO2), caused by human activities (Libby, et al., 1976, Marion, 1981). Stable isotopes (δ13C) values in tree-rings are different for each type of plant species. Because these stable isotopes are affected by the climate conditions where these plants live. According to the study by Freyer (1979), investigating that the average of δ13C values in tree-rings decreased about 1.5 ‰ in the period time of 1750 to 1980 years. Since fractionation is additive, this trend should be reflected in tree rings (McCarroll&Loader, 2004).
Numerous tree-ring studies have been applied in European, North America, and China. In China, ring patterns of still living or recently dead trees both provided evidence of paleoclimate (Reading, 1986). Thus, the authors investigated that climate data records from the tree-ring could be measured with high reliability, which is provided information on the meteorological research and climate changes. Although there are very few plants that a large amount of tree-ring which has been used to reconstruct the history of climate in the long-term.
3.1.3. Reconstruction of paleoclimate changes inferred coral analysis
The coral growth rate has been affected by a variety of factors, including an average sea surface temperature (SST) and nutrition level (Sharp, 2007). The longest record of coral growth rate is about an 800-year record from a massive coral head in Bermuda. In this area study, the coral growth rate is inversely related to SST. For example, the coral growth rate is increased in cool upwelling water which contains rich nutrient compositions. In the period from 1250 to 1470 AD year, SST was generally above the long mean. Seawater is occurred coolest in the period between 1470-1710 AD year and from 1760 AD year to the end of the nineteenth century, followed by twentieth-century warming. Besides coral growth rate also used as a factor which was characterized by the mean of SST in the Bahamas (Pätzold&Wefer, 1992). Base on the relationship between coral growth and SST, suggesting that the largest cooling of the last few centuries occurred from 1650-1730 AD year and SST was 1 °C cooler than the twentieth century (Saenger, 2009). The coral growth indicators (bone density, linear extension, calcification rates) in the long term played an important role for today, the context of the survival of coral recruits due to the double threat from human activities (seawater temperature increase and acidification of the ocean) (Mathers, et al., 1996).
The oxygen isotopic content (δ18O) of a coral skeleton is influenced by three factors, including seawater δ18O, the temperature of seawater and a biological offset from seawater. While oxygen isotope fractionations are impacted by temperature during carbonate precipitation. Based on isotope fractionations, the shell δ18O decreases by ∼0.22‰ per 1 °C increase (Epstein&Mayeda, 1953). Thus, a corresponding annual cycle in δ18O is recorded along the growth axis of the coral and it is possible to determine and restore the average sea surface temperature changes every year. According to δ18O values in the coral skeleton (Caribbean), investigating that the temperature in 1980 is lower (2-3 °C) than the late nineteenth and early twentieth century. These results were similar when combined with other methods as coral strontium/calcium (Sr/Ca) and magnesium/calcium (Mg/Ca) ratios.
δ13C value in coral tissue is influenced by various factors (biological and climatic parameters), including δ13C in inorganic carbon dissolved in seawater, carbon isotope fractionations in photosynthesis of algae, two major physiological processes in corals (photosynthesis and biomineralization) (Birks, et al., 2014, Grottoli&Wellington, 1999, Swart, et al., 1996). Algae preferentially utilize 12C which is originated from inorganic carbon dissolved in seawater. Thus, a higher photosynthetic rate leads to higher 13C in inorganic carbon dissolved, which impacts to 13C for incorporation into the skeleton (Xiangjun&Yinshuo, 1991). Several studies show that the δ13C value is decreased according to the deep of seawater and its occurrence in cloudy months. Therefore, δ13C value could be provided an index of cloudy in the long-term (Overpeck, et al., 1996). Besides, carbon isotopes also reflect the balance between photosynthesis and respiration, which could be impacted by food availability (Grottoli&Wellington, 1999, Grottoli, 2002). Although carbon isotopes in coral skeleton allow the reconstruction of ancient climate conditions, however, it can cause errors because of changes in coral structure.
3.1.4. Reconstruction Holocene climate records from speleothems
Speleothem is originated from the deposition of calcium carbonate and other minerals, which is precipitated from mineralized water solutions in limestone karst. Speleothems could be extremely sensitive to past climate conditions and it records these changes as variations in their chemistry. The core of speleothem (hanging on the ceiling of the cave) is usually empty and it develops an annual layering. Inversely, the core of stalagmite has a solid core and it is built on the cave floors due to the influence of drip water (development cycle). Thus, stalagmite most often uses for palaeoclimate reconstruction. Limestone karst is globally common so that climatic signals could be studied in many regions with various climate features (Birks, et al., 2014).
Speleothem could be dated by extreme precision using the thorium-uranium (230Th/234U) method in the uranium series. U is highly soluble, which transported in solution (drip water) and coprecipitated during speleothem deposition. As soon as the U atoms are incorporated into the lattice of the carbonate crystalsradiogenic. In contrast, Th is insoluble in water and tends to be adsorbed on particles (clay minerals) or rarely moved far away after hydrolysis. Therefore, speleothem was supplied from compounds without the insoluble matter contains Th ions or clay minerals 230Th starts to build up from U decay, which provided data for the speleothem dating method. The accepted time range of the method is from 100 years to 500,000 years BP. Sample sized have been reduced to 0.5-2.5 grams in most cases, and the standard deviation is < 1 % and accuracy up to 0.1 % in some cases.
Stable isotopes in the stalagmite (δ13C and δ18C) provided basic information about temperature and precipitation in the past time. In addition, δ13C and δ18O display some covariation along the growth axis of a speleothem, which indicated that they have been affected by the same external parameter as temperature and/or precipitation (Birks, et al., 2014).
Some proxies preserved in the environmental record which have been applied for reconstruction Holocene of paleoclimates. The study of paleoclimate along the Holocene provided some benefits especially increase knowledge of the climate change characteristics and interaction between the ecosystems and the combined effect on the ecosystems. Dataset for Climate Change studies before the climate was affected by human activities. They are collected through short-term data. Reconstruction of paleoclimate on investigating global warming during the last period. Along with monitoring indicators that allow directory comparison between amplitude, time and frequency for weather extremes. Besides using stable isotopes method, determination of grain particles and content of clay minerals in sediment cores also illustrating the depositional environment and reconstructing the paleoenvironment and paleoclimate. (Li, et al., 2017, Sionneau, et al., 2008). Clay minerals are a dominant component of most marine sediments and are mainly land-derived. Their geographic distributions and sources have been extensively investigated since the 1960s (Biscaye, 1965). The content of clay minerals provided some information about climate condition in the last time (rainfall, runoff,…) in Northern Gulf of Mexico, Bay of Bengal,…(Li, et al., 2017, Sionneau, et al., 2008). Semi-quantitative estimation of clay mineral abundances is based on peak areas, weighted by empirically estimated factors (Biscaye, et al., 1997, Biscaye, 1965). Since in most continental environments, smectite and kaolinite are generally formed by chemical weathering, whilst, illite and chlorite are mostly inherited from ancient rocks which impacted by physical or moderate chemical weathering (Sionneau, et al., 2008).
3.2. Taking sediment core samples
Table .. The list of the sediment cores in RRD and Ao Tien Lake
Two boreholes were cored in RRD (VL) and Ao Tien Lake (AT), Northern Vietnam (Table 3.1).
3.2.1. Sediment core sampling in RRD
The Basin often has big sediment thickness. They are depended on the rate of sedimentation, sea-level fluctuation and interaction of the tectonic motion which made the difference of sediment thickness (Funabiki, et al., 2007, Tanabe, et al., 2006). In the area of RRD, this concurrence of a rapid change in sediment thickness from Hanoi to the coastline of Thai Binh and Nam Dinh provinces. Some areas along with Nam Dinh, Ha Nam to Thuong Tin of the lowland have a large thickness. The thickness of the Holocene sediment is 50-70 m in some areas of Nam Dinh, Ha Nam and coastline as Tien Hai (Thai Binh province), Giao Thuy (Nam Dinh province) and about 40 m in Kien Xuong (Thai Binh province) (Tanabe, et al., 2006). Therefore, boreholes need to drill with a depth of 40 m or 70 m which are required to better understand climate features during Holocene in RRD, Vietnam.
A core sample of the RRD was collected by rotary drilling techniques (Fig 3.3) which was based on a few principles: to avoid disturbances to sediments; to preserve the status quo of drill cores; enhanced core recovery. The sediment core was immediately divided into pieces of 1 m in length. During the drilling processes, disturbing sediment surfaces (about 20 cm) were rejected. Sandy materials were often gone lost because no core catcher was mounted on the coring tube. Therefore, it was described as detail in describing as drill cores.
From this information, this study was collected drilling cores taken from the RRD with the geographical location at latitude 20°25'39.86" N, longitude 106°24'7.46" E, altitude +0.5 m (Fig 3.3). The total core depth was about 36 m and core recovery was 80 %.
Directly following a collection, the core sample was placed in PVC tubes and delivered to the laboratory in cool condition. In the laboratory, core samples were processed during 12 h of collection by first removing the outer layer (~0.5 cm in thickness), then the cores were sliced in 20 cm portions. Finally, the sediment samples were covered in labeled polyethylene bags.
3.2.1. Sediment core sampling in Ao Tien, Ba Be National Park
In this study, a sediment core was collected by a simple device that followed design by Davis and Steinman (Davis&Steinman, 1998), Somrisi et al., (Somsiri, et al., 2006). This device included of drill bit PVC at a length of 2 m and 9 cm in diameter. The tip of the drill bit was beveled edge and other tip connected with the cross hand which included four pipes which the same at length. The PVC pipes were connected by the adapter. Holes (1 cm in diameter) were drilled in a whorled pattern along the length of the 2 pipes over the drill bit with the distance between two close holes was 30 cm. These holes allow water to drain out of the corer and water pressure reducing on sediment core in a drilled bit (Fig 3.4).
Figure .. Taking sediment core by drilling machine in Ao Tien Lake
Sediment core (AT) was collected by a rotary drill. The core sample located at latitude 22°26'51.35" N, longitude 105°37'2.64" E, in 14 m deep water. The total core length was 108 cm and sediment recovery approached 100 %. Directly following a collection, the core sample was placed in PVC tubes and delivered to the laboratory in cool condition.
In the laboratory, the sediment core was split and examined on the basis of grain size, texture, colour, and fossils. The outer layer (1 cm in thickness) was removed which minimized contamination, then the cores were sliced in 2 cm portions (54 samples).
3.3. Grain size analysis
The various grain size could be an important facies indicator, sedimentary development and environmental change (Blott&Pye, 2001, Bui, et al., 1989, Folk&Ward, 1957, Friedman, 1979).
In the laboratory, all samples were first dried in an electric oven at 60 °C for 48h. The dried sample put in a mortar and slightly crushed by a rubber pestle. Organic matter, carbonate, and silica were removed using 30% H2O2, 20% Ac and 2 mol/l sodium carbonate, respectively. For the sediment grain size it was used laser beam diffraction, using a Particular LA-950 (Horiba) instrument at the GEO – CRE (Key Laboratory of Geoenvironment and Climate Change Response), Vietnam National University (VNU) (Fig 3.5). This device can measure suspension samples liquid in the grain size range of 0.05 – 3000 μm. The sediment grain size was analyzed in triplicate for each sample with a relative error of at most 1 %. The sedimentary parameters were determined by a grain size distribution and statistics program (GRADISTAT program) (Blott&Pye, 2001) (Table 3.2).
Table .. Size scale adopted in the GRADISTAT program, a modified Udden (1914), Wentworth (1922) and Friedman and Sanders (1978) (Blott&Pye, 2001)
Figure .. Preparation sample for grain size analysis
The grain size parameters include (Fig 3.6):
Mean grain size (Mz) is the average grain-size and it is a parameter related to the overall grain size.
Sorting coefficient (So) is a measure of size variation and it is calculated as the standard deviation (SD) in phi size units for each sample.
Skewness (Sk) is a measure of the asymmetry of the probability distribution of a random variable about its mean.
Kurtosis (KG) is the peakedness of the distribution and measures the ratio between the sorting in the tails and the central portion of the curve.
Figure .. Preparation and measurement of the grain size analyses were completed at the Key Lab (Vietnam National University – VNU)
3.4. X – ray diffraction analyses (XRD)
The sediment core of the RRD was studied for their clay mineral distribution on the basis of the X-ray diffraction method. The semi-quantitative determination was calculated (Biscaye, et al., 1997, Biscaye, 1965).
X-ray diffraction analyses were performed at the University of Bucharest, Romania. For the determination of clay minerals by X-ray diffraction was required to complex sample preparation to increase the intensity of diffraction reflexes. Wet sediment sample (25 g) were split and disaggregated with distilled water because dry grinding can cause changes in the phase, and in extreme cause, can lead to stress defects in the structure of crystals which will show XRD line diffracted or even structural collapses (the production of X-ray amorphous material) (Moore&Reynolds Jr, 1997) (Fig 3.7).
Figure .. The sediment samples were handled onto a glass substrate
All samples were treated to remove organic matter, carbonate and silica with 10 % H2O2, 20 % H2COO3 and 2 mol/l sodium carbonate (Kunze&Dixon, 1986, Rabenhorst, et al., 1984). At the moment the effervescence ceased, the excess acid was removed by repeated washings with distilled water and centrifugations. Particle grains were separated by settling according to Stoker’s law and concentrated by centrifugations (Fig 3.8).
After treatment with sodium polyphosphate, the samples were centrifuged several times for fraction separation < 2µ. The separation of dispersed suspension in 2 rotating test tubes with 5 ml. The first tube was saturated by MgCl2 and the second tube was saturated by KCl and each tube was transferred to two slides by wet smearing. The smear samples were performed also on oriented specimens, including air-dried condition, glycerol solvation, heated at 330 °C and 550 °C (Jackson, 2005).
Figure .. Sample processing flow chart to prepare clay films and analysis clay minerals
(Moore&Reynolds Jr, 1997)
The analysis was performed semi – quantitatively using a PANalytical diffractometer in the following conditions (Fig 3.9):
– Oriented sample was saturated by MgCl2 and air-dried at 20 °C;
– Oriented sample was saturated by MgCl2 and solvations with ethylene glycol and glycerol, heated to 80 °C for 8 -10 hours (vapour method);
– Oriented sample was saturated by KCl and air-dried at 20 °C;
– Oriented sample was saturated by KCl and heated to 330 °C for 1 hour;
– Oriented sample was saturated by KCl and heated to 550 °C for 1 hour.
Figure .. X'Pert Panalytical Diffractometer in University of Bucharest, Romania
The heated, solvated, saturated and the air-dried sample was kept in a desiccator until they were analyzed.
X-rays are electromagnetic radiation similar to light (with a great shorter wavelength). The wavelength of X-ray and the structural spacings of crystals both have dimensions about angstroms (1Å = 10-8 cm). X-rays are produced when electrons that have been accelerated to high speeds collide with a target metal (Poppe, et al., 2001). In an X-ray tube, the high voltage maintained across these electrodes draws the electrons toward a metal target (the anode). (Moore&Reynolds Jr, 1997, Poppe, et al., 2001).
If an incident X-ray beam encounters a crystal lattice, general scattering interferes with itself and is eliminated (destructive interference), diffraction occurs when scattering in a direction certain, which is in phase with scattered rays from other atomic planes. During this condition, the scattered X-rays include to form new enhanced wave fronts mutually reinforce each other (constructive interference) and they are formed at the point of impact, and radiate in all directions. The relation by which diffraction scattering occurs is known as the Bragg law or Bragg equation. Each crystalline material will diffract X-rays in a unique characteristic pattern, caused by it has a characteristic atomic structure.
This Bragg’s law is expressed in a mathematical form a equation:
nλ = 2d sinθ
where:
d – the distance (nm) between the lattice planes;
θ – the Bragg angle;
λ – a wavelength of the characteristic X-ray;
n – the order of reflection.
Based on the model of Bragg, the layers of the clay minerals can be regarded as network levels.
Oriented specimen preparation has been used for the investigation of the clay minerals. The analyses were run on a PANalytical X’Pert at the Mineralogy Laboratory. The department has a PANalytical X’Pert Pro diffractometer equipped with a Co X-ray tube and the very fast X’Celerator detector. The unit has a 15 position sample changer and a set of low background sample holders for samples about 1 milligram. Diffraction data is acquired by exposing powder samples to Cu-Kα X-ray radiation and it has a characteristic wavelength (λ) of 1.5418 Å. X-rays were generated from a Cu anode supplied with 40 kV and a current of 40 mA. The data was recorded with a scan speed of 1.0 o min-1 and a step size of 0.01 o in the 2θ range of 4-40 degrees (Fig 3.10).
During operation of the machine, the stability and accuracy of the mass spectrometry systems need to test before driving.
Clay minerals
Clay mineral occurs in all types of sediments and sedimentary rocks which modified by physical or chemical weathering at high latitudes or altitude (Sionneau, et al., 2008, Wang&Yang, 2013). Clay minerals are required to better understand the depositional conditions and provenance of the sediments (Chamley, 1989). Because clay minerals are very sensitive regarding changes in depositional conditions and especially due to the processes of weathering.
According to some previously (Chamley, 1989, Hay, et al., 1991, Meunier, 2005), these authors also agreement for clay minerals weathering pathways from source rock to residuals could be determined. In addition, the influence of hydrodynamic sorting in clay mineral in the coastal and marine environment and have been investigated by previous publishes (Chamley, 1989, Steinke, et al., 2008, Wang&Yang, 2013).
The clay minerals are fine-grained. Illite and chlorite general are inherited from accident rocks (Paleozoic and Precambrian shales) and they are would be the first clay mineral to form in young soil (Lieu, 2006). Most of the smectite material was formed by the alteration of volcanic material and basic igneous rocks (Weaver&Pollard, 2011). Kaolinite could be originated from a solution by resilicsation of aluminium-rich materials and by hydrothermal alteration (Weaver&Pollard, 2011).
Clay minerals have been used as indicators for a weathering under different climate regimes (dry/cool condition) (Chamley, 1989). Furthermore, the chemical and physical weathering factors (temperature, humid air, precipitation,…) control element (as Al, Si, and K in soil) to form other mineral types. For example, the abundance of Si in the environment can be slowly lead to the substitution of Al by Si during dry periods, which procedures illite-smectite mixed layer and smectite. While, high chemical weathering and rainfall during wet/warm conditions, Si and other cations could be released from pore solution and higher amount of kaolinite and smectite (Alexiades&Jackson, 1966, Lieu, 2006). Therefore, the clay mineral ratio has been used to indicating the type of weather (chemical or physical) which affected the erodible sediment (Sionneau, et al., 2008).
Clay minerals can have various behaviours after transportation. Clay mineral research was accomplished to study the sedimentary in general and delta in particular. According to study about the clay mineral distribution in Yangtze delta plain in different Holocene stages by Wang et al., (2005), investigating that clay mineral distribution is affected by several major factors: climate change, sea level fluctuation and the provenance of sediment (Wang, et al., 2005). The multi-sourced clayey sediment derived from the Changjiang catchment due to transport and deposition processes. Smectite distribution in the early Holocene is a correlation with the rapid sea-level rise, that caused land inundation in parts of the coastline. While the high amount of chlorite in the early Holocene, which is suggested that colder temperature. Inversely, the high amount of kaolinite in mid-Holocene is showed that the Earth's climate was becoming warmer. In addition, according to the result by Moon et al., (2000), they were investigated that the enrichment of chlorite is inversely related to that of smectite. The kaolinite content is constant, investigating that chlorite was originated from the Han River which was delivered to smectite by pedogenesis along regression periods. Therefore, the relative contents of clay minerals from the unconsolidated materials in the tidal flat of Youngjong Island have been used as indicators of transgression and regression during the sea-level fluctuation (Lieu, 2006, Moon, et al., 2000).
Mineral identification
Diffractograms were visually interpreted with the help of a computerized search via PANalytical X’Pert HighScore Plus, v3.0 program (Fig 3.11). The peak area values of clay minerals used to calculate percentages of major clay minerals. Clay minerals reflect X-ray beams in the variou direction because they are formed of numerous crystal lattices arranged in a certain manner. If a mineral has high amount in the smear sample, after analysis, its peak will appear with high intensity and high value of peak area. According to the semi-quantitative method, clay minerals in RRD sediment are determined (Biscaye, 1965, Starkey, et al., 1984). Remember that diffraction peaks could have appeared for all the other d (00l) reflections within the two theta interval scanned if there is no mixed layer.
Figure .. X'PERT PANalytical in University of Bucharest, Romania
a. Illite
Illite is one of the mica clay mineral series and it is structurally similar to muscovite. Illite is identified by a series of X-ray diffraction peaks at 10 Å (00l) and 3.3 Å (003) peaks which are not appreciably affected by heat treatment (to 550 °C), glycerol or ethylene glycol solvation and K-saturated (Fanning, et al., 1989, Poppe, et al., 2001). In many cases, the value of area of peak 10 Å (from the air-dried sample) was suggested for calculation of illite percentage. The degree of crystallization of illite was determined by illite well-ordered (10.0 Å) and illite poor ordered (10.2 Å) (Meunier, 2005) (Table 3.3).
Table .. The identification of clay minerals base on peak of X-ray diffraction
b. Kaolinite and chlorite
Kaolinite and chlorite were divided into the relative intensities of their 3.58 Å (002) and 3.54Å (004) peak, respectively (Biscaye, 1965). Heating alone will not determine the dioctahedral kaolinite group minerals from chlorite (iron-rich), caused by the 002, 003, and 004 chlorite peaks are also weakened during this heat treatment (Moore&Reynolds Jr, 1997). The area of peaks in air-dried smear samples represent the behaviour of kaolinite, chlorite, respectively. Kaolinite and chlorite can be differentiated by heating because Chlorite (about 14 Å) is typical heat stability. When the sample is heated at 550 °C to destroy the kaolinite and may weaken or destroy the chlorite (7 Å), the chlorite structure is left largely intact with only partial dehydration of the octahedral layer (Brindley&Gillery, 1956) (Table 3.3).
c. Smectite
Smectite is characterized by basal reflections (yield X-ray diffraction patterns typical). when heated to 550 °C, the 00l reflection will collapse to about 10 Å (the exchange cations) (Moore&Reynolds Jr, 1997). When Mg-saturated, the 00l reflection will swell to about 17 Å (approximately 17.8 angstroms with glycerol). The value of area of peak 17 Å has been used to identify this clay mineral in any cases (Table 3.3).
Figure .. Clay mineral identification flow diagram
(Coastal and Marine Geology Program – USGS)
3.5. The grain density analysis
This method was accomplished to determine the mass of sediment per unit of volume which is calculated by:
The grain density was analyzed and performed according to the following steps: the plastic container was marked and then it was weighted using calculations (M1). The amount of sediment was taken into the plastic container (with the unit volume – V is 1 cm3). After that, the weighing before (M2) and after (M3) drying in oven (at 60 °C for 48h) until the volume is constant.
3.6. Organic matter analysis
The organic matter (OM) content was obtained via loss on ignition measurement (LOI). About ten grams of fresh sediment was put into a beaker and completely dried at 60 °C (in the oven) and then ground to a fine powder by using an agate mill. During grinding, extraneous materials as small plant matter and shell fragments, which were manually eliminated using forceps (by stainless steel).
About 2 grams of the pulverized sediments were first dried at 100 °C in a drying oven for 2h. Then these samples heated at 550 °C in a temperature monitored muffle furnace for 3 h. Organic matter content is calculated based on the inequality between the weight from before and after heat treatment processed (at 550 °C), which divided by the initial sample weight times 100 % (Tue, et al., 2014).
3.7. Stable isotopes analysis
Figure .. The processes of pretreatment and the treating sample for Stable isotopes analysis (Using Stable Isotope Mass Spectrometry (ISMS) – Nu Instruments systems)
Figure .. Preparation sample for analysis: a – grind to fine powder; b – Eppendorf tube;
c – treatment with HCl
Figure .. Vibration mixer with 12 tubes in the laboratory
For analysis of stable carbon isotopes (δ13C), and C/N ratios, the fresh sediment was put into a beaker and completely dried at 60 °C for 48h, and then ground to a fine powder by using an agate mill. The extraneous materials (as small plant matter and shell fragments) were removed using forceps (by stainless steel). The measurement. About 0.2 g of pulverized sediment was placed in an Eppendorf tube and treated with HCl 1N for 24h to remove carbonate at room temperature. Then, the samples were rinsed by 4 ml Milli-Q filtered with distilled water. This process was repeated four times before being dried in an oven at 60 °C for 48h, following the methods outlined by Tue et al., (2014). After dried, about 10 g samples were weighed and wrapped in tin capsules (Fig 3.14 and Fig 3.15). The δ13C, C/N ratios were measured using a stable an isotope ratio mass spectrometer (Nu-Perspective Instrument) system at the GEO – CRE (Key Laboratory of Geo-environment and Climate Change Response), Vietnam National University (VNU) (Fig 3.16 and Fig 3.17). During the analysis, a certified reference material (L-alanine) has been used to quantify the analyzed results. The δ13C was expressed in ‰ (per mil) and the following equation:
Where, δX is δ13C or δ15N; R = 13C/12C or 15N/14N, Rsample is the isotope ratio of the sample and Rstandard is the isotope ratio of a standard reference to Pee Dee Belemnite (PDB) limestone carbonate.
Figure .. Stable Isotope Mass Spectrometry (ISMS) – Nu Instruments systems
(Vietnam National University – VNU)
Figure .. Gas chromatography system (Vietnam National University – VNU)
The process can be divided into steps:
– Sample analysis is pre-weighed into tin cups. After that, it is automatically dropped into the combustion chamber of Eurovector EA3000 (Fig 3.18).
– In the combustion chamber, sediment samples were transferred into carbon dioxide (CO2) and oxides of nitrogen (NO2). Then, these gases enter the reducing chamber, they are reduced to CO2 and elemental nitrogen (N2).
– CO2 and N2 peaks are separated on a gas chromatography column and carried to the IRMS system. These gases are ionized on a hot filament under vacuum, it was separated and accelerated by a magnetic field based on their mass to charge ratio (m/z). The separated ions are collected and counted in Faraday cups where the carbon and nitrogen stable isotopes are determined.
Figure .. EuroVector EA3000 system (Vietnam National University – VNU)
Figure .. The description of peaks associated between sample analysis and gas standard
Figure .. Results of stability test of Stable Isotope Mass Spectrometry (ISMS) – Nu Instruments systems
The test's sensitivity: during operation of the machine, the stability and accuracy of the mass spectrometry systems need to test before driving. This method was conducted by the tests when were by the amount of gas (CO2 and N2) standard injected into the mass spectrometry systems. Those gases standard were injected from small volume to large volume. This machine conducted analysis and determined the relationship between the electromagnetic signal and the amount of gas (CO2 and N2) which was injected (Fig 3.19).
Figure .. The sample analysis program by The Nu Instruments Perspective
The analytical errors of the isotopic measurement have minimal effect on the body composition estimates. The analytical errors must ensure 0.05 % for δ13C, 0.1 % for δ15N, 0.05 % for TN, 0.1 % for TOC.
THE LITHOLOGY, GEOCHEMICAL PROXIES, CLAY MINERAL
CONTENT OF SEDIMENT
4.1. Red River Delta (Vietnam)
4.1.1. Lithological characteristics
The parameters show sediment grained size distribution, which used to reconstruct the sediment environments (McLaren&Bowles, 1985). They are affected by the processes of sediment transportation. In this study, Mz values showed the sediments were classified from the fine silt to very fine sand with mainly of coarse silt, very coarse silt, which followed geometric Folk and Ward graphic measures (Folk&Ward, 1957).
Figure .. Photographs of typical sediment facies of the sediment core in the RRD, Vietnam
Figure .. Tri-plot for textural analysis of all sediments in core sample RRD
The sediments of core samples are comprised mainly of clay, silt, and sand, which varies from 1.5-25.5, 22.9-87.2, and 3.3-73.4%, respectively. According to textural analysis in the core sample, sediment facies could be classified as sandy silt, silty sand and silt (Fig 4.2). While based on lithology characteristics, color, and texture parameters, the sediment can be divided into six sections and two-second sedimentary units: unit 1- estuarine sediments (from 36 m to 30.1 m); unit 2 – deltaic sediment (from 30.1 m to 0 m) (Fig 4.2). The depositional condition of the RRD has resulted in the interaction between sea-level rise and fluvial inputs along the Holocene.
Figure .. Grain-size distribution in the sediment core of the RRD: (A), (B), (C) and D sediments
According to the character (lithology) of sediment core, it showed a correlation with the core VN from the last studies in RRD (Li, et al., 2006, Tanabe, et al., 2006). Besides, the location of core sediment was also nearly with the core VN. Consequently, the geochronology of the sediment core was calculated due to the sediment rates, which was identified based on accelerator mass spectrometry radiocarbon dates (AMS-14C). The result showed that the last core length corresponds to dates ranging from 11.26 cal. kyr BP (Tue, et al., 2019). With this time, the sediment core was covered along the Holocene.
Figure .. The bivariate relationship between: (a) the mean grain-size (µm) and sorting,
(b) skewness and sorting, (c) skewness and kurtorsis
Grain-size analyses for the sediment core revealed the fourth types of grain-size distribution: For type B, a large portion of the samples (46 %), has a bimodal grain-size distribution with a modal size of 10-40 µm and a small amount of fine material. Type A and C, which just account for 31 % and 19 % of the total samples, respectively and show a trimodal grain-size distribution. Type D shows bimodal grain-size distributions, with a similar modal size of the fine compound and a different modal size of the coarse compound (Fig 4.3).
According to the assumption of these statistical parameters, reliability reflects differences between different deposition settings. Figure 4.4a illustrates the correlative between mean grain-size (Mz) and sorting (So). For third deposition environments (sub- and intertidal flat, tidal flat, floodplain), there is clustering in fine-sized and very poorly sorted. Both Mz and So are hydraulically controlled (Rajganapathi, et al., 2013) so that in all sedimentary environment the poorly sorted sediments have a mean grain-size in the medium silt fraction. Figure 4.4b illustrates the correlation between skewness (Sk) and sorting. For the sub-to intertidal flat and delta front flatform, sediment is very poorly sorted and symmetrical to very fine skewed. By contrast, poorly sorted to very poorly sorted are mainly clustered around symmetrical to fine skewed and have negative skewness values. For the delta front slope and shelf prodelta sediment, poorly sorted to very poorly sorted are mainly clustered around very coarse to symmetrical skewed and have positive skewness value. In figure 4.4c illusrates the correlation between skewness and kurtosis (KG). For the delta front slope and shelf prodelta sediment, the positive skewness/very platykurtic to the very leptokurtic. Others deposition environments, the negative skewness/very platykurtic to the very leptokurtic. This pattern showed the mainly of a silt grain-size population and the subordinate of very fine sand grain-size which gives a small proportion of negative skewness.
Figure .. Grain-size distribution of the sediment core in RRD, Vietnam
a. Core section 1: at the depth from 36 to 30.1 m
In this section, sediment is characterized by reddish gray and consisted of very fine sand and clay lamination (as faint lenses 3-8 mm thick) and a large portion of this facies is bioturbated. Mud content of the sediment slightly decreases from the depth of 36 m to the smallest value (the depth of 32.2 m) (Fig 4.7). The Mz tended to slightly increase upward with average value was 18.39 µm. The average value of So was 4.45±0.4 and a large portion of value is classified as very poorly sorted. The mean of Sk ranged -0.29±0.1, it divided into very fine skewness and fine skewness sediment. KG values tended to increase upward with two high value were 1.65, 1.28 at the depth 32.3 m, 31.3 m, respectively. The KG value can be divided three, but a large portion value indicated platykurtic sediment (Fig 4.6).
Plant and shell fragments are scattered in core sediment and a large portion of shell fragments is concentrated from 31.6 to 32.5 m. Therefore, this section was interpreted as a sub-tidal and inter-tidal flat environment (Tanabe, et al., 2006). Sedimentation rates in this section interpreted as 0.42 cm/year (Li, et al., 2006).
Figure .. Depth variation of sedimentary parameters (Mz,So, Sk, KG) in the core sample of the RRD
b. Core section 2: at the depth from 30.1 to 18.9 m
In the section, surface erosion formed during a transgression and separated by others section (depth core sample about 30 m) (Tanabe, et al., 2006). The feature's sediment is blue-gray bioturbated clay and composing of laminated fine sand, silt with reddish-gray in color. Besides, sediment was bioturbated by gray clay and rich in shell fragments. Mud values in this section ranged from 69.16 to 95.48 %. Most notably, Mz values tended to gradually increase upward from the depth of 30.1 m (the erosion surface) to the depth of 27.5 m and decreased to the depth of 18.1 m. The value of So were divided into two types with a high portion of values were very poor sorted sediments (30-21.9 m of the depth sediment core) and other was poorly sorted sediments. Based on Sk values, sediments could be classified into 4 types, including coarse skewed, symmetrical, fine skewness and very fine skewness sediments. According to KG values, indicating that platykurtic sediments (the depth in core 30.1-26.5 m) and the upper layer was fluctuated in a high range which indicated form very platykurtic to leptokurtic.
In the middle of the Holocene, the sediments comprised of two parts. At the lowest part, sediment facies could be a ravinement surface, which based on transition processes (Li, et al., 2006, Tanabe, et al., 2006). At the depth from 30.1 to 27.5 m, sediment core showed the transgressive sand layer overlie the erosion surface, characteristics of Sk corresponds to lower energy condition, suggesting the abundant origin of sediment inputs. Between 26.7 and 18.7 m in depth of core sediment, the Mz and Sk values varied over a small range, which corresponds to low hydrodynamic energy conditions. A large portion of mesokurtic and leptokurtic sediments suggested the continuous addition of coarse grain-size.
Table .. Sediment parameters in grain size distribution
The environment of this section was interpreted as shelf to pro-delta sediments because shell fragments in this section were decreased than lower layers and the increased mud content (Tanabe, et al., 2006). The sedimentation rate in this section ranged from 0.06 to 1.14 cm/year (Li, et al., 2006).
c. Core section 3: at the depth from 18.9 to 11.7 m
In this section, sediment consisted of reddish gray, black and laminated silt clay. Mz values between 6.40 and 20.49 µm and tended to gradually increase upward. Mud content tended to slightly decrease with ranged from 70.77 to 96.70 %. Sk values were gradually decreased upward and classifying it as symmetrical and coarse skewed sediment. The values of kurtosis showed relative low variation from 0.69 – 1.29, classified as platykurtic, mesokurtic and leptokurtic sediment. In this section, Mz values and sand content slightly increased upward which indicated higher hydrodynamic energy.
The sediment with abundant shell and plant fragments is typical of the delta, therefore the environment of this section changed to delta front slope sediment (Tanabe, et al., 2006). The sedimentation rate in this section ranged from 0.26 to 2.13 cm/year (Li, et al., 2006).
Figure .. Sedimentary columns of the core sample in RRD. (A) Lithological characteristic of sediment core; (B) Mud content in sediment core; (C) Sedimentation rates were determined by linear interpolation between radiocarbon dating (14C) (Li, et al., 2006)
d. Core section 4: at the depth from 11.7 to 4.1 m
Sediment is consisted of gray clay and fine sand with rich of plant matter. Mud values in sediment tended to decrease and the minimum value at a depth of 8.7 m (32.4 %). Mz values tended to increase and ranged between 11.06 and 56.55 µm. So values was a small range from 2.99 to 4.97 with mainly categorized very sorted sediment. Sk value fluctuated and indicated as symmetrical, fine skewed and very fine skewed sediment. KG values fluctuated and predominantly indicating platykurtic and mesokurtic sediment.
The rate of sediment markedly up to 1.94 cm per year in this section during the increasing of delta progradation (Hori, et al., 2004, Li, et al., 2006, Tanabe, et al., 2006), indicating the sediment supplied to the river over the last 2 cal. kyr BP was different than the volume of sediment redistributed or rework which affected by longshore current and waves (Li, et al., 2006, Tanabe, et al., 2006). The coarsening succession upward which suggested that the energy of the environment increased.
The environment of this section was interpreted as delta front platform sediment (Tanabe, et al., 2006). The sedimentation rate in this section ranged from 0.82 to 1.94 cm/year (Li, et al., 2006).
e. Core section 5: at the depth from 4.1 to 0.5 m
In this section, sediment is featured by silt, fine sand, silt with gray in color and minor peat lenses 1-5 cm thick. Sand content tended to slightly increase upward with a maximum value (36.5 %) at the depth of 0.5 m, whilst, Mud content tended to decrease. Mz values varied over a wide range, continuously decreasing from underlying facies to the depth of 1.7 m. The upward increasing trends of So values and reached the maximal value at 1.7 m. In this section, the sediment is dominated by very poorly sorted. While Sk values continuously fluctuated widely which classified from very fine skewed to fine skewed sediments. The values of KG varied over a small range with the exception of the depth interval from 0.8 to 1 m. The major part of this section is covered with platykurtic sediments. The increasing of clay content and a decrease in the values of Mz suggested the decline of wave energy and tidal.
At the depth between 4 and 0.5 m, sediment core suggested decreasing tidal and wave energy. In this section, abundant plant fragment and root and laminated clay indicated it influenced by tidal environmental (Tanabe, et al., 2006). The sedimentation rate in this section interpreted 0.82 cm/year (Li, et al., 2006).
f. Core section 6: at the depth from 0.5 to the core surface
Sediments in this section consisted of reddish brown mottled clay with abundant rootlets because it corresponds to a lateritic weathering profile develop in the floodplain (Tanabe, et al., 2006). The upward increasing trends of mean of grain size. According to the values of sorting, sediment could be classified as very poorly sorted. While, the values of the kurtosis ranging between 0.64 and 0.67, suggesting that the size distribution was platykurtic sediment. Sand content was form 36.5 to 64.9 % and mud content fell to the lowest percentage (35.08 %) which suggested the sediments formed in a high energy environment. The Mz values in the surface were high because it was influenced by channel-levee sediment at the land surface of the core site.
4.1.2. Geochemical proxies (LOI, TOC, δ13C)
a. Loss-on ignition (LOI)
In the depth sediment core from 36 to 30.1 m, LOI values fluctuated with an average of 4.4 %. LOI values tended to gradually decrease from 30.1 to 26.7 m in depth and increased to the maximal values of 21.3 m in depth. LOI values continuously decreased in the depth sediment core between 21.3 and 4.1 m, then slightly increased again from 4.1 to surface sediment core.
Figure .. The variation of LOI (%), C/N ratios, TOC (%) and δ13C (‰) in sediment, Red River Delta with depth (m)
b. C/N ratios
C/N ratios varied from 3.53 to 20.08 with an average of 11.09 ± 3.4 (Fig 4.8). Most notably, the ratios fell sharply to 6.7 at the 30.3 m in depth (the erosion surface). In the depth from 36 to 30.1, C/N rations fluctuated slightly, then it tended to increase upward from 30.1 to 17.3 m in depth. At the depth from 17.3 to 7.7 m, C/N ratios tended to decrease to the lowest value (3.5) at 7.7 m in depth and then fluctuated to the core surface.
c. Total organic content (TOC)
Below the depth of 30.1 m, TOC content gradually fluctuated with an average of 1.1±0.4 % and notice a suddenly decreased (0.45) at the depth of 30.3 m. TOC content varied over a small range at the depth sediment core from 30.3 to 26.5 m, then it tended to decrease between 17.3 and 4.1 m with three exception (at the depth of 4.9, 7.5, and 14.7 m). TOC values tended to increase at the depth from 4.1 to surface sediment core.
d. Correlation between TOC (%) and LOI (%); TOC (%) and TN (%)
The linear regression of TOC content and LOI content (Fig 4.9) indicated that the LOI method could be used to determine TOC content in sediment. In addition, there were strongly significant positive correlation between TOC and total nitro (TN) (Fig 4.10). The regression line for these data approached very close to the origin, which suggested the inorganic nitrogen content was an insignificant and the total nitrogen was chiefly in organic nitrogen. Consequently, the total nitro could be used to calculate C/N ratios, which determined the origins of sediment and reconstruction of the Holocene paleoenvironment (Lamb, et al., 2006, Müller&Mathesius, 1999).
Figure .. Relationship between TOC (%) and LOI (%)
Figure .. Relationship between TOC (%) and TN (%)
f. Stable isotope carbon
δ13C values ranged from -29.09 to -21.95 ‰ with an average of −26.79±1.4 ‰. Most notably, it suddenly increases to value of -23.84 ‰ at the depth of 30.3 m (the erosion surface). δ13C values had small fluctuations from 27.02 to 29.09 ‰ at the from 36 to 30.5 m, then rose and reached the highest value (-21.9 ‰) at the depth of 28.9 m. Between 29.8 and 20.3 m in depth, δ13C values tended to decrease. At the depth between 20.3 and 8.7 m, δ13C value was relatively invariant, then fluctuated to the core surface sediment. Generally, δ13C values displayed an opposite trend with C/N ratios.
4.1.3. Clay mineral content
a. Illite
Illite contents ranged from 13.29 to 78 % with an average of 60.46±17.45 %. Below the depth of 28.1 m, illite contents fluctuated extensively around the mean 49.78±15.90 %. Illite contents teded to increase from 28.1 to 24.3 m in depth, then slightly fluctuated between 24.3 and 2.9 m with a mean of 59.37±12 %. From 2.9 m to the core surface, illite contents tended to increased (Fig 4.11).
Figure .. Variation in clay mineral (smectite, kaolinite and illite) proportion (%) in the core sediment of RRD, Vietnam
b. Kaolinite
Kaolinite contents ranged from 5 to 25 % with an average of 10.75±3.95 %. Below the depth of 30.1 m, kaolinite contents fluctuated around the mean 12.19±5.05 % and reach a peak to a maximum at 33.9 m. In the section between 30.1 and 21.6, kaolinite contents tended to increased and then suddenly decreased at the depth of 21.8 m. Kaolinite contents continuous increased at the depth between 21.6 and 11.3 m, then slightly decreased of 2.9 m in depth. From 2.9 to 0.9 m in depth, kaolinite tended to increase and then drop at the depth from 0.9 to the core surface (Fig 4.11).
c. Smectite
Smectite contents ranged from 1 to 49.33 % with an average of 14.24±13.34 %. Below the depth of 28.1 m, smectite contents fluctuate widely from 4 to 49.3 %, then suddenly dropped at the depth of 27.9 m., smectite contents fluctuated again at the depth from 27.9 to 14.2 m, then increase upward at the depth of 2.9 m. Smectite tended to fluctuated at the depth from 2.9 m to the core surface. In comparison, variations in the two most abundant ones (smectite and illite) have opposite trends (Fig 4.11).
4.2. Babe Lake
4.2.1. Lithological characteristics
The parameters show sediment grained size distribution, which used to reconstruct the sediment environments (McLaren&Bowles, 1985). They are impacted by the sediment transport dynamics. In this study, Mz values showed the sediments were classified from the sandy silt to silt, with a predominant coarse silt, following geometric Folk and Ward graphic measures (Folk&Ward, 1957).
Figure .. Grain-size distribution and density of the core AT in Ao Tien, Vietnam
The dominance of sediment composed of sand, silt and ranged from 6.04-27.67 %, 72.33-93.96 %, respectively (Fig 4.12). The clay fraction content displayed very small. The color of sediment is characterized by dark gray bands, it suggested that sediments could be formed in an aerobic environment because the depositional environment occurred in the lake bottom.
a. Core section 1: from 108 to 90 cm
In this section, sediment is characterized by sandy silt and composed of the fine sand and coarse silt. Mud content varied in cross a wide range from 76.01 to 83.58 %. The mean sediment grain size (Mz) fluctuated slightly between 23.73 and 33.63 µm with a mean of 29.12±3.21 µm. In this section, the mean sorting (So) value ranged from 2.96 to 3.42, classifying it as being very poorly sorted sediment (Fig 4.13). The skewness (Sk) values were ranged between 0.3 and 0.36 with a dominance of a very coarse skewness sediment.
Figure .. Triangle graph of grain-size composition shows that the sediment of AT core was mainly composed of sandy silt and silt
b. Core section from 90 to 23 cm
This section consisted of coarse silt, medium silt and fine sandy. The increasing trends of mud content and reached the highest value (93.96 %) at a depth of 51 cm. Mz values fluctuated widely and reached the maximum (28.97 µm) at a depth of 81 cm. So values were closely correlated with the Mz values. According to the different of So values, the dominance of sediment could be classified as poorly sorted. Sk values fluctuated widely, suggesting is as very coarse skewness to coarse skewness sediments (Fig 4.14).
c. Core section from 23 to 0 cm
In this section, sediment is characterized by fine sandy and medium silt. Mud content markedly decreased upward to the depth of 13 cm (72.33 %), before continuously increasing to the sediment core surface. Mean grain size tended to value of 30.75 µn at13 cm in depth, before decreasing upward. So values were also closely correlated with the Mz values. In this section, sediment categorized as poorly sorted. Sk values can be divided into two-part. In the first part (from 23 to 9 cm), the Sk values tended to increase with the maximum value was 0.48, however, the Sk values from 9 cm to surface sediment core tended to decrease and the dominance of sediment is classified as very coarse skewness (Fig 4.14),
Figure .. Depth variation of sedimentary parameters (Mz,So, Sk, KG) and grain size distribution in the core sample AT
4.2.2. Geochemical proxies (OM, C/N ratio, δ13C)
a. Loss-on ignition (LOI)
Organic matter is estimated from performing the loss on ignition (LOI). LOI values varied across a wide range from 5.19-20.69 %, with an average of 12.69±4.88 %. Below the depth of 91 cm, LOI values tended to slightly increase with a maximum at the depth of 95 cm (20.69 %), then decreased at the depth of 53 cm. LOI values content fluctuated with a small range at the depth from 53 to 27 cm, although LOI values displayed a two peak at depth of 41cm, 31 cm with ranged 10 %, 10.45 %, respectively. At the depth between 27 and 15 cm, LOI values tended to increase, then continuously decreased upward (Fig 4.15).
b. Organic carbon (OC) content
Organic carbon content ranged from 16.67-36.55 % with an average of 26.2±6.44 %. At the depth below 57 cm, OC content varied over a small range and tended to slightly increase. From 57 to 27 cm, OC content displayed higher variation and then decreased to the surface sediment (Fig 4.15).
c. C/N ratios
C/N ratios fluctuated between 10.68 and 17.01, with an average of 13.23±1.3. Below 91 cm, ratios tended to gradually increase upward, then slightly decreased at the depth of 23 cm. Between 23 and 9 cm, the upward increasing trends of C/N ratios and then relatively stabled from 8 m to the core surface. Generally, C/N ratios were quite closely correlated with the LOI values (Fig 4.15).
d. Stable isotopes carbon (δ13C and δ15N)
δ13C values varied from -28.57 to -36.8 ‰ with an average of -32.23±2 ‰. The data shows an inverse trend of the relationship between the δ13C values and C/N ratios. Below the depth of 69 cm, δ13C values had small fluctuations from the core bottom to 69 m in depth, then suddenly fell to a value of 34.99 ‰ at 53 cm in depth. From 53 to 23 cm, δ13C values displayed a small variation, before increasing to the depth of 11 cm and then decreased to the surface sediment (Fig 4.15).
δ15N values slightly fluctuated from 5.33 to 7.67 ‰, with an average of 6.2±0.6 ‰ and can be divided into two-part. Below the depth of 47 cm, δ15N values tended to gradually decrease, then displayed a small variation from 47 m to the surface sediment with the exception of samples near the surface (Fig 4.15).
Figure .. The variation of LOI (%), OC (%), C/N ratios, δ15N and δ13C in the sediment core with depth (cm)
e. Correlation between LOI and other sedimentary parameters
LOI values is positively correlated with density of sediment, C/N ratio and δ13C values follow by equations: Density (g/m3) = 68.34*LOI – 37.3 (R2=0.66), C/N ratios = 0.23*LOI + 10.25 (R2 = 0.69), δ13C = 0.45*LOI – 37.87 (R2=0.66), respectively. Inversely, LOI values are negatively correlated with OC content, the linear regression line has an equation of the form TOC = -0.84*LOI + 36.83 (R2=0.4) (Fig 4.16), which means the sample with high of LOI values, as well as OC content was low.
Figure .. Relationship between LOI and density of sediment (a), LOI and OC content (b), LOI and C/N ratios (c), LOI and δ13C values
f. Correlation between OC content and other sedimentary parameters
Organic carbon (OC) content is positively correlated with organic nitrogen (Norg), the linear regression line has an equation of the form Norg = 0.1*OC-0.5 (R2=0.94). The relationship between OC content and Norg suggested that organic carbon and total nitrogen in sediments mainly originated from organic matter. In contrast, OC content is negative correlated with C/N ratios and δ13C, follow by equations: C/N = -0.13OC+ 16.7 (R2=0.69), δ13C=-0.36*OC-22.82 (R2=0.75), respectively (Fig 4.16).
Figure .. Relationship between OC content and Norg (a), OC content and C/N ratios (b), OC content and δ13C values
RECONSTRUCTION OF PALEOENVIRONMENT AND PALEOCLIMATE IN NORTHERN VIETNAM AS INFERRED FROM SEDIMENTALOGICAL AND GEOCHEMICAL DATA
5.1. Reconstruction of Paleoenvironmental and Paleoclimate in Ao Tien Lake
The variation in sedimentary parameters describing sediment grain-size distribution have used to reconstruct the depositional environment of sediments (Angusamy&Rajamanickam, 2006, McLaren&Bowles, 1985, Visher&Hughes, 1969). Those studies suggested that textural parameters are affected during the transportation and depositional processes. In this paper, the mean grain size values suggested that the sediments were determined from fine sand to medium silt with mainly of coarse silt. This pattern suggested the prevalence of comparatively low energy conditions. The Mz values reflected lake water level with time (Xiao, et al., 2009) and corresponded to a stronger warmer/wetter climate in the surrounding lake (Yanhong, et al., 2006).
Figure .. Bi-plot of δ13C and C/N ratios for sediment core in Ao Tien Lake. According to Lamb et al. (2006), C/N ratios of organic matter originated from phytoplankton, bacteria and algae typically generally <10, while originating from terrestrial vegetation, C3 vascular plant material normally >12 and the δ13C values of C3 plants varies between -33 and -23 ‰
According to Weide (2012), sedimentation rates in Ba Be lake was calculated by 14C and 137Cs dating in sediment core with the depth of 212 cm. This result shows the sedimentation rate in the bottom of the core (150-165 cm) is estimated at 0.1 cm/year, from 130.5 to 139.5 cm, the sedimentation rates increase to 0.2 cm/year and slightly increasing to 0.23 cm/year from 61-26 cm (Weide, 2012). Moreover, the source of water in Ao Tien Lake exchanged with Ba Be Lake by the karst system. In core AT, the Mz values, sand contents, silt content had small fluctuations which suggested the source of sediment input from surrounding areas must be maintained in a stable condition/environment. In the present study, the average sedimentation rates in Ba Be lake below the depth of 130 cm could represent for the Ao Tien lake’s sedimentation rate, which means that the core length in Ao Tien corresponds to the last 700 years (from 1300 AD to present).
In unit 1 (from 108 to 90 cm), the Mz values and sand contents tended to slight decrease and inversely, mud contents tended to gradually increase. This pattern indicated that hydrological regimes are a favourable environment for fine-grain particles (Liu, et al., 2008). LOI values, C/N ratios and δ13C had small fluctuations and they are the highest than other sections, indicating high lake water level and warmer climate (Yanhong, et al., 2006). LOI values and C/N ratios reached a maximum at the top sediment as 20.69 %, 17.01, respectively, suggesting a high amount of organic matter input at Ao Tien Lake. In addition, δ13C values in this section were quite stable with an average of -28.9 ‰, indicating a large proportion of C3 plants in organic matter (Lamb, et al., 2007) (Fig 5.1). The core length in unit 1 corresponds to a period from 1300 to 1424 years AD. The record δ18O measurements of stalagmite calcite in southern China (Dongge Cave) showed this period of high Asian monsoon intensity with a high precipitation, a decrease of δ18O values (Dykoski, et al., 2005).
In unit 2 (from 90 to 23 cm), the sediment core could be divided into three parts. In the lowest part (from 90 to 57 cm), all LOI values, C/N ratios and δ13C, δ15N tended to decrease with an average of 13.97±2.97 %, 13.92±0.85, -30.20 ±1.04 ‰ and 6.23±0.39 ‰, respectively. In addition, the parameters as Mz, So also tended to decrease but it is not clear. Sand contents suddenly decreased with an average of 15.74±2.96 %. Mz values tended to a slight decrease. So values were classified as poorly sorted sediment. In addition, the organic matter tended to change from high C3 plants to lake microalgae (Talbot&Lærdal, 2000). C/N ratios and δ13C values tended to decrease which suggested that organic matter with a dominance of microalgae (Lamb, et al., 2007). δ15N values tended to decrease suggested that a large proportion of microalgae/cyanobacteria and they can concentrate nitrogen from the atmosphere for growing up, thus reducing δ15N values in the sediment (Fig 5.1). The record δ18O measurements of stalagmite calcite in southern China (Dongge Cave) and tree-ring base hydroclimate (reconstruction the Palmer Drought Severity Index – PDSI) showed this period of weak Asian monsoon intensity with a decrease of precipitation and variation in PDSI remained low (Buckley, et al., 2010, Dykoski, et al., 2005). In this period, the weakening of monsoonal activity in Northern Vietnam was supported by the percentage decrease in tropical rain forest and lake water level. In the middle part (from 57 to 45 cm), LOI values, C/N ratio, δ13C values, and δ15N varied over a small range and they tended to decrease. While the variation of mean grain size and sand content remained low. It is suggested that the enhanced microalgae biomass with a low level of water in Ao Tien Lake. The decreasing δ13C and δ15N values showed the dominance of chrysophyceae and cyanobacteria in lake microalgae. The results of, Dykoski et al., (2005), Buckley et al., (2010) also showed the decreasing precipitation in the first part, then continuously increased upward (Buckley, et al., 2010). Thus, the δ18O values of stalagmite calcite in southern China (Dongge Cave) tended to decrease (Dykoski, et al., 2005). In the last part (from 45 to 23 cm), the parameters of sand contents, Mz values and So values tended to slightly increase. LOI values, δ13C values, δ15N values and C/N in this part tended to increase with an average of 8.23±2.58, -34.57±0.95 ‰, 5.97±0.13 ‰, and 12.18±0.56. This pattern suggested a high amount of sediment and organic matter into the lake from the surrounding area and the increase of organic matter derived from microalgae and C3 plants which grown up in the area surrounding. Thus, the lake water level was increased due to a high amount of water meteoric input at Ao Tien Lake. Otherwise, the enhanced Asian monsoon regime. These results were similar to those of the record δ18O measurements of stalagmite calcite in southern China (Dongge Cave) (Dykoski, et al., 2005). At the end of this part, while the LOI values increased, C/N ratio and δ13C decrease, a variation of δ15N changed with time. The pattern indicated microalgae biomass developed with the high lake water level. At some point, δ13C values increased, it can occur when 13C content of inorganic carbon dissolved in water (Leng, et al., 2006). The core length in unit 2 corresponds to a period from 1424 to 1864 years AD.
In unit 3 (from 23 cm to the core surface), the sediment core can be divided into two distinguished part. In the lower part (from 23 to 9 cm), sand contents, Mz, So are strongly increased. While LOI values tended to slightly increase than the underlying part with an average of 15.31±2.73 %, C/N ratios and δ13C also tended to gradually decrease with an average of 13.49±0.41 and -33.06±0.7 ‰. These results indicated the enhanced organic matter derived from C3 plants grown up in the area surrounding due to higher lake water level. The comparative with the record δ18O measurements of stalagmite calcite in southern China (Dongge Cave) and tree-ring based hydroclimate (reconstruction the PDSI) showed an increase of precipitation at the first of the part and slightly decrease at the end of this part (Buckley, et al., 2010, Dykoski, et al., 2005). In the upper part (from 9 to surface sediment core), sand contents, Mz values and So values markedly dropped with an average of Mz as 21.52±3.93 µm. Similarly, LOI values, C/N ratio, and δ15N values tended to decrease upward. δ13C values suddenly decreased and reached a minimum at 3 m in depth. This pattern indicated the decreasing amount of water input at Ao Tien Lake. Variation of δ13C and C/N ratios remained low, suggesting the organic matter contents had derived from microalgae with a dominance of chrysophyceae (Lamb, et al., 2007, Vuorio, et al., 2006). The core length in unit 3 corresponds to a period from 1864 AD to present.
5.2. Recontruction of paleoenvironment and paleoclimate in the Red River Delta
5.2.1. Source of sediment organic matter
Investigating source of sediment organic matter provide a wealth of information on the depositional processes (Leng&Marshall, 2004, Lorente, et al., 2014), the paleoenvironment, palaeoclimate (Dykoski, et al., 2005, Reotita, et al., 2014, Tue, et al., 2011), sediment transport (Liu, et al., 2016) and sea-level fluctuation (Wilson, et al., 2005).
Figure .. Bi-plot of δ13C and C/N ratios for sediment core in the sediment core (the RRD, Vietnam)
Stable isotopes (e.g., carbon stable isotope δ13C) have been largely used to investigate the source of organic matter in sediment, including the marine environment, estuary, lagoon, inter-tidal zone and delta plain environment (Amano, et al., 2006, Liu, et al., 2014, Müller&Mathesius, 1999, Wilson, et al., 2005). According to Lamb et al., (2006), terrestrial vegetation, plant material have relatively high C/N ratios of >12, which is contained predominantly of lignin and cellulose and are nitrogen-poor. While phytoplankton, bacteria and algae typically have low C/N ratios of <10 (Meyers, 1997). Most notably, C/N ratios in organic matter of sediments could be impacted by a differentiation of the degradation processes (Lamb, et al., 2006, Wilson, et al., 2005). Plants can be divided into three major groups: Crassulacean acid metabolism (CAM), C3 and C4 plants. C3 plants have no special features to combat photorespiration whereas C4 plants minimize photorespiration by separating initial CO2 fixation and the Calvin cycle in different cell types. CAM plants are characterized by a minimize photorespiration and save water during time between night and day. They are determined base on the variation of δ13C due to their changing in photosynthetic pathways (O'Leary, 1981). C3 and C4 plants have the δ13C values vary from -33 to -23 ‰ and -15 to -9 ‰, respectively. Besides, δ13C values in marine phytoplankton ranged from -22 to -18 ‰ (Lamb, et al., 2007). Therefore, using a combination of δ13C and C/N values, they were able to distinguish sedimentary organic matter sources (Fig 5.2).
According to the geochemical proxies demonstrated a clear negative relation between δ13C values and C/N ratios in sediment cores from the RRD. This result similar of several previous research results in sediment core and surface sediments from coastal mangrove forests in Red river estuary (Tue, et al., 2014, Tue, et al., 2011). The variation of δ13C values and C/N ratios, investigating the missing of C4 plant material in sediments along the Holocene.
Using C/N ratios and δ13C was determined sedimentary organic matter sources in the previously published data, including C3 plants (Cloern, et al., 2002, Meyers&Lallier-Vergès, 1999); particulate organic matter (POM) (Tue, et al., 2012); Marine phytoplankton and freshwater aquatic plants (Cloern, et al., 2002, Liu, et al., 2007). Based on C/N ratios and δ13C values in sediment cores, suggesting that the organic matter was mainly sourced from terrestrial vegetation (in which C3 plants suggesting terrestrial sources occupied about 40% of total the sediment sample) and marine phytoplankton (occupied about 10%). The scatter diagram proved that the distribution of the other sediment samples between terrestrial sources and marine phytoplankton, showing a mixture of two distinctive organic source (Lamb, et al., 2007).
Below at the depth of 30.1 m, this sediment section was characterized by low δ13C values (< -27 ‰) and high C/N ratios (> 12), which showed that the sediment of this section was sourced from terrestrial vegetation (C3 plants) (Lamb, et al., 2007, Lamb, et al., 2006). Most notably, the variation of δ13C value and C/N ratio at the erosion surface suggested the source of organic matter varied from terrestrial (C3 plants) to marine phytoplankton. The variation of δ13C values and C/N ratios at the depth from 30.1 to 18.9 m suggested that the sedimentary organic matter was originated from marine phytoplankton, which located mainly in the lower layers of the sediment core and then gradually decreased. In which, in the part under (between 30.1 and 26.3 m in depth), the C/N ratios tended to increase and δ13C values sharply dropped, which suggested that the dominant of the marine phytoplankton in the organic carbon source. In the part below (from 26.3 to 18.9 m in depth), the upward decreasing trends of δ13C values and C/N ratios slightly increased, which investigated a decline of marine phytoplankton in the organic carbon source (Corner, et al., 1971, Liu, et al., 2007). In the core sediment section between 18.9 to 11.7 m, the variation of C/N ratios and δ13C values indicated a significant proportion of the sedimentary organic matter was originated from terrestrial C3 plants. In the last core sediment section from 11.7 m to core surface, δ13C values slightly increase and C/N ratios gradually decreased, indicating a decline of terrestrial vegetation (C3 plants) in the organic carbon source, it could be caused by a increasing of particulate organic matter or freshwater aquatic plants in the estuary environment.
5.2.2. Source of clay minerals during the Holocene
Clayey sediment, weathering provenance, depositional processes and sediment transport, are all factors affected to the homogenous association of clay minerals in marine and coastal sediments (Chamley, 1989, Pandarinath, 2009, Wang&Yang, 2013). Estuarine and delta have received huge volumes of sedimentary detrital material from a vast drainage basic. The features of these locations are defined by a region's geology and impacted by chemical, physical, and climatic conditions. Thus, the location of the receiving basin represents an average composition of clay minerals sourced from the whole catchment (Wang&Yang, 2013). In the study area, the multi-sourced clayey sediments derived from the Song Hong catchment might have been well mixed in the marginal marine environments whereas the clays in the floodplain and river channel better reflect their source characteristics due to rapid transport and deposition. The appearance of gibbsite-kaolinite assemblage suggested not only a strong chemical weathering volume of supply source during the last glacial maximum but also fluvial activity of regression stage in the wet and warm conditions (Li, et al., 2006). The concentration of smectite assemblage in sedimentary environments is controlled by sediment derivation from a different source terrain and also by grade (particle size) deposit. Because smectite remains in suspension, which transported further offshore during the long time. In addition, during the transportation process, the sediment could be carries detrital material from delta plain and passed into shallow marine.
The abundance of Si ions in the sedimentary environment along the dry conditions could be leaded to the substitution of Al ions, which formed smectite-illite mixed layer and smectite materials. Inversely, heavy rainfall and strong chemical weathering along warm/wet condition, Si ions and other cations could be released and formed a large amount Kaolinite. However, in the marine environment illite could be leached to transform to smectite-illite mixed layer, smectite, which caused by the substitution of K+ by Na+(Meunier, 2005). Finally, material is deposited in the marine environment during the sedimentary transport (Chamley, 1989).
5.2.3. Holocene evolution of the Red River Delta, Vietnam
The Holocene sedimentary system of the Red River Delta has been affected by rivers, wave and tidal processes. According to the lithological characteristics in sediment core was similar to the core VN from the last study in the RRD by Tanabe et al., (2006). The result showed that the deposition environment in the sediment core was closely correlated with the core VN which was studied by Tanabe et al., (2006). Consequently, they are likely derived due to similar mechanisms. The sediment texture was related to processes of marine and riverine, the spatial patterns of hydrological and the Holocene evolution of RRD passed through three stages: state I (from 11.26 to 8.86 cal. kyr BP); state II (from 8.86 to 2.29 cal. kyr BP); and state III (from 2.29 cal. kyr BP to present).
During the state I (corresponded from 11.26 to 8.86 cal. kyr BP), the sea level was rising (Steinke, et al., 2003, Tanabe, et al., 2006). In this sate, the sea level in Vietnam was about 31 m below prevent sea level and increased with a constant rate of about 9 mm per year (Tjallingii, et al., 2014). The reddish-gray color of the sediment showed erosion processes during low stands. The sedimentary parameters illustrated a large portion of fine sand and coarse silt sediments, suggesting the dominance of sediment is originated in high-energy condition (McLaren&Bowles, 1985, Rajganapathi, et al., 2013). The upward increasing trends of C/N ratios (with an average of 15.69) investigated that the dominance of organic matter sourced from terrestrial vegatation (Lamb, et al., 2007, Tue, et al., 2011). In addition, δ13C values fluctuated between approximately -29 ‰ and -23 ‰ and it is similar to the result in the coastline of the RRD by Tue, et al., (2012), together indicating that the sedimentary organic carbon could be formed by mangroves (Tue, et al., 2012). Thus, this period could be defined as the sub- and intertidal environment which is strong controlled by tidal flooding (Tanabe, et al., 2006).
The state II (corresponded from 8.86 to 2.29 cal. kyr BP) has divided into two sub-state according to textural sediment, geochemical proxies and feature of lithology.
– At the lowest section (sub-state IIa), C/N ratios suddenly decreased and δ13C values increased upward, together indicating that sedimentary organic carbon sources are quickly changed from C3 plants to a dominance of marine phytoplankton (Lamb, et al., 2006, Wilson, et al., 2005). The sediment suggested as an erosional surface which have been formed along the transgression (Tanabe, et al., 2003, Tanabe, et al., 2006, Tanaka, et al., 2011). In the period from 8.8 to 4.06 cal. kyr BP, the RRD was slightly submerged on the water surface caused by the rising of sea level (Tanabe, et al., 2006), leading to the depth water of sediment core (between 7 to 6 cal. kyr BP) located about 30 m lower than at present, caused by the acceleration of sea-level rise. This result is similar with the previous publish in RRD by Tanabe et al., (2006). In addition, the upward increasing trends of Mz values (between 30.1 and 27.5 m in depth) investigated the erosion surface have been overlain by sand sediment. C/N ratios decreased to the lowest value and δ13C value increased to the maximal values, together suggesting the dominance of organic carbon formed by marine phytoplankton (Fig 5.2). The pattern only explained when the sediment is formed under the high sea level during transgression period. It is similar to the previous studies, Tanaka et al., (2011) were investigated a huge of marine ostracod records in the sediment core (VN) of the RRD. Results showed that the highest of sea level were observed in the sediment layer at the depth between 28.7 and 26.7 m (about from 7.2 to 5.7 cal. kyr BP). A previous investigated that the sediment in this section formed under wave-dominated conditions by Tanaka et al., (2011). The mean grain size and Sk values are varied in a small range at the depth between 26.7 and 18.7 m, indicating that the deposition environment with a low hydrodynamic energy. This period is characterized by a dominance of mesokurtic and leptokurtic sediment which showed coarser-grained sizes are continuously deposited during the transportation and tidal activities. C/N ratios tended to gradually increase upward and the small variation in δ13C values, together indicating the sedimentary environment could be considered as a shelf-prodelta.
– At the upper section (sub-state IIb), the increasing trends of C/N ratios and δ13C values varied in a small range indicated that sedimentary organic carbon originated from diverse sources between terrestrial vegetation (C3 plants) and marine phytoplankton, or deposited from particulate organic matter. This result shows the sediment facies of the RRD is higher affected by the fluvial sediment during this period (Duc, et al., 2007, Van Maren&Hoekstra, 2005). The sand fraction and Mz values tended to increase slightly at the depth between 18.7 and 11.7 m, which indicated that the hydrodynamic energy of this sub-state is higher than the lower layers (McLaren&Bowles, 1985). Skewness values varied from coarse to fine skewed sediment which suggested that the upward increasing trend of fine-grained particles, whilst, coarse-grained particle tended to decrease and a relatively high riverine input. The gradual change of textural sediment, feature of lithology and stable isotopes (C/N ratios, δ13C values) in the underlying sediment facies, together suggesting that the sedimentary environment could be defined as delta front slope (Hori, et al., 2004, Li, et al., 2006)
In the state III (corresponded from 2.29 cal. kyr BP to present), the sea level tended to decrease to the present sea level. Notably, the sediment in the mouth have been formed shoals (Hori, et al., 2004, Li, et al., 2006). At the depth from 11.7 to 4.1 m (corresponded from 2.29 to 1.72 cal. kyr BP), the rate of sediment accumulation increased to the maximal value (1.94 cm each year) in this state (Li, et al., 2006, Tanabe, et al., 2003) and active progradation became dominant in the delta system (Li, et al., 2006, Mathers&Zalasiewicz, 1999). The increasing trends of the sand percentage, silt and clay percentages tended to decrease upward, together suggested that the coarser-grained size became predominated. Because they are concentrated in a shallower with low energy condition and river-dominated which passed into the delta system. δ13C values and C/N ratios varied in a large range which suggested that strongly interacted between C3 plants and particulate organic matter in the estuary (Fig 5.2). The sediment in this state was formed close to the paleo-estuary, consequently, the sedimentary environment corresponded to the delta front platform (Duc, et al., 2007, Tanabe, et al., 2006). According to the study by Van den Bergh et al., (2007), this state corresponded with the depth lower than 10 m which located near estuary dunes. In addition, a study by Tanabe et al., (2006) also investigated that the amount of sediment inputted to the delta system during the last 2 cal. kyr BP was higher than the sediment volume supplied from marine activities. In the depth from 4.1 to 0.5 m (corresponded from 1.72 to 0.55 cal. kyr BP), the increasing trends of clay fraction and mean grain size decreased upward, which suggested that the sediment was formed in low energy condition (Rajganapathi, et al., 2013). Moreover, C/N ratios (< 10) showed that the prevalence of particulate organic matter from sediment which was transported by tidal (Lamb, et al., 2007). While the variation of δ13C suggeted that C3 plants became predominated (Lamb, et al., 2007). The coordinated variation of both values above (C/N ratios and δ13C values) was similar to the result reported by Wilson et al., (2005) for tidal flat environment. Thus, the sedimentary environment of this core section corresponded to tidal a flat environment. At the depth from 0.5 to the surface core (corresponded from 0.55 cal. kyr BP to present), the lithological characteristics were similar to a floodplain environment (in core VN) which reported by Tanabe et al., (2006). More notable, the Mud content fell to the lowest value, suggesting that the sedimentary environment of this core section was interpreted as flood plain environment.
5.2.4. Clay mineralogy indicating the monsoon along the Holocene
The variability of monsoons along the Holocene in the Red River Delta is strongly impated by the East Asian Monsoon (EAM) system. Reconstruction of the paleoclimate from lake sediments (Sun, et al., 2017), stalagmites in caves (Dykoski, et al., 2005, Wang, et al., 2005), and marine sediment cores in the Gulf of Tonkin (Li, et al., 2010) have shown that the variation in EAM caused a strong shift in the summer and winter monsoon.
Figure .. Variations in smectite/illite, kaolinite/illite and kaolinite/smectite ratios for the core from RRD. The clay/silt ratios, TOC/TN ratios and sea level are show for comparision
Clay mineral contents of the core sediment can provide wealthy information on continental weathering under different climate regimes (Biscaye, 1965, Chamley, 1989, Thiry, 2000, Wang&Yang, 2013). According to the distribution of these (S+K)/(I+C) ratios, they can be divided into different dominant weathering regimes (cold/warm). In most continental environments, illite is mostly formed by a combination of strong physical weathering in a relatively very cold or arid climate and their predominance in the clay fraction usually indicates limited hydrolysis. While kaolinite and smectite are generally formed by chemical weathering under relative hot and humid climate with conditions of intense hydrolysis (Sionneau, et al., 2008, Sun, et al., 2017, Wang&Yang, 2013). In the studied core, smectite/illite, kaolinite/illite and kaolinite/smectite ratios present significant variation between 0.01-2.67, 0.08-0.61 and 0.16-9, respectively (Fig 5.3).
In addition, we assumed that the monsoon intensity variations along the Holocene caused fluctuations in precipitation and freshwater flux by the Red River Estuary into the sea. The paleoenvironmental proxies (Mz, TOC, C/N ratios, δ13C) in sediment core could be closely related to the monsoonal variability that caused fluctuations in freshwater discharge from major tributaries of the Red River Delta. Because sedimentary organic carbon sources preserved in sediment core were C3 plants, riverine phytoplankton, estuarine particulate organic matter (POM) and marine phytoplankton. The change in precipitation by strengthening/weakening of the monsoon has been significantly affected by the fluxes of freshwater, sediments and organic carbon along the Holocene.
From 11.26 to 9.6 cal. kyr BP, the clay minerals component of this period is characterized mainly by kaolinite-rich than others. The kaolinite widely fluctuated and reached the highest values at the top sediment layer in this section which suggested that a slightly warmer climate than at present and enhanced chemical weathering. Organic geochemical data further imply that the paleoclimate was warmer than at present (Li, et al., 2006, Tanabe, et al., 2006). Base on pollen and organic in the Changjiang, China implies that the enhanced of East Asian Summer Monsoon with abundant rainfall and dominance of chemical weathering along the early Holocene (Wang&Yang, 2013). Moreover, the clay fractions and C/N ratios were slightly higher and δ13C values were the more negative, suggesting higher terrestrial carbon inputs or an increase in freshwater flux. This is supported by pollen records from the RRD indicating a warm and wet period due to the main from tropical pollens (Li, et al., 2006, Tue, et al., 2019).
During the period between 9.6 and 8.4 cal. kyr BP, the illite values tended to increase which caused by reducing rainfall and the dominance of physical weathering. The pollen and carbonate records from northeastern Cambodian lake sediment also showed the climate was cooler and drier than the present for at least 1000 years before 8400 14C years (about 9.3 cal. kyr BP) (Maxwell, 2001). The result similar to the reported from northeast Thailand, the cooler and dry period of about 9300-8.54 cal. kyr BP (Udomchoke, 1989). In addition, the acceleration of sea-level rise, the TOC and C/N ratios were low and δ13C values markedly increased, suggesting that a decline in the freshwater flux concerning the transgression process of seawater. Pollen records from core VN also investigating that the climate during this interval was dry and cold, as inferred from the percentage decrease in both tropical semi-evergreen forest and tropical rain forest (Fig 5.4f) and the dominance of temperate taxa (Li, et al., 2006). In this period, the marked increase in stalagmite δ18O values from Dongge Cave (China) also indicated a dryer phase (Fig 5.4h) (Dykoski, et al., 2005).
From 8.4 to 5.28 cal. kyr BP, the kaolinite values tended to increase, caused by enhancing of rainfall and the dominance of chemical weather. A large reduction of δ13C values suggests that the maximum development of warm conditions and precipitation was reached at about 8 cal. kyr BP. In the Red River Delta, this period is considered as the Holocene climatic optimum with the high development of vegetation (Fig 5.4g) (Li, et al., 2006). The relatively high temperature and precipitation could be important factors intensifying erosion processes on land and transport the fine-grain size sediments to the Red River catchment. The study of the Hanjiang delta, southeast China also reported the warm climate during the middle Holocene, which began at 8.3 cal. kyr BP (Zheng&Li, 2000). Beside, Maxwell studied in northeastern Cambodia and showed conditions were relatively warm and moist during the period 8.4-5.3 cal. kyr BP (Maxwell, 2001).
Figure .. Comparison of the results from sediment core with other paleoclimate proxies. (a-d): Mean sediment grain sizes (Mz), TOC (%), C/N ratios and δ13C analyzed in sediment core; (e-f): Tropical pollen proportions (%) and total pollen concentrations in sediment core VN (Li, Saito, Matsumoto, Wang, Tanabe, et al., 2006); (g) Spelepthem δ18O record from Dongge Cave in southern China (Dykoski et al., 2005)
Between 5.28 and 3.09 cal. kyr BP, the illite values tended to increase which caused by reducing rainfall and the dominance of physical weathering. The gradual increase in Mz values, the high fluctuation of TOC and C/N ratios suggest that freshwater and organic matter fluxes from river systems may have declined. The weakening of monsoonal activity in this period was supported by pollen records which demonstrated a significant decline in the diversity of vegetation in the Red River Delta (Li, et al., 2006). Additionally, the slight increase in the stalagmite δ18O values from Dongge Cave suggested a reduction of precipitation during this period (Dykoski, et al., 2005).
From 3.09 to 2.1 cal. kyr BP, a warm and dry climate is shown by the kaolinite and smectite values increased. Although kaolinite and smectite values show a slight decline after the depth of 11.3 m, however, a large portion of kaolinite and smectite in this period were higher than the lower part. The high abundance of tropical taxa of the core sediments VN also suggested similar paleoclimatic changes with warm climate (Li, et al., 2006).
Between 2.1 and 1.54 cal. kyr BP, the kaolinite contents tended to decrease which suggesting weaker chemical weathering and also the climate became cooler than at present. The increase in Mz and δ13C values suggested a decrease in freshwater flux. This hypothesis is supported by the decrease in the percentage of tropical pollen species and total pollen concentration in core VN (Li, et al., 2006). The abrupt increase in the stalagmite δ18O values from Dongge Cave (Dykoski, et al., 2005) also suggested the continuous weakening of the Asian monsoon intensity across the East Asia continent. Base on the regular variations of crystallographic indices in the core shows the decreasing of chemical weathering and monsoon precipitation during the late Holocene in the Changjiang Delta, China (Wang&Yang, 2013).
From 1.540 cal. kyr BP to present, a warming character is indicated by the kaolinite and smectite values increased and the dominance of chemical weathering. An increasing of freshwater flux is indicated by the decrease in δ13C. The clay mineral assemblages of the Chanjiang Delta also showed a weak monsoon precipitation along the late Holocene (Wang & Yang, 2013). Moreover, human activities are considered to affect on the natural vegetation. Rice cultivation was carried out in the RRD over the past 2000 to 3000 years (Oscar, 1995). After 1540 cal. years BP, expansion of agricultural cultivation intensified as the climate became warm (Dykoski, et al., 2005).
CONCLUSIONS
1. Climate change along the Holocene has been affected by natural factors (climate cycle, monsoon, solar irradiance, orbital forcing, solar activity, volcanic forcing,…) and human activities (using fossil fuels deforestation, conversion of land use purpose,…).
Climate viability along the Holocene in the World has been divided into three major periods: between 9-8 cal. kyr BP; classic “cool poles, dry tropics”; “cool poles, wet tropics” started at ~0.6 cal. kyr BP.
– The period between 9-8 cal. kyr BP was characterized by severe climatic disruption. Widespread aridity midway through humid period in low latitudes during the early Holocene;
– Following the period between 9-8 cal. kyr BP, the climate characterized by low latitude aridity and high latitude cooling. The most extensive of these reorganizations occurred from 6-5 to 3.5-2.5 cal. kr BP.
– During the “cool poles, wet tropics”, starting at ~0.6 cal. kyr BP, the climate in both poles are cold and windy.
Some proxies preserved in the environmental record which have been applied for reconstruction Holocene paleoclimates. The study of paleoclimate along the Holocene provided some benefits especially increase knowledge on the climate change characteristics and interaction between the ecosystems and the combined effect on the ecosystems. On the Earth's surface, some types of matters could be stored some proxies which have been used to reconstructing Holocene climate records as lacustrine sediments, tree-rings, coral and speleothems.
2. According to the results of pollen, ASM-14C and facies study on two sediment core (VN and GA cores) from the RRD, the Holocene sea-level could be classified into the following three phases: from 9 to 6 cal. kyr BP (phase I), sea level was enhanced from 15 m below the PSL to 3 m above the PSL (with a rate increase of 6 mm per year); between 6 and 4 cal. kyr BP (phase II), sea level was stable; from 4 to 0 cal. kyr BP (phase III), sea level fell from 3 m above the PSL to the present levels with a mean rate of approximately 0.8 mm per year.
3. Textural parameters and grain-size distribution provide a wealth of information on the properties of sediments and depositional conditions. In addition, C/N ratios, δ13C, δ15N used to reconstruct the environment changes in Ao Tien Lake, Northern Vietnam. Sediments in Ao Tien were transported and deposited by flowing water due to climate change. In this study, the sediment in core AT could shows the changing paleoenvironments in Northern Vietnam during about 700 years (from 1300 to present). The variation of lithological characteristics, textural parameters (Mz, So, Sk, KG) and geochemical proxies (LOI, C/N ratio, δ13C, δ15N) suggested the sediment in core can be divided into three periods: unit 1 (from 108 to 90 cm); unit 2a (from 90 to 57 cm); unit 2b (from 57 to 45 cm); unit 2c (from 45 to 23 cm); unit 3a (from 23 to 9cm); unit 3b (from 9 cm to the core surface). Covariation of δ13C, δ15N and C/N ratios in the sediment core can be regarded to the organic carbon source, which showed from the dominant C3 plants (carbon fixation plants) surrounding lake at the unit 1 to both C3 plant and lake microalgae with a large proportion of chrysophyceae and cyanobacteria at the unit 2 and unit 3. Moreover, unit 3 was characterized by the dominance of chrysophyceae microalgae in the oligotrophic state.
The RRD is located in the western coastal zone of the Gulf of Tonkin (Vietnam). The Holocene sedimentary of RRD is influenced by rives, wave and tidal processes and more recently to grain-size distribution. In this study, sediment core (~ 36 m) was collected in the wave-dominated system of the RRD. The parameters of sediment used to reconstruct the sedimentation environments of the RRD and sediment facies can be classified as sandy silt, and silty sand and silt. Besides, the mean grain-size showed that the sediment included from fine silt to very fine sand with dominant coarse silt and very coarse silt. According to the lithology features, textural parameters the sediment core sample can be divided into six deposition environments which consisted of sub-tidal flats and inter-tidal flats (from 36 to 30.1 m); shelf-prodelta (from 30.1 to 18.9 m); delta front slope (from 18.9-11.7 m); delta front platform (from 11.7 to 4.1 m), tidal flat (from 4.1 to 0.5 m) and flood plain (from 0.5 m to surface of sediment). The textural characteristics of the core sample were closely related with the core sample from the previous study in RRD. In a sediment core from 30.1 to 18.9 m, sediment showed erosion surface during the transgression at 8.86 cal. kyr BP. The textural parameter of the RRD has resulted in the interaction between sea level rise and fluvial inputs. In addition, the predominance of coarse silt very and coarse silt can be indicated the prevalence of comparatively from low to high energy conditions in the RRD.
The textural parameters of sediments used to reconstruct the depositional environments of the RRD, fit the sediment facies into sandy silt, silt and silty sand. The homogenous association of clay minerals in all sequence of the core suggest a constant supply of material, regardless the kind of sedimentary system, transgression or progradation. Three cycles of warning and cooling were identified through isotopic analysis and clay mineral assosiation along the Holocene: a cool and wet climate during 9.6-8.4 cal. kyr BP, 5.28-3.09 cal. kyr BP, 2.1-1.54 cal. years BP and a warm climate during 11.260-9.6 cal. kyr BP, 8.4-5.28 cal. kyr BP, 3.09-2.1 cal. kyr BP and the present warm climate. Besides, the evolution of the Asian Summer Monsoon in the Red River Delta can be reliably reconstructed through paleoenvironmental proxies’ variation (Mz, TOC, C/N ratios, δ13C). The monsoonal intensity changed from high during the early Holocene to low during the mid-late Holocene. This study considers the paleoclimatic meaning of the clay minerals association from the estuarine and deltaic sediments of the RRD, a crucial area for the economy of Vietnam.
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