Holocene Paleo-Environmental Reconstruction in Central Tibetan Plateau Inferred from a Lacustrine Sediment Record PDF Download
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Author: Man-Ching Cheung
Publisher: Open Dissertation Press
ISBN: 9781361380703
Category :
Languages : en
Pages :
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Book Description
This dissertation, "Holocene Paleo-environmental Reconstruction in Central Tibetan Plateau Inferred From a Lacustrine Sediment Record" by Man-ching, Cheung, 張敏青, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Previous studies on reconstructing the paleoclimate on the Tibetan Plateauare commonly based on a single proxy, and have thus limited the usage of their results. Therefore this study was designed firstly to evaluate the applicability of organic carbon isotopes from modern plants in the Tibetan Plateau as a paleo-environmental proxy, and secondly to apply palynology, organic geochemistry and biomarkers to the reconstructions ofpaleo-environmental history of central Tibet. The new, comprehensive records reveal the history of regional precipitation, temperature and vegetation change in responding to Holocene climatic changesin the region. Organic carbon isotope (δ13C) and lipid n-alkanes were measured from plant samples collected from various altitudes and environments across the study area. The δ13C results indicate the importance of availability of water to C3/C4 herbs distribution in this alpine region. The molecular distributions of trees, shrubs, herbs and submergent macrophytes show distinctive features in a number of indices. These indices are considered to be useful for paleo-environmental reconstructions. A 5.8m-long peat core, which were dated back to 95,000 yr BP was obtained from a river valley at an altitude of 4300 m above sea level. The pollen data from the core reveals a sparse herbaceous vegetation in the catchment area during early Holocene, suggesting non-favourable conditions for vegetation growth. The environment became wetter during the early mid-Holocene around 8100-7000 cal. yr BP, and since then a sedge-dominated wetland habitat was developed. This vegetation changed little until about 1100 cal. yr BP; from which an increase of drought-tolerant herbs was indicated by the pollen data, implying a slightly drier condition of the latter part of late Holocene. The pollen record suggests that the optimum precipitation in central Tibet occurred later and lasted longer than that in the East Asian Monsoon region. Finally, bulk sample carbon geochemistry(TOC, δ13C) as well as the biomarkers analysis on n-alkanes and GDGTs as paleo-limnology and paleo-temperature indicators respectively were applied to the core. Before 1500 cal. yr BP, the wetland was dominated by both vascular herbs as suggested by the δ13Cdata and aquatic floating/submerged macrophytes as indicated by mid-chain n-alkanes. The relative proportion of the two sources of organic matter input varied slightly, and vascular herbs increased in two episodes(5400 -4700 and 2800 -2300 cal. yr BP) indicated by the increase of long-chain n-alkanes (>n-C25). Considering of the soil water content variation by the ratios of iGDGTs to bGDGTs, the change between the two plant communities could to be caused by different lake status. GDGTs paleo-temperature reconstruction indicates a warm period from 6700 to 6000 cal. yr BP, which is believed to be the warmest and highly humid in central Tibetan region. Then the temperature decreased towards the late Holocene, consistent with other paleo climate records across the Plateau region, i.e. largely controlled by change of solar insolation. Along the decreasing trend, a warm episode at about 1200 yr BP that inferred from MAAT was suggested corresponding with the North Hemisphere Medieval Warming. DOI: 10.5353/th_b5270537 Subjects: Paleoecology - Tibet, Plateau of Paleoecology - Holocene
Author: Man-Ching Cheung
Publisher: Open Dissertation Press
ISBN: 9781361380703
Category :
Languages : en
Pages :
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Book Description
This dissertation, "Holocene Paleo-environmental Reconstruction in Central Tibetan Plateau Inferred From a Lacustrine Sediment Record" by Man-ching, Cheung, 張敏青, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Previous studies on reconstructing the paleoclimate on the Tibetan Plateauare commonly based on a single proxy, and have thus limited the usage of their results. Therefore this study was designed firstly to evaluate the applicability of organic carbon isotopes from modern plants in the Tibetan Plateau as a paleo-environmental proxy, and secondly to apply palynology, organic geochemistry and biomarkers to the reconstructions ofpaleo-environmental history of central Tibet. The new, comprehensive records reveal the history of regional precipitation, temperature and vegetation change in responding to Holocene climatic changesin the region. Organic carbon isotope (δ13C) and lipid n-alkanes were measured from plant samples collected from various altitudes and environments across the study area. The δ13C results indicate the importance of availability of water to C3/C4 herbs distribution in this alpine region. The molecular distributions of trees, shrubs, herbs and submergent macrophytes show distinctive features in a number of indices. These indices are considered to be useful for paleo-environmental reconstructions. A 5.8m-long peat core, which were dated back to 95,000 yr BP was obtained from a river valley at an altitude of 4300 m above sea level. The pollen data from the core reveals a sparse herbaceous vegetation in the catchment area during early Holocene, suggesting non-favourable conditions for vegetation growth. The environment became wetter during the early mid-Holocene around 8100-7000 cal. yr BP, and since then a sedge-dominated wetland habitat was developed. This vegetation changed little until about 1100 cal. yr BP; from which an increase of drought-tolerant herbs was indicated by the pollen data, implying a slightly drier condition of the latter part of late Holocene. The pollen record suggests that the optimum precipitation in central Tibet occurred later and lasted longer than that in the East Asian Monsoon region. Finally, bulk sample carbon geochemistry(TOC, δ13C) as well as the biomarkers analysis on n-alkanes and GDGTs as paleo-limnology and paleo-temperature indicators respectively were applied to the core. Before 1500 cal. yr BP, the wetland was dominated by both vascular herbs as suggested by the δ13Cdata and aquatic floating/submerged macrophytes as indicated by mid-chain n-alkanes. The relative proportion of the two sources of organic matter input varied slightly, and vascular herbs increased in two episodes(5400 -4700 and 2800 -2300 cal. yr BP) indicated by the increase of long-chain n-alkanes (>n-C25). Considering of the soil water content variation by the ratios of iGDGTs to bGDGTs, the change between the two plant communities could to be caused by different lake status. GDGTs paleo-temperature reconstruction indicates a warm period from 6700 to 6000 cal. yr BP, which is believed to be the warmest and highly humid in central Tibetan region. Then the temperature decreased towards the late Holocene, consistent with other paleo climate records across the Plateau region, i.e. largely controlled by change of solar insolation. Along the decreasing trend, a warm episode at about 1200 yr BP that inferred from MAAT was suggested corresponding with the North Hemisphere Medieval Warming. DOI: 10.5353/th_b5270537 Subjects: Paleoecology - Tibet, Plateau of Paleoecology - Holocene
Author: 張敏青
Publisher:
ISBN:
Category : Paleoecology
Languages : en
Pages : 181
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Author: Yongbo Wang
Publisher:
ISBN:
Category :
Languages : en
Pages :
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The past climate in central Asia, and especially on the Tibetan Plateau (TP), is of great importance for an understanding of global climate processes and for predicting the future climate. As a major influence on the climate in this region, the Asian Summer Monsoon (ASM) and its evolutionary history are of vital importance for accurate predictions. However, neither the evolutionary pattern of the summer monsoon nor the driving mechanisms behind it are yet clearly understood. For this research, I first synthesized previously published Late Glacial to Holocene climatic records from monsoonal central Asia in order to extract the general climate signals and the associated summer monsoon intensities. New climate and vegetation sequences were then established using improved quantitative methods, focusing on fossil pollen records recovered from Tibetan lakes and also incorporating new modern datasets. The pollen-vegetation and vegetation-climate relationships on the TP were also evaluated in order to achieve a better understanding of fossil pollen records. The synthesis of previously published moisture-related palaeoclimate records in monsoonal central Asia revealed generally different temporal patterns for the two monsoonal subsystems, i.e. the Indian Summer Monsoon (ISM) and East Asian Summer Monsoon (EASM). The ISM appears to have experienced maximum wet conditions during the early Holocene, while many records from the area affected by the EASM indicate relatively dry conditions at that time, particularly in north-central China where the maximum moisture levels occurred during the middle Holocene. A detailed consideration of possible driving factors affecting the summer monsoon, including summer solar insolation and sea surface temperatures, revealed that the ISM was primarily driven by variations in northern hemisphere solar insolation, and that the EASM may have been constrained by the ISM resulting in asynchronous patterns of evolution for these two subsystems. This hypothesis is further supported by modern monsoon indices estimated using the NCEP/NCAR Reanalysis data from the last 50 years, which indicate a significant negative correlation between the two summer monsoon subsystems. By analogy with the early Holocene, intensification of the ISM during coming decades could lead to increased aridification elsewhere as a result of the asynchronous nature of the monsoon subsystems, as can already be observed in the meteorological data from the last 15 years. A quantitative climate reconstruction using fossil pollen records was achieved through analysis of sediment core recovered from Lake Donggi Cona (in the north-eastern part of the TP) which has been dated back to the Last Glacial Maximum (LGM). A new data-set of modern pollen collected from large lakes in arid to semi-arid regions of central Asia is also presented herein. The concept of "pollen source area" was introduced to modern climate calibration based on pollen from large lakes, and was applied to the fossil pollen sequence from Lake Donggi Cona. Extremely dry conditions were found to have dominated the LGM, and a subsequent gradually increasing trend in moisture during the Late Glacial period was terminated by an abrupt reversion to a dry phase that lasted for about 1000 years and coincided with the first Heinrich Event of the northern Atlantic region. Subsequent periods corresponding to the warm Bølling-Allerød period and the Younger Dryas cold event were followed by moist conditions during the early Holocene, with annual precipitation of up to about 400 mm. A slightly drier trend after 9 cal ka BP was then followed by a second wet phase during the middle Holocene that lasted until 4.5 cal ka BP. Relatively steady conditions with only slight fluctuations then dominated the late Holocene, resulting in the present climatic conditions. In order to investigate the relationship between vegetation and climate, temporal variations in the possible driving factors for vegetation change on the northern TP were examined using a high resolution late Holocene pollen record from Lake Kusai. Moving-window Redundancy Analyses (RDAs) were used to evaluate the correlations between pollen assemblages and individual sedimentary proxies. These analyses have revealed frequent fluctuations in the relative abundances of alpine steppe and alpine desert components, and in particular a decrease in the total vegetation cover at around 1500 cal a BP. The climate was found to have had an important influence on vegetation changes when conditions were relatively wet and stable. However, after the 1500 cal a BP threshold in vegetation cover was crossed the vegetation appears to have been affected more by extreme events such as dust storms or fluvial erosion than by the general climatic trends. In addition, pollen spectra over the last 600 years have been revealed by Procrustes analysis to be significantly different from those recovered from older samples, which is attributed to an increased human impact that resulted in unprecedented changes to the composition of the vegetation. Theoretical models that have been developed and widely applied to the European area (i.e. the Extended R-Value (ERV) model and the Regional Estimates of Vegetation Abundance from Large Sites (REVEALS) model) have been applied to the high alpine TP ecosystems in order to investigate the pollen-vegetation relationships, as well as for quantitative reconstructions of vegetation abundance. The modern pollen-vegetation relationships for four common pollen species on the TP have been investigated using Poaceae as the reference taxa. The ERV Submodel 2 yielded relatively high PPEs for the steppe and desert taxa (Artemisia Chenopodiaceae), and low PPEs for the Cyperaceae that are characteristic of the alpine Kobresia meadows. The plant abundances on the central and north-eastern TP were quantified by applying these PPEs to four post-Late Glacial fossil pollen sequences. The reconstructed vegetation assemblages for the four pollen sequences always yielded smaller compositional species turnovers than suggested by the pollen spectra, indicating that the strength of the previously-reported vegetation changes may therefore have been overestimated. In summary, the key findings of this thesis are that (a) the two ASM subsystems show asynchronous patterns during both the Holocene and modern time periods, (b) fossil pollen records from large lakes reflect regional signals for which the pollen source areas need to be taken into account, (c) climate is not always the main driver for vegetation change, and (d) previously reported vegetation changes on the TP may have been overestimated because they ignored inter-species variations in pollen productivity
Author: Stephen J. Brooks
Publisher:
ISBN: 9780907780717
Category : Chironomidae
Languages : en
Pages : 276
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Author: Zhang Chengjun
Publisher: Frontiers Media SA
ISBN: 2832504035
Category : Science
Languages : en
Pages : 269
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Author: Xiangjun Liu
Publisher: Frontiers Media SA
ISBN: 2889745074
Category : Science
Languages : en
Pages : 263
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Author: Liangcheng Tan
Publisher: Frontiers Media SA
ISBN: 2889664716
Category : Science
Languages : en
Pages : 206
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Author: Anoop Ambili
Publisher: Frontiers Media SA
ISBN: 288976981X
Category : Science
Languages : en
Pages : 167
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Author: Lin Li
Publisher:
ISBN:
Category :
Languages : en
Pages : 227
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"The uplift history of the Tibetan Plateau serves as key evidence for understanding the dynamic models that build the Plateau and its influence on Cenozoic climate change of East Asia. This dissertation contributes to our knowledge for the growth history of the central and northern Tibetan Plateau through sedimentary analysis of basin sequences, and stable isotope analysis of carbonate rocks and modern surface waters. Field work in the Hoh Xil basin of central Tibet, including facies analysis, paleocurrent reconstruction, detrital zircon provenance analysis, and stable isotope analysis, suggests that a unified Hoh Xil basin, including both the east and west sub-basins, experienced a transition from pre-India-Asia collision foreland basin to post-collision hinterland basin setting at around 50 Ma. The Qiangtang terrane, serving as the main source for sediments deposited in the Hoh Xil basin, experienced significant topographic growth during the Cretaceous time. Far field deformation in the Hoh Xil basin was initiated shortly after India-Asia collision; contractional deformation and concomitant filling of the Hoh Xil hinterland basin provides clues for outward and upward growth of the Tibetan Plateau during Cenozoic time. Work in the western Qaidam basin provides new lithostratigraphy and carbonate stable isotope data that records early-middle Miocene topographic growth of the northern Tibetan Plateau. A hydrological change from restricted sub-basin to open marginal basin around 20 Ma was probably caused by late Oligocene-early Miocene tectonic activity around the Qaidam basinches A major topographic growth in the northern Tibetan Plateau is inferred around 15 Ma based on a negative shift in oxygen isotopic values, sedimentary facies changes from marginal lacustrine to fluvial, and an increase in sedimentation rate. A 13-12 Ma aridification event that was observed over a large area of the northern Plateau was likely caused by continued topographic growth to a critical point to block moisture from entering the northern Tibetan Plateau. To understand the caveats of stable isotope-based paleoaltimetry in the central and northern Tibetan Plateau, a comprehensive data set of 1,315 river water samples (450 newly collected) was compiled. With this large data set, a consistently assumed, but not well-documented prerequisite is demonstrated that river waters are a good substitute for isotopic studies of precipitation on the high Tibetan Plateau on the mean annual scale. The spatial variations of [delta]18O/[delta]D and d-excess values in the plateau margins can be modeled as a Rayleigh distillation process, on which stable isotope-based paleoaltimetry is based. On the contrary, the isotopic values of meteoric waters in the interior of the plateau are controlled by the combined effects of mixing of different moisture sources, contribution of recycled moisture from local surface water evaporation, and sub-cloud evaporation. A Rayleigh distillation model modified by sub-cloud evaporation is provided to simulate the isotopic variations in the western Plateau. This new understanding indicates that stable isotope-based paleoaltimetry is most reliable in the southern margins of the Plateau, and increasingly unreliable toward the northern Plateau. In addition, the contour maps of modern isotopic variations of meteoric waters across the whole Tibetan Plateau also provide validation criteria for isotopic simulations using general circulation models"--Pages v-vi.
Author: Liviu Giosan
Publisher: John Wiley & Sons
ISBN: 1118704436
Category : Science
Languages : en
Pages : 574
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Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 198. Climates, Landscapes, and Civilizations brings together a collection of studies on the history of complex interrelationships between humans and their environment by integrating Earth science with archeology and anthropology. At a time when climate change, overpopulation, and scarcity of resources are increasingly affecting our ways of life, the lessons of the past provide multiple reference frames that are valuable for informing our future decisions and action plans. Volume highlights include discussions of multiple connotations of the Anthropocene, landscapes as a link between climate and humans, synoptic approaches to explore large-scale cultural patterns, regional studies for contextualizing cultural complexity, and environmental determinism and social theory. Straddling the fields of Earth sciences, anthropology, and archaeology and presenting research from across several continents, Climates, Landscapes, and Civilizations will appeal to a wide readership among scientists, scholars, and the public at large.
