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Holocene paleo-environmental reconstruction in central Tibetan plateau inferred from a lacustrine sediment recordCheung, Man-ching, 張敏青 January 2014 (has links)
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. / published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy
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Reconstructing the Late Holocene History of Human Activity and Climate from a Costa Rican Diatom RecordUnknown Date (has links)
This project expands on the 4200-year history of Laguna Los Mangos by adding diatom analysis to existing records of pollen, charcoal, elemental composition, and stable light isotope analysis. Diatoms were counted in peroxide-treated samples, and data were analyzed using cluster analysis, PCA, CCA, and Pearson correlation. Diatom variability was correlated with nitrogen and organic material. Before 3430 cal yr BP, diatoms reflect landscape instability with higher lake levels and macrophyte cover. This period was followed by a period of agriculture-induced nutrient pollution, reduced pH, and increased precipitation until about 2450 cal yr BP. Peaks in A. granulata may indicate periods of increased mixing driven by precipitation. Diatom composition reflects reduced agricultural activity and lake desiccation during the TCD and increased precipitation and agricultural collapse during the LIA. Overall, this record reflects a history of slightly alkaline, eutrophic conditions, increasing salinity, and human disturbance from maize agriculture and deforestation. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2021. / FAU Electronic Theses and Dissertations Collection
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Late-Holocene stream dynamics on the middle Gila River, Pinal County, ArizonaHuckleberry, Gary January 1993 (has links)
An archival-stratigraphic investigation of the middle Gila River provides insights into late-Holocene channel behavior and flood-plain formation. Historical records detail changes in channel patterns that correlate with changing frequency of large floods, but channel sensitivity is also affected by factors such as flood seasonality, changes in sediment load, human disturbances, and internal thresholds. Because the frequency of large floods is the dominant factor in channel changes, radiocarbon-dated flood deposits in late-Holocene alluvial terraces allow for a reconstruction of prehistoric channel behavior. A period of reduced large flood frequency and channel stability 4,000-1,000 years BP separates periods of increased large flood frequency and channel instability 5,000-4,000 and 1,000-0 years BP. Transformations between braided and single channel morphologies affect the conveyance of floods and change the spatial characteristics of flood hazards. These channel dynamics are also important in analyzing changes in Hohokam-Pima irrigation technology and settlement patterns.
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Application of intertidal salt-marsh foraminifera to reconstruct late Holocene sea-level change at Kariega Estuary, South Africa.Strachan, Kate Leigh. January 2013 (has links)
Unclear predictions surrounding climate change, associated sea-level rise and potential impacts upon coastal
environments have placed an emphasis on the importance of sea-level change. Past sea-level fluctuations
have been measured using biological and geomorphological forms of evidence. One such biological proxy is
salt-marsh foraminifera, which have been used as a high-resolution indicator of past sea-level change, based
on the assumption that surface foraminiferal assemblages are similar in composition to buried fossil
foraminifera. In South Africa, there is ongoing research seeking to produce high-resolution records of sealevel
change, however foraminifera remain an underutilized source of proxy evidence. This research applies
salt-marsh foraminifera as precise indicators of relative sea-level change at Kariega Estuary on the Eastern
Cape coastline of South Africa. Distributions of modern foraminiferal assemblages were investigated,
revealing vertical zonation across the intertidal zone. The foraminiferal and marsh vegetation zones were in
part similar and overlapped to a certain extent, identifying three zones; high, low and tidal flats. This
suggested foraminiferal distribution is a direct function of elevation relative to tidal fluctuation. A 94 cm
core consisting of peat, sand and clay sediments was extracted from the salt marsh. A chronological
framework for the core was based on five AMS radiocarbon age determinations of both bulk sediment and
shell fragment samples placing the record within the last 1500 years Before Present (BP). The basal shell age
was a clear outlier to all bulk sediment ages, possibly as a result of shell recrystallisation. The bulk sediment
age determinations suggested two possible age reversals, potentially linked to sedimentary hiatus or
contamination. These inconsistencies in the chronology were best viewed as separate age models. The core
was analysed at a high resolution, whereby fossil foraminifera were extracted every 2 cm’s down the core. A
transfer function was applied to calculate the former elevation at which each core sample once existed, to
produce a relative sea-level reconstruction. The reconstruction was related to the age models to produce two
possible sea-level curve scenarios. Reconstructed curves from both scenarios depict a 0.5 m (±0.16 m) sealevel
highstand at 1500 cal years BP followed by a lowstand of -0.6 m (±0.03 m). Scenario One reached its
lowest recorded sea-level between 600 cal years BP and 500 cal years BP and then fluctuated below present
day levels. Scenario Two reached its lowest recorded sea-level around 1200 cal years BP, followed by low
amplitude fluctuations and a relatively stable period from 100 cal years BP till the present day. The 1500 cal
years BP highstand recorded for both scenarios correlates well with existing palaeoenvironmental literature
from the southern African coastline. Chronological limitations associated with the remainder of the record
hinder inter-comparison with previous studies. The outcomes of this research suggest that intertidal saltmarsh
foraminifera demonstrate enormous potential for the high-resolution reconstruction of relative sealevel
change in the South African context. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.
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Peat's secret archive : interpreting the geochemical and palaeodust record from Scottish peat as a potential index of North Atlantic storminess and Holocene climate changeStewart, Helena K. January 2016 (has links)
Four continuous high-resolution peat records for the Holocene have been reconstructed across a ~300km transect from Shebster in Caithness to Yell in the Shetland Isles. These records describe the nature and extent of North Atlantic climate changes inferred from indicators of storminess and minerogenic aeolian dust, and are supported by radiogenic isotope analysis, tephrochronology and radiocarbon dating. The environmental changes at all four sites displays a significant degree of synchrony in response to changes in the position of the polar front jet (PFJ) stream and the phase of the North Atlantic Oscillation (NAO). Bromine concentrations in the peat, derived from sea spray, are used to reconstruct storm frequency and storm intensity, and mire surface wetness is used as an indicator of longer-term climate shifts. The results suggest a strong link between positive phases of the NAO and storminess. Subtle differences between the bromine concentrations and the mire surface wetness suggest that high intensity but perhaps less frequent periods of storminess are not necessarily associated with a wetter climate. Atmospheric minerogenic dust concentrations are used to reconstruct large-scale climate changes across the wider North Atlantic region. The results suggest a sympathy between dust activity and periods of glacial advance and a negative index of the NAO. Radiogenic isotope analysis suggests that the smallest particles may originate from Iceland.
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