Late Quaternary Planktic Foraminifer Fauna andMonsoon Upwelling Records from the Western South China Sea, Near the Vietnam Margin (IMAGESMD012394)
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Marine sediment core MD012394 from the Vietnam coastal upwelling area in the western South China Sea was investigated in order to reconstruct the last Quaternary monsoon upwelling based on planktic foraminifer fauna assemblages and fauna-based sea surface temperature (SST) estimates.The age model of core MD012394 was constructed using oxygen isotope stratigraphy of the planktic foraminifer G. sacculifer, with 10 accelerator mass spectrometry (AMS) 14 C dating of planktic foraminifers from the sediment samples.Our studies on the relative and absolute abundances of planktic foraminifer assemblages reveal eight dominant species in core MD012394: N. dutertrei + N. pachyderma (right coiling), G. ruber, G. glutinata, G. sacculifer, P. obliquiloculata, G. menardii + G. tumida, G. calida, and G. inflata.In a Q-mode factor analysis of the fauna abundance data, the fauna factors show variations that do not parallel the glacial/interglacial changes throughout the last 135 kyr.The relative abundance patterns of G. inflata and N. dutertrei (including N. pachyderma-R) are interpreted as hydrographic proxies for East Asian summer and winter monsoon, respectively, in the current study.We calculated the fluctuations in the SST using the Revised Analog Method (RAM) in MD012394 and found that the abundance changes of the summer monsoon upwelling indicator G. inflata were similar and nearly synchronous.This suggests that the summer monsoon-driven upwelling signal was strong near the local summer insolation maximum, which induced low SSTs, particular around ~11, 33, 59, and 83 kya.Our studies support the view that the strengths of both summer insolation and the East Asian summer monsoon have determined the relative abundance of planktic foraminifers and the SSTs in the western SCS during the last 135 kyr.Keywords:
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Quaternary Huarong uplift is located at the center of Jianghan-Dongting basin.Detailed geological mapping and bore data analysis were conducted to investigate Quaternary tectonic and sedimentary characteristics and geological evolution.EW-trending Huarong uplift is adjacent to north Jianghan basin,south Anxiang sag and Yuanjiang sag of Dongting basin.The framework of the uplift is controlled by several surrounding Quaternary normal faults such as EW-trending Shishou fault,NW-trending Nanxian-Huangshantou fault and NNE-trending Zhuanqiao fault.There generally occurred alluvial or lacustrine deposits in every period of Quaternary in the peripheral sags.Late Early Pleistocene-Holocene deposits are located in the inner lower areas of the uplift and on the edge of the mountainous area,and pre-Quaternary rocks or residual deposits are distributed in the other areas of the uplift.On the basis of studying landforms,boundary faults,distribution of Quaternary deposits and pre-Quaternary rocks,the authors recovered the Quaternary evolution of Huarong uplift and adjacent areas in such aspects as rising and subsidence,sedimentation and denudation.There had occurred original hypsographic differentiation inside Huarong uplift in springtime of Quaternary before denudation.During early Early Pleistocene the normal faults around the uplift moved.At the same time,Jianghan basin,Anxiang sag and Yuanjiang sag subsided and received deposits and Huarong uplift was denuded.During the late Early Pleistocene—middle Middle Pleistocene period the normal faults continued to move,Huarong uplift and peripheral sags subsided,and the uplift rose relative to the sags.At the same time there occurred deposits in low areas of Huarong uplift and its peripheral sags,and there existed tectonic rising and denudation at the end of Early pleistocene.During late Midle Pleistocene Huarong uplift and peripheral sags rose and were denuded,forming vermicular red earth.There was a stable period in late Middle pleistocene when fluvial and lake-frontal deposits were formed on the edge of main Huarong-Zhuanqiao body of the uplift.Alluvial and lacustrine deposits were formed on the edge of the uplift and in the peripheral plain during Late Pleistocene under the relatively stable condition.At the end of Late Pleistoncene regional sea-level was lowered,causing the denudation of Huarong uplift and its peripheral areas.There occurred lacustrine-alluvial deposits in low areas of Huarong uplift and peripheral sags in Holocene due to the rising of regional river and lake level.Controlled by early high topography,the main body of Huarong uplift and parts of south Nanshan sub-uplift and Tanshan sub-uplift were weathered and denuded throughout Quaternary,resulting in the formation of large-sizes residual soil in some areas.Controlled by overall subsidence of Jianghan-Dongting basin,Huarong uplift subsided markedly in Quaternary as a whole.
Denudation
Tectonic uplift
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Asthenosphere
Low-velocity zone
Rayleigh Wave
Mantle plume
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Diatom abundance and species composition were quantitatively studied in two latest Quaternary (≃130 ka to the Present) sequences from the continental margin of northwest Africa. Off this region, coastal upwelling is well developed under the influence of the NE trade winds. Variations in diatom abundance in these cores are inferred to represent changes caused by varying degrees of the upwelling fertility. Times of high productivity are marked by high relative frequencies of Chaetoceros , while low productivity is marked by the dominance of Aulacoseira granulata . Upwelling increased during glacial episodes (isotopic stages 2–4 and 6) relative to isotopic stages 1 and 5. During the late Holocene, primary productivity levels are similar to those for Stage 5, but in the early Holocene upwelling intensities seem to have been weaker than today. The paleoproductivity reconstruction based on the diatom record is supported by paleoproductivity estimations based on the organic carbon content of the sediments (Sarnthein et al., 1987a).
Dominance (genetics)
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Neogene
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Deglaciation
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Marine sediment core MD012394 from the Vietnam coastal upwelling area in the western South China Sea was investigated in order to reconstruct the last Quaternary monsoon upwelling based on planktic foraminifer fauna assemblages and fauna-based sea surface temperature (SST) estimates.The age model of core MD012394 was constructed using oxygen isotope stratigraphy of the planktic foraminifer G. sacculifer, with 10 accelerator mass spectrometry (AMS) 14 C dating of planktic foraminifers from the sediment samples.Our studies on the relative and absolute abundances of planktic foraminifer assemblages reveal eight dominant species in core MD012394: N. dutertrei + N. pachyderma (right coiling), G. ruber, G. glutinata, G. sacculifer, P. obliquiloculata, G. menardii + G. tumida, G. calida, and G. inflata.In a Q-mode factor analysis of the fauna abundance data, the fauna factors show variations that do not parallel the glacial/interglacial changes throughout the last 135 kyr.The relative abundance patterns of G. inflata and N. dutertrei (including N. pachyderma-R) are interpreted as hydrographic proxies for East Asian summer and winter monsoon, respectively, in the current study.We calculated the fluctuations in the SST using the Revised Analog Method (RAM) in MD012394 and found that the abundance changes of the summer monsoon upwelling indicator G. inflata were similar and nearly synchronous.This suggests that the summer monsoon-driven upwelling signal was strong near the local summer insolation maximum, which induced low SSTs, particular around ~11, 33, 59, and 83 kya.Our studies support the view that the strengths of both summer insolation and the East Asian summer monsoon have determined the relative abundance of planktic foraminifers and the SSTs in the western SCS during the last 135 kyr.
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