Sedimentary Record and Environmental Implications Since the Late Pleistocene from the Core DLC70-3 in South Yellow Sea
|School||Graduate School , Chinese Academy of Sciences ( Institute of Oceanography )|
|Keywords||South Yellow Sea Last interglaciation REE Yellow Sea Warm Current biomarker primary productivity|
The South Yellow Sea is a typical semi-enclosed epicontinental sea with complicated depositional history. It had submerged by the sea for many times in the history as a response to glacial cycles. It is an ideal place and key area for the research of paleoenvironmental evolution and paleo-land/sea interaction at the margin of East Asia, since there preserved plenty of information of past global changes in the sediments. This study took the DLC70-3 core from the northern central muddy area of South Yellow Sea as the research object, and the provenance of terrigenous materials and the records of paleoenvironmental and paleoclimatic changes were discussed based on the analysis of various depositional and geochemical indexes including granularity, element geochemistry, organic geochemistry （biomakers） etc.The core was dated with AMS14C and OSL methods and a chronostratigraphy was then established with reference to the sub-bottom acoustic profile. Results show that the sequence was deposited since the last interglaciation, and consisted of MIS5, 4, 3, with most of upper part missing. REE is usually considered as provenance tracer for its stable chemical property. Based on the study of REE distribution mode, its differentiation parameters and discrimination function（FD）, combined with microelements index analysis, a conclusion was drew that the sediments of DLC70-3 core were mainly from the Yellow River.The distribution pattern of micropaleobios especially foraminifer and ostracode is one of important proofs to trace the cold eddy associated with the Yellow Sea Warm Current. Buccella frigida and Protelphidium tuberculatum, as typical cold water species, were used as ndicators of the Cold Water Mass. The high abundances of the two species appeared in the layer of 2026.4m and 5571.2 m, responding to the warm periods of MIS5 and early MIS3 respectively. Therefore, the low temperature environment indicated by the cold water species was not caused by the paleoclimate at the time, but a response to the paleo-Cold Water Mass, suggesting the co-existence of the Yellow Sea Warm Current and paleo-Cold Water Mass during the high sea level period of MIS5 and early MIS3. Research on the element geochemistry indexes sensitive to the changes in oxidation-reduction condition , such as CaCO3（%）、MnO（%） and FeO/Fe2O3 , indicated that the formation of Yellow Sea Warm Current and Cold Water Mass also changed the oxidation -reduction condition of the study area and the effect of oxidation was obviously increased.XRF Core Scanner, as an geochemical analysis instrument appeared in recent years, has many advantages such as quick testing, high resolution, nondestructive testing etc.. It has never been applied to columnar samples in Yellow Sea areas. The comparison between XRF Core Scanner continuous scanning data and XRF testing data in the this study provided the evidence that most scanning data are reliable. Relatively large errors only occur in two elements, namely Si and Ba.The sensitive grain size of 5.511μm（%）, Na/Fe, CIA and Rb/Zr were controlled by chemical weathering, and could be considered as the proxies of summer monsoon, suggesting the appearance of a heavy monsoon and rainfall process in the northern part of China during early MIS3, which was comparable with that in MIS5.5, and with the monsoon records from other parts around the world. It is believed as a global event.The analysis of long chain n-Alkanes and n-AlcohoIs reveals that both of the contents of n-Alkanes and n-AlcohoIs were high in interglacial and low in glacial stages. They reached the maximum values in MIS3. It is supposed that the content variation was primarily controlled by the change in fluvial input caused by monsoon rainfall, with sea level change as the secondary factor. Most of the alkane was transpored in the wet period during low sea level. Long chain n-Alkanes of DLC70-3 core were mainly from the Loess Plateau at the middle and upper reaches of the Yellow River. The alkane index（AI） may reflect the plant evolution of the Loess Plateau. In this study, C31 is the main peak of most of the carbon distribution of the long chain n-Alkanes and C31/C27 values of the samples are all higher than 1, indicating that herb vegetation dominated in the source area, which coincide with the pollen data in the same core and other vegetation records of the Loess Plateau.The phytoplankton productivity induced from biomarkers and the seawater surface productivity indicated by TOC content in DLC70-3 core have the same characteristics: high in interglaciations and low in glaciations; high in warm periods and low in cold periods. The phytoplankton in the study area is mainly diatoms and dinoflagellates since the late Pleistocene. Although phytoplankton productivity changed a lot, the community structure remained rather stable, and no significant changes haves been observed. The main controlling factor of primary productivity was the nutrients variation. The nutrients transported by rivers influenced greatly the marine primary productivity of the study area. The input of terrigenous materials is the main controlling factor of the primary productivity in the South Yellow Sea. In addition, the bottom nutrients from upwelling and vertical mixing caused by Yellow Sea Warm Current also played a certain role in enhancing the productivity.