Implications Of Grain Size Characteristics In Chinese Coastal Muddy Sediments

Grain size proxy in Chinese coastal muddy sediments has been widely used for reconstruction of East Asian winter monsoon(EAWM)strength.However,many intractable problems still remain concerning the discrepancies in different studies.In recent years,doubt was continuously cast on the indication of SGS in sediments from these mud areas.While many studies have been conducted using grain size proxy to reconstruct long-term records of EAWM during the whole Holocene,especially since the mid-Holocene,few have compared high-resolution records with instrumental EAWM data.The objective of the study is to explore the factors affecting sensitive grain size(SGS)proxy and whether the proxy can be used for reconstruction of EAWM strength at different time scales.Firstly,we select the grain size data from published studies using SGS for reconstruction of Holocene EAWM strength at different time scales.These previous results are comprehensively compared and studied.Then,we investigate four sedimentary cores in Chinese coastal mud areas,including Cores 38002 and 38002-A in the Northern Yellow Sea Mud(NYSM),Core 35009 in the central Yellow Sea Mud(CYSM)and Core T02 in mud area near the Yangtze River estuary(MAYE).Various technologies including AMS 14C and 210Pb?137Cs chronologies,grain size analyses,SGS selection and geochemical element data etc.are employed.Conclusions of the thesis are summarized as follows:(1)We compare EAWM sequences indicated by SGS from different studies over the last 8000 and 3000 years,respectively,and find that there are significant inconsistencies among them.The inconsistencies could be partly caused by age uncertainties,chosen SGSs and different sedimentary environments in certain regions.(2)In the context of relatively precise dating and same SGS choice,the results show that trends in sequences of SGS covering the past?150 years in three mud areas are different.No significant correlations between each sequence and Siberian High index are found.Our further analyses indicate that due to special forming mechanisms in NYSM and CYSM,SGS results in the two areas could not be mainly influenced by costal currents driven by EAWM at decadal time scale.SGS sequences in MAYE might be directly influenced by river-derived materials.The results support the viewpoint that differences of sedimentary environments and forming mechanisms of mud areas could largely influence SGS.(3)Due to its sedimentary environment,we suspect that SGS proxy in NYSM might not be appropriate for reconstruction of EAWM strength at longer time scale.As expected,the results show that grain-size characteristics in sediments and the related element ratios from the NYSM are closely related to precipitation changes in the Yellow River drainage basin driven by East Asian summer monsoon.We suggest that grain size characteristics in sediments from some mud areas could be influenced by factors other than coastal currents driven by EAWM,such as sedimentary environments,riverine runoff and storms.Thus SGS of muddy sediments should be used with caution when reconstructing winter monsoon strength,especially those from sedimentary environments like the NYSM and near the Yangtze River estuary.Although some advancements have been achieved about the study of mud areas of Yellow and East China seas,more efforts should be made for mechanisms of sedimentary dynamic and processes,especially the mechanisms of SGS responding to climate change.This will improve our understanding of the indication of grain size proxy for East Asian monsoon history at different time scales.