Geochemical Records Of Environmental Evolution In XYC Core Of The Lower Qiantang River During The Past 8000 Years

The coastal zone is the zone where the land-sea interaction is the strongest and records a lot of climate change information.The lower reaches of Qiantang River in Zhejiang province is an important sea-land interaction area with extensive Quaternary sedimentary types and unique prehistoric cultural pedigree.It is an ideal area for exploring Holocene environmental changes and human-earth coupling models.In this paper,the borehole sediments（XYC）collected near Xingyi Village in Shangyu city in the lower reaches of the Qiantang River are taken as the research object,and the combination of geochemistry and sedimentology is adopted to try to interpret the environmental evolution information of the sea-land interaction area in Zhejiang province since 8000 years,so as to provide scientific support for understanding the Holocene environmental evolution of the Qiantang River basin and the evolution of prehistoric culture in the coastal areas of Zhejiang province.The total length of XYC Core is 34.34m,and 10 sedimentary units are interacted by four sedimentary types,namely silty sand,sandy silt,silty sand and clayey silt.In the previous study,they are classified as（1）～（10）beds from bottom to top.The borehole bottom boundary is 9690 cal.a BP.In this study,645 samples were collected at a distance of 2cm,covering only the 13.32m or higher portion of the XYC Core（corresponding to 3 depositions of the core from（8）～（9）layers）,and the age of the bottom boundary of the depositions was 8613 cal.a BP.Based on the geochemical record,the preliminary conclusions are as follows:（1）The geochemical characteristics of major and trace elements show that the sediments in XYC Core are mainly composed of Si,Al and Fe,and the contents of easily eluviable components K,Ca,Na and Mg are low,indicating that the sediments in the studied strata have experienced strong weathering and eluviation.The ratio of major element oxides shows that the weathering and eluviation of core sediments are stronger at 13.32～4.58m and weaker at 4.58～1.02m.There was a significant negative correlation between Al,Fe and Si,and a significant negative correlation between K and Na,which may have a certain response to climate changes.Ca and Mn are positively correlated,which may be indicative of sea level change.（2）The environmental evolution in the study interval can be divided into three stages:Ⅰ（8613～7330 cal.a BP）,the water environment is relatively stable,the climate is warm and humid,and the organisms are relatively active in the high sea surface environment;Ⅱ（7330～5870 cal.a BP）belongs to the peak stage of Holocene Megean period,the climate is warm and humid;Ⅲ（5870～5016 cal.a BP）can be divided intoⅢ_a（5870～5306 cal.a BP）andⅢ_b（5306～5016 cal.a BP）:SubmemberⅢ_a is an interlayer composed of silty sand deposits and peat,and there are some differences in the composition of elements between the two,indicating the terrestrial limnological environment and climate transition from warm and wet to cold and dry,and the climate in the peat period is warmer and wetter.The climate ofⅢ_b was cold,dry and stable,which may be related to the advance of global glaciers and the strengthening of East Asian winter monsoon.（3）The deposit records of XYC Core have a certain response to 8.2 ka event and5.5 ka event.At about 8200 cal.a BP and 5500 cal.a BP,the K₂O/Na₂O value and the（Al₂O₃+Fe₂O₃）/Si O₂ value indicating weathering strength decreased obviously,and the Si O₂/Al₂O₃ ratio increased.Especially at about 5500 cal.a BP,clay,redness,yellowness,TOC and other indexes fluctuated obviously,indicating a relatively dry and cold environment.8.2 ka cold event may promote the sedimentary environment in the study area to gradually change from shallow sea to limnological environment.This also contributed to the creation of the cross-lake bridge culture in Hangzhou Bay area and the Jingtou Mountain culture in Ningbo Plain.