Late Holocene Environmental Geochemical Records Of The Qiongdong Continental Shelf In The South China Sea

This thesis focuses on the sediment core collected from the eastern continental shelf of Hainan Island in the northern South China Sea.The length of the obtained sedimentary core is 45cm and the core is named as QD2.Based on the chronological anal ysis of ^{2 1 0}Pb and radiocarbon(^{1 4}C)of the core and geochemical anal ysis,we reconstructed the records of geochemical elements at QD2 site over the past 1900 years.From geochemical elements,grain size,loss of ignition and chronological records,the productivit y changes of the QD2 site in the northern South China Sea during the late Holocene and the effects of human activities on the heav y metal content in the QD2 site were discussed.The continuous record of marine productivity in the stud y area in the past 1900 years was reconstructed through biogenic Ba(Ba _bio).The results show that the marine productivit y changed evidentl y in the past 1900 years,which is relativel y high during the Medieval Warm Period and relativel y low in the Little Ice Age when the clima te is relatively cool.Marine productivit y increased rapidl y over the past 100 years,and the current Ba_bio is about 210?g/g,reaching its highest level over the past 1900 years.By comparing with the climatic and environmental records,we find that the marine productivit y in the Qiongdong upwelling area is significantl y affected b y the East Asian summer monsoon and has an inherent correlation with the temperature.During the warm climate period,the intensit y of the East Asian summer monsoon increases,wh ich leads to the enhancement of coastal upwelling and the increase in marine productivit y.It is also found that in the context of global warming,man-made climate change may have had an impact on marine productivit y in the region.In addition,we found th at human activities also have different effects on the heav y metal elements in the stud y area.Through the reconstruction of the record of continuous changes in Hg over the past 1900 years.Combined with the loss of ignition?LO I?,average grain size?Mz?,aluminum?Al?,rare earth elements?REE?and mercury?Hg?,it was inferred that the change of Hg content in sediments of the core QD2 is mainl y affected b y atmospheric deposition.In the past 1900,the concentration and anthropogenic flux of mercury sho wed significant changes.In the past 150 years,the mercury flux from anthropogenic sources has reached the highest level in history?the average content is23.7?g/m²/yr?.Through the calculation of enrichment factor?EF?,it is found that the average con tent of anthropogenic mercury flux in the past150 years is about 11 times higner than that of the background stage before 1850 AD.In general,the change of mercury concentration in most samples before 1850 AD is affected b y regional natural environmental changes.In contrast,it was found that the rapid increase in mercury flux since the middle of the 19th century is closel y related to human emissions.Since the 1980s,the anthropogenic mercury flux has shown a decreasing trend,which is about 30?g/m²/yr.The change process is consistent with the time of decline in global mercury production.Nevertheless,today's anthropogenic mercury flux is still 6.5 times higher than that of the natural background before 1850,and the impact of global and regional activities on marine Hg content cannot be ignored.Through Enrichment Factor?EF?,"Ocean Sediment Qualit y Standard",Interim Sediment Quality Guidelines?ISQG?,and Geoaccumulation In dex (I_geo),it was found that the I_geo of Cu reached 2?3 in modern times,which classified as Medium-intensit y pollution.The enrichment factor indicates strong enrichment,corresponding to the third category of sediment qualit y standards and may have a biologicall y negative effect on organisms.Rapid enrichment in the late 1990 s may be associated with the shrimp hatcheries in the southeast of the South China Sea since the 1990s.The shrimp breeding activities used a large amount of fungicide containing Cu.Although Hg is moderatel y enriched,it does not exhibit the pollution and biological negative effects of I _geo.The enrichment factors of As,Zn,and Pb elements also showed a steadil y increasing trend after 1950s.The changes were affected b y human activities,but did not show pollution and biological negative effects.The increase in Pb may be affected b y urban aerosols.The enrichment of Pb?around 1928 may be related to European metal processing activities and the use of leaded gasoline.At the same time,th e Pb content in Antarctic snow pits has also increased b y 2-4 times.Cr is mainl y affected b y natural processes.The change in Cd is specific.The reasons remains unclear and further study is required.