Climate Change During Mid-late Holocene Reflected By Tridacna Of Xisha Islands,South China Sea

In recent years,the extreme weather occurs more frequently and attracts the attention for the climate and the environment.The scholars become more interested to the study and reconstruction of paleoclimate and paleoenvironment.Scientists extend the bases of modern climate patterns research by studying the past earth environment change,finding the similar spot in earth history and provide more useful information for the prediction of future climate change.Therefore,enhancing the resolution of paleoclimatic reconstruction,seeking new paleoclimatic materials and better understanding of existing paleoclimate profiles have become the hot and main spot of present paleoclimate research.This dissertation is mainly focused on the paleoclimate information during mid-late Holocene of Xisha Islands.The geochemistry proxies of tridacna shell are used to reconstruct the ENSO and SST change in South China Sea during the Holocene and try to figure out the character of seasonality and the extreme climate event.We do also evaluate the impact of human activities to the oceanic carbon circulation and push forward the application of tridacna shell in the study of climate change and global carbon circulation.1.Mid-Iate Holocene ENSO Variability Recorded by High-resolution Oxygen Isotope of Tridacna spp in South China SeaModern El Nino South-Oscillation(ENSO)is the main climate system in the Pacific Ocean,and may cause climate change all over the planet.South China Sea(SCS)located in the west of tropical Pacific Ocean,which affected gravely by ENSO activities.After the analysis of the high-resolution oxygen isotope of modern tridacna in Xisha Islands of SCS,the tridacna,one kind of bivalve biogenic carbonate,was proved to be a good potential proxy of ENSO variability.With the records of four ancient tridacna,we have reconstructed high-resolution ENSO information of four discontinuous time period in mid-late Holocene of SCS.The results of bandpass filter analysis indicated that 2.55ka and 4.16ka have similar ENSO change with modern time only a little less in frequency,while 6.05ka is relatively lower and 6.72ka is higher in intension.The extreme ENSO and seasonality change occurred more frequently in the warm period.These ENSO information and seasonality provided additional geochemistry evidence for further research of the ancient ENSO change in both SCS and all over the world.2.Mid-late Holocene SST records of South China Sea recorded by Sr/Ca profiles of tridacnaWe analyzed high-resolution Sr/Ca ratios of ten tridacna from Xisha Islands of South China Sea.The results show good year cycle.Sr/Ca profiles were rearranged monthly and calculated to SST by Sr/Ca-SST formula from previous study.Ten SST records of South China Sea were reconstructed according to different species.When the winter and the coolest month is determined,the summer time and the hottest month differ from time which indicated the phase and range difference between winter and summer during different time periods of mid-late Holocene.2.305ka and modern have bigger SST seasonality while 2.5 5ka and 6.72ka have smaller one.It seems that the warm periods are reflected by warmer winter while cool periods are reflected by cooler summer.During the past 7000 years of mid-late Holocene,the SST trend is decreasing from Holocene Mega Thermal to present with many SST fluctuation,indicating some influence of extreme climate event.This study provides a primary results of 7000 years SST records in South China Sea and may extend globally in the future.3.Climate and human activies recorded by carbon isotope of the tridacna of South China SeaSuess effect is representing the phenomenon that the ?13C decreases due to the usage of light carbon isotope in the fossil fuel since the industrial revolution.Xisha Islands is not far from the continent which could reflect the Suess effect.We analyzed high resolution ?13C of five tridacna from Xisha Islands and got more than 50 ?13C data while AMS14C dating.The results indicated the ?13C of tridacna is decreasing with some fluctuation during the mid-late Holocene and modern tridacna is evidently negative due to Suess effect.Compared with the atmospheric CO2 concentration and?13C from the ice core,Suess effect has great impact on oceanic carbon circulation.According to other researches,we could get a formula of tridacna ?13C and ?13C of dissolved inorganic carbon of seawater.But still need more measurement to determine the coefficient for quantificational application.This research provides bases for further study of the relationship between atmospheric CO2 and oceanic carbon circulation.