Records Of Tridacna In South China Sea:Sea Surface Temperature And Climate Change

Climate changes have always been the focus of the international community.With the development of modern climate research,the study of paleoclimate and paleoenvironment is also on the rise.The changes in the tropical ocean environment play an important role in the evolution of the global climate system.In this dissertation,Tridacna samples were used to reconstruct sea surface temperature(SST)changes in the South China Sea over the past 7000 years,and we obtained Hg contents and 813C sequences in inner shells to reflect the impact of climate changes and human activities.This dissertation will discuss the following three aspects:1.Analysis of the components and biological organic characteristics of Tridacna spp.shellsThe marine bivalve,Tridacna spp.,is the largest bivalve shell in geological period,it has hard and dense aragonite shell with annual or daily growth lines in its inner shell layer,and it provides an ideal material for high-resolution paleoclimate reconstructions.Due to the long-term geological process of aragonite shells,the original recorded climatic information may be changed.Therefore,identification of shell calcilization is necessary before using Tridacnidae as paleoclimate recorders.Fifty-eight Tridacnidae specimens were collected from South China Sea.The analysis results of X-ray diffraction(XRD),Differential Scanning Calorimetry-Thermo Gravimetric Analysis(DSC-TGA)and Gas Chromatography-Mass Spectrometer(GC-MS)showed that all the Tridacna spp.inner shells studied here were entirely made of aragonite,and shell calcilization did not occur.The main inorganic compound of the inner shell is CaCO3,with a content between 96.5-97.2%;the content of the organic matter,such as hexadecane and 17 kinds of amino acids,is between 2.8-3.5%.However,calcilization took place in some outer shell layers.This study presents a method to identify the calcilization of Tridacnidae shell,and preliminarily determines composition and content in Tridacnidae shell.Results from this study provides the basis for such reconstruction using Tridacnidae shells.2.Two different measurements of Sr/Ca ratio in Tridacna spp.shells from SouthChina Sea and SST records over the last 7,000 yearsTwo different analysis methods were applied and compared for determining high-resolution Sr/Ca ratio profiles of one modern(live-caught)and four fossil(dead-collected)Tridacna spp.samples from South China Sea.The Sr/Ca profile of Tridacna gigas in the South China Sea determined by Inductively coupled plasma optical emission spectrometry(ICP-OES)had well defined annual cycles and was significantly,negatively correlated with SST,implying that they can be used as good proxies of historical SST.ICP-OES is the commonly used method for measuring Sr/Ca ratios in Tridacna spp.shells,but it needs ex-situ,time-consuming chemical pretreatments using variable acids and considerable amounts of sample.While the synchrotron radiation X-ray fluorescence(SR-XRF)technique does not have these shortcomings,it requires little sample preparation and allows rapid in situ analysis of solid-state samples at high spatial resolution(submicron scales),and it is characterized by non-destructive,high sensitivity,and multi-elemental distribution.In this study,we tested the feasibility of determining chemical elements in shell samples of Tridacna spp.using SR-XRF.Our study showed that the data of SR-XRF can reflect the elements contents and their ratios change.SR-XRF derived Sr/Ca ratio profiles are significantly correlated with those from ICP-OES(P-value for five Tridacna spp.samples are all<0.05);therefore SR-XRF can be a promising alternative to ICP-OES.And it has broad application potential in palaeoclimate reconstructions using Tridacna spp..This study also obtained the Sr/Ca ratio sequence of 60 tridacnas from the South China Sea,and reconstructed the SST changes in the region over the past 7000 years.In the Tridacna SST records,several cold events in the Middle-Late Holocene can be found,such as 5.5 ka BP,4.2 ka BP,2.8 ka BP events,and it also recorded the Medieval Warm Period(MWP).The results showed that the MWP temperature did not exceed the modern temperature,agree with the results of the IPCC fourth report,which suggested that the recent decade was the warmest in at least the past 1300 years,and also agree with the results of the IPCC fifth report,which concluded that the recent 3 decades were the warmest in the past 1400 years.3.The past 7,000 years of Tridacna shells from the South China Sea record mercury content associated with climate change and human activitiesThe contemporary global mercury(Hg)cycle has been greatly affected by human activities.Mercury pollution has become a regional and global environmental concern.Research on mercury and its impact has also become one of the hot spots in environmental science research.However,we are still lacking reliable,long-term records of Hg in seawater.Here,we report for the first time on using annually banded Tridacna spp.from the South China Sea as an archive for recording changes of seawater dissolved Hg and ?13C over the past 7000 years,and reflecting its relationship with climate change and human activities.The results show that ?13C may be closely related to the concentration of atmospheric CO2 during the historical period,and Suess effect caused by human activities can be recorded in Tridacna ?13C.The change of Hg content in Tridacna shells are related to climate change and human activities.7000-4500 a BP,Hg changes were mainly controlled by natural factors,and corresponded to climate change.During the cold period,the atmospheric Hg was deposited,then high concentration of Hg accumulated in the ocean,resulting in high Hg content in Tridacna.4500-1000 a BP is the period under the joint action of climate change and human activities.The frequent climate change,social unrest,and cultural change during this period resulted in strong fluctuations in Hg content.This meant that human activities were gradually becoming more and more important in Tridacna Hg.1000 a BP,the increasing emissions from human activities and the sharply rising atmospheric CO2 content,resulted in a sharp increase in Hg content and severe negative ?13C values.This period was dominated by human activities.Our results show that the Hg content can be affected by climate change and human activities,reflecting the change in ocean Hg concentration.And it can be used to study the reaction of surface seawater to atmospheric mercury deposition.