Elemental And Isotopic Geochemistry Of Reef-building Corals

Knowledge of climate change during Late Holocene in the Northern South China Sea（NSCS）is of great significance for understanding the laws of climate evolution in China and other East Asian regions and their relationship with the succession of human civilization.From the perspective of Earth System Science,the change of the Earth’s climate and environment and the development of lives and ecosystems are a co-evolutionary process.Therefore,understanding how the biogeochemical processes of lives and ecosystems respond to and adapt to climate change should be another concern in the research on climate and environmental evolution.Coral reefs are widely regarded as the most biodiverse and productive ecosystem in the ocean,and they are very sensitive to climate and environmental changes.Therefore,reef-building corals are excellent recorders of the co-evolution of marine climate and environment and ecosystems.In this thesis,we take reef-building corals as the research subject.On the one hand,by using the annual resolution Sr/Ca andδ¹⁸O compositions of a series of fossil Porites corals collected from the eastern coast of Hainan Island in the NSCS,the sea surface temperature（SST）and seawaterδ¹⁸O(δ¹⁸O_sw)records of this area during periods of～167-309 CE（the beginning of the first millennium CE）and～3500-4000 yr BP（Middle Bronze Age Cold Epoch,MBACE）were reconstructed.On the other hand,in order to further explore proxies that can characterize the internal biological activities of corals,δ⁶⁶Zn andδ⁶⁵Cu in the skeletons of different species of corals collected from the Great Barrier Reef（GBR）in Australia and Sanya Luhuitou（LHT）Reef in the NSCS were studied.Through the above research,the following progress has been made:（1）The long-term average values of SST andδ¹⁸O_sw in the NSCS during the period of 167-309 CE were about 25.1℃and-0.06‰,respectively.Compared with those of the current warm period(26.6℃for SST and 0.40‰forδ¹⁸O_sw,respectively),the climate of NSCS in the beginning of the first millennium CE was colder and wetter,and similar to the climate condition during the Little Ice Age.The cold climate may be associated with the weaker summer solar irradiance.The wet conditions were caused by the reduced northward shift of the intertropical convergence zone（ITCZ）/monsoon rainbelt associated with the retreat of the East Asian summer monsoon（EASM）.ENSO and the Pacific Decadal Oscillation（PDO）may have also contributed to the interannual and interdecadal variabilities in the SST andδ¹⁸O_sw records.（2）The northern SCS experienced a rapid cooling and wetting event during the period of～3800-3500 years BP.Specifically,the average SST andδ¹⁸O_sw declined rapidly by～3°C and～0.65‰,respectively,over an interval of～100 years from～3850years BP to～3750 years BP.This rapid climate change pattern recorded in coral archives broadly agrees with those in foraminiferal and stalagmite records from adjacent land and ocean areas.More precisely,the stalagmiteδ¹⁸O records showed that the climate in inland China rapidly cooled and dried during this period,while the foraminifera Mg/Ca andδ¹⁸O records showed a rapid drop in SST and a rapid increase in rainfall in the western Pacific.Combined with several records from the North Atlantic,this cold event is similar to the MBACE,may represent a global-scale climate change,and may be linked with the changes in solar irradiance and the North Atlantic climate.The results may also support the occurrence and timing of the outburst flood event during the Xia Dynasty and indicate that the fall of the Xia Dynasty might have resulted from this rapid cooling and drying climate in northern China during the MBACE.（3）Theδ⁶⁶Zn values in the skeleton of the Porites coral from GBR do not have pronounced annual cycles showing in Sr/Ca,δ¹⁸O,andδ¹³C time series over the study period,and display a large variation of 0.62‰（from-0.01‰to 0.61‰）.In addition,the bulk skeletalδ⁶⁶Zn values of different coral species from LHT reef show significant interspecies differences.Our findings indicate that the internal biological activities of zooxanthellae and polyps may cause Zn isotopic fractionation in shallow-water coral skeletons and are most likely responsible for monthly variations in skeletalδ⁶⁶Zn.The direct influence of external environmental conditions on skeletalδ⁶⁶Zn is limited.We suggest that the amount of reactive oxygen species（ROS）produced by internal biological processes may induce Zn isotopic fractionation in coral skeletons while environmental factors（e.g.,SST）only partially affect changes in skeletalδ⁶⁶Zn by mediating biological processes.While the role of coral skeletalδ⁶⁶Zn is limited as a proxy for paleoenvironmental and paleoclimatic reconstruction,it may provide valuable insight into the responses of biological activities to environmental change.（4）The monthly resolutionδ⁶⁵Cu values in the skeletons of GBR Porites coral have a relatively small variation range from-0.09‰to-0.34‰,and have no obvious variation pattern.The correlation betweenδ⁶⁵Cu and the climate/environmental factors are not significant,and therefore it is not suitable to be used as a proxy to trace the climate/environment change.In addition,the skeletalδ⁶⁵Cu values in different coral species from LHT reef vary significantly from 0.02‰to 0.57‰,and are positively correlated with theδ⁹⁸Mo values affected by coral biological process.Therefore,theδ⁶⁵Cu values in the skeletons are also affected by the vital effect.