High Resolution Climatic And Environmental Changes In Xisha Islands Of South China Sea During The Late Holocene

Knowledge of past climate changes in tropics during the late Holocene provides perspective for understanding and modeling current and future global climate. In the present dissertation, lake sediments and Tridacna samples from Xisha Islands were used to reconstruct the detailed climatic information in the South China Sea during the last2,000years.Main conclusions of this dissertation are outlined as follows:1. The impact of ENSO activity on the climate of Xisha IslandsThe relationships between ENSO activity and regional climate anomalies in the Xisha Islands during the last50years were investigated. The results suggested that there is a significant positive correlation between ENSO index (EMI) and air temperature anomalies in the Xisha Islands. The precipitation and typhoon activity are also influenced by ENSO conditions. During a La Nina event, when Pacific Walker Circulation is strengthened, and the ascending limb in the western Pacific enhances, leading to more precipitation and more tropical cyclones in the South China Sea.2. South China Sea hydrological changes over the last millenniumWe reconstructed hydrological changes over the last millennium from Dongdao Island (～16°40’N,112°44’E), South China Sea, using multiple proxies as hydrology indicators. Our multiple records consistently suggest that this region received less precipitation during the relatively warm period, AD1000-1400and AD1850-2000, and more rainfall during the cool period (AD1400-1850). Due to the locality of our records, the temporal precipitation pattern could be hardly explained by the southward movement of the ITCZ during the cool period. We suggest that while ITCZ might have indeed played an important role in controlling tropical Pacific hydrology, changes in Pacific Walker Circulation could have also influenced the hydrology over the last millennium, based on our records and some existing hydrological records.3. Reconstruction of Southern Oscillation index over the last2,000years Here we attempt to extend the multi-decadal scale Southern Oscillation Index (SOI) to2,000years ago using paleo-precipitation records. The reconstructed SOI proxy has five distinct phases over the past2,000years:persistent negative values during AD50-450, AD1000-1400, and AD1850-1955interrupted by positive values during AD500-1000and AD1400-1850. Importantly, the reconstructed SOI proxy is significantly linked to the solar irradiance and Northern Hemisphere climate over the past1,000years.4. High resolution δ18O and Sr/Ca ratio of a modern T. gigas specimen and its paleoclimate implicationHigh-resolution δ18O and Sr/Ca ratio profiles of a modern (live-caught) giant bivalve shell (Tridacna gigas) from northern South China Sea were analyzed. The high-resolution δ18O profiles, as documented in most of previous studies, showed pronounced seasonal cycles and recorded the combined influence of seawater δ18O and sea surface temperature. Two Sr/Ca profiles of modern T. gigas shell were determined by ICP-OES and LA-ICP-MS, respectively. The Sr/Ca profile derived from ICP-OES had well-defined annual cycles and significantly, negatively correlated with sea surface temperature. Similar correlations were not found in this study and in previous studies with Sr/Ca ratios of T. gigas determined by LA-ICP-MS. The difference is likely caused by the organics content in T. gigas shells; the organics, having a chemical composition different than CaCO3crystals and a heterogeneous distribution across the shell, can significantly influence the estimates of trace metal concentration when using LA-ICP-MS; and the influence could be reduced by using the wet chemical method such as ICP-OES.5. The difference of Sr/Ca ratios among three Tridacna speciesSeven Tridacna specimens of three different species (Tridacna gigas, Tridacna derasa and Hippopus hippopus) were collected from Xisha Islands of South China Sea and the high-resolution Sr/Ca ratios of their inner shells were analyzed by ICP-OES. The Sr/Ca profiles of Tridacna gigas and Tridacna derasa display regular annual cycles linking to the seasonal sea surface temperature (SST) variation of the studied area, but the Sr/Ca profiles of Hippopus hippopus didn’t show clear annual cycles. Moreover, the results also showed some big interspecific differences in mean Sr/Ca values between the three Tridacna species, indicating that species identification was necessary before using Tridacna Sr/Ca as paleoclimate recorders. 6. High sea surface temperature in the South China Sea during the natural warm periods of late HoloceneHigh resolution sea surface temperatures in the South China Sea during the late Holocene natural warm periods were reconstructed by the Sr/Ca and δ18O of Tradacna gigas specimens from the Xisha Islands. The results suggested that the mean sea surface temperature during the AD914-925and BC10-AD10were0.88℃and1.64℃higher than AD1994-2005, respectively. Hence an inference can be drawn from these results that the temperatures during the second half of the20th century were lower than that during the late Holocene natural warm periods in at least the South China Sea.7. Millennial Cu and Hg records derived from ornithogenic sediment on Dongdao Isand and its relationship with the anthropogenic Cu and Hg emissionA2000-year record of copper concentration in red-footed booby excrement was derived from the sediment core on Dongdao Island and the result showed a striking correlation between the copper content of seabird excrements and the ancient copper production in China over the past two millennia. The increased Cu content corresponds to the flourishing times of dynasties, such as the Tang and Sung Dynasties, and declines in Cu content correspond with dynastic declines. We established an approximate function to model the quantitative relationship between the copper content in the seabird excrement and the ancient copper production. We estimate that a total of～14million tons of copper was produced during the past two millennia in China. Among the14million tons of copper, about0.65million tons was released into the atmosphere.Two ornithogenic sediment cores, which have a time span of1,000years and are influenced by red-footed booby (Sula sula), were collected from Dongdao Islands, South China Sea. The determined mercury concentrations of the two cores show similar and substantial fluctuations during the past millennium, and the fluctuations are most likely caused by the changes in mercury level of the ocean environment and in anthropogenic Hg emission. The mercury concentration increased rapidly after AD1600in corresponding to beginning of the unparalleled gold and silver mining in South Central America that left a large volume of anthropogenic mercury pollution. Since the Industrial Revolution, the mercury level has increased at a fast pace, very likely caused by modern coal combustion, chlor-alkali and oil refining industries. The comparison of mercury profiles from different places on earth suggested that anthropogenic mercury pollution after the Industrial Revolution is more severe in Northern Hemisphere than in Antarctica.