The Influencing Of Monsoonal Climate On Scleractinian Coral Calcification And Isotopic Compositions Of Northern Part Of South China Sea

Coral colonies of Porites lutea and P. lobata were collected from areas to the east and south offshore of Hainan Island, South China Sea (SCS) to investigate the influence of monsoonal climate on coral growth and isotopic compositions.Annual, semiannual, seasonal and monthly growth bands were found in the massive corals of Northern SCS. The highest density bands within an annual band were developed from the last ten days of March to the middle ten days of April or from the last ten days of June to the middle ten days of Septemp, while the lowest density bands are formed from the last ten days of April to the middle ten days of June in an annual band. Coral skeleton density has decreased and the linear extension has increased during the past decades. Statistic indicate that calcification of most coral has decreased in the past decade due to arising of atmospheric CO₂ concentration. It's demostrated that the available light for photosythesis of zooxanthellate within scleractinian coral is most important variable to influence coral linear extension and the skeleton density. Meanwhile, the temperature and atmospheric CO₂ have the significant effects on the trends of coral density, linear extension and calcification.Oxygen isotope ratios, ¹⁸O/¹⁶O, were analyzed along the major axis of growth at a resolution of 25-28 samples within an annual band from three colonies. Coral δ¹⁸O value correlates well with sea surface temperature (SST) throughout most years, particularly when the Asian winter monsoon prevails over the region. It is estimatedthat most of the seasonal features of coral 518O values are controlled by changes in SST with a significant contribution of SSS. This indicates the importance of temperature, as determined by the Asian winter monsoon, as a control on seasonal coral 518O value variations in the South China Sea. Given the fact that annual SSS maxima show minor inter-annual changes, the inter-annual change of annual coral 518O maximum mainly reflects the SST induced by the winter monsoon. While the coral 818O values is mainly controlled by SST, deviations of the 8I8O values from SST correlate statistically with sea surface salinity, particularly for summer extreme events when SST reaches its annual maximum with minor inter-annual change. Therefore, we suggest that the inter-annual variation of the annual coral 8I8O minima is mainly controlled by seawater composition. Seawater composition analysis indicates that its 818O values and salinity are simultaneously affected by freshwater inputs primarily from precipitation, which is mainly supplied by the Asian summer monsoon. The three coral records consistently reveal clear inter-decadal trends in 818O values - a gradual increase from 1968 to 1987, and a subsequent decrease from 1987 to 2003. These inter-decadal trends are roughly consistent with salinity changes - but not with temperature and precipitation, suggesting that seawater composition controlled by freshwater inputs from distant source areas, rather than from local precipitation, is responsible for the trends in seawater composition and coral 518O values over the last 30 years. Comparison of the three coral records indicates that spatial variations in coral 518O values coincide with variations in seawater salinity and local precipitation, but not with temperature. This confirms the dominant role of seawater composition, mainly controlled by monsoonal precipitation, on the spatial variability of coral 818O values in the South China Sea.Based on a time scale constructed from the correlation of 818O values and sea surface temperature, seasonal changes of the coral 8I3C values are in phase with solar radiation, and regression analysis demonstrates their close correlation. Therefore, it's suggest that light-induced photosynthesis modulates most of the seasonal features of the coral S13C values. A remarkable inter-decadal trend of the coral 8I3C values record of the last 30 years is the abrupt decrease of the annual minimum of around 1987,resulting in a decreased amplitude of annual fluctuation. This coral 613C values change shift corresponds statistically with the local radiation record, suggesting a causal effect. At the same time, there is a generally decreasing trend of coral 513C values over the last 15 years. This trend is consistent with seawater composition as indicated by salinity, and is best explained as the consequence of changes in atmospheric CO2 content. The spatial differentiation of the seasonal fluctuations of the three coral 8I3C values records is characterized by a lower annual minimum, lower mean value, and larger annual amplitude at Qionghai to the east offshore of Hainan Isalnd;and by a higher annual minimum, higher mean value and smaller annual amplitude at Sanya to the south offshore of Hainan Island. Comparison with marine environmental variables supports the suggestion that these spatial differences are mainly the result of differences in seawater composition in the two regions.