南海北部全新世早期低钙事件及其古气候解释

LOW CARBONATE EVENT IN NORTHERN SOUTH CHINA SEA DURING THE EARLY HOLOCENE AND THEIR PALEOCLIMATIC SIGNIFICANCE

碳酸盐旋回是南海晚更新世以来的最重要沉积特征之一 ,它受控于表层海水生产率、陆源物质的稀释作用以及深水对碳酸盐的溶解作用。南海北部碳酸钙百分含量主要受控于陆源物质对碳酸盐的稀释作用 ,即海平面低位时陆源碎屑经过陆架进入较深海域增多 ,也称稀释旋回。对西沙海槽北侧陆坡柱状样高分辨率研究后发现 ,在冰消期 ,低海平面时期存在高碳酸钙 ,而在冰后期的早中期 ,6～ 8kaB P 的高海平面时期 ,却存在短暂的碳酸盐含量急剧下降的过程 ,即低钙事件 ,产生这种短暂的碳酸钙与其旋回性不吻合的现象应该是降雨量变化造成。在冰消期 ,出现干旱 ,陆源物质减少 ,而早中全新世的雨量 ,尤其是具较强冲刷能力的大雨量急剧增加 ,造成北部海南岛和华南陆地的冲刷增强 ,陆源物质供给增加 ,从而出现低钙事件。

Carbonate cycle,which is controlled by sea surface productivity,dilution of terrigenous sediment and dissolution of deep-sea,is the most prominent feature in South China Sea since Late Pleistocene. In the northern South China Sea,variations of carbonate content are mainly controlled by dilution of terrigenous sediment. More terrigenous sediments were transported into the deep-sea areas through continental shelves (dilution cycles) during sea-level lowstand period. Studies on the cores from the northern slope of Xisha Trough show a high carbonate content during deglaciation (sea-level highstand period) and a rapid decrease in carbonate content (the low carbonate event) during early-middle post-glacial period (sea-level lowstand period,6～8 ka BP). This low carbonate event may have resulted from changes of rainfall. During deglaciation,the dry climate caused a decrease in terrigenous sediment input. However,during Early-Middle Holocene,the rainfall,especially the heavy rain with higher erosions increased rapidly,leading to the increase of terrigenous sedimental supplies from South China mainland and Hainan islands.