南海第四纪东亚季风演化的粘土矿物指标

CLAY MINERALOGICAL PROXY OF THE EAST ASIAN MONSOON EVOLUTION DURING THE QUATERNARY IN THE SOUTH CHINA SEA

南海作为西太平洋最大的边缘海，记录着东亚季风的演化历史。本文在过去十多年研究积累的基础上，综述粘土矿物是记录南海东亚季风演化历史的良好指标。大量表层样品和多个沉积岩芯的研究显示，南海第四纪以来的蒙脱石主要是形成于周边岛屿火山岩的快速化学风化作用，记录了同时期的东亚夏季风气候条件；伊利石和绿泥石形成于周边大陆和岛屿的机械侵蚀作用，与强烈降雨引发的侵蚀能力或东亚冬季风相对寒冷的气候条件有关；而高岭石形成于周边陆地表面的强烈化学风化作用，指示地质历史时期累积的或第四纪同时期的温暖潮湿气候条件，但由于强烈的差异沉降作用，高岭石在南海主要沉积在近海陆架区域，在深海记录中多指示海平面变化的古环境特征。因此，我们提出蒙脱石／（伊利石＋绿泥石）比值是南海第四纪东亚夏季风演化的良好指标，比值高代表夏季风温暖潮湿气候条件下增强的化学风化作用，比值低指示冬季风相对寒冷气候条件下减弱的化学风化作用，或是强烈降雨气候条件下增强的机械侵蚀作用。南海东亚夏季风演化的强弱与北半球夏季日射量基本呈线性关系，表明东亚季风演化的天文驱动机制，第四纪以来的长期演化显示南海在1200～400ka期间盛行夏季风，可能是受到赤道太平洋厄尔尼诺-南方涛动（ENSO）的影响。

The South China Sea is the largest marginal sea in the western Pacific and has a great potential for recording the East Asian monsoon evolution history. Based on approaches over the past ten years, this study tends to summarize clay minerals as an effective proxy for reconstructing the East Asian monsoon evolution in the South China Sea. Investigations on plentiful seafloor surface samples and many sediment cores in the South China Sea show that smectite is formed from the rapid chemical weathering of volcanic rocks in surrounding islands during the Quaternary, and then presents contemporaneous East Asian summer monsoon climatic conditions ; illite and chlorite are produced from physical erosion in surrounding continents and islands, relating to intense rainfall triggered erosion ability or to East Asian winter monsoon cold climatic conditions; while kaolinite is formed from intensive chemical weathering of surrounding lands, and indicates warm and humid climatic conditions either accumulated in the geological past or contemporaneously during the Quaternary. However, due to strong differential settling effect, most of kaolinite is deposited on the coastal shelf, and kaolinite deposited in the deep sea mostly refers to sea-level change history. Therefore, we come up with the smectite/（ illite ＋ chlorite） ratio as an effective proxy for reconstructing the East Asian summer monsoon evolution in the South China Sea. A higher ratio stands for enhanced chemical weathering forced by summer monsoon warm and humid conditions; a lower ratio indicates weakened chemical weathering driven by winter monsoon cold conditions, or strengthened physical erosion triggered by intense rainfall. The evolution of East Asian summer monsoon in the South China Sea provides an almost linear response to the summer insolation of the Northern Hemisphere, implying an astronomical forcing of the East Asian monsoon evolution. The long-term variability during the Quaternary indicates prevailed summer monsoon during 1200-400ka in the South China Sea, suggesting the influence of El Nino-Southern Oscillation （ENSO）.