越南岸外晚第四纪上升流与东亚夏季风变迁

LATE QUATERNARY COASTAL UPWELLING AND VARIATIONS OF THE EAST ASIAN SUMMER MONSOON OFF THE VIETNAM COAST

通过对南海西部越南岸外１７９５４—３柱状样中浮游和底栖有孔虫的定量分析，结合碳酸钙、有机碳和稳定同位素数据，获得南海西部上升流区近２０万年来的古海洋学记录，进而讨论了晚第四纪东亚夏季风的变迁。结果表明，在间冰期，尤其是末次间冰期，越南岸外出现表层海水古温度低、温跃层浅、生产力高，同时底层海水富营养的现象，说明上升流的存在，且该上升流有从氧同位素５期（ＭＩＳ５期）向１期逐渐减弱的趋势。结合南海现代海流及生产力分布的研究，推断该上升流由东亚夏季风驱动，其强弱的变化说明，东亚夏季风在近２０万年来有间冰期增强、冰期减弱，且从ＭＩＳ５期向ＭＩＳ１期递减的特征。

The Asian monsoon system is of primary importance in our understanding ofglobal climate evolution. Coastal upwelling, the main oceanographic phenomena drivenby monsoonal winds, has been a point of vigorous discussion in paleoceanographyrecently. Being the region where the East Asian summer monsoon originates, theSouth China Sea(SCS) is the main studied area for reconstructing the East Asianmonsoon evolution. Previous studies were concentrated on the northern and easternpo of the SCS, with emphasis on the changes in the surface and deep cireulation,paleotemperature, carbonate cycle and productivity. The western SCS, however, wasmuch less studied. In face there is potential summer monsoonal upwelling off theVietnam coast based on the modem oceangraphic observations.Quantitative analysis of planktic and benthic foraminifers in Core 17954-3 off theVietnam coast in the western South China Sea integrated with the data of CaCO_3,organic carbon and productivity as well as Uk~'_37, estimated sea surface temperature (SST),has been carried out to monitor the variations of the coastal upwelling and hencereconstruct the East Asian summer monsoon evolution over the last 200 000 years.Our observations indicate that during the interglacial stages, the upper water colummis characterized by the conditions of low SST(Fig. 1) shallow thermocline(Fig.2) andhigh paleoproductivity (Fig. 3). This result convinces us that the existence of thecoastal upwelling there during the interglacial stages. Prticularly, the intensity of theupwelling gradually decreased from marine isotopic stage (MIS) 5 to MIS 1. Basedon the studies of the surface circulation and the distribution of productivity in themodem SCS, we suggest that the coastal upwelling at Site 17954-3 is driven by theEast Asian summer monsoon. The changes of the upwelling reflect the variations ofthe East Asian summer monsoon, which intensified at interglacial stages and graduallydecreased from the MIS 5 to MIS 1.