南海北部陆坡沉积记录的全新世早期夏季风极强事件

在AMS14C精确定年的基础上,通过南海北部MD05-2905站高分辨率的氧同位素、粒度、元素分析,其结果显示所有指标均在11.2～8.5kaB.P.期间发生异常,根据夏季风指标认为这是夏季风极强事件。将本站位相同纬度（20°N）的夏季平均日辐射量与季风指标进行对比,发现全新世早期（11.2～8.5kaB.P.）东亚夏季风极强事件对应着夏季太阳辐射量最大值和热带辐合带（ITCZ）的位置向北移动,说明全新世早期夏季风突然增强可能是在岁差周期控制下,较高的太阳辐射量驱动了热带辐合带（ITCZ）及其有关的雨场向北移动,在南海北部产生了大量的降水,导致氧同位素偏负、碳酸盐的含量较低以及线性沉积速率较高。这与前人对相邻地区湖光岩玛珥湖的孢粉研究、江西南岭地区泥炭的记录以及中国南方石笋记录的研究结果相吻合。该季风最强事件与阿拉伯海发现的早全新世季风最强事件以及根据格陵兰冰芯记录的早全新世甲烷最大值推测低纬地区湿地扩张相一致,这反映了南海北部全新世早期与全球低纬地区早全新世气候变化格局一致。

EAST ASIAN SUMMER MONSOON MAXIMUM RECORDS IN NORTHERN SOUTH CHINA SEA DURING THE EARLY HOLOCENE

High-resolution element records from core MD05-2905(20°08.17'N,117°21.61'E,w.d.1647m),located in the northern part of South China Sea,were measured by XRF Core Scanner at a 1cm resolution,while stable oxygen isotope and grain size analysis were both measured at a 4cm interval.12 AMS14C dates were made on planktonic foraminifer samples(Globigerinoides ruber and G.sacculifer),which provide a basis for the chronostratigraphy.The results of grain size analysis reveal that the content of 2-9μm,as a proxy of summer monsoon,reaches to the culmination in the Early Holocene(11.2-8.5kaB.P.)after ca.28kaB.P.The G.ruber ^δ18O c rve shows a broad plateau of negative value during this time,indicating a period of summer monsoon maximum,which brought about increased fresh-water influence by excess precipitation and runoff.It is well known that carbonate cycle is controlled by sea surface productivity,dilution of terrigenous sediment,and dissolution of deep sea,which is the most prominent feature in South China Sea since Late Pleistocene.However,during the Early Holocene(11.2-8.5kaB.P.),carbonate content is the lowest,which may be related with summer monsoon maximum,because the increasing of summer monsoon brings more terrigenous sediment supplies into the northern South China Sea and terrigenous sediments dilute carbonate.This coincides with the records of Huguangyan Maar Lake,Dahu peat and South China stalagmite nearby.The summer monsoon maximum has been also found in Arabian Sea and the inferred period of low latitude wet land expansion based on an Early Holocene methane maximum recorded in Greenland ice core.It thus indicates a major global climatic event,which coincides with climatic pattern of low latitude area during Early Holocene.Through comparing with summer insolation of 20°N and ITCZ proxy,we find this event corresponds with the highest stage of insolation and the northward position of the ITCZ moving,which may be related with the amount of solar radiation.Controlled by precession periodicity,driven by the highest insolation,the position of ITCZ and its rain belt moves northward,bringing more precipitation into the northern South China Sea.