中全新世冲绳海槽北部的水文气候变化:浮游有孔虫群落证据

The hydroclimate changes in the northern Okinawa Trough during middle Holocene:Evidence from planktonic foraminiferal assemblages

东海东北部的浮游有孔虫群落结构受长江冲淡水和黑潮暖流的显著影响。对冲绳海槽北部NOKT-3岩芯178个样品中浮游有孔虫的统计分析,共鉴定出壳径大于150μm的种属28种。其中,低温低盐种Globigerina quinqueloba的百分含量在中全新世早期波动剧烈,而中晚期呈逐渐下降趋势。G.quinqueloba是长江冲淡水指示种,长江冲淡水的强弱受到东亚夏季风降雨的控制。因此,据6330aBP以来G.quinqueloba含量的变化将中全新世长江中下游地区的降雨量变化划分为3个阶段:(1)6330～4950aBP降雨较强,为全新世适宜期;(2)4950～4600aBP期间降雨开始减少,全新世适宜期结束;(3)4600～3890aBP间降雨量明显降低,气候显著变干。降雨量这一变化过程的机制可能是全新世早期以来北半球太阳辐射量逐渐减小,导致东亚夏季风强度减弱,季风前缘锋面逐渐向东南方向退缩,因而长江中下游地区中全新世降雨量逐渐降低。此外,黑潮指示种Pulleniatina obliquiloculata含量的变化显示,中全新世早期6330～5100aBP期间,黑潮比较强,对冲绳海槽的影响持续而强劲;中全新世晚期5100～4000aBP P.obliquiloculata含量显著降低,对应着P.obliquiloculata极小值事件(PME),表明中全新世晚期PME发生时区域气候并没有明显变冷,且海槽北部的PME事件也无法用赤道太平洋长期的类El Ni1o状态来解释。

Planktonic foraminifera assemblages in the northern East China Sea are significantly influenceed by both the Changjiang Diluted Water and Kuroshio Current.A total of 28 species in the coaser faction(>150μm)are identified based on planktic foraminifera analysis of 178 samples from core NOKT-3 in the northern Okinawa Trough.The results show that variations in percentage of Globigerina quinquelobais characterized by sharp fluctuation in early Mid-Holocene and gradual decline in late Mid-Holocene.This low temperature and low salt salinity species is an indicator of the Changjiang Diluted Water,which is controlled by the East Asian summer monsoon precipitation.Therefore,the evolution of precipetation in the middle and lower reaches of the Yangtze River between6330 and 3890 aBP can be divided into three major stages based on variations in its con-tent:(1)high precipetation during the period of 6330~4950 aBP,which correspondes to the Holocene optimum;(2)a decreased trend of precipetation from4950 to 4600 aBP,implying termination of the Holocene optimum;(3)a marked decrease in the monsoon precipitation between 4600 and 3890 aBP,indicating an arid climate condition.This precipetation pattern could be related to the general weakening and southeastward retreat of the East Asian summer monsoon resulted from the decreased solar-radiation in the northern hemisphere during the Middle Holocene.Moreover,remarkably high abundance of P.obliquiloculat during 6330~5100 aBP,which is normally an indicator species of the Kuroshio Current,suggests that influence of the Kuroshio Current in the Okinawa Through was persistently strong.The abundance of P.obliquiloculata sharply decreased during5100~4000 aBP,well corresponding to the so-called Pulleniatina minimum event(PME).Our results show that there were not a cold climate condition corresponding to the PME during the period of late Mid-Holocene.In addition,enhancement of the El Ni1 o-like condition in the tropical Pacific region during this time window also can not fully explain the occurrence of PME in the northern Okinawa Trough.