全新世东亚季风变化的百年尺度周期

Centurial Cycles of Variation of the Holocene East Asian Monsoon

对位于东海内陆架闽浙沿岸泥质沉积区中部的PC-6孔进行了AMS14C年龄测试和粒度分析,综合该孔的岩性、颜色、沉积构造以及沉积物垂向上的叠加方式等特征,将该孔以450cm和540cm为界分为3段,其中上段(450cm以上)以灰褐色、灰色粘土质粉砂为主,岩性较均一,测年表明其形成于近7.64ka以来,此期间海平面变化只有约3￣4m,对应的沉积动力条件与沉积环境应该与现今基本一致,为海侵结束后高海平面以来主要受沿岸流控制的浅海沉积。通过计算PC-6孔450cm以上225个样品陆源碎屑组分中每个粒级组分标准偏差随粒级组分的变化,分离出对沉积环境敏感的粒级约为6μm和56μm,2个粒度组分的分界线约在28μm。细粒组分(<28μm)为东海冬季沿岸流携带悬浮体所沉积,而>28μm粗粒组分对应的沉积动力条件为波浪。本文采用28μm以下的细粒级组分的平均粒径作为替代指标,来揭示东亚冬季风的变化周期。使用功率谱分析软件REDFIT35对<28μm组分的平均粒径进行分析,PC-6孔揭示出70￣72、78、89、102、112、123、154a等百年尺度的强周期信号。与世界其它地区进行综合对比分析表明,全新世东亚季风的变化在百年尺度以Gleissberg周期和约70a的周期为主,且它们都是太阳辐射变化的结果。

AMS14C dating and grain-size analysis for Core PC-6, located in the middle of a mud area on the inner shelf of the East China Sea （ECS）, were used to rebuild the Holocene history of the East Asian winter monsoon （EAWM）. The 7.5-m core recorded the history of environmental changes during the postglacial transgression. The core was divided into three sections with boundaries at 450 cm and 540 cm, according to its lithology, color, sedimentary structure, and vertical stacking pattern of its sediment. The core^s mud section （the upper 450 cm） has been formed mainly by suspended sediment delivered from the Yangtze River mouth by the ECS Winter Coastal Current （ECSWCC） since 7.6 kyr BP. Using a mathematical method called ＂grain size vs. standard deviation＂, we can divide the Core PC-6＇s grain-size distribution into two populations at about 28μm. The fine population （〈28μm） is considered to be transported by the ECSWCC as suspended loads. Content of the fine population changes little and represents a stable sedimentary environment in accord with the present situation. Thus, variation of mean grain-size from the fine population would reflect the strength of ECSWCC, which is mainly controlled by the EAWM. Spectral analyses of the mean grain-size time series of Core PC-6 show statistically significant centurial periodicities centering on 70-72, 78,89, 102, 112, 123, 154 years. The EAWM and the East Asian summer monsoon （EASM） agree with each other well on these cycles, and the East Asian Monsoon （EAM） and the Indian Monsoon also share in concurrent cycles in Holocene, which are in accord with the changes of sun irradiance.