东海北部外陆架晚更新世以来两期古三角洲的时空展布特征及物源属性研究

It was reported that two stages large paleo-deltas were developed on the outer shelf in the north of East China Sea, according to analysis on the 6252km long high-resolution shallow seismic profile. In this study, we choose shallow seismic data and SHD-1 core (longest and highest coring rate in this area) obtained in the north of East China Sea as research object. By comparison seismic facies characteristics of core-passing subbottom seismic profile and paleo-deltas sedimentary characteristics recorded in core SHD-1, the thickness and distribution range of two stages paleo-deltas will be determined. The formation age of two stages paleo-deltas will be fixed, based on the AMS C-14 and Optically Stimulated Luminescence (OSL) dating data of SHD-1 core. Spatial and temporal distribution characteristics of paleo-deltas will be further be revealed. Moreover, rare earth element (REE) and Sr-Nd isotope analysis on particle <32μm from paleo-deltas sediments in SHD-1 core was conducted to obtain discrimination indicators for tracing sediment sources of paleo-deltas. By comparing with discrimination indicators of the same grain-size deposit from the large rivers(the Yellow River, the Yangtze River) around the East China Sea, the provenance attribute and sediment sources changes for the two stages paleo-deltas since late Pleistocene will be revealed. Thus, this study can not only help to deeply understanding the evolution history of the East China Sea continental shelf , but also will provide scientific evidences to determine whether the ancient Yangtze River and Yellow River once flew across the East China Sea shelf and Okinawa trough into the sea.

在对东海北部外陆架实测的6252km高分辨率浅地层剖面解释中发现，晚更新世以来该海域至少存在两期大型古三角洲。本研究以东海北部外陆架的浅地层剖面和SHD-1孔的岩心为研究对象，拟通过对比东海北部外陆架浅地层剖面的地震相特征与SHD-1孔记录的古三角洲沉积特征，确定两期古三角洲的沉积厚度、分布范围等，结合SHD孔的AMS C-14和光释光测年资料，确定两期古三角洲的形成时代，进而揭示古三角洲的时空展布特征；此外，对钻孔古三角洲层位粒径<32μm的沉积物进行稀土元素（REE）与Sr-Nd同位素的系统分析，获取两期古三角洲的物源指标，并与周边主要入海大型河流（黄河、长江）相同粒径沉积物的鉴别指标进行对比，确立古三角洲的物源属性，揭示东海外陆架晚更新世以来古三角洲的物源演变。本课题将有助于深入认识晚更新世以来中国东海陆架的形成演化，为古长江、古黄河是否经东海陆架从冲绳海槽入海提供科学依据。

利用最新采集的高分辨率浅地层剖面资料，结合SHD-1钻孔岩心资料，对东海北部外陆架MIS6以来的地层结构进行了分析，建立了MIS6以来的沉积地层格架，并对地层的地质年代进行了厘定，从而得到两期古三角洲的形成年代。东海北部外陆架MIS6以来可划分出7个不整合界面（D7-D1）和7个地震单元（SU7-SU1）。东海外陆架的海进层序与海退层序有规律地交替发育，它们与海平面变化曲线也有很好的对应关系。MIS4-MIS3阶段发育厚层且分布广泛的水下三角洲，但MIS4发育的水下三角洲的规模不及MIS3大。SHD-1孔稀土元素分布模式、Eu异常与稀土元素总量关系(δEuN-ΣREEs)判别图解和物源判别指数(PI)的综合研究, 表明沉积物物源主要来自长江, 长江从晚更新世早期到现代对东海外陆架起着主要作用, 而黄河沉积物在晚更新世晚期已开始对东海外陆架也有一定的影响。通过地层格架的建立、地层年代的厘定及物源属性的分析，探讨了MIS6以来沉积相特征，该研究可为东海外陆架晚第四纪地层的海平面变化、古环境演化等相关研究提供参考。