东海外陆架晚第四纪高精度年代地层及层序地层研究

The East China Sea (ECS) is one of the largest marginal seas over a broad continental shelf located between the largest continent (Asia) and the largest ocean (Pacific) in the world . It is considered as a natural laboratory for research of paleoclimate and paleoenvironmental evolution of late Quaternary, due to its very thick deposits. However, up to now, most of those relevant studies in ECS are focused on the landward subaqueous Yangtze delta and inner shelf area, with less attention to the more seaward outer shelf. There are also several studies on the outer shelf, but with many different viewpoints about the division of late Quaternary strata, mainly embodying in two aspects: chronostratigraphy and sequence stratigraphy. So, in this project , we plan to construct a high-precision chronostratigraphic framework , based on a new borehole core DH02 obtained in 2013 on the outer shelf of the ECS by the First Institute of Oceanography, SOA. A comprehensive study of sedimentology and micropaleontology is to be carried out to construct sedimentary facies succession through sedimentary macro-characteristics observation, grain size analysis and benthic foraminifera identification, and the high- precision chronostratigraphic framework is to be built up, incorporated with paleomagnetism dating and two high-precision dating methods: Accelerator Mass Spectroscope (AMS) 14C and Opically Stimulated Luminesecence(OSL) . In the following stage, combined with the seismic profiling, we will analyse and discuss the genesis and evolution of sequence stratigraphy during late Quaternary on the outer shelf of the ECS.

东海拥有世界罕见的宽广大陆架，发育了巨厚的沉积物，是开展晚第四纪古气候和古环境演化研究的天然实验室。迄今为止, 相关研究主要集中在内陆架和长江水下三角洲地区，而外陆架研究则相对薄弱，年代地层和层序地层的划分存在较大的分歧，没有形成对晚第四纪沉积地层格架的统一认识。基于此，本项目拟以国家海洋局第一海洋研究所2013年在东海外陆架钻取的DH02孔晚第四纪岩芯地层为研究对象，综合应用沉积学和微体古生物学的研究方法，以岩芯宏观特征描述、粒度和底栖有孔虫分析为基础，建立沉积相演化序列，同时以多种定年方法（古地磁定年、AMS 14C测年和光释光测年），构建高精度的年代地层格架；在此基础上，结合地震资料，划分层序地层单元，结合海平面变化曲线，探讨东海外陆架晚第四纪层序地层的成因和演化。

基于DH02孔岩芯，综合利用沉积构造观察和描述、粒度分析、微体古生物鉴定及高精度定年（放射性碳测年（AMS 14C）和光释光（OSL））技术和方法，对东海外陆架晚第四纪以来的层序地层成因和演化进行了研究。依据岩性和微体古生物组合特征，构建了DH02孔岩芯的沉积相序列：（1）90.6-100.1m，河流相；（2）66.5-90.6m，受潮流影响的河流相；（3）48-66.5m，潮上坪-潮下坪相；（4）37.5-48m，潮间坪相；（5）20-37.5m，前三角洲或近河口内陆架相；（6）8.8-20m, 受潮流影响的河流相；（7）0-8.8m，浅海潮流沙脊相。此外，根据高精度AMS14C和OSL定年，建立了DH02孔的年代地层格架，揭示了深海氧同位素（MIS）6期以来的4期（MIS1、3、5和6）海侵-海退事件。在沉积序列和年代地层分析的基础上，结合其他钻孔资料，探讨了东海外陆架晚第四纪以来的沉积演化历史。MIS6期，东海发生了大面积海退，外陆架暴露，古长江在此发育，形成了DH02孔底部的河流相，随后发生海侵。然而DH02孔仅保留了海侵旋回最底部的受潮流影响的河流相，推测上覆海相序列的缺失可能是后期海侵过程海洋动力侵蚀造成。MIS5期，海平面发生多次小幅度的升降，形成了的潮上-潮下-潮间坪相序列。DH02孔并未发现MIS4期的沉积记录。分析认为这可能与该时期海平面波动条件下，海洋动力侵蚀有关。MIS3期，伴随着持续的海退，古长江在陆架之上发育了三角洲和近河口内陆架泥质沉积体系。MIS2期，即末次盛冰期，海平面降至低于现今~120 m，古长江流路在陆架上广泛发育。DH02孔缺失了这一时期的沉积记录，其原因可能是低海平面时期，长江携带的大量沉积物路过大陆架，到达冲绳海槽发生沉积，亦可能是沉积地层在后期海侵过程中遭受海洋动力侵蚀所致。MIS1期，伴随着海侵的发生，沉积环境由河流河流依次转变为河口湾和潮控陆架浅海环境。陆架沉积物接受潮流侵蚀、改造和再沉积，形成潮流沙脊。DH02孔顶部潮流沙脊形成于距今1500年，这表明形成始于全新世海侵的潮流沙脊的沉积演化在距今7千年以来的高海平时期依旧活跃。