末次盛冰期以来南海东北部底层洋流的演变

The South China Sea (SCS), the largest marginal sea in northwestern Pacific, is an ideal place for investigating deep sea processes. Previous researches based on physical oceanology and marine geology have uncovered the existence of the SCS deep circulation and its great impact on deep sea sediment transport. However, less studies have been carried out on the history of SCS deep circulation and its influencing factors especially on its variation in orientation and strength. Sediment cores have been retrieved from northeastern slope of SCS, located in flowing path of the SCS deep circulation, to elucidate aforementioned problem. The variation in strength of SCS deep circulation has been reconstructed based on accurate AMS 14C dating and δ18O of benthic foraminifera, and various deep circulation proxies including the sortable silt fraction, magnetic parameters and deep circulation end member extracted from grain size matrix by using End Member Modeling Analysis (EMMA). Meanwhile, the orientation of SCS deep circulation was traced using the calibrated inclination of principal axes of anisotropy of magnetic susceptibility ellipsoid. Based on distinct proxies in orientation and strength and various stations along the deep circulation route, we could reconstruct the variation of strength, orientation of SCS deep circulation since the Last glacial Maximum in a spatial and temporal scale. The comparison between SCS deep circulation proxies and that of North Pacific intermediate and deep water is further complemented to assist in elucidating teleconnection relationship between SCS and Pacific climate and circulation system, which is critical for the interpretation of SCS deep sea transport mechanisms and the evaluation of regional climate response to deep circulation.

作为西北太平洋最大的边缘海，南海是开展深海过程研究的理想海域。尽管物理海洋学和海洋地质学研究已经揭示南海底流的存在及其对深海沉积物的搬运作用，但目前对底流流向和强度的研究非常缺乏，对底流的变迁历史及其影响因素还很不清楚。本项目以底流流经的东北部陆坡区的代表性沉积柱样为具体解剖对象，利用δ18O氧同位素及AMS14C厘定沉积年龄，综合分析沉积物粒度组成、磁化率各向异性和岩石磁学特征。采用粒度端元解析方法，剥离底流端元，结合分选性粉砂和磁化率各项异性指标，重建底流强度演化；同时，运用磁化率各向异性椭球体长轴倾向，以反演底流方向。本项目通过多点位和多指标，从时间和空间尺度上重建末次盛冰期以来底流方向与强度演变历史，并对比南海与太平洋地区中深层水指标，探讨南海底流演变与太平洋深层洋流系统之间的联系，研究成果对阐明南海沉积物输运机制以及评价底层洋流对南海区域气候变化的影响具有重要的科学意义。

深层洋流是全球气候系统重要的组成部分，其与北向的西边界流一起所构成的经向翻转流对全球气候具有十分巨大的调控作用。但关于气候突变时期北太平洋和北大西洋地区之间气候态、洋流系统的关系以及对应传递机制还存在争议。南海是西北太平洋的边缘海，位于北太平洋深层水流经的中下游，是研究太平洋与大西洋之间气候态和传递机制的重要场所。利用沉积磁学和粒度矩阵数据，结合数值模拟（LOVECLIM）方法揭示了末次盛冰期以来南海深层洋流千年尺度的动态演化历史。研究发现，南海深层洋流的快速加强对应于北大西洋气候变冷时期，即Heinrich 1和Younger Dryas事件，这与这些时期北太平洋地区深水的形成加强有关，从而揭示了太平洋-大西洋之间海盆尺度跷跷板效应。该研究对于我们理解千年尺度洋流演化机制及其对全球气候变化的影响具有重要意义，也有助于深入认识全球变暖情形下大气和海洋之间复杂的相互作用以及极端气候现象。