苏克凡^1,,
高萌¹,
胡哲¹,
王娜²,
黄宝琦^1,,
1. 北京大学地球与空间科学学院, 北京 100871
2. 北京大学深圳研究生院, 城市规划与设计学院, 广东 深圳 518055

基金项目: 国家自然科学基金项目（批准号：41376043）和广东省基础与应用基础研究基金项目（批准号：2019A1515110896）共同资助


详细信息

作者简介: 苏克凡, 女, 22岁, 硕士研究生, 古生物学与地层学专业, E-mail: sukirye@pku.edu.Cn

通讯作者: 黄宝琦, E-mail: bqhuang@pku.edu.Cn

中图分类号: P722.7,Q915.811⁺.1

收稿日期:2021-04-30
修回日期:2021-08-07
刊出日期:2021-11-30

Reconstruction of upper water structure in the northern South China Sea since 400 ka based on planktonic foraminiferal assemblages and its response to the East Asian monsoon

SU Kefan^1,,
GAO Meng¹,
HU Zhe¹,
WANG Na²,
HUANG Baoqi^1,,
1. School of Earth and Space Sciences, Peking University, Beijing 100871
2. School of Urban Planning and Design, Peking University Shenzhen Graduate School, Shenzhen 518055, Guangdong


More Information

Corresponding author: HUANG Baoqi,E-mail:bqhuang@pku.edu.Cn

MSC: P722.7,Q915.811⁺.1

--> Received Date: 30 April 2021
Revised Date: 07 August 2021
Publish Date: 30 November 2021


摘要
摘要:现代南海上层水体结构具有鲜明的季节特征，反映了季节性风场通过动力与热力过程施加的强烈影响；这种风场对上层水体结构的调控同样存在于地质历史时期，借此，长时间尺度上的东亚季风强度变化能够在南海沉积中留下记录，对相关高分辨率样品的研究继而可以从海洋的角度发掘出新的季风信息，成为对现有古季风资料的重要补充。本研究利用南海北部岩芯总长为50.8 m的MD12-3432站位（19°16.88'N，116°14.52'E；水深2125 m）高分辨率沉积样品，通过浮游有孔虫属种组合及其转换函数恢复了400 ka以来冬、夏季海水表层温度及温跃层深度，并对其进行频谱分析，同时对比现代观测资料及黄土粒度等古季风记录，讨论了海表温度、温跃层深度变化与东亚季风强度、表层环流格局等因素的关系。结果表明，晚更新世以来南海北部海表温度具有显著的100 ka冰期-间冰期旋回，冬季海表温度降幅可达8℃，是冰期冬季风增强与表层环流变化的共同结果。此外，冬季海表温度与温跃层深度重建结果均具有明显的67 ka周期，可能代表了东亚冬季风风速在南海北部的变化周期。同时，温跃层深度中未出现100 ka周期，可能指示冰期-间冰期旋回中表层环流的变化对其影响较弱，温跃层的加深主要由东亚冬季风的增强造成。
关键词: 南海北部/
浮游有孔虫/
上层水体结构/
温跃层/
东亚季风

Abstract:The upper water structure of the modern South China Sea(SCS) has distinctive seasonal features, depicting the remarkable influence of seasonal wind fields imposed through dynamic and thermal processes. In geological history, the variations of the East Asian monsoon intensity on a larger timescale could be engraved in the sedimentary record of the SCS, owing to the foregoing existence of the regulatory relationship between upper water structure and wind. Thus, related high-resolution marine records potentially possess undetected information about the East Asian monsoon, which may serve as an important complement to current paleo-monsoon data. In this work, we selected a high-resolution sediment core named MD12-3432, which was drilled from the northern SCS(19°16.88'N, 116°14.52'E; water depth of 2125 m, total core length of 50.8 m) in the 2012 CIRCEA cruise, to reconstruct the major properties of the top tier of the marginal sea. The gray-grayish green semi-pelagic sediments consists of calcareous foraminifera ooze and terrigenous clastic clay, with an age of 400 ka approximately. The samples were taken every 20 cm, with a time resolution around 2 ka, and then 252 samples went through relevant treatments and species identification for planktonic foraminifera. Sea surface temperatures(SST) in winter and summer, together with thermocline depth since 400 ka ago were estimated, using planktonic foraminiferal assemblages and their transfer functions. Spectral analysis, modern observations and paleo-monsoon proxies such as loess grain size were applied and referenced in the discussion.
It turned out that in the northern SCS, the variations of winter and summer SSTs embodied prominent glacial-interglacial cycles of 100 ka since Late Pleistocene, both being lower in the glacial periods and higher in the other. During the glacial periods, intensified winter monsoon and altered surface circulation could impel significant decline in winter SST together, by augmenting latent heat loss and inflow of Pacific temperate water. As for summer SST, the 100 ka-period may involve the exposure of continental shelves at the southern tip of SCS, which would block the supply of warm water from the ocean outside and then impair the rising trend of SST in the northern SCS in summer. Besides, an obvious 67 ka-period appeared in estimations of both winter and summer SST, probably indicating the influence of East Asian winter and summer monsoons respectively. To be specific, the 67 ka-signal in winter SST might illustrate the periodicity of wind speed of East Asian winter monsoon in the studied area, while the summer monsoon needs further discussion.
Unexpected periodicity also occurred in variations of thermocline depth. The absence of the 100 ka-cycle in thermocline depth suggested a rather dim effect brought by changes of surface circulation in glacial-interglacial rhythms, while the presence of a salient 67-ka period demonstrated that strengthened East Asian winter monsoon being the glaring cause of a deepened thermocline. Prying deeper, we found that the deepening moments generally synchronized with the spikes in grain size record far from the China Loess Plateau, which has long been labelled as a classic proxy of the East Asian winter monsoon intensity. By comparing with modern observation data, we ascribed the 67-ka period in thermocline depth to the enhancement of wind stirring and latent heat loss under a stronger monsoon, which leads to vertical mixing. To recapitulate, being different from previous works, with more accurate time resolution and more extensive time scale, our study probably dug up a forthright relevance between the intensity of monsoon and the thermocline depth on the orbital timescale, pointing toward a potential novel index of paleo-monsoon.
Key words:northern South China Sea/
planktonic foraminifera/
upper water structure/
thermocline/
East Asian monsoon



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