全新世以来琼东上升流形成演化及其对气候变化的响应机制

Coastal upwelling is a wind-driven circulation process that replenishes the nutrients needed for biochemical activity and sustains the most biologically productive ecosystems in the ocean. Coastal upwelling zones play a disproportionally important role in the global ocean carbon cycle. In addition, coastal upwelling systems may prove more resilient to changes resulting from human activities than other ocean ecosystems because of their ability to function under extremely variable conditions. Ongoing and future modifications of coastal upwelling systems, potentially linked to global climate changes, have become an issue of increasing importance. Although there is some evidence that the vigor of coastal upwelling, at least at the decadal scale, has progressively increased as a result of anthropogenic greenhouse gas emissions, most evidence is based only on the results of the Eastern Boundary coastal upwelling systems (EBUSs). However, little is known about the behavior of Western Boundary coastal upwelling systems (WBUSs) in response to the modern climate changes. .Under the southwesterly monsoon forcing, summer upwellings in the northern South China Sea (NSCS) are seasonal common phenomenon associated with basin-scale processes. The NSCS summer upwelling systems, as one of major WBUSs, together with river plumes significantly influence the biogeochemical processes and sea-air CO2 fluxes of NSCS. Among which Qiongdong Upwelling (QDU), off the east coast of Hainan Island, plays an important role in the regional climate, coastal ecosystems, as well as marine fishery and farming, and thus has received considerable attention during the past decades. Most of previous studies about QDU are by far based on short instrumental records and numerical simulation, aiming to decipher its structure and spatio-temporal variation. Longer temporal records are urgently needed to provide good constraints on the timing and magnitude of QDU change and therefore an assessment of the climate sensitivity of the QDU. This is especially important for the already stressed upwelling ecosystem in this region due to overfishing, pollution, and growing demand for seafood. In this proposal, an integrated study of the evolution of QDU during the Holocene will be conducted. We will reconstruct the multi-dimension records at different timescales (millennial vs. centennial) using multidisciplinary proxies (sedimentology, geochemistry, ecology, etc.): (1) changes in marine sedimentary environment; (2) changes in sea surface temperature (SST); (3) changes in marine productivity and phytoplankton community structure. These high temporal resolution sedimentary records can provide critical clues on the initiation, evolution and impact of QDU and its forcing mechanism. This will help us to decipher how will the QDU’s respond to climate and global change, which is most importantly for society.

沿岸上升流在全球气候变化和生物地球化学循环中扮演了重要角色。在全球变暖日益加剧情形下，深入理解沿岸上升流的一系列变异和变化规律、影响机制和未来趋势，具有重大的理论和现实意义。琼东上升流是南海北部陆架季节性上升流系统的重要组成部分，对海南岛沿岸气候、珊瑚生态系统、海洋渔业和养殖业发展具有关键性作用。本申请拟以海南岛东侧沿岸海域柱状和表层沉积物为研究对象，针对“重建琼东上升流区海洋环境演化过程及规律，揭示琼东上升流对气候变化的响应机制”这两个关键科学问题，以“现代过程→记录(规律)重建→机制区分”为主线，开展多学科交叉与综合研究，从海洋沉积环境、海水古温度、海洋生产力和浮游植物种群结构等多个角度，构建不同时间尺度上琼东上升流演化的代表性序列，揭示气候变化对琼东上升流形成与演变的驱动机制，客观评估全球变暖下琼东上升流的变异幅度及可能影响，为该区域海洋资源可持续利用提供重要的科学依据。

本项目针对上升流系统的关键物理、化学、生物过程的特异性，采用沉积学、地球化学和古生物学等方法获取多种古环境替代指标及其与关键环境参数之间的定量化关系，并通过不同指标的交叉验证提取对上升流海洋环境变化敏感的代表性演化序列。主要成果如下：1）系统采集了海南岛20条主要河流的沉积物样品，并进行了粘土矿物、地球化学等分析测试，揭示了海南岛河流沉积物的化学风化特征，开展了海南岛河流沉积物的重金属污染状况和生态风险评价工作，为海南岛生态系统的持续发展及污染防治提供参考依据。系统开展了海南岛周边浅海表层沉积物粒度和地球化学分异性，揭示了元素分布的空间差异性，为开展古环境研究工作提供了背景资料。2）对海南岛东部柱状沉积物开展了综合研究工作，整合AMS 14C定年、粒度、黏土矿物、地球化学分析，重建了中全新世以来该海域的古环境演化信息，利用重金属元素富集程度的历史演化探讨了4.0ka前后从自然状态转变岛人类活动影响增强的过程。开展了微体古生物鉴定工作（底栖有孔虫、浮游有孔虫以及介形虫），并结合浅地层剖面结果，初步划分了第四系沉积单元，重建了海南岛东南近海全新世（11.17 ka）以来环境演化过程。3）选取东海内陆架上升流系统开展了对比研究，厘定了四醚膜类脂物古温标（TEX86）和长链不饱和烯酮温标（UK37）在沿岸上升流区的指示意义，进而重建了3000年以来东海上升流区的上层水体结构历史演化记录，探讨了内在驱动机制。针对黄河入海颗粒物中的甘油二烷基甘油四醚（glycerol dialkyl glycerol tetraether, GDGTs）的季节性变化，厘定了黄河入海颗粒物中的GDGTs来源，并着重探讨了人类活动（特别是调水调沙过程）对GDGTs组成和搬运机制的显著影响及其在古环境研究中的意义。利用海南岛东侧琼东上升流区QZ6和QZ4站柱状沉积物，利用多指标综合重建了过去150年以来和全新世以来琼东上升流的演化过程及其不同时间尺度下的驱动机制，并开展了与全球东部边界沿岸上升流的对比分析。