末次冰期以来印尼穿越流次表层流时空变迁及其古海洋学意义

Indonesian Throughflow (ITF) modulates heat and freshwater budgets between the western Pacific and Indian Ocean, with potential feedbacks to East Asian monsoon and El Nino-Southern Oscillation (ENSO) phenomena. The territory of China is under significant influence of East Asian summer monsoon and devastating floods used to happen in the years next to El Nino years. Thus, understanding of East Asian monsoon and ENSO activities, through study on variability of the ITF in the past, is of global significance and is as well as of valuable reference for defense against devastating floods probably happened in our country. It is revealed by modern oceanic observations and as well as paleoceanographic records that significant change of the ITF en route mainly took place in its subsurface waters. It is further indicated by paleoceanographic records that subsurface flow of the ITF experienced two stages of strengthening after gradual recovery and increase of the ITF during the last glacial to the last deglaciation. We speculated that the two stages of strengthening of the ITF subsurface flow may be respectively caused by very different mechanisms, which we hypothesized as 'similar records due to different influencing mechanisms'. The objective of this proposal is to testify the above hypothesis. We will focus on five cores retrieved from the Makassar Strait and two cores from the margin of the western Pacific. Oxygen isotopes and Mg/Ca ratios in shells of planktonic foraminiferal subsurface dwellers from these cores will be analyzed, in combination with published data sets from the Timor Sea, to reconstruct tempo-spatial changes in ITF subsurface waters and upper ocean thermal structure en route from the entrance to the exit since the last glacial.

印尼穿越流调节着西太平洋和印度洋的热量和淡水平衡，对东亚季风和厄尔尼诺－南方涛动（ENSO）等气候现象具有反馈作用。我国受东亚夏季风影响强烈，并在厄尔尼诺次年容易发生洪涝。对印尼穿越流研究，理解东亚季风和ENSO的变化规律，不仅具有全球意义，对我国洪涝灾害的预防也具有参考价值。现代海洋观测和古海洋学记录均表明，印尼穿越流在传输中的变化主要发生在次表层。古海洋学记录揭示，末次冰期至末次冰消期印尼穿越流逐渐恢复并增强之后，次表层流经历了两个阶段的强化。我们推测其背后的影响机制可能迥异，即“相似记录、不同影响机制”的猜想。项目以望加锡海峡内5个站位为主，以西太平洋边缘2个站位为辅，分析钻孔中浮游有孔虫次表层种壳体稳定同位素和Mg/Ca比值，结合帝汶海已有数据，恢复末次冰期以来印尼穿越流从“入口”到“中途”再到“出口”次表层流海水性质以及上部水体结构的时空变迁，检验上述猜想。

印尼穿越流是连接西太平洋和印度洋上层海水的唯一载体。现代海洋学研究表明，印尼穿越流的传输主要发生在次表层，其水体的垂直分层受东亚冬季风强烈影响，次表层海水温度与ENSO活动也具有很好的对应关系。末次冰期时印尼穿越流入口和出口处表层海水的性质相似，这种趋势维持至大约16ka；之后出现明显分异，出口处海水变淡的幅度大大超过了入口处。可能指示印尼穿越流在其传输过程中或在出口附近降水或河流淡水输入的增加。但这一过程究竟如何，需要对印尼穿越流传输途中水体性质的改变进行分析。本项目便是选择来自印尼穿越流“入口”、“途中”和“出口”处钻孔沉积物进行分析，探讨整个传输路径上次表层流海水性质以及上部水体结构的时空变迁。研究结果初步显示，随着冰消期变暖印尼穿越流区域内所有站位的表层和次表层海水都表现出长期变冷的趋势，直到大约2.5ka之后微弱回暖。可能反映东亚冬夏季风均为逐渐减弱的状态，但冬季风相对夏季风较强。ENSO在长时间尺度上对印尼穿越流的上部水体热力结构具有重要的影响，11-2.5ka之间印尼穿越流区域所有钻孔热力结构记录的长期趋势变化，可能反映了类El Niño活动的增加。印尼穿越流区海水氧同位素记录与婆罗洲石笋氧同位素记录非常吻合，但与指示降水同位素指标存在差异，可能指示望加锡海峡内海水氧同位素主要受输入到海峡北部的婆罗洲降水和河流输入的影响，并且通过上部水体混合盐度的信息由表层海水向词表层海水输送。此外，海平面的快速上升导致海峡的变宽而增强了来自南海较淡的水加入印尼穿越流、或者陆架河流淡水输入增多，也可能是印尼穿越流海水氧同位素变化的重要因素。因此，印尼穿越流在从入口到出口的过程中，经历了途中的水体混合，区域降水或周围岛屿和陆架河流的输入，改变了印尼穿越流来自源头的信号，从而影响其在出口处的海水性质。