北半球冰盖扩大期印尼穿越流古海洋学记录及其意义

As the key component of the global thermohaline circulation in the low latitudes, the Indonesian Throughflow (ITF) acts as the unique connection between the western Pacific and Indian Oceans and regulates their heat and fresh water budgets, subsequently playing a crucial role in the low latitudes and even global climate change. It is indicated, by modern oceanographic observations and paleoceanographic records covering the last glacial to the present, that water structure and intensity of the ITF is closely associated with climatic phenomena such as East Asian monsoon and El Niño-Southern Oscillation. Concomitant with the Northern Hemisphere Glaciation (3.6-2.4Ma), northward movement of New Guinea caused closure of the Indonesian Seaway, possibly resulting in restricted ITF and significant change in sources of ITF seawaters, which was suggested to strongly influenced evolution of Earth’s climate. This hypothesis has been indirectly testified, but long-timescale records of the ITF are still absent. Here, we propose to work on sediments from cores of Site U1482 that were retrieved during the International Ocean Discovery Program (IODP) Expedition 363. We will reconstruct records of the ITF at 4-2 Ma ago, including the period of Northern Hemisphere Glaciation, based on stable isotopes and Mg/Ca ratios on shells of planktonic foraminiferal surface and subsurface species against benthic oxygen isotope stratigraphy. The proposal targets to investigate evolution of the ITF with respect/feedback to tectonic and climatic factors, including increase of Northern Hemisphere ice volume, restriction of the Indonesian Seaway, dynamics of the western Pacific warm pool and the East Asian paleo-monsoon.

印尼穿越流是全球温盐环流在低纬海区的关键组成部分，调节着西太平洋和印度洋之间热量和淡水的收支，在低纬海区乃至全球的气候变化中扮演着十分重要的角色。现代海洋学观测和末次冰期以来的古海洋学记录均显示印尼穿越流的水体结构和强弱变化与东亚季风和ENSO等气候现象密切相关。在3.6-2.4Ma北半球冰盖扩大期间，新几内亚向北移动增加了印尼海道的封闭程度，可能导致印尼穿越流受到限制、和海水来源的重大改变，继而影响全球气候。但目前缺乏长时间尺度的印尼穿越流古海洋学记录予以证实。项目以“国际大洋钻探计划”U1482站沉积物为材料，利用浮游有孔虫表层和次表层水种壳体的氧同位素和Mg/Ca比值等数据、结合底栖氧同位素地层，重建4-2Ma 涵盖北半球冰盖扩大期印尼穿越流的古海洋学记录，探讨印尼穿越流与北半球冰量增长、印尼海道的封闭程度、西太平洋暖池变动、以及东亚古季风演化等构造和气候因素之间的联系和反馈机制。

现代观测和晚第四纪古海洋学研究显示印尼穿越流与东亚季风和ENSO等气候现象密切相关，调节着西太平洋和印度洋之间的热量和盐度收支，继而在全球气候中扮演着重要的角色。在晚上新世至更新世北半球冰盖扩大期间，构造约束下的印尼海道封闭程度增加，一方面使得印尼穿越流较深层流受限，另一方面可能导致印尼穿越流来源的重大改变，继而影响全球气候并对冰量产生反馈。这一推测缺乏长尺度高分辨率记录的支撑。项目借助在澳大利亚西北岸外钻取的“国际大洋发现计划”U1482站高质量岩芯，通过对沉积物中底栖和浮游有孔虫壳体的碳氧稳定同位素和Mg/Ca比值分析，重建北半球冰盖扩大期印尼穿越流的古海洋学记录。根据申请书依据船测资料设计的沉积物采样深度和间隔，最终经过项目建立的底栖氧同位素年代地层框架确定共计1368个沉积物记录了3.17-1.07 Ma的地质历史，分辨率为1.53 kyr。项目查阅资料确定U1482站当前以及在北半球冰盖扩大期间所处水深受印尼穿越流中层水的影响，和印度洋其他深水站位底栖碳同位素对比显示与印度洋深层水无关。与同时间段全球深水站位底栖氧同位素记录对比，揭示U1482站记录的印尼穿越流中层水不完全随着全球冰量的增加而变化，其阶段性变化可能更多地受构造约束下的印尼海道封闭程度、地球轨道参数变化等因素的影响；1.67-1.07 Ma之间，印尼穿越流中层水呈现两个阶段的变暖变淡的变化，在底栖氧同位素记录上表现为两段负偏，可能与西太平洋暖池和古季风活动影响下的印尼穿越流传输途中受潮汐和上升流驱动的水体混合的增强有关；浮游有孔虫属种壳体Mg/Ca比值和氧同位素配对分析显示表层海水在早期呈现温度下降、盐度升高的趋势外，之后保持长时间相对稳定状态，而后呈现变暖变淡的变化，与底栖有孔虫记录的印尼穿越流中层水的变化相呼应，指示了印尼穿越流在传输途中通过水体垂向混合将区域气候信号从表层传递到深层。