新近纪以来印度季风的形成、演化及其驱动机制：基于IODP359航次U1467和U1468孔的研究

The Indian monsoon is an important part of the tropical climate system, and plays a significant role in regulating regional and global climate. So far, however, its initiation, evolution and underlying causes are still debated. There main viewpoints have been proposed, including tectonic-forcing, land-sea distribution and global climate driving. The applicant joined the IODP359 in 2015 that aims to reveal the Indian monsoon evolution since the Neogene. Two long cores have been obtained in the Maldives Inner Sea in the northeastern Indian Ocean, and preliminary results show that the history of Indian monsoon since the Early Miocene (~ 25 Ma) was recorded and suggest that the driving factors for the monsoon may be variable at different periods. In order to test the point of multi-factors driving monsoon, we choose these two long cores for studying in this project. Through detailed sedimentology, mineralogy, environmental magnetism, element and isotope geochemical analysis, along with the paleoclimate modelling at the selected time of Early (~23 Ma), Middle (~13 Ma) and Late (~ 8 Ma) Miocene, we aim to reveal the initiation and evolution of the Indian monsoon since the Neogene and test the validity of the three viewpoints above mentioned. We try to obtain the new viewpoints for the initiation and subsequent evolution of the Indian monsoon environment from the view of marine records.

印度季风是热带气候系统的重要组成部分，对区域乃至全球气候具有重要的调节作用。但迄今为止，对印度季风形成、演化及驱动机制还未形成统一的认识，存在“高原隆升说”、“海陆分布说”及“全球气候驱动说”三种主流观点。申请人于2015年参加了以揭示印度季风演化为目标的IODP359航次，在东北印度洋获得了两支长岩心，初步研究显示岩心记录了早中新世（~25 Ma）以来印度季风的演化，并推断在不同时段季风的驱动因素不是单一的。为了进一步验证印度季风演化多因素驱动的观点，本项目拟对上述岩心开展详细的沉积学、矿物学、环境磁学、地球化学等综合研究，针对三个气候重要转变期—早中新世（~23 Ma）、中中新世（~13 Ma）与晚中新世（~8 Ma）重建印度季风演化历史，并结合数值模拟结果，验证目前关于印度季风形成、演化三种科学假说的合理性。本项目预期从海相记录的角度在季风环境形成、演化及其驱动机制方面获得新认识。

印度季风对人类社会有重大影响，但对其在地质历史时期的演变历史和驱动机制却知之甚少。在本项目中，我们分析了IODP359航次在印度洋马尔代夫内海获得的U1467岩心沉积的碎屑组份的锶和钕同位素组成，用来追踪沉积物来源，并建立了Nd同位素组成与印度夏季风强度之间的联系。研究结果表明，在夏季风强盛时期，更多来自非洲东北部和阿拉伯半岛富含εNd的风尘物质被季风输送到研究区沉降。根据Nd同位素记录，我们提出了自约12Ma以来印度夏季风呈现“两步式”阶段性减弱趋势，同时，在约7-5Ma时被一次重大减弱所中断。我们这一研究结果得到了气候模拟结果的支持，表明印度夏季风的演变主要受大气压力系统变化的控制，即马斯克林高压和印度低压，同时与哈德莱环流的经向移动有关，我们认为，这主要是由中新世以来两半球间冰盖增长造成的。