南亚中新世中-晚期季风降水的演化历史

The Middle to Late Miocene (~15-5 Ma) was a period of drastic changes to terrestrial environments and ecosystems. The most spectacular change was the proliferation of C4 grasslands in large areas of the tropics and subtropics between 8 Ma and 6 Ma, indicating a shift to drier conditions. As humidity in South and East Asia is largely controlled by the intensity of the summer monsoons, it is essential to understand how the summer monsoon rainfall has evolved during that period. Whether the Late Miocene shift from C3- to C4-type vegetation in South and East Asia was driven by changes in monsoonal rainfall is not unanimously accepted. Part of the ambiguity lies in the fact that the long-term evolution of the summer South Asian Monsoon (SAM) has mostly been inferred from records of wind strength over the Arabian Sea while only a few proxy records exist for summer SAM rainfall changes. However, interpretations relying on wind based proxies have been challenged as monsoonal precipitation over the continents depends more on the moisture content of the interhemispheric monsoon winds and their transport pathways rather than wind strength. Up to now, proxy records for Middle to Late Miocene changes in SAM rainfall for the Arabian Sea region however are still sparse and virtually non-existent for the peninsular India and the eastern Arabian Sea region, limiting our understanding about the hydroclimate changes of peninsular India. For that reason, the motivation exists for targeting the salinity signal recorded in sediments in the Eastern Arabian Sea to record changes in summer SAM rainfall of peninsular India during the Middle to Late Miocene. The objective of the proposed project is to reconstruct and document the history and evolution of summer SAM rainfall in peninsular India during the Middle to Late Miocene (~14-6 Ma). Particular focus will be on major changes at ~12.9 Ma and 8-6 Ma which have previously been described as drastic changes in SAM wind strength. To this end, an integrated approach, which includes planktonic foraminiferal stable oxygen isotopes, Mg/Ca and Ba/Ca will be used to constrain sea surface salinities and continental runoff and thus changes in summer SAM rainfall.

中新世中-晚期(15-5 Ma)是地球环境和生态系统急剧变化的一个时期，最显著的变化是8-6 Ma C4植物在热带和亚热带地区的繁盛,表明气候更加干旱。南亚和东亚地区的湿润程度在很大程度上是由夏季季风控制,因此了解夏季季风降水在中新世晚期的变化规律显得非常重要。然而季风降水是否是导致南亚和东亚地区C3植物往C4植物转变的主要因素，目前并不清楚。对南亚夏季风演化历史的认识主要源于阿拉伯海季风强度的变化，而有关南亚夏季风降水变化的记录并不多，特别是印度半岛和阿拉伯海东部等地区的研究鲜有报道。另外陆地季风降水主要受季风的湿度及其路径控制，而不是季风强度。本项目拟采集阿拉伯海东部地区中新统沉积物柱样，基于浮游有孔虫的氧同位素及Mg/Ca和Ba/Ca比值，分析表层海水的盐度、陆地径流等特征，重点关注发生在~12.9 Ma和~8 Ma夏季风强度的显著变化，重建中新世中-晚期印度半岛夏季风降水演化历史。

在中新世中期，地球的气候从全球温暖期（中新世气候最佳;MCO;~16.9-14.7 Ma）随着中中新世气候转变期间南极冰盖的扩张而变冷（MMCT;~13-14.7 Ma）。这一突出的气候转变也是全球大气环流发生剧烈变化的时期。南亚季风（SAM）是地球上最强烈的气候现象之一，也是目前影响东亚的湿气和能量的半球间交换的重要输送机。然而，人们对SAM的地质历史知之甚少，SAM风和降雨对现代强度的开始和加剧仍然存在争议。该项目研究了中新世中期（~16 Ma至11 Ma）SAM的发展。为此，利用浮游有孔虫微量元素（Mg/Ca和Ba/Ca）和在东阿拉伯海（EAS）西印度近海沉积物档案中进行的稳定氧同位素分析，研究了SAM降雨变化。Ba / Ca和海水δ18O估计显示大陆径流的变化，因此西印度和邻近的EAS的季风降雨从至少约16 Ma到11 Ma加剧。降雨重建反映了夏季SAM已经加剧，约为15 Ma，这与阿拉伯海西部约14.8至14.0 Ma之间的SAM风力增强一致。最近的模型模拟证实了约在15 Ma后SAM降雨量的增加。然而，在中新世中期，EAS地区的风力强度的指标记录几乎不存在。为此，对来自同一档案的四种共存的浮游有孔虫物种进行了比较稳定的同位素分析，以确定可用于重建地表水水文条件的物种，从而重建SAM风强度。已经表明，Dentoglobigerina altispira居住在下混合层和/或上温跃层。为了测试使用D. altispira重建地表水文条件，计算了混合层栖身者T. sacculifer和D. altispira的Mg / Ca基温度（∆T）的差异，以估计上层海洋热梯度。∆T估计值显示从~16 Ma增加到~13 Ma，表明EAS的垂直混合减少和地表水分层的增加，这很可能归因于盐度分层，随着MCO之后降雨量和淡水供应的增加。

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