东阿拉伯海拉克希米盆地浊流沉积序列的粒度变化及其对中更新世气候转型的响应

GRAIN SIZE ANALYSIS OF A DEPOSITIONAL SEQUENCE IN THE LAXMI BASIN （IODP HOLE U1456A, ARABIAN SEA） REVEALS THE INDIAN MONSOON SHIFT AT THE MID-PLEISTOCENE CLIMATIC TRANSITION

阿拉伯海印度沉积扇是世界第二大海底扇，保存着来自喜马拉雅和喀喇昆仑等山系的新生代风化沉积物，是研究印度季风和南亚地区环境演化的理想材料。通过对2015年3~5月完成的IODP 355航次U1456钻孔A段总长345.36 m沉积物（约1.8 Ma以来）的沉积特征分析，发现其为典型的浊流沉积。通过对757个样品高分辨率的粒度测试，揭示研究区沉积序列在构造时间尺度上的粒度变化主要受到印度季风降水控制，阿拉伯海深海沉积物的粒度变化可作为一个新的季风演化的替代指标。从1.80~1.18 Ma到1.18~0.10 Ma，浊流沉积物的堆积速率降低，粒度变细，揭示了更新世印度季风强度的阶段性减弱。在沉积序列上，砂组分的变化周期从约41 ka向更长周期转化，推测东阿拉伯海U1456A钻孔沉积物序列在中更新世的粒度变化可能是对印度季风气候转型的响应。

The Indus fan is supplied by the eroded sediments from the Himalayas and adjacent mountain ranges in South Asia. It provides an ideal sedimentary archive for studying the Indian monsoon evolution during the Late Cenozoic. Texture and sedimentological data from the IODP Hole U1456A（345.36 m thick,〈1.8 Ma. IODP Expedition 355） show that it is mainly composed of turbidite sediments. Consistent with previous studies, our grainsize analysis of 757 samples show that the sediments mainly come from the Himalayan and the Karakorum mountains, and that the grain-size distribution is determined by the runoff of the Indus River, mostly controlled by the Indian monsoon precipitation. By contrast, tectonic evolutions and sea level changes have less impact on the sedimentation process in this area in this period. As a result, the grain-size can be used as a proxy indicator of the Indian monsoon strength, as stronger monsoon brings more rainfall to the Southwest Asia and the resulting higher discharge in the Indus River transports coarser and larger amount of sediment to the Indus delta and Arabian Sea during the Quaternary. We build a sedimentary record by interpolating the age of each sampling level constrained by biostratigraphy and paleomagnetic stratigraphy age controls. We find that both the sediment accumulation rate and grain-size decreased after ca.1.18 Ma, suggesting decreasing Indian monsoon intensity during the same time. Before ca.1.18 Ma, however, coarser particles suggest a phase with stronger Indian monsoon circulation and more precipitation. In addition, our spectral and wavelet transfer analyses of the grain-size time series reveal a change of cyclicity from dominated ca. 41 ka to ca.80 ka at ca.1.18 Ma. These observations suggest that the Indian monsoon system was influenced by the global temperature and ice volume change at the Mid-Pleistocene Climatic Transition（MPT）, and that the monsoon system shifts to a relatively weak phase after the MPT. Our findings seem to support the dominating influence of temperature and ice volume on the Indian monsoon evolution at the tectonic and orbital time scales since Late Pliocene, although further research is still needed for more conclusive results.