From Desert to Monsoon: Irreversible Climatic Transition at ~36 Ma in Southeastern Tibetan Plateau

Although there is increasing evidence for wet, monsoonal conditions in Southeast Asia during the late Eocene it has not been clear when this environment became established. Radiometrically dated Cenozoic sedimentary rocks from the Jianchuan Basin in the southeast flank of Tibetan Plateau now provide a section whose facies and climatic proxies constrain this evolution. Jianchuan Basin is located in Yunnan Province, PR China. Basin infill consists mostly of Cenozoic sedimentary rocks, which form the basic materials for this study. Mapping of lithostratigraphy and sedimentological investigation are performed in the field. Apart from facies analysis in the field, rock samples are collected for proxy investigation. These include samples for hematite and goethite measurements, SEM observation of quartz, grain size analysis, pollen counting, and 40Ar/39Ar dating. Semi-arid conditions had dominated the region since Paleocene, culminating in mid Eocene when desert dunes developed. From 36 Ma, the basin began to accumulate swamp sediments with coals, together with synchronous braided river deposits, indicating significant increase in precipitation. This remarkable transition from dry to wet conditions precedes the E/O boundary at 34 Ma, thus excluding general global cooling as a prime driver. We propose that uplift of Tibetan Plateau might have reached a threshold elevation by that time, deflecting/obstructing the Westerly Jet and thus giving way to monsoonal rains to penetrate into this downwind locality.尽管晚始新世东南亚地区存在湿润季风环境的证据日益增多，但学界此前始终未能明确该环境究竟于何时形成。本次研究依托青藏高原东南缘剑川盆地内经放射性同位素定年的新生代沉积岩，其所形成的地层剖面及其沉积相与气候代用指标，为厘清该环境的演化过程提供了关键约束。 剑川盆地位于中华人民共和国云南省境内，盆地充填序列以新生代沉积岩为主，为本研究的核心基础材料。本次研究开展了野外岩石地层填图与沉积学调查工作，除野外沉积相分析外，还采集了岩石样品用于各类气候代用指标测试，测试内容涵盖赤铁矿与针铁矿含量测定、石英的扫描电子显微镜（Scanning Electron Microscope，SEM）观察、粒度分析、孢粉计数以及40Ar/39Ar放射性定年。 古新世以来，该区域长期受半干旱气候主导，至始新世中期达到极盛，发育了沙漠沙丘沉积。至36 Ma时，盆地开始沉积含煤层的沼泽相沉积物，同时伴生同期的辫状河沉积，指示区域降水量显著提升。这次由干转湿的显著气候转变发生在34 Ma的始新世-渐新世（Eocene-Oligocene，E/O）界线之前，因此排除了全球普遍降温作为主要驱动因素的可能性。我们提出，青藏高原的隆升此时或已达到临界高程，使得西风急流发生偏转或受阻，从而让季风雨得以深入该下风方向区域。