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.

尽管已有越来越多的证据表明，始新世晚期的东南亚地区存在湿润的季风气候，但这类季风环境究竟何时形成仍未明确。本次研究依托青藏高原东南缘剑川盆地经放射性定年的新生代沉积岩，获得了一套地层剖面，其岩相特征与气候代用指标可约束该气候环境的演化过程。
剑川盆地位于中国云南省，盆地充填物以新生代沉积岩为主，是本研究的基础材料。研究团队在野外开展了岩石地层填图与沉积学调查工作，除进行野外岩相分析外，还采集了岩石样品用于气候代用指标测试，测试内容包括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，至始新世中期达到鼎盛，当时区域内发育有沙漠沙丘。自36 Ma（百万年）起，盆地开始沉积含煤的沼泽相沉积物，同时伴生同期的辫状河沉积，指示区域降水量出现显著增长。这场从干旱到湿润的显著气候转型发生在34 Ma的始新世-渐新世（E/O）边界之前，因此排除了全球普遍降温作为该转型主要驱动因素的可能性。我们提出，青藏高原的抬升此时可能已达到临界海拔高度，偏转/阻滞了西风急流，从而使季风水汽得以深入该下风区域。