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.