MIO-PLIOCENE GRAIN-SIZE RECORD OF THE ZHADA BASIN POINTS TO DECREASED INTENSITY AND INCREASED VARIABILITY OF THE INDIAN SUMMER MONSOON IN THE SW TIBETAN PLATEAU

In the Pleistocene-Holocene, the intensity of the Indian Summer Monsoon (ISM) modulates the environmental hydrology of high-elevation lakes of the Tibetan Plateau. Variation in insolation controls the moisture content and penetration of the ISM over the plateau. The Intertropical Convergence Zone (ITCZ), separating moist ISM air masses and more arid Westerly air masses, sits above the Zhada Basin in the SW Tibetan Plateau. Periods of increased insolation correspond with northward migration of the ITCZ, and increased precipitation and wetland expansion across the southern Tibetan Plateau, including the Zhada region. Lake and wetland expansion results in deposition of lacustrine lithofacies characterized by smaller mean grain sizes. Conversely, lower insolation decreases precipitation over Zhada Basin, resulting in lake and wetland contraction and progradation of coarser-grained lake-margin lithofacies. Therefore, the grain-size record of the Zhada Formation during the Miocene-Pliocene may document expansion and contraction of paleo-Lake Zhada related to long-term and short-term ISM intensity.

Grain size analysis of sediment (0.04–2500 μm) from the lower ~640 m of the Zhada Formation was conducted using the CILAS 1190 Laser Particle Size Analyzer. A low-resolution dataset, measured in increments of ~20kyrs (2m), depicts wetland expansion at ~6.2 Ma and wetland contraction at ~3.52 Ma, reflected by shifts in mean and minimum grain size. Increased grain sizes in Zhada Basin at 3.52–2.26 Ma mimic the increased grain sizes observed across and around the Tibetan Plateau at 3–4 Ma. During the same time eastern Pacific δ¹⁸O values increased, sea surface temperatures decreased, and freshening occurred, potentially linked to a shift from permanent El Niño-like conditions into an oscillating system, as it is today. We conclude that the observed increase in mean grain size starting at 3.52 Ma reflects southward migration but increased latitudinal variation in the ITCZ with an attendant decrease in the strength, but increase in variability of the ISM in the southern Tibetan Plateau. This decrease in ISM strength coeval with increasing strength of the East Asian Summer Monsoon is consistent with model predictions following uplift of the southeastern and northern Tibetan Plateau and the Tian Shan mountains.