MONSOON MODULATION OF MILANKOCITCH FREQUENCY MIO-PLIOCENE ENVIRONMENTAL CHANGES IN ZHADA BASIN, SW TIBET

The effect of middle Miocene–Pliocene weakening and subsequent Pliocene-Pleistocene strengthening of the Indian and East Asian Summer Monsoons is well documented at low elevations in the marine and non-marine record. However, the driver and effect of long- and short-term monsoon intensity changes at high elevations is poorly understood largely due to limited detailed investigation of the few well-exposed high-elevation Miocene–Pleistocene stratigraphic records. The late Miocene–Pleistocene Zhada basin, in the SW Tibetan Plateau sits at the boundary between areas dominated by moisture sourced from the Indian Summer Monsoon (ISM) to the south and drier Westerly air masses to the northwest. At this location, strengthening of the ISM would result in increased penetration of monsoon circulation onto the plateau resulting in increased precipitation, while its weakening would result in southward migration of the zone of Westerly influence and increased aridity. These changes in ISM intensity are particularly well expressed in the underfilled–balance-filled paleo- Lake Zhada through the expansion and contraction of the Miocene-Pleistocene basin-center lake and wetlands.

We reconstruct wetland expansion and contraction through analysis of grain size, sorting, and distribution of the lower ~640 m of the Zhada formation. Samples were analyzed in 2 m (~20 kyr) increments. Grain size, obtained using a CILAS 1190 Laser Particle Size Analyzer, of sediments ranging from 4 μm to 2500 μm in diameter provide the first long-term quantitative record of transport energy at this basin-center location. Changes in grain size on Milankovitch frequency scales (~10⁵ yrs) likely correlate with variations in ISM strength at this site, controlled by the penetration of the ISM into the Tibetan Plateau. However, long-term trends in ISM weakening and strengthening are masked by topographically-induced progradation of basin-margin depositional environments and associated long-term increase in average grain size.