STABLE ISOTOPIC RECORD OF MONSOON INTENSITY AND PALEOENVIRONMENTAL CHANGE IN THE ZHADA BASIN, SW TIBET

Climate and environmental changes associated with strength variations of the Indian Summer Monsoon (ISM) have been extensively studied at low elevations flanking the Tibetan Plateau (TP). However, there is little research on the effects of long- or short-term climate change on the plateau itself in the Neogene; existing records focus on the late Pleistocene-Holocene. In the southwestern TP, ISM-sourced moisture from the south mixes with drier Westerly air masses to the northwest. ISM weakening (strengthening) would result in a decrease (increase) in ISM penetration will correlated with increased (decreased) δ¹⁸O and δ¹³C values of lacustrine carbonate. Continuous late Miocene-Pleistocene sedimentation in the Zhada basin, limited post-depositional deformation, and excellent exposure makes this an ideal location to better understand climatic evolution. Initial lithofacies data from the Zhada formation suggest long-term depositional environmental changes with higher frequency 379, 91, and 23 kyr cycles. This suggests that insolation-driven global climate change, rather than elevation changes, drove high-frequency environmental changes in southern Tibet.

Here, we test the hypothesis that changes in precipitation/evaporation related to strengthening and weakening of the ISM drove the observed environmental change at Milankovitch frequencies in the high-elevation Zhada basin. We conducted stable isotope (O and C) analysis of bulk carbonate at 20 kyr intervals to provide a long-term paleoclimatic record of the TP. Initial results confirm an abrupt long-term increase in both δ¹⁸O and δ¹³C values and an increase in correlation coefficient between δ¹⁸O and δ¹³C values. We attribute these changes to tectonic damming and transition from overfilled to underfilled or balance-filled basin conditions. We further compare the δ¹⁸O record to the record of depositional environmental change to determine whether there are systematic correlations between the two records suggesting that precipitation/evaporation ratio drove changes in depositional environments. Finally, we conducted spectral analysis of the δ¹⁸O record to determine if high-frequency cyclic variations are consistent with Milankovitch cycles as observed in lithostratigraphy.