INSOLATION AND TECTONICS CONTROL ON LATE MIOCENE–PLEISTOCENE CLIMATE AND ENVIRONMENTAL VARIABILITY IN THE ZHADA BASIN, SOUTHWESTERN TIBETAN PLATEAU (Invited Presentation)

Mechanisms controlling the strength of the Indian Summer Monsoon (ISM), including both tectonics and global climate change, have largely been examined using low-elevation or marine records. We address the mechanisms controlling both ISM variability and environmental change using a high-resolution, long-term record of ISM precipitation from high-elevation, non-marine, late Miocene–early Pleistocene (~9.0-2.2 Ma) Zhada Basin, southwestern Tibetan Plateau. The Zhada Basin is particularly sensitive to changes in ISM strength because it lies near the northern extent of modern ISM precipitation where moisture-bearing ISM air masses mix with dry westerly air masses.

Long-term changes in basin hydrology at ~6.0 Ma and ~3.4 Ma reflect tectonically driven drainage reorganization. These changes include the onset of lacustrine deposition coupled with a decrease in mean grain size, and an increase in δ¹⁸O_carb and δ¹³C_carb values at ~6.0 Ma followed by a return to palustrine/fluvial deposition, an increase in mean grain size, and a decrease in δ¹⁸O_carb and δ¹³C_carb values at ~3.5 Ma.

Spectral analysis reveals systematic short-term variations in the late Miocene–early Pleistocene δ¹⁸O_carb record. Spectral analysis of our most densely sampled interval (4.23-3.54 Ma; 1 sample/2.8 kyr), tuned to the record of daily insolation (35°N) shows significant power in both the 100 kyr and 20 kyr periods. However, while the 20 kyr cycles are coherent with precession, the 100 kyr cycles are not coherent with eccentricity. Hence, whereas the 20 kyr cycles are interpreted as directly driven by precessional variation in insolation, the 100 kyr cycles are tentatively interpreted as a bundling of multiple precession cycles due to clipping. These results suggest that variations in daily insolation drove late Miocene–early Pleistocene high-frequency ISM variability and environmental changes in the high-elevation southwestern Tibetan Plateau.