Orbital Forcing of Late Miocene-Pleistocene Environmental Change in the Zhada Basin, SW Tibetan Plateau

Previous research has attributed the onset and/or variability of Indian Summer Monsoon (ISM) circulation to both tectonic and climatic changes. However, discriminating the effects of tectonics and climate on ISM circulation and high-elevation environmental change has remained a challenge. I address this question using a high-resolution, long-term record of Indian Summer Monsoon precipitation from late Miocene–early Pleistocene (~9.0-2.2 Ma) fluvio-lacustrine sediments in the Zhada Basin, southwestern Tibetan Plateau. The Zhada Basin lies near the northern extent of modern ISM precipitation where moisture-bearing ISM air masses mix with dry westerly air masses, and thus the basin is sensitive to strengthening or weakening of the ISM. Major changes in basin hydrology at ~6.0 Ma and ~3.4 Ma are indicated by long-term changes in the δ18Ocarb, δ13Ccarb, and mean grain size record, reflecting the role of regional tectonics on drainage reorganization. These long-term changes are characterized by the onset of lacustrine deposition coupled with a decrease in mean grain size, and an increase in δ18Ocarb and δ13Ccarb values at ~6.0 Ma followed by a return to palustrine/fluvial deposition, an increase in mean grain size, and a decrease in δ18Ocarb and δ13Ccarb values at ~3.5 Ma. Spectral analysis reveals that variations in the Zhada Basin’s δ18Ocarb record are dominated by 100 kyr cycles during the late Miocene–early Pleistocene. Wavelet and 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) reveals the dominance of 100 kyr and 20 kyr cycles. 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. The 100 kyr and 20 kyr periodicities observed in the Zhada record are interpreted to represent a clipped response to summer insolation forcing, which has been shown to be largely an eccentricity-modulated precession signal.