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

摘要

Mechanisms controlling the long- and short-term variability of the Indian Summer Monsoon (ISM) and high-elevation environmental change have largely been examined using low-elevation or marine records with less emphasis on high-elevation non-marine records. We address this using a high-resolution, long-term record from upper Miocene-lower Pleistocene (～9.0-2.2 Ma) fluvio-lacustrine strata in the Zhada Basin, southwestern Tibetan Plateau. Long-term changes include the onset of lacustrine deposition, a decrease in mean grain size, and an increase in δ~(18)O(carb) and δ~(13)O(carb) values at ～6.0 Ma in response to basin closure following regional extension. This was followed by a return to palustrine/fluvial deposition, an increase in mean grain size, and a decrease in δ~(18)O(carb) and δ~(13)O(carb) values at ~3.5 Ma in response to tectonically driven long-term ISM weakening. Spectral analysis reveals that high-frequency variations in the δ~(18)O(carb) record are dominated by 100 and ～20 kyr cycles from ～6.0-2.2 Ma. Wavelet and spectral analysis of the most densely sampled interval (4.23-3.55 Ma), tuned to the record of daily insolation (21 June at 35°N) confirms and highlights 100 and 20 kyr cycles. The tuned Pliocene δ~(18)O(carb) record is coherent with the record of Northern Hemisphere insolation at precession periods but not at obliquity or eccentricity periods. Additionally, the tuned δ~(18)O(carb) record is anticorrelated to the insolation record, indicating that stronger Northern Hemisphere insolation correlates with a stronger ISM. 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.