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

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 (similar to 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 delta(18)O(carb)and delta(13)C(carb)values at similar to 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 delta(18)O(carb)and delta(13)C(carb)values at similar to 3.5 Ma in response to tectonically driven long-term ISM weakening. Spectral analysis reveals that high-frequency variations in the delta(18)O(carb)record are dominated by 100 and similar to 20 kyr cycles from similar to 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 degrees N) confirms and highlights 100 and 20 kyr cycles. The tuned Pliocene delta(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 delta(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. Plain Language Summary The Indian Summer Monsoon is a planetary-scale climate system. It currently affects >1.8 billion people, and changes in the monsoon likely led to major changes in plant and animal populations similar to 7 million years ago. Whether the Himalayan mountains, Tibetan Plateau, or the Southern Hemisphere are the major players in controlling the strength of the monsoon remains unclear. We address this question using the first long-term, high-resolution isotopic and sedimentological record from the Zhada Basin on the Tibetan Plateau. Changes in oxygen isotopes in carbonate rocks track the increase or decrease in precipitation amount. The frequency of the changes in the isotopic record match the frequency of changes in Earth's orbit. Changes in the isotope record also show that when the Tibetan Plateau received more sunlight, which is determined by changes in Earth's orbit, the monsoon was more intense. These observations point to the conclusion that Earth's orbit changes the solar heating received by the plateau surface, which in turn plays a major role in the amount of precipitation on the Tibetan Plateau.