Mid-Pliocene onset of eccentricity cycles in the Zhada Basin, southwestern Tibetan Plateau.

Abstract:

Changes in the Asian Monsoon driven by Tibetan Plateau uplift have been invoked to explain late Miocene–Pliocene biological turnover and increases in erosion and sediment accumulation. However, documentation of pre-Miocene uplift of the plateau and evidence for early appearance of the Asian Monsoon system has called these conclusions into question. We present a high-resolution, long-term record of environmental change at 3.5-4.5 km essential to understanding the impact and causes of monsoon variation. The O and C isotopic record of authigenic carbonate in 566 samples from the Zhada Basin extends from 8.99 to 1.75 Ma. Both the δ¹⁸O and stratigraphic record indicate long-term changes in basin hydrology which may reflect the impact of regional tectonics on drainage reorganization. These long-term changes are characterized by an onset of lacustrine deposition coupled with an increase in mean δ¹⁸O values at 6.11 Ma followed by a return to palustrine/fluvial deposition and a decrease in mean δ¹⁸O values at 3.24 Ma. These Miocene-early Pleistocene Zhada Basin results mimic late Pleistocene–Holocene paleoclimatic records across the Tibetan Plateau suggesting similar driving mechanisms.

Frequency analysis of the δ¹⁸O record reveals that Milankovitch cycles are recorded in the Zhada Basin; suggesting that insolation-driven climate change drove high-frequency environmental changes in the southern Tibetan Plateau. Counterintuitively, non-stationary frequency analysis of the δ¹⁸O record indicates strengthening in the 100 kyr (eccentricity) band: coincident with initial onset of Northern Hemisphere Glaciation, but ~2.4 Myr prior to strengthening in the marine record. The discrepancy between marine and high-elevation records suggests that changes in the character of the ISM may be responding to cooling in the Pacific Ocean, and may be responsible for widespread mid-Pliocene changes observed on the peripheries of the Tibetan Plateau.