Neogene interaction of the westerlies and high topography north of the Plateau: Implications for Central Asia paleoclimate

The timing of high surface topography and the corresponding climatic impacts of the many high ranges north of the Tibetan Plateau, such as the Altai and Tian Shan, remain poorly constrained. Most Neogene reconstructions of Central Asia climate come from interior China, where the influences of Altai and Tian Shan uplift are difficult to deconvolve from effects due to Tibetan Plateau uplift and changes in global climate. We present a new pedogenic carbonate oxygen and carbon isotope record from terrestrial Neogene sediments of the Zaysan Basin in eastern Kazakhstan, which lies upwind of the Altai and Tian Shan, in contrast to the numerous paleoclimate records from interior China. The δ18O values of pedogenic carbonate exhibit a robust 4‰ decrease in the late Neogene—a trend that sharply contrasts with nearly all downwind records of δ18O from Central Asia. We attribute this decrease to the establishment of the modern seasonal precipitation regime whereby Kazakhstan receives the majority of its moisture in the spring and fall, which lowers the δ18O of pedogenic carbonates. The dominance of spring and fall precipitation in Kazakhstan results from the interaction of the mid-latitude jet with the high topography of the Altai and Tian Shan during its movement northward in spring and southward in fall. The late Miocene interaction of the jet with these actively uplifting northern Central Asia ranges reorganized Central Asia climate, establishing starkly different seasonal precipitation regimes, further drying interior China, and increasing the incidence of the lee cyclones that deposit dust on the Loess Plateau. To the south of the Zaysan Basin, earlier shifts in δ18O hint at early Neogene changes in climate attributable to a late Oligocene/early Miocene phase of uplift in the Tian Shan. We conclude that paleoclimatic changes in Central Asia in the Neogene are more tightly controlled by the interaction of the mid-latitude westerlies with the bounding ranges of northern Central Asia than by changes in the height or extent of the Tibetan Plateau.