The evolution of hydroclimate in Asia over the Cenozoic: A stable-isotope perspective

Links between tectonics and regional climate have been extensively studied in Asia. However, despite decades of research, the relative roles of tectonics and global climate in pacing the long-term evolution of climate in Asia remain disputed. Today, climate in Asia is characterized by large-scale monsoonal circulation over India and southern China and exceptionally arid deserts across much of Central Asia that, to the north, yield to the wetter conditions of the steppe and taiga. How, why, and if these boundaries have changed during the Cenozoic has been limited by the lack of paleoclimate proxy data that can be broadly applied over space and time. We use a large compilation of pedogenic and lacustrine carbonate stable oxygen (delta O-18) and carbon (delta C-13) isotope data (> 7,700 samples), which record large-scale atmospheric circulation and primary productivity, respectively, to reconstruct Cenozoic hydroclimate in Asia and delineate the boundaries through time of monsoonal and mid-latitude westerly moisture. The delta O-18 data indicate that atmospheric circulation has remained similar to today for at least 55 million years. Southern Tibet has always received southerly, likely monsoonal moisture, and this southerly moisture has likely rarely extended northward of the central Tibetan Plateau. In contrast, northern Tibet and Central Asia have received moisture dominantly via the mid-latitude westerlies, which have maintained a semi-arid to arid climate in Central Asia since the early Eocene. Thus, the spatial extent of large-scale atmospheric circulation appears largely unperturbed by contemporaneous changes in India-Asia collisional tectonics, extent of the Paratethys, and in global climate. The delta C-13 data support this interpretation, but yield additional details. Particularly low delta C-13 across a wide swath of eastern China during the Eocene suggests greater primary productivity due to a strengthened East Asia Monsoon, perhaps driven by high atmospheric pCO(2) and globally warm temperatures. Consistently higher delta C-13 in Central Asia indicates that Central Asia has remained more arid than surrounding regions, such as northern India and southern China, which have received abundant precipitation due to the presence of monsoonal moisture. However, increases in delta C-13 across much of Central Asia throughout the Cenozoic-including particularly pronounced increases in Mongolia and Kazakhstan during the Neogene-imply that primary productivity has declined across much of Asia. This large-scale "de-greening" is attributable to the combined effects of Cenozoic global cooling and the late Cenozoic uplift of the Tian Shan and Altai mountains, which block westerly moisture. The uplift of these ranges in the late Miocene is supported by decreasing Neogene delta O-18 in eastern Kazakhstan and sharply increasing delta C-13 in the lee of the Altai. These regional data demonstrate that the Tian Shan and Altai reached sufficient elevations to interact with the westerly jet by the late Miocene, establishing a substantial rain shadow, and shifting the seasonality of precipitation in western Central Asia. We conclude that paleoclimatic changes during the Cenozoic in Central Asia are more tightly controlled by global climate and by the interaction of the mid-latitude westerlies with the Tian Shan and Altai rather than by changes in the height or extent of the Tibetan Plateau or Paratethys.