ROLE OF THE MID-LATITUDE WESTERLIES IN DRIVING THE CLIMATIC EVOLUTION OF CENOZOIC CENTRAL ASIA

Despite a plethora of field studies and modeling efforts, there remains substantial debate concerning the relative roles of Tibetan Plateau uplift and of global climate change in pacing the Cenozoic evolution of aridity in Central Asia. Distinguishing between these two mechanisms requires knowledge of moisture transport pathways to Central Asia through time. Presently, Central Asia—from the northern Tibetan Plateau to central Mongolia—receives highly-recycled, ¹⁸O-enriched moisture that has been transported across Eurasia by the westerlies. Here, we reconstruct the long-term spatial distribution of oxygen isotopes in precipitation in Asia since the early Eocene to constrain the influence of changing topography and global climate. We use both new paleosol δ¹⁸O data from Mongolia and a compilation of δ¹⁸O data from 2,650 paleosol and lacustrine carbonate samples and compare these data with modern precipitation/river δ¹⁸O. Across Asia, the spatial distribution of paleo-precipitation δ¹⁸O remains remarkably similar through time, with low δ¹⁸O in the lee of the Himalaya in southern Tibet, intermediate values in central Tibet, and constant, high δ¹⁸O in Central Asia. These results suggest that to first order the Himalayan range and/or southern Tibet are long-standing topographic features that have continuously blocked southerly moisture and driven monsoonal-like circulation since the early Eocene; in contrast, subsequent uplift of the Plateau has had little impact on moisture transport pathways. As a result, the mid-latitude westerlies have remained the dominant moisture source in Central Asia since the early Eocene. We combine these results with an isotope-enabled reactive transport model and sedimentary aridity data to show that the westerlies have maintained extensive recycling of the cross-Eurasian moisture flux, which has kept Central Asia arid to semi-arid for more than 50 Ma. Finally, our analysis suggests that only a high, southern topographic barrier is necessary to both block southerly moisture and drive monsoonal circulation, supporting recent, modern GCM studies. We conclude that the westerlies have translated global climatic changes to Central Asia and that cross-Eurasian, westerly moisture, rather than Plateau uplift, controlled the Cenozoic evolution of aridity in Central Asia.