Monsoonal circulation and Central Asian aridity set by a high Eocene Himalaya and the mid-latitude westerlies: Stable isotopic evidence

Abstract:

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 climate in Asia. Distinguishing between these two mechanisms requires knowledge of both moisture transport pathways and topography in Asia through time. Here, we reconstruct the long-term spatial distribution of oxygen isotopes in precipitation in Asia since the early Eocene to examine the relative influence of changing topography and of 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. The long-term consistency in the spatial distribution of δ¹⁸O strongly suggests that the same atmospheric processes that today govern Asian climate have been operating since the early Eocene; in contrast, uplift of the Plateau over the Cenozoic has had little impact on moisture delivery to Asia. It thus seems that only a high, southern topographic barrier is necessary to both block southerly moisture and drive monsoonal circulation, supporting recent, modern GCM studies. We combine these results with an isotope-enabled reactive transport model to show that the mid-latitude westerlies have maintained extensive recycling of the cross-Eurasian moisture flux through evapotranspiration, which has kept Central Asia arid to semi-arid for more than 50 Ma. Further, any topographic δ¹⁸O signal due to uplift of the northern Plateau has been overprinted by this westerly evapotranspirative recycling flux. We conclude that the climatic impact of the India-Asia collision was set by the early Eocene with subsequent Plateau uplift having little effect on monsoonal circulation or Central Asian aridity. Instead, it is the mid-latitude westerlies that have translated the effects of global climatic changes into Central Asia and driven the evolution of Central Asian aridity.