Role of the westerlies in Central Asia climate over the Cenozoic

- We use a compilation of δ18O data to reconstruct Central Asian moisture sources.
- Modern precipitation δ18O influenced by the mid-latitude westerlies and monsoon.
- Similar δ18O spatial distribution through time and constant δ18O in Central Asia.
- High, southern Tibet/Himalaya has blocked southerly moisture since the Eocene.
- The mid-latitude westerlies have been the dominant moisture source since the Eocene.

There remains substantial debate concerning the relative roles of tectonics and of global climate in pacing the evolution of aridity in Central Asia over the Cenozoic. Tibetan Plateau uplift, variable monsoonal strength, Paratethys retreat, and reduced moisture transport from the North Atlantic have all been hypothesized to drive aridity in Central Asia. Distinguishing between these mechanisms requires knowledge of moisture transport pathways to Central Asia through time. Presently, Central Asia receives recycled, high δ18O moisture that has been transported across Eurasia by the westerlies, while southern Tibet receives low δ18O moisture distilled by passage over the Himalaya. Here, we reconstruct the spatial distribution of oxygen isotopes in precipitation since the early Eocene, using a compilation of δ18O data from 2750 sedimentary carbonate samples. Across Asia, the spatial distribution of paleo-precipitation δ18O remains remarkably similar through time, with low δ18O values in the lee of the Himalaya in southern Tibet, constant, high δ18O values in Central Asia, and intermediate values on the central Plateau despite independent evidence for substantial changes in both topography and climate through time. These results suggest that a long-standing topographic feature has continuously blocked southerly moisture, and subsequent progressive uplift of the Tibetan Plateau has had little impact on Central Asian climate. In turn, southerly monsoonal moisture has never persistently extended northward of the central Plateau. As a result, the westerlies have remained the dominant moisture source in Central Asia since at least the early Eocene. Therefore, sedimentary aridity indicators in Central Asia, such as loess deposition, are controlled by the trans-Eurasian, westerly moisture flux rather than Tibetan Plateau uplift.