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 δ 18 O moisture that has been transported across Eurasia by the westerlies, while southern Tibet receives low δ 18 O 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 δ 18 O data from 2750 sedimentary carbonate samples. Across Asia, the spatial distribution of paleo-precipitation δ 18 O remains remarkably similar through time, with low δ 18 O values in the lee of the Himalaya in southern Tibet, constant, high δ 18 O 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.

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Central Asia has become increasingly arid during the Cenozoic, though the mechanisms behind this aridification remain unresolved. Much attention has focused on the influence and uplift history of the Tibetan Plateau. However, the role of ranges linked to India-Asia convergence but well north of the Plateau-including the Altai, Sayan, and Hangay-in creating the arid climate of Central Asia is poorly understood. Today, these ranges create a prominent rain shadow, effectively separating the boreal forest to the north from the deserts of Central Asia. To explore the role of these mountains in modifying climate since the late Eocene, we measured carbon and oxygen stable isotopes in paleosol carbonates from three basins along a 650 km long transect at the northern edge of the Gobi Desert in Mongolia and in the lee of the Altai and Hangay mountains. We combine these data with modern air-parcel backtrajectory modeling to understand regional moisture transport pathways at each basin. In all basins, ␦ 13 C increases, with the largest increase in western Mongolia. The first ␦ 13 C increase occurs in central and southwestern Mongolia in the Oligocene. ␦ 13 C again increases from the upper Miocene to the Quaternary in western and southwestern Mongolia. We use a 1-D soil diffusion model to demonstrate that these ␦ 13 C increases are linked to declines in soil respiration driven by dramatic increases in aridity. Using modern-day empirical relations between mean annual precipitation and soil respiration, we estimate that precipitation has likely more than halved over the Neogene. Given the importance of the Hangay and Altai in steering moisture in Mongolia, we attribute these changes to differential surface uplift of the Hangay and Altai. Surface uplift in the Hangay began by the early Oligocene, blocking Siberian moisture and aridifying the northern Gobi. In contrast, surface uplift of the Altai began in the late Miocene, blocking moisture from reaching western Mongolia. Thus, the northern Gobi became increasingly arid east to west since the late Eocene, likely driven by orographic development in the Hangay during the Oligocene and the Altai in the late Miocene through Pliocene.

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