ARIDIFICATION OF CENTRAL ASIA AND UPLIFT OF THE ALTAI AND HANGAY MOUNTAINS, MONGOLIA: STABLE ISOTOPE EVIDENCE

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 back-trajectory modeling to understand regional moisture transport pathways at each basin. In all basins, delta C-13 increases, with the largest increase in western Mongolia. The first delta C-13 increase occurs in central and southwestern Mongolia in the Oligocene. delta C-13 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 delta C-13 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 and 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.