Clumped isotope constraints on warming and precipitation seasonality in Mongolia following Altai uplift

Abstract

The timing of surface uplift of the Altai Mountains in northern Central Asia—and the climatic consequences—remains controversial. Today, the Altai Mountains cast a substantial rain shadow, effectively separating the western Gobi Desert and steppe from the Siberian Taiga. We take advantage of this stark climatic gradient to trace the interaction of climate and topography in the lee of the Altai. First, we present new water stable isotope data that demonstrate that—along with this climatic gradient—the Altai modify the δ¹⁸O of precipitation via rainout on the leeward side of the range. Second, we present a new paleosol carbonate clumped isotope (Δ₄₇) record that spans much of the Neogene from the immediate lee of the Altai in western Mongolia to address how surface temperatures may have responded to potential uplift during the Neogene. We find that Δ₄₇-derived temperatures have, overall, declined by approximately 7 °C over the course of the Neogene, though the precise timing of this decrease remains uncertain. Third, we pair our Δ₄₇ record with previously published stable isotope data to demonstrate that the timing of decreasing temperatures corresponds with long-term stability in paleosol carbonate δ¹³C values. In contrast, increases in paleosol carbonate δ¹³C values—linked to declining vegetation productivity—are correlated with intervals of increasing temperatures. We speculate that declines in vegetation biomass and leaf area changed the partitioning of latent and sensible heat, resulting in rising surface temperatures during Altai uplift. In contrast, long-term Neogene cooling drove the overall decline in surface temperatures. Reconstructed soil water δ¹⁸O values (based on carbonate δ¹⁸O and Δ₄₇ values) remain surprisingly stable over our Neogene record, differing from our expectation of decreasing δ¹⁸O values due to progressive uplift of the Altai Mountains and Neogene cooling. We demonstrate that the shift in precipitation seasonality that likely accompanied Altai uplift obscured any change in lee-side precipitation δ¹⁸O that would be expected from surface elevation change alone.