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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.
