Palaeo-environmental evolution of Central Asia during the Cenozoic: new insights from the continental sedimentary archive of the Valley of Lakes (Mongolia)

Abstract

The Valley of Lakes basin (Mongolia) contains a unique continental sedimentary archive, suitable for constraining the influence of tectonics and climate change on the aridification of Central Asia in the Cenozoic. We identify the sedimentary provenance, the (post)depositional environment and the palaeo-climate based on sedimentological, petrographical, mineralogical, and (isotope) geochemical signatures recorded in authigenic and detrital silicates as well as soil carbonates in a sedimentary succession spanning from ~34 to 21 Ma. The depositional setting was characterized by an ephemeral braided river system draining prograding alluvial fans, with episodes of lake, playa or open-steppe sedimentation. Metamorphics from the northern adjacent Neoarchean to late Proterozoic hinterlands provided a continuous influx of silicate detritus to the basin, as indicated by K-Ar ages of detrital muscovite (~798-728 Ma) and discrimination function analysis. The authigenic clay fraction is dominated by illite-smectite and “hairy” illite (K-Ar ages of ~34-25 Ma), which formed during coupled petrogenesis and precipitation from hydrothermal fluids originating from major basalt flow events (~32-29 and ~29-25 Ma). Changes in hydroclimate are recorded in [delta]18O and [delta]13C profiles of soil carbonates and in silicate mineral weathering patterns, indicating that comparatively humid to semi-arid conditions prevailed in the late(st) Eocene, changing into arid conditions in the Oligocene and back to humid to semi-arid conditions in the early Miocene. Aridification steps are indicated at ~34-33, ~31, ~28 and ~23 Ma and coincide with some episodes of high-latitude ice-sheet expansion inferred from marine deep-sea sedimentary records. This suggests that long-term variations in the ocean-atmosphere circulation patterns due to pCO2 fall, reconfiguration of ocean gateways and ice-sheet expansion in Antarctica could have impacted the hydroclimate and weathering regime in the basin. We conclude that the aridification in Central Asia was triggered by reduced moisture influx by westerly winds driven by Cenozoic climate forcing and the exhumation of the Tian Shan and Altai Mountains and modulated by global climate events.