Contrasting late Miocene to present weathering regimes across the Khangay Mountains, Mongolia

Abstract

Our understanding of climate change impacts on the geomorphology of terrestrial landscapes is often derived from proxy sedimentary records preserved in depositional fluvial and lacustrine basins that integrate landscape responses. At million-year time scales, most mountainous regions are characterized by net erosion and the export of chemically and physically weathered bedrock. For example, in the Khangay Mountains of central Mongolia, late Cenozoic valley-conforming lava flows preserve “snapshots” of hillslope weathering regimes in the headwaters of the Selenga-Baikal depositional system during the consequential climate transition from the late Miocene (ca. 12 Ma) into the Quaternary.This research aims to characterize the relative importance of chemical and physical weathering to landscape development in this upland intracontinental setting through an investigation of geochemical major and minor trace elemental composition of well-developed paleosols formed in metasediments (middle Orkhon), granite (upper Orkhon), and Miocene fluvial deposits (upper Chuluut) preserved beneath basaltic lavas at 11.9, 7.5 and 3.1 Ma, respectively.We used the Chemical Index of alteration (CIA), Plagioclase Index of Alteration (PIA), and Chemical Index of Weathering (CIW) to derive an integrated paleoclimate regime from the three lava flow-buried paleosol locations. Results reveal that from the late Miocene into the Pliocene, the climate was warmer and slightly more humid than today in the upland continental interior of west-central Mongolia. This result matches the long-term paleo records from Lake Baikal and late Cenozoic global cooling trends from other proxies.

Keywords

• Khangay
• Chemical weathering indicies
• Paleosols
• Basalt flows