CONTRASTING LATE MIOCENE TO PRESENT LANDSCAPE EVOLUTION ACROSS MONGOLIA’S KHANGAY MOUNTAINS THROUGH THE LENS OF CHEMICAL AND PHYSICAL WEATHERING PROCESSES

Our understanding of the impacts of climate change 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 scales of 10² to 10⁴ km². At million-year time scales, most mountainous regions are characterized by net erosion and the export of chemically and physically weathered bedrock. Thus, the direct observation of near-surface weathering, sediment and soil production, and geomorphic character of the mountainous regions is often asserted through examination of physical sedimentation and bio-geochemical proxies preserved in depositional basins hundreds to thousands of meters below and tens-to-hundreds of kilometers distant from the bedrock and hillslopes from which they were originally liberated.

In the Khangay Mountains of central Mongolia, late Cenozoic valley-conforming lava flows preserve “snap-shots” of hillslope weathering regimes in the headwaters of the Selenga-Baikal depositional system during the critical climate transition from the late Miocene (ca. 12 Ma) into the Quaternary.

Our research focuses on characterizing 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 analysis of well-developed saprolitic paleosols formed in metasediments (middle Orkhon), granite (upper Orkhon), and Miocene fluvial deposits (upper Chuluut) preserved beneath basaltic lavas at c.11.9, 7.5 and 3.1 Ma, respectively. Today, the Khangay Mountains region is dominated by a continental climate resulting in cold-region physical weathering processes.

We use the Chemical Index of alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW) to reconstruct integrated paleo-MAT and MAP from the saprolite locations. Preliminary results reveal that from the late Miocene into the Pliocene, climate was 5-10°C warmer, slightly more humid and less variable than today in the upland continental interior of west-central Mongolia. This result matches with both long-term paleoclimate records from Lake Baikal and late Cenozoic global cooling trends from other proxies.