THE INTERPLAY OF LAKE-LEVEL FLUCTUATIONS AND ALLUVIAL FAN ACTIVITY IN AN INTRACONTINENTAL SETTING: KHYARGAS NUUR (LAKE), WESTERN MONGOLIA

The Great Lakes Depression in western Mongolia is among the most continental regions on Earth, exhibiting extreme temperature variations and scant precipitation. Endorheic Khyargas Nuur (KN) is the ultimate water and sediment sink in a sequential cascade from the adjacent Altai and Khangai Mountains. The intercalated lakes (Khar-Us, Khar, Durgun, Airag) formerly joined as one major paleolake, evidenced by various morphological paleo-shoreline features. The overall catchment of the KN comprises an area of 186,110 km². Paleo-shoreline sequences reveal multiple well-expressed lake-level transgressions and regressions since the mid-Pleistocene, driven by climatic cyclicity, including extensive Pleistocene glacial advances within the catchment’s higher elevations. The northern shore KN is confined by the Khankhukhii mountain range, where mesoscale mid-to-late Pleistocene alluvial fans grade to the modern lake surface, creating a dynamic base level.

Here we present an approach to (i) establish a geochronological framework for KN lake-level highstands through optically stimulated luminescence (OSL) and cosmogenic radionuclide (CRN; ¹⁰Be) exposure ages, (ii) interpret Quaternary alluvial fan evolution based on morphostratigraphy and ¹⁰Be CRN exposure and burial ages, and (iii) reconstruct regional paleoclimate from the interplay between lake level fluctuations and fan aggradation.

Our K-feldspar (pIRIR150/pIRIR225) OSL results indicate major lake expansion up to +129 m compared to the modern lake level (1029 m a.s.l.) during MIS5 (104 ± 14 ka; 87.2 ± 12.3 ka). Another transgression up to a well-preserved, prominent +118 m beach ridge is dated to the demise of the Last Glacial Period, MIS2 (13.62 ± 2.1 ka), suggesting an influence of local glacial meltwater pulses for KN expansions. Lower elevated shorelines at +87 m (14.1 ± 1.1 ka), +65 m (13.4 ± 1.0 ka), +49 m (12.7 ± 1.1 ka), +39 m (11.6 ± 0.8 ka), +20 m (11.7 ± 0.9 ka) point to a rapid lake-level regression throughout the late Pleistocene to early Holocene transition. These lower shorelines indicate a decreasing meltwater inflow which is ultimately superseded by a climatic signal exhibiting rather small-scale lake level fluctuations since the mid Holocene (+15 m at 8.8 ± 0.7 ka; +14 m at 2.0 ± 0.1 ka), that correspond to other regional archives.