Late Pleistocene outburst floods from Issyk Kul, Kyrgyzstan?

Elevated shorelines and lake sediments surrounding Issyk Kul, the world’s second largest mountain lake, record fluctuating lake levels during Quaternary times. Together with bathymetric and geochemical data, these markers document alternating phases of lake closure and external drainage in the Late Pleistocene. The uppermost level of lake sediments requires a former blockage of the lake’s western outlet through the Boam gorge. Previous studies hypothesised that failures of Pleistocene ice or landslide dams in the gorge generated partial outburst floods of Issyk Kul. We test this hypothesis by exploring possible links between late Quaternary lake levels and outbursts. We dated stranded shorelines using ¹⁴C in shells, snails, and plant detritus, as well as sand lenses in delta and river sediments using Infrared Stimulated Luminescence. Our dates are consistent with lake levels expanding into Boam gorge between ~46 ka and 22 ka. Cosmogenic ¹⁰Be and ²⁶Al exposure ages of fan terraces containing erratic boulders downstream of the gorge constrain the timing of possible outburst floods to 22-24 ka, postdating a highstand of Issyk Kul. A flow competence analysis gives a peak discharge of >10⁴ m³ s^–1 for entraining and transporting these boulders. Palaeoflood modelling, however, shows that naturally dammed lakes unconnected to Issyk Kul could have produced such high discharges upon sudden emptying. Hence, although our data are consistent with hypotheses of catastrophic outburst flooding, we caution against directly these to Pleistocene lake levels of Issyk Kul. Average lake-level changes of up to 90 mm yr^–1 in the past 150 years were highly variable without any outburst event, so that attributing catastrophic lake-level drops to dam breaks is ambiguous using sedimentary archives alone. Nevertheless, the Pleistocene flood events that we reconstruct are among the largest reported for the Tien Shan mountains, and motivate further research into the palaeoflood hydrology in Central Asia.