Würm glaciation of Lake Issyk-Kul area, Tian Shan Mts.: A case study in glacial history of Central Asia

Abstract

Recent field research and modeling experiments by the authors suggest that Wurm glaciation of Tian Shan Mountains had much larger extent than it was previously believed. Our reconstruction is based upon the following evidence: 1. a till blanket with buried glacier ice occurring on mountain plateaus at altitudes of 3700 to 4000 m asl; 2. trough valleys with U-shaped profiles breaching the border ridges and thus attesting to former outlet glaciers spreading outwards from the plateaus; 3. morphologically young moraines and ice-marginal ramps which mark termini of the outlet glaciers at 1600–1700 m asl (near Lake Issyk-Kul shores) and farther down to 1200 m asl (in Chu River valley); 4. clear evidence of impounding the Chu River by former glaciers and turning Lake Issyk-Kul into an ice-dammed and iceberg-infested basin; 5. radiocarbon dates attesting to the Late Pleistocene age of the whole set of glacial phenomena observed in the area. Our data on past glaciation provide a solution for the so called “paleogeographical puzzle of Lake Issyk-Kul”, in particular they account for the lake-level oscillations (by ice dam formations and destructions), for the origin of Boam Canyon (by impact of lake outbursts), and the deflection of Chu River from Lake Issyk-Kul (by incision of the canyon and build-up of an ice-raft delta near the lake outflow). The Wurm depression of regional snowline was found to be in the range of 1150–1400 m. While today's snowline goes above the plateaus of Tian Shan touching only the higher ridges, the Wurmian snowline dropped well below plateau surfaces making their glacierization inevitable. The same change in snowline/bedrock relationship was characteristic of the interglacial-to-glacial climate switches on the Tibetan Plateau resulting in similar changes of glaciation. The whole history of central Asian glaciations seems to be recorded in the Chinese loess sequences. A finite-element model was used to test two climate scenarios — one with a gradual and another with an abrupt change in snow-line elevation. The model predicted that an equilibrium ice cover would form in 19,000 (first scenario) or 15,000 (second scenario) years of growth. It also yielded ice thicknesses and ice-marginal positions which agreed well with the data of field observations.