IRSL FELDSPAR DATES FOR PALEOLAKE SEDIMENTS FROM DARHAD BASIN, MONGOLIA

Drainage from Darhad Basin in northern Mongolia was blocked by Pleistocene glaciers, impounding ephemeral lakes ~160 m deep that episodically overtopped the dams and drained suddenly. High stands of the paleolakes occurred only at times of maximum ELA depression, whereas low stands occurred at other times also. The timing of maximum glaciations in Mongolia is important in the effort to understand climatic asynchrony across Asia. Although ¹⁰Be cosmic ray exposure dating of tillstones gives an age of ~18 ka (MIS 2) for the last major advance, the dating has only been on the reconnaissance level, and has been criticized as unreliable due to possible exhumation of dated boulders by solifluction due to permafrost. Dating the paleolake sediments should also yield the age of the glacier advances. However, the uppermost sediments exposed in cut banks dated from MIS 3, either because later lakes did not form, or because their sudden drainage eroded the upper few meters of silty MIS 2 sediment. To resolve this geologic uncertainty, a 92 m sediment core was extracted from Darhad Basin in 2004 and sub-sampled for optical dating. Single-grain quartz OSL ages from the two youngest sand layers at 9.15 and 9.50 m were 18.3±3.6 and 23.7±3.8 ka, respectively (Batbaatar et al., 2008), supporting the ¹⁰Be results. However, the luminescence sensitivity of the quartz was low, so we have now attempted IRSL dating of the more sensitive feldspar from the same samples. Density-separated feldspar grains measured by single-grain IRSL (central age model) and corrected for anomalous fading yielded higher ages of 38.2±2.7 and 43.7±2.2 ka, but a minimum age model (Lepper and McKeever, 2000) applied to analyze non-uniformly distributed equivalent doses gave concordant results: 25.3±0.3 and 20.1±0.2 ka. This model considers only fully bleached grains and thus may give the deposition age. Bleached low-stand beach sands gave central and minimum ages of 14.8±1.0 and 14.4±4.4 ka, respectively, and it thus appears that the glacially eroded silts of the lake sediments were variously bleached in the lake environment. Our results support the hypothesis that the last maximum glaciations around Darhad Basin did occur during MIS-2. In contrast, the last maximum glaciations in Kyrgyzstan occurred earlier, probably before MIS 3 (Koppes et al., 2008).