2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 11
Presentation Time: 8:00 AM-4:45 PM

First Direct Dating of MIS-2 Paleolake Sediments from Darhad Basin, Mongolia

GILLESPIE, Alan R., Department of Earth & Space Sciences, University of Washington, Seattle, WA 98195, BATBAATAR, J., Earth and Space Sciences, University of Washington, Seattle, 98195-1310 and FEATHERS, James K., TL Dating Laboratory, Department of Anthropology, University of Washington, Seattle, WA 98195, arg3@u.washington.edu

Darhad Basin, at the headwaters of the Little Yenesei River, hosted glacier-dammed paleolakes with volumes ≤809 km3 and depths ≤285 m. The basin drained repeatedly, sending deep floods into Siberia. Cosmic-ray exposure dates of drowned moraines indicate that the last Pleistocene paleolake was impounded during MIS 2, but this inference has been challenged based on: 1) the presence of deep permafrost in the basin, supposedly incompatible with the presence of lakes; 2) telecorrelation of glacial sequences in Darhad Basin to elsewhere in Asia; and 3) the absence of MIS-2 lake sediments from studied sections. None of these objections is incontrovertible: 1) glacier-dammed lakes may be short-lived, and permafrost occurs in the drained basins of other such lakes; 2) ELA depression in arid regions is strongly and nonlinearly variable as a function of precipitation (Rupper, 2007), such that telecorrelation of glacial traces based on extent is unreliable; and 3) strong currents during outburst flooding may have locally eroded MIS-2 sediments. To test this last conjecture, in 2004 we extracted a 92-m sediment core from Darhad Basin at 1545 m asl. The core is dominated by rhythmically laminated gray silt, with fine sandy layers at ~9.1 – 9.6 m, 41-44 m, and 76-78 m depths. Above 1.5 m, lighter-toned silts resemble to sediments capping other sequences, 14C-dated to ~10,700 cal yr BP. OSL single-grain (SAR) dates at 9.15, 9.50, 43.75, and 46.87 m from the core gave ages of 18.3 ± 3.6, 23.7 ± 3.8, 43.7 ± 7.2 and 45.2 ± 4.9 ka, respectively. A wave-cut notch on a moraine at Jarai Gol gave an OSL age of 14.4 ± 4.4 ka. These findings confirm the presence of a paleolake in Darhad Basin in the ~20 ka timeframe, and reinforce the cosmic-ray exposure dating of related moraines.