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

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.