Deciphering old moraine age distributions in SE Tibet showing bimodal climatic signal for glaciations: Marine Isotope Stages 2 and 6

Determining the timing and extent of past glaciations in Tibet is essential to reconstruct regional paleoclimate and understand how atmospheric circulation varies due to the high altitude low latitude Tibetan Plateau. In SE Tibet, geomorphological field observation of glacial deposits shows two main imbricated moraines. We apply statistical analyses to a compilation of eight new Be-10 cosmogenic exposure ages from two moraine crests at GMX site and 128 previously published but recalculated exposure ages from 30 additional crests in the region. The results show that ages from the sharpest inner moraines range from 14-25 ka, corresponding to the full range of Marine oxygen Isotope Stage (MIS)-2 (i.e., Last Glacial Maximum, LGM) with less than 2% of older outliers. The outer moraines have a fundamentally different distribution with scattered ages from 10 to 200 ka, obtained using the same method of sampling, dating, and age modeling proven robust for dating the LGM inner moraines, therefore excluding a methodologic artifact. This large scatter prevents the application of any statistical analysis to the age distribution. At a site with well-developed and preserved imbricated moraines (Cuopu), the outer moraine's oldest ages are MIS-6, with the oldest one being at the MIS-6/MIS-7 limit, identical to what is observed in the regional compilation. Following our observations for the LGM moraines where <2% of older outliers are present, the outer moraines in SE Tibet could not be younger than MIS-6. This implies that no glacial advance occurred during MIS-3 which is surprising because MIS-3 moraines have been reported to be the most extensive elsewhere in the Himalayan-Tibetan orogen. Indeed, glaciers are sensitive to both precipitation increase and temperature decrease but whether one factor is prevalent remains debated especially on the Tibetan Plateau. Considering negligible erosion of the boulders, as observed in the field at Cuopu, the most conservative interpretation of our observations is that the true emplacement age of the outer moraines external to the LGM moraines is MIS-6. In that case, glacial advances in SE Tibet correlate with the two coldest periods of the Northern Hemisphere cooling cycles, MIS-2 and MIS-6, indicating that these glaciers are mostly sensitive to a decrease in temperature. (C) 2018 Elsevier B.V. All rights reserved.