The Challenge of Deciphering Old Moraine Age Distribution for Paleoclimate Reconstruction in SE Tibet

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. Statistical analyses of a compilation of ¹⁰Be cosmogenic ages from the sharpest inner moraines shows that they range from 14-25 ka, corresponding to the full range of Marine Isotope Stage MIS-2 (i.e. Last Glacial Maximum, LGM) with a peak around 19 ka and less than 2% of 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 proved robust for dating the LGM inner moraines, therefore excluding a methodologic artifact. In that case the statistical analyses of the age distribution for individual outer moraines is not pertinent, favoring sites with few clustered samples per moraine (n<4), which is not representative of the inherent large age scatter observed in the regional compilation. 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 theMIS-6/MIS-7 limit. Following our observations for the LGM moraines where few older outliers are present, we conclude that outer moraines in SE Tibet are minimum MIS-6. The age scatter on old outer moraines is due to post-depositional processes, which is more important on older deposits. Importantly, we find no glacial advance during MIS-3 which is surprising because they were the most extensive elsewhere in the Himalayan-Tibetan orogen. Taking the most conservative hypothesis for the outer moraines as being MIS-6 and the inner ones as MIS-2, we conclude that glacial advances in SE Tibet correlate with the two coldest periods of the Northern Hemisphere cooling cycles, suggesting that glaciers are more sensitive to a decrease of temperature rather than an increase of precipitation.