Limit of monsoonal precipitation in southern Tibet during the Last Glacial Maximum from relative moraine extents

Constraining the timing and extent of past glaciations can help understand how glaciers respond to climate change as well as study past changes in atmospheric circulation patterns, precipitation and temperature. Southern Tibet, thanks to its numerous, well-preserved, glacial deposits, is an ideal place to conduct such study, as it receives occasional winter precipitation from the mid-latitude Westerlies and abundant summer precipitation (snowfall at high elevation) from the Indian and East Asian monsoons. The fact that amounts of precipitation varied in space and time over the plateau, greatly influenced past glaciations, now recorded as successive moraine deposits. Here, we study two large moraines from the southern Yadong rift (China-Bhutan border) and determined (using 19 new 10Be cosmogenic samples) that at Jichu, which is located at the base of high summits (~7000 m), glacial deposits were abandoned during Marine oxygen Isotope Stage (MIS)-3 (~29–57 ka), and that at Qudui, located at the base of lower summits (~6000 m), the moraine was deposited during the Last Glacial Maximum (LGM, 19–26.5 ka). Both moraines directly rest on top of presumably MIS-6 (~130–191 ka) remnant surfaces. We suggest that while abundant MIS-3 precipitation allowed MIS-3 moraines to be deposited at the base of the highest summits (such as at Jichu) despite warmer temperature than during the LGM, smaller amounts of precipitation occurred during colder MIS-2, blocked by high summits, preventing deposition of large LGM moraines. By contrast, where restricted LGM precipitation can easily reach across lower summits, large MIS-2 moraines exist at their base (such as at Qudui). In addition, we compiled published ages from 22 most extensive moraines (≤MIS-6) from the northern Himalaya to central Tibet (211 samples) along three N-S transects (~50–400 km-long). We found that the relative extent of LGM and MIS-3 moraines varies between different regions, that we infer is due to spatial variations in precipitation: in general, MIS-3 (or maybe MIS-4 at places) moraines are more extensive than MIS-2 moraines, except in the Yadong rift, where LGM monsoon was able to reach, thanks to low topographic barrier and proximity to the monsoonal front. This thus allows to draw the limit of monsoonal precipitation during the LGM.