10Be evidence for extensive, low-elevation Late Pleistocene glaciation in far eastern (Arunachal) Himalayas - timing and palaeoclimate reconstruction

The glacial history of the eastern Himalayas has remained undocumented compared to its western and central regions. In this study, we present 63 Be-10 exposure ages from glacial deposits in the Dri Valley, situated at the eastern extremity of the Himalayas, to reconstruct the first Pleistocene glaciation history of the ~900 km long Arunachal Himalayas. The most extensive glaciation occurred before ~48 ka (and possibly earlier than ~58 ka), when the (inferred) trunk glacier reached 100 km in length and descended to around 1500 to 1300 m above sea level (a.s.l) – among the lowest elevations recorded for glaciers in the Himalayan-Tibetan Orogen. The main valley glacier remained substantial during the Last (Global) Glacial Maximum (LGM) and had a length of ~76 km and terminated around 1680 m a.s.l. 

Despite dense forest cover, paraglacial and postglacial erosion obscuring (prominent) glacial landforms in the Dri Valley, an integrated approach combining geomorphological evidence and high-resolution satellite imagery with cosmogenic-based geochronological data has enabled a detailed reconstruction of its Pleistocene glaciation. Glacial deposits covering elevations from 3700 to 1600 m a.s.l were subdivided based on morphostratigraphy, revealing four periods of postglacial exposure dating to ≥58 ka, ~48 ka, ~19 ka and ~13 ka. The results indicate that the Dri Valley cirques (elevations ~3700 to ~3800 m a.s.l) became ice free between ~14 – 13 ka. Reconstruction of the cirque glacier yields an equilibrium line altitude (ELA) of approximately 3750 m, corresponding to a ∆ELA of ~900 m compared to today which is among the largest ELA depressions in the Himalayas for this period (~14 ka to present). 

Our findings reveal that Late Quaternary glaciation in the Dri Valley was primarily temperature-driven, influenced by long-term orbital forcing. With approximately 90% of the region’s abundant annual precipitation today occurring in summer, a positive mass balance for the Dri glacier is maintained up to a temperature-sensitive threshold. Beyond this threshold, even minimal warming would have caused glacier retreat (or collapse) due to a larger percentage of the summer precipitation falling as rain rather than snow, reducing the glacier’s accumulation. This is reflected in the termination of Dri Valley glacial phases correlating with known regional warm periods. This study offers critical insights into the climate-glacier interactions in the understudied eastern Himalayas and enhances our understanding of broader Himalayan-Tibetan palaeoclimate.