Climate and Glaciation in the Nepalese Himalaya

Tightly constrained cosmogenic exposure ages, equilibrium line altitude (ELA) changes, modern weather data, and modeling of glacial mass balance shed light on the possible range of paleoclimate changes in the Annapurna region of central Nepal over the last ~25 ka and demonstrate that quite different climates can lead to very similar ELA depressions. Fifty-three 10Be, 2 36Cl, and 2 OSL dates show times of maximum ice advance during the wet early Holocene (~7 ka), the drier Younger Dryas chronozone (~12 ka), and possibly during the LGM (~24-19 ka). ELA depressions were calculated by applying an accumulation-area-ratio (AAR) of 0.6 to small (<5 km2) reconstructed glaciers, uncomplicated by extensive avalanching. Early Holocene ELAs were ~800 m lower than modern on the monsoon-soaked southern slopes and ~450 m lower to the north. During the Younger Dryas the depressions were ~1000 m and ~550 m, respectively. These paleo glaciers occupied the lowest identifiably glaciated terrain in the region, suggesting that even during Marine Isotope Stages 2, 3 and 4, ELAs did not descend significantly lower. These ELA gradients were used to estimate climatic variables in a cellular automata model of glacial mass balance. With these inputs, the model predicted that, given sufficient time to reach equilibrium, the most extensive valley glaciers developed during wetter periods, but that significant glaciers could also develop when it was cooler and drier. These asynchronous glacial advances within a single region are attributable to differing catchment hypsometries. The distribution of modern precipitation and temperature from 19 weather stations across this region (altitudes of 530-4950 m) suggest that the ~1000 m ELA depression is not due solely to huge regional increases in precipitation or decreases in temperature. Rather, changes in temperature of just 2-4°C could cause a significant portion of summer precipitation now falling as rain to fall as snow at glacial levels, thus increasing the effective glacial snowfall, even if total regional precipitation decreases.