Comparing the evolution of debris-free and debris-covered glaciers during the end of the Lateglacial and the Holocene in Dudh Koshi basin, Everest region, Nepal

Abstract

Debris-covered glaciers are very frequent geomorphological features in Khumbu Himal (Nepal). Rock debris on the glacier surface play a significant role in glacier-climate relationships and glacier dynamics. These effects may cause an asynchronous evolution of debris-covered glaciers compared to debris-free glaciers at a multicentennial to millennial scale. Here, we explore this hypothesis by documenting and comparing the multi-millennial Holocene evolution of a debris-free glacier, Sabai glacier, and two debris-covered glaciers, Dig and Huuku glaciers, from adjacent catchments in Dudh Koshi basin (Everest region, Nepal). To do so, we dated rock samples collected from moraine boulders on both debris-covered and debris-free glaciers using the ¹⁰Be cosmic ray exposure (CRE) dating method. ¹⁰Be CRE ages obtained from 41 moraine boulder samples provide time constraints from ∼13.5 ka to 0.1 ka. While at Dig (debris-covered) and Sabai (debris-free) glaciers, no moraines from the Lateglacial and the Early Holocene are preserved, debris-covered Huuku glacier evidenced a large glacier extent during the Bølling-Allerød and Early Holocene with two moraines dated respectively to ∼13.5 ka and 11 ka, synchronously with most debris-free and debris-covered glaciers in this region. These two glacier advances are concomitant with enhanced monsoon precipitation supporting a qualitative relationship. The absence of debris landforms in the main valley question the nature of Huuku glacier during the Bølling-Allerød and Early Holocene, which could have been either debris-free or covered by a thin debris layer only. During the Mid Holocene, significant differences are observed in the evolution of the two glacier types. The two debris-covered glaciers recorded a significant advance at ∼4.8 ka, synchronous with that observed on other debris-covered glaciers in Khumbu valley. However, such glacier advance during the Mid Holocene was not evidenced on debris-free glaciers in the Dudh Koshi valley. Such a Mid Holocene glacier advance may have a spatial signature with frequent cases reported from both types of glaciers in the western part of High Mountain Asia, which are however infrequent in the arid and semi-arid southern and north-eastern Tibet. During the Late Holocene, both types of glaciers evolved similarly again, with moraines spanning the last two millennia, including the Little Ice Age, concomitant with enhanced monsoon precipitation.