Continuous borehole optical televiewing reveals variable englacial debris concentrations at Khumbu Glacier, Nepal

Katie E Miles,Duncan J Quincey,Evan S Miles,Ann V Rowan,Martin Kirkbride,Josephine Hornsey,Bryn Hubbard

doi:10.1038/s43247-020-00070-x

Abstract

Surface melting of High Mountain Asian debris-covered glaciers shapes the seasonal water supply to millions of people. This melt is strongly influenced by the spatially variable thickness of the supraglacial debris layer, which is itself partially controlled by englacial debris concentration and melt-out. Here, we present measurements of deep englacial debris concentrations from debris-covered Khumbu Glacier, Nepal, based on four borehole optical televiewer logs, each up to 150 m long. The mean borehole englacial debris content is ≤ 0.7% by volume in the glacier’s mid-to-upper ablation area, and increases to 6.4% by volume near the terminus. These concentrations are higher than those reported for other valley glaciers, although those measurements relate to discrete samples while our approach yields a continuous depth profile. The vertical distribution of englacial debris increases with depth, but is also highly variable, which will complicate predictions of future rates of surface melt and debris exhumation at such glaciers.

Highlights

Surface melting of High Mountain Asian debris-covered glaciers shapes the seasonal water supply to millions of people
We drilled by pressurised water[28] and logged by optical televiewer (OPTV)[29] four boreholes located across the ablation area of debris-covered Khumbu Glacier, Nepal Himalaya (Fig. 1), in 2017 and 2018 (Supplementary Table 1)
Precise debris disposition and association with structure is visible in the OPTV images

Summary

Introduction

Surface melting of High Mountain Asian debris-covered glaciers shapes the seasonal water supply to millions of people This melt is strongly influenced by the spatially variable thickness of the supraglacial debris layer, which is itself partially controlled by englacial debris concentration and melt-out. The rate of atmospherically driven surface melting at debriscovered glaciers, which comprise ~30% of glacial ablation areas in High Mountain Asia[5], is strongly influenced by the spatially variable supraglacial debris thickness, described by the shape of the Østrem curve[9]. The debris layer insulates the ice surface and suppresses, rather than enhances, atmospherically driven ablation[9,10,11,12,13] Due to this melt suppression, debris-covered glaciers are expected to persist longer than climatically equivalent clean-ice glaciers[14], becoming increasingly important water stores as cleanice glaciers disappear.

Methods

Results

Conclusion