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

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.

Remote sensing of the mountain cryosphere: Current capabilities and future opportunities for research

“…Advances in robotics and robot design may offer insights into mountain glaciers that could open a significant new sub-branch of the discipline. Presently, the englacial system of a glacier remains somewhat of an enigma, with access limited to conduits that are safe and accessible enough for physical exploration (Gulley and Benn, 2007), using ground penetrating radar (Church et al, 2019) or the drilling of boreholes (Miles et al, 2019a;Miles et al, 2021). In the mining industry, robots and automated UAVs have been used to explore and map underground conduits (Mitchell and Marshall, 2017) -similar to the englacial systems of a glacier.…”

Modelling supraglacial debris-cover evolution from the single glacier to the regional scale: an application to High Mountain Asia

“…), and were obtained through a simplified surface mass-balance inversion procedure similar to (Ragettli et al, 2015;Rounce et al, 2018). In this procedure, Østrem curves were created for each glacier using an energy-balance model for debris-covered glaciers, then inverted using 'observation'-based surface mass balance data from Miles et al (2021). The energy-balance model was initiated at randomly chosen points on the surface of each glacier.…”

“…where b is the local surface mass balance, i debris and k debris are free parameters, and h is the debris thickness (m). Comparison to surface mass balance data inferred by Miles et al (2021) finally allowed determining the actual debris thickness.…”

“…3). Note that ponds and ice cliffs, known to influence the surface mass balance of debris-covered glaciers as well (Ragettli et al, 2016;Miles et al, 2018;Rounce et al, 2018), are implicitly accounted for during the Østrem curve fitting procedure (see section 2.2), since their effect is already accounted for in the mass balance data (Miles et al, 2021). We do not model ponds and ice cliffs explicitly because (1) of the lack of detailed information that would be needed for accurate calibration and evaluation at the regional scale, and (2) their long-term and future evolution is uncertain (Narama et al, 2017;Watson et al, 2017;Chand and Watanabe, 2019;Mölg et al, 2020), and requires small-scale, specific process models to be captured (e.g.…”

“…The limited retreat can be attributed to the 0.5-1 m thick insulating debris cover present on the entire glacier tongue, which reduces Langtang Glaciers's ice melt by a factor of about three compared to the hypothetical situation with no debris. The approximately linear dependence between surface mass balance and elevation, which is typical for clean-ice glaciers, is suppressed for Langtang Glacier since debris cover is thicker at lower elevation than it is for higher ones (Bisset et al, 2020;Miles et al, 2021). This leads to a nearly homogeneous downwasting of the glacier (e.g.…”