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The origin of supraglacial debris covers is often conceptualised as the formation of a surface lag by melt‐out of englacial debris from slow‐moving ice, where complexity arises from feedback between debris thickness and sub‐debris ice melt. Here, we examine the origin of a debris cover from the perspective of debris provenance and changing tributary supply in a high‐elevation compound valley glacier. Geochemical analysis of 11 major elements in 21 debris samples from six tributaries of Khumbu Glacier (Nepal) shows unambiguous statistical differentiation of debris sources reflecting lithological differences between tributary catchments. Twenty‐four samples from transects across the ablation area are partitioned according to their source areas using the FR2000 sediment unmixing model. We estimate the age of ice at each transect using a higher order ice flow model. The results show greater proportions of debris from lateral tributaries in downglacier locations that have experienced longer flowline histories. More recently, ice from the Main Himalayan Divide (Western Cwm) has become relatively more important. This suggests a change in the state of the lower glacier's structure depending on the relative ice discharges of lateral and divide sources. Ice flux from lower elevation tributaries was more important probably prior to a weakening of the Indian Summer Monsoon at around 1420 CE. The lower elevation tributaries lie within the range of late Holocene equilibrium line altitude variation and therefore respond most sensitively to climatic drivers of the glacier's flow structure. Negative glacier mass balance since around 1900 CE caused tributary glaciers to detach and high‐elevation catchments to re‐establish as the dominant ice source to Khumbu Glacier. [ABSTRACT FROM AUTHOR]

Characterising the structures within glaciers can give unique insight into ice motion processes. On debris-covered glaciers, traditional structural glaciological mapping is challenging because the lower glacier is hidden by the supraglacial debris layer. Here, we use high-resolution optical televiewer (OPTV) image logs from four boreholes drilled into Khumbu Glacier, Nepal, to overcome this limitation and investigate englacial structural features within a Himalayan debris-covered glacier. The OPTV logs show structural features that are up to an order of magnitude thinner than those observed at the glacier surface and reveal five structural units: (I) primary stratification of ice; (II) debris-rich planes that conform with the primary stratification; (III) water-healed crevasse traces; (IV) healed crevasse traces; and (V) steeply dipping planes of basally derived fine sediment near the glacier terminus. The OPTV logs also reveal that the primary stratification both decreases in dip with depth (by up to 56° over 20 m) and rotates with depth (by up to 100° over 20 m) towards parallelism with the proximal lateral moraine. This transformation and the presence of relict layers of basally derived sediment raised into an englacial position – possibly involving thrusting – near the glacier's now stagnant terminus reveal a previously more dynamic glacier regime. [ABSTRACT FROM AUTHOR]

The dynamic response of large mountain glaciers to climatic forcing operates over timescales of several centuries and therefore understanding how these glaciers change requires observations of their behavior through the Holocene. We used Be‐10 exposure‐age dating and geomorphological mapping to constrain the evolution of glaciers in the Khumbu Valley in the Everest region of Nepal. Khumbu and Lobuche Glaciers are surrounded by high‐relief lateral and terminal moraines from which seven glacial stages were identified and dated to 7.4 ± 0.2, 5.0 ± 0.3, 3.9 ± 0.1, 2.8 ± 0.2, 1.3 ± 0.1, 0.9 ± 0.02, and 0.6 ± 0.16 ka. These stages correlate to each of the seven latest Holocene regional glacial stages identified across the monsoon‐influenced Himalaya, demonstrating that a coherent record of high elevation terrestrial palaeoclimate change can be extracted from dynamic mountain landscapes. The time‐constrained moraine complex represents a catchment‐wide denudation rate of 0.8–1.4 mm a−1 over the last 8 kyr. The geometry of the ablation area of Khumbu Glacier changed around 4 ka from a broad, shallow ice tongue to become narrower and thicker as restricted by the topographic barrier of the terminal moraine complex. Plain Language Summary: Satellite observations indicate that glaciers in the monsoon‐influenced Himalaya are changing rapidly in response to climate change. However, understanding why glaciers are changing requires observing glacier behavior over longer timescales using the glacial geological record. We mapped the geometry and measured the ages of ice‐marginal moraines built by two adjacent glaciers in the Everest region of Nepal. The moraines represent a complete record of glacier expansion during cold periods in the monsoon‐influenced Himalayan region over the last 8,000 years. Moraines formed by the large Khumbu Glacier and the smaller, steeper Lobuche Glacier also reveal differences in how these glaciers have changed since the last Ice Age. The results are useful to understand how monsoon‐influenced Himalayan glaciers respond to climate change, and to improve projections of their future behavior. Key Points: Ice‐marginal moraines in the upper Khumbu Valley represent a complete record of regional glacier change between 7.4 ka and the present dayMoraine building modified the response of Khumbu Glacier to climate change after 8 kaHolocene moraine volume indicates a catchment‐wide denudation rate of 0.8–1.4 mm per year [ABSTRACT FROM AUTHOR]

Sustained mass loss from Himalayan glaciers is causing supraglacial debris to expand and thicken, with the expectation that thicker debris will suppress ablation and extend glacier longevity. However, debris‐covered glaciers are losing mass at similar rates to clean‐ice glaciers in High Mountain Asia. This rapid mass loss is attributed to the combined effects of; (a) low or reversed mass balance gradients across debris‐covered glacier tongues, (b) differential ablation processes that locally enhance ablation within the debris‐covered section of the glacier, for example, at ice cliffs and supraglacial ponds, and (c) a decrease in ice flux from the accumulation area in response to climatic warming. Adding meter‐scale spatial variations in supraglacial debris thickness to an ice‐flow model of Khumbu Glacier, Nepal, increased mass loss by 47% relative to simulations assuming a continuous debris layer over a 31‐year period (1984–2015 CE) but overestimated the reduction in ice flux. Therefore, we investigated if simulating the effects of dynamic detachment of the upper active glacier from the debris‐covered tongue would give a better representation of glacier behavior, as suggested by observations of change in glacier dynamics and structure indicating that this process occurred during the last 100 years. Observed glacier change was reproduced more reliably in simulations of the active, rather than entire, glacier extent, indicating that Khumbu Glacier has passed a dynamic tipping point by dynamically detaching from the heavily debris‐covered tongue that contains 20% of the former ice volume. Plain Language Summary: Glaciers in the Himalaya are shrinking rapidly in response to ongoing climate change. Many of these glaciers are covered with thick layers of rock debris that insulate the ice surface from atmospheric warming. Recent observations suggest that, contrary to expectations, debris‐covered glaciers are losing mass at similar rates to clean‐ice glaciers. We explore the processes driving the rapid loss of ice from a debris‐covered glacier using a glacier model with a novel representation of sub‐debris melt. Our model shows that the rapid ice loss from Khumbu Glacier, Nepal, since 1984 cannot be explained solely by accounting for variations in debris thickness and the presence of ice cliffs and ponds across the glacier surface. Instead, the glacier has passed a dynamic tipping point in the last 100 years where the upper active glacier no longer provides ice to the stagnant debris‐covered tongue. We expect that Khumbu Glacier will pass another tipping point in the next 100 years and detach completely from the debris‐covered tongue at the base of the icefall as the active glacier shrinks in response to climate change. Key Points: Debris‐covered glaciers are losing mass at a similar rate to clean‐ice glaciers in High Mountain AsiaParameterizing differential ablation in an ice‐flow model increases net mass loss from Khumbu Glacier by 29%–47%Dynamic detachment of the upper active glacier from the stagnant debris‐covered tongue after 1900 CE could have accelerated recent mass loss [ABSTRACT FROM AUTHOR]

While hot-water drilling is a well-established technique used to access the subsurface of ice masses, drilling into high-elevation (≳ 4000 m a.s.l.) debris-covered glaciers faces specific challenges. First, restricted transport capacity limits individual equipment items to a volume and mass that can be slung by small helicopters. Second, low atmospheric oxygen and pressure reduces the effectiveness of combustion, limiting a system's ability to pump and heat water. Third, thick supraglacial debris, which is both highly uneven and unstable, inhibits direct access to the ice surface, hinders the manoeuvring of equipment and limits secure sites for equipment placement. Fourth, englacial debris can slow the drilling rate such that continued drilling becomes impracticable and/or boreholes deviate substantially from vertical. Because of these challenges, field-based englacial and subglacial data required to calibrate numerical models of high-elevation debris-covered glaciers are scarce or absent. Here, we summarise our experiences of hot-water drilling over two field seasons (2017–2018) at the debris-covered Khumbu Glacier, Nepal, where we melted 27 boreholes up to 192 m length, at elevations between 4900 and 5200 m a.s.l. We describe the drilling equipment and operation, evaluate the effectiveness of our approach and suggest equipment and methodological adaptations for future use. [ABSTRACT FROM AUTHOR]

Abstract While the supraglacial hydrology of debris-covered glaciers is relatively well studied, almost nothing is known about how water is transported beneath the glacier surface. Here, we report the results of sixteen fluorescent dye tracing experiments conducted in April–May 2018 over the lowermost 7 km of the high-elevation, debris-covered Khumbu Glacier, Nepal, to characterise the glacier's surface and subsurface drainage system. Dye breakthroughs indicated a likely highly sinuous and channelised subsurface hydrological system draining water from the upper part of the ablation area. This flowpath was distinct from the linked chain of supraglacial ponds present along much of the glacier's lower ablation area, through which water flow was extremely slow (∼0.003 m s−1), likely reflecting the study's timing during the pre-monsoon period. Subsurface drainage pathways emerged at the glacier surface close to the terminus, and flowed into small near-surface englacial reservoirs that typically delayed meltwater transit by several hours. We observed rapid pathway changes resulting from surface collapse, indicating a further distinctive aspect of the drainage of debris-covered glaciers. We conclude that the surface and subsurface drainage of Khumbu Glacier is both distinctive and dynamic, and argue that further investigation is needed to refine the characterisation and test its regional applicability to better understand future Himalayan debris-covered glacier meltwater delivery to downstream areas. Highlights • Dye tracing used to investigate hydrology of debris-covered Khumbu Glacier, Nepal. • A subsurface drainage system exists, and is likely highly sinuous and channelised. • The subsurface drainage system does not link to the chain of supraglacial ponds. • Flow through the pond chain is slow and comprises a small part of total discharge. • Rapid pathway changes are observed, resulting from variable surface lowering. [ABSTRACT FROM AUTHOR]

Predicting the response of Earth's high-elevation debris-covered glaciers (HEDCGs) to anticipated climate change represents an important current research challenge. While 2D and 3D computer models of glacier motion and geometrical change are now highly sophisticated, their application to HEDCGs is hampered by a dearth of empirical information for model calibration and validation. This knowledge gap principally reflects logistical difficulties associated with carrying out research at such glaciers, particularly in terms of obtaining information relating to subsurface conditions and processes. Thus, key information that could be used to guide models such as englacial temperature fields, the occurrence and nature of glacier drainage, sediment incorporation and transfer, and 3D dynamic characteristics remain largely unknown.Here, we report on 27 boreholes drilled and instrumented at five sites across the debris-covered tongue of Khumbu Glacier, Nepal, for the EverDrill research project. We report on our drilling experiences, including challenges that were anticipated and encountered and the adaptations made to overcome them, and on the results of experiments undertaken in the boreholes. The latter includes measurements of ice temperature, ice deformation, and the glacier's englacial debris load. Temperatures were measured to be approaching melting point. Ice deformation was found to be low but variable. While the englacial ice contained surprisingly little distributed debris – allowing two boreholes to be drilled to greater than ~150 m - discrete shallow layers of impenetrable debris, interpreted to originate from valley-side avalanches, were encountered during drilling. [ABSTRACT FROM AUTHOR]