Debris thickness patterns on debris-covered glaciers

Buri

et al. 2019

Preprint

Self Cite

Abstract. The mass balance of many Alaskan glaciers is perturbed by debris cover. Yet the effect of debris on glacier response to climate change in Alaska has largely been overlooked. In three companion papers we assess the role of debris, ice dynamics, and surface processes in thinning Kennicott Glacier. In Part A, we report in situ measurements from the glacier surface. In Part B, we develop a method to delineate ice cliffs using high-resolution imagery and produce distributed mass balance estimates. In Part C we explore feedbacks that contribute to glacier thinning. Here in Part A, we describe data collected in the summer of 2011. We measured debris thickness (109 locations), sub-debris melt (74), and ice cliff backwasting (60) data from the debris-covered tongue. We also measured air-temperature (3 locations) and internal-debris temperature (10). The mean debris thermal conductivity was 1.06 W (m C)−1, increasing non-linearly with debris thickness. Mean debris thicknesses increase toward the terminus and margin where surface velocities are low. Despite the relatively high air temperatures above thick debris, the melt-insulating effect of debris dominates. Sub-debris melt rates ranged from 6.5 cm d−1 where debris is thin to 1.25 cm d−1 where debris is thick near the terminus. Ice cliff backwasting rates varied from 3 to 14 cm d−1 with a mean of 7.1 cm d−1 and tended to increase as elevation declined and debris thickness increased. Ice cliff backwasting rates are similar to those measured on debris-covered glaciers in High Mountain Asia and the Alps.

Section: Debris Thicknesssupporting

confidence: 91%

“…But the melt-out of debris is limited by the fact that melt rates are reduced strongly as debris thickens ( Fig. 7; Anderson and Anderson, 2018). The lowest 4 km of the glacier-that appears to not be actively deforming-was once underlaid by active ice (Rickman and Rosenkrans, 1997).…”

Section: Debris Thicknessmentioning

confidence: 99%

Debris cover and the thinning of Kennicott Glacier, Alaska, Part A:in situ mass balance measurements

Buri

et al. 2019

Preprint

Self Cite

“…If melt hotspots control the location of the ZMT then we should expect their effect to be maximized in the ZMT. But if melt hotspots are not dominant then we should expect the mass balance profile to be dictated by the debristhickness melt relationship (or Østrem's curve ) downglacier (e.g., Anderson and Anderson, 2018). We therefore address:…”

Section: Introductionmentioning

confidence: 99%

Debris cover and the thinning of Kennicott Glacier, Alaska, Part B: ice cliff delineation and distributed melt estimates

Armstrong

et al. 2019

Preprint

Self Cite

Abstract. The mass balance of many valley glaciers is enhanced by the presence of ice cliffs within otherwise continuous debris cover. We assess the effect of debris and ice cliffs on the thinning of Kennicott Glacier in three companion papers. In Part A we report in situ measurements from the debris-covered tongue. Here, in Part B, we develop a method to delineate ice cliffs using high-resolution imagery and use empirical relationships from Part A to produce distributed mass balance estimates. In Part C we describe feedbacks that contribute to rapid thinning under thick debris. Ice cliffs cover 11.7 % of the debris-covered tongue, the most of any glacier studied to date, and they contribute 19 % of total melt. Ice cliffs contribute an increasing percentage of melt the thicker the debris cover. In the lowest 4 km of the glacier, where debris thicknesses are greater than 20 cm, ice cliffs contribute 40 % of total melt. Surface lake coverage doubled between 1957 and 2009, but lakes do not occur across the full extent of the zone of maximum glacier thinning. Despite abundant ice cliffs and expanding surface lakes, average melt rates are suppressed by debris, the pattern of which appears to reflect the debris thickness-melt relationship (or Østrem’s curve). This suggests that, in addition to melt hotspots, the decline in ice discharge from upglacier is an important contributor to the thinning of Kennicott glacier under thick debris.

“…Debris-covered tongues are schematized with a convex-concave up-glacier thickness pattern, causing lower tongues to be covered in thick debris that causes debris-covered tongues to stagnate (Anderson and Anderson, 2018;Watson et al, 2017;Kirkbride, 2000;Kirkbride and Deline, 2013). This can be explained with debris being accumulating continuously at the snout at higher rates than it can be evacuated ( Figure 10A2).…”

Section: Consequences For Terminus Retreat and Debris Thicknessmentioning

confidence: 99%

“…This process could be an important negative feedback as the further a glacier downwastes the faster debris is supplied to its surface and the more melt is reduced. This is important to take into account in glacier flow and energy balance models (Carenzo et al, 2016;Anderson and Anderson, 2018;Rowan et al, 2015), which often use a uniform debris cover derived solely from headwall erosion and do not take lateral moraine debris supply into account.…”

Section: Consequences For Terminus Retreat and Debris Thicknessmentioning

confidence: 99%

Estimating lateral moraine sediment supply to a debris-covered glacier in the Himalaya

Woerkom

Steiner

Kraaijenbrink

et al. 2018

Preprint

Abstract. Debris-covered glacier tongues in the Himalaya play an important role in the high-altitude water cycle. The thickness of the debris layer is a key control of the melt rate of those tongues, yet little is known about the relative importance of the three potential sources of debris supply to those glaciers: the headwalls, the glacier bed or the lateral moraines. In this study we hypothesize that erosion from the lateral moraines is a significant debris supply to the debris-covered tongues, in particular when the tongue is disconnected from the headwall due to glacier downwasting. To test this hypothesis eight high-resolution and highly accurate digital elevation models for the lateral moraines of the debris-covered Lirung glacier in Nepal derived from an unmanned aerial vehicle between May 2013 and April 2018 are used. The analysis shows that the lateral moraines erode at an average rate of 0.31 ± 0.26 m yr−1 during this period driven by different erosion processes. There is also a higher erosion rate observed in the monsoon season (0.42 m yr−1) than in the dry season (0.25 m yr−1). In addition the loose lower parts of the lateral moraines erode at a faster rate during both seasons. These erosion rates translate into an annual increase in debris thickness ranging from 0.17 m yr−1 when the eroded material is distributed over the entire glacier to 0.29 m yr−1 in case the material is deposited in a narrow runout zone. It is concluded that the lateral moraines provide an important source of surface debris for glaciers in an advanced state of mass loss, and the source needs to be incorporated into models of glacier evolution. Further research should focus on how large this negative feedback is in controlling the melt of the tongues and a better understanding of the redistribution of debris on the tongue is therefore required.