Debris cover and the thinning of Kennicott Glacier, Alaska: in situ measurements, automated ice cliff delineation and distributed melt estimates
2021
Abstract. Many glaciers are thinning rapidly beneath melt-reducing
debris cover, including Kennicott Glacier in Alaska where glacier-wide
maximum thinning also occurs under debris. This contradiction has been
explained by melt hotspots, such as ice cliffs, scattered within the debris
cover. However, melt hotspots alone cannot account for the rapid thinning at
Kennicott Glacier. We consider the significance of ice cliffs, debris, and
ice dynamics in addressing this outstanding problem. We collected abundant in situ measurements of debris thickness, sub-debris
melt, and ice cliff backwasting, allowing for extrapolation across the
debris-covered tongue (the study area and the lower 24.2 km 2 of the
387 km 2 glacier). A newly developed automatic ice cliff delineation
method is the first to use only optical satellite imagery. The adaptive
binary threshold method accurately estimates ice cliff coverage even where
ice cliffs are small and debris color varies. Kennicott Glacier exhibits the highest fractional area of ice cliffs (11.7 %) documented to date. Ice cliffs contribute 26 % of total melt across
the glacier tongue. Although the relative importance of ice cliffs to area-average
melt is significant, the absolute area-averaged melt is dominated by debris. At Kennicott Glacier, glacier-wide melt rates are not maximized in the zone
of maximum thinning. Declining ice discharge through time therefore explains
the rapid thinning. There is more debris-covered ice in Alaska than in any
other region on Earth. Through this study, Kennicott Glacier is the first
glacier in Alaska, and the largest glacier globally, where melt across its
debris-covered tongue has been rigorously quantified.
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