Abstract
The cause of debris-covered glacier thinning remains controversial. One hypothesis asserts that melt hotspots (ice cliffs, ponds, or thin debris) increase thinning, while the other posits that declining ice flow leads to dynamic thinning under thick debris. Alaska’s Kennicott Glacier is ideal for testing these hypotheses, as ice cliffs within the debris-covered tongue are abundant and surface velocities decline rapidly downglacier. To explore the cause of patterns in melt hotspots, ice flow, and thinning, we consider their evolution over several decades. We compile a wide range of ice dynamical and mass balance datasets which we cross-correlate and analyze in a step-by-step fashion. We show that an undulating bed that deepens upglacier controls ice flow in the lower 8.5 km of Kennicott Glacier. The imposed velocity pattern strongly affects debris thickness, which in turn leads to annual melt rates that decline towards the terminus. Ice cliff abundance correlates highly with the rate of surface compression, while pond occurrence is strongly negatively correlated with driving stress. A new positive feedback is identified between ice cliffs, streams and surface topography that leads to chaotic topography. As the glacier thinned between 1991 and 2015, surface melt in the study area decreased, despite generally rising air temperatures. Four additional feedbacks relating glacier thinning to melt changes are evident: the debris feedback (negative), the ice cliff feedback (negative), the pond feedback (positive), and the relief feedback (positive). The debris and ice cliff feedbacks, which are tied to the change in surface velocity in time, likely reduced melt rates in time. We show this using a new method to invert for debris thickness change and englacial debris content (∼0.017% by volume) while also revealing that declining speeds and compressive flow led to debris thickening. The expansion of debris on the glacier surface follows changes in flow direction. Ultimately, glacier thinning upvalley from the continuously debris-covered portion of Kennicott Glacier, caused by mass balance changes, led to the reduction of flow into the study area. This caused ice emergence rates to decline rapidly leading to the occurrence of maximum, glacier-wide thinning under thick, insulating debris.
Highlights
Debris cover thicker than a few centimeters insulates the ice surface beneath it and reduces melt (Østrem, 1959; Nicholson and Benn, 2006)
Within the debris-covered tongue of Kennicott Glacier the pattern of ice flow is dominated by an overdeepened bed
The annual mass balance gradient in the debris-covered tongue is reversed and melt rates decline towards the terminus
Summary
Debris cover thicker than a few centimeters insulates the ice surface beneath it and reduces melt (Østrem, 1959; Nicholson and Benn, 2006). The low melt rates within debris-covered surfaces seem to conflict with the common observation that glacier-wide thinning is often highest beneath melt reducing debris (e.g., Kääb et al, 2012; Gardelle et al, 2013) This phenomenon has been referred to as the ‘debris-cover anomaly’ (Pellicciotti et al, 2015); it appears to occur globally and has been reported in High Mountain Asia, the European Alps (Mölg et al, 2019), and Alaska (Anderson et al, 2021). We approach this topic hoping that the study of a single glacier in detail will effectively reveal the underlying processes that control the debris cover anomaly more generally. For this case study we consider Kennicott Glacier, a large, dynamic glacier in the Wrangell Mountains of Alaska (Figure 1)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
