Structurally controlled englacial origin of supraglacial debris cover and relief at a debris-covered Alpine glacier

It is common for temperate glaciers in mountainous regions to exhibit an extensive ablation-zone supraglacial debris cover. Although secondary reworking of surface debris and its role in modifying rates of glacier melt is receiving increasing attention, debris origin and primary distribution is poorly understood. Arguably, studies have tended to uncritically assume that debris supply is dominated by the passive transport of rockfall material that is dispersed within the ice (englacially) or is deposited onto the surface directly. We show that a substantial portion of the debris cover at Miage Glacier, Italy, can be attributed to release from medial moraine (MM) structures that can be observed englacially in debris-free ice cliffs and as supraglacial ‘melt out’ ridges containing vertically oriented clasts, occasionally supported by a fine matrix. Two MM types displaying contrasting debris characteristics were observed: one arising from the tributary confluences located near or below the equilibrium line position, and another derived from accumulation basin confluences. The former were reasonably continguous supraglacial features that in the upper- and mid-ablation area develop considerable relief that clearly acts as a primary control on debris redistribution across the glacier surface. The latter type are traceable for limited distances, and form more isolated areas of high topography in the mid-ablation area. We argue that ablation area debris cover and relief complexity in the upper- and mid-ablation area largely reflects debris entrainment at the point of medial moraine origin, though additional factors include the recent detachment of tributaries, the decline in mass contributed by specific accumulation basins, and the stochastic nature of headwall debris supply. Secondary debris redistribution processes appear to increase as glacier surface elevation declines, meaning in the lower ablation area surface relief decreases as debris distribution complexity increases.