Cold-based debris-covered glaciers: Evaluating their potential as climate archives through studies of ground-penetrating radar and surface morphology

We describe the morphology and internal structure of Mullins and Friedman Glaciers, two cold-based, debris-covered alpine glaciers that occur in neighboring valleys in the McMurdo Dry Valleys, Antarctica. Both glaciers are overlain by a single, dry supraglacial debris layer (8-75cm thick); each mantling debris layer is marked with near-identical patterns of arcuate ridges and steps, as well as corresponding changes in bulk grain size, meter-scale surface topography, and thermal contraction crack polygons. Results from 24km of ground-penetrating radar data show that the ice within the uppermost 1-2km of Mullins and Friedman Glaciers is essentially free of englacial debris (<1% by volume) but thereafter is interspersed with bands of englacial debris (each <3m thick and dipping up glacier) that intersect the ice surface at all major surface ridges and steps. The similarity in number and pattern of englacial debris bands and corresponding surface ridges and steps across both glaciers, along with model results and observations that call for negligible basal entrainment, is best explained by episodic environmental change at valley headwalls. Our working hypothesis is that layers of englacial debris originate as supraglacial lags that form in ice-accumulation areas during times of reduced net ice accumulation; following renewed net ice accumulation, these lags are subsequently buried by snow and ice, flow englacially, and intersect the ice surface to impart distinctive changes in the texture of supraglacial debris and topographic relief. The implication is that the englacial structure and surface morphology of these cold-based, debris-covered glaciers preserves a consistent record of climate and environmental change.