Stacking of Basal Debris Layers Without Bulk Freezing-on: Isotopic Evidence from West Greenland

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Abstract
This paper tests and falsifies the theory that the development of thick sequences of vertically stacked clean and debris-laden ice layers at the margin of the Greenland ice sheet can be attributed solely to simple freezing-on of material at the bed. Isotopic analysis in δD and δ18O of ice from the ice-sheet margin near Søndre Stramfjord indicates that the debris-rich and debris-poor elements of the basal sequence have different origins. While the debris bands display isotopic fractionation consistent with a freezing origin, the intercalated clean ice layers do not. The clean ice layers have isotopic values indistinguishable from debris-bearing ice immediately above the debris-band sequence and from unaltered glacier ice, and are entrained by a different process from the debris bands. Debris may be entrained by freezing at the bed, but the development of a vertically stacked sequence of debris bands must be attributed to some other mechanism.