On the origin of debris-bearing basal ice, West Greenland
This project was desgined to ascertain the origin of debris-bearing basal ice exposed in thick sequences at the margin of the Greenland ice sheet; an understanding of basal processes is fundamental to realistic modelling both of ice sheet behaviour and of the development of glaciated landscapes. Stratigraphic, isotopic (delta18O, deltaD), structural, dynamic and sedimentological analyses of ice and debris from the ice sheet margin indicate two zones of basal ice formation and debris entrainment beneath the ice. At some point in the interior, water freezes to the bed in small increments across a transition zone between warm and cold based areas. This interior derived basal ice re-crystalises during flow, may undergo pressure melting and regelation, and appears at the ice sheet margin as an isotopically distinctive ice facies with large clear crystals and with gas and debris pockets at crystal boundaries. Close to the margin, particularly in zones of faster flowing ice, some of this basal ice melts from the bed. Water derived from this melting, and from penetration of surface meltwater, re-freezes at the bed in a narrow freezing zone at the very edge of the ice sheet, forming a sequence several metres thick of ice and debris laminae. Compressive flow at the margin, related to seasonal freezing as well as to marginal thinning of the ice, causes folding and thrust-faulting within the glacier. This thickens the basal sequence, and raises material from the basal layers and from the basal transport zone into the body of the glacier to form debris bands. At the very margin, accumulations of snow, superimposed ice, and debris are overriden and incorporated into the lowest part of the basal sequence. These findings have implications for basal thermal conditions, ice rheology, the distribution of zones of sub-glacial geomorphic activity, and the structure of ice sheet sediments.