The basal ice layer of glaciers and ice sheets

In many glaciers and ice sheets there is a basal ice layer (BIL) in which the ice is conditioned primarily by processes operating at the bed. The BIL is chemically and physically distinctive, and is characterised by a component of basally derived sediment. The BIL is: a rheological control on ice sheet dynamics; an indicator of subglacial conditions and processes; an agent of subglacial geologic processes; the source of a substantial proportion of glacial sediments; a limit to the downward extension of the climate record from deep ice cores. If debris characteristics of the BIL are preserved in glacial sediments, former glacier conditions can be inferred. Analysis of stable isotopes, gas and cation composition, ice crystallography, debris composition and structural glaciology has elucidated many mechanisms for BIL formation. Common elements can be identified from a range of locations. Key issues are entrainment of ice and debris from the bed, formation of new ice, metamorphism of existing ice in the BIL, and thickening of the BIL by deformation. A distinction can be made between basal ice formed by accretion of material from the bed ('stratified facies') and basal ice formed by metamorphic processes within the ice close to the bed ('dispersed facies'). Copyright (C) 1998 Elsevier Science Ltd.