SEDIMENT TRANSFER BY PALAEO-ICE STREAMS

The anatomy of the ice stream system can be idealised as a series of zones though which sediment may be transferred from source-to-sink. Material may be eroded and entrained over large areas of the catchment zone which is fed into the ice stream trunk via the onset zone (where flow acceleration occurs). Given high velocities in the onset and trunk zones this is where most erosion is expected. Ice stream termini may be marine with a rapidly-calving margin or feeding an ice shelf, or terrestrial, with a splayed lobe of low relief, or discharging into an impounded glacial lake. If the ice stream was a powerful agent of erosion and transport the terminus zone should yield a spatially-focused accumulation of sediment in contrast to the neighbouring margin. For marine-terminating ice streams, a far-travelled component may be recorded in ocean cores by ice-rafted debris (Heinrich layers).

We have compiled estimates of sediment transfer from the literature and use our own observations of palaeo-ice stream tracks to explore the question of how effective ice streams are in redistributing sediment at continental scales. One view of ice streams found in deep topographic troughs is that they eroded these and moved considerable volumes of sediment. This is supported by huge trough mouth fans which can provide valuable estimates of sediment flux, although the chicken and egg problem remains to be resolved, i.e. which came first the ice stream or the trough?. Other records of ice stream activity indicate that they functioned by merely smearing existing sediments without transferring sediment in significant quantities. We illustrate a continuum of sediment redistribution by ice streams with reference to three ice streams that drained the north-western sector of the Laurentide Ice Sheet. The M’Clintock Channel Ice Stream eroded and transferred large volumes of sediment, The Dubawnt Lake Ice Stream transported very little, and the MacKenzie Corridor Ice Stream System attests to extremely variable sediment redistribution. It is concluded that sediment transfer beneath ice sheets is highly variable and patchy in time and space, with the main controls being the thermal regime (warm vs. cold based), emergence of ice streams and their longevity, and the nature of ice stream basal processes (basal sliding, sediment deformation, groove-ploughing).