Subglacial Sediment Dispersal and till Composition

Abstract

Dispersal of subglacial sediment down-ice from its bedrock source assumes the form of a negative exponential decay curve. Dispersal decay curves can be quantified by their decay constants, half-distances ( = half-lives), and total transport distances. Mapped dispersal decay curves associated with Pleistocene ice sheets in North America and Europe identify several scales of dispersal, ranging from local (kilometers) to continental (100's to 1000's km) scales. Till composition reflects the various scales of dispersal. Local scales (short transport distances) result in tills that reflect underlying bedrock, whereas continental scales (long transport distances) result in tills derived largely from sources far up-ice. A number of parameters proposed as influencing subglacial sediment dispersal and transport distances do not explain the range of scales observed, although they may explain minor variations (kilometers) in transport distances. Topography and basal ice velocity (sliding velocity) are interpreted as primary controls on the observed scales of transport. Topography directs transport paths of basal debris toward topographic lows, resulting in longer transport distances through depressions than across intervening obstructions. Sliding velocities determine rates of entrainment, dilution, and deposition. Relatively high sliding velocities decrease mixing rates by transporting basal debris and result in long transport distances. Relatively low sliding velocities result in short transport distances, as debris is lodged, new debris is entrained, and mixing rates are high.