Antarctic cryogenic sediments: Biotic and inorganic facies of ice shelf and marine-based ice sheet environments

Abstract Cryogenic sediments, defined as marine deposits that form in association with ice (icebergs, sea ice, ice shelves or marine-based ice sheets), are widespread in high latitude regions. Existing models for cryogenic depositional processes are of limited utility because of (1) lack of direct observational data, (2) reliance on older land-exposed sections from which inferential conclusions have been drawn, (3) employment of inadequate glaciologic and oceanographic concepts, and (4) failure to incorporate adequately information on the biotic content of these deposits. As a first step toward developing a synthetic model for cryogenic deposition, we relate glaciologic, sedimentologic, and biotic processes of the modern ice shelf and marine-based ice sheet environments using data from Antarctic continental shelves. The principal deposit beneath grounded marine-based ice sheets is lodgement till, which often contains reworked and fragmented fossil material. This till is identical in all important sedimentologic respects with terrestrial lodgement deposits. Marine lodgement tills commonly form geographically extensive sheets which are restricted to continental shelf sites, and which may be differentiated from other cryogenic deposits by their high degree of compaction and reworked fossils. Waterlain till may form beneath grounded ice in areas where debris melting out of the glacier sole falls through a thin water layer (up to a few m) between the ice and its bed. Waterlain till usually lacks fossil material, unless the source material for the subglacial debris is fossiliferous, and is less compact than lodgement till. Deposition beneath large polar ice shelves (e.g., Ross, Filchner-Ronne) is largely restricted to a zone (probably km in width) near the grounding line. In this zone, a complex of primary and secondary cryogenic deposits (including waterlain till, ice shelf rafted detritus (ISRD), flow till, slumps, etc.) is likely to interfinger. Processes controlling this deposition include: local basal conditions of the ice sheet and ice shelf (freezing or melting); presence or absence of subglacial outflow, marine currents, and tidal action; sediment supply; long- and short-term grounding line movement; sea-floor topography; and water depth. Deposition of ISRD is limited elsewhere beneath these large ice shelves because high surface accumulation rates cause downward particle paths for ice and debris. Thus most debris is released soon after passing the grounding line. Biotic activity beneath ice shelves declines rapidly, in diversity and abundance, between the calving margin and grounding line. Deposition rates seaward of the calving margin are much higher than beneath the large ice shelves. Here, icebergs derived from outlet glaciers and ice streams that terminate directly in the sea, may carry significant loads of subglacial, englacial, or superglacial debris, which is released by a variety of mechanisms as the icebergs melt. This ice-rafted detritus mixes with biotic material from planktonic organisms as it falls through the water column to form compound IBRD. If currents are present in the water column or at the seafloor, sorting may occur to form residual IBRD. On shallow banks, grounded icebergs carrying subglacial debris may melt in situ forming iceberg till, and iceberg ploughing of sediment may form iceberg turbate. Both these deposits may be difficult to distinguish from lodgement till but both are restricted to areas with modern water depths less than ≈ 450 m .