Benthic communities and the potential fossil record at polar glacier grounding lines
MetadataShow full item record
Benthic communities have been analyzed at grounding lines of two contrasting polar, marine-ending glaciers: the Mackay Glacier, a fast flowing outlet glacier from the East Antarctic Ice Sheet terminating as a floating glacier tongue, and Blue Glacier, a slow-moving valley glacier terminating as a tidewater cliff. Distinct patterns in distributions of benthos provide information on modern grounding-line processes. An abundant, species-rich epifauna containing large, slow-growing sponges occurs beyond the margins of and in some areas beneath Mackay Glacier Tongue. This epifauna indicates low sedimentation rates (only a few cm/100 years) and no advance of grounding lines into these areas for a hundred or more years. A sparse, species-poor epifauna in iceberg/glacier tongue scours and near a local grounding (pinning) zone, plus gradients of increasing abundance and/or species richness of epifauna in two areas, imply recent (probably within the last 100 years) retreat of grounding lines. This grounding-line retreat correlates with a loss of 60% from the length of Mackay Glacier Tongue since the early part of this century. A sparse, species-poor benthos at Blue Glacier tidewater cliff probably reflects a limited organic matter supply, associated with the water circulation pattern in McMurdo Sound. Biotic components of sediments form potential fossil assemblages useful for identifying particular grounding-line environments in the stratigraphic record. Rare diatoms and rare, fine sponge spicules characterize shelfstone diamictons deposited from melting basal debris layers close to grounding lines. Pecten shells, ostracodes, the proportion of calcareous benthic foraminifera and total abundance of benthic foraminifera all increase with distance from a glaciertongue grounding line. Radiolaria and planktonic foraminifera first occur at the transition from a sub-glacier tongue to an open water environment. In the absence of indicator species, stratigraphic sequences produced at grounding lines will be best interpreted by evaluating trends in a variety of fossil components. Rarity of biota at the grounding line of Mackay Glacier, only a few hundred meters from a productive open water environment, implies that biota at grounding lines of large ice shelves are probably rare or absent and thus of limited use for stratigraphic interpretation.