No abstract

Surveys were conducted seaward of all the major drainage outlets of the Antarctic ice sheet from the Pennell Coast, northVictoria Land, to Marguerite Bay, Antarctic Peninsula. The results show that the ice sheet extended onto the outer shelf. Glacial troughs occur offshore of all major glacial outlets.Where the substrate is crystalline bedrock, ice flow tended to follow the structural grain of the bedrock, deposited little sediment and eroded the underlying bedrock. Where ice flowed over relatively soft, more easily eroded, sedimentary strata, the direction of ice flow was more directly offshore, and depositional features characterize the sea-floor. In these areas the signature of the grounded ice consists of till deposits and large-scale geomorphic features. Drumlins occur within the region of contact between crystalline and sedimentary substrates. The different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets. The troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast-flowing ice, ice streams, and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower-moving ice. The area between these two zones was characterized by ice acceleration and is marked by drumlins.

No abstract

Marine-geological and -geophysical data collected from the continental shelf in Pine Island Bay, Antarctica, reveal a complex paleo-subglacial drainage system controlled by bedrock topography and subglacial meltwater discharge. Significant amounts of freely flowing meltwater existed beneath former ice sheets in Pine Island Bay. Subglacial drainage is characterized by descriptions of glacial landforms imaged on the sea floor and sedimentary deposits collected in piston cores. Bedrock geology is characterized using seismic data. Large-scale landforms on the shelf include channels and cavities incised into impermeable crystalline bedrock. There is a transition from randomly oriented channels on the inner shelf to a dendritic pattern of elongate channels on the middle shelf. On the outer shelf, a change in basal conditions occurs where sedimentary deposits bury crystalline bedrock. No evidence for flowing meltwater exists on sedimentary substrates. Instead, meltwater formed at the ice^sediment contact was incorporated into the sediments, contributing to development of a deforming bed, which was sampled in piston cores. Characterization of subglacial meltwater processes that occurred in the past may aid in understanding the role meltwater plays in stability of the West Antarctic ice sheet today.

Evaluating the stability of the West Antarctic Ice Sheet and its regulating role in global climate and eustasy hinges on our ability to understand the interaction of ice streams and the bed on which they rest. Rapid streaming of ice is enabled by flow across a deformable till bed produced by the incorporation of basal meltwater into unconsolidated sedimentary material. These ice streams are shown to have flowed across extensive deformable till beds characterized by megascale glacial lineations composed of soft deformation till. The onset of rapid ice discharge occurs at the transition from crystalline bedrock to seaward-dipping sedimentary strata. In most locations, the deformed bed extends tens of kilometers to the outer continental shelf, which implies a thin ice sheet margin. Furthermore, most of the lateral boundaries of these ancestral ice streams were not constrained geologically, and there is evidence that these boundaries migrated a few tens of kilometers. The extent of the deformable till bed, the nature of the boundaries, and the location of grounding-zone wedges, which record grounding-line positions of individual ice streams, vary from trough to trough, implying unique ice advance and retreat histories. These are all critical parameters in glaciological models and, therefore, predictions of the West Antarctic Ice Sheet's stability.