Structure and Dynamics of the Lambert Glacier-Amery Ice Shelf System: Implications for the Origin of Prydz Bay Sediments

Abstract

Depositional processes in Prydz Bay during the past 40 m.y. or so have been strongly influenced by glacier ice. Therefore, to understand these processes better, and to define the source areas of the sediment, it is necessary to determine the role of the different ice masses entering the bay. Ice thickness, topography, and ice velocity data indicate that the Lambert Glacier-Amery Ice Shelf system is one of the most important routes for the discharge of ice from the East Antarctic Ice Sheet, and in the past has been the dominant influence on sedimentation in Prydz Bay. Most of the flow is concentrated through the Lambert Graben, which has been overdeepened to a depth of 2500 m below sea level. Glaciological work has indicated that close to the grounding line there is considerable melting, but from a short distance seaward of this position, basal freeze-on of ice of oceanic origin occurs. Thus nearly all the basal debris load in the Lambert Glacier system may be deposited close to the grounding line, and that there is probably negligible deposition beneath the major part of the Amery Ice Shelf. Englacial debris, delivered to the open sea through the interior of the ice shelf, will be deposited from icebergs. There have been conflicting reports concerning the dynamics of the Lambert Glacier-Amery Ice Shelf system. One hypothesis is that the system is not in steady state and thus may be prone to surging. The alternative hypothesis is that the glacier system is in steady state, with no evidence of surging. Consideration of the pattern of ice structures can help to resolve this question. Mapping of structures in glaciers yields fundamental information about ice dynamics. For the Lambert GlacierAmery Ice Shelf system, Landsat images have permitted the mapping of ice foliation, moraines, and crevasses. As the geometry of these structures is incompatible with pulsating ice flow, it is argued that surging has not occurred during the residence time of ice in the Lambert Glacier-Amery Ice Shelf system, which is estimated as lasting up to several thousand years. The structures are also used to define eight major flow units within the entire system. The proportion of ice reaching Prydz Bay from the inner reaches of the system through the main trunk glacier, the Lambert, is only about 4O°7o, a figure further reduced by half if the gain in ice from basal freeze-on and snow accumulation is taken into account. Thus, a considerable proportion of the ice originates much more locally in ice streams that enter the Amery Ice Shelf in the north. Reconstruction of particle paths for advanced grounded ice conditions suggests that most debris, which was deposited as diamictite at the inner and outer continental shelf drill sites, originated from the eastern side of the Lambert Graben, with little influence from the western side. The floor of the Lambert Graben also provided much of the material that makes up the diamictites at these sites. The inner continental seif sites were probably largely influenced by ice flowing off Princess Elizabeth Land, northeast of the Amery Ice Shelf. In times of reduced ice, as at the present day, little debris from the Prydz Bay hinterland may be deposited near the drill sites, and the main clastic component may be derived from icebergs that originate far to the east.