Hydrological controls on the formation of basal ice layers beneath the Antarctic Ice Sheet

In the austral summer 2000-2001, a borehole camera system developed at the Jet Propulsion Laboratory was used to image the ice compositions in West Antarctica. This resulted in the identification of a 15-m-thick layer of accreted basal ice in Kamb Ice Stream. The debris content of the basal ice layer has a high spatial variability. The upper c. 90 % of the basal ice layer is composed of multiple layers of several-m-thick clean accretion ice. The clean ice layers are separated by layers of banded ice facies or ice facies with inclusions of dispersed sediment. The lower c. 10 % is composed by debris-rich basal ice similar to frozen sediment.

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Figure 1. Borehole camera images of the basal ice layer in Kamb Ice Stream. From left to right: (a) a banded sequence of clean ice and bubbly ice, (b) laminated basal ice, (c) massive basal ice with dispersed debris, and (d) solid debris-rich basal ice. Images are approximately 4 cm in the vertical.

To better understand the ice accretion processes, a numerical model was constructed. The model can be used to predict accretion ice facies associated with different subglacial conditions. Clear accretion ice with little or no debris forms when inflow of subglacial water equals the freezing rate. Fine bands of debris regelates into basal ice if the supply of subglacial water becomes restricted. Alternatively, the debris-bearing basal ice develops a loose framework of uniformly distributed sediment. Debris-rich basal ice develops when the supply of subglacial water cannot satisfy the basal heat budget. Freezing becomes a run-away process that relies on continuous extraction of till pore water. Ice lenses and debris in stratified arrangement develop in fine-grained subglacial sediments when run-away freezing has been triggered.

Subglacial accretion processes is an important glaciological mechanism because the stratigraphic variability of basal ice layers may be used to infer pre-existing subglacial conditions from borehole camera data. Such knowledge is important with respect to understanding ice stream dynamics and ice sheet evolution.

Figure 2. Schematic diagrams illustrating (first) arrangement of debris in different basal ice facies observed beneath the West Antarctic Ice Sheet, and (second) subglacial conditions predicted by numerical modelling. Roman numerals (I-VI) and letters (a-d) correspond to the image series shown in Figure 1.

Related papers:

• Christoffersen, P., S. Tulaczyk, F. Carsey, and A. Behar, 2006. A quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment, Journal of Geophysical Research, F01017, doi:10.1029/2005JF000363.
• Christoffersen, P., and S. Tulaczyk, 2003a. Response of subglacial sediments to basal freeze-on: I. Theory and comparison to observations from beneath the West Antarctic Ice Sheet, Journal of Geophysical Research, 108(B4), 2222, doi:10.1029/2002JB001935.
• Vogel, S.W.. S. Tulaczyk, B. Kamb, H. Engelhardt, F. D. Carsey, A. E. Behar, L. Lane, and I. Joughin, 2005, Subglacial conditions during and after stoppage of an Antarctic Ice Stream: Is reactivation imminent?, Geophysical Research Letters, 32, L14502, doi:10.1029/2005GL022563.

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