Basal mechanics and geologic record of ice streaming, West Antarctica.

摘要

Piston coring in boreholes drilled at the UpB camp through Ice Stream B, West Antarctica, provided the first samples of sediments ever recovered from beneath an active ice stream. Sedimenological analyses indicate that the samples come from the layer of weak, subglacial till underlying this ice stream (the UpB till). Textural properties of the till and the Tertiary diatoms found in it suggest that the UpB till is recycled from the sediments of the inferred eastern subglacial extension of the Ross Sea sedimentary basin. Geotechnical tests show that the UpB till can be modeled as a compressible, Coulomb-plastic material whose strength is practically independent of deformation rate but is determined by effective stress which also determines the water content. Simulations of the subglacial behavior of such till have successfully reproduced fundamental features of the observed subglacial till kinematics, e.g., viscous-like vertical distribution of strain and oscillations in tilt rates. The compressible-Coulomb-plastic till model offers a framework for understanding and modeling of ice stream motion and ice-till interactions. The high porosity of the UpB till (≈0.4) suggests that effective stress is consistently very low, ca. 0.1 to 30 kPa, in the subglacial zone of Ice Stream B. These conditions are explained by the `undrained-bed' model of sub-ice-stream hydrology that includes only local exchange of water between the water stored in the till pore space and the water stored as basal ice. In this model, there is a negative feedback effect between the basal melting rate and till strength which forces a steady-state in which the basal melting rate is zero and the till is water-rich and weak. Coupling of the undrained-bed model with an equation for the velocity of ice stream sliding yields the undrained-plastic-bed model of ice streaming (the UPB model). In accordance with the existing observations, the physics of the UPB model produces two stable modes: an active `ice-stream' mode and an `ice-sheet' mode. The model may experience thermally-triggered switches between the two modes and it can be used to test the hypothesis that the West Antarctic Ice Sheet will become unstable in the near-future.