A subglacial aquifer bed model and basal sliding relationship is constructed for Ice Stream B West Antarctica. The calculated subglacial water discharge is 3 to 18 m3/s at the grounding line. The inferred subglacial water pressure is greater than 90 % of the ice overburden pressure for the entire 300 km length of the ice stream, and greater than 96 % of the ice overburden pressure for 230 km upglacier from the grounding line. This suggests that the high pore-water pressure mechanism proposed as an explanation of overthrust faulting also facilitates the rapid motion of the ice stream through the slower-moving mass of the ice sheet; that is, the ice stream is effectively decoupled from its bed by high water pressure within the subglacial sediments. In addition, this result suggests that subglacial water pressure in excess of 86 % of the ice overburden pressure, which is within the range found to be the cause of the 1982–1983 surge of Variegated Glacier in Alaska, may be typical of the flow regime of this ice stream. The bed model and inferred subglacial water pressures are consistent with (1) the calculated subglacial water flux, (2) the thickness of a porous water-saturated subglacial layer at a location where it was measured recently by geophysicists from the University of Wisconsin, (3) Darcy’s law, which governs water flow through a porous medium, and (4) an equation relating the basal sliding velocity of the ice stream to the inverse effective normal pressure on the bed, as well as to an empirical bed-smoothness function and the driving stress. The inferred distribution of subglacial water pressure is not a unique solution, but it falls within the range of physically-possible solutions.The ratio R of the inferred subglacial water pressure to the ice overburden pressure beneath Upstream B camp (about 190 km upglacier from the grounding line) is 0.91 ≤ R ≤ 0.97. The effective normal pressure within the subglacial layer at this location, calculated from seismic velocity measurements by the University of Wisconsin geophysicists, is equivalent to 0.986 ≤ R ≤ 0.998. Both of these results suggest that if surge velocity is defined as abnormally high velocity for an ice mass of given geometry, due to minimal coupling at the bed caused, in turn, by high subglacial water pressure, then Ice Stream B is moving at surge velocity. This implies that ice streams may be expressions of ice-sheet surges. If so, the question of whether the West Antarctic Ice Sheet can surge (in a conventional sense), in response to warming climate caused by increasing CO2 and other “greenhouse” gases, should be replaced by the question of whether the ice streams can accelerate, such that the rate of discharge across grounding lines exceeds the rate of replenishment over catchment areas. This question is of similar significance, because if ice-stream acceleration causes the mass balance of the West Antarctic Ice Sheet to become negative, thinning will occur, grounding lines will retreat, and sea level will be affected.
Weitere Kapitel dieses Buchs durch Wischen aufrufen
- A Subglacial Aquifer Bed Model and Water Pressure Dependent Basal Sliding Relationship for a West Antarctic Ice Stream
Craig S. Lingle
Timothy J. Brown
- Springer Netherlands