Drainage pathways beneath ice sheets and their implications for ice sheet form and flow: the example of the British Ice Sheet during the Last Glacial Maximum

作者：Boulton Geoffrey^*

来源：Journal of Quaternary Science, 2010, 25(4): 483-500.

DOI：10.1002/jqs.1407

• Ice streams
• ice sheet drainage
• eskers
• groundwater flow
• Vale of York
• British Ice Sheet

The drainage system beneath ice sheets is poorly understood, even though it is a major determinant of their basal boundary condition, with implications for their form, flow and stability, and particularly of the ice streams that are crucial dynamic components. Understanding can be enhanced by evidence from the beds of former ice sheets. Glaciological principles are applied to geological evidence in the Vale of York that demonstrate occupancy by a major ice stream of the British ice sheet, with deduced gravitational shear stresses in the range 4-24 kPa, compared with a typical glacier shear stress of about 80-100 kPa, and horizontal velocities in the terminal zone in the range of 400-950 ma(1). It is shown that the ice stream was in an almost buoyant state, with effective pressures in the range 50-100 kPa for a range of ice pressures between 2000 and 5000 kPa. Similar deductions are made for other ice streams on the southern flank of the British ice sheet. The esker system in the area of the ice stream represents locations of subglacial drainage tunnels that are suggested to be the key drainage elements in ice sheets where excess surface water penetrates to the bed. A theory of channel-groundwater coupling which self-organises in response to the rate of water recharge and the transmissivity of the subglacial bed is applied to the sub-ice stream esker system. When matched to local geological evidence, the theory predicts the distribution of basal water pressures, the rate of recharge of surface water to the glacier bed, local shear stresses, the areal pattern of sedimentary processes including bed deformation, and their seasonal fluctuations. Predictions of the annual recharge rate of 950 mm are consistent with the modern values of modem mean annual rainfall in the Ouse (Vale of York) catchment of 906mm. The calculated mean annual discharge of the four major subglacial rivers in 55 compared with a mean flow of the modern Ouse of 49 m(3)s(1). A transient analysis shows the response time of water pressures in the underlying aquifer to changes in surface recharge to be of the order 10-100 days, suggesting the possibility of instabilities at the ice-bed interface. It is not possible, however, to determine whether the Vale of York ice stream was characterized by stable or instable (surging) flow.

• 出版日期2010-5