A RELATIONSHIP BETWEEN SLIDING SPEED AND EFFECTIVE PRESSURE BASED ON OBSERVATIONS OF DEGLACIATED BEDROCK

Vast areas of the Laurentide ice sheet (LIS) were warm-based, with water at the bed allowing basal slip and fast glacier flow. A central challenge in modeling the LIS, as well as modern ice sheets, is assessing the relationship between effective pressure, as modulated by subglacial water pressure, and basal slip velocity. Glacial landscape evolution models also require that this relationship be known.

With this study, we seek to establish a relationship between sliding speed and effective pressure using data that describe the sizes of cavities between a glacier and its rock bed. Retreat of Castleguard Glacier in Banff National Park, Alberta, Canada, has exposed its former bed, characterized by limestone steps. Dissolution features at the bases of steps serve as evidence of former water-filled cavities, while striations and calcite precipitant indicate ice-rock contact that defines the maximum extent of ice-bed separation. Measurements indicate centimeter to meter scale cavities that increase in length with step height, and striations show that the direction of ice flow was approximately perpendicular to steps. In this case, where ice flow was largely two-dimensional, a simple theoretical relationship, tested experimentally, describes cavity length as a function of step height, sliding speed, effective pressure and the flow-law parameters of temperate ice. Thus, the measured relationship between step height and cavity length, together with estimates of ice rheological parameters, allows us to determine the desired relationship between sliding speed and effective pressure. We compare this relationship, which has the advantage of being time-integrated, with those determined by measuring ice velocity and effective pressure beneath glaciers.