The role of the margins in the dynamics of an active ice stream

Abstract

ABSTRACT. A transverse profile of velocity was measured across Ice Stream B, West Antarctica, in order to determine the role of the margins in the force balance of an active ice stream. The profile extended from near the ice-stream center line, through a marginal shear zone and on to the slow-moving ice sheet. The velocity profile exhibits a high degree of shear d eformation within a marginal zone, where intense, chaotic crevassing occurs. Detailed analysis of the profile, using analytical and numerical models of ice flow, leads to the following conclusions regarding the roles of the bed and the margins in ice-stre a m dynamics:(i) The overall resistive drag on the ice stream is partitioned nearly equally between the margins and the bed and, thus, both are important in the force balance of the ice stream. (ii) The ice within the chaotic zone must be about 10 times softer than the ice in the central part of the ice stream. (iii) The average basal shear stress is 0.06 X 10 5 Pa. This implies that the entire bed cannot be blanketed by the weak, deformable till observed by Engelhardt and others (1990) near the center of the ice stream -there must be regions of increased basal drag. (iv) High strain rates and shear stresses in the marginal zones indicate that strain heating in the m a rgins may be significant.While the exact quantitative values leading to these conclusions are somewhat model and location-dependent, th e overall conclusions are robust. As such, they are likely to have importa nce for ice-strea m dynamics in gen eral.

Streamlined bedrock terrain and fast ice flow, Jakobshavns Isbrae, West Greenland: implications for ice stream and ice sheet dynamics

“…The steep, up-ice surface gradient of the Isbrae drives large shear stresses within it. As there is no deformable bed, initial research suggested that internal ice deformation was the main mechanism of ice motion (Echelmeyer & Harrison 1990;Iken et al 1993;Echelmeyer et al 1994). More recent work, however, suggests that large driving stresses (c. 200-300 kPa) cause basal melt rates that account for up to 60% of surface movement through basal motion (Truffer & Echelmeyer 2003), while deformation of the lowest 1700 m of 'softer' Wisconsin-age ice is thought to account for the rest of the Isbrae's high surface velocity (Luthi et al 2002).…”

Glacial erosion beneath ice streams and ice-stream tributaries: constraints on temporal and spatial distribution of erosion from numerical simulations of a West Antarctic ice stream

“…Calculations of ice velocity can be easily extended in the transverse direction, yielding distribution of ice velocity along a transverse crosssection (Raymond 1996, equation 38). This approach is appropriate for firstorder modeling of an ice-stream flow, whenever longitudinal stresses can be ignored (Echelmeyer et al 1994;Harrison et al 1998;Jackson & Kamb 1997;Van der Veen & Whillans 1989b;Whillans et al 1989). This approach is appropriate for firstorder modeling of an ice-stream flow, whenever longitudinal stresses can be ignored (Echelmeyer et al 1994;Harrison et al 1998;Jackson & Kamb 1997;Van der Veen & Whillans 1989b;Whillans et al 1989).…”

Glacial erosion beneath ice streams and ice‐stream tributaries: constraints on temporal and spatial distribution of erosion from numerical simulations of a West Antarctic ice stream

“…Equation (1) assumes that the driving stress is balanced by a combination of spatially uniform basal resistance and marginal stress. This approach is appropriate for firstorder modeling of an ice-stream flow, whenever longitudinal stresses can be ignored (Echelmeyer et al 1994;Harrison et al 1998;Jackson & Kamb 1997;Van der Veen & Whillans 1989b;Whillans et al 1989).…”