HR: 0830h
AN: C11D-0854    [PDF]
TI: Crevasse Patterns and Grounding Line Change Along the Siple and Gould Coasts, West Antarctica
AU: * Hulbe, C L
EM: chulbe@pdx.edu
AF: Portland State University, Department of Geology Box 751, Portland, OR 97207 United States
AU: Fahnestock, M A
AF: University of New Hampshire, Institute for the Study of Earth, Oceans, and Space; 9 College Road;, Durham, NH 03824 United States
AB: Crevasses and strealklines observed in composite MODIS imagery of the Ross Ice Shelf have been used to infer changes in flow across the transition from ice sheet to ice shelf. We focus on changes in crevasse type and orientation as a guide to recent (100s of years) changes grounding line dynamics and location at the now-quiescent Kamb, and fast flowing Whillans and Mercer Ice Stream outlets. Across the grounding line of a rapidly flowing ice stream, the transition in the basal stress condition is slight so few (if any) crevasses are formed. In contrast, along-flow tension is relatively large across downstream no-slip/slip transitions (i.e. the downstream ends of ice rises and interstream ridges, and the current Kamb grounding line) and will produce crevasses transverse to flow. This is distinctly different from the upstream pointing orientation of crevasses that form due to shear at lateral boundaries. At a no-slip/slip grounding line that is transverse to flow, only tensional crevasses may form so the presence of other crevasse types in the ice stream effluent, or the transition from one type to another, indicates a change in flow style. The Kamb Ice Stream grounding line is now generating transverse crevasses while most of the Mercer/Whillans ice plain grounding line is not. The southern end of the current Kamb grounding line was established as a no-slip/slip boundary sometime after Steershead became an ice rise, as evidenced by the change from shear crevasses to tension crevasses about 20 km downstream from its present location. At the northern end of the grounding line, the first tensional crevasses are only a few km downstream from its present location. If, as seems likely, ice stream deceleration coincided with the transition from a Mercer/Whillans type grounding zone to a no-slip/slip grounding line, then the oldest tensional crevasses should have advected about 1.5 km downstream (the present speed is ~10 m/a and the stream shut down ~150 years ago). The observed and computed advection distances are similar at the northern end of the Kamb grounding line, but crevasses are an order of magnitude too far downstream at its southern end. Previously measured grounding line retreat of ~30 m/a (Thomas and others,1988) in combination with downstream advection of crevasses still cannot account for the change in crevasse style at the southern edge of the ice stream. The implication is that the grounding line was substantially seaward of its present location several hundred years ago and that it has retreated rapidly since that time.
DE: 1699 General or miscellaneous
DE: 1827 Glaciology (1863)
SC: Cryosphere [C]
MN: 2003 Fall Meeting