The marginal shear stress of Ice Stream B , West Antarctica

To ascertain whether the , ·elocity of Ice Stream B, \\'est Antarctica, may be controlled by the stres e in its marginal shear zones \ the .. Snake" and the .. Dragon'· ), we undertook a determination of the marginal hear stress in the Dragon near Camp up B by using ice itself as a stress meter. The ob erved marginal shear strain rate of0.14 a I is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of300 min the Dragon, obtained by using a hot-water icecoring drill. The test-specimen orientation relative to the stress axes in the tests is chosen on the basis of c-axis fabrics so that the test applies horizontal hear across veni<;:al planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) x 10" Pa. This imp! ie that 63-100% of the ice stream's support against gra,·itationalloading comes from the margins and only 37-Q% from the base. o that the margin play an important role in controlling the ice-stream motion. The marginal shear-stres value is twice that given by the ice-stream model of Echelmeyer and others (1994} and the corresponding strain-rate enhancement factors differ greatly (E ~ 12 vs 10-12.5). This large discrepancy could be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time-scale bracket the .....,1 h time-scale of coring and leave the likelihood of recrystallization uncertain. H owe,·er, the observed two-maximum fabric type is not what is expected for annealing recry tallization from the harp single-maximum fabric that would be expected in situ at the high shear strains im·olved ('y ....., 20). Experimental data from \ Vii son (1982) suggest that, if the core did recrystallize, the prior fabric was a two-maximum fabric not substantially different from the observed one, which implies that the measured flow law and deri,·ed marginal shear stres are applicable to the in situ situation. These issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress.