The marginal shear stress of Ice Stream B, West Antarctica
The marginal shear stress of Ice Stream B, West Antarctica
复制标题
南极洲西部冰流 B 的边缘剪应力
作者：
M. Jackson;B. Kamb
为了确定西南极洲冰流B的速度是否受其边缘剪切带（"蛇"和"龙"）的应力控制，我们用冰本身作为应力计测定了Camp Up B附近的"龙"的边缘剪切应力。利用观测到的0.14 a-1的边缘剪切应变率，根据冰的流动规律计算边缘剪切应力，该冰的流动规律是通过使用热水冰芯钻获得的300 m深的冰芯蠕变试验确定的。根据c轴织物选择试验中相对于应力轴的试样方向，以便试验在平行于边缘的垂直平面上施加水平剪切。由此得到的边缘剪应力为（2.2 ± 0.3）× 105Pa。这意味着，63 - 100%的冰流的支持，对重力负荷来自边缘，只有37 - 0%，从底部，所以边缘在控制冰流的运动中起着重要的作用。边界剪应力值是Echelmeyer等人（1994）的冰流模型给出的值的两倍，相应的应变率增强因子也相差很大（E = 1 - 2vs10 - 12.5）。这种巨大的差异可以用取芯过程中或取芯后不久冰的重结晶来解释。预计的重结晶时间尺度的估计支架的101小时的时间尺度的核心和离开重结晶的可能性不确定。然而，所观察到的双峰组构类型并不是在高剪切应变（γ ≥ 20）下原位预期的尖锐单峰组构退火再结晶的预期类型。Wilson（1982）的实验数据表明，如果岩心确实发生了重结晶，那么先前的组构是一个两峰组构，与观察到的组构没有实质性差异，这意味着测得的流动定律和推导出的边缘剪应力适用于原位情况。这些问题需要通过进一步的工作来解决，以获得更明确的边缘剪应力的观测评估。
Abstract To ascertain whether the velocity of Ice Stream B, West Antarctica, may be controlled by the stresses in its marginal shear zones (the “Snake” and the “Dragon”), we undertook a determination of the marginal shear stress in the Dragon near Camp Up B by using ice itself as a stress meter. The observed marginal shear strain rate of 0.14 a−1 is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the Dragon, obtained by using a hot-water ice-coring 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 shear across vertical planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) × 105 Pa. This implies that 63–100% of the ice stream’s support against gravitational loading comes from the margins and only 37–0% from the base, so that the margins play an important role in controlling the ice-stream motion. The marginal shear-stress 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 ≈ 1–2 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. However, the observed two-maximum fabric type is not what is expected for annealing recrystallization from the sharp single-maximum fabric that would be expected in situ at the high shear strains involved (γ ∼ 20). Experimental data from Wilson (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 derived marginal shear stress 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.