Softening of Temperate Ice by Interstitial Water
Softening of Temperate Ice by Interstitial Water
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DOI：
10.3389/feart.2021.702761
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发表时间：
2021-07-15
影响因子：
2.9
通讯作者：
Bate, Charlotte E.
中科院分区：
文献类型：
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作者：
Adams, Conner J. C.;Iverson, Neal R.;Bate, Charlotte E.
冰流剪切边缘深度的冰通常被认为是温带的，间质融化的冰使冰变软了。包括这种软化的模型推断了一项实验研究的结果，在该研究中，冰有效粘度在与0.01-0.8％的水含量上降低了3倍。建模表明，水的软化可以定位于剪切边缘的应变，并通过剪切加热会增加床上的融水，从而增强了基础滑动。为了将数据扩展到更高的水内容物，我们用大型环形装置将实验室制造的冰剪切在密闭压缩中。平均晶粒尺寸为2-4 mm的冰环保持在压力熔化的温度下，并以受控速率剪切，峰值应力与0.06-0.20 MPa相似，涵盖了大多数估计的剪切压力范围。最终的平均晶粒尺寸为8-13毫米。水含量是通过在冰圈边缘诱导冰冻前端，用热敏电阻跟踪其运动的前部，并与相关Stefan问题的解决方案一起拟合数据。结果表明，在水含量下方和高于0.6％的水含量下方和高于蠕变状态。比较次级蠕变中的有效粘度值与早期实验研究的第三次蠕变的粘度值表明，对于0.2-0.6％的水含量，次级蠕变的粘度对水含量的敏感性是对冰的敏感的两倍，而不是对第三次蠕变的冰层。高于0.6％的水含量，次级蠕变中的粘度值在第三次蠕变的25％以内，这表明水含量大于0.6％的应力限制机制对第三纪念蠕变中的冰织物发育不敏感。在类似于0.6-1.7％的水含量下，有效粘度与水含量无关，冰几乎是线性粘性的。通过晶粒尺度的熔化和重新冻结以接近上限的速率，随着晶粒结合水膜变厚的速度，将晶层应力异质性最小化可能是两种状态。
Ice at depth in ice-stream shear margins is thought to commonly be temperate, with interstitial meltwater that softens ice. Models that include this softening extrapolate results of a single experimental study in which ice effective viscosity decreased by a factor of similar to 3 over water contents of similar to 0.01-0.8%. Modeling indicates this softening by water localizes strain in shear margins and through shear heating increases meltwater at the bed, enhancing basal slip. To extend data to higher water contents, we shear lab-made ice in confined compression with a large ring-shear device. Ice rings with initial mean grain sizes of 2-4 mm are kept at the pressure-melting temperature and sheared at controlled rates with peak stresses of similar to 0.06-0.20 MPa, spanning most of the estimated shear-stress range in West Antarctic shear margins. Final mean grain sizes are 8-13 mm. Water content is measured by inducing a freezing front at the ice-ring edges, tracking its movement inward with thermistors, and fitting the data with solutions of the relevant Stefan problem. Results indicate two creep regimes, below and above a water content of similar to 0.6%. Comparison of effective viscosity values in secondary creep with those of tertiary creep from the earlier experimental study indicate that for water contents of 0.2-0.6%, viscosity in secondary creep is about twice as sensitive to water content than for ice sheared to tertiary creep. Above water contents of 0.6%, viscosity values in secondary creep are within 25% of those of tertiary creep, suggesting a stress-limiting mechanism at water contents greater than 0.6% that is insensitive to ice fabric development in tertiary creep. At water contents of similar to 0.6-1.7%, effective viscosity is independent of water content, and ice is nearly linear-viscous. Minimization of intercrystalline stress heterogeneity by grain-scale melting and refreezing at rates that approach an upper bound as grain-boundary water films thicken might account for the two regimes.