COLLABORATIVE RESEARCH: Subglacial Deforming Beds as Erosive and Sedimentary Agents: An Experimental Study of Particle Comminution and Rock Erosion

9819346Tulaczyk冰下沉积物变形越来越被人们接受为快速冰流和冰川生成的新的、独特的、甚至可能是主要的机制。通过这种机制，可以产生大范围的耕作层，以快速的速度运输，并完全沉积在冰川以下。目前，在冰流和冰层生成的变形床模型中有两个基本假设：(1)这些快速变形的床层足够有效地侵蚀下面的基岩，以维持所需的高泥沙通量；(2)它们可以产生碎屑粉碎的证据，这是在冰层中常见的观察到的。对西南极B冰流下冰壶的观测和实验研究结果表明，这些假设极大地改变了目前对变形床的理解。然而，目前还不清楚从B冰流到TIL的观测结果是否可以外推到其他冰下环境。本文通过一系列的试验，研究了剪切耕作对颗粒粉碎和基岩侵蚀的影响。在这些实验中，现有的环形剪切装置将被用来在适用于变形床模型(6kPaN 100kpa)的有效正应力下将六种不同类型的冰犁剪切成大应变(~10万)。剪切前和剪切后的粒度分析、粘土粒度组成以及砂粒形状和微观形态的比较将决定颗粒粉碎的程度。岩石侵蚀速率将在安装在环形剪切装置中的常见岩性的岩板上测量。结果将被用来检验假设(1)和(2)的总体有效性。所收集的数据将为侵蚀和粉碎的定量公式提供实验基础，这些公式可用于未来变形床的模拟。最后，这些实验有可能产生可靠的沉积学标准，用于测试给定的冰层是否完全由冰下变形产生。

9819346TulaczykSubglacial sediment deformation is becoming increasingly accepted as the new, distinct, and perhaps even predominant mechanism of fast ice flow and till generation. By this mechanism, extensive layers of till may be generated, transported at fast rates, and deposited entirely subglacially. At present, two fundamental assumptions are made in the deforming-bed models of ice flow and till generation: (1) that these rapidly deforming beds erode underlying bedrock efficiently enough to sustain the required high sediment fluxes; and (2) that they can produce the evidence of debris comminution which is commonly observed in tills. Results of observational and experimental investigations of the till from beneath Ice Stream B. West Antarctica suggest that these assumptions changes significantly the presently existing understanding of deforming beds. However, it is not clear whether the observations from Ice Stream B till can be extrapolated to other subglacial environments. A series of experiments is proposed here to study the rates of particle comminution and bedrock erosion by shearing till. In these experiments, an existing ring shear device will be used to shear six different types of till to large strains (~100,000) under effective normal stresses which are applicable to the deforming-bed model (6 kPa N 100 kPa). Comparison of pre-shear and post-shear granulometry, composition of clay-size fraction, as well as sand grain shape and micromorphology will determine the extent of particle comminution. The rate of rock erosion will be measured on rock plates of common lithologies mounted in the ring shear device. The results will be used to test the general validity of the assumptions (1) and (2). The collected data will provide an experimental basis for quantitative formulations of erosion and comminution that can be used in future modeling of deforming beds. Finally, these experiments have the potential to yield solid sedimentological criteria for testing whether a given till may or many not have been generated fully by subglacial deformation.