Sediment Transport by the Laurentide Ice Sheet: Testing the Bed-Deformation Hypothesis Using Till Microstructural Characteristics

摘要更新世冰盖底部沉积了大量的冰，但这些冰的搬运方式尚不确定。一种主要的假说将沉积物的输运归因于冰下坡地的剪切变形，这导致了高坡地通量和具有冰川标志的地貌，如鼓丘和冰碛。脆弱的冰下土层的剪切也可能导致了冰盖的快速流动和随之而来的气候变化。尽管显微结构特征提供了高分辨率、时间集成的变形记录，但它们与应变量级的关系基本上是未知的。结果是，从地质记录中没有提取出检验床层变形假说所必需的变形幅度和变形模式。我们用环剪装置研究了土体微观结构特征随剪切应变大小的变化，该环剪装置可将大型土体剪切至高应变（100）。结果表明了剪切应变的几个定量指标：光学测量的颗粒织物强度和微剪切方向，x射线织构几何测量的粘土矿物织物强度，以及由最大磁化率取向定义的织物强度。将这些指标校准为应变大小提供了以前无法用于解释基垄起源的工具。请支持将我们的校准应用于我们实验研究过的三个被认为在劳伦蒂德冰盖下被剪切的丘：密歇根湖瓣的贝斯敦丘、绿湾瓣的霍里肯丘和苏利伯尔湖瓣的道格拉斯丘。其中第一个已被建模为床层变形假说的原型；第二种被雕刻成鼓形，通常归因于地层变形；第三个有不寻常的微纤维特征。这些工位将沿着多个垂直剖面密集取样。显微结构特征将被测量，以确定变形的大小和模式。如果床层普遍剪切到至少中等应变，则将满足三个标准：应在大部分厚度上显示通过显微结构特征（20-40）可检测到的最大剪切应变；微剪切应该是可见的（在碳酸盐蚀刻之后），表明冰下而不是冰川变形，直到周围的大碎屑应该包含由碎屑旋转对称偏转的微观结构元素。这些数据将为床层变形假说提供迄今为止最完整的地质检验。更广泛的影响：这项研究将使其他学科受益，并为教育和推广提供工具。由于河床变形对快速冰川流动的潜在影响，该项目将影响对冰盖-气候相互作用的模拟工作。它也可以使其他领域受益，其中剪切颗粒材料的各向异性通常是一个中心问题，包括构造地质学，石油地质学和岩土工程。此外，这项研究将支持三名研究生的教育：84%的项目。美国的直接费用将为这些学生提供津贴、学费、差旅费和研究支持。工作成果将融入本科教学。此外，正如我们在斯瓦蒂森冰盖下的研究所证明的那样，这个项目可以作为一个平台，向非科学家强调冰川在地球上的作用。S过去和现代的环境。

AbstractVast sheets of till were deposited at the bases of Pleistocene ice sheets, but the mode of transport of this sediment is uncertain. A leading hypothesis attributes sediment transport to shear deformation of subglacial till, resulting in high till fluxes and landforms emblematic of glaciation, such as drumlins and moraines. Shearing of weak, subglacial till may have also helped instigate fast flow of ice sheets and consequent climate change. Although till microstructural characteristics provide a high-resolution, temporally-integrated record of deformation, their relationship to strain magnitude has been essentially unknown. The result is that magnitudes and patterns of deformation necessary to test the bed-deformation hypothesis have not been extracted from the geologic record.We have studied the evolution of till microstructural characteristics as a function of shear-strain magnitude with a ring-shear device that shears a large till specimen to high strains ( 100). Results indicate several quantitative indices of shear strain: particle-fabric strength and microshear orientation measured optically, clay-mineral fabric strength measured with x-ray texture goniometry, and fabric strength defined by orientations of maximum magnetic susceptibility. Calibrations of these indices to strain magnitude provide tools previously unavailable for interpreting origins of basal tills.Support is requested to apply our calibrations to three tills that we have studied experimentally and that are thought to have sheared beneath the Laurentide Ice Sheet: the Batestown Till of the Lake Michigan Lobe, the Horicon Till of the Green Bay Lobe, and the Douglas Till of the Lake Superior Lobe. The first of these has been modeled as an archetype for the bed-deformation hypothesis; the second has been sculpted into drumlins.frequently attributed to bed deformation; the third has an unusual microfabric signature. These tills will be densely sampled along multiple vertical profiles. Microstructural characteristics will be measured to determine magnitudes and patterns of deformation. Three criteria will be met if the bed sheared pervasively to at least moderate strains: the maximum shear strain detectable with microstructural characteristics (20-40) should be indicated over most of the till thickness; micro-shears should be visible (after etching of carbonate) indicative of subglacial rather than englacial deformation, and till surrounding large clasts should contain microstructural elements deflected symmetrically by clast rotation. These data will provide the most complete geologic test, to date, of the bed-deformation hypothesis.Broader Impacts: This research will benefit other disciplines and provide a vehicle for education and outreach. Owing to the potential influence of bed deformation on fast glacier flow, this project will impact efforts to model ice sheet-climate interactions. It could also benefit other fields, in which anisotropy of sheared granular materials is commonly a central issue, including structural geology, petroleum geology, and geotechnical engineering. Also, this research will support the education of three graduate students: 84% of the project.s direct costs will provide stipends, tuition, travel, and research support for these students. Results of the work will be integrated into undergraduate teaching. In addition, as we have demonstrated with our research beneath the Svartisen Ice Cap, this project can serve as a platform for emphasizing to nonscientists the role of glaciers in Earth.s past and modern environment.