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冰川，但这些沉积物的搬运方式尚不清楚。一种主要的假说将沉积物的运输归因于冰下耕作的剪切变形，导致高冰川通量和象征着冰川的地貌，如鼓状和冰雹。脆弱的冰下耕作的剪切可能也有助于引发冰盖的快速流动和随之而来的气候变化。尽管到目前为止，显微构造特征提供了高分辨率的、时间上完整的变形记录，但它们与应变大小的关系基本上是未知的。结果是没有从地质记录中提取验证床层变形假说所需的变形大小和模式。我们利用环形剪切装置将大的TIL试件剪切到高应变(100)，研究了TIL微观结构特征随剪应变大小的变化。结果表明，剪切应变的几个定量指标：用光学方法测量的颗粒组构强度和微观剪切取向，用X射线织构测角仪测量的粘土-矿物组构强度，以及由最大磁化率方向定义的组构强度。这些指数对应变大小的校准为解释基础耕作的起源提供了以前不可用的工具。请支持将我们的校准应用于我们通过实验研究的三种耕作，它们被认为是在劳伦蒂冰盖下剪切的：密歇根湖的贝斯特敦冰层、绿湾冰盖的Horcon Till和苏必利尔湖的Douglas Till。其中第一个被模拟为床变形假说的原型；第二个被雕刻成鼓形，通常被归因于床的变形；第三个具有不同寻常的微结构特征。这些地块将沿着多个垂直剖面进行密集采样。将测量微观结构特征，以确定变形的大小和模式。如果地层被普遍剪切到至少中等应变，将满足三个标准：在大部分冰耕厚度上应显示可检测到的具有微观结构特征的最大剪切应变(20-40)；(碳酸盐蚀刻后)指示冰下变形而不是冰下变形的微剪切应可见；直到周围的大碎屑应包含因碎屑旋转而对称偏转的微结构元素。这些数据将提供迄今为止对床层变形假说最完整的地质测试。广泛的影响：这项研究将有益于其他学科，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，该项目将影响模拟冰盖-气候相互作用的努力。它还可以使其他领域受益，在这些领域，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：该项目84%的直接成本将为这些学生提供津贴、学费、旅行和研究支持。工作成果将整合到本科教学中。此外，正如我们在斯瓦蒂森冰帽下的研究中所展示的那样，这个项目可以作为一个平台，向非科学家强调冰川在地球过去和现代环境中的作用。

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.