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

Abstract<br/>Vast 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.<br/>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.<br/>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.<br/>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.

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