A Field and Theoretical Study of Sediment Transport near the Basal Thermal Transition of a Polythermal Glacier

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A FIELD AND THEORETICAL STUDY OF SEDIMENT TRANSPORT NEAR THE BASAL THERMAL TRANSITION OF A POLYTHERMAL GLACIERNeal Iverson and Denis Cohen, Iowa State UniversityABSRACT1) Description of the projectIce flow and sediment transport will be studied near the frozen margin of Storglaciaren, a soft-bedded glacier in Sweden. The goal is to fully characterize the processes that entrain basal sediment and carry it the glacier surface; these processes result in hummocky end moraines and other supraglacially-developed landforms of that are emblematic of continental glaciation. Debris derived from the bed of Storglaciaren is accumulating in an ice-cored moraine down-glacier from the basal thermal transition (BTT): the transition from melting to cold basal ice. Our hypothesis is that resistance to basal movement increases sufficiently abruptly at the BTT to cause large stress and velocity gradients in the ice. Associated enhancement of normal stress on the bed entrains sediment by regelation infiltration. Reoriented stresses concentrated near the BTT cause debris-rich ice to be lifted off the bed and carried to the glacier surface, perhaps along zones of localized shear strain (i.e., thrust faults). Measurements will be focused in two regions that straddle the BTT. Surface velocity will be measured, together with borehole measurements of ice temperature, shear deformation of ice, depth-averaged vertical strain, basal sliding speed, effective normal stress on the bed, and strain localization near and within debris-rich ice bands. Oxygen and tritium isotopic analyses of ice in debris bands will allow debris entrainment by regelation to be distinguished from that due to freeze-on of basal water. To determine the far-field ice velocity, ice deformation and surface velocity will be measured upstream, across the glacier width. These measurements, together with the measured sliding speed, will provide upstream and basal boundary conditions for a 3-D finite-element model of steady ice flow in the lowermost ablation area. The model will include rheological heterogeneity caused by variable ice temperature and debris-rich ice bands. Model results will be tested with measurements near the BTT of surface velocity, borehole deformation, vertical strain, and normal stress on the bed. 2) Non-technical explanation of importanceFormer ice sheets transported and deposited huge volumes of debris that dominate the landscapes and near-surface geology of formerly glaciated regions, such as New England and the Upper Midwest. In addition to affecting the form of the landscape, these sediments greatly impact agriculture, subsurface hydrology, and engineering design. This research will help explain the processes that lift debris from the beds of ice sheets and carry that debris to the glacier surface, where it melts out and is eventually deposited on the land surface during glacier wastage. The engineering and hydrological properties of these sediments directly reflect these processes. Through measurements on a small, particularly well-studied glacier and use of the resultant data to drive and test mathematical models of ice flow and sediment transport, this project will help clarify how sediment is moved and deposited by glaciers. In particular, the work will demonstrate the role of spatially-variable glacier temperature. An important ancillary objective of this work is to illuminate the effects of ice-temperature variability on glacier-flow velocity; this objective will be increasingly important in the coming decades as the ice of glaciers warms in response to atmospheric warming, thereby accelerating glacier wastage with impacts on sea level and climate. This project will also support the Ph.D. research of a graduate student and will involve undergraduates with the goal of stimulating post-graduate careers in Earth science.