Collaborative Research: Influence of natural ice microstructure on rheology in general shear: in-situ studies in the Alaska Range

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Understanding the loss of ice from glaciers and ice sheets, and the resulting sea-level rise, is of critical importance. Both the Greenland and Antarctic Ice Sheets, as well as mountain glaciers, discharge primarily though rivers of ice; understanding what controls the type of flow that occurs in these rivers of ice is therefore central to understanding and predicting sea-level rise. Among the least-understood factors that are thought to be important in affecting ice flow is internal strength of the ice near the sides of a flowing glacier. This viscous strength, in turn, may be affected by the micro-scale structure of the ice crystals in the glacier. The investigators propose to examine these relationships in detail on Jarvis Glacier, in the eastern Alaska Range, with the ultimate goal of being able to represent the effects of microstructure in numerical models of glacial flow.To do this, the investigators will first use surface velocity measurements, knowledge of the glacier geometry derived from ground penetrating radar, and numerical modeling to identify a site for drilling. They will then collect surface-to-bed cores across lateral and vertical flow gradients. Velocity and temperature measurements derived from the boreholes will complement the surface measurements and allow the investigators to produce a more sophisticated three-dimensional numerical model to test the sensitivity of flow patterns to the mechanical structure within the study area. They will compare the microstructure (e.g., grain size distribution, crystallographic fabric) in the ice cores to the in-situ and modeled velocities and temperatures. Although experiments suggest that variations in the intensity and orientation of the crystallographic fabric can result in up to a ten-fold difference in flow strength, there are very few in-situ observational studies of the microstructural architecture of streaming ice; most studies of ice microstructure come from ice divides, where flow rates are slowest. At the end of this project, the investigators aim to have determined (1) the degree to which fabrics formed in the study area are predictable based on ice kinematics, and (2) the relationship among measured crystallographic orientation fabric intensity, grain size, temperature, and ice viscosity as calculated through numerical models. A correlation between fabric and viscous strength would suggest that remote sensing techniques such as radar and seismic anisotropy could become an even more powerful method for identifying the rheological structure of ice. Alternatively, the lack of a strong link between viscous strength and fabric indicates that other factors exert significant control on the rheological properties. Therefore, the results of the proposed project, whatever the correlation between microstructure and viscous strength, should improve quantitative understanding of the physical laws governing streaming ice and improve future predictions of ice mass balance. The project would support and involve both graduate and undergraduate students. The project's numerical models will be developed into a publicly available web-based graphical user interface for use by other researchers and in the classroom.