Modeling ice-crystal fabric for use as a proxy for ice-stream stability

Abstract:

Ice deformation causes reorientation of the constitutive ice crystals, so crystal fabric is a proxy for ice-flow history. While much interest in crystal fabric focuses upon its strong control upon ice flow, comparing modeled and measured crystal fabrics can both constrain the models and perhaps provide information about past ice flow. Potential for this type of application is growing as seismics and polarimetric radar measurements augment ice-core observations of crystal fabric. However, the complexity of crystal fabric evolution, and the lack of model results with which to compare data, hinder interpretation of these fabric measurements. Here, we use a coupled model of ice flow and crystal fabric evolution (implemented in Elmer/Ice) to identify the fabric characteristics associated with ice-stream flow. We focus on characteristics of ice-crystal fabric that may be used diagnose instability in an ice stream’s speed or lateral position. We find that within an ice stream, flow convergence generally results in a crystal fabric that is weaker in the vertical than that formed purely by internal deformation, with horizontal components strongly dependent on the rate of flow convergence. This fabric arises over thousands of years after ice-stream activation and persists for tens of thousands of years after ice-stream deactivation. Lateral changes are manifest in rotation of the horizontal orientation of the fabric as well as in weakening of the vertical, and are likely identifiable for thousands of years after lateral movement or width changes of the ice stream. Synthetic radargrams indicate that many of these transient fabrics deviate sufficiently from steady-state so as to be measurable with ApRES, suggesting that surface observations of fabric could be used to diagnose past flow changes. These results could be utilized to design phase-sensitive radar studies to identify changes to ice flow over periods as long as the Holocene.