Experimental Recreation of the Sub-Glacial System, an Analysis of Hold Time versus Strengthening to Shed Light on the Mechanics of Stick-Slip Ice Streams

McBrearty, Ian
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Geological and Atmospheric Sciences

Ice streams in Antarctica are known to move ice rapidly, in a narrow band, over a loose permeable substrate to the sea- contributing to sea-level rise and the decay of the Antarctic Ice Sheet. It has been observed that some of these ice streams move in a “stick-slip” fashion, lurching ahead small steps at a time. Past workers have proposed this observation is indicative of temporal strengthening in the ice stream when it’s sitting at rest, but have not satisfactorily concluded which components of the glacial system are directly causing the strengthening. To understand the mechanics dictating ice stream motion, and in turn put better constraints on predictions regarding the continued decay of the Antarctic Ice Sheet, we have resorted to experimentally recreating the sub-glacial system in the lab, in two “ringshear” devices designed by Dr. Iverson at Iowa State University. Two main questions we wanted answered were: (A) “Do we observe a significant relationship between hold time and strengthening?” and (B) “Which component(s) (ice, water, and sediment) contribute to the strengthening, and by how much does each contribute?” To do this we ran a suite of “slide-hold-slide” experiments, in each case adding one more component of the glacial system, where time held still (corresponding to the “stick” phase of motion), was measured against the maximum strength the material achieved upon forcing it to undergo further shear (corresponding to the “slip” phase of motion). Our results indicate that there is indeed a significant relationship between hold times and strengthening, and that all three components (ice, water, sediment) contribute to strengthening- however, with each component contributing differently depending on how long of a hold time they have undergone. These results are in agreement with first order physical principles, but further work is required to develop the mathematical relationships relating strengthening to hold time for each component of the system.