Experimental Constraints on Subglacial Abrasion

Basal drag for temperate, hard-bedded glaciers is intrinsically linked to subglacial abrasion, the erosive byproduct of rock friction at the ice-bed interface. Previous estimates suggest this process may be an important energy sink; however, the amount of sliding energy consumed by abrasion and its sensitivity to subglacial conditions remain poorly constrained. To address this, we conducted a series of experiments to quantify the energy dissipated during subglacial abrasion and explore the relative influence of its controlling mechanisms. A direct shear apparatus housed in a cold room was repurposed to simulate abrasion. Limestone beds were slid beneath a stationary slab of debris-laden ice under a vertical load at realistic sliding speeds and effective stresses. A range of parameters (i.e effective stress, basal melt rates, debris concentration, and bedrock lithology) were tested, all of which are believed to control subglacial abrasion. The total excavated volume for each experiment was determined by scanning striations with a white light interferometer at 12 μm line spacing with 31 nm vertical accuracy. To compute the total change in surface area and, by extension, the abrasion energy, the grain size distribution of the resultant rock flour was characterized via laser diffraction method (LDM). For our experimental setup, we find that subglacial abrasion consumes approximately 2-5% of the total sliding energy, nearly an order of magnitude less than previous estimates. Abrasion rates scale with shear traction; however, the relative contribution of effective stress and melt at the ice-bed interface to basal drag depends on the sliding velocity.