Intermittent glacial sliding velocities explain variations in long-timescale denudation

Abstract

Quantifying controls on glacial erosion over geologic timescales is central to understanding the role of Cenozoic climate change on the development of modern mountain belts, yet the mechanisms that produce the distinct relief and topography visible in glaciated regions remain poorly constrained. We test the hypothesis that commonly assumed glacial sliding parameterizations control denudation rates over geologic timescales. We do this by modeling glacier dynamics over a glacial–interglacial cycle and compare with a dense dataset of (U–Th)/He thermochronometer derived denudation rates from the southern Coast Mountains, BC. Results indicate zones of rapid Quaternary erosion correspond to locations where the model predicts the highest averaged sliding velocities. The results are consistent with the hypothesis that sliding influences the rate of glacial erosion. Regression between sliding predicted by the model and erosion rates shows a statistically significant correlation (r2=0.6). The coefficient of the regression (10−5) is smaller than previous estimates based on data from much shorter timescales. The model results also reveal that for a specific location, active subglacial sliding, and hence erosion, occurs for only ∼10–20% of a glacial–interglacial cycle, suggesting high temporal variations in erosion rates. This intermittency of erosion requires instantaneous erosion rates to be greater than long term averages, explaining how timescale averaging can impact estimates of glacial erosion rates.