Abstract

Glacial erosion rates inferred from total sediment budgets in recently deglaciated fjords, which are the highest reported erosion rates worldwide, have received considerable attention in fields as diverse as tectonics, glacial sedimentation, and climate. These record rates, however, are representative only of tidewater glaciers during their extensive retreat of the post–Little Ice Age period; erosion rates averaged over glacial-interglacial cycles and longer periods are likely to be substantially smaller. We examine the influence of retreat rate on sediment yields from tidewater glaciers by reconstructing the history of sediment output from retreating glaciers necessary to produce sediment packages observed in contemporary fjords. Using a simple numerical model of proglacial sedimentation in front of a retreating glacier, seismic profiles of proglacial sediments, and the history of terminus retreat of Muir Glacier, Glacier Bay, Alaska, we calculate the sediment flux as a function of time from this glacier between 1900 and 1979, and conclude that sediment flux scales with retreat rate. The corresponding basin-wide erosion rate during this 79 yr period averages 37 mm/yr, and exceeds long-term erosion rates by a factor of 5 ± 1. For Muir Glacier and, by inference, for other calving glaciers, the general drastic retreat and the marked regional drawdown of ice since the Little Ice Age are both linked to unusually rapid calving and fast ice motion, which is conducive to rapid erosion.

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