Short- and long-term subglacial erosion rates from 10Be measurements in recently deglaciated bedrock at Jakobshavn Isbrae

Abstract:

Subglacial erosion and sediment removal drive landscape development and play an important role in Cenozoic climate change. Quantifying subglacial erosion rates can help inform ice-sheet model basal sliding parameters, for which empirical data are sparse. However, the controls on these processes remain unclear because they are notoriously difficult to study given that they take place beneath modern glaciers. Most estimates of subglacial erosion rates rely on sediment flux through proglacial rivers and sediment volume in fjords, which obscure the spatial pattern of erosion within a glacier catchment and the relative importance of abrasion, quarrying and sediment removal. In contrast, cosmogenic ¹⁰Be concentrations in recently deglaciated bedrock with a well-constrained glacial history at Jakobshavn Isbrae, western Greenland, afford: 1) point measurements of abrasion and sediment removal rates during the most recent period of ice cover, and 2) erosion rates on glacial-interglacial timescales. We find abrasion rates throughout the study area of ~0–1 mm yr^-1, which scale to a basin-wide erosion rate of ~0.5–0.9 mm yr^-1 (abrasion + quarrying). In addition, we present a novel method to estimate longer-term erosion rates in the Jakobshavn Isbræ forefield by quantifying the build up of muon-produced ¹⁰Be below ~2 m depth in a 4m-long bedrock core. Our measurements are consistent with ~10 m of subglacial erosion during each of the last eight glacial cycles, equating to a rock erosion rate of ~0.1 mm yr^-1 during each glacial period. Lastly, some of our ¹⁰Be measurements indicate that prior to late Holocene ice advance ~30–60 cm of sediment likely covered some sites that have emerged from the ice sheet in the last few decades. Ultimately, our measurements allow us to evaluate subglacial erosion and sediment removal rates on centennial and glacial-interglacial timescales, enhancing our understanding of the temporal and spatial patterns of these basal processes.