Constraining subglacial erosion in Greenland using estimates of fjord sediment volumes and ice-flow modeling

Fjord landscapes along the margins of past and present ice sheets testify to the significant long-term erosive power of large outlet glaciers. Yet, our understanding of the rates and processes of subglacial erosion and sediment transport beneath ice sheets remains incomplete. Quantifying these processes is crucial for reconstructing past ice dynamics, estimating sediment fluxes to the ocean, and understanding long-term landscape evolution.

Greenland’s narrow, steep-sided fjords act as natural sediment traps, preserving erosion products delivered by large outlet glaciers during deglaciation. These fjord sediments constitute a valuable constraint on past erosion rates and glacial sediment fluxes when combined with ice catchment areas and retreat histories in a source-to-sink framework. However, most Greenlandic fjords remain unmapped in terms of sediment thickness because sediment cores rarely penetrate deeply and seismic data acquisition is sparse. In contrast, accurate bathymetric data are increasingly available for many fjords. We use a geomorphological approach to estimate sediment infill volumes based on fjord cross-sectional profiles, where deviations from the expected U-shape and the slope of the sidewalls are used to infer sediment thickness.

We quantify fjord infill volumes for several fjords and use these to estimate average catchment-wide erosion rates. The timing of deposition (starting when the ice retreated into the fjord) is constrained by available deglaciation models. To further explore the temporal and spatial variability of subglacial erosion, we employ a coupled ice-flow and erosion model (iSOSIA), driven by paleoclimate forcing, to simulate erosion beneath marine-terminating outlet glaciers during the last deglaciation (~21–0 ka BP). Modeled sediment outputs are compared with our estimates of sediment volumes and accumulation rates from sediment cores to calibrate the model erosion parameters.

Our results indicate that average deglacial erosion rates are largely independent of catchment size but vary significantly through time and space within ice-sheet catchments. Rates can exceed 10 mm yr⁻¹ for topographically steered, fast-flowing outlet glaciers, while much lower rates (<0.1 mm yr⁻¹) occur in slower-flowing interior regions with slow-moving ice. Quantifying and linking offshore sediment volumes with numerical modeling provides an opportunity to constrain subglacial erosion rates, sediment transport and ice-sheet reconstructions. This work demonstrates the value of integrating glaciological modeling with marine sediment archives to refine erosion estimates and improve predictions of future sediment fluxes under continued ice-sheet retreat.