DIFFERENTIAL ICE SHEET EROSION AT A FJORD ONSET ZONE

As fjords are unique to glacial landscapes, their evolution through the repeated glacial occupation of the continental margin requires an understanding of ice sheet processes. Recent applications of cosmogenic radionuclides to glacial landscapes have contributed to a rapidly evolving understanding of Pleistocene ice sheet dynamics and history that point toward a strong control of erosion patterns by the basal thermal regime. Our group has recently reconstructed ice sheet basal thermal regimes of northeastern Baffin Island fjord landscapes based on patterns of glacial erosion deduced from cosmogenic radionuclide concentrations. Here, we report new findings from the fjord onset zone at a site 20 km inland from the head of Clyde fjord, >150 km from the LGM ice limit, and only 25 km from the Barnes Ice Cap, the remnant of Laurentide Ice Sheet. We use the density of bedrock fractures (obscured where covered with blockfield) and lakes (on exposed bedrock or impounded by drift) as proxies for glacial erosion. By combining these data with 12 cosmogenic radionuclide analyses collected along a cross-valley profile, we document the vertical transition from sliding to frozen basal conditions. For example, where lake and bedrock fracture density is high in the valley bottom, bedrock surfaces yield deglacial ¹⁰Be exposure ages between 4 and 5 ka. In contrast, at higher elevations, weathered bedrock that we know to have been covered by LGM ice yields apparent ¹⁰Be exposure ages between 20 and 55 ka. This study provides field data that we use to constrain numerical ice sheet simulations aimed at addressing glacial landscape evolution. Preliminary results suggest that the combination of field, cosmogenic and glacial modeling efforts should allow us to test hypotheses regarding feedbacks between ice sheet processes and fjord landscape evolution.