In situ cosmogenic 10Be and 26Al reveal the complex exposure and erosion history of the landscape once covered by the Quebec-Labrador Ice Dome

Abstract

The rate at which ice sheets erode rock and produce sediment is poorly known. Here, we use paired cosmogenic nuclides in both deglacial and modern sediment to understand better the efficacy with which the Quebec-Labrador Ice Dome (QLID) of the Laurentide Ice Sheet eroded bedrock and generated sand and boulders across the landscape of eastern Canada. We sampled deglacial sediment (esker and delta sand, n = 10), sediment from modern streams (n = 11), one bedrock outcrop, and a bedrock depth profile (n = 7), measuring concentrations of 10Be and 26Al in quartz isolated from all samples. We also collated published cosmogenic nuclide measurements of boulders and bedrock from eastern Canada (n = 237 samples), and using independent estimates of deglaciation timing, calculated initial nuclide concentrations when the material was exposed by the most recent deglaciation, between 6.3 to 15.2 ka. At the time of deposition, all 10 deglacial sand samples contained 10Be and 26Al, on average equivalent to several thousand years of surface exposure. The ubiquitous presence of 10Be and 26Al in eastern Quebec deglacial sediment is consistent with older-than-expected exposure ages for bedrock outcrops (n = 26 of 46 samples) and boulders (n = 65 of 192 samples) once covered by the QLID. Error-weighted averages of 26Al/10Be ratios for both deglacial (6.1 ± 0.3, all uncertainties 1 SD) and modern sediment samples (6.6 ± 0.5) are lower than the measured production ratio at high latitudes (Greenland, 7.3 ± 0.3), suggesting cumulative burial of at least some sediment grains for at least hundreds of thousands of years. This burial history suggests that ice at the middle of the QLID either survived some interglacials and/or that the average sediment residence time on the landscape is several times longer than a 100 kyr glacial cycle, allowing storage and burial of sediment over multiple glacial cycles, either under ice and/or in thick deposits such as deltas and moraines. Modern river sand contains on average only slightly higher nuclide concentrations than deglacial sediment, suggesting that contemporary river sand is predominately recycled from glacial deposits. Together, the new sediment data (which amalgamate across large areas of the landscape), and our compilation of bedrock and boulder point data, suggest that the average depth of bedrock erosion by ice and the speed of glacial sediment transport in eastern Canada were insufficient to remove material containing cosmogenic nuclides produced during prior interglacial(s).