Measuring Multiple Cosmogenic Nuclides in Glacial Cobbles Sheds Light on Greenland Ice Sheet Processes

The behavior of the Greenland Ice Sheet during the Pleistocene remains uncertain due to the paucity of evidence predating the Last Glacial Maximum. Here, we employ a novel approach, cosmogenic nuclide analysis of individual subglacially-derived cobbles, which allows us to make inferences about ice sheet processes and subglacial erosion. From three locations in western Greenland, we collected 86 cobbles from the current ice sheet margin and nine cobbles exposed on the modern proglacial land surface. We measured the concentration of in situ Be-10 in all cobbles (n = 95) and Al-26 and C-14 in a subset (n = 14). Cobbles deposited during Holocene retreat have Be-10 exposure ages generally consistent with the timing of ice retreat determined by other methods. Conversely, most of the 86 subglacial cobbles contain very low concentrations of Be-10 (median 1.0x10(3) atoms g(-1)), although several have similar to 10(4) and one has similar to 10(5) atoms g(-1). The low concentrations of Be-10 in most subglacial cobbles imply that their source areas under the Greenland Ice Sheet are deeply eroded, preserving minimal evidence of surface or near-surface exposure. The presence of measurable C-14 in ten of the cobbles requires that they experienced cosmogenic nuclide production within the past similar to 30 ka; however, C-14/Be-10 ratios of similar to 6 suggest that nuclide production occurred during shielding by overlying material. Only two of the 86 subglacial cobbles definitively have cosmogenic nuclide concentrations consistent with prior surface exposure. Overall, isotopic analysis of subglacial cobbles indicates that much of western Greenland's subglacial landscape is characterized by deep erosion and minimal subaerial exposure. (C) 2020 Elsevier B.V. All rights reserved.