Direct evidence for thinning and retreat of the southernmost Greenland ice sheet during the Younger Dryas

During the last deglaciation, North Atlantic climate abruptly warmed at the Belling (similar to 14.7 ka), cooled into the Younger Dryas (similar to 12.9 ka) and abruptly warmed again into the Holocene (similar to 11.7 ka). While these events are defined by Greenland ice cores, there is still considerable uncertainty on Greenland ice-sheet margin responses to abrupt climate change. To refine the ice sheet's deglacial history, we present new cosmogenic nuclide surface exposure ages from boulders on bedrock at five sites in southernmost Greenland fjords located midway between the coast and inland ice margin. We find ice-sheet thinning below three local topographic highs at 12.7 +/- 0.3 ka (n = 3),13.1 +/- 0.4 ka (n = 1, 2 outliers), and 12.3 +/- 0.2 ka (n = 3), with up-fjord retreat at 12.5 +/- 0.3 ka (n = 3) and 12.7 +/- 0.2 ka (n = 4) based on two sites just above the mid-fjord marine limit. These mid-fjord Be-10 ages therefore show southernmost Greenland ice-sheet thinning and retreat during the Younger Dryas. We hypothesize that this thinning and retreat was a response to ocean warming prior to the Holocene and/or summer shortwave radiative forcing during the Younger Dryas due to peak boreal summer insolation. Our results also support a previously hypothesized winter bias in proxy records of Younger Dryas atmospheric cooling, since a large summer cooling during the Younger Dryas could have counteracted the effects of ocean warming and direct radiative forcing, inhibiting ice-sheet retreat. (C) 2021 Elsevier Ltd. All rights reserved.