Rapid early Holocene retreat of a Laurentide outlet glacier through an Arctic fjord

Ice-sheet behaviour is disproportionately controlled by the dynamics of outlet glaciers that terminate in the ocean1,2. However, outlet-glacier dynamics—particularly over timescales longer than the observational record—are not well understood3, leading to uncertainties in our models of ice-sheet response to climate change. Here we use 10Be exposure ages and radiocarbon dating from the Sam Ford Fjord in the Canadian Arctic to reconstruct the retreat chronology of an outlet glacier of the Laurentide ice sheet, following the last glacial termination. We find that Sam Ford Fjord, which has a similar morphology to the troughs holding many outlet glaciers of the Greenland ice sheet, was rapidly deglaciated about 9,500 years ago, with retreat rates ranging from 5 to 58 m yr−1. The highest rates occurred in the deepest part of the fjord (900 m), whereas regions beyond the fjord mouth and up-valley of the head of the fjord experienced the lowest rates of retreat. We conclude that in such a fjord setting, there is a strong bathymetric control on the retreat of marine outlet glaciers: once the terminus of the outlet glacier retreated into deeper waters, increasing calving rates and basal sliding speeds caused the glacier to rapidly thin and retreat, stabilizing only when it reached the shallow inland head of the fjord. Ice-sheet mass balance is significantly influenced by the behaviour of outlet glaciers and ice streams, which dominate overall discharge from ice sheets1,4. Large marine-terminating ice streams that drain the Greenland ice sheet have recently undergone significant fluctuations in velocity and terminus position2,5–7, but the response of ice streams to climate change is complicated by poorly understood dynamic controls on ice motion7–10. A significant limitation to our understanding of the dynamics of ice sheets and their outlets is that direct observations are limited to the past few decades. Centennialand millennial-scale perspectives of ice-sheet behaviour arise from ice-sheet reconstructions that constrain the timescale of ice-sheet change. Geological reconstructions of ice sheets reveal that positive feedbacks lead tomuch faster ice-sheet decay than growth on glacial–interglacial and millennial timescales11. Ice-thickness reconstructions of the West Antarctic ice sheet based on cosmogenic exposure dating provide a millennial-scale context for the rapid changes that are taking place there at present12,13. In other contexts, palaeodata provide long-term perspectives of ongoing ice-sheet change based on Quaternary ice-sheet behaviour in settings analogous to present ice sheets. For example, retreat of palaeo-ice shelves in Antarctica and marine-based margins of the Laurentide ice sheet during deglaciation14,15 are potentially analogous to the present deglaciation ofGreenland andWest Antarctica. Here, we report a detailed chronology of ice-sheet retreat through Sam Ford Fjord, Arctic Canada (Figs 1 and 2), one of dozens of fjords that were conduits for the discharge of ice from