Meltwater beneath the large ice sheets can influence ice flow by lubrication at the base or by softening when meltwater refreezes to form relatively warm ice1,2,3. Refreezing has produced large basal ice units in East Antarctica4. Bubble-free basal ice units also outcrop at the edge of the Greenland ice sheet5, but the extent of refreezing and its influence on Greenland’s ice flow dynamics are unknown. Here we demonstrate that refreezing of meltwater produces distinct basal ice units throughout northern Greenland with thicknesses of up to 1,100 m. We compare airborne gravity data with modelled gravity anomalies to show that these basal units are ice. Using radar data we determine the extent of the units, which significantly disrupt the overlying ice sheet stratigraphy. The units consist of refrozen basal water commonly surrounded by heavily deformed meteoric ice derived from snowfall. We map these units along the ice sheet margins where surface melt is the largest source of water, as well as in the interior where basal melting is the only source of water. Beneath Petermann Glacier, basal units coincide with the onset of fast flow and channels in the floating ice tongue. We suggest that refreezing of meltwater and the resulting deformation of the surrounding basal ice warms the Greenland ice sheet, modifying the temperature structure of the ice column and influencing ice flow and grounding line melting.
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The authors acknowledge support from NASA and NSF for this manuscript. The Operation IceBridge mission provided critical data for this analysis. The radar data from the CReSIS radar systems and the Sander Geophysics Ltd. AirGrav gravity data were central to this work. L. Altman, B. Bell and S. Starke provided support in analysis of the data and production of the figures. R. Buck provided feedback that improved the paper substantially. LDEO contribution number 7800.
The authors declare no competing financial interests.
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Bell, R., Tinto, K., Das, I. et al. Deformation, warming and softening of Greenland’s ice by refreezing meltwater. Nature Geosci 7, 497–502 (2014). https://doi.org/10.1038/ngeo2179
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