Ice speed of a calving glacier modulated by small fluctuations in basal water pressure

Ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in Alaska(1,2), Greenland and Antarctica(3,4). Glaciers that calve in water flow much faster than those that terminate on land, as a result of enhanced basal ice motion where basal water pressure is high(5). However, a scarcity of subglacial observations in calving glaciers limits a mechanistic understanding. Here we present high-frequency measurements of ice speed and basal water pressures from Glaciar Perito Moreno, a fast-flowing calving glacier in Patagonia. We measured water pressure in boreholes drilled at a site where the glacier is 515 +/- 5m thick, and where more than 60% of the ice is below the level of proglacial lakes. We found that the mean basal water pressure was about 95% of the pressure imposed by the weight of the overlying ice. Moreover, changes in basal water pressure by a few per cent drove nearly 40% of the variations in ice flow speed. The ice speed was strongly correlated to air temperature, suggesting that glacier motion was modulated by water pressure changes as meltwater entered the system. We conclude that basal water pressure in calving glaciers is important for glacier dynamics, and closely connected to climate conditions.