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

Abstract

Ice flow acceleration has played a crucial role in the recent rapid retreat of calving glaciers in Alaska 1,2 , as well as in Greenland and Antarctica 3,4 . Fast flow of such glaciers is due primarily to basal ice motion 5 , but its mechanism is poorly understood because subglacial observations are scarce in calving glaciers. Here we show high-frequency ice speed and basal water pressure measurements performed in Glaciar Perito Moreno, a fast-flowing calving glacier in Patagonia. The water pressure was measured in a borehole drilled through the 515±5 m thick glacier at a site where more than 60% of ice is below the proglacial lake level. We found that mean basal water pressure reached 94-96% of the ice overburden pressure, and that a few percent of pressure changes were driving nearly 40% of ice speed variations. The ice speed was strongly correlated to air temperature, suggesting the glacier motion was modulated by water pressure 1 under the influence of changing meltwater input. Our observations demonstrate the great importance of basal water pressure in the calving glacier dynamics and its close connection to climate conditions. It is thus crucial to take into account the elevated basal water pressure for predicting future evolution of calving glaciers.Acceleration of fast-flowing calving glaciers is the focus of attention as it is responsible for the rapid retreat of large tidewater glaciers in Alaska 1,2 as well as the recent wastage of Greenland and the Antarctic ice sheets 3,4 . Calving glaciers flow much faster than those terminating on land as a result of basal ice motion enhanced by high basal water pressure 5 . A commonly used basal flow law stateswhere u b is the basal ice speed, τ b is the basal shear stress, P i and P w are ice overburden and basal water pressures, and k, p and q are empirical parameters 6,7 . Because τ b is primarily controlled by ice thickness and surface slope, changes in basal water pressure play a critical role in short-term ice speed variations. Observations in mountain glaciers have shown rapid acceleration as basal water pressure approaches ice overburden pressure 8−10 , which confirms the non-linear dependence of the basal ice speed on the effective pressure defined by P e = P i − P w .The hydraulic head within a calving glacier is expected to be higher than the surface level of the proglacial water body, which maintains basal water pressure closer to ice overburden.According to the inverse proportionality of u b to P e , small perturbations in P w near P i result in large ice speed variations. Moreover, changes in P i due to glacier thinning or thickening have a great impact on the ice speed as well. These characteristics make calving glacier dynamics more susceptible to external forcing than land terminating glaciers. Studying the response of ice speed to the changes in P e is thus crucial for predicting the future evolution of calving glaciers. In the austral summer 2008/09 and 2010, we operated three GPS (Global Positioning System) receivers on GPM at hourly intervals ...

Modelling environmental influences on calving at Helheim Glacier in eastern Greenland

“…The results presented here indicate that changes in basal water pressure leading to velocity changes of this magnitude can in fact have a strong influence on modelled calving rates. The finding supports previous tidewater glacier modelling work (Vieli et al, 2000) and observations at other tidewater glaciers (Kamb et al, 1994;Sugiyama et al, 2011;Danielson and Sharp, 2013) which had already observed a significant dynamic response to changes in basal water pressure, and highlights that changes in basal conditions should not be excluded as an important influence on tidewater glaciers. The link between changes in air temperature and run-off, and changes in velocity may, however, be complicated.…”

Recent Advances in Our Understanding of the Role of Meltwater in the Greenland Ice Sheet System

“…In order for a dramatic change in the ice dynamics of grounded land-terminating catchments, the ice sheet will need to accelerate substantially in response to rising meltwater inputs. For that to occur requires the maintenance of hydraulically inefficient subglacial drainage and thus a positive relationship between meltwater input and ice motion [61,66]. In contrast, the weight of evidence suggests a seasonal increase in drainage efficiency beneath the GrIS [62,64] with no net acceleration on annual timescales [84,87].…”

“…For a number of stable tidewater glaciers in west Greenland, Howat et al [111] reached similar conclusions based on remote sensing of ice velocity 4-6 km from their termini. In stark contrast, Sugiyama et al [66] found a strong correlation between air temperature and ice velocity over two summers at lake-terminating Perito Moreno in Patagonia, indicative of a static hydrological system showing little seasonal evolution, and in which increases in surface melting consistently result in ice acceleration. At the ice sheet scale, Moon et al [112] found evidence for both behaviours.…”

“…That this evacuation is likely the result of efficient subglacial drainage is further supported by horizontal ice flow velocities which quickly drop to similar or below those observed prior to the establishment of surface to bed hydraulic connection, despite continuously high supraglacial melt rates [9,55]. If the drainage system had remained inefficient following lake drainage, velocity enhancement relative to predrainage values would instead be expected [61,66]. Nevertheless, the precise form(s) of these efficient drainage systems remains unclear with the rapid evacuation at the bed of supraglacially draining lake waters being proposed via both a transient subglacial turbulent distributed sheet [67] as well as drainage via hydraulically efficient channels [64].…”

Seasonal Variation of Drainage System in the Lower Ablation Area of a Monsoonal Temperate Debris-Covered Glacier in Mt. Gongga, South-Eastern Tibet

“…Recent observations and modeling studies on either mountain valley glaciers or ice sheets have highlighted the role of subglacial water in ice flow processes (e.g., [1][2][3][4][5]). Studies on polar glaciers have suggested that enhanced melting has induced ice-sheet flow accelerations [6], whereas some modeling analysis indicated that those accelerations were primarily determined by melting supply variability [7] and the speed-up may be offset by efficient subglacial drainage [8].…”