Abstract
Abstract. Over the past decade, satellite observations of ice surface height have revealed that active subglacial lake systems are widespread under the Antarctic Ice Sheet, including the ice streams. For some of these systems, additional observations of ice-stream motion have shown that lake activity can affect ice-stream dynamics. Despite all this new information, we still have insufficient understanding of the lake-drainage process to incorporate it into ice-sheet models. Process models for drainage of ice-dammed lakes based on conventional R-channels incised into the base of the ice through melting are unable to reproduce the timing and magnitude of drainage from Antarctic subglacial lakes estimated from satellite altimetry given the low hydraulic gradients along which such lakes drain. We have developed an alternative process model, in which channels are mechanically eroded into the underlying deformable subglacial sediment. When applied to the known active lakes of the Whillans–Mercer ice-stream system, the model successfully reproduced both the inferred magnitudes and recurrence intervals of lake-volume changes, derived from Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter data for the period 2003–2009. Water pressures in our model changed as the flood evolved: during drainage, water pressures initially increased as water flowed out of the lake primarily via a distributed system, then decreased as the channelized system grew, establishing a pressure gradient that drew water away from the distributed system. This evolution of the drainage system can result in the observed internal variability of ice flow over time. If we are correct that active subglacial lakes drain through canals in the sediment, this mechanism also implies that active lakes are typically located in regions underlain by thick subglacial sediment, which may explain why they are not readily observed using radio-echo-sounding techniques.
Highlights
Since the initial observation of “large, flat, circular basins” in the ice surface of Antarctica by Russian pilots during the International Geophysical Year (Robinson, 1964), there has been significant interest about the role of lakes within the larger ice-sheet system
Our experiments began with several tests comparing the simulated lake-volume time series output by the R-channel model against the volume change time series inferred from observations for subglacial Lake Whillans (SLW) (Fricker and Scambos, 2009; Siegfried et al, 2014)
In cases where runaway growth in outflow rate led to unrealistically large ice surface drawdown, the model was stopped once the lake level had decreased by over 30 m
Summary
Since the initial observation of “large, flat, circular basins” in the ice surface of Antarctica by Russian pilots during the International Geophysical Year (Robinson, 1964), there has been significant interest about the role of lakes within the larger ice-sheet system. Beginning in 1972, radio-echo sounding (RES) began confirming that these surface features reflect storage of free water at the ice-sheet base (Oswald and Robin, 1973) and subsequently continued to be a primary technique to identify subglacial lakes, including Lake Vostok, one of the largest freshwater lakes in the world (Ridley et al, 1993; Kapitsa et al, 1996). With increased availability of RES data, and as our ability to observe the ice surface precisely has improved, the number of known subglacial lakes in Antarctica has increased. Until the mid-2000s RES was the primary technique for identifying subglacial lakes (e.g., Siegert et al, 2005; Carter et al, 2007). Most of the lakes found in RES surveys tended to be located beneath the slowmoving ice near the divides (Fig. 1a), driving initial research questions on whether lakes were open or closed systems (e.g., Bell et al, 2002; Tikku et al, 2005), with considerable speculation about their impact on local ice dynamics (e.g., Dowdeswell and Siegert, 1999; Bell et al, 2007; Thoma et al, 2012)
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