Icestream instability

Despite the shortcomings noted in an earlier section, two recent assessments using the credit/debit approach (Joughin & Tulaczyk, 2002; Rignot & Thomas, 2002) found one additional basin that is significantly out of balance. Both studies considered the part of the West Antarctic Ice Sheet that drains through the five major ice streams of the Siple Coast (Fig. 42.8), an area beyond the coverage of current satellite altimetry. Although, the studies disagree in the detail of their assessments and their estimated uncertainty, taken together, these studies indicate that most of the ice streams (A, Whillans, D, E and F) are not unambiguously out of balance at this time. Both studies do, however, agree that the basin of Ice Stream C is thickening at a mean rate of around 14cmyr-1— a significant fraction (38%) of the total annual accumulation in this basin.

Although this represents a fairly gross imbalance, it was not entirely unexpected. Evidence of buried crevassing from the downstream portion of Ice Stream C, which is clearly visible in ice-penetrating radar data, suggests that the ice stream effectively shut down around 140-150yr ago (Retzlaff & Bentley, 1993; Anandakrishnan et al., 2001). The upstream parts of the glacier, which were once tributaries to the main glacier, are still active and the log-jam between active and stagnant ice flow is likely to be causing an area of considerable thickening, perhaps approaching 1myr-1. The apparent rapidity with which flow terminated has been a puzzle to glaciologists for some time and the observation of this single glacier has been most influential in driving the study of basal conditions beneath ice streams. Anandakrishnan et al. (2001) usefully summarized the several mechanisms that have been proposed to explain the stagnation of Ice Stream C, and although several of these mechanisms can now be discounted, it appears that some combination of diversion of subglacial water, evolution of thermal conditions at the bed, and possible changes in the distribution of subglacial 'sticky-spots' was responsible.

Although Ice Stream C remains the only example of a dated ice stream switch-off, there are other glaciers (e.g. Carlson Inlet, Doake et al., 2001) that may well have behaved in a similar fashion. Even if it were alone, the significance of the stagnation of Ice Stream C is undeniable; it is evidence that an ice-stream basin, once close to balance and which might have looked near to steady-state, changed rapidly and in a direction away from balance. This implies that 'non-linearity' in the internal dynamics of the ice sheet was sufficiently strong to cause reorganization of flow away from the stable state. Such non-linear behaviour (i.e. a response that is not proportional to its cause) is the first requirement for a system to become chaotic. This potential for chaotic behaviour is important because it implies that the apparent proximity to balance, which we see in much of the ice sheet, cannot be taken to imply that rapid and unpredicted changes in flow will not occur.

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