Pleistocene Ice Sheets

There have been several models of recent (last glacial cycle) ice-sheet behaviour (e.g. Ritz et al., 2001; Huybrechts, 2002). Huybrechts' model demonstrated the differences in behaviour between East and West Antarctica over the last 200,000 years, and showed these differences to be controlled to a large degree by topography (Fig. 6.5). The West Antarctic Ice Sheet is known as a marine-based ice sheet, with a bed that is well below sea level in most places (mean bed elevation 400 m). This is in contrast to the terrestrial ice sheet in East Antarctica, where the bed elevation is generally above sea level (mean bed elevation ~+50m). Marine ice sheets are thought to be susceptible to greater change than terrestrial ice sheets as a consequence of (1) grounding line retreats caused by the disintegration of ice shelves, (2) the deepening of the bed inland of ice-sheet margins, which can lead to a positive feedback of ice decay, (3) the level to which the ice sheet is held buoyant by the surrounding sea, which may be enhanced if sea level rises, and (4) the nature and stability of ice streams, which may be underlain by weak marine sediments leading to enhanced flow speeds and complex dynamics.

In the Pleistocene, the lower topography in West Antarctica was associated with significant fluctuations in ice-sheet size in contrast with a relatively stable East Antarctic Ice Sheet. While the ice stream configurations in East Antarctica remain largely similar (although they may reduce in flux), significant adjustments to the ice flow regime are predicted in West Antarctica. Consequently, ice flow in East Antarctica is associated strongly with topography, in which ice streams are most often constrained in topographic channels. In West Antarctica, the association between flow and topography is less clear. Here, ice streams flow over relatively flat terrain where weak water-saturated sediments dominate. Lateral migration of ice stream margins, without topographic controls, is entirely feasible in such places. The outcome is that the spatial pattern of glacial erosion and deposition in East Antarctica appears to have been relatively consistent over the past few glacial cycles, and indeed back through several million years, whereas substantial changes are likely to have occurred in West Antarctica.

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Figure 6.5: Numerical modelling of Antarctic ice surface elevation during the last glacial interglacial, including the ice-sheet configuration at the last glacial maximum. Taken from Huybrechts (2002) with permission.

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Although Huybrechts' (2002) investigation represents the most advanced numerical analysis of Pleistocene ice sheets in Antarctica, the model used has several limitations that prevent it from resolving ice streams well, or ice shelves at all. Increasing the complexity of the model to solve this issue might not lead to improved results, however, due to greater uncertainties that remain in terms of bed configuration and accumulation rates. The lesson to be learned is that glaciologists have an important choice concerning model complexity and the robustness of the model's outputs, which must be made through consideration of the aims of the investigation. A broad-scale reconstruction at a continental level may still be best suited to a simple model, whereas information concerning time-dependent changes in discrete ice-sheet processes might be better served through a high-order model.

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