Isostatic Modelling of Post Glacial Uplift and Contribution to Global Sea Level

The Earth's crust is displaced by the loading of ice sheets, which recovers when these loads are taken away. In Antarctica, the pattern of recovery following removal of an ice load has been reconstructed by investigating and dating records of relative sea level, such as of emergent shorelines (e.g., Adamson and Pickard, 1986; Baroni and Orombelli, 1994), the transition from marine to freshwater sediment deposition in enclosed basins (Goodwin, 1993; Zwartz et al., 1998; Verleyen et al, 2005) and by determining the present day rate of crustal motion by measuring the change in height of bedrock surfaces over time using a Global Positioning System (GPS; e.g., Tregoning et al., 2000).

These uplift data have been compared with the response predicted by numerical Earth models to constrain the spatial and temporal pattern of former ice loads. However, some of the parameters (such as mantle viscosity) used in the models are not independently known, and it is often possible to model the measured isostatic response at any one site with a range of former ice loads (e.g., Bassett et al., 2007). Thus, care must be taken when interpreting these results in regions like Antarctica where relative sea level reconstructions are few and far between.

Despite these issues, the technique is useful in constraining former ice heights where evidence is ambiguous, or difficult to reconstruct by other means. This has resulted in numerous reconstructions of former ice heights across the continent (Nakada and Lambeck 1988; Tushingham and Peltier, 1991; Colhoun et al., 1992; Peltier, 1994), which provide a very large range of post-LGM sea level contributions from Antarctica, of between 2 and 37 m. The more recent models, which incorporate a greater degree of constraints derived from the geological studies detailed above, provide estimates of 17.3 m (ICE-5G model of Peltier, 2004) and 6.6-16.7 m (ANT5 and ANT6 of Nakada et al., 2000).

These models have also been used to provide estimates of former ice loads at individual sites around the continent. Zwartz et al. (1998) used relative sea-level data from the Vestfold Hills to estimate that the ice sheet retreated 30-40 km, and thinned 600-700 m in this region since the LGM. Goodwin and Zweck (2000) used uplift information from raised beaches and the marine-lacustrine sediment transition in basins in the Windmill Islands and Budd Coast as input to an isostatic model of the Law Dome ice cap in East Antarctica. Their numerical Earth model predicts that the area near the present day margin of Law Dome formerly hosted an ice load of between 770 and 1000 m, and that ice expanded out 40 to 65 km across the continental shelf (Goodwin and Zweck, 2000). Goodwin and Zweck (2000) also used the similarity in the height of the marine limit along the coast between Wilkes and Oates land to postulate that the East Antarctic Ice Sheet in this area has retreated by a similar amount since the LGM.

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