BASIC MECHANISMS
3.3. THE LAST GLACIAL CYCLE
3.3.2. The Last Glacial Maximum
3.3.2.2. data acquired since CLIMAP
Since the theoretical CLIMAP reconstruction, based in rnany places on little data, more geornorphological and glacial geological observations have becorne available. Some pieces of new field evidence agree well with the CLIMAP reconstruction, but others irnply that sorne arnendrnents are necessary.
With respect to the Weddell Sea region, there appears to be little doubt that the late Wisconsin ice sheet extended to the edge of the continental shelf.
Elverhai (1981) describes dated sedimentological data which clearly suggest grounding of the Ronne-Filchner ice shelf down to a water depth of 500 m.
Further support for widespread grounding is provided by the presence of fresh striations and erratics on nunataks. These have been found at elevations up to 500 rn above the current ice surfaces along the Orville Coast (Carrara, 1981) and the Lassitter Coast (Waitt, 1983). Both areas lie On the east side of the southern Antarctic Peninsula, respectively adjacent to the Ronne ice shelf and the Weddell Sea. Sirnilar evidence for a thickening of 1000 to 2000 m has been reported at the head of the Ronne-Filchner ice shelf, based On trirnline elevations in the Heritage Range, Ellsworth Mountains (Rutford et al, 1980).
According to the latter study, the surface of the 'Whitrnore dome' of the West Antarctic ice sheet rose by 300
-
500 rn, but rernained at the Same location throughout the Wisconsin.Also in the Antarctic Peninsula area, Clapperton and Sugden (1982) postulate a late Wisconsin rnaxirnurn ice Cover in broad agreernent with CLIMAP, although their geornorphologic observations did not support ice flowing from the Peninsula axis across Alexander Island to the edge of the continental shelf in the West. Rather, they believe that separate ice dornes were centred over both Alexander Island and Palmer Land, with the ice converging into George VI Sound prior to flowing northwards. Grounded ice probably expanded to the continental shelf break elsewhere in the Pacific sector as well. This is indicated by the presence of basal tills in such critical locations as Marguerite Bay along the Antarctic Peninsula (Kennedy and Anderson, 1989) and Pine Island Bay in the Arnundsen Sea (Kellogg and Kellogg, 1987b). In the latter area, grounded ice filled the entire bay for distances of up to 400 km from the present ice sheet rnargin, although Kellogg and Kellogg emphasize that dating control was not firm.
Further support for the theoretical CLIMAP reconstruction cornes from additional, though still sparse, evidence for Wisconsin rnargin positions in East Antarctica. Adarnson and Pickard (1983) review data (rnainly glacial striations) for a late Wisconsin advance of the ice sheet across the Vestfold Hills region, an oasis near the Australian research station of Davis. The striations clearly indicate recent (25000-10000 years BP) overriding in a
direction pointing radially outwards. Also submarine terminal moraines seaward of the Ninnis and Mertz outlet tongues are in good agreement with the Stuiver et al. reconstruction (Domack et al., 1989). Here, the ice sheet expanded over a distance of 150 km to a position where the water depth is some 400 m below contemporary sea level.
In contrast, it appears that the controversy which centred around the history of the marine ice sheet in the ROSS Sea is still not resolved. The available data are still Open to a considerable degree of interpretation and at best only constrain a maximum and minimum reconstruction. The problem is that dated evidence on the seafloor is still lacking. The ice sheet margin therefore has to be estimated from the altitude of side moraines in the Transantarctic Mountains. Detailed descriptions of late Wisconsin drift sheets have recently been made for Beardmore glacier (Denton et al., 1989a) and Hathertonl Darwin glacier (Bockheim et al., 1989). In accordance with the CLIMAP interpretation, they both show late Wisconsin longitudinal profiles that are close to the current surface near the plateau, but rise far above the ROSS Sea ice shelf near the glacier mouth. A typical value for the late glacial surface elevation near the present grounding line is 1300-1400 m.
On the basis of these new ice suriace profiles, Denton et al. (1989b) have presented new reconstructions for a minimum and maximum extent of grounded ice in the ROSS Embayment. They are shown in fig. 3.6. They both depict grounded ice along the entire inner ROSS Embayment together with little change (compared to the present) of the inland plateau surface adjacent to the Transantarctic Mountains. The major difference between the two reconstructions lies in the areal extent of grounded ice in the outer ROSS Embayment and was due to different glaciological assumptions. The minimum reconstruction was based on the assumption that West Antarctic ice streams persisted at the last glacial maximum, hence the surface morphology of the ice flowing from West Antarctica did not change significantly. The maximum reconstruction assumes that ice streams did not exist during the late Wisconsin glaciation (Denton et al., 1989b). Compared to the CLIMAP reconstruction (fig. 3.5), both new reconstructions share a considerably less extensive and thinner grounded ice Cover. However, neither of the reconstructions in fig. 3.6 are based on ice flow models or make use of seafloor sediments, because they lack numerical chronology.
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W.., ,wo ea.3 t S , ~ I="fig. 3.6: Minimum (a) and maximum (b) reconstructions of grounded Wisconsin ice in the ROSS Ernbayment based On the glacial geology of the Transantarctic Mountains. The position of the ice shelf front in both diagrarns is arbitrary. From Denton et al. (1989b).
Figure 3.7 shows another sketch (not a numerical reconstruction) of the entire Antarctic ice sheet during the Wisconsin maximum, also by the Maine geology group (Denton et al., 1986). It takes into account some of the more recent geological data just mentioned and, where available, data on total-gas content from deep ice cores. The latter Parameter is closely related to the temperature and atmospheric pressure at the time of pore-closure, and hence, is indicative of changes in ice-surface elevation. Noteworthy differences between fig. 3.7 and the older CLIMAP reconstruction include a lower ice- sheet surface in West Antarctica, which features several divides with domes and saddles. Peripheral domes in the ROSS and Weddell Seas are common.
Also ice-surface elevations in inferior East Antarctica are little different from those of today. This is assumed to be a direct consequence of lower accumulation rates. The position of the ice sheet margin, however, is almost similar to the CLIMAP reconstruction.
fig. 3.7: Sketch of the entire Antarctic ice sheet during the LGM taking into account rnore recent glaciological and geological data. The outer heavy line shows the tentative reconstruction of the grounding line. Frorn Denton et al. (1986), who Stress that this figure is only a working sketch and not a nurnerical reconstruction.
Finally, in addition to the expansion of grounded ice, mention should be made of the ice extent in the Southern Ocean. From lithological changes, sedimentation rate changes and distribution of ice-rafted detritus in Antarctic deep-sea sediments, Cooke and Hays (1982) predict that the winter sea-ice entirely encircled by a vast ice shelf. According to their reconstruction, this ice shelf may have extended to 55's.
To summarize then, it seems that available geomorphological and glacial geological data suggest a late Wisconsin ice sheet which was grounded at the edge of the continental shelf. Such an ice sheet would have shown only minimal surface elevation changes over the vast East Antarctic polar plateau and have been completely surrounded by an enormous ice shelf. Only in the ROSS basin does the evidence not permit to reconstruct a maximum ice sheet with sufficient confidence. Here, the reconstructed late glacial extent varies from a position close to the continental break to several ice lobes with low surface slopes covering only the inner Embayment.