An example from the Ford Ranges of West Antarctica

The Ford Ranges of West Antarctica consist of scattered nunataks that separate large outlet glaciers draining the interior of the West Antarctic Ice Sheet into the Sulzberger Ice Shelf (Figs 43.1 & 43.2). Ice-free surfaces in the Ford Ranges consist mostly of steep slopes of bare granite and phyllite bedrock or locally derived talus and blockfields. Striated surfaces at all elevations indicate past overriding by the ice sheet. Ice retreat is recorded only by a few patches of thin till and by erratic cobbles scattered on rock surfaces (Fig. 43.2). We collected such erratic cobbles from a range of elevations on several nunataks, measured 10Be concentrations, and found that they yielded a precise record of continuous Holocene lowering of the ice-sheet surface and consequent exposure of the peaks (Stone et al., 2003; Fig. 43.2). Analyses of adjacent cobbles from a few sites agreed within analytical uncertainty, and cobbles very close to the ice margin had exposure ages of only a few hundred years, reinforcing the idea that these cobbles arrived at the ice margin with no inherited nuclide inventory. Sets of cobbles from single nunataks yielded smooth deglaciation histories, and adjacent nunataks produced similar results, indicating that the cobbles had not been significantly covered by snow or sediment, and had not moved since they were emplaced. A few nunataks, however, yielded more complicated results. At Mount Rea (Fig. 43.2) we found that samples at lower elevations, as well as the youngest sample near the summit, recorded the same Holocene deglaciation as on nearby nunataks. At higher elevations, we found many erratics that were physically indistinguishable from Holocene erratics, but had very much greater exposure ages. Some of these old erratics also had 26Al/10Be ratios below the surface production ratio of 6.1, indicating that they had been covered by ice for long periods (Cowdery, 2004; also see Stroeven et al., this volume, Chapter 90). It appears that these erratics were deposited during long-past episodes of ice retreat, and remained undisturbed by subsequent ice advances: their 10Be concentrations integrate several periods of exposure in addition to the most recent one. These repeatedly exposed erratics persist through multiple glacial-interglacial cycles, not only because of the ineffectiveness of weathering processes that might degrade or destroy them during ice-free periods, but also because of the common occurrence of cold-based ice at higher elevations where overriding ice was thinner. Although some parts of Antarctica (the McMurdo Dry Valleys in particular, where surfaces have been exposed for millions rather than thousands of years, and weathering and periglacial disturbance are correspondingly more important) present additional complexities in interpreting cosmogenic-nuclide measurements, much of Antarctica is similar to this example from the Ford Ranges. The near absence of ice-free areas that might supply pre-exposed erratics, the minimal ice-marginal sediment accumulations, and the very slow rates of weathering provide the best possible environment for reconstructing ice-

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Figure 43.2 Exposure ages of glacial erratics from the Ford Ranges. Photographs: (A) View of the western Sarnoff Mountains and the Mount Rea massif from the east. (B) View of Mount Darling seen from the south. Ice flow is from right to left. (C) Typical granite erratic, resting on granodiorite, in the eastern Sarnoff Mountains. Panels to right: 10Be exposure ages of erratic cobbles from Ford Ranges nunataks. These data were previously published in Stone et al. (2003) and Sugden et al. (2005).

sheet history by exposure-age dating. If the goal is to accurately reconstruct the most recent deglaciation, it is important to keep in mind that glacial erratics may survive many ice advances and retreats without being disturbed. On the other hand, these per sistent erratics are a unique source of other useful information: evidence of the past subglacial temperature distribution, and a potential record of not one but many past ice advances and retreats.

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