A solution exposureage dating with cosmicrayproduced nuclides

The chief recent advance in understanding the history of the Antarctic Ice Sheets, therefore, has been the development of a dating technique that is perfectly suited to the Antarctic landscape: exposure-age dating with cosmic-ray-produced nuclides. This relies on the measurement of rare nuclides such as 10Be, 26Al and 3He, which are produced within mineral grains by cosmic-ray bombardment of rocks exposed at the Earth's surface. These nuclides are useful for dating ice-marginal deposits because nearly all cosmic rays stop within a few metres below the rock (or ice) surface, so any clast that is quarried by subglacial erosion at the bed of the ice sheet and brought to the ice margin arrives there with a negligible nuclide concentration. The surface production rate of these nuclides varies in a predictable way with altitude (Stone, 2000), and once this is determined, the nuclide concentration in an erratic cobble or boulder is related only to the duration of subsequent surface exposure, that is, the time since the ice margin lay at that position. Thus any erratic, lying on any previously glaciated surface, is a record of the past ice-sheet configuration. Inferring deglaciation ages from nuclide concentrations in erratics relies only on the two assumptions that the rock samples of interest have been emplaced with zero nuclide concentration, and that they have not been eroded, moved, or covered with a significant thickness of soil or snow since exposed by ice retreat. In principle, these assumptions could be true of bedrock surfaces eroded subglacially and exposed by deglaciation as well as erratic clasts. The important difference is that, in practice, there is generally no assurance that subglacial erosion was sufficient to remove any nuclide inventory that might date from a previous period of exposure (e.g. Briner & Swanson, 1988). Cosmic-ray-produced nuclide concentrations in bedrock surfaces can provide some information about glacial history (e.g., Stroeven et al., this volume, Chapter 90); however, they are hard to interpret. For Antarctic erratics, on the other hand, both assumptions are nearly always met. First, the extreme scarcity of exposed rock, and near absence of supraglacial debris, in most of the continent means that any glacially transported clast was almost certainly derived

from the glacier bed and thus will have negligible inheritance. In the case of erratics of lithologies that do not crop out at all above the ice surface, this assumption is always true. Second, once such a clast arrives at the ice margin, the typical occurrence of only thin and patchy glacial deposits, and the windy, arid climate of most nunataks, mean that it is unlikely to be covered by soil or snow, and the extremely slow rates of erosion ensure that it is preserved unmodified for very long periods of time.

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