Discoveries in the Transantarctic Mountains The Sirius Formation Group

On land, wet-based glacial deposits at high elevations in the Transantarctic Mountains were providing new evidence of past Antarctic climate. The deposits had been found in the late 1960s, and named the Sirius Formation by Mercer (1972), who saw them as the products of temperate glaciation pre-dating the present ice sheet. However, Mayewski (1975) mapped and analysed more of these deposits, and thought they more likely represented a pre-Pliocene over-riding phase of the East Antarctic Ice Sheet. Later in the mid-1980s, yet another view was proposed by Webb et al. (1984) as the result of a laboratory study of samples collected earlier by Mercer, Mayewski and others and reported in detail by Harwood (1986). In this study, samples had been processed from tens of localities along the mountains. Some were barren but others yielded a variety of types and ages of microfossils with some as old as late Cretaceous and a few marine diatoms as young as Pliocene. The latter fossils were thought to provide a maximum age for the Sirius Formation, having been deposited in East Antarctic interior seas and subsequently transported by a growing ice sheet flowed through the mountains at levels more than 1,000 m higher than today's outlet glaciers. The discovery of plant material, and later leaves and stems, in the deposits (Webb and Harwood, 1987, 1993; Francis and Hill, 1996; Hill et al., 1996) indicated a mean annual temperature almost 20°C warmer than today. Mercer's (1978) case for a collapse of the West Antarctic Ice Sheet through a doubling of atmospheric CO2 had heightened interest in ice-sheet behaviour, but the loss of the East Antarctic Ice Sheet in recent geological times seemed less credible to many, who queried the age of the Sirius Group from recycled diatoms. Indeed, some (e.g. Clapperton and Sugden, 1990) queried the age of the diatoms themselves. However, the Pliocene age of the diatoms was confirmed by dating a volcanic ash in mudstone with the same taxa and cored in CIROS-2 (Barrett et al., 1992).

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