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Glaciovolcanic Studies: a Pivotal Role in Characterising Past ice Sheets [*John Smellie*] (British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK; ph: +44 1223 221418; fax: +44 1223 362616; email: [email protected]) Glaciovolcanic sequences can be used as proxies for a uniquely wide range of palaeo-ice parameters. They are thus potentially highly useful archives of palaeoenvironmental information, particularly for pre-Quaternary periods, and the parameters derived can be incorporated by climate and ice sheet modellers in the same way as other environmental proxies. However, despite their unique potential, there are still few published environmental studies using glaciovolcanic methodology. On Earth, at least nine different types of terrestrial subglacial volcanic successions can be identified using landform characteristics, lithofacies and sequence architecture. They include mafic tuyas, felsic tuyas (2 types), pillow mounds/ridges, pillow sheets, tephra mounds/ridges, felsic lobes/domes, and subglacial sheet-like sequences (2 types). When tectonic influences are removed (e.g. eruptions from fissures), the different landforms are relatively well distinguished on morphometric diagrams using height versus width (i.e. aspect ratio). The potential for each landform and/or succession type for yielding useful environmental information is variable and is poorly known for several. Conversely, because of the abundance of terrestrial basaltic volcanism, the only mature palaeoenvironmental studies published so far are those that have focused on products of mafic magmas. This is illustrated here by using the results of a five-year investigation of the Antarctic Ice Sheet using information from basaltic sequences in northern Antarctic Peninsula. It is the most detailed palaeoenvironmental glaciovolcanic study conducted to date. The James Ross Island Volcanic Group is a large (c. 6000 km2) basaltic volcanic field situated in northern Antarctic Peninsula and dominated by the long-lived (> 6.25 m.y.) Mount Haddington stratovolcano. It is in a pivotal position (northerly latitude, low elevation) to record the dynamics of the adjacent Antarctic Peninsula Ice Sheet. At least 50 eruptive phases have been identified and Mt Haddington is now the most intensively isotopically dated volcanic centre in Antarctica. Eruptions were mainly of large-volume (individually tens of km3) lava-fed deltas, which gave the volcano its low-profile shield-like form. Most of the deltas show well preserved and beautifully displayed structural features diagnostic of eruption in association with erosive, wet-based ice, but a few deltas and several tuff cones were also probably constructed in the sea during interglacial periods, particularly during the early Pliocene. More than 90% of eruptions took place in association with a glacial cover. However, because of high errors associated with dating young Kpoor basalts, it is currently impossible to determine at what stage(s) in any glacial cycle eruptions took place. The lack of a precise dating method is now a major factor limiting further environmental research in these and similar sequences worldwide. However, the overwhelming evidence for a former ice cover affecting most eruptions suggests that both glacial and interglacial conditions are being preserved. Thus, the so-called interglacial periods are best regarded as icepoor rather than ice-free, and may have been similar in appearance to today. The thickness of the glacial cover was typically just 200-300 m, with maximum thicknesses of about 700 m achieved rarely. The ice sheet would thus have had a low profile, with a large dome centred on Mount Haddington dominating the local morphodynamics. There is no evidence for a giant ice sheet at any stage during the eruptive period. These results are the first evidence for the morphology, thickness and thermal regime of the Antarctic Peninsula Ice Sheet for pre-Quaternary periods. The study illustrates the utility and importance of the new glaciovolcanic methodology as an important palaeoenvironmental proxy. It is uniquely capable of yielding more quantifiable parameters of past ice sheets than any other methodology currently used. ORAL Abstract for VI-2, session on “Using glaciovolcanic rocks to constrain former ice conditions and palaeoclimate”, Vancouver, 19-22 June 2007