Six million years of environmental (glacial – interglacial) conditions preserved in volcanic lithofacies of the James Ross Island Volcanic Group, northern Antarctic Peninsula

The Neogene geological record in the James Ross Island region (northern Antarctic Peninsula) is dominated by the products of at least 50 mainly effusive basaltic volcanic eruptions that are preserved predominantly as lava-fed deltas and a smaller number of tuff cones. The volcanism was persistent over more than 6 million years resulting in construction of an extensive volcanic field and one of the largest and longest-lived stratovolcanoes in Antarctica. Most of the eruptions took place during glacial periods, and interpretation of the deltas has enabled critical parameters of the palaeo-ice cover to be deduced for the first time, for multiple time slices. However, the resolution of 40Ar/39Ar dating of young basaltic lavas is relatively poor compared with the duration of glacial—interglacial periods and precludes any Milankovitch-scale cyclicity being identified - a problem that is now becoming acute in palaeoenvironmental investigations of this type. The period was characterised by a relatively thin glacier cover in this area, typically just 200-350 m, interspersed with fewer periods of thicker ice c. 600-750 m in thickness. The glacier cover increased in thickness toward the present. Significantly, no evidence was found for the “giant” ice sheets predicted by some studies, at any time during the last 6 m.y. The glacier cover was formed predominantly of ice (sensu stricto) that was wet-based, erosive and probably sub-polar (polythermal). If it reached the continental shelf edge, it must have had a low profile dominated for most of the period by a local ice cap that draped James Ross Island and was presumably confluent with the Antarctic Peninsula Ice Sheet along its western margin. These results are the first evidence for the morphology, thickness and thermal regime of the glacier cover in the northern Antarctic Peninsula region for the late Neogene period.

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