Six million years of glacial history recorded in volcanic lithofacies of the James Ross Island Volcanic Group, Antarctic Peninsula

Basaltic volcanism in the James Ross Island region has been persistent over the last 6 million years resulting in at least 50 mainly effusive eruptions that are preserved predominantly as lava-fed deltas and a small number of tuff cones. Most of the eruptions took place during glacial periods, and the deltas have enabled the characteristics of the palaeo-glacier cover to be deduced for the first time, for multiple time slices. 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. Our results indicate that glacial periods were characterised by a relatively thin glacier cover in this area, typically just 200–350 m. They were interspersed with fewer periods of thicker ice c. 600–750 m in thickness. These are likely maximum estimates and they may be too high by a few tens of metres. The glacier cover increased in thickness toward the present. However, as evidenced by 4.6 myr-old surfaces at c. 620 m a.s.l. that are glacially unmodified other than frost shattering, no evidence has been found for a substantially thicker ice sheet at any time during the last 6 myr. The glacier cover was formed predominantly of ice (sensu stricto) that was wet-based and erosive. Thus it had a temperate or, probably more likely, sub-polar (i.e. polythermal) thermal regime and, if the ice reached the continental shelf edge, it must have had a low profile. After an early history (c. 6.2–4.6 Ma) dominated by an areally extensive Antarctic Peninsula Ice Sheet (APIS), the local glacial morphodynamics were determined by a local ice cap that draped James Ross Island and was presumably confluent with the APIS 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–Quaternary period.