Basaltic Glacivolcanic Sequences: Under-utilised Proxies Of Global Change

Basaltic eruptions beneath glaciers have unique characteristics, and the resulting deposits and landforms can be used to document the presence, distribution and properties of glaciers over geological time. Many glacier characteristics are uniquely preserved in glacivolcanic sequences for pre-Quaternary periods. They are therefore important proxies for palaeoenvironmental conditions, yet they are generally neglected, probably through a lack of awareness by the geological community. At least five different types of basaltic volcanic edifices have been identified, each with differing usefulness for deciphering past ice sheets. They comprise sheet-flow sequences (2 types), pillow volcanoes, tephra mounds and tuyas. The distinctive characteristics of each glacivolcanic sequence are largely determined by the simple presence of surface ice and its characteristics (thermal regime, rheology, hydraulics). Significant advances in our understanding of products of subglacial eruptions have been made particularly over the past decade, but few glacivolcanic investigations have addressed global change issues. Neogene volcanic sequences are common in Antarctica, the region that contains the world's most important and longest-lived ice sheet. Interpretations of two, situated on east and west flanks of northern Antarctic Peninsula, illustrate the potential power of glacivolcanic research. Sequences on James Ross Island (< 7 Ma) are tuya-like lava-fed deltas and tephra mounds formed mainly in association with wet-based ice sheets, none of which exceeded 600 m in thickness, alternating with ice-poor periods of marine inundation. By contrast, Quaternary (< 300 ka) volcanism on Brabant Island is represented by multiple sheet-flow sequences indicative of an even thinner wet-based ice cover (not exceeding 100-150 m). The evidence for wet-based ice sheets throughout Late Neogene and Quaternary time is novel, particularly considering the cold polar regime prevailing today, with glaciers frozen to their beds. The new information also provides the only direct evidence that Neogene Antarctic Peninsula ice sheets had very low profiles during periods of extension to the shelf edge.