| Using Volcanoes to Track Ice Sheet History |
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My name is John Smellie and I am the Senior Volcanologist working for the British Antarctic Survey. Together with my colleague, Jo (Joanne) Johnson, a Geochemist, we are investigating a volcano in Antarctica, known as James Ross Island. For me, it is the third and final season spent working on the island; Jo has completed two of the three. Because of difficult access conditions, each season was supported entirely by HMS Endurance, using helicopters. The James Ross Island volcano is situated at the northern tip of the Antarctic Peninsula, which is the conspicuous long narrow landmass that points north towards southern South America. The island is about 1500 m high and 40-60 km wide. It is almost entirely clothed in snow and ice, with thicknesses of 100-400 m, except for several large almost snow-free areas mainly on its western and northern periphery. Our purpose in investigating this volcano is to discover more about its very poorly known eruptive history and to extract detailed evidence for climate change from the volcanic rock record preserved there.
Most of the volcanic formations on James Ross Island are what are called volcanic deltas, in which rivers of lava have poured into water. Just as in sedimentary deltas, each formation on James Ross Island preserves a fossil water level, and features of that fossil water level give us critical information that enable us to interpret whether that water was the sea or ponded water derived from melting an ice sheet. For example, this year we mapped a rock formation that showed the fossil water level moving up and down through 100 m, many times in rock faces only a few hundred metres long. As eruptions on James Ross Island probably last no more than a decade or so, the fluctuating water cannot be the sea– sea levels take thousands to millions of years to vary significantly. Anyway, 100 m of relative sea level change would probably involve simultaneous melting of both polar ice caps, an extremely unlikely event. So the formation was certainly erupted in association with an ice sheet that was more extensive than at present. Other formations showed similar evidence, less well displayed, whereas still others contain sedimentary beds with marine fossils that indicate that those deltas flowed into the sea and the ice sheet was much reduced in extent. So our story so far, which still has gaps and needs laboratory follow-up studies, is already indicating that the Antarctic Ice Sheet in this region has had a dynamic history of advances and retreats during the past 6 million years. It is demonstrably unstable and our first thoughts suggest that we should be concerned about the effects of current global warming. However, our study is also suggesting that ice in the Antarctic Peninsula region has always been quite thin, probably only a few hundreds of metres thick. This is contrary to previous suggestions that huge ice sheets formerly overrode the region – ice that may supposedly have been more than 1000 m thick. So despite its likely sensitivity and fragility, our results suggest that we are probably not facing a global deluge from melting. We can still sleep safely – perhaps! |
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