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Ice Sheet History

John Smellie and Jo Johnson
John Smellie and Jo Johnson

Using volcanoes to track Ice Sheet history

Recovering survey equipment by helicopterRecovering survey equipment by helicopter 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.

The volcano began life at least 6 or 7 million years ago and, although it has never erupted in historical times, we now believe that it may still be very much alive and merely "sleeps" for exceptionally long periods, perhaps a few hundred thousand years between eruptions. Our results from this season's fieldwork have enabled us to correlate the individual volcanic rock formations these are the "building blocks" of the volcano. We can now connect them up across large parts of the island and our laboratory studies will produce precise dates for each eruption. The volcano's life also includes a period known as the "Warm Pliocene", during which the world was significantly warmer than now, on average by perhaps 3 or 4 degrees Celsius. Those are the kinds of temperature increases that we are currently facing globally during the next 50-100 years, and we should like to know what sort of effect they might have on the world. In particular, we wish to know: what kind of world are we facing and how might the warmer temperatures influence the stability of the Antarctic Ice Sheet? Melting of the entire ice sheet in Antarctica (which is an extremely unlikely event) would raise sea levels globally by about 70 m. However, long before that might happen, the more climatically sensitive parts of the ice sheet, particularly in the Antarctic Peninsula, would disappear first. The situation of the James Ross Island volcano, in a relatively northerly position peripheral to the main ice mass, is particularly fortuitous. It is where the slightest effects of a contracting and expanding ice mass should be most obvious and therefore likeliest to be preserved in the rock record.

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!

Read all the latest news about the ship's deployment by following the links below, with additional pictures in the Picture Gallery.. HMS Endurance Tracking Project pages