Spectacular cliff section hundreds of metres high at Cape Washington, constructed of multiple lava sequences formed during subglacial eruptions 2.8 million years ago.
The death of the Sirius debate
11 July 2014
John Smellie (right) explains how research on Antarctic volcanoes may have resolved a 30-year-old controversy over the history of the continent's ice sheet.
A debate about the historical stability of the Antarctic ice sheet has raged for decades, splitting the community of scientists studying the ancient climate along increasingly partisan lines. It concerns the evolution of the ice sheet's thermal condition - that is, how fixed it is to its bed, how stable it is, and how it has contributed to global sea levels through geological time.
Our efforts to settle these questions have included several expensive offshore and onshore drilling campaigns. Yet until recently, no resolution has been in sight. Now the definitive answer has come from an unexpected direction - from studies of Antarctic volcanoes that have erupted under the ice cap.
It may seem counterintuitive that volcanic hot spots can tell us about past ice sheets, but studying them has revealed for the first time that the ancient Antarctic comprised a mosaic or patchwork of warm and wet ice alongside areas of cold ice frozen to its bed. This is called a polythermal ice sheet and the result, just published, represents a radical change in our thinking.
The author standing in front of subglacially erupted volcanic sequences at Cape Washington.
The old view was that the ice sheet has existed over time in one of two major thermal states: it was either warm, wet and mobile or cold, stable and frozen to its bed. This occurred even though the past world was mainly warmer than the present, because thermal regime is affected by a variety of factors such as how thick the ice is, the amount of snow accumulating at any time and now much heat is escaping from the world's interior. Warm mobile ice can react more rapidly than cold ice to changes in the global climate, feeding ice into the ocean and contributing to sea-level rise.
Until now it was thought this ice sheet made a sudden and irreversible change in its thermal state in the distant geological past. Arguments have raged about the timing of that change. Either it occurred 14 million years ago and the ice has been an essentially stable monolith since then, or the change was much more recent, at around 2.5 million years ago when the world plunged into the Pleistocene ice age. If the second possibility is true, it implies the ice sheet might be much more responsive to climate change and could therefore become unstable in a warming world.
Thus, the idea that even the supposedly highly stable East Antarctic Ice Sheet might be capable of reverting to a warm and mobile regime has major implications for mankind. Are we facing an unexpected, rapid rise in sea level as the world heats up?
A Sirius question
Geologists call this controversy the Sirius debate after a group of important glacial deposits found in many places along the 2000km-long Transantarctic Mountains: the so-called Sirius Group. The Sirius Group came to prominence in the 1970s because there is abundant evidence that it formed under the influence of wet-based or warm ice, in a region that is today characterised by extreme cold. If we could get reliable ages for the Sirius Group deposits, they might shed important light on the thermal evolution of the world's greatest ice sheet. But this has never been possible.
Although the Sirius rocks contain fossils, attempts to identify them have yielded frustratingly ambiguous ages. Related studies aimed at dating rocks that are exposed on the surface have also provided conflicting results. Even multiple offshore drilling campaigns to examine the sedimentary record of environmental change have not resolved the controversy.
It was clearly time to try something more radical. Victoria Land, which includes a large segment of the Transantarctic Mountains, contains many large central volcanoes active during the past 15 million years, including ones that remain active today such as Mount Erebus and Mount Melbourne. These volcanoes show clear evidence that they erupted under ice.
View of Adare Peninsula, a very remote extinct shield volcano erupted beneath a cover of glacial ice 3.9 million years ago.
Such eruptions are known as glaciovolcanic. The study of glaciovolcanism is a very young geological science, only achieving prominence in the past decade - it was only defined for the first time in 2006. Since then it has developed rapidly into an important source of information about the past environment that can provide more insight into the history of the ice than any other method.
Working closely with other researchers from Italy and the USA, I spent three seasons in Antarctica systematically examining the Victoria Land volcanoes and their environmental record. Most of them are very remote and hard to visit, but the rewards are great. The volcanoes are exceptionally well exposed in cliff sections nearly 1000m high and extending many kilometres.
Using helicopters and often working from the sea ice at their foot, we accessed the volcanoes and obtained an unrivalled and hitherto completely unknown record of Antarctic ice, its thickness and thermal state.
Unlike the sediments of the Sirius Group, volcanic rocks can be easily and precisely dated. This is because they contain radioactive minerals, whose 'radioactive clock' began ticking from the moment they erupted and quickly cooled. Sedimentary rocks contain mixtures of radioactive minerals indiscriminately eroded from a variety of sources, all of which are older than the time when the sediment was deposited, so we cannot date them in the same way. We hoped our study of the volcanic rocks would let us verify the step change in the ice sheet's thermal state, and obtain the first reliable and exact age for that change.
The results were a revelation. Unexpectedly, we began discovering evidence that the thermal state switched back and forth between warm and cold conditions and it also varied from place to place. In other words, ancient Antarctica was polythermal - a patchwork of thermal states. We were stunned. The paradigm that had existed for many decades had to change.
In some respects the result was less surprising than it initially seemed. After discovering many subglacial lakes in Antarctica, we now know that the modern ice sheet is polythermal, including in East Antarctica. But this is the first clear sign that the ice sheet has probably been polythermal throughout its history, or at least for the last 12 million years.
Our results are just in, and they are still being assessed. Modelling the future flow of an ice sheet with such variable thermal states may be extremely difficult. Although we now know that the ice was polythermal, it will be some time before we have a detailed idea of when, where and how the changes happened. In the light of our discoveries from the volcanic studies, recreating a poorly-defined patchwork ice sheet that is slippery and fast-moving in one area while cold and slow in others is mathematically very complicated.
However, our studies have clearly demonstrated that volcanoes are a major repository for environmental information that was largely ignored until recent years. That is a mistake we will not make again - we will be back in Antarctica in 2014 further evaluating past conditions of the Antarctic ice sheet.
John L Smellie is a professor in the Department of Geology at the University of Leicester, specialising in glaciovolcanism.http://planetearth.nerc.ac.uk/features/story.aspx?id=1672&cookieConsent=A
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