Planet Earth Online

Greenhouse gases shrouded Snowball Earth

12 January 2009

New evidence from ancient Arctic rocks suggests that high levels of carbon dioxide in the atmosphere - normally associated with a warm climate - coexisted with the vast ice sheets.

Scientists have speculated for some time that an ice-bound Earth would lead to high atmospheric CO₂ levels, but the theory lacked independent evidence. Now they have it in the form of rocks from Svalbard - a collection of islands between Norway and Greenland - that date from around 630 million years ago, when researchers believe the world was coming to the end of a so-called Snowball Earth period.

The paper, published in the American journal Science, states that 'the atmosphere [630 million years ago] had either an exceptionally high atmospheric CO₂ level or an utterly unfamiliar oxygen cycle.' Both possibilities fit with an Earth largely covered in ice and snow.

The team carried out chemical analyses of limestone rocks that contained traces of sulfates.

Co-author, Professor Ian Fairchild from the University of Birmingham, says, 'We found several aspects of the chemistry of the rocks that broke world records.'

'For example, some dolomites [limestone that contains magnesium carbonate] showed a strange chemical signal. They contained the heaviest oxygen yet found on Earth's surface, attesting to a hyper-arid glacial environment,' explains Fairchild.

Researchers piecing together ancient climates use the ratios of different oxygen isotopes to help complete the puzzle. Oxygen on Earth is made up of three stable isotopes, ¹⁶O, ¹⁷O and ¹⁸O. By far the most abundant is ¹⁶O making up 99.763% of all oxygen, the heavier ¹⁸O takes up 0.1995% and ¹⁷O makes up the balance, 0.0375%.

The ratios of these isotopes provide a telling picture of past climates. During ice ages, the lighter ¹⁶O evaporates first from the colder oceans, leaving the heavier ¹⁸O behind.

Extreme aridity

In the Svalbard rock samples, the scientists found this process went to extremes in small lakes in the barren, dry landscape surrounded by glaciers. The situation could be compared to the conditions in the McMurdo Dry Valleys in Antarctica today, but probably at much lower latitudes - the valleys are famous for their exceptionally dry air.

The heavy oxygen found by the team contained more of the weightier ¹⁸O than any sample yet analysed, indicating extreme aridity, just like the Dry Valleys.

In the same rock collections, the researchers also found significantly less ¹⁷O than expected. To date, scientists have largely ignored ¹⁷O because it was not thought to provide any additional information than that provided by ¹⁸O.

But recent research by lead-author Huiming Bao from Louisiana State University - published in the journal Nature in 2008 - showed for the first time that high levels of atmospheric carbon dioxide can lead to a drop in ¹⁷O in atmospheric oxygen. Scientists can locate this drop in sulfate (so4) trapped in rocks.

Other research indicates that atmospheric greenhouse gas levels were very high following the Snowball Earth episode 630 million years ago. The question is: did the greenhouse gases build up while Earth was glaciated? Or was there a huge release of methane (a greenhouse gas more potent than CO₂) following the ice age which led to elevated levels?

Fairchild, who reviewed Bao's Nature paper late in 2007, realised the significance of Bao's work to his own.

'Having refereed his paper, I contacted him. I thought we had the perfect samples to apply his technique,' explains Fairchild.

In the early 1980s, Mike Hambrey (now at the University of Aberystwyth), Fairchild and colleagues including Nick Cox, now NERC's Svalbard station manager, took part in a number of polar expeditions. Venturing for weeks over the Svalbard ice in under-powered skidoos, they collected some highly unusual samples from protruding rocks in an isolated region of the island.

Throughout the 80s they published evidence that these samples were very similar to samples from the Dry Valleys of Antarctica.

But in 2002, Fairchild realised that there was something unique about these rocks. He started on more detailed analyses with several collaborators, including the paper's co-authors Dr Peter Wynn, formerly at the University of Birmingham, now at the University of Lancaster, and Professor Christoph Spötl from the University of Innsbruck, Austria.

In the first half of 2008, the work with Bao intensified. The ¹⁷O analyses quickly exhausted the best remaining samples from the early expeditions. They hoped their result would settle the issue. It did.

'We found this signal much more strongly developed than in the original examples studied by Bao,' says Fairchild.

The surprisingly low levels of ¹⁷O found supports high levels of CO₂ in the atmosphere. The link between CO₂ levels and low levels of ¹⁷O is understood, but there is no obvious mechanism for how methane could lead to a similar situation.

Baked Alaska?

So how can Snowball Earth work in harmony with high CO₂ levels?

The world 630 million years ago was very different from today's planet: the sun did not radiate as much heat. A fall in temperature, which may have been sparked by a drop in greenhouse gases, probably got out of control.

The white surface of ice and snow reflects heat back out to space. Theory and models suggest that as snow and ice cover increases it can reach a point where temperatures spiral downwards. More ice reflects more heat, cooling the planet further, and extending the ice sheets towards the equator.

CO₂ levels still grow during Snowball Earth because atmospheric CO₂ comes largely from volcanoes, which would still be active.

But, if ice covered the planet, the process that removes CO₂ naturally from the atmosphere - weathering - would fail. CO₂ dissolves into water creating a weak acid that dissolves rocks removing carbon dioxide from the water as it goes.

An ice sheet covering water and rocks prevents weathering allowing CO₂ to build in the atmosphere, even while the rest of the planet freezes. A stable state is reached at around -45ºC. Average temperatures on the planet today are a relatively balmy +15ºC.

In addition, most water on the planet would have been locked up in the ice, creating an extremely arid environment: water vapour in the atmosphere would be a fraction of today's value. Water vapour is a powerful greenhouse gas. Together with clouds it forms the largest percentage of the greenhouse effect. So elevated CO₂ levels, without additional water vapour, would not necessarily remove the chill completely from the air.

Atmospheric and ocean circulations would also be limited in this extreme environment. Without an efficient way of distributing the sun's heat north and south from the equator, any warmth would tend to remain there.

It is possible that conditions may not have been quite so harsh. The team's results also accord with the Slushball Earth theory where average temperatures would be around 0 ºC - ice sheets reach down to lower latitudes, glaciers at sea level in the tropics, but some areas remain ice-free.

'In either case, the hyper-arid environment we found in Svalbard is entirely consistent with the idea of ice sheets largely covering the continents, even close to the equator,' says Fairchild.

Keywords: Antarctic, Arctic, Environmental change, Europe, Geology, Polar,

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