Lower carbon dioxide triggered Antarctic ice sheet formation
14 September 2009, by Tamera Jones
East Antarctica started to ice over around 34 million years ago when atmospheric levels of the greenhouse gas carbon dioxide (CO2) dropped to twice the levels they are at today, report researchers in Nature.
Beautifully preserved microscopic plankton shells (foraminifera) from the Eocene - Oligocene boundary in Tanzania (scaning electron micrographs).
'This is the largest and most important climate tipping point of the last 65 million years,' says Professor Paul Pearson from Cardiff University, who led the study.
There is plenty of geological evidence that the Earth's climate cooled during the Eocene to Oligocene transition, 33.5 to 34 million years ago. This triggered the Antarctic ice sheet to form.
Further evidence comes from the Gamburtsev mountain range beneath the East Antarctica ice sheet. Scientists showed just this year that the range supported mountain glaciers before ice cap took hold.
'This is the largest and most important climate tipping point of the last 65 million years.'
Professor Paul Pearson, Cardiff University
Climate models suggest CO2 levels had to drop below a critical threshold before the ice sheet could form, while some scientists think changes in ocean circulation around Antarctica could've helped to trigger the ice sheet's formation.
But evidence for when CO2 levels fell, and by how much, has until now been sketchy. 'There have been no detailed studies on fossils from this period of time, so it's been difficult to explain what was going on in the atmosphere,' says Pearson.
Coring the Eocene - Oligocene climate transition near the village of Stakishari, Kilwa District, Tanzania.
Pearson and his team estimated atmospheric CO2 levels during the Eocene-Oligocene transition by analysing exceptionally well-preserved carbonate fossils recently discovered in Tanzania.
'Tanzania is unique in its superb preservation of plankton shells. We knew the rocks there were roughly the right age, but we had to first make detailed geological maps of the area,' he adds.
To estimate CO2 levels, the team used the ratio of boron isotopes in the fossils as a climate proxy. Not every atom of an element is the same: some are heavier than others and the ratio of different atom types, or isotopes, tells us about the environmental conditions at the time.
Scientists use proxies to estimate the amount of CO2 in the atmosphere millions of years ago, because there are no samples of air from that time. The oldest air trapped in bubbles in Antarctic ice cores is around a million years old.
'The ratio of boron isotopes would've mirrored what was happening to atmospheric CO2 levels 34 million years ago and we knew this would be recorded in the fossils' shells,' says Pearson.
Pearson's team showed that CO2 levels dropped to around 760 parts per million just before the ice sheet started to take hold. But levels of the gas bounced back soon after, before beginning a steady decline towards today's levels.
'Our work shows that there does appear to be a relationship between ice sheet formation and atmospheric CO2 levels, which is what the climate models suggest,' adds Pearson.
'Growth in ice caps causes profound changes in the global climate system - sea levels drop, this changes world weather patterns and the water around Antarctica cools. When ocean currents change, the way heat is transported around the planet also changes. This very often leads to extinctions and evolution of new species,' explains Pearson.
'At the moment, we don't know why CO2 levels bounced back and then fell again once the ice sheet started, but we're working on trying to understand that right now.'
Pearson et al., 2009. Atmospheric carbon dioxide through the Eocene-Oligocene climate transition. Nature (published online 13 September 2009) doi:10.1038/nature08447
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