Fossilised leaves found at an ancient crater lake in New Zealand show that a major collapse of the Antarctic ice sheet 23 million years ago was caused by a sharp increase in atmospheric carbon dioxide levels.
Researchers found that changes in the stomatal cells and carbon isotope ratios in the leaves indicated a major increase in the levels of carbon dioxide, rising from about 500 parts per million (ppm) to between 750 and 1,550 ppm over a span of less than 10,000 years.
“What surprised us was how such large carbon dioxide fluctuations happened over geologically, relatively short time scales,” said Beth Fox, from the University of Waikato in New Zealand.
“We found that atmospheric carbon dioxide levels began to rapidly increase around the same time as the ice-sheet began to decline, and more importantly, even when the carbon dioxide levels dropped back to previous levels, the ice kept on melting,” she said.
“Once the process of destabilisation of the ice-sheet was kick-started, it could keep going by itself,” she added.
This information is important as scientists study today’s carbon dioxide concentrations and the melting ice in Antarctica, Fox said.
“We don’t yet know at which point between 500 and 1550 ppm that destabilisation of the ice took place and we would also like to look at different plant species to confirm what we have found so far,” she said.
Some models have shown that at the rate we are going right now, the Antarctic ice sheet might reach a critical tipping point and start destabilising very quickly, said Tammo Reichgelt from Colombia University in the US.
The study was made possible by the fantastic level of fossil preservation in Foulden Maar, where soft tissues are preserved right down to the cellular level, along with the fact that the sediment contains annual layers, allowing for much more accurate dating than is normal for such an ancient deposit, Fox said.
“It is an amazing site, with many more stories to tell us about how climate has changed in the past,” she said.