The team analyzed core sediment samples collected in 2000 from an area in the Southern Ocean known as the East Tasman Plateau at (ODP) Site 1172. Researchers who set out to re-create the opening between the subcontinents of Antarctica and Australia originally took the samples, Bijl says. Some 40 million years ago, Australia was still connected to Antarctica.
The group reconstructed the ancient CO2 concentrations by examining the fossil molecules of algae. The carbon isotopic composition of these algae is strongly controlled by the atmospheric CO2 concentration during growth, or during photosynthesis. The group also studied the changes in the abundance of different groups of fossil plankton in the sediment to help refine their CO2 estimate, Bijl said.
In a mostly positive commentary accompanying the Science study, Paul Pearson, of the School of Earth and Ocean Sciences at the UK’s Cardiff University, cautioned that the actual level of atmospheric CO2 rise and the sea surface temperatures should be considered best estimates because they rely on a series of assumptions made by the researchers. Nonetheless, “what the study did show was that the CO2 was certainly higher during this period, and it at least doubled during the MECO,” he wrote.
What caused the rise in atmosphereric CO2 levels remains unknown. Scientists have proposed a number of potential causes, including the disappearance of an ocean between India and Asia that occured as the Himalayas rose, significant volcanic activity or the recycling of carbon from carbonate sediments in the dying ocean through “extensive metamorphic decarbonation reactions,” Pearson wrote.
“To put it briefly, the change in CO2 40 million years ago was too large to have been the result of temperature change and associated feedbacks,” Bijl said. “Such a large change in CO2 certainly provides a plausible explanation for the changes in Earth’s temperature.”