Fossil leaves yield extinction clues

Analyses of fossil leaves provide more evidence that the mass extinction that wiped out the dinosaurs 65 million years ago was sudden and probably brought about by an extraterrestrial impact.

The demise of the dinosaurs coincided both with a major extraterrestrial impact and massive volcanism in India. Both of those events–one sudden, the other unfolding over as many as 2 million years–would have dramatically influenced Earth’s climate by pumping carbon dioxide and other gases into the atmosphere. Some scientists have proposed that volcanism alone was enough to have caused the extinctions.

David J. Beerling of the University of Sheffield in England and his colleagues looked to fossil leaves for clues about how quickly the mass extinctions may have occurred. The proportion of leaf area occupied by stomata–the pore cells that leaves use to take in carbon dioxide and send out oxygen–is directly related to the concentration of planet-warming carbon dioxide in the air. The scientists’ analysis of ginkgo leaves from the million years before the worldwide dinosaur die-off suggests that the carbon dioxide concentrations in the air fluctuated between 350 and 540 parts per million (ppm) during that period.

Evergreen ferns that lived just after the mass extinctions grew in a much different atmosphere, which held at least 2,300 ppm carbon dioxide. Such a sudden spike in carbon dioxide doesn’t match what would be expected from a long period of sustained volcanism, says Beerling. However, an asteroid or comet splashing down in the shallow sea off ancient Mexico could have instantly vaporized enough carbonate sediments to boost carbon dioxide to the higher concentration. That change would have raised average global temperatures about 7.5C, says Beerling.

After another half-million years, the stomata on leaves of the same ginkgo species–which had vanished from the fossil record for a while–showed that concentrations of carbon dioxide stabilized at around 340 ppm. Beerling and his coworkers present their results in the June 11 Proceedings of the National Academy of Sciences.