It took a volcanic eruption and the loss of half of Earth's plant life 200 million years ago to tip the scales in favour of the dinosaurs over crocodiles, say researchers.
The idea is not new, but connecting the eruption to a 200-million-year-old mass extinction event has not been easy. Now that link is confirmed in a new study published in the latest Proceedings of the National Academy of Sciences.
The research looks at ancient plant substances and other evidence in lake and ocean sediments from both sides of the 9 million-square-kilometre Central Atlantic Magmatic Province (CAMP) eruption zone, better known nowadays as the Atlantic Ocean.
"We weren't convinced that volcanism caused the extinctions," says palaeobiologist Assistant Professor Jessica Whiteside of Brown University.
But that all changed when she and her colleagues found and accurately dated some unusual changes in the kind of carbon available to plants during the eruption. "We actually did a complete 180," she says.
Lake sediments to the west of the eruption in New England contain leaf waxes, pollen, wood and other plant materials that record what sorts of carbon was being incorporated by plants from the atmosphere during the eruption. The sediments are particularly useful because they - as well as some ocean sediments of the same age in England - are physically interwoven with some of the earliest lava from the giant eruption. So their overlap in time is undeniable.
"I think they've tied into a very nice section (of sediments), so they have very good timing for this thing," says Earth scientist Professor Michael Rampino of New York University. In fact their resolution is about 20,000 years, which is pretty good considering the 200 million years that have passed since the events took place.
An analysis of the plant material suggests there was a rapid and dramatic rise in climate-warming carbon compounds in the atmosphere. That warming likely caused die-offs, as well as new opportunities that early dinosaurs were apparently in a good position to exploit.
The CAMP eruption itself was very long-lived, and as such was unlikely to have released enough of the gases in short order to cause a rapid climate change, says Rampino.
What's more, the carbon that made it into the atmosphere, as shown by Whiteside, was primarily carbon-13. That's important because that is the lighter-weight isotope of carbon, which is associated not with volcanic eruptions but with living things.
So how did a volcanic eruption lead to the release of greenhouse gases that are typically associated with burning forests and fossil fuels?
Whatever causes of the mass extinctions and climatic changes, this new research makes a convincing link between the die-offs and the CAMP eruption, says Rampino, and should serve as a model for looking at other mass extinctions where volcanoes appear to be culprits.