The mass extinction that took out the dinosaurs was already well underway by the time a six-mile asteroid slammed into Mexico's Yucatán Peninsula 65 million years ago, it appears.
New University of Washington research indicates that life on the sea floor was already dying, thanks to volcanic eruptions on India's Deccan Plateau that warmed the planet and killed life on the ocean floor.
"The eruptions started 300,000 to 200,000 years before the impact, and they may have lasted 100,000 years," says doctoral student Thomas Tobin.
The eruptions would have filled the atmosphere with fine particles, or aerosols, that initially cooled the planet. As time passed, though, they would have spewed out carbon dioxide and other greenhouse gases.
"The aerosols are active on a year to 10-year time scale, while the carbon dioxide has effects on a scale of hundreds to tens of thousands of years," says Tobin.
During this period, it was primarily life on the ocean floor that died, while the asteroid impact appears to have killed many more free-swimming species.
"The species in the first event are extinct but the groups are all recognizable things you could find around on a beach today," he said.
The researchers took small surface core samples from rocks and fossils in the Antarctic sediment and used a technique called magnetostratigraphy, employing known changes over time in Earth's magnetic field to determine when the fossils were deposited.
"I think the evidence we have from this location is indicative of two separate events, and also indicates that warming took place," says Tobin .
There's no direct evidence that the first extinction event had any effect on the second, but Tobin believes it will have weakened species, making it harder for them to survive the asteroid impact.
"It seems improbable to me that they are completely independent events," he says.