The most-studied mass extinction in Earth history happened 65 million years ago and is widely thought to have wiped out the dinosaurs. New research has discovered that a separate extinction came shortly before that, triggered by volcanic eruptions that warmed the planet and killed life on the ocean floor.
The dinosaur extinction is believed to have been triggered by an asteroid at least 6 miles in diameter slamming into Mexico’s Yucatán Peninsula. But new evidence shows that by the time of the asteroid impact, life on the seafloor–mostly species of clams and snails–was already perishing because of the effects of huge volcanic eruptions on the Deccan Plateau in what is now India.
Researcher Thomas Tobin says, "The eruptions started 300,000 to 200,000 years before the impact, and they may have lasted 100,000 years."
Climate change has happened in the past as well. The eruptions would have filled the atmosphere with fine particles, called aerosols, that initially cooled the planet but, more importantly, they also would have spewed carbon dioxide and other greenhouse gases to produce long-term warming that led to the first of the two mass extinctions.
Tobin says, "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." There is no direct evidence yet that the first extinction event had any effect on the second, but he believes it’s possible that surviving species from the first event were compromised enough that they were unable to survive the long-term environmental effects of the asteroid impact.
"It seems improbable to me that they are completely independent events," he says.