What killed off the dinosaurs? According to a new study by an international team of researchers, it certainly wasn’t volcanic activity. Although floods of fire did emerge from the Deccan Traps around the time of the famous mass extinction event 66 million years ago, the chemistry of the oceans shows that they didn’t change the world as much as previously thought.

“Although Deccan volcanism caused a short-lived global warming event and some ocean acidification, the effects were cancelled out by natural carbon cycling processes long before the mass extinction that killed the dinosaurs,” Michael Henehan, a postdoctoral researcher at Yale University and lead author of the study, said in a statement.

Mass extinction events never have just one cause. They may have a coup de grâce – a huge asteroid in the case of the demise of the non-avian dinosaurs – but they always have multiple antagonists.

Paleontologists know this as the Murder on the Orient Express model, named after the eponymous Poirot murder-mystery novel by Agatha Christie in which (spoiler alert) everyone on the locomotive is behind the grisly homicide. When it comes to the dinosaurian mass extinction event, one of the most frequently cited additional perpetrators is volcanism, which was thought to have significantly changed the chemistry of the oceans and altered the climate.

However, recent studies have cast some doubt on how influential this volcanism was, with one pointing out that the climate would have warmed quickly but would have returned to normal soon after. Now this new study, published in the journal Philosophical Transactions of the Royal Society B, has again criticized the idea that volcanoes were as guilty as some think.

^{Spinosaurus would have experienced the effects of this massive volcanic eruption. Herschel Hoffmeyer/Shutterstock}

As it points out, massive volcanism did indeed occur towards the end of the Cretaceous period, pumping out continental-sized lava flows and unleashing vast amounts of carbon dioxide. Much of this would have found its way to the oceans, where it would have turned into an acidic compound just as it still does today.

The research team analyzed sediment cores from the depths of the ocean dating back to this time, looking for signs of broken down calcium carbonate shells, pH decreases and other chemical changes that would indicate how quickly the oceans acidified following the onset of the vast volcanism. They initially found that there was indeed ocean acidification happening – in fact, it was far more severe and went on for a longer duration than previous models suggested.

However, they also discovered that the oceans were already on their way back to normal before the mass extinction event even began to occur. By the time species began to rapidly die off, the oceans were relatively habitable to marine life. So, although volcanism released vast amounts of carbon dioxide, it was over too long a timescale to make much of a difference – both on the land and in the sea.

More than anything, this study shows how powerful the asteroid impact actually was: Not only did it create giant tsunamis and a huge fireball, but it darkened the sky and cooled the climate at a remarkable speed. Consequently, up to 75 percent of life died out, including almost all marine life.

The non-avian dinosaurs were actually extremely unlucky: The asteroid may have destroyed them, but something else, perhaps the rise of mammals, was already killing them off long before the space-borne apocalypse arrived.