Some 66 million years ago, the dinosaurs had an apocalyptically bad day. An asteroid measuring about six miles (10 kilometers) across collided with Earth in what is now Mexico's Yucatán Peninsula, devastating life on the planet. Every dinosaur lineage except for birds was snuffed out in the aftermath, along with three-quarters of all plant and animal species—a disruption that ultimately enabled the rise of our own mammalian family.
Scientists generally agree on these basic points, but when it comes to reconstructing the fallout of the impact, the devil is in the details. Were the majority of dinosaurs wiped out instantly, or over a longer geological period? To what extent did wildfires, volcanism, and acid rain contribute to the disaster? Why did some species survive where others did not?
A new hypothesis on the extinction, published Thursday in Scientific Reports, tackles this last question by examining why dinosaurs were killed, while their fairly close relatives, crocodilians, were spared. These Mesozoic crocodilians have since evolved into modern alligators, crocodiles, caimans, and gharials.
Led by Tohoku University geochemist Kunio Kaiho, the research proposes that atmospheric soot kicked up by the impact may have played a much larger role in the event than previously estimated. This would account for the odd distribution of extinctions. Basically, "the dinosaurs may have gone up in smoke," as a press release rather theatrically puts it.
There's no doubt that the asteroid catapulted all kinds of hazardous material into Earth's atmosphere, which either ballistically reentered, battering the planet with scattered detritus, or was trapped in the skies, blocking out sunlight and disrupting the global climate. Previously, it has been suggested that the impact completely vaporized Yucatán's sulfur-rich rock, producing sulfuric acid aerosols that accumulated in the atmosphere. These condensed aerosols became a major driver of global dimming, cooling, and acid precipitation.
But according to Kaiho and his colleagues, this scenario doesn't explain crocodilians' survival: Global darkness would have seriously disrupted their equatorial ecosystems, just like it did to the dinosaurs. For the team, the main culprit had to be both milder and more long-lived than sulfate aerosols. A slow burn, in other words, rather than a flash fire.
"Recent impact experiments and model calculations have demonstrated that condensed sulfuric acid aerosols cannot form and persist over long periods following asteroid impacts," Kaiho explained to me via email. "If darkness (no sunlight) had occurred for a few years after the impact, the resulting low temperatures would have caused extinction of the crocodilians, birds, and mammals."
Hydrocarbon soot, however, has a longer lifespan in the stratosphere, and its milder effects could account for the survival of some species over others. Using atmospheric simulations along with samples collected from Haiti and Spain, the team estimated that if about 1,500 teragrams (1.5 billion metric tons) of sooty debris were released by the impact, it would be enough to wipe out the dinosaurs within a period of just two years, while allowing crocodilians to pull through in low latitude river systems.
The intricacies surrounding such a complex event will have to be examined further. Kaiho told me he is already thinking about new approaches to clarify the extent to which soot contributed to the massive death toll that capped off the Cretaceous period.
The only thing that the authors claim for certain is that their results "show that rapid global climate change can play a major role in driving extinction," which is a handy lesson to keep in mind in the warming Anthropocene age.