Dinosaurs disappeared under a blanket of smoke: study
A sooty stratosphere explains why most dinosaurs died but crocodilians pulled through. Belinda Smith reports.
Mega volcanic eruptions, sulfuric acid falling from the sky ... Earth wasn't a pretty place 66 million years ago, following the meteorite that blasted the Chicxulub impact crater in Mexico, which wiped out three quarters of species including non-avian dinosaurs.
Now, researchers from Japan's Tohoku University led by Kunio Kaiho have thrown another cause for widespread death into the ring: a layer of atmospheric soot, produced from burning oil, which they say cooled the planet longer than sulfur aerosols.
The mechanism also produced different conditions at different latitudes, explaining why most dinosaurs died but crocodilians, for instance, did not.
When the asteroid smashed into our planet, it kicked up huge amounts of dust and sulfuric aerosols into the stratosphere – the second-closest layer of the atmosphere to Earth, just above the troposphere.
The sulfuric component not only produced acid rain, but also reflected sunlight back into space, plunging the planet into a period of global cooling.
Without sunlight, plants were unable to photosynthesise and grow. Some 90% of land species on Earth died. Under this scenario, though, crocodilians would have been wiped out too. Yet they survived. Was something else at play?
Kaiho and colleagues put forward the idea that this cooling wasn't primarily caused by aerosols. Experiments suggest sulfuric acid aerosols don't last the vast periods after meteorite impacts. Rather, they think the dinosaurs disappeared in a puff of smoke (albeit a very long-lasting puff).
The team examined hydrocarbons trapped in 66-million-year-old pieces of rock from Haiti and Spain, grinding the samples to a powder and measuring ingredients of soot called polyaromatic hydrocarbons, which form when hydrocarbons aren't completely combusted.
They think intense heat from the impact ignited fires in oil-rich rocks. Black smoke billowed into the stratosphere and spread around the globe and stayed there for years. Any soot in the troposphere would have rained out in a matter of days.
Even though fires would have raged around the impact site, they suggest all the soot came from burning oil. Smoke from even the biggest forest fires today doesn't reach the stratosphere and the polyaromatic hydrocarbons found in their rock samples were from the same source – which, they reason, must have been the crater.
The pattern of stratospheric soot, they modelled, caused latitude-dependent death. Mid-high latitudes suffered the worst cooling and most species went extinct thanks to rapid climate change. But droughts and mild cooling at the lower latitudes would have wiped out dinosaurs but not crocodilians.
Some 1.5 billion tonnes of soot, they calculated, would be enough to wipe out the non-avian dinosaurs while keeping the crocdilians around. "The climate change could have led to the terrestrial extinctions within a few years, followed by the marine extinctions over several years," they write.
The work was published in Scientific Reports.