Settling the Soot
Fossils from New Mexico and Colorado show that the doomsday asteroid of 66 million years ago may have caused entire forests to burn to the ground. And for decades, some scientists thought that happened because the atmosphere superheated the planet, igniting fires everywhere on Earth as fireballs rained down from the skies.
But David Kring of the Lunar and Planetary Institute has modeled Chicxulub’s immediate aftermath and shown forest fires were likely more regional — some forests lived, while others died. His team proposes that a thermal pulse is to blame, an explosion of heat reaching more than 36,000 degrees Fahrenheit as it spread from the impact site, igniting nearby areas. If that’s correct, forest fires likely spread across southern North America, but stopped before destroying the continent’s northern reaches. The extended fallout also would have started fires on the opposite side of the planet. “There would have been a huge number of ecosystems on Earth at the time, and those ecosystems would have reacted differently,” says Kring.
In research published last year, Joanna Morgan of Imperial College London and her colleagues put that idea to the test by setting pine needles on fire in the lab. The team showed that the thermal pulse from the impact couldn’t ignite the kind of global canopy-replacing wildfires commonly associated with the asteroid. Instead, dry forest litter likely sparked wildfires like the ones forests had evolved with. In Morgan’s version of events, reduced sunlight and re-entering debris may have dried out many of Earth’s plants. Those dead trees and plants later burned as a result.
Either way, we’re sure Earth burned. “What we know is there’s lots and lots of soot, so there must have been lots of fires all across the place,” Morgan says. How did these forests bounce back? A 2014 University of Arizona study of fossilized leaves in North Dakota showed a surprising shift in plant populations. Deciduous plants — those that lose their leaves — fared better than slow-growing evergreens, thanks to their live-fast-die-young strategy. This implies that evergreens were more common before the impact, but fast-growing flowering plants thrived immediately afterward.
Fossil records also commonly show a fern spore spike following the impact, indicating that some spores and seeds survived the fires. This helps explain why some avian dinosaurs lived and others died. A paper published in the journal Current Biology earlier this year looked at birdlike creatures living at the end of the Cretaceous and notes that the survivors — those species that went on to become modern birds — had beaks without teeth, ideal for seeds. Carnivorous species died as their food sources did, but those dinosaurs with toothless beaks could feast on fallen seeds long after plants died.