The Extinction Event That Wiped Out the Dinosaurs Almost Didn’t Happen

By Joel Hruska

Sixty-six million years ago, an asteroid roughly 9 kilometers in diameter slammed into the Gulf of Mexico near the Yucatán Peninsula. The massive impact created the Chicxulub crater (named for a nearby town) and wiped out the dinosaurs. It’s one of the most-studied mass extinction events. A new analysis of the impact suggests that we, by which I mean mammals and other modern species, were extremely lucky. If the impact had happened a few minutes later, we might never have existed at all.

Here’s why: In 1980, a team led by Luis Alvarez discovered evidence of a thin layer of iridium deposited across the Earth. That was a noteworthy discovery because iridium is comparatively rare on the planet’s surface–finding a thin layer of it distributed across huge amounts of the planet at a specific moment in geologic time suggested a massive impact by an asteroid comparatively rich in iridium. This boundary layer is referred to as the K-T or K-Pg boundary.

But that’s not all we’ve found. There’s a layer of soot distributed across the world as well. One of the coolest things about the K-PG boundary is that you can actually see it in various rock formations with no prior geological training:

Image courtesy of Wikipedia

A recent paper by Kunio Kaiho and Naga Oshima investigated why the K-Pg impact was so destructive and came to a startling conclusion: If the impact had happened just a few minutes later, it might not have kicked off a mass extinction at all. According to their study, the Chicxulub impact happened in an area of the Earth that was unusually rich in hydrocarbons and decayed organic matter, as shown below:

The Chicxulub impact smashed into one of the few spots on Earth where there were huge concentrations of hydrocarbons laid down by the decay of animals and plants over millions of years. Just 13 percent of the planet held those deposits at the time. A huge volume of soot from the burned material was ejected into the air, leading to a catastrophic drop in global temperatures, possibly aided by high levels of sulfur.

While Kaiho and Oshima argue that this global layer of soot drove the mass extinction event, other scientists aren’t so sure. “The 13 percent number they’re quoting has a lot of assumptions based around it,” Sean Gulick, a geophysicist at the University of Texas at Austin, told the Washington Post. The asteroid churned up soot, he said, but soot was “not the driver” that killed the dinosaurs.

In truth, there are multiple potential drivers that could have collectively contributed to the event. The asteroid struck a relatively shallow body of water, increasing the amount of ejected material flung back into the atmosphere. The impact event could have fed the ongoing eruption of the Deccan Traps, a massive volcanic formation in India that may have played a role in multiple extinction events. If the Chicxulub impact event had occurred over the deepest part of the Pacific, the asteroid would have had to vaporize seven miles of water before hitting the bottom of the ocean. While that’s still a tremendous impact, it would have bled off a non-trivial amount of the asteroid’s impact energy and limited the amount of material released into the atmosphere.

In short, this is an interesting argument for the uniqueness of the Chicxulub impact and the evolution of mammals leading to the existence of our own species. But it’ll be difficult to ever come up with a single unified explanation that absolutely answers our questions about what led to the extinction of the dinosaurs and our own existence.