NASA / Reuters

The worst day in the history of life on Earth, so far, happened almost exactly 66 million years ago, when an asteroid roughly the size of Manhattan slammed into the Yucatán Peninsula.

You may know the story. The asteroid—which arrived, probably, in June or July—immediately drilled a 20-mile hole into the planet’s surface, vaporizing bedrock and spewing it halfway to the moon. The planet shuddered with magnitude-12 earthquakes, loosing tsunamis across the Gulf of Mexico. Some of the ejected debris condensed in orbit and plunged back to Earth as searing spheres of molten glass, which torched the land and turned forests into firestorms. Other debris remained high in space, where it blocked the sun’s rays and began to chill the surface of the planet.

By the time it was over, about 75 percent of all species on Earth had died, including all nonavian dinosaurs. The event, which ended the Cretaceous Period and began the Tertiary Period, is named the K-T extinction.

Since 1980, when the K-T impact hypothesis was first proposed, the Day the Dinosaurs Died has attained almost mythic significance. But questions remain about the theory. None of the Earth’s other big mass extinctions were caused by an asteroid impact. Why did this one end the 180-million-year reign of the dinosaurs?

A new paper, published this week in the Proceedings of the National Academy of Sciences, offers a possible answer: The impact changed the chemical content of the ocean, rendering seawater more acidic and inhospitable to the tiny plankton that form the base of the marine food chain. Combined with the other effects of the asteroid—darkened skies and a snap of global cooling—this ecologic disruption doomed much of life on Earth.

The finding may be satisfying for asteroid fans, but it is an ominous one. Ocean acidification, a hallmark of the planet’s previous mass extinctions, is happening again today.

How does an asteroid prompt an extinction? It chooses the right location. The Yucatán Peninsula was an excellent one, says Pincelli Hull, an author of the paper and a geology professor at Yale. The peninsula is essentially an “old buried reef,” she told me, an accumulation of dead coral and other sea life that is now more than a mile thick. When the asteroid hit, untold megatons of that old organic material—rich in nitrogen and sulfur—instantly became dust and shot up into the atmosphere.

Soon it began to fall back down, now as nitric oxide and sulfuric acid. “It was raining brimstone and acid from the sky,” Hull said. The air would have reeked of acrid smog and burnt matches. The acid accumulated in the oceans, wearing away the shells of the small, delicate plankton that serve as the basis of the marine food chain. Within a few centuries of the impact, ocean acidity had jumped by at least 0.3 pH units.

This spike in ocean acidification may have lasted for less than 1,000 years. But even that pulse “was long enough to kill off entire ecosystems for sure,” Hull said. Ocean acidification also likely worsened other sweeping environmental changes wrought by the impact, such as the years-long darkness caused by orbiting debris and ash from the global wildfires.

With this new finding, it now appears that all three of the worst mass extinctions in Earth’s history featured huge spasms of ocean acidification. They include the K-T extinction; the End-Triassic Extinction, when volcanoes in New Jersey killed 75 percent of all species; and the dread End-Permian Event, the worst extinction in the history of the planet, which killed roughly 85 percent of all species and nearly sterilized the oceans. Scientists call that event “the Great Dying.”

And that pattern is worrying, because the oceans are acidifying again today. Carbon dioxide—the same air pollutant that causes global warming—also dissolves in the oceans and increases the acidity of seawater. Since the late 1980s, the planet’s oceans have become about 0.02 pH units more acidic every decade, according to a report last month from the Intergovernmental Panel on Climate Change. More than a fifth of all modern carbon pollution has already dissolved into the oceans, the report also found.

The researchers collected more than 7,000 of these tiny fossils—each half the size of a grain of sand—from Geulhemmerberg Cave in the Netherlands, the only place on Earth that contains fossils from the oceans in the decades and centuries after the K-T impact. (Michael J. Henehan / PNAS)

Modern acidification is not yet at the same magnitude as the K-T pulse. It’s “moving toward that scale, but it’s not quite there yet,” Hull said. What unites our world and the K-T period, she said, is that a number of environmental catastrophes can overlap with ocean acidification to produce a major upheaval.

“You should think of [ocean acidification] as the straw that broke the camel’s back” during the K-T extinction, she said. “It’s dark, it’s really cold after the impact—and the ocean has acidified.”

Chris Lowery, who studies the oceans of the past at the University of Texas at Austin, told me that the paper represents “a big leap” in our understanding of the extinction. “We’ve known for a while that there was some amount of ocean acidification due to the Chicxulub impact, but this is the first time that the acidification has actually been quantified,” he said in an email, referring to the town in the Yucatán Peninsula for which the impact crater is named.

While paleontologists have long hypothesized about how an asteroid impact could produce the K-T extinction, this is some of the first evidence that supports those mechanisms, he added. And while the asteroid struck Mexico, the crucial evidence for this study came from a cave in the Netherlands that preserves fossils from the oceans in the decades or centuries immediately after the impact. Michael Henehan, a scientist who was then a postdoc in Hull’s lab, collected more than 7,000 tiny plankton fossils from the cave—each half the size of a grain of sand—and crushed them to analyze their chemical signatures.

“It was a herculean effort to get these measurements,” Hull said. “There’s just one place in the world where we think these fossils preserved.” (Henehan is now a professor at the German Research Centre for Geosciences.)

Two years ago, another study found the first geological evidence of global cooling, another proposed mechanism, following the impact.

Notably, the study’s findings do not support the idea that enormous eruptions from volcanoes in modern-day India, called the Deccan Traps, prompted the surge in ocean acidification and resulting mass extinction. That hypothesis has a small number of ardent advocates, among them Gerta Keller, a geologist at Princeton who was the subject of a profile in this magazine last year.

But “this study pretty definitively shows that those eruptions had no effect on ocean chemistry,” Lowery said. In an email, Keller disputed the paper’s dating of the impact, arguing the asteroid actually struck Earth “over 100,000 years” prior to the extinction’s start.

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