A death shroud swept over the planet 66 million years ago, after a giant asteroid crashed into Earth, leaving a crater more than 110 miles wide on the Yucatán Peninsula centered on what is now the Mexican town of Chicxulub. This impact launched more than 12,000 cubic miles of material into the air, which caused a winter that lasted for decades and acidified the oceans. When the shroud lifted, three-quarters of all the species on Earth, including many dinosaurs, were dead.
Yet, with the benefit of hindsight, the fallout wasn’t all bad.
According to research conducted by an international collaboration of three dozen scientists, the mass extinction that marked the end of the Cretaceous period may have allowed the oceans to soften the blow from a massive volcanic eruption that occurred around the same time as the asteroid collision. The asteroid primed the seas to absorb some of the massive amounts of greenhouse gases emitted from a volcanic plateau in ancient India known as the Deccan Traps, dampening the warming that might otherwise have spelled trouble for early mammals and many of the other species that persisted after the impact.
Studies suggest this epic eruption had already been underway for 400,000 years when the asteroid struck, and some scientists argue that the volcanic gases may have been partly responsible for the mass extinction. But based on new estimates of the global temperatures around this time—published Thursday in the journal Science—it seems unlikely the massive volcanoes contributed to the dinosaurs’ demise.
The asteroid appears to have acted alone—and its dramatic influence on oceanic plankton may have tempered subsequent global warming as the volcanic eruptions continued for another 300,000 years.
Planktonic palm trees
In muddy sediments sampled from the ocean floor, the sudden disappearance of plankton species with calcareous shells usually coincides with a layer of tiny glass beads that are known to have rained down after the asteroid hit, says Pincelli Hull, a paleoceanographer at Yale University who co-authored the study.
“Since those species appear to have been hit hardest by the asteroid impact, we believe the sulfur and nitrous oxide released by the impact may have acidified the ocean, dissolving the creatures’ shells,” Hull says. It’d be akin to what happens when you throw a piece of chalk—which consists of the remains of calcareous plankton—into a glass of vinegar. The ocean was never that acidic, however, so the plankton shells would have dissolved much slower, and there would have been no fizz.
The same sediments can also teach us how global temperatures have changed through time, Hull explains—changes that should reflect any large effects of the gases emitted by the Deccan volcanoes, including CO2.
“The fine mud from the deep sea that we find in those samples has a consistency comparable to toothpaste,” Hull says. “It isn’t made up of rock, like the mud we are familiar with on land, but of microscopic fossils of calcareous plankton species that rain down to the seafloor after they die.”
Merely identifying the plankton embedded in the various layers of the seafloor sediment can give an indication of the ocean climate at the time, with some species being “equivalent to finding palm trees on the North Pole,” Pincelli says.
But the chemical compositions of their shells contain even more information.
Ocean temperatures affected which kinds of carbon and oxygen isotopes the plankton incorporated into their protective shells. By combining data from deep sea mud collected around the world, the researchers could reconstruct how global temperatures changed over hundreds of thousands of years.
To do this, the researchers adapted a computer model using equations that capture the relationship between the planet’s temperature changes and the carbon cycle during various periods of time, including the present.
This model helps address a 40-year-old debate that was reignited by a pair of studies published last February, also in the journal Science.
The best-supported scenarios posit that the Deccan’s greenhouse gases were either mostly released 200,000 to 350,000 years before the end-Cretaceous extinction, or about equally before and after the event. The latter possibility of an equal split was first hinted at by geochronologist Courtney Sprain of the University of Florida and her colleagues in one of the papers published last February.
“I’m of course very excited to see that this study supports our findings,” Sprain says, and adds that both of last year’s studies allowed for half of the gases to have erupted after the asteroid hit. The main difference is the study led by geochronologist Blair Schoene proposed that a pulse of volcanism occurred within the 100,000-year timespan immediately before the extinction event. This pulse would have disrupted the environment and conspired with the asteroid to drive the global devastation of the planet’s species.
But this scenario isn’t supported by the new computer model, which instead insists that global temperatures cooled in the period leading up to the asteroid collision.
That still leaves the question of how much of the gases were emitted before and after the mass extinction. Looking at clear periods of warming closest to the asteroid impact reveals one peak of about 2 degrees Celsius (about 3.6 degrees Fahrenheit) around 200,000 years before the mass extinction. A second, much less pronounced spell of warming occurred about 200,000 years after the event.
But less warming does not necessarily mean that the Deccan volcanoes were emitting less gas, says Donald Penman, a Yale geochemist and co-creator of the new models. There may be a more intriguing explanation.
“After most of the calcareous plankton went extinct, the model suggests the build-up of compounds they would otherwise have used in their shells allowed the oceans to absorb more volcanic CO2, reducing its global warming effect,” Penman says.
Heather Birch, a micropaleontologist at the University of Bristol in England who wasn’t involved in making the models, agrees the plankton composition looked very different in the time after the asteroid impact, and that this may have affected the uptake of carbon. But Birch warns that “only a small fraction of the plankton fossilizes, so more research is needed for us to learn how these large amounts of CO2 would have been absorbed.”
But given that oceans are currently acidifying again, this time due to a human-caused rise in CO2, could another mass extinction of calcareous plankton save us from the worst of climate change?
Hull says don’t hold your breath. After the end-Cretaceous plankton die-off, temperatures rose for thousands of years before the oceans started absorbing more CO2. On a timescale relevant to human society, this means we’d have millennia of upheaval ahead.