The Palisades cliffs west of New York City rear up from the Hudson River like the spine of some ancient beast—and that impression is not far off. Their basalt backbone is a remnant of an immense lava flow that engulfed what is now New Jersey, Nova Scotia, West Africa, and Brazil, among other places, 200 million years ago when they were neighbors on the great super-continent of Pangaea.
Scientists have suspected for years that this lava flow, which welled up through the Earth’s crust and spread over millions of square kilometers, was linked to the end-Triassic extinction—a mass die-off around the same time that wiped at least half of the species then living on Earth. Among the victims were the crocodile-like creatures and long-jawed proto-mammals that had dominated the planet, and their sudden disappearance allowed the dinosaurs—at that point relative newcomers—to take over. But difficulties in lining up the dating of the basalt with the dating of the fossil layers had left the exact timing, and hence the possible connection between the two events, up in the air.
But now researchers using an exquisitely precise dating technique have brought the timing into much better focus. In a recent paper in Science they reveal that there were four successive lava flows over a period of hundreds of thousands of years—and that the first one coincides closely with the extinction. These findings will allow scientists to investigate further how the eruptions might have led to the extinction, and paint the surprising picture that life began to recover even as the lava was still pouring out of the ground.
A bit of fuzziness in the exact dates hasn’t stopped scientists from investigating the connection between the lava flows and the end-Triassic extinction in the past. Researchers have long discussed whether the doubling of carbon dioxide in the atmosphere that occurred around that time was the result of hot lava reacting with rocks around it, triggering catastrophic global warming and the resulting die-off. But as Paul Olsen, a paleontologist at Columbia University and an author of the Science paper, notes: “What was lacking was any completely independent way of determining the amount of time that had lapsed” between the fossils that record the extinction and the laying down of the basalt.
This is because there are different methods for establishing the ages of layers of rock, depending on whether they are volcanic, like the basalt, or sedimentary, like fossil-bearing rock. Among other methods, volcanic rocks can be dated by examining substances trapped within crystals when the rock formed, using their decay as a clock (called radioisotope dating); sedimentary rocks, on the other hand, can be assigned relative dates using signs left in rocks as the Earth’s orbit shifted over eons (called astrochronology). Large margins of error meant it was hard to tell whether the two layers were laid down at close the same time or separated by ten thousand years.
To fill this gap, Olsen and his coauthors—including Terrence Blackburn, a fellow at the Carnegie Institute—embarked on a years-long rock hunt in search of zircon crystals trapped in the basalt. When zircon crystals form, they lock within them a large helping of uranium, and over time, the uranium decays at a regular rate into lead. The amount of each substance trapped in these sealed time capsules from the deep past allows dating that’s much more precise than other decay-based methods—such as argon-argon dating —which can be compromised by leaks or limited by a small initial deposit of the decaying material.
Such crystals are rare in basalt, but this team was ready to look anywhere. The hunt took them to open pit mines in Morocco and suburban New Jersey, among other places. “Some of the samples from New Jersey are literally taken from people’s houses in the suburbs of New York—rock outcroppings in their front yards,” says Blackburn. “We’d go up there and kind of bang on rocks and run off real quick.” Back in the lab, Blackburn pulverized the samples, extracted any zircon crystals—less than 0.1% of the total rock—and ran them through a chemical gauntlet, including 60 hours at 900 C and a bath in hydrochloric acid to isolate the innermost parts of the crystals, which had not been exposed to the environment for millions of years. A final dousing with acid separated the zircon from the uranium and lead so their respective amounts could be determined.
Using their new dates and existing data, the team was able to build a chronology of the events taking place at the end of the Triassic. At around 201.564 million years ago, the first lava flow and the end-Triassic extinction both occur. Then, over the next 600,00 years, three more flows bubble up. These later flows, or pulses, raise some interesting questions. “We’ve had a good idea for quite a while that [this] volcanism is more or less synchronous with all of these interesting biological events. This paper really nails that,” says Jennifer McElwain, a paleobotanist at University College Dublin who has studied the connection between the lava flows and the mass extinction. “But what I think is really interesting is that there’s something unusual or interesting about the first basalt event.” There do not seem to be further waves of extinction at the time of the later basalt flows, she notes. “It sets up lots of interesting questions. Perhaps biological systems are really intensely effected by the first pulse of volcanism but perhaps they then adapt.” Alternatively, it’s possible that the chemistry of the basalt flows was different, such that later eruptions did not have the same effect on the atmosphere.
What exactly happened during this extinction is of more than academic importance: The release of tremendous amounts of CO2 into the atmosphere over a brief period of time bears some similarity to our current situation, as we create billions of tons of CO2 from the burning of fossil fuels like coal and oil. “It behooves us to find out what’s going on,” says Olsen. A mass extinction event of our own is something we’d best avoid.