Scientists studying lava formations in western India are casting increasing doubt on how important a 66-million-year-old asteroid impact was in the extinction of the dinosaurs.
When geologists first found evidence of the event, called the Chicxulub impact, it looked like a smoking gun.
After all, the asteroid was 10 kilometres in diameter and came down with a big enough wallop to severely disrupt ecosystems, worldwide. It also hit at roughly the same time as the Cretaceous-Paleogene (K-Pg) extinction, when not only the dinosaurs but many other species went extinct.
“We thought bingo, this is an open and shut case,” says Seth Burgess, a geologist and geochronologist with the US Geological Survey, in Menlo Park, California.
Then scientists got to looking for asteroids associated with other mass extinctions and came up blank. Instead, the extinctions appeared to be associated with giant volcanic eruptions, almost incomprehensibly larger than anything ever seen in human history.
And one these super-eruptions also occurred somewhere around the time of the K-Pg extinction, spreading lava flows two kilometres thick across India’s Deccan Plateau over the course of about a million years. In total, there was one million cubic kilometres of lava, enough to cover all of Australia to a depth of 130 metres. The result is known as the Deccan Traps.
Volcanism on that scale, Burgess says, would generate massive quantities of greenhouse gases such as carbon dioxide and methane, drastically changing the climate.
“That’s what people think drove other mass extinction events,” he says.
In an effort to determine which of these disasters actually killed the dinosaurs, two independent teams of scientists have spent years scouring the Western Ghats – a mountain range formed of Deccan Traps lavas – using sophisticated dating techniques in an effort to determine how much of this giant pile of lava erupted before the K-Pg extinction, and how much afterward.
If most of the lava had erupted afterward, then it obviously played no role – the extinction had already occurred. If it erupted beforehand, it could at least have been a contributing cause.
Both teams published their results in the journal Science – and drew different conclusions.
Burgess also has a paper in the same issue, commenting on the others.
One group, led by Courtney Sprain, a geoscientist at the University of Liverpool, UK, found that the vast majority of the lava, 75%, erupted after the K-Pg boundary, much reducing the chances that magmatism was the primary trigger of the extinction.
The finding was a surprise, because previous, less precise, studies had suggested the opposite: that only 20% of the lava erupted after the impact.
The other group, led by Blair Schoene of Princeton University, US, found that half the magma erupted before the K-Pg extinction.
They also found that the K-Pg extinction came right after a major surge in Deccan Traps activity, in which something like 160,000 cubic kilometres of magma erupted in a rapid pulse, only a few tens of thousands of years before both the Chicxulub impact and the K-Pg extinction.
“[This] is a big step in the direction of volcanism as primary cause for the K-Pg mass extinction,” says one of Schoene’s co-authors, Gerta Keller, an interdisciplinary geologist at Princeton, who has never bought the killer-asteroid theory. “I always thought the Deccan Traps played a role.”
The difference between the studies stems largely from the fact they used different methodologies, with different strengths and weaknesses.
Schoene’s and Keller’s measured the radioactive decay of uranium trapped in zircon crystals found in ash deposits between successive layers of lava. There are no zircons in the lava itself.
This is an extremely precise dating technique, capable of revealing the age of a single zircon to within 40,000 years, but zircons are durable crystals and might have formed before they were trapped in the ash.
Sprain’s team avoided this problem by using a mineral called plagioclase, found within the lava itself. But, because plagioclase contains no uranium, her team dated it using a method based on the decay of radioactive argon – which, at least for the Deccan Traps plagioclases – gave less precise dates than Keller was able to get from her zircons. Not only did this put the K-Pg boundary at a different time, but it gave no indication of the massive pre-extinction pulse found by Schoene’s team.
Reconciling these two results will probably take years of additional study, starting with looking more deeply at whether the lava indeed erupted in giant pulses.
Schoene’s team found several such pulses, separated by interludes as long as 300,000 years.
That’s long enough, Sprain notes, for the boundaries between them to show weathering layers, erosional features, or even shifts in magnetic signatures due to changes in the Earth’s magnetic field.
“All of which would suggest that there was a large period of time when nothing was erupting,” she says.
It’s also possible, she adds, that the early phases of the eruption released more climate-changing gases than the later ones.
If so, that would mean that the early phases were disproportionately climate-disruptive, increasing the likelihood that the Deccan Traps played a major role in the extinction.
“We know that in Hawaii, gas is released not just in lavas, but in fractures when lava is not actively erupting,” she says.
Or, the Chicxulub impact and the Deccan Traps might have acted in concert.
“Volcanism [could have] weakened ecosystems and these weakened systems were more susceptible to the abrupt climate change from the asteroid,” Sprain says.
“We are currently in the belief that it was a one-two punch scenario.” Though she adds, “The asteroid probably did a lot of the heavy lifting.”
Burgess offers an analogy to a murder mystery.
“What started out as a simple story now has two possible killers,” he says, “and we don’t know if they acted in tandem, or who’s the guiltier party.
“It’s a fantastic mystery that these two world-class geochronology laboratories are both trying to solve.”
And it’s of interest not only to dino-lovers. “The more we learn about how the Earth responds to these rapid, large-scale changes, the better we can inform our hypotheses about how the biosphere is going to respond to anthropogenic changes,” Burgess says.
