Richard A Lovett
Scientists studying ocean cores have found new clues as to whether it was indeed an asteroid strike that killed off the dinosaurs.
For years, most scientists presumed that the asteroid, which hit near Mexico’s Yucatan Peninsula 66 million years ago, was the culprit.
But mounting evidence has shown that the demise of the dinosaurs – part of a larger die off known as the Cretaceous/Paleogene (K-Pg) extinction – occurred at the same time as a series of massive volcanic eruptions.
These eruptions, known as the Deccan Traps, occurred over a 710,000-year period, burying much of India in 500,000 cubic kilometres of lava.
Asteroid-theory sceptics have argued that the climate effects of these eruptions were more than potent enough to drive the K-Pg extinction, without help from the asteroid, due to the vast amounts of climate-changing gases, such as carbon dioxide, that would have been released with them.
“A lot of results coming out of [this] community have been pointing toward the timing being closer and closer to the [asteroid] impact,” says Pincelli Hull, a paleooceanographer at Yale University, US. “That has raised all kinds of questions about the relative role of the impact and volcanism in the extinction.”
These studies, however, had focused on the lava, trying to determine how much erupted before the extinction – when it might have contributed to it – and how much afterwards, too late to be a cause.
But it’s possible, Hull says, that the timing of the lava flows and the largest releases of volcanic gases weren’t the same.
In fact, she says, volcanic gases are often released ahead of the lava itself. “The work [on the Deccan Traps] could be 100% correct on the age of the lava flows, but the outgassing could have occurred before,” she says.
To unscramble this, her team turned to ocean cores from the Atlantic and Pacific oceans, looking to see what they showed about global temperature changes before and after the extinction.
They then ran climate models of gas releases from the Deccan Traps, before, during, and after the extinction, looking to see how their results compared to what they’d found.
“There’s only a couple of scenarios that seem to work,” she says, adding that neither allows for a giant pulse of volcanic gases shortly before the extinction.
“If that happened, we’d expect a big warming event, or acidification right before the impact – and we don’t see that.”
That said, it’s possible that there might have been a lot of volcanic outgassing well before the boundary (as well as more, later on).
In fact, there is a well-known warming event that fits this model, called the Late Maastrichtian Warming Event. But it peaked 200,000 years too early, and by the time of the asteroid impact, global temperatures had come back to normal, Hull says.
Not to mention that there’s no sign of mass extinctions during the Late Maastrichtian warming. “It’s only the impact that coincides with extinction.”
After the extinction, however, changes in the ocean from the die off of important marine organisms would have altered it in ways that would have allowed it to absorb much of the carbon dioxide emitted by subsequent pulses of Deccan Traps volcanism.
This would have limited the amount of global warming these pulses could have induced after the extinction.
Thus, her team writes in the journal Science, “Deccan volcanism might have contributed to shaping [the rise of Cenozoic species and communities] during the extinction aftermath”.
The new paper isn’t Hull’s only foray into studying the extinction.
Last October, in a paper in the journal Proceedings of the National Academy of Sciences, her team used a “pretty cool” geochemical technique involving boron isotopes to determine ocean acidity in the first 1000 years after the asteroid struck.
It’s all part of a continuing effort, she says, to address the K-Pg extinction, “and try to understand the mechanisms of the extinction its aftermath”.
