The mass extinction event that ended the Cretaceous period 66 million years ago has long generated a lively back-and-forth debate among geologists. Wild episodes of volcanism line up with earlier mass extinctions, and the end-Cretaceous saw the Deccan Traps eruptions, which covered much of what is now India in lava. The asteroid impact that formed the Chicxulub crater quite obviously goes in the “bad things for life” category, too. But the Deccan eruptions can’t be ignored, so debates on the relative contributions of these events have been unavoidable.

In the last few years, more precise dating techniques have made the timing of the eruptions clearer. It’s obvious where the asteroid impact fits into the timeline, as a layer of dust and soot appears in the rocks around the world, but tying in the eruptions has been more difficult. And the fossil evidence and climate indicators have also left some room for interpretation about the effects of the volcanism.

A new study led by Alessandro Chiarenza at Imperial College London and Alexander Farnsworth at the University of Bristol tries to get some answers through a slightly different approach—creating a dinosaur habitat model.

Modeling the climate gone insane

To create the climate scenarios for changing habitat, the researchers used a handful of climate model simulations driven by estimates of the eruptions and the asteroid impact. This included different amounts of sulfate aerosols (reducing incoming sunlight), carbon dioxide increases, and ashfall on land.

They tried volcanic aerosol scenarios ranging from five to 20 percent sunlight reductions. That would have a massive impact on temperatures, producing 10°C to 67°C global cooling. Simulations of the volcanic CO₂ release, meanwhile, produced 5°C to 9°C warming. Because the researchers say the paleoclimate evidence seems to point to about 2°C warming during the eruptions, even the five percent sunlight reduction experiment is likely too extreme.

It’s particularly difficult to estimate the volcanic aerosol effect of the eruptions. For one, aerosols have to reach the stratosphere to have a chance of hanging around for a meaningful length of time, and it’s simply unknown whether the Deccan eruptions were violent enough to lift them that high. And even then, aerosols don’t last very long, so sustained eruptions would be required to maintain a given amount of aerosols.

The asteroid impact simulations, on the other hand, have a pretty unavoidable result—sudden and drastic global cooling—drastic meaning in the neighborhood of 34°C. The world starts to warm up within a few years, but it takes decades to return to the pre-impact temperatures.

This is where things get a little more interesting. In simulations with both the volcanic and impact effects, the volcanic CO₂ actually causes a slightly quicker return to warm temperatures after the asteroid impact—recovering in about 20 years rather than 30 years. So rather than a climatic “double whammy,” it could be that the eruptions helpfully offset some of the freezing temperatures.

But what would a dinosaur like?

To connect this idea to habitats, the researchers started by comparing annual temperature and precipitation ranges to fossil locations in Cretaceous simulations pre-catastrophes. Using the climates dinosaurs were living in, the researchers could then see what the various climate change scenarios would do to available habitat.

Apart from seemingly implausible volcanic aerosol scenarios, the volcanic warming simulations actually increase the area of suitable habitat. In fact, warming roughly doubles global dinosaur habitats. The asteroid impact simulation, on the other hand, basically erases any place a dinosaur might thrive.

The results also show that the volcanic warming would result in more habitat, returning more quickly, after the asteroid.

This study only evaluates habitat in terms of climate—other nasty things were definitely or potentially associated with these events. There were huge tsunamis, submarine landslides, widespread wildfires, and likely insufficient sunlight to sustain photosynthesis after the asteroid hit, for example. And the volcanism might have had toxic local effects. But the researchers say the climatic effect of the impact alone seems sufficient for a mass extinction.

The researchers also highlight pieces of evidence that seem to point to the asteroid rather than the volcanism. There’s some fossil evidence for dinosaur diversity increasing during the phase of eruptions that occurred prior to the asteroid impact. And in the oceans, the extinctions only affected shallow water species—which makes more sense for a short-lived climatic event than a drawn-out one where changes could work their way into the deeps. It also helps explain evidence of rapid recoveries, like the return of life to the Chicxulub crater itself.

And this is not just another academic argument siding with the asteroid as the dominant cause of the extinction, relegating the volcanism to something more like a coincidence. The researchers go as far as to “suggest that Deccan volcanism might have contributed to the survival of many species across the [end-Cretaceous] boundary and potentially fostered the rapid recovery of life from the most iconic of mass extinctions.”

PNAS, 2020. DOI: 10.1073/pnas.2006087117 (About DOIs).

https://arstechnica.com/?p=1688478