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SYFY WIRE Bad Astronomy

Hey, maybe the dinosaur-killer asteroid really *did* act alone!

By Phil Plait
Someone's about to have a VERY bad day. Credit: Shutterstock/Esteban De Armas

We're still not really sure what killed the dinosaurs.

I mean that both in general and specifically. Yes, there was an asteroid impact. We know that 66 million years ago, an object roughly 10 kilometers across, either an asteroid or a comet, slammed into what is now the Gulf of Mexico just off the coast of Yucatan. The crater is something like 150 kilometers across and 20 deep, and the impact was so catastrophic that 75% of all species died out, including the non-avian dinosaurs. This is called the Cretaceous-Paleogenic, or K-Pg, extinction.

For specifics, it's not clear what all the effects were and what damage they did. Climate change, ocean acidification, destruction of the ozone layer… the list goes on at length, and how much damage each effect did isn't well understood, just that these things did happen and there was a huge impact on life. So the specifics are hazy.

Part of the Deccan Traps in western India. See the layering? That's not sedimentary, that's igneous rock deposition, and it goes on for more than 1,000 kilometers. This was no ordinary volcanic event. Wikipedia / Nichalp

But so is the general picture. At the same time as the impact, a supervolcano called the Deccan Traps was erupting in India. The eruption lasted for hundreds of thousands of years (!!) and the layer of lava in that area covers 500,000 square kilometers up to 2 kilometers deep. The scale of this eruption is really difficult to grasp. There’s also evidence the volume of lava output increased right after the asteroid impact, and that may be due to the colossal seismic energy of the impact opening up the throat of the eruption.

So recently the science has been leaning more toward both events doing in the dinosaurs, the asteroid and the volcano.

But a new paper has come out contradicting that conclusion. The evidence, the authors say, points squarely at the asteroid being the root cause, with the volcano eruption being at most a secondary consideration, not really involved with the K-Pg extinction at all.

Global temperature changes before and after the K-Pg imact (vertical red line) show that there were warming pulses before and after the impact, likely due to the eruptions of the Deccan Traps, but no corresponding mass extinction. Credit: Hull et al.

They looked at the global temperature changes before and after the extinction event, using various well understand proxies to measure that. For a few hundred thousand years before the impact event there was a warming pulse of a few degrees Celsius that was already substantially subsiding when the asteroid hit. Then, after the impact, there was a slow rise in temperature again, though only of about 1°C.

They then looked at the amount of carbon dioxide expected to be released from the Deccan volcano as the lava outgassed, modeling what it would actually do to global temperatures. Their computer models looked at five different scenarios: One was if the eruptions happened mostly before the impact, one where it was split half and half before and after the impact, another with big pulses of eruptions spread out before and after the impact, one where the output was mostly after the impact, and the fifth with continuous eruptions before and after.

What they found is that the last three scenarios didn’t match the temperature record at all, so were relatively easy to discount as the cause of the temperature changes. That left the first two, with eruptions mostly before the impact and split 50/50. They dig into some details, and find that either scenario works, though the one where the eruptions were split 50/50 before/after fits the data a little better.

But this brings up a glaring discrepancy. That first warming pulse, the one that peaked roughly 200,000 before the asteroid impact, isn’t reflected in the marine fossil record as an extinction event. The fish at that time did fine*.

Because of this, the authors posit that the volcanic eruption had at best a minor contribution to the extinction event. There've been some ideas that the volcano "softened up" the ecosystems, weakening life on Earth, making it more susceptible to the effects of the impact, but they find no evidence of that in the fossil record.

Therefore, their conclusion is that the extinction was indeed caused by the asteroid impact. The eruptions likely had a big effect after the impact, but it was more about how animals adapted to the new conditions versus killing off huge amounts of them as part of the extinction event.

So this is all very interesting! Given how many times the issue has moved from one side (asteroid + volcano) to the other (just asteroid), I doubt this is the last word we’ll get on this matter. I’d love to see this part of the debate resolved, though, because then we can understand better the details of the extinction. How important was it that the asteroid impacted in a shallow continental shelf loaded with things like chlorine and bromine, which, when flung into the upper atmosphere, eat away at the ozone layer? How big an effect was the impact on the Deccan Traps? How did huge amount of rocks ejected by the impact into ballistic trajectories affect the atmosphere, even on the other side of the planet, as this material came back down burning like meteors?

We still have more questions than answers, but every time we get more evidence, every time the models improve or are run in a different way, we learn something that, in part, tells us how another piece of this puzzle fits in.

*Note: It may be tempting to think that our current situation with global warming is therefore not a big deal, since a big temperature change didn't result in a mass extinction. But that's wrong; note the timescale. The warming pulse before the K-P impact took a hundred thousand years to peak. Our current rise in temperatures is screaming up hundreds of times faster. That's the difference; if it's slow species have time to adapt. Our current rate is so fast we don't have time to adapt, and we're reaping that whirlwind.