Asteroid or Eruption: New Research Sheds More Light on the Demise of the Dinosaurs

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Extinction: the end of the dinosaurs.

An asteroid impact is thought to have been instrumental in the extinction of the dinosaurs. Image by NASA.

It was interesting to read in the news this week about an apparently revolutionary theory about the demise of the dinosaurs. According to the Washington Post: “A new report suggests that a massive lava flow could share the blame with an asteroid impact credited with killing the dinosaurs at the end of the Mesozoic era.

Whatever the impression given by the news reports, the story is anything but new; and it was mentioned in passing here in Decoded Science way back in 2011. (In fairness to the Washington Post, it does concede that “there has long been an alternate theory, espoused by a rump caucus of researchers who think they’ve never been given a fair hearing”.)

The Late Cretaceous Mass Extinction

Mass extinctions are points in the Earth’s history, identified from the geological record, in which, for a variety of reasons, a significant proportion of the planet’s flora and fauna have met a spectacular and (in geological terms) rapid decline. Some may have recovered but significant numbers of species have been lost forever.

There have been several such major episodes in the planet’s history but the Late Cretaceous is the one that everybody knows about, even if they don’t recognise the name. It’s the one that did for the dinosaurs, the one which is represented in a hundred television documentaries by a computer-generated clip of a vast meteorite crashing to Earth as T-Rex looks up at the sky in what might be surprise.

The dinosaurs my have been the most spectacular casualties, but they were by no means the only ones. Although we can’t know for certain, geologists estimate that 80-90% of all marine species and up to 64% of terrestrial vertebrate species disappeared around 65 million years ago.

The Case For the Asteroid

Chixulub impact crate

The Chixulub impact crater in Mexico is the likely site of the asteroid impact. Image by USGS.

The causes of mass extinctions are varied and uncertain but they are all associated with large-scale and dramatic changes in the earth system, causing environmental change and placing enormous stresses on living organisms to adapt — or die.

Rapid changes in temperature or atmospheric composition are key aspects and the causes of these can vary.

But external factors — such as asteroid impacts — are also key factors.

Scientists first proposed the asteroid impact theory as a cause for the Late Cretaceous mass extinction in 1980 and the theory seemed to be validated by the discovery some years later of the ‘smoking gun’ — an impact crater off Mexico with the appropriate dimensions and date.

Such an impact would have vaporised almost unbelievable amounts of rock, the dust from which would have created short term global cooling; but it would also have produced vast quantities of carbon dioxide, with associated implications for both atmospheric composition and climate.

The Case For The Volcanoes

Dinosaurs

Dinosaur extinction: whodunnit? Image by North Dakota State Government.

That isn’t the full story. At around the same time a vast volcanic eruption, on a scale unimagined during human history, was taking place in what’s now India, forming the plateau known as the Deccan Traps.

Several such major outpourings of lava have occurred through geological time and are often contemporaneous with major extinctions.

The production of vast quantities of volcanic gases are the mechanism for generating environmental stress as they can have significant cooling impacts on the planet — in the the same way as the vast quantity of material produced by an asteroid impact.

The new information uses geological techniques to refine what we know about the scale and timing of the eruption of the Deccan Traps. The researchers found that this major eruption occurred around 250,000 years before the Late Cretaceous mass extinction and also produced data on its scale — with over 1.1 million cubic km of lava. In criminal parlance, the Deccan Traps were at the scene of the crime when it was committed.

So Does the Meteorite Go Free?

Inevitably, the real situation is probably rather more complicated. As the researchers note: “While the temporal relationship between large igneous provinces and mass extinctions is well established, potential kill mechanisms remain debated.”

All the available evidence suggests that the two events occurred at the time of the mass extinction but the exact role played by each is unclear, although the study concludes that the results “are consistent with the hypothesis that the eruption of the Deccan Traps contributed to the latest Cretaceous environmental change and biological turnover that culminated in the maritime and terrestrial mass extinctions.”

Which doesn’t mean the asteroid is innocent. In all probability — they were in it together.

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