Dinosaurs went out with a bang, says study

8 March 2010

Dinosaur

The dinosaurs died out as a result of a huge asteroid strike rather than the eruption of a super volcano, according to a study published today in the journal Science.

Dr Paul Bown (UCL Earth Sciences) was part of a panel of researchers who analysed more than two decades’ worth of evidence to determine the cause of the Cretaceous–Tertiary (KT) mass extinction.

The KT extinction, which happened around 65 million years ago, wiped out more than half the species on Earth, including the dinosaurs, pterosaurs and large marine reptiles, clearing the way for mammals to become the dominant species.

This study of all the available evidence concludes that the KT extinction was caused by an asteroid slamming into the planet at Chicxulub in Mexico with a force one billion times more powerful than the atomic bomb at Hiroshima.

The asteroid – about the size of the Isle of Wight – would have blasted material into the atmosphere at high velocity, triggering a chain of events that caused a global winter, wiping out much of life on Earth in a matter of days.

Some scientists have argued that the KT extinction was caused by volcanic activity in the Deccan Traps in India, where a series of eruptions lasted approximately 1.5 million years. These eruptions spewed 1,100,000 cubic kilometres of basalt lava across the Deccan Traps, which would have been enough to fill the Black Sea twice, and caused a cooling of the atmosphere and acid rain on a global scale.

In the new study, researchers from UCL, Imperial College London, the University of Cambridge, and the Open University analysed the work of palaeontologists, geochemists, climate modellers, geophysicists and sedimentologists who have been collecting evidence about the KT extinction over the last 20 years.

One key piece of evidence was the abundance of iridium in geological samples around the world from the time of the extinction. Iridium is very rare in Earth’s crust and very common in asteroids. Immediately after the iridium layer, there is a dramatic decline in fossil abundance and species, indicating that the KT extinction followed very soon after the asteroid strike.

Another direct link between the asteroid and the extinction is evidence of ‘shocked’ quartz in geological records. Quartz is shocked when hit very quickly by a massive force and these minerals are only found at nuclear explosion sites and at meteorite impact sites. The researchers said that an abundance of shocked quartz in rock layers all around the world at the KT boundary lent further weight to their conclusions.

Despite evidence for volcanism in Deccan Traps at the time, marine and land ecosystems showed only minor changes in the 500,000 years before the time of the KT extinction. Furthermore, computer models and observational data suggest that the release of gases such as sulphur into the atmosphere after each volcanic eruption would have had a short-lived effect on the planet. These would not cause enough damage to create a rapid mass extinction of land and marine species.

Dr Bown, co-author of the study, said: “The hypothesis that it was an asteroid was first put forward in the 1980s, so we’ve had over two decades of intense scientific study – in fact, this is one of the most intensely studied hypotheses in all of science.

“We now have about 350 sites centred on Yucatan in Mexico, the Gulf of Mexico and the Caribbean that show geological evidence of the impact, many more around the world display a unique chemistry that can be traced to that area of impact, and also the fossil record, which shows an instantaneous geological change in marine and land animals.”

The panel discounted previous studies suggesting the Chicxulub impact occurred 300,000 years prior to the KT extinction. These studies, they said, had misinterpreted geological data gathered close to the impact site. This is because the rocks close to the impact underwent complex geological processes after the initial collision, making it difficult to interpret the data correctly.

Dr Bown said: “The reason there has been debate and controversy is that the evidence at those sites is very confusing. The impact generated a huge amount of energy, triggering all sorts of subsequent geological activity such as earthquakes, huge fires, tsunamis, and continental and oceanic landslides. Some people have argued that the sheer range of geological activity suggested a series of smaller asteroid impacts over a long period of time rather than a single strike.

“But I think this study is as close to consensus as we can get – and don’t forget it’s the work of a broad range of scientists. I hope it’s strong enough to silence some of the debate about whether the KT extinction was the result of a single unique event, or a series, or the result of volcanic activity, which was certainly happening, but had started hundreds of thousands of years earlier.”

For more information about UCL Earth Sciences, follow the link above.

Image: Half of all life, including the mighty dinosaurs, perished during the KT extinction


UCL context

Research at UCL Earth Sciences spans a diverse range of activities including crustal processes, Earth and planetary evolution, mineral physics, palaeobiology and palaeoclimatology, polar observation and modelling, natural hazards, environmental geochemistry and sedimentology.

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