Scientists have previously argued about whether the extinction was caused by the asteroid or by volcanic activity in the Deccan Traps in India, where there were a series of super volcanic eruptions that lasted approximately 1.5 million years. These eruptions spewed 1,100,000 km3 of basalt lava across the Deccan Traps, which would have been enough to fill the Black Sea twice, and were thought to have caused a cooling of the atmosphere and acid rain on a global scale.
But the evidence shows, researchers say, that the extinction was caused by a massive asteroid slamming into Earth at Chicxulub in Mexico. The asteroid, which was around 15 kilometres wide, is believed to have hit Earth with a force one billion times more powerful than the atomic bomb at Hiroshima. It would have blasted material at high velocity into the atmosphere, triggering a chain of events that caused a global winter, wiping out much of life on Earth in a matter of days.
For the Science article, researchers analyzed the work of palaeontologists, geochemists, climate modelers, geophysicists and sedimentologists who have been collecting evidence about the KT extinction over the last 20 years. Geological records show that the event that triggered the extinction destroyed marine and land ecosystems rapidly, according to the researchers, who conclude that the Chicxulub asteroid impact is the only plausible explanation for this.
Despite evidence for relatively active volcanism in Deccan Traps at the time, marine and land ecosystems showed only minor changes within 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 sulfur into the atmosphere after each volcanic eruption in the Deccan Traps 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.
One key piece of evidence linked the asteroid impact with the KT extinction 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 hit.
Evidence of 'shocked' quartz in geological records also offers a direct link between the asteroid impact and the extinction. Quartz is shocked when hit very quickly by a massive force and these minerals are only found at nuclear explosion sites and at meteorite impacts sites. The team say that an abundance of shocked quartz in rock layers all around the world at the KT boundary lends further weight to their conclusions that a massive meteorite impact happened at the time of the mass extinction.
The researchers were able to discount previous studies that suggested that the Chicxulub impact occurred 300,000 years prior to the KT extinction. The researchers say that these studies had misinterpreted geological data that was gathered close to the Chicxulub impact site. This is because the rocks close to the impact zone underwent complex geological processes after the initial asteroid collision, which made it difficult to interpret the data correctly.
Citation: Schulte et al., 'The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary', Science, March 2010, 327(5970), 1214 - 1218; doi: 10.1126/science.1177265