Morgan and Collins Confirm Dinosaur Killer

In a paper published in the journal Science a panel of 41 internationally leading scientists, including Dr Morgan and Dr Collins from ESE, reviewed 20 years worth of research to settle the controversy once and for all

The boundary between the Cretaceous and Paleogene periods marks one of the Earth's largest and best known mass extinctions. A significant proportion of species on Earth, famously including the dinosaurs, and flying and marine reptiles, became extinct. Amongst the casualties of this abrupt event are less well know, but highly successful groups, such as the ammonites. Since the discovery of an impact-produced iridium layer at the K-Pg boundary the primary cause of the K-Pg (formally K-T) mass extinction has been hotly debated. Did the impact of an asteroid or comet blast the dinosaurs out of existence, were they poisoned by volcanic gasses from the massive volcanic eruptions of the Deccan flood basalts, or was their another as yet unidentified cause?

In a paper published in the journal Science a panel of 41 internationally leading scientists, including Dr Joanna Morgan and Dr Gareth Collins from ESE, reviewed 20 years worth of research to settle the controversy once and for all. By considering all the evidence together they conclude that the impact at Chicxulub in Mexico was the principal factor that led to the K-Pg mass extinction.

The panel based their conclusion of the strong correlation between the abrupt mass extinction and the global iridium-layer, the close match of modelled kill mechanisms and extinction patterns seen in the fossil record, and most importantly of all, on the undeniable link between the impact layer and the Chicxulub layer due to its change with distance from the crater. Based on these arguments alternative hypotheses could be discounted as significant factors in the K-Pg mass extinction. The Deccan flood basalts erupted for around 1.5 million years, beginning 500,000 yrs before the abrupt K-Pg extinction and did not match the extinction kill mechanisms. These suggested global deforestation in a matter of hours or days and an extended period of global darkness, that mostly affected photosynthetic organisms and the food chains they supported. Suggestions that the Chicxulub impact structure, a 180 km wide buried crater on the Yucutan penisular of Mexico, occurred 300,000 years earlier than the iridium layer at the K-Pg boundary, was found to be untenable. The impact layer shows a very strong change in character with distance from the Chicxulub crater, that closely matches computer models of the impact, confirming that Chicxulub is the source of the layer.

Dr Joanna Morgan has led seismic investigations of the structure of the Chicxulub crater and drilling programs to directly observe the nature of the crater and its target rocks. Dr Morgan emphasises that the research confirms the direct link between the impact and extinction kill mechanisms: "The final nail in the coffin for the dinosaurs happened when blasted material was ejected at high velocity into the atmosphere. This shrouded the planet in darkness and caused a global winter, killing off many species that couldn't adapt to this hellish environment."

Dr Gareth Collins, who has investigated the impact at Chicxulub through computer models to recreate the effects of the impact, says even the immediate effects would have been dramatic: "The explosion of hot rock and gas would have looked like a huge ball of fire on the horizon, grilling any living creature in the immediate vicinity that couldn't find shelter."

Both Morgan and Collins agree that the ultimate reason the Chicxulub impact was so destructive was the chance of its location. The impact occurred into a 3-4 km thick accumulation of carbonate and sulphate-rich rocks instantly releasing 100 to 200 Gt of atmosphere changing carbon dioxide and sulphur dioxide gas.

Despite the consensus on the cause of the mass extinction, the paper stresses that much still has to done in understanding the exact affect of the Chicxulub impact on the global environment and the biological kill mechanisms during the mass extinction.

Associated Personnel 

• Gareth Collins
• Joanna Morgan