When a 10-kilometre-wide asteroid struck Earth 65 million years ago, a huge searing cloud of vapourized rock and other materials fanned out across the Earth and into the atmosphere, setting North and South America on fire. Dust from the impact enveloped the globe for three to six months, blocking out the Sun and starving all the plants and animals in the sea that relied on photosynthesis. The hot ejecta thrown into the air burned the nitrogen in the atmosphere, which in combination with water, produced acid rain. All living things at the surface of the oceans were killed. As the plants died or became dormant from lack of sunlight, the plant-eating dinosaurs starved to death. They were soon followed by the meat-eaters. The dinosaurs didn’t have a chance.
“What happens to plant life is that there is a tremendous extermination event and a great release of organic carbon into the world oceans that coincides with the [dinosaur] extinction,” says Dale Russell, senior curator of paleontology at the North Carolina Museum of Natural Sciences and a strong proponent of the asteroid impact theory. “Dinosaurs were effectively adapted to eating green plants and dinosaurs. The greatest stress that I see is one of ecosystem collapse and starvation for dinosaurs.”
The asteroid impact theory was first proposed in 1980 by Louis Alvarez, a Nobel Prize-winning physicist, and his son Walter Alvarez, a geologist. They discovered high concentrations of iridium – an element rarely found on Earth but found in abundance in extraterrestrial bodies such as asteroids and meteorites – in a thin layer of clay from Italy. The iridium was found at the Cretaceous-Tertiary (K/T) boundary, the layer of geological deposits dated at 65 million years when the dinosaurs became extinct.
Over the years, iridium has been found at the K/T boundary at more than 100 sites around the world. Some of those sites also revealed glass spherules – evidence of a hugely violent impact. (The ballistic cloud would fall back to Earth in the form of tiny glass droplets the size of sand grains).
The clincher was the discovery of an immense crater under the Yucatan Peninsula in Mexico in 1991. The crater was formed 65 million years ago and at 170 kilometres across and 600 to 800 metres deep, it is just the right size predicted by the asteroid theory.
But there are holes in the theory. David Archibald is a biologist at San Diego State University who studies the paleoecology of extinction. Based on the fossil record of North America, he points out that 76 per cent of the 49 species of aquatic animals continued to thrive, while only 28 per cent of the 58 species of land animals did. Acid rain, a result of the asteroid impact, should have affected more of the marine animals.
He also points out that there have been other asteroid impacts but no mass extinctions. There are also mass extinctions where there are no asteroid hits. While Archibald agrees that an asteroid did strike the Earth 65 million years ago, he thinks the dinosaurs were already in decline – because of volcanism and receding sea levels.
“I think it was a factor, yes,” says Archibald. But, “I don’t think it was sufficient by itself.”
Russell disagrees. “I’m definitely in the minority and I’m happy to be there… I don’t understand why there’s a controversy on this, I really don’t.”