In 1980, when examining sediments dating from the time at which dinosaurs suddenly ceased roaming the Earth, a Nobel Prize-winning physicist, Luis Alvarez, and his geologist son Walter made an extraordinary discovery. Right at the junction between rocks of the Cretaceous and Paleogene periods (the “K-T boundary“), marking one of the greatest mass extinctions of all time, they found a thin clay layer (first near Gubbio, Italy, subsequently the world over) rich in the rare element iridium. Only a sudden massive extraterrestrial impact could have created that layer. Dino-world had ended with a very big bang indeed.
Walter described the discovery in his 1997 blockbuster T. rex and the Crater of Doom. It was a huge success that did more than just make dinosaurs even sexier. It handed the public a real “nuclear winter” scenario. Carl Sagan, no less, made the connection. The Alvarezes had also helped us realise that nuclear war was even deadlier than we thought.
Never was a more exciting, timely, better aimed science story backed by scientists and writers with higher credentials. And it was not long before the “crater of doom” was found – conveniently close to the USA, in the Gulf of Mexico, offshore from a place called Chicxulub. As the dino-loving public eagerly embraced the “killer impact” theory, Chicxulub crater became irresistible to researchers and their funders.
Unfortunately, like all those things we believe because we want to, it was wrong. At least, so said a group of researchers led by Professor Gerta Keller of Princeton University. Keller, an expert on the faunal changes at this time, never denied the existence of an end-Cretaceous impact. She challenged only the linkage between it and the Chicxulub crater – which she held was 300,000 years too old. Unfortunately that connection constituted Chicxulub’s funding paydirt. Keller’s contention was not popular among other scientists (by then running the peer-reviewed journals and organising conferences) whose careers were being built on proving the connection true.
As they closed ranks against her, Keller began finding it difficult to gain a hearing. Indeed, even for writing about her work, a share of reflected odium found its way to me. I had, I was once told, “done a disservice to science” by giving “oxygen of publicity” to “that bloody woman”. When one of the scientific sisterhood (for such it was) is moved to language of that kind about another, you know there’s something far more serious than science at stake. Keller had threatened to dynamite the gravy train. The empire was striking back.
Dating craters precisely is not easy because of the mess they make of the rocks in which they form. So Keller looked for, and has now found, a record of continuous, quiet sedimentation at a locality in Texas far enough from Chicxulub to escape its most disruptive effects. These sediments cover the period from over a million years before the impact to about 300,000 years after the iridium anomaly that precisely marks the K-T boundary worldwide. They have confirmed her belief; the Chicxulub impact predated the boundary by 300,000 years, and didn’t kill off a single microfossil species.
As Keller told me: “Conventional wisdom holds that any large impact leaving a 175km-diameter crater must cause major mass extinctions. But this hypothesis is based solely on the assumption that Chicxulub was the K-T killer. No other major mass extinction in Earth history is associated with major impacts. This hypothesis has no empirical support and must be considered false – at least with respect to Chicxulub.”
So what did kill the dinosaurs (and all the other things)? Well, impacts certainly helped. But they were part of a global environmental crisis (including massive volcanism in India) that together mounted an almost lethal assault upon the whole edifice of life over several million years. In Earth history, there are no single causes.
Luis Alvarez died in 1988. Walter, now 68 and one of the most decorated geologists in the USA, is soon to publish another enjoyable and entertaining book, In the Mountains of St Francis (WW Norton). It is not about the great 1980 discovery but, being set in the Apennines around many of the key localities, it contains many references to it – not one of which gives the slightest hint that Chicxulub’s “smoking gun” status has even been challenged.
Science claims to be an internally consistent, self-correcting system of knowledge. And so it is. But it isn’t a perfect, well-oiled machine. It is a human enterprise. Like the Earth, it takes time.
