Mass extinctions

I am become Death, destroyer of worlds
Oct 22nd 2009
From The Economist print edition

The story of how the dinosaurs disappeared is getting more and more complicated

EVERYONE knows that the dinosaurs were exterminated when an asteroid hit what is now Mexico about 65m years ago. The crater is there. It is 180km (110 miles) in diameter. It was formed in a 100m-megatonne explosion by an object about 10km across. The ejecta from the impact are found all over the world. The potassium-argon radioactive dating method shows the crater was created within a gnat’s whisker of the extinction. Calculations suggest that the “nuclear winter” from the impact would have lasted years. Plants would have stopped photosynthesising. Animals would have starved to death. Case closed.

Well, it now seems possible that everyone was wrong. The Chicxulub crater, as it is known, may have been a mere aperitif. According to Sankar Chatterjee of Texas Tech University, the main course was served later. Dr Chatterjee has found a bigger crater—much bigger—in India. His is 500km across. The explosion that caused it may have been 100 times the size of the one that created Chicxulub. He calls it Shiva, after the Indian deity of destruction.

Dr Chatterjee presented his latest findings on Shiva to the annual meeting of the Geological Society of America in Portland, Oregon, on October 18th. He makes a compelling case, identifying an underwater mountain called Bombay High, off the coast of Mumbai, that formed right at the time of the dinosaur extinction. This mountain measures five kilometres from sea bed to peak, and is surrounded by Shiva’s crater rim. Dr Chatterjee’s analysis shows that it formed from a sudden upwelling of magma that destroyed the Earth’s crust in the area and pushed the mountain upwards in a hurry. He argues that no force other than the rebound from an impact could have produced this kind of vertical uplift so quickly. And the blow that caused it would surely have been powerful enough to smash ecosystems around the world.

Double whammy

In truth, agreement on the cause of the mass extinction at the end of the Cretaceous (when not only the dinosaurs, but also a host of other species died) has never been as cut and dried among palaeontologists as it may have appeared to the public. One confounding factor is that the late Cretaceous was also a period of great volcanic activity. In India, which was then an island continent like Australia is today (it did not collide with Asia until 50m years ago), huge eruptions created fields of basalt called the Deccan Traps. Before the discovery of Chicxulub, the climate-changing effects of these eruptions had been put forward as an explanation for the death of the dinosaurs. After its discovery, some argued that even if the eruptions did not cause the extinction, they weakened the biosphere and made it particularly vulnerable to the Chicxulub hammer-blow.

There are also puzzling anomalies in the pattern of extinction. The greatest of these is that, as fiery and horrible as the impact would have been, the survivors included many seemingly sensitive animals like birds, frogs and turtles. Moreover, close inspection of the fossil record shows that many “Cretaceous” species disappear both well before, and well after, the signs of the impact that are found in the rocks.

Ironically, it was while he was investigating the Deccan Traps that Dr Chatterjee came across the evidence for Shiva. First, he found dinosaur nests that had been built between lava flows 10-15 metres thick—evidence that the animals were coping well with the volcanic activity rather than being weakened by it. Then, quite suddenly, 65m years ago, a layer of lava nearly 2km thick appears. This led him to wonder what could possibly have caused such a sudden volcanic surge.

He knew that the west coast of India had been the site of an ancient impact of unknown age and size. It was not until he was reading through a paper published by an oil company that had collected geological information in the area that he realised the volcanic surge he had seen might be related to a cosmic collision.

Further examination revealed a crater rich in shocked quartz and iridium, minerals that are commonly found at impact sites. (These are also the telltales in distant layers of ejecta that the rock in question has come from an impact.) Most important, the rocks above and below Shiva date it to 65m years ago. Dr Chatterjee therefore suggests that an object 40km in diameter hit the Earth off the coast of India and forced vast quantities of lava out of the Deccan Traps. As well as killing the dinosaurs the impact was, he proposes, responsible for breaking the Seychelles away from India. These islands and their surrounding seabed have long looked anomalous. They are made of continental rather than oceanic rock, and seem to be a small part of the jigsaw puzzle of continental drift rather than genuine oceanic islands.

This story, though, raises the question of why there is but a single ejecta layer of iridium and shocked quartz in late Cretaceous rocks around the world. One answer might be that the two impacts were, in effect, simultaneous—that the objects which created Shiva and Chicxulub were the daughters of a comet that had broken up in space and hit the Earth a few hours apart, as the pieces of comet Shoemaker-Levy 9 hit Jupiter in 1994. Other, smaller craters in the North Sea and Ukraine have been prayed in aid of this theory. Recent research suggests that there was, in fact, no impact in the North Sea at all, but the Ukrainian site does appear to be the result of a genuine collision that took place at the time of the dinosaur extinction and could therefore be connected with Shiva. But even if it is, it is so much smaller that its environmental effect would have been both minor and local.

Extensive dating research at Chicxulub, however, now suggests that the object which created that crater actually struck 300,000 years earlier than the dinosaur extinction, meaning there really should be two ejecta layers. That there are not could be explained by the fact that the accumulation of sediment in most rocks is so slow that the two layers are, in effect, superimposed. Alternatively, it could be that no one has been looking for two layers, so they have not seen the double signature or have ignored its significance. Indeed, two iridium layers have been found in some places. Anjar, an Indian town north of the impact site, is one. That is leading Dr Chatterjee to suggest that the two big impacts did take place at different times.

The picture that is emerging, then, is of a strange set of coincidences. First, two of the biggest impacts in history happened within 300,000 years of each other—a geological eyeblink. Second, they coincided with one of the largest periods of vulcanicity in the past billion years. Third, one of them just happened to strike where these volcanoes were active. Or, to put it another way, what really killed the dinosaurs was a string of the most atrocious bad luck.

'Hey look over there...a 500km crater!'

i know right?

My Jewish boss explains that the earth is only roughly 6,500 years old. So there could be no dinosaurs. End of discussion.

The bones were put in the Earth by God to test our faith, I guess?

Arrogant scientists wrong again!

There were no dinosaurs. God planted dinosaur bones to test our faith.

Ok, so that cartoon has probably broken into my top 5 favourite cartoons.

really? the most cliche' comment about extinct/imaginary animals is now in your top 5. congrats.

change dinosaurs to unicorns, minotaurs, quaggas, dodos, black role models, or pegasus' and you have a coffee table book!

flew into the sun