Killer asteroid

Though sedimentary rocks formed during the Cretaceous and Tertiary are limestone, there is a layer of clay at the K-T boundary. Geologists call it the boundary clay. Dinosaurs and other living things that disappeared in the mass extinction 65 million years ago left tell-tale signs of their existence in fossils in this layer of clay. This clay is the site of the mass murder.

Dead bodies - fossils in this case - are not the only evidence of murder. Killers also leave other clues. One day, an American geologist stumbled on a thin layer of clay. The evidence he unearthed opened up a new line of inquiry into the death of the dinosaurs.

In the late 1970s, Walter Alvarez was studying a limestone rock in a gorge outside the northern Italian town of Gubbio. The rock resembled a sandwich. The bottom or older layer consisted of white limestone full of tiny fossils from the Cretaceous. Next there followed a dull red layer of clay about 2 centimetres thick, after which began the top layer consisting of greyish pink limestone, but almost devoid of the Cretaceous fossils. Undoubtedly the clay layer was the boundary clay. Below this layer are the remains of the dinosaurs. Above this layer they are missing.

Some 100,000 tonnes of dust from outer space rain on Earth every year. This invisible cosmic dust is deposited with other sediments when sedimentary rocks are formed. If geologists know the rate at which the cosmic dust falls and how much dust is present in a certain layer of rock, they can find out how long it took to deposit that layer of rock.

The age of the boundary clay can also resolve the question of whether the K-T extinction was a sudden or a slow event that took place over millions of years. 'Sudden' in the geological sense ranges from a few days to hundreds or even thousands of years.

Fossil records show that ammonites, tiny spiral-shelled marine animals, lived right up to the K-T boundary and then disappeared suddenly. It is believed that dinosaurs, who lived at the same time as ammonites and whose fossils are rare, also disappeared suddenly. But fossil records fail to tell us how long it took for the extinction to occur. Alvarez hoped that the Gubbio layer would provide an answer.

When he analysed the clay for cosmic dust, Alvarez failed to find out how long the Gubbio layer took to be deposited. But he discovered something very strange, which provided a crucial first clue to the identity of the mass killer. 'That is what detectives and scientists need: a lot of hard work and occasional lucky break', he remarked.

The lucky break was the discovery of iridium - very large amounts of iridium - in the Gubbio layer. Luis Alvarez, Walter's father, a Nobel-Prize-winning physicist, suggested that the iridium had an extra-terrestrial source. He also predicted that the iridium anomaly, as the whopping amounts of iridium came to be known, should be worldwide. Since this prediction in 1980, Walter Alvarez and other scientists have discovered the same concentrations of iridium as in the Gubbio layer in the K-T boundary clay in Denmark, Spain and New Zealand, and in deep-sea cores from both the Atlantic and the Pacific.

Alvarez, father and son, concluded that some 65 million years ago a large asteroid plunged out of the sky and hit Earth, throwing up a great cloud of dust that quickly covered the planet like a blanket, blocking sunlight for several years. The dust cloud slowly deposited its iridium-rich debris worldwide. This extra-terrestrial impact wiped out the dinosaurs, along with nearly 75 per cent of all other species.

If an asteroid did hit Earth 65 million years ago, where is the impact crater? In 1980, an oil company drilling off the coast of Mexico's Yucatan peninsula stumbled across a great near-circular structure, buried under the surface near the village of Chicxulub, which means 'tail of the devil' in ancient Mayan. It had not been spotted earlier because it was buried beneath 1,100 metres of limestone. No one bothered about the structure until the American geologist Alan Hilderbrand learnt about it from a local reporter in 1990. Investigations by Hilderbrand and other geologists revealed that the bowl-shaped structure was indeed an impact crater rather than some kind of volcanic structure. It is about 180 kilometres across and 20 times as deep as the Grand Canyon. This estimate of the size is based on boreholes drilled in the search for oil.

The powerful argon-argon technique of dating a rock retrieved from a borehole confirmed that the crater was formed 65 million years ago, when a shallow sea covered the region. Scientists estimate that it was formed by an asteroid about the size of San Francisco zooming at 40 times faster than the speed of sound. The asteroid packed the energy of a 100-million-megaton bomb.

When a giant meteorite or asteroid hits rocks on the

Earth's surface, it leaves some evidence of impact. Scientists have found evidence for tektites, shocked quartz and iridium associated with the Chicxulub crater. The K-T boundary layer of the rocks also has large concentrations of iridium, as suggested for an extra-terrestrial impact by Walter Alvarez. Alvarez, who has dubbed the Chicxulub crater 'the crater of doom', believes that it is the best evidence for his theory: 'It looks to me like this is the smoking gun.'

Two other smoking guns have also been found. In 1996 the American scientist Frank Kyte claimed that he had found a pebble in mid-ocean 900 kilometres due west of the Chicxulub crater. The coarse-grain pebble, which is about 2.5 millimetres long, contains iron and iridium in quantities similar to meteorites. The pebble was found in a layer of rock deposited at the same time as the dinosaurs disappeared. Kyte believes it came from outer space: 'There was no way a rock a few millimetres across could get there other than falling from the sky.'

In 1996, after sifting through rocks in the Chicxulub crater, the American scientist Benjamin Schuraytz found two nuggets of iridium. They weigh a few trillionths of a gram and are 99 per cent pure. Schuraytz believes the impact was so powerful that it vaporised other metals, leaving almost pure iridium, which vaporises at more than 4,400 degrees Celsius.

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