Clues to catastrophe

MARK RIDLEY

Michael J. Benton When Life Nearly Died, The Greatest Mass Extinction of all Time, 336 pp., Thames and Hudson, 16.95. 0 500 05116X.

The Times Literary Supplement, No. 5238, August 22, 2003, page 28 (back page).

Life on Earth is probably about 4,000 million years old, but the fossil record is poor for the first 3,500 million years. The drama of life during the 500 relatively well documented million years contains several (perhaps five) catastrophes: short scenes in which the dominant life forms die out and are replaced by something else. The most famous, and best understood, of these occurred about 65 million years ago. Then, the dinosaurs became extinct, and the dominant large animals on land since then have been mammals - horses, elephants and monkeys, for example - rather than reptiles.

The catastrophe of 65 million years ago looks almost minor compared with the larger, but more obscure, event of 250 million years ago. About 50 per cent of the then living species became extinct 65 million years ago; but more like 90 per cent of species succumbed 250 million years ago. A few million years later, perhaps 225 million years ago, the dino-saurs started their rise to prominence. If the 65 million-year-old catastrophe is remembered for taking the dinosaurs out, the 250 million-year-old catastrophe was the one that let them in. Michael J. Benton is an expert on dinosaurs, and more enthusiastic about the event that caused the rise, rather than the fall, of his favourite animals. His new book is about the mass extinction 250 million years ago. However, that extinction event is only just emerging from scientific obscurity. Scientific research on it has inevitably been influenced by what is known about other, better understood, extinctions.

The 250-million-year-old event lies under the scientific shadow of the smaller event 185 million years later. Work on the mass extinction 65 million years ago has been a model of successful science for the past twenty years. In 1980, some 65-million-year-old rocks in Gubbio, Italy, were found to contain large amounts of the rare element iridium. Meteors can contain iridium: maybe, it was suggested, a large meteor hit the Earth at that time. The meteor theory has been one of those ideas that, perhaps because they are right, seem to carry everything before them. The theory progressed from science fiction to scientific orthodoxy in about ten years. It is still going strong.

Meanwhile, science was advancing only slowly, if at all, with the earlier, larger event. The extinction of 250 million years ago occurred at the end of the period called the Permian. The fossil record is divided into ten or so major periods. Many of them were recognized and named by British geologists, working on British rock strata, in the early nineteenth century. But British rocks are none too revealing about the Permian. In the 1840s, British geologists were puzzled by what they called the New Red Sandstone, with the Carboniferous below, and perhaps blurring into the Triassic above - though the Triassic was only clearly defined in German rocks. One of the British experts, Roderick Murchison, duly trav-elled to the Urals. The rocks there solved the puzzle, and he named the period between the Carboniferous and the Triassic as the Permian, after the city of Perm in the Russian Urals.

Therein lay the problem, for the next 150 years. The rocks were in the wrong place. Western scientists were effectively shut out of the important geological sites during Soviet times. Benton describes how from time to time some Western scientists would imagine they had negotiated a visit. They would fly to Moscow, be politely shown round the Kremlin . . . and then the permissions to travel further afield would somehow not be forthcoming. Benton himself came in the mid-1990s. It was clearly an exhilarating visit, but had its challenges: "I was not used to drinking vodka by the pint", he says. Outside Russia, the other major Permo-Triassic sites are in places that were politically shut off until recently, such as China, or otherwise difficult to access, such as Greenland.

Thus, although it has been known for over 150 years that some kind of catastrophe occurred at the end of the Permian, the key facts about it have been discovered very recently. The date of the catastrophe, 251 million years ago to be exact, was only established in the year 2000. Given the date, it becomes possible to work out whether extinctions at different sites were simultaneous or staggered in time. (This matters, because the extinctions should be simultaneous if they were caused by something like a meteor impact.) The best study of how the extinctions occurred over time is for a site in China, and again was only published in 2000. The extinctions do look sudden and simultaneous there, but their exact timing relative to other sites in Greenland and the Urals remains unsettled.

The fossil record shows the catastrophe not only by the extinctions, but also by the kind of fossils that are present - and absent. The Carboniferous and Permian are great ages of coal deposition; but the end of the Permian has a "coal gap". Coal consists mainly of fossilized plants, and the coal gap probably means that plant production collapsed at this time. Atmospheric oxygen also seems to have thinned out. Photosynthesis may have reduced following the loss of plants. On the other hand, the end of the Permian has a "mushroom spike". Fungi live on dead animals and plants, and a fungal increase is inverted evidence of death.

The obvious question is, what caused the catastrophe? My impression, after reading Benton's book, is that we do not know yet. The tempting answer - that it was due to another meteor impact - can just about be ruled out. Scientists have looked hard for the clues, including an iridium anomaly, but the results are negative. A few positive claims, initially greeted by trumpet blasts from the mass media, subsequently failed when tested with repeat research.

Benton argues for another kind of catastrophic cause, of mass volcanic eruption. In east Siberia, there is a huge area of volcanic rock known as the Siberian Traps. They cover an area the size of the EU. They date to the end of the Permian, though again this has only been known recently - since 1993. The volcanic erup-tions that left us the Siberian Traps would have been massive. Benton suggests they would have released enough carbon dioxide to cause 6oC of global warming. This in turn would have melted ice into the ocean. Inside the ice there were probably pockets of poisonous gas, such as methane. When the ice melted, the gas was released into the ocean and bubbled up to the surface, killing life. He concludes: "much of this is very new work, and it might be modified in the future.... However, I'll bet on the Siberian Traps coupled with gas hydrates for the moment".

Clearly this is a fast-moving subject, and research on it is unfinished. Benton is an expert - he is a palaeontologist, and knows the science. Unlike many palaeontologists, who are content to describe individual fossils, Benton also likes to think about big questions. He knows how to communicate with a general audience. I suspect he could have translated out one or two lapses of academic-ese in this book, but in any case he always explains technical terms as they arise. He is good at leavening the science with personal stories. The Russians drink him under the table, the British media frame him with a mad theory about pigs, but the bad girls fail to penetrate his wallet. When Life Nearly Died is now the book of choice for non-specialist readers who want to find out about the biggest catastrophe in the history of life. It does not (and does not claim to) solve the prob-lem. But the Permian mass extinction could become one of science's hot topics in the next ten years. Michael Benton's book is an ideal preparation to follow the unfolding story.


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