Universe a grand tour of modern science Phần 5

extremophiles Antarctica seemed more challenging. Penguins huddle together in the winter darkness to minimize their heat loss. On the other hand the nematode Panagrolaimus davidi, a worm almost too small to see, which lives among algae and moss on ice-free edges of Antarctica, regularly freezes solid each winter. It can chill out to minus 358C with virtually all its metabolism switched off, and then revive in the spring. In laboratory tests, it can go down to minus 808C without problems. Investigating the nematode’s survival strategy, Wharton found that the rate of cooling is critical. It survives the rather slow rate experienced in the wild but fast freezing in liquid nitrogen kills it. Cryptobiosis is the term used for such suspended or latent life. Various animals and plants can produce tough larvae, seeds or spores that seem essentially dead, but which can survive adversity for years or even millennia and then return to life when a thaw comes, or a shower of rain in the desert. To Wharton, these cryptobiotic organisms are not true extremophiles. Assessing the ability of larger animals to cope with extreme conditions, as compared with what archaea and bacteria can do, Wharton judged that only a few groups, mainly insects, birds and mammals, are much good at it. Insects resist dehydration with waxy coats. Warm-blooded birds and mammals contrive to keep their internal temperatures within strict limits, whether in polar cold or desert heat. On the other hand, fishes and most classes of invertebrate animals shun the most severe habitats—the big exception being the deep ocean floor. ‘We think of the deep sea as being an extreme environment because of the high pressures faced by the organisms that live there,’ Wharton commented, a quarter of a century after the discovery of the animals of the hydrothermal vents. ‘Now that the problems of sampling organisms from this environment have been overcome, we have realized that, rather than being a biological desert, as had been assumed, it is populated by a very diverse range of species....Perhaps we should not consider the deep sea to be extreme.’ E For related subjects, see Global enzymes, Life’s origin, Tree of life and Extraterrestrial life. 296 T he road from mumbai to pune, or Bombay to Poona as the British said during their Raj, takes you up India’s natural rampart of the Western Ghats. It’s not a journey to make after dark, when unlit bullock carts compete as hazards with the potholes and gullies made by the monsoon torrents. Natural terraces built of layer upon layer of volcanic rock give the scarp the appearance of a staircase, and Ghats is a Hindi word for steps. In the steep mountains and on the drier Deccan Plateau beyond them is the triangular heartland of the Indian peninsula. It is geologically odd, consisting mainly of black basalt,uptotwokilometresthick,whichnormallybelongsonthedeepoceanfloor. Preferring a Scandinavian word for steps, geologists call the terraced basalt ‘traps’. The surviving area of the Deccan Traps is 500,000 square kilometres, roughly the size of France. Originally the plateau was even wider, and rounder too. You have to picture this region as hell on Earth, 65 million years ago. Unimaginable quantities of molten rock poured through the crust, flooding the landscape with red-hot lava and spewing dust and noxious fumes into the air. It was not the only horrid event of its kind. Flood basalts of many different ages are scattered around the world’s continents, with their characteristic black bedrock. In the US states of Washington and Oregon, the Columbia River Plateau was made in a similar event 16 million years ago. The Parana flood basalt of south-east Brazil, 132 million years old, is more extensive than the Deccan and Columbia River basalts put together. Plumb in the middle of Russia are the Siberian Traps. Around 1990 several investigators confirmed that the flood basalt there appeared almost instantaneously, by geological standards. Through a thickness of up to 3500 metres, the date of deposition was everywhere put at 250 million years ago. This was not a rounded number. The technique used, called argon–argon dating, was accurate to about 1 million years. The basalt builds the Siberian Plateau, which is flanked to the east by a succession of unrelated mountain ranges. To the west is the low-lying West Siberian Basin, created by a stretching, thinning and sagging of the continental crust. During the 1990s, prospectors drilling in search of oil in the basin kept hitting basalt at depths of two kilometres or more. 297 flood basalts Geologists at Leicester arranged with Russian colleagues to have the basalt from many of the West Siberian boreholes dated by argon-argon at a Scottish lab in East Kilbride. Again, it all came out at almost exactly 250 million years old. So a large part of the flood basalt from a single event had simply subsided out of sight. This meant that the original lava flood covered an area of almost 4 million square kilometres, half the size of Australia. The speed and magnitude of the event make it ghoulishly fascinating. In Iceland in 1783 the discharge of just 12 cubic kilometres of basalt in a miniature flood killed the sheep by fluoride vapour and caused ‘dry fog’ in London, 1800 kilometres away. In Siberia, you have to imagine that happening continuously for a million years. The Siberian affair’s most provocative aspect was that the huge volcanic event coincided precisely with the biggest disaster to befall life on the Earth in the entire era of conspicuous animals and plants. At the end of the Permian period, 250 million years ago, the planet almost died. About 96 per cent of all species of marine animals suddenly became extinct. Large land animals, which were then mammal-like reptiles, perished too. ‘The larger area of volcanism strengthens the link between the volcanism and the end-Permian mass extinction,’ the British–Russian team reported. Again the dating was good to within a million years. And it forced scientists to face up to the question: What on Earth is all this black stuff really telling us? I A tangled web The facts and theories about flood basalts had become muddled. In respect of the recipe for the eruptions there were two conflicting hypotheses. According to one, a hot plume of rock gradually bored its way upwards from close to the molten core of the Earth, and through the main body, the mantle. When this mantle plume first penetrated the crust, its rocks melted and poured out as basalt. The other hypothesis was the pressure cooker. The rock below the crust is quite hot enough to melt, were it not squeezed by the great weight of overlying rock. Crack the crust, by whatever means, and the Earth will bleed. The relief of pressure will let the basalt gush out. That happens all the time, in a comparatively gentle way, at mid-ocean ridges where plates of the Earth’s outer shell are easing apart. Basalt comes up and slowly builds an ever-widening ocean floor. According to the pressure-cooker idea, just make a bigger crack at a point of weakness in a continent, and basalt will haemorrhage all over the place. There are old fault-lines everywhere, as well as many regions of stretched and thinned crust. The pressure cooker is much more flexible about candidate localities for flood-basalt events. With the mantle-plume hypothesis you need a pre-existing plume. 298 flood basalts Flood basalts often herald the break-up of a continent. Both in the eastern USA and West Africa are remnants of 200-million-year-old basalts released just before the Atlantic Ocean began to open between them, in the break-up of the former supercontinent of Pangaea. The South Atlantic between south-west Africa and Brazil originated later, and its immediate precursor was the 132-million-year-old flood basalt seen in Brazil’s Parana. A sector of the Atlantic that opened relatively late was between the British Isles and Greenland. The preceding basalt flood dates from 60 million years ago. Famous remnants of it include Northern Ireland’s Giant’s Causeway and Fingal’s Cave on the island of Staffa. The latter inspired Felix Mendelssohn to compose his Hebrides Overture, in unconscious tribute to the peculiarities of flood basalts. When the Deccan Traps formed, 65 million years ago, India was a small, free-range continent, drifting towards an eventual collision with Asia. The continental break-up that ensued was nothing more spectacular than the shedding of the Seychelles, as an independent microcontinent. Whether the effect on worldwide plate motions was large or small, in the mantle-plume theory the basaltic outbursts caused the continental break-ups. The pressure-cooker story said that a basalt flood was a symptom of a break-up occurring for other reasons. Another tangled web of ideas concerned the mass extinctions of life. In the 1980s, scientists arguing that the dinosaurs were wiped out by the impact of a comet or asteroid, 65 million years ago, had to deal with truculent biologists, and also with geologists who said you didn’t need an impact. The disappearance of the dinosaurs and many other creatures at the end of the Cretaceous Period coincided exactly with the great eruption that made the Deccan Traps of India. Climatic and chemical effects of so large a volcanic event could be quite enough to wreck life around the world. The issue did not go away when evidence in favour of the impact became overwhelming, with the discovery of the main crater, in Mexico. Instead, the question was whether the apparent simultaneity of impact and eruption was just a fluke. Or did the impact trigger the eruption, making it an accomplice in the bid to extinguish life? I Awkward coincidences Space scientists had no trouble linking impacts with flood basalts. The large dark patches that you can see on the Moon with the naked eye, called maria, are huge areas of basalt amidst the global peppering by impact craters large and small. And in 1974–75, when NASA’s Mariner 10 spacecraft flew past Mercury three times, it sent home pictures showing the small planet looking at first glance very like the Moon. 299 flood basalts The largest crater on Mercury is the Caloris Basin, 1500 kilometres wide. Diametrically opposite it, at the antipodes of the Caloris Basin, weird terrain caught the attention of the space scientists. It had hummocky mountain blocks of a kind not seen elsewhere. The Mariner 10 team inferred a knock-on effect from the impact that made the Caloris Basin. Seismic waves reverberating through the planet came to a strong focus at the antipodes, evidently with enough force to move mountains. Translated to terrestrial terms, a violent impact on Brazil could severely jolt the crust in Indonesia, or one on the North Pole, at the South Pole. This remote action enlarges the opportunities for releasing flood basalts. The original impact might do the job locally, especially if it landed near a pre-existing weak spot in the crust, such as an old fault-line. Or the focused earthquake waves, the shocks from the impact, might activate a weak spot on the opposite side of the planet. Either way, the impact might set continents in motion. Severe though it may be, an impactor hasn’t the power to drive the continents and the tectonic plates that they ride on, for millions of years. The energy for sustained tectonic action— earthquakes, volcanoes, continental collisions—comes from radioactivity in the rocks inside the Earth. What impactors may be able to do is to start the process off. In effect they may decide where and when a continent should break. Advocates of impacting comets or asteroids, as the triggers of flood basalts, had plenty of scope, geographically. There was evidence for craters in different places with very similar ages, suggesting either the near-simultaneous arrival of a swarm of comets or a single impactor breaking up before hitting the Earth. So you could, for example, suggest that something hit India, or the Pacific seabed at the antipodes of India, 65 million years ago, to create the Deccan Traps, irrespective of what other craters might be known or found. In 1984, Michael Rampino and Richard Stothers of NASA’s Goddard Institute for Space Studies made the explicit suggestion, ‘that Earth’s tectonic processes are periodically punctuated, or at least modulated, by episodes of cometary impacts.’ Many mainstream geologists and geophysicists disliked this challenge, just as much as mainstream fossil-hunters and evolutionary theorists abhorred the idea of mass extinctions being due to impacts, or flood basalts. In both cases, they wished to tell the story of the Earth in terms of their own preferred mechanisms, whether of rock movements or biological evolution, concerning which they could claim masterful expertise. They wanted neither intruders from space nor musclers-in from other branches of science. The glove thrown down by Rampino and Stothers therefore lay on the floor for two decades, with just a few brave souls picking it up and dusting it from time to time. The crunch came with the new results on the Siberian Traps, and especially from the very precise dating that confirmed the match to the end-Permian 300 ... - tailieumienphi.vn