Universe a grand tour of modern science Phần 5 doc

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.

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extremophiles

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, up to two kilometres thick, which normally belongs on the deep ocean floor.

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.

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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.

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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.

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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

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catastrophe to life. There was no longer any slop in the chronological

accounting, which previously left Earth scientists free to choose whether or not

they wished to see direct connections between events. The time had come for

them to decide whether they were for or against cosmic impacts as a major

factor in global geology as well as in the evolution of life.

By 2002, the end-Permian event of 250 million years ago had a basalt flood and

a mass extinction but no crater, although there were other hints of a possible

impactor from outer space. A clearer prototype was the end-Cretaceous event of

65 million years ago, with a global mass extinction, a basalt flood in India, and a

crater in Mexico.

‘To some Earth scientists, the need for a geophysically plausible unifying theory

linking all three phenomena is already clear,’ declared Paul Renne of the

Berkeley Geochronology Center. ‘Others still consider the evidence for impacts

coincident with major extinctions too weak, except at the end of the Cretaceous.

But few would dispute that proving the existence of an impact is far more

challenging than documenting a flood basalt event. It is difficult to hide millions

of cubic kilometres of lavas.’

There will be no easy verdict. Andrew Saunders of Leicester, spokesman for the

dating effort on the buried part of the Siberian Traps, was among those sceptical

about the idea that impacts can express themselves in basalt floods. ‘Some

scientists would like to say that the West Siberian Basin itself is a huge impact

crater,’ Saunders said, ‘but except for the presence of basalt it looks like a

normal sedimentary basin. And if crust cracking is all you need for flood basalts,

why don’t we see them in the biggest impact craters that we have?’

The controversy echoes a broader dispute among Earth scientists about the role

of mantle plumes, which could provide an alternative explanation for the

Siberian Traps. For that reason, the verdict about impacts and flood basalts will

depend in part on better images of the Earth’s interior, expected from a new

generation of satellites measuring the variations in gravity from region to

region. Neither side in the argument is likely to yield much ground until those

images are in, from Europe’s GOCE satellite launched in 2005. Meanwhile, the

search for possible matches between crater dates and flood basalts will continue.

E A closely related geological topic is Hotspots. For more about impacts, including the

discovery of the 65-million-year-old crater in Mexico, see Impacts. Catastrophes for life

are dealt with also under Extinctions.

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T he flowers on display in the 200-year-old research garden in Valencia,

Jardı´ Bota`nic in the Catalan language, change with the seasons, as is usual in

temperate zones. The Valencia oranges for which the eastern coast of Spain

is famous flower early in spring, surrounded by blooming rockroses, but in

summer the stars of the garden are the water hyacinths, flowering in the middle

of the shade. In winter the strawberry trees Arbutus unedo will catch your eye.

‘All flowering plants seem to use the same molecular mechanisms to govern

their dramatic switch from leaf-making to flower-making,’ noted Miguel

Bla´zquez of the Universidad Polite´cnica de Valencia. ‘I want to know how the

control system is organized, and linked to the seasons that best suit each

species.’

For 10,000 years the question of when plants flower has been a practical concern

for farmers and horticulturalists. Cultivated wheat and barley, for example, were

first adapted to the seasons of river floods in the Middle East, but they had to

adjust to spring rains and summer sunshine in Europe. The fact that such

changes were possible speaks of genetic plasticity in plant behaviour. And year￾round floral displays in well-planned gardens like Valencia’s confirm that some

species and varieties take advantage even of winter, in the never-ending

competition between plants for space and light.

During the 20th century, painstaking research by physiologists and biochemists

set out to clarify the internal mechanisms of plant life. Special attention to the

small green chloroplasts in the cells of leaves, which capture sunlight and so

power the growth and everyday life of plants, gradually revealed the molecular

mechanisms. The physiologists also discovered responses to gravity, which use

starch grains called statoliths as sensors that guide a seed to send roots down

and stems up. They found out how growth hormones concentrate on the dark

side to tip the stem towards the light. Similar mechanisms deploy leaves

advantageously to catch the available light.

To help it know when to flower, a plant possesses light meters made of proteins

and pigments, called phytochromes for red light and cryptochromes and

phototropins for blue light. By comparing chemical signals from the

phytochromes and the cryptochromes with an internal clock, like that causing

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