IT'S a striking feature when seen from above, a circular lake 75 kilometres wide. Manicouagan, also known as the Eye of Quebec, was formed when an asteroid around 5 kilometres across struck northern Canada, gouging out a crater that was originally 100 kilometres wide.
That makes Manicouagan (below right) the fifth largest impact crater on Earth, not much smaller than the 170-kilometre Chicxulub crater in Yucatán, Mexico, site of the impact that ended the reign of the dinosaurs 65 million years ago. Many blame the extinction on a "nuclear winter" caused by the dust and sulphate aerosols thrown up by the impact. Clearly the Manicouagan strike, too, must have had a similar impact.
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| ©NASA |
| Manicouagan Impact Crater on Earth
Credit: STS-9 Crew, NASA |
For a while it was blamed for the mass extinction that marks the end of the Triassic period 200 million years ago. But a 1992 study produced a surprise: it showed the crater is 214 million years old, too old to be the culprit.
Amazingly, it now looks as if life recovered quickly after the impact, which left hardly a trace in the fossil record. "This was something probably as large as the Chicxulub impact, but there was barely a whimper in terms of climatic or biotic response," says geologist Michael Arthur of Pennsylvania State University in State College.
Manicouagan is not the only puzzle. About 35 million years ago, a massive asteroid slammed into Siberia, forming the 100-kilometre-wide Popigai crater. This, too, had no apparent long-term effect on life. How is this possible?
The mystery deepens when you consider the immense volcanic events known as flood basalt eruptions. An eruption 250 million years ago covered huge areas of Siberia in basalt up to 6 kilometres thick and has been blamed for the worst ever mass extinction, known as the Great Dying, which marks the end of the Permian period. Global warming triggered by the release of vast amounts of CO2 from the lava is thought to have played a big role in the extinction.
The Siberian event, however, was dwarfed by a flood basalt eruption that occurred in the western Pacific 120 million years ago. This produced more than 10 times as much lava, covering vast areas of the sea floor in a layer 30 kilometres deep, known as the Ontong Java Plateau. It is the largest known flood basalt eruption and should have released even more CO2 into the seas and thence the atmosphere than the Siberian lavas, yet resulted in only a minor marine extinction.
So why do some asteroid impacts and flood basalt eruptions devastate life on our planet while others make little difference? The answer, some researchers now think, is location: what really matters is what's in the rocks vaporised by meteorites or roasted by massive eruptions.
Some of the first hints of this came from studies of the Paleocene-Eocene Thermal Maximum (PETM) 55 million years ago, which caused a minor marine extinction. At the time, the Earth was as much as 5 °C warmer than it is today. Temperatures shot up a further 6 to 8 °C in just a few thousand years, with the Arctic Sea surface reaching a balmy 23 °C.
Many deep-sea species vanished, probably because the seas were starved of oxygen. Warmer waters hold less oxygen and can stall ocean circulation, cutting the oxygen supply to the depths - an oceanic anoxic event.
Carbon isotopes show the sudden warming was caused by huge amounts of fossil carbon being released into the atmosphere, but where did it come from? One suspect is the methane hydrate ice buried on the continental shelves, which is very sensitive to changes in temperature. If deep waters warmed rapidly by just 1 or 2 °C, the hydrates could release billions of tonnes of methane - a potent greenhouse gas. Such a "methane burp" could warm the globe.
Then again, what triggered the methane burp in the first place? One idea is that slight warming allowed it to occur spontaneously; another is that an impact is to blame. But the oceans don't warm for no reason, and no crater of the right age has been found.
Enter Henrik Svensen of the University of Oslo in Norway, whose team was studying the oil and gas-rich Norwegian Sea. The ocean floor there is part of the massive lava fields known as the North Atlantic Igneous Province, formed by the violent opening of the north Atlantic Ocean 55 million years ago. Svensen found that the lava from this huge flood basalt eruption had found its way into carbon-rich mudstone in hundreds of places, insinuating itself between the mudstone layers to form sheets, or sills, between 100 and 300 metres thick. Nearby he found 735 pipe-like hydrothermal vent complexes.
Exploding upwards
Svensen realised that the sills would have heated the mudstones, turning their carbon into methane. This methane would have built up deep below the seabed and eventually exploded upwards, forming the hydrothermal vents. The size and force of the explosions would have sent most of the methane to the sea surface without it dissolving or oxidising to CO2, as would happen if it was released slowly (Nature , vol 429, p 542).
While basalt lava does contain some CO2, Svensen calculates that the lavas would have baked off at least 10 times as much carbon from the mudstone as if they had simply degassed straight into the atmosphere. This, he thinks, was the trigger for the initial warming, perhaps leading to a massive methane burp later on, which sent temperatures soaring even higher.
The debate over the causes of this event certainly isn't settled, but more precise dating of rocks supports Svensen's idea. If he is right, the real cause of the soaring temperatures and the subsequent extinctions during the Paleocene-Eocene Thermal Maximum was the sudden release of large amounts of fossil carbon into the atmosphere. Sound familiar?
Sensing he was on to something, Svensen next went to South Africa to study the 183-million-year-old Karoo-Ferrar large igneous province. This eruption coincided with the Toarcian extinction, which hit marine species hardest, probably due to an oceanic anoxic event like that caused by the PETM.
The Karoo lavas forced their way into huge underground coal and oil shale deposits in the ancient continent of Gondwana, and their remnants are now found in parts of South Africa, Antarctica, Argentina and Tasmania. In the Karoo desert, Svensen found pipe-like structures much like the vents in the Norwegian Sea dotting the landscape, 150 metres across and almost perfectly round. Locals had drilled as deep as a kilometre for groundwater, allowing Svensen to peer into the massive structures.
Here, too, Svensen thinks intruding lava cooked the coals and carbon-rich shale, building up gases over 200,000 years that eventually exploded through to the surface, forming the thousands of pipe-like structures. The pipes are chock-full of baked black shale.
In all, Svensen calculates the magmas could have produced as much as 27,400 gigatonnes of carbon, though far less is likely to have made it into the atmosphere (Earth and Planetary Science Letters , vol 256, p 554). Human activity today produces about 13 gigatonnes of carbon each year.
Studies of fossil leaves by Jennifer McElwain of University College Dublin in Ireland back the idea. "There are two lines of evidence that support the theory of a large spike in atmospheric carbon dioxide," she says. "There's the geologic data and our plant CO2 data."
Such findings are encouraging researchers to take a new look at other extinction events, including the biggest. While climate change has long been a suspect in mass extinctions, the idea that flood basalt eruptions can trigger the sudden release of huge amounts of methane or CO2 by heating carbon-rich rocks provides a new mechanism for it to happen. "For a long time, people noticed that large igneous provinces and large environmental changes co-occurred," says geologist Stephen Hesselbo of the University of Oxford. "It was like 'why didn't we think of that?'"
He has been studying the mass extinction that marks the Triassic-Jurassic boundary around 200 million years ago, when between 50 and 80 per cent of all species disappeared. This coincides with the flood basalt eruption on the Gondwana supercontinent that formed the Central Atlantic Magmatic Province, when, McElwain's work suggests, atmospheric CO2 rose from 600 to 2100 parts per million.
The Great Dying
The lava may have erupted into lake sediments rich in carbon, Hesselbo suggests. However, because the rocks were torn apart and buried as America and Africa parted, reconstructing the details is difficult.
Svensen, meanwhile, has turned his attention to the Siberian Traps and the Great Dying, during which 95 per cent of all marine species and two-thirds of all land species vanished. Climate models show how higher atmospheric CO2 concentrations could have triggered an anoxic oceanic event, which may have been especially bad because atmospheric oxygen levels were already low . Riddled with anaerobic bacteria, the oceans might have belched enough poisonous hydrogen sulphide into the atmosphere to poison species on land.
Some researchers think the oceans warmed enough to trigger a methane burp, which led to yet more warming. Analysis of fossil leaves at the time indicate that CO2 levels in the atmosphere may have soared as high as 8000 parts per million, compared with today's 380.
The release of carbon from the lavas that formed the Siberian Traps has long been a prime suspect in the Permian extinction. But rather than look at the lavas, Svensen's team looked at the surrounding rocks. Along with carbon-rich mudstones and coals were some things he hadn't seen before - layers of gypsum, halite and other minerals up to 2 kilometres thick. "The main thing we had to ask ourselves was: is there something special about Siberia that could cause such a big extinction? The answer is yes," he says.
Salts are rich in chlorine, which can combine with methane to form the ozone-destroying gas methyl chloride. If enough methyl chloride escaped into the atmosphere during the mega-eruption, increased ultraviolet levels would add to the woes of species trying to cope with other changes.
Still, Svensen needed a smoking gun - the pipes. On a return trip last year, he found them. As in the Karoo, the Siberian pipes have bits of baked coals and shales in them. The difference is they are much, much larger. Some were 1.5 kilometres in diameter, topped by ancient craters 700 metres deep with roots reaching down 4 kilometres into the bedrock. There were hundreds of them, too many for Svensen to count reliably.
This huge outpouring of gases could have helped turn the Siberian eruptions into one of the biggest killers ever. By contrast, the much larger Ontong Java eruption occurred in an area where the lava was unlikely to encounter any carbon or chlorine-rich rocks, possibly explaining why it was less harmful.
Asteroids and comets may also need to hit carbon-rich targets to wreak havoc on Earth's biosphere. The Manicouagan asteroid landed in ancient gneissic rocks, which contain little carbon, Arthur points out. The same is true of the Popigai impact.
On the other hand, the Chicxulub crater is in a continental shelf thick with limestone, carbon-rich sediments and salt deposits. Vaporising these rocks would have sent vast amounts of sulphur dioxide into the atmosphere as well as carbon, initially triggering acid rain and cooling, followed by long-term warming. This might account for the death of more than half of all species on the planet, including the dinosaurs.
Some geologists are applying the idea not only to Earth's great calamities, but to smaller events as well. As many as 40 volcanic events over the past 300 million years might have led to significant long-term changes in climate, says Gregory Retallack of the University of Oregon. He cites the Columbia River Flood Basalts in the north-west US. This eruption 17 million years ago was small by flood basalt standards. Because the lavas infiltrated and baked large coal seams in central Oregon, though, the eruption could have driven CO2 levels as high as 600 parts per million. "It was not a mass extinction, but still a significant event," says Retallack.
Warming at the time was enough to push plants northward that had previously grown at only lower latitudes, similar to what is happening today as a result of human-induced warming. "We have this amazing fossil flora in Oregon that looks like that of Tennessee, and basically we're going to be there again fairly shortly," he says.
Fossils of plants throughout the Permian period also suggest that there were several greenhouse warming events - perhaps as many as five - leading up to the Great Dying. These smaller events may also have been caused by volcanism intruding into carbon-rich rocks, Retallack says, and by analysing carbon isotopes in fossil peat bogs around the world, he thinks he can pinpoint the sources. Bogs closest to the source of gases should have the most dramatic shifts in carbon isotope composition, enabling him to trace ancient CO2 or methane plumes back to their vents.
Such work could change the way we think about extinctions. If the researchers are right, in many cases what turned an impact or mega-eruption into a disaster for life was the release of huge amounts of fossil carbon and other gases. Global warming is emerging as a killer.
Is warming really so bad for life on Earth? Yes, suggests a recent study by Peter Mayhew of the University of York, UK. His team compared temperature estimates over the past half-billion years with biodiversity. They found not only that rapid temperature rises are associated with extinctions, but that there is less diversity when temperatures stay high, though why this should be is not clear.
Even so, Svensen and his like-minded colleagues have their work cut out to convince other geologists. There is no shortage of ideas about what caused past extinctions, and the sparse evidence makes it hard to distinguish between the possibilities - especially given that some mass extinctions may have been caused by a combination of several factors, including "nuclear winters", low atmospheric oxygen, ozone destruction and ocean anoxia.
Still, compared with the other hypotheses about mass extinctions, Svensen's ideas do have one thing going for them: we're about to find out what happens to life on Earth when huge amounts of fossil carbon are suddenly injected into the atmosphere. Check back here in a century or two for the preliminary results of this mega-experiment.
Comment: Recent indications are that you won't have to wait that long.