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Mass Extinctions

Mass extinctions are a feature of the planet’s history. From time to time, a huge upheaval occurs which kills off vast numbers of animals and plants. The magnitude of the extinction is not measured by the number of deaths. There’s no way of knowing this. Instead it is measured by the number of species that disappear from the fossil record.

carbon isotope ratio Organic biomarkers Cambrian extinction Ordovician extinction Ordovician diversification Late Devonian extinction P-T extinction Triassic extinction K-T extinction

What causes mass extinctions?

It’s been known for more than 200 years that these mass extinctions happened. Palaeontologists originally regarded them as gradual events, and attributed them to various earthly upheavals, not fully understood.

Then, in 1980, along came the great physicist Luis Alvarez, and his son Walter. They proposed that the most famous extinction event, the one that killed off the dinosaurs (except for the birds) was, on the contrary, very sudden. It was caused by a giant meteor strike. Other space aficionados produced evidence that in fact all the five major extinction events (which we’ll come on to) were produced by impacts or other astronomical phenomena. And, because Alvarez was so famous, we all had to believe it. But gradually evidence began to accumulate that didn’t fit. At least two of the extinctions were long drawn-out events after all. And the space aficionados began to recant on some of their impact-evidence.

According to an article in New Scientist (8.12.07) there are two main causes of mass extinctions, one cosmic and one earthly. We all know about the Alvarez’s K-T event that we’ve just mentioned. What is less well known however are the Deccan Traps in India which seems to have been well under way when Alvarez’s meteorite struck. These ‘traps’ are huge outpourings of mantle material which last hundreds? of thousands of years, and produce millions of cubic kilometres of basalt lava. A single volcanic eruption, however terrible, is as nothing in comparison.

The most famous are the Siberian Traps of 250 million years ago, which caused ‘the great dying’ of the P-T extinction. We’ll be coming on to it later.

What is striking however is that the magnitude of the event itself seems to bear little relation to the magnitude of the extinction that it causes. To be sure Alvarez’s Chicxulub impact seems to have been one of the larger ones. And it did indeed cause a significant extinction (though nothing like as great as you may have been led to believe). But several other impacts, not very much smaller, seem to have had no effect at all.

The ‘great dying’ was caused by 6 million cubic kilometres of lava erupting over ?? years. But the incomparably greater Ontong Java outpouring of some 120 million years ago produced 100 cubic kilometres of basalt. And it caused “a minor marine extinction”.

The explanation seems to lie in the location. The Chicxulub meteorite landed on a continental shelf, rich in limestone, carbon-rich deposits and other nasties. Other major asteroids landed on ancient rocks that contained little carbon. This must by why their effect was so slight.

And the Siberian traps also erupted through layers of carbon-rich deposits, which would have produced carbon dioxide and the much more potent greenhouse gas methane. But that wasn’t all. There were also chlorine-rich strata. The chlorine emanating from these would have combined with the methane to produce the potent ozone destroying gas methyl chloride. By contrast the Ontong Java basalts flooded an area of the ocean floor, where there will have been far less organic material to have been decomposed into nasties. There was some of course. The ocean floor is being covered by kilometres-deep deposits of mud and dead remains. It seems to have been enough to do some mischief to marine life, but little more.

There’s more about a possible cause of some of the worst mass extinctions at the bottom of the page.

The carbon isotope ratio (Only bother with this if you are interested.)

An important line of evidence on extinction events is the carbon isotope ratio. There are 3 different forms of carbon. The vast bulk of the planet’s carbon is carbon-12. But a tiny amount of carbon-14 is produced in the upper atmosphere by cosmic rays, and it rains down to the surface. Carbon-14 doesn’t interest us here. But it’s slightly radioactive, and is useful in dating archaeological finds.

We are more interested in the carbon-13. Carbon-13 is not radioactive, and it makes up about 1% of the carbon on the planet. Normal chemical reactions don’t care which isotope of carbon they are dealing with. But what’s exciting about carbon-13 is that life’s chemistry does care. Life’s chemistry doesn’t really like carbon-13 because it doesn’t quite fit into the little niches in the enzymes that do its chemistry. So organic remains contain slightly less carbon-13 than does inorganic carbon. Scientists can analyse the carbon isotope ratio of a sample in great detail, and can estimate how much once-living matter it contained.

Organic biomarkers (Ditto.)

The latest evidence is very Hi-Tech. Ancient microbes don’t leave fossils, except in the most unusual conditions. But they do often leave ‘organic biomarkers’; in particular the phospho-lipids that they make their cell membranes with. These are made of tougher stuff. Presumably it’s totally indigestible to other microbes. And it survives in the sediments. So scientists can often get a handle on what microbes were around, and how many of them, right back to the earliest times.

The Cretaceous-Tertiary (K-T) event, 65 My ago

The most famous, and also most recent, extinction is the one that killed off the terrestrial dinosaurs – the one that separates the Cretaceous and Tertiary (or Palaeocene) periods. According to the N.S article it wiped out 50% of all species. Overall, as extinctions go, this makes it a bit of a non-event. We think it was important because it wiped out the terrestrial dinosaurs, and left the field reasonably clear for us mammals. We had been around for as long as the dinosaurs had. But we couldn’t really compete with them and remained bit players until they went. The flying dinosaurs – the birds – would think it important because it wiped out the flying reptiles and left the skies completely clear for them. The insects would probably wonder what all the fuss was about.

The K-T event does genuinely seem to have been the result of a meteor strike. It pains me to say this, because the arrogance of my fellow physicists upsets me greatly. But, the smoking gun has been found. And the latest evidence, both from carbon isotope ratios and the organic biomarkers, indicates that it was a sudden event. The amount of plant life dropped rapidly, but it recovered equally rapidly.

We should mention the great flood basalt outpourings of the Deccan Traps, which we cover in more detail elsewhere. These appear to have started before the strike, and to have continued long after it. They used to be thought a major cause of the extinction. But if it had been, then neither the extinction nor the recovery would have been sudden. Incidentally I’ve read recently that it generally takes more than one dire event to cause many creatures actually to go extinct.

The Triassic-Jurassic (T-J) event, 200 My ago

The dinosaurs got their own big chance when the previous extinction killed off most of the weird and wonderful ‘Triassic’ fauna. No doubt it killed off some species of dinosaur too. But the dinosaurs recovered and the Triassic fauna never really did. Over half the species of land animals died out. It was also a major disaster for marine life.

What caused it? We discuss this in detail below. The ‘from space’ aficionados favour an impact, of course. But I’m pleased to say that they seem to be on a hiding to nothing with this one. The recent evidence shows it to have been a long drawn out affair, spanning hundreds of thousand of years. There’s evidence of plant life suffering a number of disasters, separated by gradual recoveries, lasting tens or even hundreds of thousands of years. It may be significant that the supercontinent of Pangaea was beginning to break up around this time, setting off massive volcanic eruptions.

According to New Scientist, the event was caused by ‘the Central Atlantic eruption’. Like other flood basalt eruptions, it must have lasted a long time, although the amount of lava that poured out is ‘unclear’. And the lava must have pushed through some pretty potent strata, because temperatures soared and 95% of plant species were wiped out too. Those that survived swapped their large leaves for small or multi-lobed ones. Apparently these are better at dissipating heat. There’s evidence that carbon dioxide levels shot up by over 3½ times.

It seems to have been the T-J extinction that gave flowering plants their opportunity. Latest evidence suggests that they had been around for a while. But the extinction cleared the way for them to become much bigger players.

The Permian-Triassic (P-T) event, 250 My ago

Before the T-J event came ‘the great dying’, the worst mass extinction since Cambrian explosion. It was the one that separated the Permian from the Triassic period. Ninety five percent and sixty six of land animal species went extinct. By contrast the plant kingdom suffered much less.

The P-T extinction was another long drawn-out event. It lasted 160,000 years.

This event also coincided with another great flood basalt outpouring, this time the ‘Siberian Traps’. It was also during the life of the great supercontinent of Pangaea. It cleared the way for brand new flora and fauna to emerge, in particular the strange Triassic animals that preceded the dinosaurs.

The late Devonian event, 365 My ago

The late Devonian extinction is a bit of a mystery. It came in two bites about a million years apart. And it wiped out at least 70% of all marine animal species, but mainly in the tropics. What about the land animals? Well there weren’t any yet, apart from creepy-crawlies.

And it doesn’t seem to be clear that there was any major cataclysm to have caused it. Some scientists blame global warming and others blame global cooling. More recently a huge impact crater has been found in Australia. However there are a lot of impact craters around, and relatively few of them are associated with extinctions.

Some palaeo-botanists reckon that the extinction was caused by the evolution of trees. These days of course, trees are credited with the exact opposite effect. But conditions then were quite different. The massive tree roots broke up the rocks and caused terrible erosion. This released huge amounts of nutrients into the rivers and oceans and created huge blooms of algae. The algae took all the oxygen from the water and suffocated much of the marine life.

It was soon after this that the first fish invaded the land (more). Maybe this isn’t a co-incidence. Before crawling out on to the mud, some fish had already learned to breathe air to a certain extent. What better incentive to acquire this ability, than living in water that was desperately short of oxygen?

The Ordovician event, 440 My ago

This appears to have been the second worst event of all time, when about 85% of species were wiped out.

Its cause also seems to be a bit of a mystery. It’s a mystery that may never be solved, on account of it being so long ago.

There was an ice age on at the time. The Gondwana (Sahara) region was at the South Pole, which helped ice sheets to form. The sea level fell which uncovered large areas of coastal margins, destroying rich coastal habitats. I was surprised to read however, that it was also allegedly a time of high CO₂, which should have made it warm.

This is the event that gave vascular plants their big chance.

The Great Ordovician Diversification, 465 My ago.

We keep telling you not to expect things to be simple in this game. Here is an example of a dramatic expansion of certain sea creatures – which is attributed to a smashed up asteroid raining debris down on earth.

The diversification involved a group of sea-floor-dwelling filter feeders that exploded into life’s biggest ever burst of evolutionary variety. Other forms of marine life don’t seem to have benefited from whatever gave these filter feeders their dramatic leg up.

It seems that an asteroid collision caused the debris rain. First came a cloud of dust. Then within ½ million years larger bits of debris appeared. Later still the incidence of crater-forming impacts rose 5- to 10-fold.

The timing is so perfect that the asteroid event almost has to be the cause of the diversification. The only snag is that there is yet no convincing explanation of how these events could have brought such huge benefits to just this single class of marine life. But that’s science for you.

The Cambrian event, 520 My ago

The first extinction of, as it were, modern times came quite shortly after the Cambrian explosion itself. There will have been many before then, in particular during the great Varangerian ice age. But this far back, the fossil record is sparse and difficult to read.

Information is sparse about the Cambrian event too, and experts differ on exactly how severe it was. Some palaeo-biologists reckon that up to 80% of all ‘genera’ disappeared. (I’m not sure what the significance is of their being ‘genera’ and not ‘species’.) They all lived underwater because life hadn’t invaded the land at that time. Others point out that the event cannot have been too bad because the trilobites sailed through unscathed.

More about the cause of mass extinctions

As we’ve seen there is no one cause of mass extinctions. It’s now clear that the famous K-T event was almost certainly caused by an impact from space. It may also have been helped by the massive noxious outpourings from the Deccan traps.

The first three known extinctions, the Devonian, Ordovician and Cambrian events, are all probably too long ago for any particular cause to be established.

But the common cause of the more recent T-J and P-T events is now becoming clearer. I reckon that it’s reasonable to speculate that it could have had a hand in the earlier extinctions too.

The process starts with global warming, triggered by outpourings of CO₂ caused by a flood basalt event. You tend to get these events, plus a great deal of volcanism during certain stages of the supercontinent cycle. The volcanoes play their part but it’s the flood basalts that do the real damage.

This global warming warms up the oceans. Now there are vast deposits of methane hydrate lying around on the seabed today, and there probably always have been. And it’s the cold water above it that keeps it there. If the oceans warm up too much, then incredible amounts of methane would be released into the atmosphere. And methane is a far more potent greenhouse gas than CO₂. So what may have started out as mildly serious global warming would rapidly become deadly serious global heating. Indeed the release of the current deposits is one of the fears that scientists have for our future today.

But, according to an article in Scientific American (October 06), that’s not the worst of it. And if it’s in Sci. Am. then you have to take it seriously. This picture comes from the Sci. Am. article. The writing explains the scenario in a little more detail than we do. To see a full-sized version, click on the picture.

Serious global heating would release the H₂S (hydrogen sulphide) bugs. This is terrible news because H₂S is both a potent destroyer of the ozone layer and also toxic to all life. Without the ozone shield, UV radiation from the Sun destroys any life that is left.

There are H₂S bugs lying in wait in the Black Sea, just waiting for a bout of global warming to release them. Why the Black Sea? Because it’s stagnant. Today’s oceans are stirred up by the ocean currents, so that the entire water column shares in the oxygen that is absorbed at the surface. The H₂S bugs can’t abide oxygen, so they don’t get a look in.

But presumably the Black Sea is too small to generate these vital currents, and there’s a layer of oxygen-free water at the bottom, that is home to a large population of oxygen-hating sulphur-eating bugs. They live at the bottom, turning the sulphur into H₂S. However, at the boundary between the oxygen-free water and the oxygenated water above it, is a population of H₂S eaters. They operate their own form of photosynthesis. They too can’t abide oxygen, but they need light to convert the H₂S back into sulphur. The sulphur then rains down to the depths and feeds the H₂S makers.

These sulphur bugs operate a system almost equivalent to the better known oxygen bugs. The famous cyano-bacteria, other types of bacteria and of course plants, use the energy of sunlight to split water into hydrogen and oxygen. They use the hydrogen, and excrete the oxygen as a waste product. They die, and all sorts of scavenging bugs, and animals, effectively burn the dead remains; obtaining the energy they need to live – and using up oxygen in the process.

Periodic Table aficionados will not be surprised to learn of this sulphur system; because sulphur is in the same group as oxygen, one period down.

The boundary between the two waters is crucial. As the water warms, the boundary moves up towards the surface. If it reaches the surface then it’s Armageddon.

So far we’ve only talked about the Black Sea. But researchers have found biomarkers of these H₂S bugs in sea-floor deposits, from mass-extinction times, all around the world. This indicates that entire oceans became stagnant, and took part in the process we’ve just described.