Under the Microscope:Mass extinction of life on earth has occurred several times since life arose nearly four billion years ago. The best known extinction, in which the dinosaurs died, occurred 65 million years ago and there is good evidence it was caused by an asteroid collision with earth.
Asteroid collision has also been the conventional working hypothesis to explain most of the other extinctions. However, recent evidence, described by Peter Ward in Scientific American (October 2006), challenges the asteroid hypothesis and indicates that several of these other extinctions were caused by killer-greenhouse conditions that caused the seas to belch toxic gases into the atmosphere.
Unless we arrest the current trend of global warming we could trigger the next mass extinction by 2200.
There have been five mass extinctions over the past 500 million years. The first one happened 443 million years ago at the end of the Ordovician period. The second occurred 374 million years ago, the third 251 million years ago (Permian), the fourth 201 million years ago and the last 65 million years ago. The Permian extinction, the biggest, killed 90 per cent of ocean life and 70 per cent of land life.
The extinction 65 million years ago, at the boundary between the Cretaceous and Tertiary periods, is called the K/T event. There is considerable evidence it was caused when a 10km-diameter asteroid hit earth in the Yucatan Peninsula of Mexico. The evidence for the asteroid hypothesis includes a distinct layer of iridium, naturally uncommon on earth but a common metal in extraterrestrial rock, laid down around the globe at the time, and also pressure-shocked stone scattered around the earth. Vast amounts of debris thrown into the atmosphere from the collision shielded earth from the sun's heat and the freezing dark conditions killed off more than half the life on earth.
Up until recently, dilute evidence led most geologists to conclude that four of the five big extinctions listed above were caused by asteroid or comet collisions with earth (the Ordovician event is explained by radiation from an exploding nearby star). But contrary evidence has gradually accumulated. For example, the extinction at the K/T event happened suddenly, after which things quickly started to recover, as one would expect from a sudden dramatic impact. But the extinctions of the Permian and Triassic periods were drawn out over hundreds of thousands of years.
Studies of plant communities show they were decimated, re-formed, decimated again, re-formed and decimated again over intervals exceeding 50,000 to 100,000 years. For this to be explained by asteroids would call for a whole succession of asteroid strikes, each thousands of years apart. But there is no geological evidence for such a string of impacts.
There is now considerable evidence that three of the four extinctions in question were caused by earthly events, not by extraterrestrial intervention. Large-scale volcanic activity was associated with most extinctions, adding much warming greenhouse gases, carbon dioxide and methane, into the atmosphere. This raised the temperature of the earth, including the oceans. As ocean temperature rises, the ocean absorbs less oxygen from the atmosphere, becoming oxygen-poor (anoxic).
In our present oceans the concentration of oxygen is relatively constant in the water column from top to bottom because of ocean circulation carrying oxygen rich water downwards. However, under some unusual conditions, e.g. the Black Sea, anoxic conditions that support many types of oxygen-hating (anaerobic) organisms can exist deep beneath the surface. These anaerobic organisms generate lots of the gas hydrogen sulphide which dissolves in the water and diffuses upwards to meet the oxygen-bearing water falling downwards. Under normal circumstances the oxygenated and hydrogen sulphide bearing water stay separated and the stable interface is called the chemocline.
Photosynthetic green sulphur bacteria live near the chemocline. These organisms generate energy by oxidising to sulphur the hydrogen sulphide coming from below and they also use the sunlight filtering down from above in the daylight to photosynthesise carbohydrate and other complex biochemicals.
The relative anoxia in the oceans resulting from volcanic activity described previously favoured the deep-sea anaerobic organisms which proliferated producing great amounts of hydrogen sulphide. This would destabilise the chemocline which could float up to the top of the ocean releasing great bubbles of highly toxic hydrogen sulphide to the atmosphere. Samplings taken of the mass extinction boundaries, except the K/T boundary, show the widespread remains of green and purple sulphur bacteria, indicating that the oceans, even at the surface, had reverted to anoxic conditions enriched in hydrogen sulphide. Computer simulations indicate that enough hydrogen sulphide was emitted at the end of the Permian to cause extinctions on land and sea. Hydrogen sulphide in the atmosphere would also have attacked the earth's ozone layer allowing lethal levels of ultraviolet light to come to earth to play havoc with remaining life on earth.
So, the new hypothesis is that several mass extinctions in the past were driven by global warming when carbon dioxide levels in the atmosphere rose to about 1,000 parts per million (ppm). Carbon dioxide levels today are around 385ppm and climbing. If this trend continues at the present rate, carbon dioxide levels will reach 900ppm by 2200. This could trigger a new greenhouse extinction, which would be a neat example of the Gaia mechanism proposed by James Lovelock. According to Gaia thinking, if we abuse the planet enough it will retaliate and destroy us. We are fortunate that we have discovered the dynamics of how the earth system works. The question remains - are we wise enough to allow this knowledge guide our actions?
William Reville is associate professor of biochemistry and public awareness of science officer at UCC: understandingscience.ucc.ie