By their very nature extinction events are a big deal, but two really stand out, one for its sheer scale and the other for its sudden, spectacular and shocking cause
When life nearly died
One mass extinction truly dwarfs all the others. Whereas earlier and later events each seem to have extinguished about 50 per cent of species, the end-Permian extinction was associated with a loss of 80 to 90 per cent of species in the sea and on land. Several major groups disappeared, including trilobites and giant sea scorpions called eurypterids.
The vast scale of the extinction is shown by the fact that two major structural ecosystems disappeared – reefs and forests. Nothing like that has happened in any of the other mass extinctions.
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Reefs first appeared in the Cambrian, and by the Permian had become a major ecosystem hosting substantial biodiversity, as they do today. With the loss of the dominant reef-builders, the rugose and tabulate corals, the Earth was cleared entirely of reefs. It took 15 million years for new groups of coral to evolve and build reefs once more.
Forests likewise virtually disappeared. There is a famous “coal gap” in the early and middle Triassic when no forests anywhere became sufficiently established to produce coal deposits. Key groups of forest insects, soil churners and vertebrates disappeared too.
Such a huge devastation of life might seem to imply a colossal impact. Evidence for this, however, is weak to non-existent. The most-favoured explanation is volcanic eruptions: 252 million years ago, massive volcanoes erupted in Siberia and they continued to belch forth viscous basalt lava and massive clouds of gases for 500,000 years. These were not conventional cone-shaped volcanoes but great rifts in the Earth’s crust. The rock from the eruptions now forms a vast formation known as the Siberian Traps.
Sulphur dioxide caused flash freezing for a short time by blocking the sun, but this gas dissipated rapidly. More long-lasting was the greenhouse gas carbon dioxide, which caused global warming and ocean stagnation. Repeat eruptions kept pumping carbon dioxide into the atmosphere, perhaps overwhelming the normal feedback in which plants mop up the excess through photosynthesis. The warming probably also released frozen masses of methane, an even more potent greenhouse gas, from the deep oceans.
The earliest Triassic rocks contain evidence of repeat cycles of ocean stagnation: their black colour and rich supply of pyrite indicate oxygen-poor conditions. These dark, sulphurous rocks contain very few fossils, in contrast to the abundant and diverse fossils in the limestones just below the extinction level. On land, the volcanic gases mixed with water to produce acid rain. Trees died and were swept away together with the soils they anchored, denuding the landscape. Land animals perished as their food supplies and habitats disappeared.
The slaughter of life in the sea and on land left a devastated Earth. Pulses of flash warming continued for 5 million years, delaying the recovery of life. Some “disaster taxa” such as Lystrosaurus, a pig-sized herbivore, gained a foothold here and there, but it took 10 to 15 million years for complex ecosystems to become re-established.
The demise of the dinosaurs
The extinction of the dinosaurs 65 million years ago, at the Cretaceous-Tertiary (KT) boundary, is the most recent of the major mass extinctions and the one most amenable to study. Rocks from before, during and after the event are more abundant, detailed and datable than those for older events. So its cause was just waiting to be resolved.
Up to the 1970s the best evidence suggested that the dinosaurs – along with pterosaurs, mosasaurs, plesiosaurs, pliosaurs, ammonites and many other groups – declined slowly over some 10 million years as a result of cooling climates.
Then came the bombshell. In 1980 Luis Alvarez, who had already won a Nobel prize in physics, his geologist son Walter and other colleagues published an astounding paper in Science (). The team had set out to use the element iridium as a geological timekeeper, but ended up with remarkably different findings.
Iridium is very rare on Earth’s surface, and the minute quantities that are present arrived on meteorites. These hit the Earth at a low but steady rate, so iridium can be used to mark the passage of time: the concentration of iridium in a sedimentary rock indicates how long the rock took to form.
The method worked well when the team applied it to thick sections of sedimentary rock on either side of the KT boundary at Gubbio in Italy. But at the boundary itself they found a sharp spike in iridium, 10 times the normal amount. If they had stuck to their original hypothesis, they would have concluded that the rocks were laid down by unusually slow sedimentation over a vast time span. But they rejected that in favour of the idea that the spike indicated a sudden influx of iridium from a very large meteorite or asteroid. This, they argued, was what had caused the mass extinction.
The team reasoned that such an impact would have sent up a vast cloud of dust that encircled the globe, blacking out the sun, preventing photosynthesis and so causing massive loss of life. They calculated that a crater some 100 to 150 kilometres in diameter was required, implying an asteroid 10 kilometres across.
The paper caused an outcry, mainly because it drew such a remarkable conclusion from modest evidence – but such is the stuff of the most daring scientific advances. As the 1980s progressed, geologists found more and more evidence for an impact, including iridium spikes in dozens of locations around the world, the high pressure minerals coesite and stishovite, “shocked” quartz grains, glassy spherules of melted rock and the sudden extinction of many groups of plankton worldwide. Around the Caribbean they also found ancient tsunami debris, and in 1991 the crater itself was identified at Chicxulub on Mexico’s Yucatán peninsula (see map). As predicted, it was 130 kilometres across.
There are still some serious loose ends to tie up, not least the role played by massive volcanic eruptions on the Deccan plateau of India around the time of the extinction. A handful of geogists dispute whether the impact coincides with the extinction. Even so, the consensus now is that the Alvarez team was right.