The God Species_ How the Planet Can Survive the Age of Humans - Mark Lynas [111]
The major markers in geological time are the mass extinctions, of which there have been five major episodes in the last half-billion years. Corals, it turns out, were hit hard in all of them. At the end of the Ordovician period, about 434 million years ago, living reefs disappeared and did not reappear in the fossil record for 4 to 6 million years. Another “reef gap” can be seen after the Late Devonian mass extinction, 360 million years ago. Yet another appears after the biggest mass extinction of all, at the end of the Permian, 251 million years ago. In that case, reef-builders disappeared for 10 million years, the biggest hiatus in all the Earth’s history. When reefs eventually reappeared in the early Triassic, the old tubular corals had gone forever, to be replaced by the scleractinian—or stony—corals that still dominate reefs today. The end of the Triassic saw another extinction pulse and reef gap, while the extinction that killed off the dinosaurs also exterminated the majority of corals.32
So was there a common cause? Most of the major mass extinctions correlate well with periods of sustained volcanism, when vast basalt “provinces” flooded out from deep within the Earth’s mantle, releasing colossal quantities of CO2 in the process. The late Triassic extinction, 200 million years ago, wiped out as much as half of life on Earth—and took place at the same time as an enormous volcanic episode tore apart the supercontinent of Pangaea, laying down a 7-million-square-kilometer area of “flood basalts” that can today be found as far apart as the eastern U.S., Brazil, and Morocco.33 Termed the Central Atlantic Magmatic Province by geologists, as this lava flooded out of cracks in the Earth’s crust it simultaneously released between 8,000 and 9,000 billion tonnes of carbon in the form of CO2, which in turn may have triggered another 5 trillion tonnes of carbon to belch out from the seafloor in thawing deposits of methane hydrate.34 Even spread over ten or twenty thousand years, that is an awful lot of carbon—and the ensuing extreme greenhouse effect and bout of ocean acidification are recorded in both carbon isotopes35 and a worldwide “calcification crisis” as calcium carbonate-forming plankton were wiped out. In their stead, the hot and acidic oceans bloomed with green “disaster” plankton species, whose remains depleted oxygen levels in the waters below and left them stagnant and anoxic.36
Fifty million years earlier, at the end of the Permian period, the biggest mass extinction of all time wiped out as many as 95 percent of species—and once again volcanic CO2, followed by a massive oceanic methane hydrate release, has been fingered as the main cause. All coral reefs, and most calcifying organisms, simply ceased to exist. This time the “flood basalt” eruption took place in modern-day Siberia and was also the largest of all time, releasing perhaps as much as 30 trillion tonnes of carbon. The rate of release has been estimated at around 1–2 billion tonnes per year for around 50,000 years.37 Although the total carbon release was much greater than anything humans are likely to manage, the all-important rate of release was twenty times slower than modern fossil fuel burning. Even so, the ensuing reef gap in the fossil record lasts for ten million years. Where corals did survive as organisms, they may have done so in “naked” form in refuges, without their carbonate skeletons;38 there is also evidence that calcium carbonate rocks were dissolving in more acidic ocean waters.39
A less ancient carbon spike—taking place a mere 55 million years ago—is perhaps the best analogue for the oceanic consequences of human fossil fuel combustion.