The worst mass extinctions of all time, a team of paleontologists argued last July, were caused not by an exploding asteroid but by simple carbon dioxide: so much of it flooded the ocean, the researchers claim, that it began poisoning the tissues and dissolving the materials that marine animals use in their shells. The mass extinctions in question occurred at the end of the Permian Period, 250 million years ago, and they wiped out 95 percent of all species. (In contrast, the asteroid or comet that eliminated the dinosaurs 65 million years ago took out a mere 50 percent of Earth’s species.) People have been arguing about the Permian extinctions for decades--just a couple of years ago there was evidence that massive volcanic eruptions in Siberia could have been the culprit. But this year’s theory, presented by Richard Bambach of the Virginia Polytechnic Institute in Blacksburg and his co-workers, makes a good story that fits nicely with much of the evidence.
During Permian times, Earth’s landmasses formed a single supercontinent known as Pangaea, and an unbroken sea covered the rest of the planet. The ocean circulation was different from that of today, in which water near the poles becomes so cold and dense that it sinks to the ocean bottom and flows around the world before surfacing again. The late Permian climate was too warm for that, and so there was little flow in the deep at all. As dead organisms rained down from the surface into this nearly stagnant water, the decay of all that material gradually sucked the oxygen out of it. With few currents flowing into the deep, there was no way to bring in fresh oxygen from the surface. Some researchers have suggested that the Permian extinctions were a case of death by suffocation, triggered when anoxic deep water was somehow mixed into shallower regions, where most of the animals were. But these theorists could never explain how anoxia might have caused the particular patterns that the extinctions took.
Bambach and his colleagues think that the anoxia theory overlooks a much more likely culprit: carbon dioxide, which is a byproduct of organic decay. While the oxygen disappeared, CO2 must have been building up in the Permian deep, and for the same reason--the ocean wasn’t being stirred up much. Bambach and his co-workers calculate that the CO2 concentration reached 30 times its present-day level.
The deep ocean, they argue, became a disaster waiting to happen. What finally unleashed it was a cooling of the climate--caused in part by the buildup of CO2 in the oceans and the subsequent reduction of CO2 in the atmosphere, which weakened the greenhouse effect. For the first time in tens of millions of years, polar glaciers appeared, chilling the neighboring surface water and sending it coursing through the deep. That circulation pushed noxious CO2-laden water from the deep into shallower regions. The extinctions began.
The CO2 not only killed marine life, the researchers suggest, but also slowly returned to the atmosphere and heated the climate back up again. The polar ice melted, and the deep-ocean circulation in turn slowed down, creating a brief respite from extinctions before the whole cycle repeated itself. While the CO2 poisoning decimated life in the sea, the rapid climate swings may have made times rough for organisms on land-- although in truth the new theory doesn’t say much about the extinctions that happened there.
And unfortunately, the Permian record doesn’t offer much in the way of sedimentary rock that might prove the researchers’ CO2 scenario actually happened. But they do find evidence of its happening at least four times during the Neoproterozoic Era, from 800 million to 543 million years ago. Chemical analysis of Neoproterozoic rocks shows that time after time, millions of tons of CO2 were stored in the deep oceans, glaciers were born, and most of the CO2 was then suddenly released back into the shallow ocean and atmosphere. Maybe the same thing happened 300 million years later in the Permian, the researchers say.
The pattern of the Permian extinctions in the sea, they argue-- which species lived and which died--supports their claim. The losers, Bambach and his colleagues say, tended to be animals with passive respiration: the ones who couldn’t control the rate at which gases go in and out of their body--and thus the rate at which acidic CO2 accumulated in their tissue and dissolved the calcium carbonate they used to build their exoskeletons. The survivors were the active breathers who could flush out the excess CO2. For instance, passively respiring corals suffered heavy losses in the Permian extinctions, while active breathers like snails and clams fared much better. The things with higher extinction rates, Bambach concludes, were those which were less able to handle carbon dioxide poisoning. qed--at least until a better theory comes along.