tion of purple oceans). As anyone who has ever taken chemistry lab knows (or anyone who has had to smell rotten eggs in a closed area can attest), hydrogen sulfide is nasty stuff. Many humans have been killed by high concentrations of hydrogen sulfide found around natural gas wells, especially, coincidentally enough, around the town of Permian, Texas. This hypothesis is thus like the oxygen-drop heat-spike idea above but adds on the pulse of poisonous hydrogen sulfide entering the oceans and atmosphere as the kill mechanism at the boundary itself. As this book is being written, scientists are scrambling to confirm this intriguing proposal as an add-on to cause 4 above. As can be imagined, this new hypothesis is despised by the impact camp but is gaining favor among the rest of us working on the Permian extinction problem. Better than any of the other hypotheses, it explains the relatively sudden death at the Permian boundary, a spike of extinction overlaying a much longer-term interval of heightened species extinction. But we can improve on this.

A NEW SCENARIO FOR THE PERMIAN EXTINCTION

Here is how the Permian extinction might have occurred. First, it was caused by a succession of similar events, some smaller, some larger. The most damaging (to animals and plants) occurred 251 million years ago, but there were others both before and after, or from about 255 million to 248 million years ago.

Each event began with heat from greenhouse gases rising into the atmosphere. At the same time, the warming ocean began to favor the growth of sulfur-metabolizing bacteria as oxygen levels dropped. If deep-water hydrogen sulfide concentrations increased beyond a critical threshold during oceanic anoxic intervals (times when the ocean bottom and perhaps even its surface regions lose oxygen), then the oceanic conditions (such as those in the modern Black Sea) separating sulfur-rich deep waters from oxygenated surface waters could have risen abruptly to the ocean surface. The horrific result would be great bubbles of highly poisonous hydrogen sulfide gas rising into the atmosphere. The amount of hydrogen sulfide gas entering the late Permian atmosphere would be more than 2,000 times greater than the small



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