Last gasp: toxic gas could explain great extinction.

Poisonous gas bubbling up from the deep ocean could have caused the largest extinction of species in Earth's history. A new model describes how hydrogen sulfide gas produced by marine microbes might suddenly have built up in the atmosphere 250 million years ago, poisoning land animals. The same event would have destroyed the planet's protective ozone shield and thus killed many land and marine plants.

Researchers have debated the cause of that ecological disaster, which extinguished 95 percent of marine and 70 percent of land species at the end of the Permian period. Scientists have proposed as possible culprits meteor impacts (SN: 11/22/03, p. 323), global warming from major volcanic eruptions, and changes in ocean chemistry (SN: 2/1/97, p. 74). In the May Geology, Lee Kump of Pennsylvania State University in State College and his colleagues argue that ocean venting of hydrogen sulfide gas could have transformed the Permian event from a moderate extinction into a massive one.

Using estimates of atmospheric and ocean conditions, the researchers mathematically modeled the behavior of hydrogen sulfide during low-oxygen periods of Earth's past, such as the Permian. AS the atmosphere's makeup varies over time, swaths of the deep sea periodically lose all their dissolved oxygen, says study coauthor Alexander Pavlov of the University of Colorado in Boulder. This condition permits bacteria that live without oxygen to flourish and produce hydrogen sulfide gas.

This sulfide usually changes into a benign sulfate salt when the dissolved gas encounters oxygen to flourish at an underwater boundary called a chemocline. However, if enough hydrogen sulfide accumulates in an oxygen-free, or anoxic, zone, the gas can abruptly rise and belch into the atmosphere, Kump says.

Although oxidizing gases in the atmosphere would at first destroy the toxic gas, Pavlov notes, the hydrogen sulfide would consume all the atmospheric gas available to convert it. Then, hydrogen sulfide concentrations would skyrocket within just a few hundred years, he adds.

Moreover, the model suggests that the gas would destroy ozone molecules in the upper atmosphere. Ozone blocks the sun's ultraviolet rays from reaching Earth. Loss of that protection would have killed off vulnerable species, particularly plants, that survived the toxic blast.

Most previous theories assigning the Permian extinction to changes in ocean or atmospheric chemistry have readily explained only widespread marine extinctions, says Paul Wignall of the University of Leeds in England. While widespread ocean anoxia could account for marine die-offs, the new theory provides "a nice kill mechanism for life on land," he says.

Global warming during the Permian-probably from volcanism--played a role in producing ocean anoxia, Kump says. In warm water, oxygen-consuming microbes thrive and less oxygen dissolves.

Despite today's global warming, water-circulation patterns and abundant atmospheric oxygen keep the ocean well supplied with that gas, so harmful releases of hydrogen sulfide are unlikely, Pavlov says.

Kump suspects that even during the Permian extinction, hydrogen sulfide belches were limited. The new extinction mechanism of hydrogen sulfide releases might also have prevented life from making the evolutionary jump onto land during the Proterozoic period 0.8 billion to 1.8 billion years ago.