Toxic Seas Spurred Mass Die-Offs

Poisonous seawater probably may have driven two of the earth’s best-known mass extinctions. The origins of the two toxins are worlds apart, but one of them is making a comeback.
We all know that an asteroid the size of Mt. Everest ended the reign of the dinosaurs when it struck Mexico’s Yucatan Peninsula 65 million years ago. And we’ve heard all about the resulting firestorms, darkness and deadly gases that made trouble for life on land.

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Geochemist Lee Kump of Penn State University says something more was going on in the oceans, where 93 percent of nannoplankton—the base of the marine food web—went extinct. Dust and smoke kicked up by the asteroid would have throttled photosynthesis for several months, but it took some 270,000 years for plankton populations to bounce back.
Even in the Northern Hemisphere, which suffered a direct hit, recovery should have been much faster. In 2010 Kump and some of his Penn State colleagues explained this lag by proposing that toxic metals from the asteroid contaminated the oceans. When the super-heated debris from the disintegrating space rock hit the ocean, metals such as copper, chromium, aluminum, mercury and lead would have dissolved into the seawater at plankton-lethal levels, the team asserted. It wouldn’t take trace metal concentrations higher than a few parts per billion to inhibit plankton recovery.
 
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Kump is quick to point out that errant asteroids aren’t the only way to poison the oceans. As he reminded the audience at his plenary talk Thursday at the 2012 Goldschmidt Conference in Montréal, Earth is perfectly capable of concocting a toxic brew all on its own.
Kump thinks toxic seas were taking their toll at the end of the Permian period, about 250 million years ago. At that time, the planet suffered its Great Dying, the most extreme biological crisis yet known: 95 percent of all species were extinguished forever.
The poison that time around was hydrogen sulfide, a stinky gas toxic to plants and animals alike. And the underlying source of this massive dose of toxic gas, Kump says, was a prolonged episode of global warming. Here’s the basic process:
A prolonged, mega-dose of carbon dioxide spewing from massive volcanoes in Russia’s Siberian Traps triggers the warm spell. As the oceans warm, they cannot hold as much oxygen. As portions of the ocean lose oxygen, bacteria that produce hydrogen sulfide take over.
Computer models Kump and his colleagues created back in 2005 suggested that the late Permian seas were so full of hydrogen sulfide that they literally belched out massive clouds of toxic gas that wafted over the continents. The latest simulations reveal a far less extreme gas release, but that doesn’t mean the hydrogen sulfide didn’t still cause serious trouble.
Even if much of the toxic gas was normally trapped in deep water, hurricanes—which are more intense during warm spells—could stir it up to the surface, Kump says.
“It may be that 99.9 percent of the time the atmosphere is perfectly breathable, but then these storms come along and you continue to impose these chronic stresses,” he told Discovery News. And chronic stress over hundreds of thousands of generations of organisms can lead to mass extinctions just as easily as a quick-kill scenario can.