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Ancient sea life faced toxic brew: lots of sulfur, little oxygen stalled burst of biodiversity.

Soon after complex animals made their first great strides onto the stage of life, the oceans brewed up a toxic chemical mix that put the brakes on evolutionary innovation, suggests a paper in the Jan. 6 Nature.

The culprits: Too little oxygen and too much sulfur in coastal waters, reports a team led by geochemist Benjamin Gill of Harvard University. Ancient creatures such as trilobites and brachiopods could not cope with the changes, and many went extinct.

The "remarkable" new data are the first to link a changing ocean environment to some of the extinctions that took place between about 490 million and 520 million years ago, says Graham Shields-Zhou, a geologist at University College London who was not on the research team.

Perhaps not surprisingly, marine creatures are exquisitely sensitive to oxygen levels. Other big extinctions, such as one occurring around 400 million years ago and another around 250 million years ago, have been blamed on low levels of oceanic oxygen. But the more ancient extinctions that Gill studied are of particular interest because they came soon after the "Cambrian explosion," during which animals blossomed in biodiversity.

Gill's team decided to look at a subset of extinctions that began 499 million years ago and lasted for 2 million to 4 million years. Other researchers had proposed that low oxygen levels could be involved, but no one had marshaled enough evidence to prove it. Gill and his colleagues are the first to look at sulfur, which at high levels can kill marine creatures.

The researchers traveled the globe to collect rock samples from Nevada, Utah, Missouri, Australia and Sweden and analyzed isotopes--different forms of an element that vary in atomic mass--of sulfur and carbon.

The amount of carbon in the rocks, as compared with the amount of sulfur, could have come about only if the water were low in oxygen and high in the sulfide form of sulfur, the team reports. Today, Gill says, a similar environment can be found in the oxygen-starved Black Sea.

Although the researchers think the Cambrian oceans were toxic, they don't know why. "What we're looking at is the aftermath of the crime scene," Gill says. "We don't have the cause for why the oceans suddenly went anoxic."

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Title Annotation:Earth
Author:Witze, Alexandra
Publication:Science News
Geographic Code:1USA
Date:Jan 29, 2011
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