Oceans may have poisoned early animals

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 dissolved 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 of them 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 their surroundings, suffocating when oxygen levels drop. Other big extinctions, like one that occurred around 400 million years ago and another around 250 million years ago, have also been blamed on low levels of oceanic oxygen. But the more ancient extinctions studied by Gill, which took place in the later part of the Cambrian period, are of particular interest because they came soon after the “Cambrian explosion” in which animals blossomed in biodiversity.

Gill’s team decided to look at a specific subset of Cambrian extinctions that began 499 million years ago and lasted for 2 million to 4 million years. Other researchers had proposed that low oxygen levels — a condition known as anoxia — could be involved. But no one had marshaled enough evidence to prove that. Gill and his colleagues are the first to look at sulfur along with other elements, to piece together a complete picture of what was happening in the oceans. High levels of sulfur can kill marine creatures.

The researchers traveled the globe to collect rock samples of the proper age from Nevada, Utah, Missouri, Australia and Sweden. By analyzing sulfur and carbon isotopes — different forms of an element that vary in atomic mass — the scientists could track changes such as sediment being buried on the ocean bottom, a process that alters chemistry in the waters above.

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

Most of the toxic waters would have shoaled up along the coasts, where the majority of animals lived. Overall, the new data suggest how pulses of rising and falling oxygen and sulfur could have repeatedly devastated marine ecosystems, says Shields.

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,” says Gill. “We don’t have the cause for why the oceans suddenly went anoxic.”

His team plans to broaden the search to see if some of the other late-Cambrian extinctions could also be linked to toxic oceans.

Poisonous waters could have even been the norm through much of the planet’s history, adds Isabel Monta±ez, a geologist at the University of California, Davis. The new work, she says, “begs the question as to whether ocean anoxic events were a recurrent theme in the oceans that hosted Earth’s early metazoan life.”