In sediment traces and fossil records from one of Earth's most tumultuous periods, geologists have found a narrative linking mass extinctions with planetary biological and geological change.

After dramatic oceanic extinctions 250 million and 200 million years ago, the global carbon cycle turned chaotic. Earth's biogeochemistry went boom and bust for millions of years thereafter, as if some regulating mechanism were lost – which is exactly what happened.

"People talk about saving biodiversity, and isn't it good to have a variety of all these creatures. But the reason it matters is because ecosystem function is itself dependent on diversity in the face of normal environmental changes," said geologist Jessica Whiteside of Brown University. "Lower diversity too much, and the system will lose its resiliency. It will become a slave to otherwise minor environmental changes."

Whiteside specializes in reading the geological record of past extinctions, teasing from rocks and fossils the story of those times in Earth's history when, for one reason or another, most forms of life ceased to exist.

In the new study, published Jan. 5 in Geology, Whiteside and University of Washington biologist Peter Ward focus on two mass extinctions with especially catastrophic marine consequences: the Permian-Triassic extinction event 250 million years ago, when 96 percent of all ocean species went extinct, and the Triassic-Jurassic extinction 200 million years ago, which extinguished 20 percent of all marine families.

Scientists say that another mass extinction is now underway, with extinction rates an order of magnitude higher than normal, both on land and at sea. Studies like Whiteside's suggest what the extinction's consequences could be – not just for people, on a scale of decades or centuries, but for how the planet will work, millions of years in the future.

"Mass extinction events give us a whole suite of experiments that demonstrate what happens when you have catastrophic diversity loss," said Whiteside.

She and Ward analyzed seafloor sediments from the coast of British Columbia that accumulated during and between the two extinctions, measuring the ratios of different types of carbon.

Because living creatures metabolize only certain types of carbon, the sediment record becomes a proxy for large-scale patterns in the carbon cycle. It distinguishes those eras when ocean ecosystems – which, at the most fundamental level, serve as giant conduits of carbon between the ocean floor and atmosphere – were productive, and when they were not.

Then Whiteside and Ward looked at the fossil record of ammonoids, a class of creatures that resemble shelled squid and dominated Earth's oceans from 400 million to 65 million years ago. Their fossils are common and well-preserved enough to provide not just a record of biodiversity, but of functional diversity: how different species likely occupied similar ecological niches, providing a built-in redundancy that helps ecosystems weather the loss of individual species.

After each extinction, a rich variety of ammonoid species and body plans was replaced by a few free-floating types. In tandem with this loss of diversity, global carbon cycles oscillated wildly for millions of years. The researchers don't think this was a coincidence. Ecosystems act as thermodynamic stabilizers, and "the carbon cycle integrates biological processes with physical Earth processes," Whiteside said.

As the thinner ecosystems were strained and overwhelmed by the inevitable perturbations of volcanic activity or changes in Earth's orbit, the planet went a bit haywire. The chaos lasted for about 6 million years after each extinction, until new ecosystems formed and stabilized the carbon cycle.

Even though contemporary time is just a blink in geological terms, the findings still have modern relevance, said Whiteside. One might see, at a far smaller scale, similar patterns in regions like the Sea of Japan or off the coast of North Carolina, where overfishing and pollution have produced stripped-down ecosystems devoid of the large predators needed to maintain the rich food webs crucial to a stable carbon cycle.

"What's wonderful about looking at the past is the long lens of geological history," said Whiteside. "There is evidence that food web collapse is starting to occur in some marine ecosystems. It will take a long time for systems to recover."

*Images: 1) Ernst Haeckel's illustrations of ammonites./Wikimedia.org. 2) Courtesy *Geology.

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Citation: "Ammonoid diversity and disparity track episodes of chaotic carbon cycling during the early Mesozoic." By Jessica H. Whiteside and Peter D. Ward. Geology, online publication, Jan. 5, 2011. DOI: 10.1130/G31401.1