A Story of Climate Change Told Through Seashells

Cross section of a California mussel shellBy Matt Wood

Sometimes scientific discoveries happen by accident. Henri Becquerel discovered radioactivity when a uranium rock he left wrapped up in a drawer with some X-ray equipment imprinted itself on a photographic plate. Alexander Fleming discovered penicillin when he noticed that mold growing in a staphylococcus culture was killing all the bacteria around it. In 2007, Catherine Pfister and her colleague Timothy Wootton made their own accidental discovery. They were studying how species interact in the coastal waters around Tatoosh Island off the northwestern tip of Washington state when they found something alarming about the chemistry of the seawater—a discovery that points to the adverse effects of increasing amounts of fossil fuel carbon in the atmosphere.

As a routine part of their work, Pfister and Wootton were measuring pH levels in the waters around Tatoosh Island. Such readings are usually the boring part of field research, providing context for the rest of the experiments. “If you were studying tree growth you’d always have to be measuring the weather,” said Pfister, an associate professor of ecology and evolution at the University of Chicago, “And this is the weather for us, what the ocean is like.”

Instead of the slight declines predicted by models, however, Pfister and Wootton, a professor of ecology and evolution, found that pH levels in the water were dropping at an order of magnitude faster than expected.

The pH levels in seawater are part of the basic chemistry of the ocean. Plants and animals develop and interact in a certain pH and evolve as it changes naturally over time. What concerned Pfister and Wootton was how the rapid drop in pH they recorded would affect the ecosystem. “We all know that there’s a strong pH dependence of biological reactions, and we don’t know how those reactions will change if pH changes rapidly,” Pfister said.

Rapid decline in seawater pH is a symptom of what’s called “ocean acidification,” or decreasing alkalinity as ocean water absorbs increasing amounts of carbon dioxide from the atmosphere generated by burning fossil fuels. Scientists estimate that the ocean absorbs at least a third of that carbon dioxide.

In 2008, they published their findings about the declining pH levels in PNAS. What they didn’t know at the time was whether those readings were part of a sustained trend or just an unexpected natural variation. “We have a lot of concern about that,” Pfister said. “We haven’t been measuring [pH levels] for that long in the ocean. There’s a very short instrumental record in the ocean, and the instrumental record only goes back to the 1990s.” Most of the pH data on record was also from tropical waters and the open ocean, in areas with less species diversity than the rich coastal waters around Tatoosh Island.

To find out if their measurements represented a new trend, they turned to a tried and true method of measuring historical ocean environments: sea shells, specifically the shells of the California mussel (cross section pictured above). Mussel shells are made of calcium carbonate and grow annual layers or bands just like trees. Scientists often use hard structures like this to infer things about the environment in which an organism lived, so the chemical composition and growth patterns of these mussel shells can be used to study the chemical composition of the ocean.

The results of this study are published in a new paper in PLoS ONE. Pfister, Wootton, graduate student Sophie McCoy and colleagues from the University of Chicago Department of Geophysical Sciences analyzed carbon and oxygen isotopes in shells that had been growing near their instruments for the past decade, as well as shells collected by researchers 30 to 40 years ago and ones provided by the local Makah tribe from over 1,000 years ago. Carbon isotope levels drop in conjunction with pH. When they compared the shells from the three different time periods, they found that the ones from the last decade showed a precipitous drop in carbon isotope values similar to the pH decline recorded by their instruments in the earlier research. The findings both confirmed the earlier measurements and demonstrated that shells might be used to measure historical pH levels in water when no instrumental record exists.

Unfortunately, Pfister says that they’re left with another mystery. “We right now really have this enigma that we have a declining pH, and a declining pattern of carbon isotopes in these mussel shells that is greater than we expect based on our models of how the oceans should be changing,” she said. “We’re very worried about that, but we don’t have the answer in this paper for why that is.” She said ocean acidification as a result of increased carbon dioxide in the atmosphere is certainly part of that enigma.

For their next project, Pfister and her team are studying how low pH levels in seawater affect shelled organisms. They’ve already observed that the ancient Native American shells are much thicker than the ones from the last decade, and it’s known that calcium carbonate shells are harder to make at low pH. But that doesn’t mean Pfister knows how this next phase is going to turn out.

“We have been surprised at every step along the way,” she said. “In fact, in general, people have been skeptical of the results. But now other scientists are finding the same things in other locales and there’s less skepticism and much more concern about explaining these results.”

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Pfister, C., McCoy, S., Wootton, J., Martin, P., Colman, A., & Archer, D. (2011). Rapid Environmental Change over the Past Decade Revealed by Isotopic Analysis of the California Mussel in the Northeast Pacific PLoS ONE, 6 (10) DOI: 10.1371/journal.pone.0025766

About Matt Wood (450 Articles)
Matt Wood is a senior science writer for the University of Chicago Medicine and editor of the Science Life blog.

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