Salish Sea pH is dropping as carbon dioxide levels rise

By MEREDITH GRIFFITH, Islands Sounder Reporter, DECEMBER 5, 2011 — 1,340-year-old mussels tell the story, says SeaDoc-funded study


A bed of California mussels. Contributed by Cathy Pfister, PhD

 California mussels from the Salish Sea have undergone a serious transformation over the past millenium, indicates a recent scientific study funded by The SeaDoc Society. Their shells have become 30 percent thinner and are increasingly made of fossil fuel-sourced carbon.

“Corrosive water has recently been documented in the northeast Pacific, along with a rapid decline in seawater pH over the past decade,” wrote University of Chicago researcher Cathy Pfister and colleagues in the study abstract.

Fossil evidence indicates that ocean pH has fluctuated over the past 20 million years, but only within the range of 8.1 to 8.3. Pfister and fellow researcher Tim Wootton have measured pH values as low as 7.7 and 7.8 in the Strait of Juan de Fuca.

“What long-term effects in response to these drops in pH mean for ocean life is the million dollar question, but we are certain that some shelled organisms are going to be affected,” said SeaDoc scientist Ignacio Vilchis.

Concerned about rising carbon dioxide levels and plunging pH levels in the Salish Sea, The SeaDoc Society recently funded Pfister’s analysis of 1,340 to 1,000-year-old mussel shells from Makah middens on Tatoosh Island.

Because the earliest seawater pH recordings were taken in the 1990s, Pfister turned to the shells as a much longer-standing recording device for clues about the historic state of the ocean.

Probing carbon cycle changes

Many scientists believe that roughly a quarter of atmospheric carbon dioxide is absorbed into the world’s oceans. Higher sea CO2 levels result in a higher concentration of free hydrogen ions, which lowers seawater pH and takes up normally available carbonate, making it difficult for organisms to form their calcium carbonate shells.

Pfister’s study aimed to determine whether these observed carbon cycle changes have a historical precedent, or are caused by the carbon dioxide that enters the atmosphere when humans burn fossil fuels.

“Combusted fossil fuel carbon is composed of more of the lighter, naturally occurring isotope of carbon (12c), so we are able to see the signature of fossil fuel burning in current CO2 measurements,” she said.

When Pfister and colleagues compared modern, decade-old and midden shells, they found the carbon makeup has been rapidly changing from the heavier 13c form of carbon to the lighter combusted fossil fuel carbon, 12c.

Pfister said the changes have no historical precedent. …

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