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Timekeepers Ancient clamshells provide clues to the Atlantic Ocean’s past by Jessica Bloch

Because shifts in temperature and salinity can change the chemistry of calcium carbonate shells, scientists in UMaine’s Climate Change Institute can use them to understand changes in the Gulf of Maine in the last 1,000 years.

For schoolchildren, dendrochronology is a common lesson. Simply put, by counting the growth rings of a tree, you can tell how old it is. But trees aren’t the only living things with growth rings, and age is just a small bit of information that those rings can communicate.

University of Maine researchers Karl Kreutz and Douglas Introne are reading the rings of clamshells — a practice known as sclerochronology — as a crucial tool in understanding how the characteristics of the Gulf of Maine have changed in the last 1,000 years. Those changes provide links to a broader picture of how the climate has changed in the same time frame.

“When something happens in the North Atlantic, it’s transmitted to Maine,” says Kreutz, a professor in UMaine’s School of Earth and Climate Sciences, and the director of the university’s Stable Isotope Lab, where the shells are analyzed. The lab is a UMaine Climate Change Institute facility specializing in the measurement and interpretation of the light-stable isotopic ratio of environmentally relevant elements, such as hydrogen, carbon, nitrogen, oxygen and sulfur.

“If we can use these shells to see what has happened in the Gulf of Maine, we can interpret what’s happened in the Atlantic. And that’s really the big prize, to try to figure out if and how the North Atlantic oceanography has changed over the past 1,000 years.

“The behavior of the North Atlantic, we now know after many people looking at this over the past 20 years, has a big role to play in global climate. The Gulf Stream current and the way it changes certainly influences the entire North Atlantic region, but has more global implications as well.”

Kreutz, Introne and colleagues at Iowa State University have been collecting clamshells from the Gulf of Maine to build a detailed chronology of how the water temperature in the gulf has changed, which they can see in chemical analyses of the clamshells. Although the researchers have been able to document some results for centuries’ worth of information, a National Science Foundation (NSF) grant of $500,000 has allowed them in the last two years to continue to collect clamshells and fill in hundreds of years more of data that were missing.

“So far it looks to us as though the Gulf of Maine has actually been cooling over the past 1,000 years and was at least cooler today than 1,000 years ago,” Kreutz says. “That generally lines up with what we think was happening in the North Atlantic with the Gulf Stream. But we have huge gaps in the middle, and the results we’re getting now will fill in those gaps. As long as you have the data, you can stitch it together and can tell what’s going on year to year, or summer to summer, or summer to winter, for a detailed record.”

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Winter 2012

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