Frosted forams foil radiocarbon dating

by Mary Caperton Morton
Thursday, October 20, 2016

Climate studies often rely on radiocarbon dating of tiny shells in seafloor sediments to pinpoint the timing of when warming or cooling events began and ended. But a new study indicates that chemical reactions that take place on the seafloor may affect the accuracy of such radiocarbon dates, with potential implications for the dates published by past studies.

Seafloor diagenesis occurs when seafloor sediments are chemically and physically altered over time by chemical reactions, weathering and bioturbation of the sediments. Tiny shells left by microscopic creatures called foraminifera are often used for radiocarbon dating, but how diagenesis might affect the carbon ratios — and the accuracy of the radiocarbon dates — has not been well quantified, says Jody Wycech, a graduate student at the University of Wisconsin-Madison (UW) and lead author of the new study, published in Geology.

“This a problem that we have been aware of for decades, but people haven’t been able to directly test these diagenesis reactions as they are happening on the seafloor,” she says. The UW team of Wycech, her advisor, paleoceanographer Clay Kelly, and Shaun Marcott, studied a core sample of seafloor sediments collected off the coast of South Carolina. In the lab, they noticed a mixture of 50 percent clear, glassy shells and 30 percent white, frosted shells, with the remainder falling between the two.

The clear and frosted shells belong to the same species of foraminifera, Wycech says. The difference comes from the addition of an extra layer of carbonate that gives some of the shells an opaque, frosted appearance.

When the team radiocarbon dated the glassy shells and frosted shells in separate batches, they found the dates gleaned from the clear shells to be consistent across samples — within a few hundred years. But the dates produced from repeated analyses of the same sample of frosted, or white, shells were vastly inconsistent, varying by as much as 6,000 years. The white shells also tended to skew toward producing older dates by as much as 15,000 years. The shells appear older because the calcium coating on the shells is older, having likely come from deeper sediments and liberated by compression over time.

Paleoclimatologists already knew that the buildup of carbonate on shells after deposition could interfere with the age dating of seafloor samples that are millions of years old — but it’s a relatively new finding for more recent samples that are only a few thousand years old. Wycech says that carbonate could coat shells within a thousand years of their deposition. In most deep-sea sediments, the majority of foraminifera shells are frosted, and frosted shells are more typically selected for radiocarbon dating due to their availability. Such selection of the frosted shells can introduce notable inaccuracies in radiocarbon dating, Wycech says.

The findings have important implications for paleoclimate studies that look at short-term changes that take place over hundreds to thousands of years, such as temperature fluctuations in the ocean and the atmosphere, says Figen Mekik, a paleoceanographer at Grand Valley State University in Allendale, Mich., who was not involved in the new study.

“Paleoclimate studies depend on accurate dating. Did it happen 5,000 or 7,000 years ago? If you’re working on that level of detail, shell selection becomes very important. Studies working on timelines over tens of thousands of years are less problematic,” Mekik says. Radiocarbon dating works on samples up to about 50,000 years old.

The study also highlights the importance of being selective with samples used for radiocarbon dating, Mekik says. “Radiocarbon dating isn’t cheap. It’s important to be efficient with your samples. You need to be aware of the species when working with foraminifera,” she says. The shape and size of the shells have an effect on how the shells will age over time and how easily they will mix with other shells and sediments on the seafloor, both factors that play a role in diagenesis.

“We know that certain species are more susceptible to diagenesis than others,” Wycech says. “The species we studied are all very similar in shape, but more work will need to be done with other species and other shapes to determine how carbonate alters their radiocarbon dates.”

“As a paleoceanographer, I’m interested in this kind of work because we deal with fine timescales,” Mekik says. “If we can find an ideal species, meaning an ideally preserved species, then that helps everybody in the field.”

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