Climate Change News

Boiled or Raw, Snail Shells Keep an Environmental Archive

Snail shells discovered at archaeological sites might still accurately record past weather and vegetation despite being the leftovers of a past meal.

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Do you like your snails raw or cooked? Either is good if you’re a paleoclimatologist.

New research shows that snail shells retain their carbon and oxygen isotope signatures after being boiled and, through those proxies, likely still preserve an accurate record of past temperature, rainfall, and vegetation. In essence, boiled snails may be just as good as raw ones if you’re trying to reconstruct a paleoclimate record.

The research comes from a team led by Ola Kwiecien, an assistant professor of sediment and isotope geology at Ruhr-Universität Bochum in Bochum, Germany, and lead scientist on the study. Kwiecien knew that although snail shells are typically good environmental archives, shells collected near past human dwellings may have been cooked prior to being discarded. She wanted “to ensure that the isotopic composition of snail shells records a pristine environmental, and not thermally altered, signal, even if boiled.”

Just how do you test whether the isotopic composition of snail shells is altered under heat? “Boiling experiments were essential,” Kwiecien said.

In their boiling experiments, Kwiecien and her team found that the snail shells had the same carbon and oxygen isotope signatures within a single growth band before and after being boiled. The results are encouraging for reconstructing the paleoclimate near archaeological sites, Kwiecien explained, because snail shells recovered from human settlements may be remnants of long-ago meals and therefore were likely cooked.

Kwiecien will present these results today at the American Geophysical Union’s 2017 Fall Meeting in New Orleans, La.

Escargot as an Environmental Archive

Because of their limited mobility and ever-growing carbonate shells, snails are a valuable archive of past environmental data for any region in which they live. Two proxies for environmental conditions are particularly useful: the ratio of heavy to light oxygen isotopes (δ18O), which records the temperature and moisture level at the time a shell band grew, and the ratio of heavy to light carbon isotopes (δ13C), which records the type of vegetation eaten by the snail and then converted into shell material.

Snails “record environmental conditions as they grow,” Kwiecien said, “so the variability in the shell reflects the variability the snail experienced during its life span.”

Kwiecien explained that discarded snail shells are relatively common at archaeological sites, and their use as paleoclimate archives has been promising for connecting environmental conditions and human behavior. Humans have been eating the tiny critters for more than 20,000 years in some parts of Europe and the Mediterranean, and the leftover shells have been found at coastal archaeological sites spanning the globe. If scientists and anthropologists consider “longer time series instead of individual shells, δ13C might be an important indicator of vegetation and land use changes over millennia,” she added.

But there’s a catch: The shells that have been proven to provide this rich paleoclimate information have all been uncooked. Snails discovered near past human dwellings might have been cooked and eaten before their shells were discarded.

A question naturally arises: Can the cooking process affect paleoenvironmental proxies like δ13C and δ18O?

Bring to Boil, Measure Isotopic Signature

To answer this question, the team examined empty shells of modern, recently deceased snails. Kwiecien started her experiments with large European snails typically seen at the dinner table (Helix pomatia) but moved on to Zachrysia provisoria, a common Florida pest with more distinctive growth bands.

For each shell, the researchers drilled samples along growth bands before boiling and again after boiling the shells for 15 and then 60 minutes. Each growth band should have a single set of geochemical properties, Kwiecien explained, so drilling the same band before and after boiling should reveal whether thermal processing changes the chemical signature of the shell.

Zachrysia provisoria snail shell with drill holes
Two views of one Zachrysia provisoria snail shell used in this study, with six 0.3-millimeter drill holes along each of three growth bands. Each band contains drill holes extracted before boiling, after 15 minutes of boiling, and after 60 minutes of boiling. Credit: Ola Kwiecien and Dieter Buhl

After measuring the isotopic signatures of the samples before and after boiling, the researchers found that the δ13C and δ18O values in most of the shells were not significantly different after boiling.

What Does This Mean for Paleoclimate Studies?

Although Kwiecien’s team compared isotope values before and after boiling, they did not analyze what the shells suggest about weather or vegetation. Nonetheless, on the basis of other research involving the effectiveness of snail shells as a paleoclimate record, Kwiecien believes that the results are encouraging for scientists looking for data on past climates.

Kaustubh Thirumalai, a postdoctoral research associate at Brown University in Providence, R.I., agrees. “Snails found at archaeological sites are valuable archives of past environmental change through their geochemistry,” he said. “Studies such as this one help refine our understanding of proxy climate reconstructions and are important for robust estimates of past environmental change.”

Boiled, Poached, Roasted, or Grilled

Kwiecien would like to take the shells’ newly discovered imperviousness to boiling on the road and analyze snail shells found at well-dated archaeological sites. At sites where other forms of carbonate are rare, she explained, a shell’s paleoclimate archive would fill a critical data gap. She added that “sites [dating] from the Holocene to last glacial, covering the climate extremes, are of great potential interest.”

The researchers also plan to expand on this work by sampling more snail shells to improve the statistical significance of their results. They will also test different snail species and branch out to other cooking methods.

“Each time I was excitedly reporting preliminary results of my boiling and cooking experiments, my colleagues were laughing that without garlic butter and white wine my results simply cannot be significant,” Kwiecien said.

“Boiling is only one way of thermal processing,” she added. “I still have roasting on my agenda.”

—Kimberly M. S. Cartier (@AstroKimCartier), News Writing and Production Intern

Citation: Cartier, K. M. S. (2017), Boiled or raw, snail shells keep an environmental archive, Eos, 98, https://doi.org/10.1029/2017EO089133. Published on 15 December 2017.
© 2017. The authors. CC BY-NC-ND 3.0