11 December 2013
The shell of a tiny marine mollusk carries evidence of the ocean conditions that formed it, researchers have found. These “butterflies of the sea” could be used to determine the temperature and carbon dioxide levels of ancient oceans, they said this week at the American Geophysical Union’s Fall Meeting
“When you want to know something about our future climate, people like to go back into the past, because we had conditions on Earth where we had high temperature and high (carbon dioxide),” said Nina Keul during a poster session Tuesday afternoon. Keul is a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York.
Traditionally, scientists have estimated historical ocean characteristics, such as temperature and dissolved carbon dioxide levels, by studying the carbonate-rich shells of foraminifera, a small plankton species. Studies of these shells from marine sediment cores reveal details of ocean conditions through time. But foraminifera are not found in certain geographic areas and time periods. In some areas, such as parts of the Arctic, these sediments are rich in pteropods, a small marine mollusk.
To verify that pteropods can record ocean conditions in their shells, Keul grew a single species, Limacina helicina, in glass jars with varying temperatures and levels of dissolved carbon dioxide. The jars contained a fluorescent dye that incorporated into newly formed parts of the shells.
Keul then exposed the fluorescent areas to a laser, which blasted off the carbonate-rich material and sent it into a mass spectrometer. The resulting analyses of trace minerals in the shells showed that certain elements correlated with ocean conditions. Strontium levels increased with temperature, and uranium increased with both temperature and dissolved carbon dioxide.
To validate that this method could reveal conditions in a natural environment, Keul collected pteropod shells from the Fram Strait in the North Atlantic Ocean, east of Greenland, over a 10-year period. She set out a sediment trap 100 meters below the surface that caught sinking ocean debris. When she compared the levels of strontium and uranium in the shells from 2001 to shells from 2010, she found detectable differences in the mineral levels.
“We see the variation out in the real world just in a 10-year time frame,” said Keul. The results show that strontium and uranium levels in shells may be useful as a proxy for determining ocean temperature and carbon dioxide levels.
Although Keul only looked at a few species of pteropods, she expects other species would likely show the same relationship between trace minerals and ocean conditions, but perhaps to different degrees. “Every species forms the calcium carbonate a little bit differently,” she said.
In future experiments, Keul plans to examine the uranium and strontium levels in pteropods from Antarctic sediment traps. Eventually, she hopes to apply these techniques to pteropods in sediment cores, to see what ancient shells can reveal about historical ocean conditions.
“We’re very hopeful that this is a promising new w
ay to tell something about past climates,” said Keul.
– Patricia Waldron is a science communication graduate student at UC Santa Cruz