DURHAM, N.C. --A new study suggests that differing calcium-magnesium ratios in the shells of tiny North Atlantic Ocean marine fossils may finally provide scientists with an unambiguous way to reconstruct how bottom sea water temperatures have changed during and between past ice ages.
In an article published in the Nov. 24 issue of the journal Science, Duke University geologist Gary Dwyer and colleagues from Duke and other institutions described how the fossilized remains of marine crustaceans called ostracods seem to provide more reliable measurements of ancient water temperatures than the usual indicators.
The usual method involves comparing the ratios of two forms -- or isotopes -- of oxygen present in other fossilized shell-bearing marine organisms known as foraminifera. But the Science article said those results have proven to be "equivocal." The ratios of oxygen 16 to oxygen 18 in foraminifera "have been the primary tool for reconstructing past temperatures in the deep ocean, but that's a convoluted signal," Dwyer added in an interview.
A Duke Ph.D. candidate who performed the chemical analyses for the study, Dwyer said that more oxygen 16 is indeed present in warmer seawater, whereas oxygen 18 is more abundant in colder seawater. That's because the lighter oxygen 16 tends to get preferentially stored in glaciers when seawater evaporates and later precipitates as snow or ice.
Since foraminifera also reside in seawater, and preserve an isotopic record of its composition, those that lived during glacial times should have higher ratios of oxygen 18 than those that didn't. Foraminifera shells also take up more of one or the other isotope depending on the water temperature, with more oxygen 16 being taken up by shells formed in warmer sea water.
But according to the Science article, some researchers think that ocean temperature changes reflected in the oxygen ratio data for the last glacial period seem improbably large. Those scientists suggest th
Contact: Monte Basgall