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 that what foraminifera oxygen ratios are really measuring is a combination of ocean temperature changes and changes in the ice volumes.
In contrast, Dwyer said the ratio of calcium to magnesium in seawater is not changed by the buildup of glaciers. In fact, this ratio is thought to have remained unaltered over at least the past several million years under any climatic conditions. But the relative amounts of magnesium and calcium deposited in ostracod shells do vary according to the temperature of the ocean water.
"That's what we hope will be one of the beauties of this technique," he said. "If it works, it will be a direct paleothermometer. It won't be convoluted."
For the research described in the Science article, Dwyer worked with ostracod expert Thomas Cronin of the U.S. Geological Survey in Reston, Va. With Cronin's help, Dwyer obtained ostracod samples from a Deep Sea Drilling Project site just west of the mid-ocean ridge at roughly the same latitude as New Jersey. That is a location where deep sea circulation patterns are thought to shift markedly between glacial and interglacial periods. The ostracod in the samples were alive between 3.2 million and 2.5 million years ago, an interval that featured both glacial and interglacial intervals.
"We wanted to take this new technique somewhere where we knew there was a temperature change, so we would be likely to see something," Dwyer said. "And indeed we do see a strong signal in the magnesium-calcium ratio, which we attribute mainly to temperature change."
The research was funded by the National Science Foundation as well as the USGS Global Change Program.
Dwyer also analyzed more contemporary ostracod fossil samples collected at another North Atlantic site. Those were alive during the past 200,000 years, representing the last two glacial-to-interglacial cycles.
The study found evidence that deep ocean water cooling has intensified in the past 3.2 million years, which the authors suggest is due to greater influence of bottom currents of colder waters flowing from the south polar region.
The ostracod calcium-magnesium records also show changes in bottom temperatures that are consistent with the geological record of glacial cycles as well as with known shifts in the Earth's orbit that are suspected to drive the glacial cycles.
"What Gary is trying to understand is how deep ocean circulation drives climate change," said Duke geology professor Paul Baker, another author of the Science paper and a supervising geochemist. "The temperature of the deep water through time is very poorly understood because there hasn't been any unambiguous paleothermometer developed."
The researchers cautioned that more work needs to be done before the calcium-magnesium method is thoroughly tested. Among other things, the calcium-magnesium ratios in fossil shells could be unacceptably changed as the specimens deteriorate over time.
"What Gary is trying to do is determine temperature changes independent of changes in the isotopic composition of sea water," Baker added. "Frankly, our method is quite untested. But if we are really able to do that, that would be monumental."