Carbon dioxide not to blame in ice age mystery

Scientists have peered back in time with a new analytical technique to see atmospheric concentrations of carbon dioxide more than 2 million years into the past. The findings indicate that a long-term decline in the levels of that greenhouse gas isn’t to blame for a geologically recent shift in the frequency of ice ages, scientists say.

The record of ice ages in North America stretches back 2.4 million years (SN: 2/5/05, p. 94). Until about 1.2 million years ago, ice ages in the Northern Hemisphere occurred about every 40,000 years, says Jerry F. McManus, a paleoclimatologist at Lamont-Doherty Earth Observatory in Palisades, N.Y. But for the past 500,000 years or so, ice ages have occurred, on average, only once every 100,000 years, he notes.

Several explanations for this shift have been proposed, but one of the most popular ones — a long-term decline in carbon dioxide levels — isn’t to blame, McManus and his colleagues suggest in the June 19 Science.

By chemically analyzing air bubbles trapped in ice, researchers can get a direct measurement of atmospheric composition. But so far, the longest ice core available reaches back only 800,000 years, McManus says. So, he and his colleagues sought a technique that could look back further into time — one that could take advantage of the multimillion-year records available in seafloor sediments.

The team’s lab tests indicate that the ratio of boron-11 and boron-12 isotopes incorporated from seawater into the calcium carbonate shells of a marine microorganism called Globigerinoides sacculifer depends on the pH of the water in which the creatures lived. That pH, in turn, depends on the concentration of carbon dioxide in the air, because the gas forms an acid when it dissolves in water. So, the scientists propose that chemical analyses of G. sacculifer shells in ancient sediments should give an indirect measure of the atmosphere’s past carbon dioxide levels.

Then the researchers analyzed G. sacculifer shells taken from a sediment core drilled from the equatorial region of the North Atlantic Ocean, about 1,000 kilometers off the coast of Africa. They used ice-core data from the past 800,000 years to validate their boron-analysis technique and then analyzed shells that had fallen to the ocean floor during the past 2.1 million years.

“This [boron isotope] proxy seems to be amazing,” says Maureen Raymo, a paleoclimatologist at Boston University. “It really does a good job of capturing the trends seen in the ice core data” and has potential to tell scientists much more about how climate works.

Ice-core data suggest that carbon dioxide levels at the coldest points of ice ages ranged between 172 and 180 parts per million, and the higher concentrations in the warm periods between those ice ages varied from 260 to 300 ppm. The new findings for the period between 800,000 and 2.1 million years ago show the same pattern, McManus says. More important, the analyses show that the highest concentrations of carbon dioxide, those found during warmer interglacial periods, haven’t declined at all, he notes.

 “There’s a long history of thinking that a decline in carbon dioxide was responsible for this shift [in the frequency of ice ages],” says Peter U. Clark, a geoscientist at Oregon State University in Corvallis. As CO2 declined, scientists had suggested, the ice that built up during ice ages could last longer, he notes. “It’s a straightforward explanation, but there’s no evidence for it,” he adds.

In extending the record of carbon dioxide measurements, the study also shows that today’s levels — now above 380 parts per million and rising higher each year — are unprecedented during the past 2 million years.