Word: foraminifera
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...according to a new study just published in Science, there may not be any mystery after all. By looking at the chemistry of fossilized foraminifera - tiny sea creatures no bigger than a grain of sand - a team led by Aradhna Tripati, of University of the California, Los Angeles, has detected a significant CO2 bump during both warming episodes...
These scientists aren't the first to look at the chemistry of foraminifera; the fossils are abundant in ancient ocean sediments, so they're a particularly good tracer of the past. But they used a new technique to measure CO2: looking at how much of the element boron was present in the foraminifera's shells. When there's lots of CO2 in the air, there's also more in the top layers of seawater, where the relevant species of foraminifera live. That makes the water more acidic, which in turn makes the tiny animals incorporate less boron into their shells...
...reasonable, in theory, and Tripati and her colleagues made sure to use two species of foraminifera that are still around ("You can grow them in the lab," she says), just in case the effect varied from one species to another. Beyond that, they compared their own foraminifera-based CO2 estimates for the past 800,000 years with the measurements from the ice caps - and, says Tripati, "they matched to within 20 p.p.m." That makes her and her colleagues confident that the older measurements are valid as well...
...into it completely. "It's a really worthwhile and bold effort to understand a period we have a hard time explaining," says Ed Boyle, a professor of ocean geochemistry at MIT. "My cautious view is that this looks promising right now, but I've been studying chemical tracers in foraminifera for pretty much my whole career, and there are often unexpected twists and turns." It is, he says, "the kind of thing where they may turn out to be right, and we'll look back in five years and say, 'that was when it all began to come together...
What researchers found was a stretch of a few hundred thousand years during which foraminifera shells were unusually rich in oxygen-18, suggesting the presence of glaciers. Though changes in ocean temperature can also alter the oxygen balance, sea-bottom temperatures don't vary much no matter what's happening up top, yet the bottom-dwelling foraminifera still exhibited an oxygen imbalance, implying that the ice effect was more likely. Nobody can explain how you can have glaciers in a superhot world. But then, nobody can really explain how the world got quite that hot in the first place...