What's the Big Idea?

Ice, Ice Baby (Circa 90 Million Years Ago)

This occasional feature, written by researchers around the world, highlights new discoveries in the sciences and what they mean for your Outlook.

THE BIG IDEA: Scientists have long envisioned the Cretaceous "super-greenhouse" period, the era some 90 million years ago when crocodiles roamed the Arctic and the temperature of tropical oceans soared to 98 degrees Fahrenheit (14 degrees warmer than they are today), as a sort of anti-Ice Age. Yet my research reveals a dramatic drop in the sea level during those years -- a telltale sign that glaciers were forming somewhere on the planet. I believe that glaciers covered slightly more than half of Antarctica during the Cretaceous hothouse period, causing the sea level to fall some 131 feet and stay there for 200,000 years.

HOW WE DISCOVERED IT: My team and I analyzed deep-sea sediment samples drilled from the Demerara Rise, located northeast of South America in the tropical region of the Atlantic Ocean. First, we studied fossils of plankton called foraminifera, tiny organisms that, when alive, absorb different oxygen isotopes depending on what's happening in their environment. When ice sheets form, lighter oxygen isotopes are drawn into the glaciers and heavier isotopes wind up in the foraminifera. We also studied the fossilized fatty membranes of other marine organisms, which change in response to temperature -- acting as a kind of paleothermometer.

WHAT WE FOUND: When we discovered many heavy isotopes in the ancient plankton, and when we found that the other organisms contained certain membrane compounds associated with ice formation, we concluded that glaciers had caused the abrupt drop in the sea level.

WHY IT MATTERS: Taken by itself, this discovery can't explain modern global warming; the planet is heating much more rapidly today than during the Cretaceous Period, so comparing the two phenomena would be a scientific non-starter. Moreover, another team of scientists used similar research methods and reached an opposite conclusion last year about the Cretaceous Period. But our work shows that the "super-greenhouse" period may have been far more complex than we ever imagined. I'm convinced that further research about this ancient hothouse will help us better understand today's climate change.

-- André Bornemann is researcher at the University of Leipzig, Germany.

In Cooperation with Science and the American Association for the Advancement of Science. http://www.sciencemag.org/wpoutlook/">www.sciencemag.org/wpoutlook.