How a Warming Earth Once Cooled Off

"What was special about our location is we can get three proxies in one place," Katz told Discovery News. She and her colleagues published their discovery in the April 13 issue of Nature Geoscience.

The first line of evidence is oxygen-18, which is trapped in the fossil shells of microscopic organisms called forams. During cold times there is more oxygen-18 in the oceans because the lighter oxygen-16 evaporates more readily and ends up locked up in polar ice. So an up-tick in the oxygen-18 in the shells over time is a strong signal that the climate is cooling globally.

The proxy is the relative amount of magnesium and calcium in the shells that formed in waters once more than 650 feet (200 meters) deep. It turns out that the amounts of those two elements, used by animals to make their shells, are closely tied to temperature. So the magnesium-calcium ratios in the shells reveal ancient seafloor temperatures.

Finally, there are the rocks themselves. Such things as silty, deep-water muds and near-shore sands reveal how deep the water was and suggest a 300-foot (100-meter) drop in sea level. That matches what's expected to happen on a cooling planet where lots of water is being locked up in ice sheets.

"As far as I know, this is the first study to measure all three proxies from one study site," said earth scientist Katharina Billups of the University of Delaware. "So all in all, yes, I think it is unusual that they were able to do this."

As for what might have caused the step-wise cooling, that's a tougher question. One likely factor was the opening of the Drake Passage, which severed the connection between South America and Antarctic.

That allowed ocean currents to encircle Antarctica and cool it down dramatically.

Another suspect is what's known as Milankovitch cycles -- combinations of cycles in Earth's orbit and tilt that can periodically reinforce each other to push climate in one direction or another.

"It just kind of nudged it a little more," said Katz of the these cycles.

Related Links:

Larry O'Hanlon's blog: Earth Impacts

Realclimate.org

Planet Green

More on global warming