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Schmidt collaborated with Jean Lynch-Stieglitz of the Georgia Institute of Technology to study foraminifera, tiny single-celled organisms about the size of the head of a pen, gathered from a sediment core recovered from the Florida Straits. Lynch-Stieglitz studied the benthic foraminifera, or those on the bottom of the ocean, that lived on the margins of the Florida Straits as a means to reconstruct past changes in ocean circulation. Meanwhile, Schmidt analyzed the foraminifera that lived near the surface of the ocean to help determine past atmospheric circulation changes.
 Oceanographer Matthew Schmidt studies microscopic organisms to reconstruct past climatesThe idea behind the study was to resolve the timing of atmospheric and oceanic circulation changes that occurred during an era known as the Younger Dryas, an abrupt cooling that occurred right as the world was coming out of the last ice age about 13,000 years ago. The Younger Dryas is the period when the wooly mammoth and the saber tooth tiger go extinct, Schmidt said.
"It's a unique event," Schmidt said, "because many of the Pleistocene megafauna that had made it through multiple ice ages in the past suddenly get wiped out. The question is what was different about the Younger Dryas?"
The foraminifera help Schmidt because they are surrounded by calcium carbonate shells, similar to seashells found on the beach, that lock in the clues as to the temperature and salinity of their surroundings. The foraminifera living close to the surface of the water give clues about the atmosphere, while those living on the bottom of the ocean reflect changes in ocean circulation.
"Key to our study is that both records come from the same core, so there's no uncertainty as to the relative timing of when one changed relative to the other," Schmidt said. "Most records of past climate variability will only tell you something about the atmosphere or the ocean, but not both."
Scientists have long thought that the ocean's conveyor belt circulation that cycles warm water through the Tropical Atlantic and the Florida Straits and then into the northern Atlantic Ocean was the driver for abrupt climate change during the last ice age. Schmidt and Lynch-Stieglitz's findings seem to confirm that the North Atlantic's conveyor belt circulation dramatically decreased at the start of the Younger Dryas, which means that the trigger for the event resided in the ocean. Atmospheric circulation changes followed close behind.
 Foraminifera, like the one seen here, are tiny creatures in the ocean about the size of the head of a pen that are surrounded by calcium carbonate shells, similar to the shells around other sea creatures. (Photo by Howard Spero at University of California Davis.)However, the story is different at the end of the Younger Dryas. On this transition, it looks like the atmosphere started to recover before the ocean's conveyor belt restarted, suggesting that the tropics play an important role in bringing an end to cold intervals.
"Something was going on in the tropics that was causing the tropics to rebound or recover, and possibly that was what triggered the ocean's conveyor belt to come on to really bring us back to the warmer conditions," Schmidt said. "A lot of people would be shocked by that. They think that when the conveyor is strong, it's warm; and when it's weak, it's cold. This calls that into question."
Schmidt said that it's clear from this study that the climate state has distinct equilibrium modes, which lead to abrupt transitions. He said that one thing this type of study tells us is that we should be cautious about what we're doing to the climate state.
"You might be going along and there might not be much change, but that doesn't mean you're not getting close to a threshold where there's going to be a rapid change," Schmidt said. "It's kind of like pulling a rubber band. You can pull it, and pull it, and pull it, and not much happens, but suddenly it snaps."
By Katie Cowart
Dec. 1, 2011
Contact: Matthew Schmidt, 979.862.8342,
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or Karen Riedel, 979.845.0910,
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