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Images: Ocean Circulation and a Clam Far From Home

17,500-year-old shells from a clam found in North Atlantic seafloor sediment helped WHOI geologist Lloyd Keigwin learn about ocean circulation and climate changes. Clams and other shelled organisms incorporate the chemical characteristics of the deep-ocean water that existed when their shells and skeletons formed. (Photo by Lloyd Keigwin, Woods Hole Oceanographic Institution)

An 82-foot (25-meter) seafloor sediment core is recovered during a 2004 cruise onboard the research vessel Knorr. The hollow tube plunges into the seafloor, where it is filled with a sample of layers of sediments that accumulate over hundreds and thousands of years. The sediments contain clues to past ocean and climate changes. (Photo by Lloyd Keigwin, Woods Hole Oceanographic Institution)

Today (top), the oceans? overturning circulation carries a tremendous amount of heat northward, warming the North Atlantic region. It also generates a huge volume of cold, salty water called North Atlantic Deep Water?a great mass of water that flows southward, filling up the deep Atlantic Ocean basin and eventually spreading into the deep Indian and Pacific Oceans.

Paleoceanographers have found evidence for very different patterns of ocean circulation in the past. About 20,000 years ago (bottom), waters in the North Atlantic sank only to intermediate depths and spread to a far lesser extent. When that occurred, the climate in the North Atlantic region was generally cold and more variable. (Illustration by E. Paul Oberlander, Woods Hole Oceanographic Institution)