In corals dredged from the bottom of the North Atlantic, scientists have found evidence of a drastic shift in deep currents 15,000 years ago, as the last Ice Age began to wane. The finding, reported in today’s Science, marks the first time oceanographers have been able to clock currents flowing through the ancient ocean and supports the idea that changes in climate can suddenly shift ocean circulation.
Deep-sea sediments hold chemical clues that suggest how water flowed in the ancient ocean, but the deposits are too jumbled to reveal exactly when any shifts in currents might have taken place. But in 1993, oceanographer Edward Boyle of the Massachusetts Institute of Technology (MIT), in Cambridge, and Jess Adkins, then his student, realized that carbon-14 levels in small corals that grow in the deep sea might hold a more detailed record of circulation.
Cosmic rays create radioactive carbon-14 in the atmosphere, which is absorbed by the water on the ocean’s surface. The abundance of carbon-14 in seawater declines as soon as surface water sinks, so the isotope abundance serves as a kind of clock that records how long ago water left the surface.
Boyle and Adkins, now at Lamont-Doherty Earth Observatory in Palisades, New York, looked at four samples of coral that grew about 15,000 years ago in the western North Atlantic ocean. By measuring carbon-14 concentrations along the corals’ growth layers, the duo was able to tell how long it took water from the surface to wend its way down to the sea floor. They saw a sharp decline in carbon-14 abundance from the corals’ oldest parts to their youngest–a period of less than 160 years. Adkins thinks the higher carbon-14 concentrations were deposited by the so-called North Atlantic Deep Water, a large water mass that sinks nearby in the North Atlantic. The lower concentrations, he thinks, came from the far away Antarctic Bottom Water, which sinks in Antarctica. Much of the carbon-14 would have decayed away during its long journey.
The shift took place at a time when the ocean surface was starting to warm–an early sign that the ice age was losing its grip. “The corals are some of the first proof that the deep ocean can change very quickly” in response to climate, says Adkins. “The importance of these corals as climate recorders can’t be overemphasized,” adds coral expert Michael Risk of McMaster University in Hamilton, Ontario.