A new study, reported April 22 in the journal Nature, suggests that when the rate of the Atlantic Ocean's north-south overturning circulation slowed dramatically following an iceberg outburst during the last deglaciation, the climate in the North Atlantic region became colder. When the rate of the ocean's overturning circulation subsequently accelerated, the climate warmed abruptly.
Study author Jerry McManus and colleagues Roger Francois, Jeanne Gherardi, Lloyd Keigwin and Susan Brown-Leger at the Woods Hole Oceanographic Institution and in France report that the coldest interval of the last 20,000 years occurred when the overturning circulation collapsed following the discharge of icebergs into the North Atlantic 17,500 years ago. This regional climatic extreme began suddenly and lasted for two thousand years. Another cold snap 12,700 years ago lasting more than a thousand years and accompanied another slowdown of overturning circulation. Each of these two cold intervals was followed by a rapid acceleration of the overturning circulation and dramatically warmer climates over Northern Europe and the North Atlantic region.
McManus and colleagues studied a seafloor sediment core from the subtropical North Atlantic that was retrieved from an area known as the Bermuda Rise. The core contains sediments deposited over tens of thousands of years that include shells of small animals called foraminifera that record surface water conditions in their shells when alive. The researchers measured oxygen isotope ratios in each individual sandgrain-sized shell to determine climatic changes that occurred since the last ice age. They used a new tool, based on two daughter isotopes of uranium that occur naturally in seawater, as a proxy for changes in the rate of ocean circulation. The technique has been used for other purposes in the past, but this is the first time it has been used to generate a detailed time series that provides a history of variations in the strength of ocean circulation.
The isotopes, protactinium and thorium, are produced at constant rates in seawater by radioactive decay from dissolved uranium and are removed quickly by adhering to particles settling to the ocean floor. Thorium is removed so rapidly by particles that it resides in the water column no more than a few decades before nearly all of it is buried on the sea floor below where it was produced. Protactinium is removed less readily and thus remains in the water column 100 to 200 years. As a result, about half of the protactinium produced in North Atlantic water today is exported into the Southern Ocean as part of the ocean circulation system known as the great conveyor. At times when the rate of overturning circulation slows, the proportion of protactinium buried in the North Atlantic sediments increases, thus preserving the record of such changes in the accumulating sediments.
The research team found that the rate of ocean circulation varied remarkably following the last ice age, with strong reductions and abrupt reinvigorations closely tied to regional climate changes. McManus says this is the best demonstration to date of what many paleoclimatologists and ocean scientists have long suspected. "Strong overturning circulation leads to warm conditions in the North Atlantic region, and weak overturning circulation leads to cold conditions," he said. "We've known for some time from changes in the chemistry of the seawater itself that something was different about the ocean's circulation at times of rapid climate changes, and it now appears that the difference was related to changes in the rate of ocean circulation. One big question is why the circulation would collapse in the first place and possibly trigger abrupt climate change. We think it is the input of fresh water to the surface ocean at a particularly sensitive location."
McManus says the team is now applying this same technique to sea floor cores collected in other regions of the North Atlantic. "We've made a little step forward in understanding the ocean's role in the climate puzzle, but there are more pieces to fill in."
The WHOI study was funded by the National Science Foundation, the Institution's Ocean and Climate Change Institute and an Interdisciplinary and Independent Study Award, and by the Comer Science and Education Foundation.