Rate of Ocean Circulation Directly Linked
to Abrupt Climate Change in North Atlantic Region
A new study strengthens evidence that the oceans and climate are linked in an
intricate dance, and that rapid climate change may be related to how vigorously
ocean currents transport heat from low to high latitudes.
|Great Ocean Conveyor
Belt (From the Intergovernmental Panel on Climate Change)
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
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.