have long speculated that the Gulf Stream slows during glacial periods.
An important component of the global oceanic "conveyor," the Gulf
Stream carries tropical warmth up the eastern seaboard of the United
States to the North Atlantic and Europe. Without its warming influence,
a summer swim off the coast of Long Island would resemble a dip from
an iceberg, and the perennial winter rains of London would likely
all be snow.
The entire conveyor works
to distribute heat from the equator toward the poles, thus moderating
global climate. So while the notion of a diminished ice-age Gulf Stream
makes sense, it has never been independently demonstrated until
In the December 9th issue
of Nature, Lamont-Doherty Earth Observatory geochemist Jean Lynch-Stieglitz
presents evidence that the Gulf Stream which originates in the Gulf
of Mexico and wraps around Florida to head northward operated at
about two-thirds its current rate during the height of the last ice
age. This in turn suggests that the entire oceanic conveyor also slowed,
and so moved less heat moving into the icy upper latitudes.
In order to make this discovery,
Lynch-Stieglitz and her colleagues combined two established techniques
of modern oceanography, one geochemical the other physical, in reconstructing
the Gulf Stream as flowed through the Straits of Florida more than
10,000 years ago. "I think Nature accepted this article as much
for the novelty of our methodology, as the importance of our result,"
she said. "As far as I know, this is the first time that oxygen
isotope data from sedimentary deposits of foraminifera have been used
to quantify the flow rate of an ancient ocean current."
Foraminifera are tiny, amoeba-like
creatures that leave behind a record of ocean chemistry in the calcareous
shells they construct from the seawater around them. Isotopes are
atoms of a given element, like oxygen, with different numbers of neutrons
in their nuclei. The first step of the process involved measuring
the ratio of oxygen isotopes recorded by foraminifera across the straits
during the peak of the last ice age. These ratios are known to vary
with the temperature and salinity of the water in which the creatures
lived. The density of the water, in turn, increases with greater salinity
and lower temperature. Working backwards from the isotope ratios,
Lynch-Stieglitz was thus able to reconstruct a profile of the density
contrast across the glacial-era Gulf Stream near Florida.
The second step consisted
of feeding this water density data into a mathematical model that
calculates density-driven current flow. Known generally as the geostrophic
method, this technique is widely used to calculate the rate of modern
current flow using direct water-density measurements. "This is
an elegant and powerful new tool that can and will be applied to reconstructing
ancient shallow ocean currents in many places around the world,"
said Wallace Broecker, senior Lamont-Doherty geochemist and a founding
father of the conveyor model of ocean circulation. "It's an entirely
new way of using oxygen data that provides real flow-rate numbers,
not just qualitative interpretations."
The elegance of the new technique
aside, Lynch-Stieglitz's finding that the Gulf Stream was a third
less vigorous during the last ice age is important in its own right,
Broecker said. The ocean conveyor belt operating today is driven by
the sinking of dense water in the polar regions. These deep-water
currents flow toward the equator, pulling warm surface water back
toward the poles in compensation. This overturning conveyor mechanism
mixes the oceans while distributing heat and moderating climate. In
trying to work out the complex relationship between patterns of ocean
circulation and climate, it is extremely important to understand what
happens during glacial periods whether the conveyor slows, shuts
down, or even changes configuration.
While other research has shown
that the formation deep water in the North Atlantic all but stopped
during the last ice age, the question remained as to whether the entire
conveyor stopped, or simply became shallower. "Jean's work suggests
that conveyor circulation virtually stopped during the last glacial
maximum," Broecker said.
The Gulf Stream as it passes
Florida today is powered approximately two-thirds by prevailing winds
and one-third by conveyor circulation, Lynch-Stieglitz explained.
During the last ice age, the winds driven by equator-to-pole temperature
differences would likely have been stronger, if anything. Thus the
one-third drop in the strength of the Gulf Stream can be directly
attributed to a cessation in conveyor circulation. "Without compensatory
return flow in surface currents like the Gulf Stream, the conveyor
had to be weak or shut down altogether," she said "regardless
of whether it was shallower than today. Of course we want to corroborate
this by applying the technique elsewhere, but it is now valid to ask,
'how does a weak conveyor contribute to or result from glacial climates?'"