Although it's said that still waters run deep, now scientists find that deep waters aren't still — in fact, the deep waters of the Atlantic Ocean seem to have reversed their direction of flow since the last time ice dominated the Earth.
Instead of heading southward as they do now, these abyssal waters once flowed northward roughly 20,000 years ago, back when the world saw ice sheets more than a mile high, a new study suggests.
The change in flow could have accompanied profound changes in climate, researchers explained.
In the Atlantic, the Gulf Stream brings warm surface water northward from the tropics to high latitudes, where it cools, sinks and flows southward in the deep ocean. The way that water flows in the ocean helps redistribute large amounts of heat — and in this way is critical to how the world's climate works.
The manner in which this Atlantic circulation behaved when global climate was different has proven controversial. Contradictory lines of evidence from during the cold peak of the ice age — the last glacial maximum — make it difficult for scientists to determine whether this ocean circulation was strongly or weakly southward back then.
Now scientists have evidence that such circulation was not only weak, but actually flowed in the opposite direction. The current pattern apparently only arose in the past 10,000 years.
Peek into the past
To reach these conclusions, scientists investigated a pillar of sediment roughly 128 feet (39 meters) long, extracted from the seafloor by use of a ship off the coast of the tip of Africa under about 8,000 feet of water. The upper 15 feet or so of this core sample contains material reflecting ocean conditions over the last 50,000 years.
"It is very tricky to take such long cores from such depth without breaking the metal tube in the open ocean, which usually has bad weather conditions," said researcher César Negre, a paleoceanographer at the Autonomous University of Barcelona in Spain. However, the staff onboard the vessel, the Marion Dufresne, were very expert, he noted.
Negre and his colleagues analyzed levels of the isotopes protactinium-231 and thorium-230 within the sample. (Isotopes are atoms of a chemical element that have different numbers of neutrons in their nuclei.) The isotopes the study looked at are produced in ocean waters from the decay of dissolved uranium at predictable levels. The thorium attaches to particles that settle through the water column very rapidly, while the protactinium resides in the water column long enough for currents to transport it horizontally.
Measuring the amounts of protactinium and thorium in seafloor sediments could therefore help deduce details about past water circulation in the deep. Past studies yielded contradictory results regarding these isotopes because they did not take into account the effects that biologically produced opal material could have on concentrating protactinium levels.
To explain this switch, one must first understand that the deep Atlantic is filled with waters from two opposite sources — waters from the North Atlantic that flow south, and waters from the Southern Ocean around Antarctica that flow north.
Today, North Atlantic water dominates over that of the Southern Ocean in the deep Atlantic, but during the last glacial maximum, the Gulf Stream was weaker and the Southern Ocean was saltier than it is now. This led southern sources to dominate the flow, Negre explained.
The scientists detailed their findings in the Nov. 4 issue of the journal Nature.