May 30, 2013
Deep Freeze Couldn’t Keep The Arctic Current From Flowing During The Last Ice Age
April Flowers for redOrbit.com - Your Universe Online
When thick ice covered the Arctic during the last ice age, many scientists assumed that the deep currents below the ice that feed the North Atlantic Ocean and help drive the global ocean currents slowed or possibly even stopped. A new study reveals that the deep Arctic Ocean has been churning briskly for the last 35,000 years, through the chill of the last ice age and warmth of modern times.
These findings, published in the journal Nature, suggest that, even under vastly different climates, at least one arm of the system of global ocean currents that move heat around the planet has behaved similarly.
"The Arctic Ocean must have been flushed at approximately the same rate it is today regardless of how different things were at the surface," said Jerry McManus, a geochemist at Columbia University's Lamont-Doherty Earth Observatory.
By measuring radioactive trace elements buried in sediments on the Arctic seafloor, the team reconstructed the Arctic circulation through deep time. Uranium erodes from the continents and is delivered into the ocean by rivers. Over time it decays into sister elements thorium and protactinium, which eventually attach to particles falling through the water and wind up in the sediment at the bottom. The researchers were able to show that protactinium was being swept out of the Arctic before it could settle to the ocean bottom by comparing expected ratios of thorium and protactinium in those ocean sediments to observed amounts. The amount of missing protactinium allows the researchers to infer how quickly the overlying water must have been flushed at the time the sediments accumulated.
"The water couldn't have been stagnant, because we see the export of protactinium," said Sharon Hoffmann, a geochemist at Lamont-Doherty.
North Atlantic currents flush the upper part of the modern Arctic Ocean while the deep basins of the Arctic are flushed by salty currents formed during sea ice formation at the surface.
"The study shows that both mechanisms must have been active from the height of glaciation until now," said Robert Newton, an oceanographer at Lamont-Doherty. "There must have been significant melt-back of sea ice each summer even at the height of the last ice age to have sea ice formation on the shelves each year. This will be a surprise to many Arctic researchers who believe deep water formation shuts down during glaciations."
Sediment cores collected during the US-Canada Arctic Ocean Section cruise in 1994 - a major Arctic research expedition that involved several Lamont-Doherty scientists - were analyzed by the team who found that at each location, the protactinium has been lower than expected for at least the past 35,000 years. Using core samples from a range of depths, including the bottom of the Arctic deep basins, the team was able to show that even the deepest waters were being flushed out at about the same rate as in the modern Arctic.
The Fram Strait, which divides Greenland and Norway´s Svalbard islands, is the only deep exit from the Arctic. The modern Arctic´s deep waters flow into the North Atlantic through the Nordic seas. They contribute up to 40 percent of the water that becomes North Atlantic Deep Water – known as the "ocean's lungs" for delivering oxygen and salt to the rest of world's oceans.
The researchers hope to focus future research on finding out where the missing Arctic protactinium of the past ended up. "It's somewhere," said McManus. "All the protactinium in the ocean is buried in ocean sediments. If it's not buried in one place, it's buried in another. Our evidence suggests it's leaving the Arctic but we think it's unlikely to get very far before being removed."