From this aquatic "lung", oxygen flows into the deep ocean interior all around the world, and sustains the delicate balance of life in the high seas.

This process of "breathing" happens because wintertime cooling at the surface makes oxygen-rich, near-surface waters denser, and heavy enough to sink to the depths.

Now, in a new study in Biogeosciences, a team of researchers from Canada and Germany have measured the flow of oxygen through this cycle, which they liken to measuring the flow of oxygen through our bodies, pumped via the aorta.

"We wanted to know how much of the oxygen that is breathed in each winter actually makes it into the deep, fast-flowing currents that transport it across the globe," explains lead author Jannes Koelling, an oceanographer at Dalhousie University, Canada.

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The mixing of oxygen in the Labrador Sea is the first step in a chain reaction of deep-ocean life-support. Strong currents in the depths carry that oxygen to the rest of the Atlantic and then beyond; oxygen "breathed" in the Labrador Sea can sustain deep ocean life as far away as the Pacific and Indian Oceans.

"The newly inhaled oxygen was clearly noticeable as a pulse of high oxygen concentration that passed our sensors between March and August," Koelling says.

The research took two years; the team mounted sensors that could detect dissolved oxygen onto anchored cables which reach from the seafloor to the near-surface. The measurements revealed that about half of the oxygen taken up from the Labrador Sea in the winter months was injected into the deep currents over the following five months.

What happened to the other half? While some of the remaining oxygen may have been consumed by fish and other organisms, the team believe the bulk most likely took an alternate route out of the mixing region.

Climate models predict a huge incursion of freshwater into the oceans as glaciers in the Arctic melt. That's worrying, because this freshwater mixing with the seawater in the Labrador could reduce the depth of wintertime mixing, making the breathing shallower, reducing the life-supporting supply of oxygen to the deep oceans.

Not even the deep ocean, then, is safe from the devastating effects of anthropogenic climate change.

"This is an example of how monitoring enabled by the latest ocean technology can help us fill in knowledge gaps in this important region," says Dariia Atamanchuk, who leads the oxygen program at Dalhousie.

Now, Koelling wants to turn his attention to other pathways that oxygen might be taking away from the Labrador Sea and outwards to the rest of the world.

"The circulation of the Labrador Sea is complex, and we've only focused, so far, on the most direct export route," Koelling says. "Some oxygen-rich water may be transported eastwards, instead of to the southwest, and it may enter the boundary current off Greenland before returning southwards, over a longer time-period."

These other pathways, shown as dashed lines in the map, are being investigated already, using additional oxygen sensors mounted on moorings.

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