News in Science

Seals help solve deep water mystery

Elephant seals have helped scientists unravel a 30-year-old mystery around the sources of the ocean's deepest waters.

In a paper published today in Nature Geoscience, scientists from Australia and Japan reveal a fourth source of Antarctic bottom water lying off Cape Darnley.

Antarctic bottom water - cold, dense water that sits in the abyssal zone between 4000 metres and 6000 metres below the ocean's surface - plays a plays a key role in global water circulation and the transport of carbon dioxide to the deepest layers of the ocean.

The discovery of a fourth source of deep water is critical to our understanding of Antarctica's contribution to global ocean circulation, and will improve modelling of its response to climate change, says study co-author Dr Guy Williams, of the Antarctic Climate and Ecosystem Co-operative Research Centre.

Williams says the Cape Darnley deep water contributes about 10 per cent of volume to the Antarctic bottom water.

The discovery of a fourth source is like "finding a new component in the engine," he says.

Until recently only three sources of the deep waters were known - the Weddell and Ross seas and off the Adelie Coast.

But in 1977 a US study uncovered bottom water with high oxygen levels in the Weddell-Enderby Basin off shore of Antarctica's Mac Robertson Land.

"That was the smoking gun - it was a signal that this water had recently been near the surface [of the ocean] and had somehow been converted into this dense, deep ocean water," says Williams.

Satellites and seals

About 10 years ago, Williams' Japanese colleagues, led by Kay Ohshima from Hokkaido University, used satellite analysis to identify areas of high sea-ice production, known as polynyas, that could be possible sources of the Antarctic bottom water.

The dense deep waters are formed beneath polynyas because during the creation of sea ice, salt is leached from the ice in a process known as brine rejection. This salt goes back into the water making it heavier and causing it to flow down to the abyss once it leaves the continental shelf.

Through the satellite analysis Cape Darnley, northwest of the Amery Ice Shelf, emerged as the lead candidate and in 2008 moorings were placed in the area to prove it as a fourth source.

Due to the coastal region's inaccessibility the moorings were placed "downstream" of Cape Darnley - in canyons on the continental shelf where the researchers believed the deep waters flowed.

"[The Japanese researchers] found the down slope flow and proved bottom water was being produced, but they wanted to get to the 'top of the waterfall' if you like, where it all began," he says.

Williams says the data returned by the southern elephant seals - which are being monitored as part of an ecological study by the Institute of Marine and Antarctic Studies - was the "icing on the cake".

"Their foraging behaviour revealed the missing part of the puzzle," he says.

In 2011 the researchers noticed the tagged seals "hanging around" and were able to gain insights into seasonal variations at the polynya, including temperatures and salinity.

Later that year elephant seals dived into the 'waterfall', Williams says.

"They dived 1800 metres down that layer of overflowing cold water travelling down to the abyss."

This data also gave rare winter-time measurements of the process, he says.

New mechanism

The discovery of the polynya off Cape Darnley not only confirms a significant site of bottom water, but identifies a new mechanism for the generation of such waters, writes Michael Meredith, from the British Antarctic Survey in a separate commentary in Nature Geoscience.

"The shelf off Cape Darnley is, by comparison with the Weddell and Ross Sea shelves, remarkably narrow. Instead, the key factor in the Cape Darnley source is that the exposure of the ocean to the atmosphere during winter allows unusually intense and persistant sea-ice production, during which large amounts of salt water are released to the surrounding waters," says Meredith.

While the amount of bottom water formed at this site is not as much as at other sites, it does have important implications for understanding ocean circulation, he says.

"We need to know if this source also contributes significantly to the transfer of carbon dioxide and other climatically important gases to the abyssal oceans," he says, adding that it is also critical to understand whether this source of bottom water is changing in response to shifts in climate.

Meredith says oceanographers should now investigate whether other regions with narrow shelves but persistent polynyas, such as those found around East Antarctica, also show bottom water formation.