Photo: Elephant seals with satellite trackers provide scientists with detailed information about water flows in Prydz Bay. (Clive R. McMahon )

 

A group of elephant seals in Antarctica has helped show how freshwater from melting ice shelves affects a key part of the engine that drives the circulation of the world's oceans...

An international team of researchers led by Dr Guy Williams of the University of Tasmania analysed temperature and salinity data captured by sensors attached to the elephant seals.

The data, published in the journal Nature Communications, shows how melting ice shelves in Prydz Bay in East Antarctica are interfering with the production of Antarctic bottom water.

Bottom water is formed when seas around Antarctica freeze over in winter causing salt to leach out of the sea ice. This process forms dense surface water that eventually sinks to form cold dense water that sits in the abyssal zone 4,000 metres to 6,000 metres below the surface.

"It's a key part of the global circulation," Dr Williams explained.

"If you think of a conveyer belt, [bottom water] is really the gear that drives the engine that is pumping that circulation."

The finding suggests that increased melting of ice shelves with future warming could affect the production of bottom water, Dr Williams said.

"If we can anticipate this melting will increase in the future under global warming, if Antarctic bottom water is already being suppressed, it is likely to be further impacted by this down the track," he said.

In the long term, changes to bottom water would not only affect the circulation of water around the globe but have an impact on sea life that relies on the dense water to provide nutrients and gasses such as oxygen.

"It's not just the speed of circulation, but it's the depth change," Dr Williams said.

"So the conveyor belt continues but it operates at a shallower level."

The mystery of Prydz Bay

Satellite analyses have shown the majority of bottom water is formed in open areas of water surrounded by sea ice known as polynyas.

In a previous study, Japanese and Australian researchers including Dr Williams used data from sensors attached to a solo elephant seal to confirm a source of bottom water lay underneath the Cape Darnley polynya in East Antarctica.

At the time the researchers suspected a fresher source of water was flowing into the Cape Darnley bottom water from the adjacent Prydz Bay.

 

But, as the seal only hung around for a short time, it was unclear what was happening in the bay, which has three polynya systems, and two ice shelves — permanent shelves of ice attached to land.

"If you look at the satellite data and add up all the polynya in Prydz Bay they produce more sea ice than the Cape Darnley polynya, so Prydz Bay should produce more dense water, " Dr Williams said.

To get to the bottom of this conundrum, Dr Williams and his team studied three years of data captured by elephant seals that were tagged by other research teams, who were trying to understand where the seals were going and what they were doing.

Seals provide unprecedented view of winter waters

Between 2011 and 2013 they captured data from up to 20 young male elephant seals that hung out in Pryzd Bay between March and October.

"We've never really been able to get such amazing spatial and temporal coverage before," Dr Williams said.

"That's a period of time where we would never get down with a ship. The last time we were there in a winter time ... was 1999."

While it's still a mystery what the seals were doing in that location, they dived down into the ocean depths up to 60 times a day.

"When they move around it's a little hard to interpret their data but when they stay in one location, especially in the polynya we get this lovely record of how the salinity increases in the winter time."

The data confirmed that Prydz Bay is a secondary source of Cape Darnley's bottom water.

"It's fresher than Cape Darnley, but the two join up and ultimately become Cape Darnley bottom water," Dr Williams said.

"As water moves around the bay in a clockwise direction the salinity is going up as soon as you go under a polynya then it's getting hit the other way by the ice shelves and it's knocking it back or holding it back by adding freshwater."

But, he said, more research is needed to understand the complexity of the systems.

"To get models [of ocean circulation] right they'll have to consider this combination of processes not just the polynya but the polynya and ice shelves."

In the meantime, the seals have provided a good start.

"At the moment it's all about filling gaps. The [seals] have gone to areas where we've never had an observation before."

"But it's opportunistic. We can't control where the seals go."

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