Seals get to the source of Antarctic Bottom Water

Southern elephant seals have inadvertently helped oceanographers discover a new source of Antarctic Bottom Water, solving a 35 year mystery surrounding the formation of this cold, salty, and globally important water mass.

Carrying small IMOS* oceanographic instruments on their heads, the seals, which were part of an unrelated ecological study, foraged to depths up to 1800m off Cape Darnley, East Antarctica, into a ‘waterfall’ of newly formed bottom water heading for the abyssal zone, more than 5000m below.

The data they collected, along with data from oceanographic moorings and satellites, confirm Cape Darnley as the location of a fourth source of Antarctic Bottom Water — a cold, dense (salty) water mass that drives ocean currents around the world.

Writing in Nature Geoscience in February, Dr Guy Williams, from the Antarctic Climate and Ecosystems Cooperative Research Centre, and colleagues from Japan, Sweden and Australia, said scientists knew Antarctic Bottom Water originated at three different locations in Antarctica — the Weddell Sea, the Ross Sea and the Adélie Coast (see map). Thirty-five years ago, a fourth source was speculated to exist somewhere in the Prydz Bay region. But until now, scientists have been unable to confirm if, where and how it is being formed.

‘Since 1977 we suspected there was a source of Antarctic Bottom Water off Mac Robertson Land (60–70°E), but the inaccessibility of the area has meant that it has taken another 30 years to locate and observe how it is being formed,’ Dr Williams said.

Antarctic Bottom Water is created when sea ice forms in winter, close to the continent. As it forms, the sea ice rejects salt, making the water below salty and therefore dense, causing it to sink down the continental shelf to the abyssal depths.

‘While sea ice grows everywhere around Antarctica in winter, there are only a handful of regions where enough sea ice forms to produce the dense water necessary for Antarctic Bottom Water,’ Dr Williams said.

‘Typically this occurs in large ‘polynya’ regions, also known as “ice factories”, where the shape of the coastline and associated glaciology, together with complementary winds, conspire to create an ongoing cycle of sea-ice production-removal-production. This results in enhanced levels of sea ice production and dense water formation on the continental shelf.’

In 2008–09, as part of the International Polar Year, Japanese scientists from the Institute of Low Temperature Science, Hokkaido University and the Tokyo University of Marine Technology, deployed moorings measuring salinity and temperature in the Daly and Wild canyons, off the continental shelf at Cape Darnley. These sites were selected based on satellite analysis, which showed the Cape Darnley Polynya was the second highest producer of ice around Antarctica, after the Ross Sea Polynya.

Data from the moorings showed that cold, salty water did indeed appear ‘after the onset of active sea ice production’, drifting west along the continental slope and down into Daly Canyon. Instruments attached to the seals confirmed the origin of this water.

‘The moorings found the down slope flow and proved bottom water was being produced, but the Japanese team also wanted to get to the top of the “waterfall” where it all began,’ Dr Williams said.

‘So in 2011 we were excited to learn of the IMOS-instrumented seals at nearby Davis, which were involved in an unrelated ecological project into seal behaviour.’

The ‘CTD’ (conductivity, temperature and depth) instruments attached to each seal had a small satellite relay which transmitted data on a daily basis during the five to 10 minute intervals when the seals surfaced.

‘Several of the seals travelled into the Cape Darnley polynya in the middle of winter, providing information on the dense shelf water source of this bottom water, in an area that has so far proven impossible to access by ship,’ Dr Williams said.

‘Some additional seals tagged by French colleagues at Kerguelen Island also foraged on the continental slope as far down as 1800m, punching through into a layer of this dense water cascading down to the abyss.

‘The seals have returned very rare and invaluable winter-time measurements of this process.’

The team estimate that the Cape Darnley source of Antarctic Bottom Water represents between six and 13 per cent of the circumpolar total.

The newly identified source is different from the other three sources in that it has a relatively narrow section of continental shelf, and therefore low storage volume, for the accumulation of dense shelf water — previously thought to be necessary for bottom water formation. This opens the door for further discoveries of Antarctic Bottom Water production from the other polynya regions around the Antarctic coastline.

Dr Williams said the discovery redraws the map of large-scale Antarctic oceanography in the Atlantic sector.

‘It is now vital that this new information be incorporated into the assessment of Antarctic Bottom Water variability and change, and its input to the global overturning circulation,’ he said.

‘This will improve numerical modelling efforts to predict its response to long-term climate change.’

Wendy Pyper1 and Miranda Harman2

1. Australian Antarctic Division
2. Antarctic Climate and Ecosystems Cooperative Research Centre

* Instruments were provided by Australia’s Integrated Marine Observing System (IMOS), a national array of observing equipment to monitor the open oceans and coastal marine environment around Australia, covering physical, chemical and biological variables. The instrumented seals were part of an ecological study conducted by the Institute for Marine and Antarctic Studies and Macquarie University.