Scientists solve 30-year mystery of the deep

Scientists, with the help of seals, have solved a 30-year mystery surrounding the formation of the global ocean’s coldest, deepest waters. Until now they had known that the cold, dense bottom waters of the global ocean originated at three different locations in Antarctica - the Weddell Sea, the Ross Sea and the Adélie Coast of East Antarctica. Thirty 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. Now, through sophisticated satellite data, oceanographic moorings and tagged seals, they have discovered that a fourth stream of so-called ‘Antarctic bottom water’ is being produced from intense sea ice formation in the Cape Darnley Polynya, north-west of the Amery Ice Shelf.  Scientists estimate that this source of Antarctic bottom water represents between six and 13 per cent of the circumpolar total. The research is published today in the journal Nature Geoscience. Its significance is underlined by the fact that Antarctic bottom water is a key driver of the global ocean circulation and therefore of the earth’s climate. Co-lead author Dr Guy Williams, sea-ice specialist at the Antarctic Climate & Ecosystems CRC in Hobart, said the search for the fourth source of the Antarctic Bottom Water dated back to the discovery in 1977 of recently formed bottom water offshore of Mac Robertson Land (60 – 70ºE). “So since then we have had a smoking gun – we suspected there was a source of Antarctic bottom water in the area, but the inaccessibility of the area has meant that it has taken another 30 years to locate and observe how it is being formed.” In 2008-9, 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 in the Daly and Wild canyons off the continental shelf at Cape Darnley. Data from these moorings is now being examined in conjunction with data provided by southern elephant seals. “The seals went to an area of the coastline that no ship was ever going to get to, particularly in the middle of winter, and measured the most extreme dense shelf water anywhere around Antarctica,” Dr Williams said. “Additionally several of the seals foraged on the continental slope as far down as 1800 metres, punching through into a layer of this dense water cascading down to the abyss. They gave us very rare and valuable wintertime measurements of this process.” The oceanographic data being collected by the seals is part of a larger ecological project at Institute for Marine and Antarctic Studies to study their behaviour. When the seals surface, their tags relay information (via satellite) back to land, where it is collected by the Integrated Marine Observing System (IMOS), also based in Hobart.  By chance, the data from these seals allowed scientists to pinpoint the path of the dense salty water and therefore identify the area from which it originates. The newly identified source is different from the other three sources. Its existence demonstrates that polynyas (areas of open water surrounded by sea ice) are capable of forming sufficiently dense shelf water over a narrow section of continental shelf without the traditional assistance of a large ice shelf or coastal storage volume. 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 was missing piece of a 30-year old puzzle. “This 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. This will improve numerical modelling efforts to predict its response to long-term climate change. “ Media release Link to paper

Authorised by the CEO of the Antarctic Climate and Ecosystems Cooperative Research Centre October 2019.

The ACE CRC was established and supported under the Australian Government’s Cooperative Research Centres Program.

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