Going in deep

by Rebecca Priestley / 14 February, 2013
A Niwa-led voyage aims to get to the bottom of a vital salty ocean water source.
Going, going: satellite images showing iceberg B-909B (right of image) colliding with and breaking off the tongue of the Mertz Glacier three years ago
Going, going: satellite images showing iceberg B-909B (right of image) colliding with and breaking off the tongue of the Mertz Glacier three years ago. Photo/Nasa

When a massive iceberg smashed into the tongue of East Antarctica’s Mertz Glacier in 2010, the 78km-long floating ice chunk snapped off. An international team of scientists, concerned that this event is affecting global ocean circulation, is sailing to the Mertz region on the RV Tangaroa for a closer look

The voyage leader, Niwa oceanographer Mike Williams, says global ocean circulation is, in part, driven by Antarctic bottom water. The Mertz area is one of only three places in the world where it is made.

Although most of the ocean around Antarctica is covered in a 2m layer of sea ice, there are some areas, polynyas, that remain ice-free all year round. Sea ice still forms in polynyas, but strong winds blow the ice away, leaving a new ocean surface on which ice continues to grow. “Because you’re getting this constant new ice formation, you can grow much more ice than if you grew just one thick layer,” says Williams. A polynya can generate 16-17m of sea ice a year.

“If you keep taking fresh water out of the ocean and turning it into ice, the ocean will get saltier.” This salty water is very dense, so it sinks, forming the Antarctic bottom water that drives global ocean circulation. This dense water flows off the continental shelf and north towards less dense masses of ocean, and east with the Earth’s rotation. “This Antarctic bottom water ends up all around the globe.”

But the Mertz polynya seems to have disappeared. When the Mertz glacial tongue was intact, “it created a shadow from the currents that allowed the wind to keep pushing out the ice”. Satellite photographs now show only a small patch of ice-free water. Williams wants to know if this is a temporary change, or “the new normal”.

Although the Mertz polynya was the smallest of the three areas generating Antarctic bottom water, it still accounted for 20% of the deep water in the ocean. “If we’re not taking oxygen down into the deep ocean, there are implications for marine life. It also means we’re not burying CO2 as deep or for as long – that natural buffer the ocean has provided for absorbing CO2 will be affected.”

Once the Tangaroa reaches the Mertz region – with some help from a Danish ice pilot – an Australian and French team, working in outdoor air temperatures of about -10°C, will collect moorings that have been gathering data on ocean temperature and salinity over the past two years. Oceanographers will map the different water masses to try to track the meltwater coming from beneath the glacier and any Antarctic bottom water still being generated in the remaining area of polynya.

One of Williams’s concerns is that increased meltwater coming from the glacier could be compounding the problem. If sea ice is now being formed from freshwater flowing from the glacier rather than from seawater, the salinity of the ocean will not be increasing, further reducing the amount of Antarctic bottom water being formed.

A geology team led by Niwa marine geologist Helen Bostock will use the ship’s multibeam sonar to create a high-resolution map of the sea floor and will gather sediment cores from the Mertz region. Looking at the rate and types of marine sediment deposited over time will allow Bostock to determine if and when the Mertz glacier tongue has broken off in the past.

“I guess this is the big question: has this system shut down permanently, or is this temporary?” says Williams. “In 50 years, if the glacier tongue grows back, will we go back to where we were before?”

Updates on the Tangaroa’s progress can be followed at sciblogs.co.nz/fieldwork.
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