When a gigantic iceberg collided with the Mertz Glacier ice tongue in February 2010, it exposed a part of the sea floor that had been hidden from view for decades. The calving of the 2500km² ice tongue also changed the geography of the region, with unknown consequences for the global ocean overturning circulation and climate. But within a year of the calving the Australian Antarctic Division assembled a team of scientists to make the first observations of the new environment.
Scientists have been studying the Mertz Glacier region and its associated ‘polynya’ since the early 1990s. A polynya is an area within the sea ice pack that remains free of ice even in the depths of winter. Because there is no ice to insulate the ocean from the cold atmosphere, the ocean loses huge amounts of heat. The rapid cooling causes sea ice to form, but strong winds coming off the Antarctic continent blow the ice offshore as rapidly as it forms. This makes polynyas important sea ice ‘factories’, constantly forming and exporting sea ice. The Mertz polynya, for example, covers only 0.001% of the Antarctic sea ice zone at its maximum winter extent, but is responsible for one per cent of the total sea ice production in the Southern Ocean.
According to CSIRO oceanographer Dr Steve Rintoul, the salt that is left behind when new sea ice forms creates cold, dense water that sinks to the ocean bottom and forms ‘Antarctic Bottom Water’. This bottom water forms one branch of the global overturning circulation, which influences global climate patterns by transporting heat, carbon dioxide and fresh water around the world’s oceans.
‘The Mertz polynya contributes about 15 to 25 per cent of Antarctic Bottom Water and we think it may have been the presence of the glacier tongue that made it such an active polynya,’ Dr Rintoul says.
‘The loss of the ice tongue may mean that the area is less effective in forming dense, salty Antarctic Bottom Water, which could have flow-on effects on ocean currents.’
To find out, scientists participating in the Mertz Glacier voyage between January and February this year, set out to measure the changes in temperature, salinity, oxygen, carbon and nutrients, from the sea surface to the sea floor. On previous voyages to the region Dr Rintoul and his oceanographic team have shown that Antarctic Bottom Water has become less salty and less dense since the early 1970s, probably as the result of an inflow of glacial melt water.
‘One of the main goals of this voyage was to see if this trend is continuing and to test the idea that increased melting of Antarctic ice is contributing to the fresh water signal we see in the ocean,’ he says.
The team deployed three moorings within the region to measure the outflow of Antarctic Bottom Water from the Mertz polynya. These moorings will collect data on current speed and direction, temperature, salinity, and oxygen, before they are retrieved in two years time.
The team also measured water properties using a CTD (conductivity, temperature and depth) profiler instrument. The CTD profiler was lowered at 149 stations during the month-long expedition and almost 100 profiles were collected over the continental shelf and slope in the Mertz polynya region.
‘A preliminary analysis of the CTD results suggests that the calving of the glacier tongue has had an impact on the salinity of dense water on the continental shelf,’ Dr Rintoul says.
‘The dense waters sampled this summer were much fresher and less dense than samples taken at the same locations three years earlier. The surface waters were also much fresher than in 2008.
Dr Rintoul says the fresh surface layer probably reflects the melting of thick multi-year sea ice floes present over much of the shelf. This thick sea ice used to be trapped on the eastern side of the Mertz Glacier tongue, but when the ice tongue broke free, the thick sea ice started to move out of the area.
Sampling the line
CTD measurements were also made along a defined line between Hobart and the Antarctic continental shelf, with the ship stopping every 30 nautical miles to lower the CTD profiler to the sea floor, to depths of up to 4700m. This ‘SR3 transect’ has been studied since 1991, providing a long time series for tracking climate change in the oceans.
‘The only way we can detect and understand changes throughout the full depth of the oceans is to carry out repeat transects like this one,’ Dr Rintoul says.
‘We collected 55 CTD profiles from the sea surface to the sea floor, with the CTD package travelling about 400km through the water column. More than 1000 water samples were collected for various chemical analyses, including oxygen, nutrients, salinity, and carbon dioxide.’
Biologists also deployed nets to capture zooplankton along the SR3 transect; in particular marine snails known as pteropods. These organisms are an important food source for marine predators in the Antarctic food web and are at risk from ocean acidification, caused by increased atmospheric carbon dioxide dissolving in the ocean.
‘When carbon dioxide dissolves in sea water it forms a weak carbonic acid, which is capable of dissolving shells,’ says marine biologist Dr Will Howard of the Antarctic Climate and Ecosystems Cooperative Research Centre.
As the ocean becomes more acidic, scientists expect to see changes in the number and type of shell-forming zooplankton in sampling areas, and decreases in the thickness and integrity of their shells (Australian Antarctic Magazine 18: 4–5, 2010).
Phytoplankton bloom
One surprise of the voyage was the discovery of large phytoplankton blooms in the Mertz Glacier region.
‘Pretty much everywhere we went in the shallow waters of the continental shelf, we encountered intense phytoplankton blooms,’ Dr Rintoul says.
‘The sea looked greyish-green, like coastal waters, rather than the intense blue more typical of offshore Southern Ocean waters. In some places there was so much organic material raining down through the water column that we could measure chlorophyll (a photosynthetic pigment) down to depths of more than 200 metres. Usually we only find chlorophyll in the upper 50 to 75 metres.’
As a result, CSIRO chemical oceanographer Dr Bronte Tilbrook recorded some of the lowest carbon dioxide values ever measured in the Southern Ocean.
‘Our measurements indicated that the phytoplankton were growing rapidly, taking up lots of carbon as they photosynthesised and making that part of the ocean a strong “sink” for atmospheric carbon dioxide,’ he says.
‘It may be that the melt of all the old ice released by the calving of the Mertz Glacier was supporting the bloom. We estimated that about one million tonnes of carbon had been taken out of the water. We normally don’t see that in the Mertz region.’
Biodiversity mapping
The geographic change in the Mertz Glacier region, as well as the iceberg scouring and exposure of the sea floor caused by the tongue calving, will have an impact on the organisms that live on the sea floor (known collectively as the ‘benthos’). On previous voyages to the region scientists discovered diverse and dense communities of corals and other creatures at depths of 400 to 800 metres on the continental slope. Two areas, each about 400km2, were later protected by the Commission for the Conservation of Antarctic Marine Living Resources, which declared them ‘Vulnerable Marine Ecosystems’ (Australian Antarctic Magazine 15: 19, 2008).
On this latest voyage, the scientific team deployed cameras on the CTD equipment to take photos of the benthos, to map the location of coral gardens, and to see how this location relates to ocean currents.
‘So far the evidence seems to support our hypothesis that the corals grow where dense water, produced in winter polynyas, cascades off the shelf and into the deep sea,’ Dr Rintoul says.
‘These dense waters transport small particles of food that are scavenged by filter feeders. When we shone a light beam through the water and measured how much light was transmitted and how much was blocked by particles, we found that the dense water was often full of particles.’
The observations from the voyage will provide a useful benchmark for tracking future changes resulting from the Mertz Glacier tongue calving event. If the calving results in less dense water flowing off the shelf, this may affect the food supply to these benthic communities.
Autonomous science
Along with the CTD measurements and oceanographic moorings, scientists deployed four Argo floats, which will join some 3000 others throughout the world’s oceans measuring temperature, salinity and ocean current speeds. These free-drifting floats rise up and down between the ocean surface and two kilometres depth every 10 days, and beam their information to ground stations via satellites. Two of the Argo floats deployed were ice floats that are able to sample under the winter sea ice, providing valuable data from the seasonal ice zone which, historically, has been poorly sampled.
‘The data we’ve collected from this voyage and from the instruments that will remain in the ocean for some years to come will give us a better understanding of the role of the Southern Ocean in the climate system and the sensitivity of the region to future change,’ Dr Rintoul says.
‘This information in turn will allow us to anticipate and respond more effectively to climate change.’
WENDY PYPER1, STEVE RINTOUL2,3, BRONTE TILBROOK2,3 and ESMEE VAN WIJK3
1 Australian Antarctic Division, 2 Antarctic Climate and Ecosystems Cooperative Research Centre, 3 CSIRO
CTDs explained
The Conductivity-Temperature- Depth (CTD) instrument is the workhorse of oceanography. Almost all the samples analysed on the voyage were collected by the CTD system. A CTD is made up of two parts: sensors that continuously measure temperature, oxygen and conductivity (salinity is calculated from the temperature and conductivity measurements) as the instrument descends, and a carousel of 24 plastic cylinders (Niskin bottles) that are used to collect water samples at different depths. The CTD is lowered on a 6.4mm diameter wire that contains an electrical cable. The electrical connection provided by the cable supplies power to the instrument and allows two-way communications: profiler data is sent to the ship, and signals sent from the ship tell the instrument when it is time to collect a water sample. Different Niskin bottles are closed at different depths, trapping water samples that are analysed back on board the ship for oxygen, pH, carbon, alkalinity, fluorescence and nutrient concentrations. The first CTD was invented by an Australian, Neil Brown. Before his invention, oceanographers could only collect discrete samples at a small number of depths.
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