Saturday, August 25, 2012
Ocean Study Reveals Carbon not Sinking
British and Australian researchers have found that one of the world's largest carbon sinks stores carbon differently than first thought.
The Southern Ocean contains about 40 per cent of all carbon dioxide emissions absorbed by the world's oceans.
Researchers from the CSIRO and British Antarctic Survey examined the way the Southern Ocean sucks carbon absorbed from the surface layer into the deeper ocean.
Research co-author Richard Matear from the CSIRO says the study shows the method through which carbon is drawn down from the surface of the Southern Ocean to the ocean's interior - or deep waters.
He says it was previously thought this process, known as subduction, happened uniformly across the ocean.
"A conventional thought would be that once it gets out of this surface layer, it's kind of been tucked away and won't appear for a long time; many years of hundreds of years," he said.
"But with this re-ventilation, there's some places where actually it doesn't get put away into the deep ocean for long at all, re-ventilating in the time-scale of a decade."
Using information collected across 10 years from robotic probes known as Argo floats and various sensors, the team has shown subduction happens at specific locations as a result of interplay between winds, currents and massive whirlpools.
Dr Matear says the study also shows the Southern Ocean is not as efficient as first thought in capturing anthropogenic carbon dioxide.
"Once [the carbon] is out of the surface layer it is no longer communicating with the atmosphere so it is buried in the ocean and out of the equation," he said.
"But in many places it is a shallow burial and the carbon gets re-introduced into the atmosphere."
The largest reventilation occurs in the Indian Ocean sector in a band extending eastwards from South Africa to the middle of the basin.
Another hotspot for reventilation occurs east of New Zealand and in the Atlantic zone east of South America.
The findings have particular implications for "ocean fertilisation" projects as it can help pinpoint regions where the carbon-capture approach is most likely to be successful.
Ocean fertilisation schemes involve scattering iron particles on the ocean surface to create a feeding ground for microscopic marine vegetation called phytoplankton.
As the plants gorge on the iron, they suck up atmospheric carbon thanks to natural photosynthesis and create a giant plankton bloom.
These phytoplankton then die and sink to the deep ocean floor - taking the carbon to the ocean floor where it can lie for centuries.
"It actually makes you think about where in the Southern Ocean you could actually implement something like iron fertilisation to enhance carbon uptake because you'd want to avoid these places where you have re-ventilation," Dr Matear said.
Dr Matear says an improved understanding of how the Southern Ocean draws down the carbon will give greater insights into the impact of climate change and future carbon absorption by the ocean.
He says while the movement of carbon from the atmosphere to ocean surface happens rapidly, the transport of the carbon to the deep ocean is a slower process creating a bottleneck.
"The ocean can't keep up with the amount of carbon dioxide we are putting in the atmosphere," he said.
ABC News: Australia
Reporting this week in the journal Nature Geoscience, scientists from British Antarctic Survey (BAS) and Australia's national research agency, the Commonwealth Scientific and Industrial Research Organisation (CSIRO), reveal that rather than carbon being absorbed uniformly into the deep ocean in vast areas, it is drawn down and locked away from the atmosphere by plunging currents a thousand kilometres wide.
Winds, currents and massive whirlpools that carry warm and cold water around the ocean -- known as eddies -- create localised pathways or funnels for carbon to be stored.
Lead author, Dr Jean-Baptiste Sallée from British Antarctic Survey says, "The Southern Ocean is a large window by which the atmosphere connects to the interior of the ocean below. Until now we didn't know exactly the physical processes of how carbon ends up being stored deep in the ocean. It's the combination of winds, currents and eddies that create these carbon-capturing pathways drawing waters down into the deep ocean from the ocean surface.
"Now that we have an improved understanding of the mechanisms for carbon draw-down we are better placed to understand the effects of changing climate and future carbon absorption by the ocean."
CSIRO co-author, Dr Richard Matear says the rate-limiting step in the anthropogenic carbon uptake by the ocean is the physical transport from the surface into the ocean interior.
"Our study identifies these pathways for the first time and this matches well with observationally-derived estimates of carbon storage in the ocean interior," Dr Matear says.
Due to the size and remote location of the Southern Ocean, scientists have only recently been able to explore the workings of the ocean with the help of small robotic probes -- known as Argo floats. In 2002, 80 floats were deployed in the Southern Ocean to collect information on the temperature and salinity. This unique set of observations spanning 10 years has enabled scientists to investigate this remote region of the world for the first time.
The floats are just over a metre in length and dive to depths of 2km. Today, there are over 3,000 floats in the oceans worldwide providing detailed information used in oceanic climate models.
The team also analysed temperature, salinity and pressure data collected from ship-based observations since the 1990s. The instrument used for this is called a CTD profiler which is a cluster of sensors taking measurements as it's lowered deep down into the ocean to depths of more than 7km.