Australian and British researchers have found that one of the world’s largest carbon sinks stores carbon differently than first thought. Utilising data collected over ten years from robotic – Argo – probes, 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. 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 Dr 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 ::::
Dr Matear 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,” Dr Matear told ABC’s Dani Cooper “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,” Dr Matear 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,” Dr Matear said.
The research has been published in the journal Nature Geoscience.
Subduction: In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate and sinks into the mantle as the plates converge. Regions where this process occurs are known as subduction zones. Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately 2 to 8 centimeters per year.
Plates include both oceanic crust and continental crust. Stable subduction zones involve the oceanic crust of one plate sliding beneath the continental crust or oceanic crust of another plate. That is, the subducted crust is always oceanic while the overriding crust may or may not be oceanic. Subduction zones are often noted for their high rates ofvolcanism, earthquakes, and mountain building.
Orogenesis, or mountain-building, occurs when large pieces of material on the subducting plate (such as island arcs) are pressed into the overriding plate. These areas are subject to many earthquakes, which are caused by the interactions between the subducting slab and the mantle, the volcanoes, and (when applicable) the mountain-building related to island arc collisions
Argo Floats: Argo is an observation system for the Earth’s oceans that provides real-time data for use in climate, weather, oceanographic and fisheries research. Argo consists of a large collection of small, drifting oceanic robotic probes deployed worldwide. The probes float as deep as 2 km. Once every 10 days, the probes surface, measuring conductivity and temperature profiles to the surface. From these, salinity and density can be calculated. The data are transmitted to scientists on shore via satellite. The data collected are freely available to everyone, without restrictions. The initial project goal was to deploy 3,000 probes, completed in November 2007.
British Antarctic Survey, Cambridge UK
- Jean-Baptiste Sallée
CMAR – CSIRO, Tasmania Australia
- Richard J. Matear,
- Stephen R. Rintoul &
- Andrew Lenton
Antarctic Climate and Ecosystems Cooperative Research Centre, Tasmania, Australia
- Stephen R. Rintoul