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
New Discovery of How Carbon Is Stored in the Southern Ocean
A team of British and Australian scientists has discovered an important method of how carbon is drawn down from the surface of the Southern Ocean to the deep waters beneath. The Southern Ocean is an important
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.
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