Southern Ocean Carbon Sink
A team of British and Australian scientists has discovered an important carbon sink from water drawn down from the surface of the Southern Ocean to the deep waters beneath. The Southern Ocean is an important carbon sink in the world — around 40% of the annual global CO2 emissions absorbed by the world’s oceans enter through this region. 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 Organization (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 kilometers wide.
The Southern Ocean (also known as the Great Southern Ocean, Antarctic Ocean, South Polar Ocean, and Austral Ocean) comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. As such, it is regarded as the fourth-largest of the five principal oceanic divisions (after the Pacific, Atlantic, and Indian Oceans, but larger than the Arctic Ocean). This ocean zone is where cold, northward flowing waters from the Antarctic mix with warmer subantarctic waters.
Winds, currents and massive whirlpools that carry warm and cold water around the ocean — known as eddies — create localized pathways or funnels for carbon to be stored in the ocean deepness.
Oceans are at present CO2 sinks, and represent the largest active carbon sink on Earth, absorbing more than a quarter of the carbon dioxide that humans put into the air. On longer timescales they may be both sources and sinks — during ice ages CO2 levels decrease to ~180 ppmv, and much of this is believed to be stored in the oceans.
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."
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 meter 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 7 kilometers.
This study compliments a paper published in Nature 487, 313—319 (19 July 2012) — Deep carbon export from a Southern Ocean iron-fertilized diatom bloom — by Victor Smetacek, Christine Klaas, Volker H. Strass et al, which outlines the biological processes involved in how carbon is absorbed in the Southern Ocean.
Argo floats are small, robotic probes that collect high-quality temperature, pressure and salinity data. There are more than 3,000 underwater robots swimming in the Earth’s oceans at this moment. The probes dive as deep as 2,000m into the ocean for 10 days at a time, after which they re-surface and transmit the data they have collected via satellites. Each float is designed to make around 150 such cycles.
For further information see Carbon Sink.
Map image via University of Maine.