Enseignement - Observatoire océanologique de Banyuls sur mer
Laboratoire Arago

      The most important greenhouse gas, excluding water vapor is carbon dioxide (CO₂). The levels of this gas were changing over time naturally or caused by humans. Much of the carbon dioxide produced by humans is not in the atmosphere but stored in the oceans, land, plants or soils.

The sediments are by far the largest carbon pool on Earth, the continents and the oceans, presented on the form of calcium carbonate (CaCO₃), mainly. The second major reservation is that consists of the deep ocean, where carbon is presented on the form of dissolved carbonate (CO₃^2-) ions and bicarbonates (HCO^3-). It is estimated that one third of carbon dioxide resulting from combustion of fossil fuels is stored in the oceans through physical and biological processes.

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    "We need to reduce not only the emissions but also present levels of CO₂ in the atmosphere."

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The solution must:

       - Deal with very large quantities of carbon;

       - Be repeatable over centuries;

       - Be permanent;

       - Have no side effects;

       - Be quickly applied;

       - Not too much expensive...

Some methods have been proposed:

       - Crop residue carbon in agricultural soils;

       - Growing forests;

       - Ocean Fe fertilization;

       - Deep ocean injection of liquid CO₂

       - Burial of crop residues in the deep ocean

but why deep oceans?

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     Carbon dioxide dissolves more easily in cold water than in hot water (see graph). It dissolves also more easily in sea water than in pure water, because sea water naturally contains carbonate ions.

CO₂ + CO₃^2- + H₂O <=> 2 HCO₃^-

     The reaction of carbon dioxide with carbonate ions produces bicarbonates ions (see reaction above). Due to this reaction, carbon dioxide is not abundant in seawater. The levels of CO2 in seawater, then, are so low that the atmospheric carbon dioxide can enter the oceans (chemists call this process by Le Chatelier's Principle). If the water remains on the surface and warms as it travels around the globe, the carbon dioxide quickly returned to the atmosphere. But if the water sinks to the deep ocean, the carbon may remain there, and stored for more than 1000 years before the ocean circulation make it up the surface and thus back into the atmosphere. The cold water sink to the bottom of the ocean at high latitudes in the Southern Ocean and North Seas and the Labrador in the North Atlantic Ocean. These areas represent the main areas where carbon dioxide is absorbed by the ocean through physical processes.

      On the other way, there are also biological processes of absorption of carbon dioxide: CO₂ is also consumed by phytoplankton through photosynthesis and thus converted into organic matter. Most of the carbon dioxide uptake by phytoplankton go back into the atmosphere when the phytoplankton die or are consumed, but some is deposited in the sediment on the ocean floor, when these small particles sink. This process is called "biological pump" because carbon dioxide is transported from the atmosphere to the ocean floor. As in the physical, is primarily at high latitudes that this pump is more effective, because the phytoplankton that live in cold water is heavy enough to sink in deep water after death.

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       This figure gives a very simple idea about how the biological pump works. Phytoplankton need carbon dioxide to make the realization of their photosynthesis. The bacteria feed on phytoplankton and release, then, nutrients and carbon dioxide back into the water. This process is called remineralization and takes place mainly in surface waters. Carbon dioxide is then absorbed again by phytoplankton for photosynthesis or released into the atmosphere. If the phytoplankton dies and sink to the depths of the oceans, the carbon dioxide released into the stores to remineralization, then, for centuries, thus reducing the impact of global warming.