Solubility pump

In oceanic biogeochemistry, the solubility pump is a physico-chemical process that transports carbon (as dissolved inorganic carbon) from the ocean's surface to its interior.

The solubility pump is driven by the coincidence of two processes in the ocean :

• The solubility of carbon dioxide is a strong inverse function of seawater temperature (i.e. solubility is greater in cooler water)
• The thermohaline circulation is driven by the formation of deep water at high latitudes where seawater is usually cooler and more dense

Since deep water (that is, seawater in the ocean's interior) is formed under the same surface conditions that promote carbon dioxide solubility, it contains a higher concentration of dissolved inorganic carbon than one might otherwise expect. Consequently, these two processes act together to pump carbon from the atmosphere into the ocean's interior.

One consequence of this is that when deep water upwells in warmer, equatorial latitudes, it strongly outgasses carbon dioxide to the atmosphere because of the reduced solubility of the gas. For an overview, see Raven & Falkowski (1999).

The solubility pump has a biological counterpart known as the biological pump. See also the continental shelf pump.

Carbon dioxide solubility

Further to the description above, some other aspects of carbon dioxide solubility are worth noting.

Carbon dioxide, like other gases, is soluble in water. However, unlike many other gases (oxygen for instance), it reacts with water and forms a balance of several ionic and non-ionic species (collectively known as "dissolved inorganic carbon"). These are dissolved free carbon dioxide (CO_{2 (aq)}), carbonic acid (H₂CO₃), bicarbonate (HCO₃^-) and carbonate (CO₃^2-), and they interact with water as follows :

CO_{2 (aq)} + H₂O H₂CO₃ HCO₃^- + H⁺ CO₃^2- + 2 H⁺

The balance of these carbonate species (which ultimately affects the solubility of carbon dioxide), is dependent on factors such as pH. In seawater this is regulated by the charge balance of a number of positive (e.g. Na⁺, K⁺, Mg²⁺, Ca²⁺) and negative (e.g. Cl^-, SO₄^2-, Br^-) ions. Normally, the balance of these species leaves a net positive charge. With respect to the carbonate system, this excess positive charge shifts the balance of carbonate species towards negative ions to compensate. The result of which is a reduced concentration of the free carbon dioxide and carbonic acid species, which in turn leads to an oceanic uptake of carbon dioxide from the atmosphere to restore balance. Thus, the greater the positive charge imbalance, the greater the solubility of carbon dioxide. In carbonate chemistry terms, this imbalance is referred to as alkalinity.

References

• Raven, J. A. and P. G. Falkowski (1999). Oceanic sinks for atmospheric CO₂. Plant Cell Environ. 22, 741-755.