2.3.2.2 Biological pumps

In addition to this purely thermal effect, biological processes also play a significant role in the distribution of surface fluxes of CO₂ by affecting DIC and Alk. A first important reaction is the photosynthesis in which phytoplankton uses solar radiation to form organic matter from CO₂ and water:

Conversely, organic matter could be dissociated to form inorganic carbon (the reverse process of photosynthesis) by respiration and remineralisation of dead phytoplankton and detritus.

Reaction 2.46 is a highly simplified representation of the complex biological processes associated with photosynthesis. In particular, it hides the fact that, in order to produce organic matter, phytoplankton need nutrients (mainly nitrates and phosphates) as well as minor elements such as iron. As those nutrients generally have low concentrations in surface water where light is available for photosynthesis, their concentration is often the limiting factor for biological production.

Because particles whose density is more than that of water settle out, and some particles are transported by ocean currents, a fraction of the organic matter is exported downward out of the surface layer. The net downward flux of carbon associated with this transport of organic matter is called "the soft tissue pump". A significant part of the remineralisation thus occurs in the deep layers where it produces an increase in DIC and the release of nutrients. The deep waters are thus rich in nutrients. Where they upwell toward the surface, the surface concentration of nutrients increases, generally leading to high biological production, such as that observed off the coasts of Peru and Mauritania.

A second important biological process is related to the production of calcium carbonate (in form of calcite or aragonite) by different species, in particular to form their shells:

This production influences both the DIC and the Alk and can thus have a large influence on the carbon cycle. For instance, CaCO₃ production implies a reduction in Alk (see Eq. 2.45), which in turns lead to an increase in oceanic p^CO₂ and reduces the uptake of atmospheric CO₂ by the ocean. An alternative way to view this mechanism is to say that CaCO₃ production reduces the concentration of CO₃^2- in the ocean and thus the availability of this ion to combine with H₂CO₃ to produce HCO₃^- (reaction 2.44), so increasing [H₂CO₃] and p^CO₂.

The dissolution of calcite and aragonite mainly occurs at great depth (see section 4.3.1), following the precipitation of particles and dead organisms. This leads to the Alk and DIC being transported downwards, a system called the carbonate pump . A third pump, called the solubility pump, is associated with the sinking of cold surface water, characterised by a relatively high solubility of CO₂ and thus high DIC, to great depths at high latitudes. All these downward transports have to be compensated for at equilibrium by an upward flux of inorganic carbon by the oceanic circulation.

Because of the three pumps briefly described above, DIC is about 15% higher at depths than at the surface. The soft tissue pump plays the largest role in the observed vertical gradient. This distribution has a profound influence on the atmospheric CO₂ concentration. Indeed, if DIC were perfectly homogenous in the water column (i.e. had higher surface values and lower depth values than currently observed), the concentration of atmospheric CO₂ would be much higher. More realistically, when deep water upwells to the surface, CO₂ will tend to escape from the ocean because of the high DIC. However, as the deep waters are rich in nutrients, the biological uptake associated with photosynthesis can compensate for the influence of a higher DIC. The net effect depends on the regions, generally resulting in positive ocean-atmosphere CO₂ fluxes at high latitudes and negative ones at high latitudes, partly offsetting the direct temperature effect (Fig. 2.25).