Feedback from the solid carbonate " counter-pump

The synthesis of carbonate skeletons by biological organisms also influences the acid-base balance of seawater. To form, calcium carbonate consumes dissolved ions, notably carbonate ions _CO32-^. To compensate for this loss, ^HCO3- ions (bicarbonates or hydrogen carbonates) are converted into ^CO32- ions and dissolved_CO2.

The precipitation of solid _CaCO3 thus shifts the acid-base equilibrium and leads to a drop in seawater pH. If the _pCO2 of surface water is in equilibrium with that of the overlying atmosphere, the excess_CO2 linked to calcification will diffuse from the ocean into the atmosphere. This phenomenon is often referred to as the carbonate "counter-pump", to underline the fact that calcification tends to increase_atmosphericpCO2, unlike the organic biological pump described above.

After the death of organisms, carbonate skeletons sediment in the water column and pile up on the ocean floor to form calcareous sludge. Some of the carbonates will redissolve in the form of carbonate and calcium ions. Dissolution takes place during the fall in the water column, as well as in the interstitial water of surface sediments. The reaction is favored because limestone solubility increases with pressure and decreases with temperature. High pressures and low temperatures on the ocean floor therefore favor the dissolution of carbonates formed in the surface layers of the ocean. What's more, as the degradation of organic matter lowers the pH of deep waters, the effect of limestone undersaturation is further enhanced, particularly in the North Pacific where aged water masses are characterized by high _TCO2, low pH and low _CO32- levels^.

As with organic carbon, the ocean was in a state of dynamic equilibrium with the natural atmospheric_CO2 content. Recent contamination of the ocean by anthropogenic_CO2 has altered acid-base equilibria. The pH of surface waters has fallen by 0.1 pH units and the degree of calcium carbonate saturation has decreased significantly, although most of the surface ocean is still oversaturated with regard to the precipitation of calcium carbonates, calcite and aragonite. However, some regions naturally close to surface saturation (the Arctic Ocean or certain coastalupwelling zones in the eastern Pacific) have now become clearly undersaturated for aragonite, preventing the formation of this allotropic form of _CaCO3 (less stable than calcite).

Laboratory and mesocosm experiments suggest that a significant increase in atmospheric _pCO2 would induce a decrease in calcification in organisms such as reef corals and phytoplankton coccolithophorid algae. A possible general drop in oceanic calcification would therefore reduce the " counter-pump " effect, i.e. it would tend to lower atmospheric _pCO2 (a negative feedback).

However, studies in natural environments indicate that the biological response is complex, depending in particular on the biological species in question. Naturally more acidic oceanic zones have revealed adaptations in certain organisms (e.g. morphological and genetic adaptation of coccolithophorids, acclimatization of certain corals to acidification, etc.). Mesocosm studies are still needed to investigate the influence of these adaptations on the calcification of an entire ecosystem.

In parallel to possible changes in biological calcification at the surface, the carbonate system could retroact on excess anthropogenic_CO2 via its neutralization by dissolution of sedimentary carbonates at depth. As we saw above, most of the deep ocean has not yet been affected by the spread of anthropogenic_CO2, and most of the deep limestone remains undisturbed.

Nevertheless, anthropogenic_CO2 has effectively reached the natural saturation depth of aragonite in certain areas located at sub-polar latitudes in the Southern Ocean and North Pacific. In the latter case, the_CO2 anomaly even reached the saturation depth of calcite, which is more stable than aragonite. The consequence of this dissemination of anthropogenic_CO2 is that the saturation depth has risen by more than 100 m in these areas. We also note the vertical and horizontal extension of the undersaturation zone located at mid-depth in the subtropical Atlantic. This spatial extension of carbonate undersaturation exposes new limestone sediment surfaces to undersaturated water, which is more aggressive than normal.

It is important to note that limestone dissolution is not limited to the undersaturated zone of the ocean, but is significant as early as the first kilometer of depth. Significant dissolution in sediment pore waters is linked to remineralization of organic carbon, which creates undersaturated conditions in the first few centimeters of the sediment column, even if the overlying deep water is supersaturated. The complexity of dissolution at shallow depths has been demonstrated using particle traps, in situ measurements (benthic chambers and microelectrodes), and geochemical balances based on measurements of _O2, _HCO3-^,^CO32- and ^14C. Anthropogenic_CO2 is therefore already having an impact on shallow areas, which should further increase the effect of neutralization by limestone dissolution.

Nevertheless, the predicted drop in atmospheric _pCO2 is not yet perceptible, as the kinetics of dissolution are very slow. This first-order reaction depends on the degree of undersaturation, pressure and temperature conditions. Taking these parameters into account in ocean and sediment geochemical models shows that the residual excess of atmospheric_CO2 will be partially neutralized by limestone dissolution over several millennia. After 10,000 years, a rapid injection of 100 GtC in the form of atmospheric_CO2 would therefore be 90 % gone from the atmosphere, the bulk of the sequestration being linked to geochemical pumping from the ocean.

Recent studies of the ocean carbon cycle show that most of the biogeochemical feedbacks identified or detected have not yet had a significant impact on the pumping of anthropogenic_CO2. From the early 1960s to the present day, the annual increase in atmospheric _pCO2 has roughly tripled (from 0.8 to 2.4 ppm per year), mechanically increasing the net flux of_CO2 at the air-sea interface. Over the same period, the latest estimates of the oceanic sink also indicate a tripling of_CO2 sequestration (from less than 1 to almost 3 GtC per year). However, positive and negative feedbacks do exist. Their respective intensities will become clearer in the future, as predicted by models coupling the climate system to the carbon cycle.