In parallel with the study of the air-sea interface, it makes sense to try and directly monitor the invasion of anthropogenicCO2 into the deep ocean. The problem is that dissolvedCO2 content varies greatly from surface to 5 km depth, and also according to the ocean basin under consideration. The expected perturbation is therefore smaller than the natural gradients, notably the contrasts between surface and bottom, and the systematic differences between deep ocean basins, from low levels in the North Atlantic to maxima in the North-East Pacific.
In order to quantify recent anomalies, it is important to understand the natural distribution of inorganic carbon in the ocean. AtmosphericCO2 dissolves in seawater, and its hydration leads to the formation of carbonic acid with two dissociations. The totalCO2 content (TCO2) is therefore made up of aqueous and hydratedCO2, as well as carbonate and bicarbonate ions, which are by far the most concentrated. At depth, TCO2 increases due to remineralization of the soft and hard tissues of plankton: biological oxidation of organic matter by marine bacteria and physicochemical dissolution of solid calcium carbonate. This explains the fine covariations (Redfield ratios) observed between TCO2 and dissolved nutrient salts, particularly phosphate and nitrate ions. This deep remineralization, coupled with the ocean's physical dynamics, helps explain the distribution of nutrients at the surface, and consequently primary biological productivity. This distribution is consistent with the surface chlorophyll maps produced by satellites(Coastal Zone Color Scanner CZCS followed by Sea-viewing wide field-of-view Sensor SeaWIFS). On-board spectrometers measure sea color, with chlorophyll concentration correlated to the ratio of blue to green reflectance.
