The ocean and climate change : relations with marine chemistry and biology (2)

In parallel with the study of the air-sea interface, it makes sense to try and directly monitor the invasion of anthropogenic_CO2 into the deep ocean. The problem is that dissolved_CO2 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. Atmospheric_CO2 dissolves in seawater, and its hydration leads to the formation of carbonic acid with two dissociations. The total_CO2 content (_TCO2) is therefore made up of aqueous and hydrated_CO2, 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.

Natural _TCO2 gradients can be explained by taking into account_CO2 solubility and phytoplankton productivity at the surface, followed by remineralization at depth. As the oxidation of organic matter consumes dissolved oxygen, there is an anticorrelation between increasing _TCO2 levels and decreasing dissolved _O2 during the transport of deep water masses. This_CO2 transport can be studied independently by measuring the ^14C/12C_TCO2 ratios of the water masses. By comparing recent measurements with those taken prior to thermonuclear testing, we can identify the penetration of artificial ^14Cand differentiate it from the natural ^14Cused to date water masses.

Unfortunately, it is not possible to use the same approach to inventory anthropogenic_CO2. Indeed, the concentration anomaly is weaker, in relative terms, and older than for the anthropogenic ^14Canomaly, which began in the late 1950s. We have no measurements of _TCO2 prior to the industrial period. However, it is possible to reconstruct the pre-anthropic state of the ocean that prevailed when atmospheric_CO2 levels were around 280 ppm. This indirect method is based on calculating the remineralized fraction at depth. It involves taking into account measurements of other dissolved species (oxygen, phosphate, nitrate, alkalinity), their advection by circulation and empirical remineralization balance relationships (chemical balance equations based on the average " stoichiometry " of marine organisms with Redfield ratios).

Over the past 40 years, thousands of measurements of _TCO2, ^14Cand other dissolved species have made it possible to quantify the inventory of anthropogenic_CO2 in the ocean and calculate the current increase. Estimates converge on an inventory of 155 GtC, with an error of around 20 %. The ocean has therefore absorbed around 45 % of cumulative fossil carbon emissions (or 30 % if deforestation is taken into account). This technique leads to a current increase of 2.5 ± 0.5 GtC/year, to be compared with fossil carbon emissions (8.5 GtC/year) or total emissions including deforestation (10 GtC/year). This estimate is compatible with that derived from the mapping of fluxes at the air-sea interface. The ocean therefore currently absorbs 30 % of fossil carbon emissions, or 25 % including deforestation. Sequestration percentages for accumulated stocks are higher than for recent fluxes (45 vs. 30 %). This recent decrease is mainly linked to the rise in emissions : less than 2 GtC/year until the 1950s, whereas they currently stand at more than 10 GtC/year.