Ocean carbon cycle and physical pumping of carbon dioxide

Since the beginning of the industrial era, the partial pressure of carbon dioxide in the atmosphere has increased by more than 100 parts per million (_pCO2 from 280 ppm in the ^18th century to 400 ppm today). Precise_CO2 measurements since the late 1950s show that the increase is slightly greater in the northern hemisphere than in the southern. The inter-hemispheric_CO2 gradient increases linearly with emissions from fossil fuel combustion. However, a comparison of anthropogenic emissions and the average atmospheric stock clearly illustrates that half of the_CO2 emitted is no longer in the atmosphere. Carbon dioxide molecules are constantly diffusing towards other reservoirs in the carbon cycle. Four independent techniques show that the ocean currently absorbs 25 to 30 % of fossil carbon emissions. Since the beginning of the industrial era, cumulative oceanic absorption has been around 45 %, accompanied by a drop of around 0.1 pH units in surface waters.

Carbon 14^(14C) is used as a tracer to track the rate of exchange between reservoirs in the global carbon cycle. Cosmic rays naturally produce ^14Cin the atmosphere, which slowly decays through radioactivity. A second source of ^14Chas disrupted the natural state of equilibrium. In fact, around a ton of artificial ^14Cwas created by atmospheric nuclear testing in the early 1960s. The injection was concentrated mainly in the stratosphere of the northern hemisphere, but after ten years or so, the atmosphere became virtually homogeneous. The atmospheric ^14Ccontent then decreased, mainly through diffusion of_CO2 into the ocean. Compared with pre-anthropic equilibrium, the atmospheric excess in ^14Cwas around 600 ‰ in the 1970s and is only 100 ‰ today. Global contamination of surface waters has been measured directly in dissolved bicarbonates as well as in the annual rings of massive corals, which show ^14Cexcesses of 250 to 100 ‰ for dates ranging from the early 1970s to the 1990s. The intensity and date of the maximum ^14Cconcentration depend on the geographical area concerned, including local conditions of vertical mixing of water masses.

Tounderstand and quantify the penetration of anthropogenic_CO2 into the ocean, it is important to consider the chemical reactions of the carbonate system._{Atmospheric CO2}dissolves in the ocean. Its hydration leads to the formation of carbonic acid _H2CO3, which undergoes two acid dissociations with pK values of around 6 and 9 respectively. The neutralization of a_CO2 molecule shifts acid-base equilibria, resulting in the transformation of a carbonate ion ^CO32-into a bicarbonate ion ^HCO3- (or hydrogen carbonate), accompanied by a relatively limited pH decrease.

Thanks to the carbonic and boric acid systems, seawater behaves like a buffer solution, maintaining the same pH after the addition of small quantities of acid or base, or despite dilution. Consequently, the relative impacts are different on dissolved_CO2 (_pCO2 or, more precisely, its fugacity expressed in µatm) and on the total species of the carbonate system (_TCO2= _CO2+ ^HCO3-+ ^CO32- in mol/L). This difference is well illustrated by the calculation of the factor introduced by Roger Revelle and Hans Suess in 1957, expressing the power of the acid-base buffer.

On average, Revelle's factor is 10 at the ocean surface, meaning that if _pCO2 rises by 10%, total _TCO2 content will increase by only 1%. This factor clearly illustrates the role of the acid-base buffer in slowing the spread of excess atmospheric_CO2 into the ocean. Numerical models representing the carbon cycle by box plots can correctly simulate the joint evolution of atmospheric and oceanic_CO2 and ^14Ccontents over the last century. Ignoring the buffering factor in the simulation leads to massive sequestration of_CO2 in the ocean and an excessive reduction of more than a factor of 2 in the atmospheric anomaly, in contradiction with direct measurements.

In addition to the neutralization of carbonic acid by the acid-base system of dissolved carbonates, excess atmospheric_CO2 can, in principle, be reduced by other complementary biogeochemical reactions: increased formation of organic matter, dissolution of sedimentary solid carbonates or chemical alteration of silicate rocks. The latter only occurs on very long time scales. Numerical models that take all these reactions into account show that an instantaneous and significant disturbance of the atmospheric stock (e.g. 100 GtC) gradually disappears into the ocean: one-third after a century, and practically two-thirds after a thousand years.