CO₂ air-sea exchange due to calcium carbonate and organic matter storage: pre-industrial and Last Glacial Maximum estimates

International audienceRelease of CO2 from surface ocean water owing to precipitation of CaCO3 and the imbalance between biological production of organic matter and its respiration, and their net removal from surface water to sedimentary storage was studied by means of a model that gives the quotient ?=(CO2 released to the atmosphere)/(CaCO3 precipitated). The surface ocean layer is approximated by a euphotic zone, 50 m thick, that includes the shallower coastal area and open ocean. ? depends on water temperature, CaCO3 and organic carbon mass formed, and atmospheric CO2 concentration. At temperatures between 5 and 25°C, and three atmospheric CO2 pressures ? 195 ppmv corresponding to the Last Glacial Maximum, 280 ppmv for the end of pre-industrial time, and 375 ppmv for the present ? ? varies from a fraction of 0.38 to 0.79, increasing with decreasing temperature, increasing atmospheric CO2 content, and increasing CaCO3 precipitated mass (up to 45% of the DIC concentration in surface water). For a surface ocean layer that receives input of inorganic and organic carbon from land, the calculated CO2 flux to the atmosphere at the Last Glacial Maximum is 20 to 22×1012 mol/yr and in pre-industrial time it is 45 to 49×1012 mol/yr. In addition to the environmental factors mentioned above, flux to the atmosphere and increase of atmospheric CO2 depend on the thickness of the surface ocean layer. The significance of these fluxes and comparisons with the estimates of other investigators are discussed. Within the imbalanced global carbon cycle, our estimates are in agreement with the conclusions of others that the global ocean prior to anthropogenic emissions of CO2 to the atmosphere was losing carbon, calcium, and total alkalinity owing to precipitation of CaCO3 and consequent emission of CO2. Other pathways of CO2 exchange between the atmosphere and land organic reservoir and rock weathering may reduce the imbalances in the carbon cycle on millenial time scales