CARBON DIOXIDE SEA-AIR EXCHANGE: THE ROLES OF CARBONATE AND ORGANIC CARBON STORAGE

Precipitation of calcium carbonate and biological production of organic matter in surface ocean water modify the distribution of the dissolved carbonate species and thereby affect the transfer of carbon dioxide across the air-sea interface. Sea-air transfer of CO₂ owing to the removal of inorganic and organic carbon to the sediments, calculated as a ratio θ = (CO₂ flux to the atmosphere)/(CaCO₃ precipitated), depends on water temperature, atmospheric CO₂ concentration, and the CaCO₃ and C_org masses removed from ocean water. In CO₂ generation by CaCO₃ precipitation alone, θ varies from 0.44 to 0.79, but it can be >1 if additional C_org is remineralized or <0 for CO₂ uptake at high C_org/CaCO₃ deposition ratios. θ increases with decreasing temperature (25° to 5°C), increasing atmospheric CO₂ (195 to 375 ppmv), and increasing CaCO₃ deposition (up to 45% of the initial DIC concentration in surface water). The CO₂ flux to the atmosphere is a non-linear function of the water-layer thickness because of the back-pressure of the rising atmospheric CO₂.

The calculated CO₂ flux to the atmosphere from a model 50-m-thick euphotic zone near the Last Glacial Maximum (LGM) is 7 to 17×10¹² mol/yr (0.08 to 0.2 Gt C/yr), reflecting the range of C_org storage rates in sediments, and for pre-industrial time it is about 40×10¹² mol/yr (0.48 Gt C/yr). About 50% of the pre-industrial sea-air flux is attributable to the coastal ocean, a region of very significant calcification and organic matter production and remineralization. The magnitude of this changing flux suggests that the coastal ocean played an important role in the rise of atmospheric CO₂ concentration since the LGM. The CO₂ net flux between coastal surface waters and the atmosphere in the future is expected to reverse from net evasion to net invasion owing to increased ecosystem production, decreased ecosystem calcification, and increased atmospheric CO₂ concentration from burning of fossil fuels. Surface-water saturation state with respect to carbonate minerals would decrease and consequently the rate of biological calcification would also decrease. Weaker calcification, stronger production and storage of organic matter, and an increase in alkalinity due to dissolution of metastable carbonate minerals would favor the role of the coastal zone as a CO₂ sink in the future.