2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 11
Presentation Time: 11:10 AM


LERMAN, Abraham, Geological Sciences, Northwestern University, Evanston, IL 60208 and MACKENZIE, Fred T., Oceanography, University of Hawaii, Honolulu, HI 96822, alerman@northwestern.edu

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 Corg 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 Corg is remineralized or <0 for CO2 uptake at high Corg/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 Corg 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.