Carbon isotope mass balance modelling of atmospheric vs. oceanic CO2

Category Environmetal Geology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Segalstad, Tom V
Holding Date 28 September 2008

Stable 13C/12C isotope ratios, expressed as δ13C in permil vs. PDB, give us the only way to unequivocally determine the fraction of anthropogenic CO2 in the atmosphere. The natural atmospheric CO2 reservoir has δ13C -7 when in isotopic equilibrium with marine HCO3- and CaCO3. CO2 from burning of fossil-fuel and biogenic materials has δ13C -26. A pre-industrial CO2 value of 280 ppmv in air was assumed by IPCC based on selected low value CO2 data from ice cores (omitting measured high values up to 7,400 ppmv), matched with contemporary measurements by transposition of data with different ages (Jaworowski et al. 1992). IPCC claims that the rise in CO2 to 353 ppmv in 1990 and 379 ppm in 2005 is only due to anthropogenic CO2 (IPCC 1990, 2007).
δ13C reported for atmospheric CO2 was -7.489 in Dec. 1978, decreasing to -7.807 in Dec. 1988 (Keeling et al. 1989). If the decreasing δ13C was only caused by mixing natural CO2 with CO2 from burning of fossil fuels or plants (79%/21% CO2 mix; lifetime 50-200 years; IPCC 1989), the current atmospheric CO2 δ13C should be -11, much lower than reported. The December 1988 atmospheric CO2 composition was computed for its 748 GT C (GT = 1015 g) total mass and δ13C = -7.807 for 3 components: (1) natural fraction remaining from the pre-industrial atmosphere; (2) cumulative fraction remaining from all annual fossil-fuel CO2 emissions; (3) carbon isotope mass-balanced natural fraction. The masses of component (1) and (2) were computed for different atmospheric lifetimes of CO2.
The result fits a lifetime of 5 years, in agreement with 14C studies. The mass of all past fossil-fuel and biogenic emissions remaining in the current atmosphere was 30 GT C or less, i.e. maximum 4%, corresponding to an atmospheric concentration of 14 ppmv. The implication of the 5 year lifetime is that 135 GT C (18%) of the atmospheric CO2 is dynamically exchanged each year.
Partitioning of CO2 between atmosphere and hydrosphere is governed by Henry’s Law, implying that 50-60 times more CO2 is dissolved in the oceans than in the atmosphere. Due to the retrograde solubility of CO2 in water, less CO2 will be dissolved in water at higher temperatures.
At least 96% of the current atmospheric CO2 comes from non-fossil-fuel sources, i.e. natural marine and juvenile sources. Hence for the atmospheric CO2 budget marine degassing and juvenile degassing from e.g. volcanic sources must be much more important, and burning of fossil-fuel and biogenic materials much less important, than hitherto assumed.
Thermochemical modelling of calcium carbonate in seawater shows that pH would have to be decreased by 2 units, and H+ activity increased by 100, for Ca carbonate to dissolve at 25°C. By increasing atmospheric CO2 this would by itself require 100 times the present CO2 level. However, mineral buffers in the sea constitute an infinite buffer capacity (Stumm & Morgan 1970), making carbonate destructive ocean acidification from anthropogenic CO2 impossible.