On the total CO₂–titration alkalinity–oxygen system in the Pacific Ocean

Abstract

Decomposition of organic matter changes the concentrations of carbon, nitrogen, phosphorus, oxygen and titration alkalinity (TA) in the ratio 106:16:1:138:–17 (refs 1–3), so the combined effect of decomposing x mol of CaCO₃ and y mol of organic matter in 1 kg of seawater on the preformed total CO₂ (ΣCO₂⁰), preformed TA (TA⁰), biogenerated ΣCO₂ [ΔΣCO₂(biol)] and the apparent oxygen utilisation (AOU) can be represented as⁴ ΔΣCO₂=ΣCO₂(measured)−ΣCO₂⁰=x+106y ΔTA=TA(measured)−TA⁰=2x−17y ΔΣCO₂(biol)=106y; AOU=138y Eliminating x and y from the above equations yields Several early workers^5–8 have successfully correlated ΔΣCO₂(biol) to AOU with slopes close to the Redfield, Ketchum and Richards (RKR) model¹. However, the methods used often have not satisfactorily taken into account the variation of the preformed values for ΣCO₂ and TA with sample depth or have mistakenly taken ΔΣCO₂(biol) to be equal to ΔΣCO₂−O.5ΔTA. The inconsistency between the correlations above and below the thermocline is also not well explained. A modified computational scheme^5,8 has recently been developed for calculating the ΣCO₂-TA-oxygen correlation with the depth dependent variations of ΣCO₂⁰ and TA⁰ accounted for. The method of calculating ΔΣCO₂ is the same as that described in ref. 4 but further useful information has been obtained by plotting ΔΣCO₂ against AOU, rather than depth. The results in the Pacific Ocean, presented here, suggest a linear correlation between the bio-generated CO₂ and AOU with a slope of 0.722±0.05, in good agreement with that predicted from the RKR model. The deviation of the data from this linear correlation for shallow water can largely be explained by the influences of human induced CO₂.