The combined impact of CO₂-dependent parameterisations of Redfield and Rain ratios on ocean carbonate saturation

Abstract. Future changes to the organic carbon and carbonate pumps are likely to affect ocean ecosystem dynamics and the biogeochemical climate. Here, biological dependencies on the Rain and Redfield ratios on pCO₂ are implemented in a coupled Biogeochemistry-Ocean Model, the CSIRO-Mk3L, to establish extreme-case carbonate saturation vulnerability to model parameterisation at year 2500 using IPCC Representative Concentration Pathway 8.5. Surface carbonate saturation is relatively insensitive to the combined effects of variable Rain and Redfield ratios (an anomaly of less than 10 % of the corresponding change in the control configuration by year 2500), but the global zonally-averaged ocean interior anomaly due to these feedbacks is up to 130 % by 2500. A non-linear interaction between organic and carbonate pumps is found in export production, where higher rates of photosynthesis enhance calcification by raising surface alkalinity. This non-linear effect has a negligible influence on surface carbonate saturation but does significantly influence ocean interior carbonate saturation fields (an anomaly of up to 45 % in 2500). The strongest linear and non-linear sensitivity to combined feedbacks occurs in low-latitude remineralisation zones below regions of enhanced biological production, where dissolved inorganic carbon rapidly accumulates.