Ocean carbonate system computation for anoxic waters using an updated CO2SYS program Marine Chemistry

In anoxic/hypoxic waters, the presence of hydrogen sulfide (H2S) and ammonia (NH3) influences results of the computation of parameters in the ocean carbonate system. To evaluate their influences, H2S and NH3 contributions to total alkalinity are added to CO2SYS, which is a most often used publicly available software package that calculates oceanic carbonate parameters. We discuss how these two metabolites affect the carbonate parameters and compare the differences in total alkalinity, dissolved inorganic carbon, pH, fCO2, and aragonite saturation state between the CO2SYS packages with and without considering the acid-base systems of H2S and NH3. The results show that, without considering these two acid-base systems, even low to moderate concentrations (e.g., 2–20 μmol kg−1) of these metabolites cause errors in the calculated carbonate parameters larger than the accuracies of the best measurements, and thus it is important to include contributions from these metabolites. The outputs from this updated version of CO2SYS agree well with outputs from AquaEnv, which is the only other computation program for the ocean carbonate system that includes the acid-base systems of H2S and NH3. Users are encouraged to use the updated version of CO2SYS to calculate carbonate parameters in anoxic/hypoxic waters.