Comment on “The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: implications for Eocene and Cretaceous ocean carbon chemistry and buffering” by Hain et al. (2015)

Hain et al. [2015] (H15 hereafter) calculated effects of changing Ca2+ and Mg2+ on the equilibrium constants of the CO2‐CaCO3 system in seawater based on a Pitzer ionic interaction model [Pitzer, 1973; Millero and Pierrot, 1998]. In their Supporting Information, H15 tabulated parameters for stoichiometric equilibrium constants (K‘s) to be used by the community. Furthermore, H15 contended that previously published, simplified Ca‐Mg corrections for K‘s should be avoided [e.g., Tyrrell and Zeebe, 2004]. The main application of such corrections are carbonate chemistry reconstructions in deep‐time, when the major‐ion chemistry of seawater was different from modern. Clearly, for such an approach, the complexity and accuracy of the tool employed should be in proportion to the inherent uncertainty of the target application. For example, the reconstructed changes of past temperature, salinity, CO2‐system parameters, and Ca2+ and Mg2+ themselves have large uncertainties, which calls into question the suitability of a seemingly convoluted approach such as a Pitzer model in the first place. More importantly, however, we show here that critical results presented in H15 pertaining to the first dissociation constant of carbonic acid are fundamentally wrong. We therefore do not advise users to apply H15’s stoichiometric equilibrium constants to correct for past changes in major‐ion seawater composition.

Zeebe R. E. & Tyrrell T., in press. Comment on “The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: implications for Eocene and Cretaceous ocean carbon chemistry and buffering” by Hain et al. (2015). Global Biogeochemical Cycles. Article (subscription required).

 

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