Inclusion of MyAMI-derived Mg ∕ Ca corrections to the marine carbonate system in the cGENIE.cookie Earth system model (v.0.91)

Abstract

Abstract. The concentrations of the major cations (esp., Ca2+, Mg2+) in Earth's oceans have undergone large-scale fluctuations in the geological past. This is important because the key geochemical properties of the marine environment that underpin the global carbon cycle – the aqueous carbonate system equilibria and solubility of solid calcium carbonate (CaCO3) – are heavily influenced by ion-pairing, which in turn depends on the activity of the major cations and anions. An appropriate interpretation of marine proxies as well as the reconstruction of past states of ocean geochemistry and carbon cycle across geologic events requires that these effects are considered. However, most current global carbon cycle models use empirical carbonate system dissociation constants fitted to laboratory experiments with present-day seawater major cation and anion concentrations and in the few simulations of global carbon cycling that have accounted for paleo-seawater composition, only relatively simplified empirical adjustments of the equilibrium constants (from Ben-Yaakov and Goldhaber, 1973; Tyrrell and Zeebe, 2004) have been implemented (e.g., Panchuk et al., 2008). Here we develop and evaluate a new scheme in the cGENIE Earth system model for correcting carbonate system equilibrium constants and accounting for variations in the dissolved calcium and magnesium concentrations in the ocean. We base our new parameterization on the MyAMI specific ion interaction model of Hain et al. (2015) and implement this in cGENIE by means of linear interpolation within a 4-dimensional parameter look-up table of pre-calculated carbonate system equilibrium constant values. For modern seawater composition, our implementation of MyAMI-based equilibrium constants yields no meaningful deviation from model results using empirically-based equilibrium constants, validating our look-up/interpolation approach. However, for simulations conducted under non-modern Mg/Ca, we find substantial differences in carbon chemistry and CaCO3 saturation when using our new MyAMI-based equilibrium constants as compared to the existing (default) correction scheme in cGENIE. Specifically, our new MyAMI-based correction scheme exhibits a much lower sensitivity to an instantaneous change in Mg/Ca from modern to Eocene and an approximate doubling of Ca2+ concentration, in both surface ocean pH (0.01) and calcite saturation state (9.41), which were overestimated by 0.05 and 4.04, respectively, with the previous correction scheme. Any bias in carbonate chemistry and CaCO3 saturation will also affect the preservation and burial of CaCO3 in deep-sea sediments and hence potentially impact model-data comparisons. We illustrate this by contrasting the ocean carbon inventory arising under Eocene Mg/Ca with the same total weathering (and hence CaCO3 burial) flux for the different possible equilibrium constant corrections. We find that the new MyAMI-based and previous default corrections give rise to a dissolved inorganic ocean carbon inventory 20 µmol kg−1 (348 PgC) higher and 59 µmol kg−1 (950 PgC) lower, respectively, relative to the same experiment conducted using empirical equilibrium constants without any Mg/Ca correction. Applying no correction at all for a different-from-modern Mg/Ca ratio in the ocean would appear to be better than applying an overly-approximated correction but explicitly accounting for past dissolved calcium concentrations remains of fundamental importance. We provide this new carbonate system equilibria correction as an option in cGENIE.muffin version 0.9.76, and as standard in a forthcoming new cGENIE code release – cGENIE.cookie v.0.91.