Predicting Effects of Cations (Mg, Ca, Na, and K) on 13C-18O Clumping in Dissolved Inorganic Carbon Species and Implications for Carbonate Geothermometry

Abstract:

¹³C-¹⁸O bond abundance in carbonates is becoming more widely used as a geothermometer; this proxy is affected by various environmental factors. Here we report the influence of cations (Mg²⁺, Ca²⁺, Na⁺, and K⁺) at high concentrations (~2 mol/liter) on the isotopologue composition of the DIC pool. Clumped isotope fractionation in CO₃^2– groups of dissolved species and carbonate minerals is reported using the notation Δ₆₃ corresponding mainly to the enrichment in per mil of H_x¹³C¹⁸O¹⁶O₂^x–2 (plus H_x¹²C¹⁸O¹⁷O ¹⁶O^x–2, H_x¹²C¹⁷O¹⁷O ¹⁷O^x–2, and H_x¹³C¹⁷O¹⁷O ¹⁶O^x–2) above the amount expected for a random distribution of isotopes among all CO₃^2–, HCO₃^– and H₂CO₃ isotopologues. The Δ₆₃ of a solution of dissolved inorganic carbon (DIC) depends upon the relative abundances of each DIC species (CO₂(aq) or H₂CO₃, HCO₃^-, and CO₃^2-) since each DIC species has a distinct equilibrium clumped isotope signature. These abundances depend primarily upon solution pH and secondarily upon temperature and salinity (fresh water vs. sea water vs. brine).

Solvated DIC species with additional ions and the composite DIC solutions were modeled as a series of supermolecular clusters, each with a single DIC molecule, an added cation, and 21 to 32 surrounding H₂O molecules. As in our previous work (Hill et al., 2014, GCA 125, 610–652), we developed electronic structure models at different levels of theory to ensure the best possible reliability at reasonable computational efficiency. Overall, the models predict that common aqueous cations will slightly increase the ¹³C-¹⁸O clumping signature of both individual DIC species and the total DIC pool at a given pH, salinity, and temperature. Predicted Δ₆₃values are also dependent upon cation concentration.

The perturbing effect of Mg²⁺ > Ca²⁺ > K⁺ > Na⁺. Dissolved cations increase the clumped crossover pH (pH at which the composite Δ₆₃ of the DIC pool equals the Δ₆₃ of calcite at equilibrium). Our models predict that a DIC solution of low to moderate pH (i.e. dominantly HCO₃^– or H₂CO₃) will have Δ₆₃ greater than the equilibrium Δ₆₃ of calcite; at higher pH (i.e., mostly the CO₃^2- species in solution) the composite DIC Δ₆₃ will be lower than the equilibrium Δ₆₃ of calcite. As salinity and/or temperature increase, the crossover pH (between higher or lower Δ₆₃) decreases.