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

Friday, 19 December 2014: 4:45 PM
Pamela S Hill, Aradhna Tripati and Edwin A Schauble, University of California Los Angeles, Los Angeles, CA, United States
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 CO32– groups of dissolved species and carbonate minerals is reported using the notation Δ63 corresponding mainly to the enrichment in per mil of Hx13C18O16O2x–2 (plus Hx12C18O17O 16Ox–2, Hx12C17O17O 17Ox–2, and Hx13C17O17O 16Ox–2) above the amount expected for a random distribution of isotopes among all CO32–, 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 CO32-) 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 CO32- 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.