Determining the Equilibrium Fractionation Factor Between Calcite and Dissolved Ca in Solution: Ca Isotope Measurements of Pore Fluids and Sediments From ODP Site 807A

Recent laboratory experiments report that the Ca isotope fractionation factor between calcite and aqueous Ca2+ is about --1.5‰ at 20°C. However, there is disagreement regarding whether this is an equilibrium or a kinetic isotope effect. Knowledge of the fractionation factor is critical to understanding the Ca cycle and the paleoceanographic implications of Ca isotopes in marine carbonates. Due to the well-known difficulty of precipitating calcite in the laboratory at equilibrium, we have measured the equilibrium fractionation factor in a natural setting where calcite and fluid have millions of years to exchange Ca and attain equilibrium. We measured the Ca isotopic composition of pore fluids and carbonates from an 800 m section of nearly pure carbonate (ODP Site 807A, Ontong Java Plateau). The average sedimentation rate is ~20 m/m.y. and the entire record spans 35 m.y. We reason, and confirm by modeling, that carbonate in the section recrystallizes so that the pore fluid Ca2+ is in isotopic equilibrium with the solid carbonate at depths greater than 20 m. Since the solid carbonate does not vary greatly in its δ44Ca value, we use pore fluid data to estimate the equilibrium fractionation factor to within ±0.1‰. Carbonate recrystallization rates are constrained by the Sr concentration and isotopic compositions of the pore fluid using the approach of Richter and Liang (EPSL 117, p.553, 1993). The bulk recrystallization rates vary from about 4%/m.y. at the top of the section to 0.2%/m.y. at the bottom, with an age-dependence that is similar to that of other sites. The inferred rates are fast enough to ensure that Ca isotopes are at equilibrium throughout the sediment column. The measured pore fluid δ44Ca values are within ±0.1‰ of the solid carbonate values at all depths, except for one sample at 4 meters. This shallow sample occurs in the boundary layer where the pore fluid δ44Ca values are transitional between seawater (+1.0) and deeper pore fluids (--0.5). A numerical model applied to the data indicates that the equilibrium fractionation factor is 1.0000±0.0001, or Δ = 0.0±0.1‰. This result suggests that the experimentally-observed Δ of --1.5‰ is not an equilibrium value. The pore fluid Ca isotope data can also be used to estimate the carbonate recrystallization rate in the upper 5 meters of the sedimentary column, where Sr isotopes are less sensitive. The model rate for the upper 5 m is 30%/m.y., which extends the age dependence of the recrystallization rates to 0.1 m.y. The modeling suggests that the Ca concentration in pore fluids is determined by carbonate solubility at all depths. As a result, Ca sources and sinks cannot be discerned but we are able to calculate pH values for the pore fluids using carbonate system equilibria and solubility constraints.