Constraining sulfur incorporation in calcite using inorganic precipitation experiments

The sulfur over calcium ratio (S/Ca) in foraminiferal shells was recently proposed as a new and independent proxy for reconstructing marine inorganic carbon chemistry. This new approach assumes that sulfur is incorporated into CaCO₃ predominantly in the form of sulfate (SO₄²⁻) through lattice substitution for carbonate ions (CO₃^2–), and that S/Ca thus reflects seawater [CO₃^2–]. Although foraminiferal growth experiments validated this approach, field studies showed controversial results suggesting that the potential impact of [CO₃^2–] may be overwritten by one or more parameters. Hence, to better understand the inorganic processes involved, we here investigate S/Ca values in inorganically precipitated CaCO₃ (S/Ca_(cc)) grown in solutions of CaCl₂ − Na₂CO₃ − Na₂SO₄ − B(OH)₃ − MgCl₂. Experimental results indicate the dependence of sulfate partitioning in CaCO₃ on the carbon chemistry via changing pH and suggest that faster precipitation rates increase the partition coefficient for sulfur. The S/Ca ratios of our inorganic calcite samples show positive correlation with modelled [CaSO₄⁰]_(aq), but not with the concentration of free SO₄²⁻ ions. This challenges the traditional model for sulfate incorporation in calcite and implies that the uptake of sulfate potentially occurs via ion-ion pairs rather than being incorporated as single anions. Based on the [Ca²⁺] dependence via speciation, we here suggest a critical evaluation of this potential proxy. As sulfate complexation seems to control sulfate uptake in inorganic calcite, application as a proxy using foraminiferal calcite may be limited to periods for which seawater chemistry is well-constrained. As foraminiferal calcite growth is modulated by inward Ca²⁺ flow to the site of calcification coupled to outward H⁺ pumping, sulfate incorporation as CaSO₄⁰ ion-pair in the foraminifer's shell also provides a mechanistic link for the observed relationship between S/Ca_(cc) and [CO₃^2–].