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Abstract Foraminifera are unicellular organisms that inhabit the oceans. They play an important role in the global carbon cycle and record valuable paleoclimate information through the uptake of trace elements such as strontium into their calcitic shells. Understanding how foraminifera control their internal fluid composition to make calcite is important for predicting their response to ocean acidification and for reliably interpreting the chemical and isotopic compositions of their shells. Here, we model foraminiferal calcification and strontium partitioning in the benthic foraminiferaCibicides wuellerstorfiandCibicidoides mundulusbased on insights from inorganic calcite experiments. The numerical model reconciles inter-ocean and taxonomic differences in benthic foraminifer strontium partitioning relationships and enables us to reconstruct the composition of the calcifying fluid. We find that strontium partitioning and mineral growth rates of foraminiferal calcite are not strongly affected by changes in external seawater pH (within 7.8–8.1) and dissolved inorganic carbon (DIC, within 2100–2300 μmol/kg) due to a regulated calcite saturation state at the site of shell formation. 

Abstract Most Earth surface carbonates precipitate out of isotopic equilibrium with their host solution, complicating the use of stable isotopes in paleoenvironment reconstructions. Disequilibrium can arise from exchange reactions in the DIC‐H₂O system as well as during crystal growth reactions in the DIC‐CaCO₃system. Existing models account for kinetic isotope effects (KIEs) in these systems separately but the models have yet to be combined in a general framework. Here, an open‐system box model is developed for describing disequilibrium carbon, oxygen, and clumped (Δ₄₇, Δ₄₈, and Δ₄₉) isotope effects in the CaCO₃‐DIC‐H₂O system. The model is used to simulate calcite precipitation experiments in which the fluxes and isotopic compositions of CO₂and CaCO₃were constrained. Using a literature compilation of equilibrium and kinetic fractionation factors, modeledδ¹⁸O and Δ₄₇values of calcite are in good agreement with the experimental data covering a wide range in crystal growth rate and solution pH. This relatively straightforward example provides a foundation for adapting the model to other situations involving CO₂absorption (e.g., corals, foraminifera, and high‐pH travertines) or degassing (e.g., speleothems, low‐pH travertines, and cryogenic carbonates) and/or mixing with other dissolved inorganic carbon sources.