⁸⁸Sr/⁸⁶Sr fractionation in inorganic aragonite and in corals

Conflicting results have been reported for the stable Sr isotope fractionation, specifically with respect to the influence of temperature. In an experimental study we have investigated the stable Sr isotope systematics for inorganically precipitated and biogenic (coral) aragonite (natural and laboratory-cultured). Inorganic aragonite precipitation experiments were performed from natural seawater using the CO₂ diffusion technique. The experiments were performed at different temperatures and different carbonate ion concentrations. ⁸⁸Sr/⁸⁶Sr of the inorganic aragonite precipitated in the experiments are 0.2‰ lighter than seawater, but showed no correlation to the water temperature or to CO₃^2- concentration. Similar observations are made in different coral species (Cladocora caespitosa, Porites sp. and Acropora sp.), with identical fractionation from the bulk solution and no correlation to temperature or CO₃^2- concentration. The lack of ⁸⁸Sr/⁸⁶Sr variability in corals at different environmental parameters and the similarity to the ⁸⁸Sr/⁸⁶Sr fractionation in inorganic aragonite may indicate a similar Sr incorporation mechanism in corals skeleton and inorganic aragonite, and therefore the previously proposed Rayleigh-based multi element model (Gaetani et al., 2011) cannot explain the process of Sr incorporation in the coral skeletal material. It is proposed that the relatively constant ⁸⁸Sr/⁸⁶Sr fractionation in aragonite can be used for paleo reconstruction of seawater ⁸⁸Sr/⁸⁶Sr composition. The seawater ⁸⁸Sr/⁸⁶Sr ratio reconstruction can be further used in calcite samples to reconstruct paleo precipitation rates.