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

Abstract

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 CO32− 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 CO32− 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.