The influence of seawater carbonate ion concentration [CO 32−] on the stable carbon isotope composition of the planktic foraminifera species Globorotalia inflata

Abstract

We sampled the upper water column for living planktic foraminifera along the SW-African continental margin. The species Globorotalia inflata strongly dominates the foraminiferal assemblages with an overall relative abundance of 70–90%. The shell δ 18O and δ 13C values of G. inflata were measured and compared to the predicted oxygen isotope equilibrium values ( δ 18O eq) and to the carbon isotope composition of the total dissolved inorganic carbon ( δ 13C DIC) of seawater. The δ 18O of G. inflata reflects the general gradient observed in the predicted δ 18O eq profile, while the δ 13C of G. inflata shows almost no variation with depth and the reflection of the δ 13C DIC in the foraminiferal shell seems to be covered by other effects. We found that offsets between δ 18O shell and predicted δ 18O eq in the surface mixed layer do not correlate to changes in seawater [CO 3 2−]. To calculate an isotopic mass balance of depth integrated growth, we used the oxygen isotope composition of G. inflata to estimate the fraction of the total shell mass that is grown within each plankton tow depth interval of the upper 500 m of the water column. This approach allows us to calculate the Δ δ 13C interval added-DIC; i.e. the isotopic composition of calcite that was grown within a given depth interval. Our results consistently show that the Δ δ 13C IA-DIC correlates negatively with in situ measured [CO 3 2−] of the ambient water. Using this approach, we found Δ δ 13C IA-DIC/[CO 3 2−] slopes for G. inflata in the large size fraction (250–355 μm) of − 0.013‰ to 0.015‰ (μmol kg − 1) − 1 and of − 0.013‰ to 0.017‰ (μmol kg − 1) − 1 for the smaller specimens (150–250 μm). These slopes are in the range of those found for other non-symbiotic species, such as Globigerina bulloides, from laboratory culture experiments. Since the Δ δ 13C IA-DIC/[CO 3 2−] slopes from our field data are nearly identical to the slopes established from laboratory culture experiments we assume that the influence of other effects, such as temperature, are negligibly small. If we correct the δ 13C values of G. inflata for a carbonate ion effect, the δ 13C shell and δ 13C DIC are correlated with an average offset of 2.11.