Carbon isotopic fractionation in live benthic foraminifera—comparison with inorganic precipitate studies

Carbon and oxygen isotopic analyses have been performed on live-stained aragonitic and calcitic benthic foraminifera and dissolved inorganic carbon (DIC) from the Southern California Borderland to examine carbon isotopic fractionation in foraminifera. Temperature, salinity and pH data have also been collected to permit accurate determination of the δ¹³C of bicarbonate ion and thus aragonite-HCO₃ and calcite-HCO^?₃ isotopic enrichment factors (?_ar-b and ?_cl-b, respectively). Only species which precipitate in ¹⁸O equilibrium have been considered.?_ar-b values based on Hoeglundina elegans range from 1.9%. at 2.7°C to 1.1%. at 9.5°C. Only the lower temperature values agree with a tentative carbon isotope equilibrium equation for aragonite based on the data of Rubinson and Clayton (1969) and Emrich et al. (1970). The temperature dependence of ?_ar-b is considerably greater than the equilibrium equation would predict and may be due to a vital effect.The calcitic foraminifera Cassidulina tortuosa, Cassidulina braziliensis, and Cassidulina limbata, Bank and Terrace dwellers, have similar δ¹³C values and yield an average ?_cl-b value of ?0.2 ± .1%. between 8° and 10°C. Calcitic Uvigerina curticosta, Uvigerina peregrina, and megalospheric B. argentea, Slope and Basin dwellers, are ?0.7 ± .1%. enriched relative to ambient bicarbonate for 3 to 9°C. No temperature dependence for ?_cl-b was observed for the species in either habitat. The ?_cl-b values for Cassidulina species are close (± 0.3%.) to the values given by the tentative equilibrium curve for calcite, while Uvigerina and Bolivina species give values 0.2–0.8%. less. The ?_cl-b difference between the Cassidulina species and the Uvigerina and Bolivina species is attributed to the incorporation of ¹³C-depleted pore water DIC by the latter group rather than to taxonomic or temperature differences.