Field examination of the oceanic carbonate ion effect on stable isotopes in planktonic foraminifera

We determined the δ 18 O and δ 13 C of individual Globigerinoides ruber and Pulleniatina obliquiloculata from sediment traps located from 5°N to 12°S along 140°W in the Pacific Ocean to evaluate the effects of varying [CO 3 = ] on shell δ 18 O and δ 13 C. Variations in the offset between shell δ 13 C and δ 13 C DIC (Δδ 13 C s ‐DIC ) are attributed to differences in [CO 3 = ], temperature, and shell size between sample sites. When Δδ 13 C s ‐DIC of G. ruber was corrected for variations in [CO 3 = ] using the experimental slope of Bijma et al. [1998], the residual Δδ 13 C s ‐DIC was correlated with mixed layer temperature (+0.10±0.04‰ °C −1 ). The slope of this temperature effect is consistent with experimental results. In P. obliquiloculata , Δδ 13 C s ‐DIC and temperature were strongly anticorrelated (−0.14±0.03‰ C −1 ). We are unable to separate the influences of [CO 3 = ] and temperature in this species without independent experimental data. Correcting for [CO 3 = ] variability on δ 18 O s of G. ruber improves the accuracy of estimated sea surface temperatures.