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

We determined the delta(18)O and delta(13)C of individual Globigerinoides ruber and Pulleniatina obliquiloculata from sediment traps located from 5 degrees N to 12 degrees S along 140 degrees W in the Pacific Ocean to evaluate the effects of varying [CO3=] on shell delta(18)O and delta(13)C. Variations in the offset between shell delta(13)C and delta(13)C(DIC) (Delta delta(13)C(S-DIC)) are attributed to differences in [CO3=], temperature, and shell size between sample sites. When Delta delta(13)C(s-DIC) of G. ruber was corrected for variations in [CO3=] using the experimental slope of Bijma et al. [1998], the residual Delta delta(13)C(s-DIC) was correlated with mixed layer temperature (+0.10+/-0.04 parts per thousand degrees C-1). The slope of this temperature effect is consistent with experimental results. In P. obliquiloculata, Delta delta(13)C(s-DIC) and temperature were strongly anticorrelated (-0.14+/-0.03 parts per thousand C-1). We are unable to separate the influences of [CO3=] and temperature in this species without independent experimental data. Correcting for [CO3=] variability on delta(18)O(s) of G. ruber improves the accuracy of estimated sea surface temperatures.