Laboratory experiments with living planktic foraminifers show that the δ 13 C and δ 18 O values of shell calcite decrease with increasing sea water pH and/or carbonate ion concentration. The effect has been quantified in symbiotic (Orbulina universa) and non-symbiotic (Globigerina bulloides) species and is independent of symbiont activity and temperature. It is concluded that a kinetic fractionation process affects both the carbon and oxygen isotopic composition of the shell simultaneously. At

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... resent it cannot be determined definitively whether the relationship is controlled by the pH dependent balance between hydration and hydroxylation of CO 2 or by [CO 3 2 -] related variations in the calcification rate. However, independent of which factor ultimately controls the relationship between the carbonate chemistry and isotopic fractionation, in the real ocean [CO 3 2 -] and pH covary linearly across the relevant pH range. The true relationship between shell isotopic composition and the bulk carbonate chemistry is masked by the fact that host respiration and symbiont activity locally modify the carbonate system. Respiration lowers and photosynthesis increases ambient pH and [CO 3 - ]. This translates into modified absolute shell values but leaves the slope between the shell isotopic composition and the bulk carbonate chemistry unaffected. A second level of shell isotopic modification is introduced by the incorporation of respired carbon, enriched in 12 C, which depletes the shell δ 13 C value. In symbiont bearing species this depletion is partially negated by a shell δ 13 C enrichment in the light. As an alternative to the RUBISCO hypothesis (enrichment via preferential removal of 12 CO 2 ), we propose that scavenging of respired CO 2 during photosynthesis, raises the shell δ 13 C value. Our results have partly been documented before (Spero et al. 1997) and demonstrate that the carbonate chemistry is undoubtedly a major control on temporal geochemical variability in the fossil record. For instance, the sea water carbonate system of the pre-Phanerozoic world (Berner 1994; Grotzinger and Kasting 1993) or during glacials (Sanyal et al. 1995) was significantly different from today confounding direct interpretation of foraminiferal stable isotope data using existing relationships (see companion paper in this volume by Lea et al.).