Abstract. Cultured planktonic foraminifera, Orbulina universa (symbiotic) and Globigerina bulloides (nonsymbiotic), are used to reexamine temperature:fi180 relationships at 15ø-25øC. Relationships for both species can be described by

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essentially induced by local modifications of the electronic properties of a surface near chemisorbed particles and hence, we believe, are of general importance for chemical processes at surfaces whenever the diffusion lengths of adsorbing species reach critical values compared with their lateral distribution.

Abstract. Two haptophyte algae, Emiliania huxleyi and Gephyrocapsa oceanica, were cultured at different temperatures and salinities to investigate the impact of these factors on the hydrogen isotopic composition of long chain alkenones synthesized by these algae. Results showed that alkenones synthesized by G. oceanica were on average depleted in D by 30‰ compared to those of E. huxleyi when grown under similar temperature and salinity conditions. The fractionation factor, α alkenones−H 2 O , ranged from 0.760 to 0.815 for E. huxleyi and from 0.741 to 0.788 for G. oceanica. There was no significant correlation of α alkenones−H 2 O with temperature but a positive linear correlation was observed between α alkenones−H 2 O and salinity with ∼3‰ change in fractionation per salinity unit and a negative correlation between α alkenones−H 2 O and growth rate. This suggests that both salinity and growth rate can have a substantial impact on the stable hydrogen isotopic composition of long chain alkenones in natural environments.

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Abstract: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 present 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 32 -] 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 32 -] 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 prePhanerozoic 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.).

Seeded calcite growth experiments were conducted at fixed pH (10.2) and two degrees of supersaturation (X = 5, 16), while varying the Ca 2+ to CO 3 2À solution ratio over several orders of magnitude. The calcite growth rate and the incorporation of Sr in the growing crystals strongly depended on the solution stoichiometry. At a constant degree of supersaturation, the growth rate was highest when the solution concentration ratio, r = [Ca 2+ ]/[CO 3 2À ], equaled one, and decreased symmetrically with increasing or decreasing values of r. This behavior is consistent with the kink growth rate theory for non-Kossel crystals, assuming that the frequency factors for attachment to kink sites are the same for the cation and anion. Measured Sr partition coefficients, D Sr , ranged from 0.02 to 0.12, and correlated positively with the calcite growth rate.

a b s t r a c t a r t i c l e i n f oIn this paper, we review the current understanding of biomineralization in perforate foraminifera. Ideas on the mechanisms responsible for the flux of Ca 2+ and inorganic carbon from seawater into the test were originally based on light and electron microscopic observations of calcifying foraminifera. From the 1980s onward, tracer experiments, fluorescent microscopy and high-resolution test geochemical analysis have added to existing calcification models. Despite recent insights, no general consensus on the physiological basis of foraminiferal biomineralization exists. Current models include seawater vacuolization, transmembrane ion transport, involvement of organic matrices and/or pH regulation, although the magnitude of these controls remain to be quantified. Disagreement between currently available models may be caused by the use of different foraminiferal species as subject for biomineralization experiments and/or lack of a more systematic approach to study (dis)similarities between taxa. In order to understand foraminiferal controls on element incorporation and isotope fractionation, and thereby improve the value of foraminifera as paleoceanographic proxies, it is necessary to identify key processes in foraminiferal biomineralization and formulate hypotheses regarding the involved physiological pathways to provide directions for future research.