Calibration of δ¹⁸O of cultured benthic foraminiferal calcite as a function of temperature

Abstract. The geochemical composition of deep-sea benthic foraminiferal calcite is widely used to reconstruct sea floor paleoenvironments. The calibration of the applied proxy methods has until now been based on field observations in complex natural ecosystems where multiple factors are interfering. However, laboratory experiments with stable physico-chemical conditions appear to be the ideal way to evaluate the influence of a single parameter. In this paper, we present the oxygen isotopic composition of deep-sea benthic foraminiferal shells entirely calcified under controlled experimental conditions over a large temperature range (4 to 19 °C). The new laboratory protocols developed for this study allowed us to produce large quantities of shells in stable conditions, so that also the shell size effect could be investigated. It appears that when considering a narrow test size range, the curve describing the temperature dependency of δ¹⁸O in Bulimina marginata is parallel to the thermodynamically determined curve observed in inorganically precipitated calcite (−0.22‰ °C⁻¹). This observation validates the use of δ¹⁸O of this benthic species in paleoceanographical studies. Over the studied size range (50 to 300 μm), the effect of test size was 0.0014‰ μm⁻¹, confirming previous suggestions of a substantial test size effect on δ¹⁸O of benthic foraminifera. This study opens new perspectives for future proxy calibrations in laboratory set-ups with deep-sea benthic foraminifera (e.g. quantification of the influence of the carbonate chemistry).