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Abstract.We investigated the effect of the calcium concentration in seawater and thereby the calcite saturation state ( ) on the magnesium and strontium incorporation into benthic foraminiferal calcite under laboratory conditions. For this purpose individuals of the shallow-water species Heterostegina depressa (precipitating high-Mg calcite, symbiont-bearing) and Ammonia tepida (low-Mg calcite, symbiont-barren) were cultured in media under a range of [Ca 2+ ], but similar Mg/Ca ratios. Trace element/Ca ratios of newly formed calcite were analysed with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and normalized to the seawater elemental composition using the equation D TE =(TE/Ca calcite )/(TE/Ca seawater ). The culturing study shows that D Mg of A. tepida significantly decreases with increasing at a gradient of −4.3×10 −5 per unit. The D Sr value of A. tepida does not change with , suggesting that fossil Sr/Ca in this species may be a potential tool to reconstruct past variations in seawater Sr/Ca. Conversely, D Mg of H. depressa shows only a minor decrease with increasing , while D Sr increases considerably with at a gradient of 0.009 per unit. The different responses to seawater chemistry of the two species may be explained by a difference in the calcification pathway that is, at the same time, responsible for the variation in the total Mg incorporation between the two species. Since the Mg/Ca ratio in H. depressa is 50-100 times higher than that of A. tepida, it is suggested that the latter exhibits a mechanism that deCorrespondence to: M. Raitzsch (raitzsch@uni-bremen.de) creases the Mg/Ca ratio of the calcification fluid, while the high-Mg calcite forming species may not have this physiological tool. If the dependency of Mg incorporation on seawater [Ca 2+ ] is also valid for deep-sea benthic foraminifera typically used for paleostudies, the higher Ca concentrations in the past may potentially bias temperature reconstructions to a considerable degree. For instance, 25 Myr ago Mg/Ca ratios in A. tepida would have been 0.2 mmol/mol lower than today, due to the 1.5 times higher [Ca 2+ ] of seawater, which in turn would lead to a temperature underestimation of more than 2 • C.

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In order to investigate the interindividual and ontogenetic effects on Mg and Sr incorporation, magnesium/calcium (Mg/Ca) and strontium/calcium (Sr/Ca) ratios of cultured planktonic foraminifera have been determined. Specimens of Globigerinoides sacculifer were grown under controlled physical and chemical seawater conditions in the laboratory. By using this approach, we minimised the effect of potential environmental variability on Mg/Ca and Sr/Ca ratios. Whereas temperature is the overriding control of Mg/Ca ratios, the interindividual variability observed in the Mg/Ca values contributes 2-3°C to the apparent temperature variance. Interindividual variability in Sr/Ca ratios is much smaller than that observed in Mg/Ca values. The variability due to ontogeny corresponds to À0.43 mmol/mol of Mg/Ca ratio per chamber added. This translates into an apparent decrease of $1°C in Mg/Ca-based temperature per ontogenetic (chamber) stage. No significant ontogenetic effect is observed on Sr incorporation. We conclude that the presence of a significant ontogenetic effect on Mg incorporation can potentially offset Mg/Ca-based temperature reconstructions. We propose two new empirical Mg/Ca-temperature equation based on Mg/Ca measurements of the last four ontogenetic (chamber) stages and whole foraminiferal test: Mg/Ca = (0.55(±0.03) À 0.0002(±4 Â 10 À5 ) MSD) e 0.089T and, Mg/Ca = (0.55(±0.03) À 0.0001(±2 Â 10 À5 ) MSD) e 0.089T , respectively, where MSD corresponds to the maximum shell diameter of the individual.