IMIS

The chemical composition of foraminiferal calcite reflects seawater variables and is therefore a popular paleoceanographic tool. The sedimentary record of foraminiferal shell chemistry is, however, mostly interpreted using empirical calibrations. Since geochemical patterns in foraminifera often deviate from inorganic analogues, there is an ongoing need for a more mechanistic understanding of foraminiferal biomineralization . One of the most elusive, but potentially important parameters characterizing foraminiferal biomineralization is the rate of calcite precipitation. Using a combination of labelling experiments and sub-micrometer imaging of the incorporated label with NanoSIMS , we show that the benthic foraminifer Ammonia beccarii precipitates its calcite at a rate of ∼24 ± 4 nmol/cm²/min. These values are close to maximum reported rates for inorganic calcite precipitation from Mg-depleted seawater, which is consistent with the strong fractionation against Mg during biomineralization. At the same time, the measured precipitation rate is in accordance with the similarity between the foraminiferal Sr/Ca values and ratios from calcite precipitated inorganically at these rates. Our results also show that the observed precipitation rate is surprisingly uniform among specimens and within chamber walls, indicating that the small-scale element banding is not reflecting variability in precipitation rate. Based on our results, we present a conceptual model where foraminiferal calcification is characterized by two major processes: first, active ion transport determines the composition of the calcifying fluid, whereas thermodynamics and process kinetics dictate fractionation and partitioning during the subsequent calcium carbonate precipitation. This model also accounts for a role of seawater transport, which may be important in the first steps of calcification to explain geochemical signatures of other foraminiferal taxa.