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.

有孔虫方解石的化学成分反映了海水变量，因此是一种流行的古海洋学工具。然而，有孔虫壳化学的沉积记录主要是使用经验校准来解释的。由于有孔虫的地球化学模式通常与无机类似物不同，因此需要对有孔虫生物矿化有更机械的理解。表征有孔虫生物矿化的最难以捉摸但潜在的重要参数之一是方解石沉淀率。使用 NanoSIMS 结合标记实验和结合标记的亚微米成像，我们表明底栖有孔虫Ammonia beccarii以 ~24 ± 4 nmol/cm ²的速率沉淀其方解石/分钟。这些值接近从贫镁海水中无机方解石沉淀的最大报告速率，这与生物矿化过程中对镁的强分馏相一致。同时，测量的沉淀速率与有孔虫 Sr/Ca 值和以这些速率无机沉淀的方解石的比率之间的相似性一致。我们的结果还表明，观察到的降水率在样本之间和室壁内出奇地均匀，表明小尺度元素带没有反映降水率的变化。基于我们的研究结果，我们提出了一个概念模型，其中有孔虫钙化具有两个主要过程：首先，活性离子传输决定了钙化液的组成，而在随后的碳酸钙沉淀过程中，热力学和过程动力学决定了分馏和分配。该模型还解释了海水运输的作用，这在钙化的第一步中可能很重要，以解释其他有孔虫类群的地球化学特征。