Foraminiferal biomineralization: Culture experiments on trace/minor element uptake, ontogenetic, and calcification rate effects

Benthic foraminifera, collected from sediments off the North and South Carolina coast, were maintained in mono- and multi-species cultures for four to eight months. Physicochemically-controlled artificial seawater was circulated through 3 mL acrylic culture chambers that contained 80--100 foraminifera living in ∼1-mm-thick trace-metal-inert silica substrate.; Pooled cultured-foraminifer samples (30--150 individuals each) consisted of two species, Bulimina aculeata and Rosalina vilardeboana. Bulimina aculeata DCd was consistent with previously published cultured and intermediate-depth (∼700--1200 m) core-top samples of seven other species. Bulimina aculeata DBa was consistent with previously reported culture-derived values, but was lower than published deep-water (>2000 m) core-top values. Bulimina aculeata collected from our shallowest field site had similar DCd and DBa values to the cultured samples. The lower DBa of cultured and 220-m field-collected foraminifera, when compared to published DBa, suggest a pressure-related or site-specific geochemical effect similar to that observed in DCd. Significant differences in the four elemental signatures (DCd, DBa, DMg, and DSr) were observed between identically cultured B. aculeata and R. vilardeboana. The signature variation of DCd, DBa, and DSr between the two foraminiferal species was consistent with increases in precipitation rate, and agreed with the observed growth rate estimations for B. aculeata and R. vilardeboana. The signature variation of DMg between the two species suggested that growth rate influences the mechanism of precipitating low-Mg-calcite.; Ontogenetic (life-stage) measurements of Mg and Sr were made on cultured Bulimina aculeata, utilizing two methods of ontogenetic sampling, whole-specimen sample length, and partial specimen high-resolution laser microsectioning. DMg was elevated during a mid-development (175--225 mum; "juvenile") stage compared to earlier (<175 mum) and later (>225 <500 mum) stages, while DSr did not vary. The elevated Mg-calcite was estimated to alter the whole individual DMg by 4--13%, which equates to +1.4°C error in temperature prediction. The ontogenetically-enriched Mg-calcite solubility indicates that these phases dissolve readily in relatively shallow waters (>1,000 m Atlantic, >200 m Pacific). Deep-sea samples likely have altered trace/minor element composition based on this partial dissolution. Additionally, reductive foraminiferal cleaning techniques may dissolve these unstable phases to an unknown degree, further altering the Mg/Ca signature.