Incorporation of Mg and Sr and oxygen and carbon stable isotope fractionation in cultured Ammonia tepida

The shallow water benthic foraminiferal species Ammonia tepida was cultured in controlled temperature conditions (20 °C) at three different salinities (29.8, 32.2, and 35.5). The calcite shells of single individuals of different sizes that calcified entirely in controlled experimental conditions were analysed for their magnesium and strontium concentrations (LA-ICP-MS) as well as for their oxygen and carbon isotopic composition (pooled individuals). The approach used in this study allows us to investigate the effect of seawater salinity, inter-test variability and test size on the studied parameters. Inter-test Mg/Ca variability is large (24–28%) in each salinity experiment. The size of the individuals seems to have an effect on Mg/Ca only in the lowest salinity experiment (29.8 salinity units). In contrast, Sr/Ca ratios show a strong size dependency in all salinity experiments, with Sr/Ca ratios decreasing with the size of the individuals. There is no response of Mg/Ca and Sr/Ca ratios to a 5 unit salinity increase. The carbon isotopic composition of cultured A. tepida shows an important size dependency with the smallest individuals (~ 200 μm) being around 1‰ 13C-depleted compared to the largest individuals (~ 600 μm). Interestingly, the oxygen isotopic composition of A. tepida, which does not show an ontogenetic effect, is ~ 0.30–0.45‰ enriched relative to the corresponding δ18Oeq. values. We hypothesise that heavier than predicted δ18O values may be the result of depleted carbonate ion concentrations in the microenvironment surrounding the foraminiferal shell during calcification (i.e., food cyst).