Experimental determination of strontium and magnesium partitioning in calcite of the freshwater ostracod Herpetocypris intermedia

Cultures of the freshwater ostracod Herpetocypris intermedia (Crustacea) we re kept under controlled conditions in order to study the influence of the chemistry and the temperature of the water, as well as the influence of the calcification process, on the incorporation of strontium and magnesium in their calcite carapaces. These experiments pointed out to three factors tha t induced variations in the partition coefficients D(Sr) and D(Mg): (1) the calcification process: light-weight shells were usually depleted in Sr con tent when compared to well calcified valves; (2) a high water Sr/Ca ratio: in this case an increase of D(Sr) with ostracod Mg content was observed; (3 ) the water Mg/Ca ratio: above a water Mg/Ca ratio of similar to 1, D(Mg) w as nearly constant (0.0042 +/- 0.0007). In low water Mg/Ca ratios (<0.4), D (Mg) increased. Our results also show that temperature does not influence t he D(Mg) value for Herpetocypris. We hypothesise that freshwater taxa such as Herpetocypris have developed mechanisms to control the temperature effec t on calcite Mg content, because they do not have to exert a control on Mg content during calcification to avoid high-Mg calcite formation.