Chamber formation and trace element distribution in the calcite walls of laboratory cultured planktonic foraminifera (Globigerina bulloides and Globigerinoides ruber)

Two species of spinose planktonic foraminifera (Globigerinoides ruber and Globigerina bulloides), cultured under controlled seawater temperatures (19, 21, 23, and 25 °C) with newly precipitated walls labeled with Ca isotopes, were subjected to quantitative analyses of Mg/Ca, Sr/Ca and Ba/Ca ratios at a spatial resolution of 2 μm. Mg mapping of calcite walls showed banding structures parallel to the wall surfaces for both species. Significant Mg accumulation was evident, particularly on walls precipitated at higher temperature, together with irregularly shaped chambers. Our experiment reproduced little cross-sectional variation in Sr/Ca, suggesting constant ratio levels in the calcification fluid and seawater. The Ba/Ca, however, showed different behavior in the two species: a relatively constant profile in Gs. ruber but a positive correlation with Mg/Ca in Ga. bulloides. The different geochemical properties in the two species cultured under the same conditions suggest different elemental regulation processes in the calcification sites. Positive coupling of Mg-Ba (but constant Sr/Ca) is unlikely to be explained only by a Mg-specific removal process, whereas Sr and Ca transport may follow a similar pathway.