Water chemistry control of cadmium content in Recent benthic foraminifera

The micropalaeontology and isotopic chemistry of foraminifera and other fossils have provided a detailed picture of the surface ocean environment during glaciation^1,2. Less is known about climate-related changes in the deep ocean, although Weyl³ and Newell⁴ have proposed significant variations on theoretical grounds. It is clear that significant variations in the population assemblages and δ¹³C of benthic foraminifera occur^5,6 but these changes do not yet constrain a unique hydrographical solution. Adsorption of phosphate onto calcite and unfavourable solid solution behaviour^7,8 preclude direct palaeonutrient determination. Because CdCO₃ and CaCO₃ form a continuous solid-solution series⁹ it has been suggested that the trace cadmium variability in foraminifera may provide an additional tool for probing changes in deep ocean circulation¹⁰. Here we demonstrate that certain benthic foraminifera, Uvigerina spp. and C. kullenbergi, show a consistent relationship between the Cd/Ca of the bottom water and of their calcite shells.