Antje Mewes

• Paleo-oceanographers use element to calcium ratios of foraminiferal tests to reconstruct past climate conditions. Calcite Mg/Ca is used to reconstruct past seawater temperatures. In addition to temperature Mg/CaSW also influences Mg/CaCC and - although not yet shown for foraminifers – Sr/CaCC. To reliably use such proxy relationships it is necessary to understand calcification mechanisms. Here we culture two benthic foraminifers at different seawater [Mg] and [Ca]. Growth rates are monitored during the culture experiment. Organic matter is removed with the bleaching agent NaOCl and test weight as well as calcite Mg/Ca and Sr/Ca is analyzed with LA-ICP-MS. Results show optimal growth and test thickness at Mg/CaSW near ambient. Different timespans of bleaching or concentrations of NaOCl had no influence on element to calcium ratios. Calcite Mg/Ca increases with increasing Mg/CaSW, while the partition coefficient DMg decreases. Relative Mg/CaSW, and not the absolute [Mg] and [Ca] play the crucial role in determining growth, thickness as well as Mg/CaCC. Results allowed to test a conceptual biomineralization model by Nehrke et al. (2013) and confirmed that a mixture of transmembrane transport and passive transport via leakage or vacuolization provides the necessary ions needed for chamber formation. A refined mixing model is presented, which is based on these assumptions, but assumes a weaker ion fractionation during transmembrane transport and accounts for the Mg/CaCC and DMg observed. Results furthermore showed that increasing Mg/CaCC increases Sr/CaCC, which can be explained by stress in the crystal lattice, caused by increasing Mg2+. This stress can be compensated by additional incorporation of Sr2+. This inorganic process is not yet accounted for in the mixing model. The mixing model needs further attention in future studies to finally develop a generalized biomineralization model which can explain the incorporation of all elements into foraminiferal tests.