Carbonate ion effect on Mg/Ca, Sr/Ca and stable isotopes on the benthic foraminifera Oridorsalis umbonatus off Namibia

We investigate the influence of carbonate system parameters (carbonate ion concentration, [CO32−]; carbonate ion saturation, Δ[CO32−]) on the trace element and stable isotope ratios in the endobenthic foraminifera Oridorsalis umbonatus. Data from modern core top samples from the Namibian continental slope suggest that the shell composition of this species is influenced by the chemistry of the pore-water. For these organic-rich sediments, the impact of ocean bottom water properties on both pore-water and shell chemistry is surprisingly small. Sr/Ca correlates positively with [CO32−] and to a lesser extent with Δ[CO32−], which is opposed to previous results. A [CO32−] decrease of 10 μmol/kg leads to an increase of 0.05 mmol/mol in Sr/Ca. We observe a correlation between shell δ18O (corrected for temperature and δ18O seawater) and [CO32−], however, the variability of the corrected δ18O is close to the analytical limit. No clear dependences were observed for δ13C and Mg/Ca.