Testing the Influence of Changing Seawater Ca Concentration on Elements/Ca Ratios in Planktic Foraminifera: A Culture Experiment

AbstractElement/Ca ratios in foraminifera shells are used to assess oceanic physical‐chemical properties (e.g., paleo‐temperature, paleo‐pH…). On geological time scales, these proxies may also depend on changes in the chemical composition of seawater. Here we investigate the effect of changing seawater calcium concentration on the Li/Ca, Mg/Ca, Sr/Ca, B/Ca, and Na/Ca ratios in low‐Mg calcite shells of the planktic foraminifer Globigerinella siphonifera. Specifically, we evaluate the possibility that Na/Ca in planktic foraminifera shells may serve as a proxy for Ca concentrations in the past ocean. Since Ca concentration in today's open ocean is largely conservative with relation to salinity, field calibrations are not possible. Therefore, we cultured the planktic foraminifer (G. siphonifera) collected from the Gulf of Eilat (Red Sea) in variable Casw (10.5–29 mmol/kg), at constant temperature (25°C) and under two salinities (37 and 40.6). When changing the Casw of the growth medium, the relationships between Mg/Ca and Na/Ca in the shells and their corresponding ratios in the seawater are best described by power functions implying a variable distribution coefficient function. For Sr/Ca and Li/Ca, the relations between the shells and the seawater are described by linear functions intersecting the origin implying their distribution coefficients are insensitive to changes in Casw. The results also confirm that foraminiferal B/Ca is not influenced by seawater Casw. We conclude that Na/Ca ratios in planktic foraminifera may serve as reliable proxy for Cenozoic ocean Ca, and that other El/Ca ratios may be used to determine their past concentrations relative to Ca.