Factors controlling the fluoride content of planktonic foraminifera: An evaluation of its paleoceanographic applicability

Abstract

Abstract This work is an assessment of the effects of seawater salinity and temperature on the incorporation of fluoride into calcitic shells of planktonic foraminifera and of the subsequent modification of shell chemistry by postdepositional dissolution. A strong correlation between the fluoride content of most planktonic species and their calcification depth was observed; deeper-calcifying foraminifera have lower fluoride content and heavier 18O composition, perhaps suggesting a temperature control on the partition coefficient. However, species deviations from this general trend and the absence of correlation between fluoride and sea-surface temperature within an individual species suggest that the coprecipitation of fluoride in foraminiferal calcite is more strongly controlled by biological processes rather than by simple chemical relationships. Foraminiferal fluoride shows weak or no correlation with sea-surface salinity. Core-top planktonic foraminifera from the eastern equatorial Atlantic (Sierra Leone Rise) show a substantial decrease of fluoride and, to a lesser extent, Mg content with increasing water depth (2931–5104 m). These decreasing trends with depth are attributed to selective dissolution, i.e., removal of the more dissolution-susceptible fraction with a different chemical composition than the residual, a process which previously has been suggested to account for variations in the isotopic composition of planktonic foraminifera. These findings indicate that foraminiferal fluoride cannot serve as a reliable tracer of oceanic paleo-temperature or salinity. It is possible, however, that fluoride may be useful for the study of dissolution in deep-sea sediments.