Sr/Ca in multiple species of planktonic foraminifera: Implications for reconstructions of seawater Sr/Ca

Abstract

Sr/Ca ratios were measured on eight species of planktonic foraminifera from a core top transect of the North Atlantic. Five of the species (Globigerinoides ruber, Globigerina bulloides, Neogloboquadrina pachyderma, Globigerinoides sacculifer, Globigerinella siphonifera) show remarkably little within-species variability in Sr/Ca (1.2 ± 0.1%). Three globorotaliid species (Globorotalia hirsuta, Globorotalia inflata, Globorotalia truncatulinoides) show somewhat greater variability (3-7%). Interspecies variations are of ∼10%. The variability in globorotaliid Sr/Ca is explained either by a greater temperature sensitivity than for other species or by depth-dependent dissolution. Other than this effect, the overwhelming control on foraminiferal Sr/Ca appears to be seawater Sr/Ca. In contrast, time series Sr/Ca records of four species from a North Atlantic sediment core over a glacial-interglacial cycle show very significant differences. N. pachyderma shows large-amplitude changes in Sr/Ca (9.7%), in phase with δ18O, whereas G. ruber shows the smallest amplitude changes (2.6%) out of phase with δ18O and most similar to model predictions for temporal changes in seawater Sr/Ca. Records of G. inflata and G. bulloides are intermediate (amplitude changes of 3.3-4.7%) and out of phase with δ18O. The lack of coherence in the time series records between the species shows that factors in additional to changing seawater Sr/Ca must affect planktonic foraminiferal Sr/Ca.