Trace element proxies for surface ocean conditions

Abstract

The trace element composition of planktic foraminiferal calcite provides a useful means of determining past surface ocean conditions. We have assembled the results of culture experiments for three species of symbiont-bearing planktic foraminifera, Globigerinoides ruber, Globigerinoides sacculifer, and Orbulina universa, and one symbiont-barren species, Globigerina bulloides, to evaluate their responses to temperature, salinity, pH, carbonate ion, and dissolved inorganic carbon (DIC) growth conditions. Trace element ratios (Li/Ca, B/Ca, Mg/Ca, Sr/Ca, Mn/Ca, Cd/Ca, Ba/Ca, Na/Ca, and U/Ca) were measured simultaneously on samples grown with the same culture techniques, which provides robust, relatable calibrations that may be used together in multi-proxy paleoceanographic studies. Our data confirm that temperature is the dominant control on foraminiferal Mg/Ca under the ranges of conditions studied and that the potential effects of salinity and CO32− on Mg/Ca of these tropical species across late Pleistocene glacial cycles are relatively small. Carbonate system experiments suggest that Sr/Ca may be useful for reconstructing large DIC changes. Na/Ca increases with salinity in G. ruber (pink), but not in G. sacculifer. As these emerging proxy relationships become more firmly established, the synthesis of multiple trace element ratios may help paleoceanographers isolate the effects of different environmental parameters in paleo records. Calcification rates (μg/day) vary among species and do not respond consistently to any experimental parameter. Comparison of our calcification rates with those observed in inorganic calcite precipitation experiments suggest that foraminifera calcify ∼100× more slowly than inorganic calcites grown in similar solutions. We suggest that calcification rate does not typically exert a dominant control on trace element partitioning in planktic foraminiferal calcite, though it may play a role for some elements under certain circumstances. Differences in average growth rate cannot explain composition differences among species, pointing to alternative controls that may be biological in origin.