Spatially-resolved trace element records in cultured and fossil foraminifera: Cenozoic ocean chemistry, palaeoseasonality and tropical temperature reconstruction

The incorporation of trace elements into biogenic carbonates, particularly foraminifera, forms a substantial portion of our knowledge of Cenozoic ocean temperatures. However, analogously to isotopic systems, trace element partioning into carbonates is also dependent on the composition of the solution from which biomineralisation takes place. Seawater chemistry, in particular the concentration of the alkali earth metals, is known to have undergone significant secular variation during the Phanerozoic. In order to form the basis of more accurate ocean temperature and seawater chemistry reconstructions throughout the Cenozoic, detailed laboratory calibrations of the relationship between temperature, seawater chemistry and test chemistry for two very different foraminifera were conducted. The long-lived large benthic foraminifer Operculina ammonoides was targeted for its potential as a seasonal proxy archive, whilst an improved understanding of these relationships in the widely utilised planktic foraminifer Globigerinoides ruber have the potential to improve Plio-Pleistocene palaeoceanic reconstructions. These data constrain the complexity of the controls on trace element incorporation into foraminifera. In particular, seawater-test Mg/Ca were found to vary nonlinearly, demonstrating that such coupled calibrations are a prerequisite for quantitative palaeoceanic reconstruction before the Pleistocene, when seawater chemistry cannot be assumed to be the same as at present. Based on highly spatially-resolved laser-ablation trace element depth profiles, tracks and 2D mapping, these calibrations are applied to the abundant Eocene nummulitids, which are shown to have excellent potential as an archive of seawater chemistry and palaeoseasonality. These data indicate a 1.5-2× increase in surface ocean temperature seasonality compared to present, and provide new constraints on secular seawater chemistry variation. The observed seasonality increase in a greenhouse world is interpreted to be related to the frequency and/or intensity of tropical storms. Finally, applying these G. ruber calibrations to the offset between Mg/Ca and biomarker-derived palaeotemperatures for published PlioPleistocene datasets spanning the last 5 million years enables Pliocene seawater Mg/Ca to be more accurately constrained. This seawater chemistry reconstruction indicates that the ocean calcium concentration was 20-25% higher in the Pliocene, with the direct implication that deep ocean temperatures and ice volume may have been previously underestimated.