Planktonic foraminifera stable isotopes and water column structure: Disentangling ecological signals

Differential carbon and oxygen stable isotope (δ13C and δ18O) fractionation between planktonic foraminifera test calcite and sea water related to ecology and life stage confound the potential for reconstructing palaeo-water column temperature and carbon gradients. Multi-species analysis and strict selection of test sizes are useful methods for identifying these fractionation processes, also known as ‘vital effects’, in fossil taxa. However, there are a limited number of species with adequate size-controlled data sets, needed for ground truthing the approach in the modern. Here we report δ13C and δ18O measurements made on twelve species of modern planktonic foraminifera across a range of fourteen tightly constrained size windows from a tropical Indian Ocean core top sample. This data set includes more test size windows per species, especially from the smallest (identifiable) test size-classes, and a wider range of species than previously attempted. We use the size controlled δ18O calcite trajectories to infer depth habitats and calculate species-specific calcification temperatures. The temperatures are then used to constrain species-specific calcification depths along the modern vertical temperature profile in the western tropical Indian Ocean. By overlaying the per species δ13C calcite trajectories on local water column δ13CDIC profiles, we estimate if and when (i.e. at which test sizes) the planktonic foraminifera species investigated approach ambient δ13CDIC values. The profiling shows significant size-controlled δ13C deviation from seawater values in all species at some life/growth stage, which we attribute to (i) metabolic fractionation in tests < 150–300 μm (juveniles of all species and small adults), and; (ii) photosymbiont fractionation, affecting large tests (>~ 300 μm) of mixed layer photosymbiotic taxa. For most species there is a size-window where these effects appear to be at a minimum, and/or in balance. Exceptions are Globigerinita glutinata, a small (< 200 μm) surface living species, Globigerina bulloides, which is highly opportunistic, and deep living Globorotalia tumida and Globorotaloides hexagonus, the latter two species being affected by various unexplained δ13C vital effects. Use of our refined guidelines for test-size selection should improve the potential for making realistic reconstructions of water column δ13CDIC in a modern tropical stratified setting and potentially in the distant geological past when there are no living analogues present.

► Size-controlled planktic foraminifera isotopes of 12 modern species are presented.
► Photosymbiosis and metabolic disequilibrium effects can be disentangled.
► Strict size control allows realistic reconstruction of water column DIC gradient.