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

• Published in: Marine Micropaleontology
• Main Authors: Birch, Heather, Coxall, Helen K., Pearson, Paul N., Kroon, Dick, O'Regan, Matthew
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2013
• Subjects: Carbon gradient; Ecology; Metabolic fractionation; Photosymbiosis; Planktonic foraminifera; Stable isotopes; Test size; Thermal structure; Water column; Western tropical Indian Ocean
• Online Access: https://hdl.handle.net/1983/620379c7-ef1d-44fd-a527-6b8c6acdfc2d; https://research-information.bris.ac.uk/en/publications/620379c7-ef1d-44fd-a527-6b8c6acdfc2d; https://doi.org/10.1016/j.marmicro.2013.02.002; http://www.scopus.com/inward/record.url?scp=84879109015&partnerID=8YFLogxK

Differential carbon and oxygen stable isotope (δ 13 C and δ 18 O) 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 δ 13 C and δ 18 O 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 δ 18 O 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 δ 13 C calcite trajectories on local water column δ 13 C DIC profiles, we estimate if and when (i.e. at which test sizes) the planktonic foraminifera species investigated approach ambient δ 13 C DIC values. The profiling shows significant size-controlled δ 13 C deviation from seawater values in all species at some life/growth stage, which we attribute to (i) metabolic fractionation in tests <150-300μm (juveniles of all species and small adults), and; (ii) photosymbiont fractionation, affecting large tests (>~300μm) 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 (<200μm) surface living ...