Boron isotope pH calibration of a shallow dwelling benthic nummulitid foraminifera

The boron isotope palaeo-pH/CO₂ proxy is one of the key quantitative tools available to reconstruct past changes in the concentration of CO₂ in the atmosphere. In particular, marine calcifying organisms have been shown to be useful archives of this proxy, enabling quantitative variations in pH/CO₂ to be reconstructed throughout the Cenozoic. In order to provide an alternative proxy archive to the widely used planktonic foraminifera, we investigated the symbiont-bearing, high-Mg, shallow-dwelling, tropical large benthic foraminifera (LBF) species Operculina ammonoides and present a calibration of the relationship between the shell boron isotopic composition and seawater pH. We investigated specimens collected from both several reefs as well as grown in laboratory culture experiments in which pH and DIC were decoupled from each other, measuring newly-formed chambers using laser-ablation as a sample introduction technique. Based on our laboratory culture samples, the resulting linear relationship between the in situ boron isotopic composition of aqueous borate ion (B(OH)₄⁻) and the shells of O. ammonoides is characterised by a gradient of 0.38_-0.10^+0.12. In contrast, the boron isotopic composition of the field collected samples displays a near 1:1 relationship with B(OH)₄⁻. We suggest that the shallow slope of the laboratory culture regression is the result of the difference between their micro-environment carbonate chemistry and that of the surrounding seawater driven by a pH dependence of the relative rates of calcification and photosynthesis. Based on a model of the effect of these processes on the diffusive boundary layer, we show that this effect is expected in laboratory culture experiments free from micro-turbulence, but not in the foraminifer's natural environment. As such, we demonstrate the utility of these organisms as proxy archive, while also highlighting how laboratory experimental design has the potential to drive important changes in the micro-environment and resulting shell chemistry of organisms of this size. Given that the genus Operculina originated in the late Palaeocene, this work paves the way towards deep-time palaeo-pH/CO₂ reconstructions using foraminifer species which have a very closely related modern representative.