Diffusion-reaction modelling to predict the boron isotope composition of photosymbiont species O. universa from observed physiological fluxes

The boron isotope composition (δ¹¹B) of planktic foraminifera tests is primarily controlled by ambient seawater pH and is therefore a well-established proxy for reconstructing surface ocean carbonate chemistry in the geological past. Reconstructions of past seawater pH from planktic foraminifera underpin estimates of atmospheric pCO₂ over geological time and have driven recent advances in using past climate states to improve projections of Earth’s future climate. However, biological “vital effects” necessitate empirical, species-specific, δ¹¹B-pH proxy calibrations for accurate pH (and pCO₂) palaeo-reconstructions. Specifically, physiological processes including respiration, photosynthesis and calcification alter the pH of the diffusive boundary layer (DBL) surrounding living foraminifera. Given it is this pH that is thought to be recorded by foraminferal calcite δ¹¹B, shell composition typically deviates from what is predicted based upon equilibrium seawater borate δ¹¹B. This can be mechanically understood using diffusion-reaction modelling, which predicts pH and associated boron systematics within the DBL, and thus the expected δ¹¹B at the shell surface, if respiration, photsynthesis and calcification fluxes are known. Here we report modelled pH and δ¹¹B of individual O. universa specimens using respiration and photosynthesis rates calculated from direct observations using light-dark microelectrode and respiration chamber measurements of [O₂] within the DBL. Together with modelled DBL pH/δ¹¹B, these provide valuable insight into the drivers of species-specific and inter-specimen offsets between δ¹¹B of foraminiferal calcite δ¹¹B and seawater borate, addressing a critical limitation in reconstructing past seawater pH, particularly during greenhouse high-CO₂ intervals, when vital effects and metabolic behaviour may have differed from the present. Ultimately, this work opens avenues to reconstruct past changes in seawater pH using single shell δ¹¹B analysis, providing a methodology to significantly improve the temporal resolution of palaeo-pH and CO₂ records relative to traditional, monospecific, bulk population analyses.