Decoupled carbonate chemistry controls on the incorporation of boron into Orbulina universa

In order to fully constrain paleo-carbonate systems, proxies for two out of seven parameters, plus temperature and salinity are required. The boron isotopic composition (delta 11B) of planktonic foraminifera shells is a powerful tool to reconstruct changes in past surface ocean pH. As [B(OH)]4- is substituted into the biogenic calcite lattice in place of [CO3]2-, it has been suggested that B/Ca ratios in biogenic calcite are a possible proxy for [CO3]2-. However, differentiating between the effects of pH and [CO3]2- is problematic, as they co-vary closely in natural systems, and so the major control on boron incorporation remains unclear. To deconvolve the effects of pH and [CO3]2- on the B/Ca ratio and to test whether d11B remains constant at constant pH, but under changing [CO3]2- (pH 8.05 with 238, 285 and 532 µmol/kg [CO3]2-) and vice versa, we decoupled pH and [CO3]2- (276 +/- 19.5 µmol/kg [CO3]2- with pH 7.7, 7.9 and 8.05) and grew the planktonic foraminifer Orbulina universa in these manipulated culture media. Measurements of the isotope composition of boron and the B/Ca ratio were performed simultaneously using a femtosecond laser ablation system coupled to an MC ICP-MS. Results show that delta11B is controlled by pH and does not respond to changes in [CO3]2-. On the other hand, the B/Ca ratio is driven by [HCO3]- independently of pH. This suggests that B/Ca ratios in foraminiferal calcite may be used as a second, independent, proxy for paleo-carbonate system reconstructions.