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.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2016) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation is 2016-11-30.

Supplement to: Howes, Ella L; Kaczmarek, Karina; Raitzsch, Markus; Mewes, A; Bijma, N; Horn, Ingo; Misra, Sambuddha; Gattuso, Jean-Pierre; Bijma, Jelle (2017): Decoupled carbonate chemistry controls on the incorporation of boron into Orbulina universa. Biogeosciences, 14(2), 415-430