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

Abstract. 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 (δ¹¹B) of planktonic foraminifera shells is a powerful tool for reconstructing changes in past surface ocean pH. As B(OH)₄⁻ is substituted into the biogenic calcite lattice in place of CO₃²⁻, and both borate and carbonate ions are more abundant at higher pH, it was suggested early on that B ∕ Ca ratios in biogenic calcite may serve as a proxy for [CO₃²⁻]. Although several recent studies have shown that a direct connection of B ∕ Ca to carbonate system parameters may be masked by other environmental factors in the field, there is ample evidence for a mechanistic relationship between B ∕ Ca and carbonate system parameters. Here, we focus on investigating the primary relationship to develop a mechanistic understanding of boron uptake.

Differentiating between the effects of pH and [CO₃²⁻] is problematic, as they co-vary closely in natural systems, so the major control on boron incorporation remains unclear. To deconvolve the effects of pH and [CO₃²⁻] and to investigate their impact on the B ∕ Ca ratio and δ¹¹B, we conducted culture experiments with the planktonic foraminifer Orbulina universa in manipulated culture media: constant pH (8.05), but changing [CO₃²⁻] (238, 286 and 534 µmol kg⁻¹ CO₃²⁻) and at constant [CO₃²⁻] (276 ± 19.5 µmol kg⁻¹) and varying pH (7.7, 7.9 and 8.05). Measurements of the isotopic composition of boron and the B ∕ Ca ratio were performed simultaneously using a femtosecond laser ablation system coupled to a MC-ICP-MS (multiple-collector inductively coupled plasma mass spectrometer). Our results show that, as expected, δ¹¹B is controlled by pH but it is also modulated by [CO₃²⁻]. On the other hand, the B ∕ Ca ratio is driven by [HCO₃⁻], independently of pH. This suggests that B ∕ Ca ratios in foraminiferal calcite can possibly be used as a second, independent, proxy for complete paleo-carbonate system reconstructions. This is discussed in light of recent literature demonstrating that the primary relationship between B ∕ Ca and [HCO₃⁻] can be obscured by other environmental parameters.