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 B-11) 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 delta B-11, 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 mu mol kg(-1) CO32-) and at constant [CO32-] (276 +/- 19.5 mu mol 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, delta B-11 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.