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

• Published in: Biogeosciences
• Main Authors: E. L. Howes, K. Kaczmarek, M. Raitzsch, A. Mewes, N. Bijma, I. Horn, S. Misra, J.-P. Gattuso, J. Bijma
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2017
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Planktonic foraminifera
• Online Access: https://doi.org/10.5194/bg-14-415-2017; https://doaj.org/article/cd12e37a181a48c1a276654f36f9db49

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 ( δ 11 B) 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 CO 3 2− , 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 [CO 3 2− ]. 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 [CO 3 2− ] 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 [CO 3 2− ] and to investigate their impact on the B ∕ Ca ratio and δ 11 B, we conducted culture experiments with the planktonic foraminifer Orbulina universa in manipulated culture media: constant pH (8.05), but changing [CO 3 2− ] (238, 286 and 534 µmol kg −1 CO 3 2− ) and at constant [CO 3 2− ] (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, δ 11 B is controlled by pH but it is also modulated by [CO 3 2− ]. On the other hand, the B ∕ Ca ratio is driven by [HCO 3 − ], independently of pH. This suggests that B ∕ Ca ratios in foraminiferal calcite can possibly be used as a second, independent, proxy for complete ...