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

• Published in: Biogeosciences
• Main Authors: Howes, Ella L., Kaczmarek, Karina, Raitzsch, Markus, Mewes, Antje, Bijma, Nienke, Horn, Ingo, Misra, Sambuddha, Gattuso, Jean-Pierre, Bijma, Jelle
• Other Authors: Observatoire océanologique de Villefranche-sur-mer (OOVM), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Pierre et Marie Curie - Paris 6 (UPMC), Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung (AWI), Center for Marine Environmental Sciences Bremen (MARUM), Universität Bremen, Christian-Albrechts-Universität zu Kiel (CAU), Leibniz Universität Hannover Hannover (LUH), University of Cambridge UK (CAM), Laboratoire d'océanographie de Villefranche (LOV), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Centre National de la Recherche Scientifique (CNRS), Sciences Po (Sciences Po)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2017
• Subjects: [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; [SDU.STU.MI]Sciences of the Universe [physics]/Earth Sciences/Mineralogy; Planktonic foraminifera
• Online Access: https://hal.sorbonne-universite.fr/hal-01501470; https://hal.sorbonne-universite.fr/hal-01501470/document; https://hal.sorbonne-universite.fr/hal-01501470/file/bg-14-415-2017.pdf; https://doi.org/10.5194/bg-14-415-2017

International audience 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 CO3 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 [CO3 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 [CO3 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 [CO3 2−] 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 [CO3 2−] (238, 286 and 534 µmol kg−1 CO3 2−) and at constant [CO3 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, δ11B is controlled by pH but it is also modulated by [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 can possibly be used as a second, independent, proxy for complete paleo-carbonate ...