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

Howes, Ella L. and Kaczmarek, Karina and Raitzsch, Markus and Mewes, Antje and Bijma, Nienke and Horn, Ingo and Misra, Sambuddha and Gattuso, Jean-Pierre and Bijma, Jelle (2017) Decoupled carbonate chemistry controls on the incorporation of boron into Orbulina universa. Biogeosciences, 14 (2). pp. 415-430. ISSN 1726-4189 DOI 10.5194/bg-14-415-2017

[img]
Preview
Text
bg-14-415-2017.pdf - Published Version
Available under License Creative Commons Attribution.

Download (2MB) | Preview
Official URL: http://doi.org/10.5194/bg-14-415-2017

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.

Item Type: Article
Uncontrolled Keywords: 2016AREP; IA71
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
07 - Gold Open Access
Journal or Publication Title: Biogeosciences
Volume: 14
Page Range: pp. 415-430
Identification Number: 10.5194/bg-14-415-2017
Depositing User: Sarah Humbert
Date Deposited: 26 Jul 2017 17:21
Last Modified: 26 Jul 2017 17:21
URI: http://eprints.esc.cam.ac.uk/id/eprint/3994

Actions (login required)

View Item View Item

About cookies