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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-180
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-180
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 23 May 2019

Research article | 23 May 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

The pH dependency of the boron isotopic composition of diatom opal (Thalassiosira weissflogii)

Hannah K. Donald1, Gavin L. Foster1, Nico Fröhberg1, George E. A. Swann2, Alex Poulton3,4, C. Mark Moore1, and Matthew P. Humphreys1,5 Hannah K. Donald et al.
  • 1School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, SO14 3ZH, UK
  • 2School of Geography, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
  • 3Ocean Biogeochemistry and Ecosystems, National Oceanography Centre, Southampton, SO14 3ZH, UK
  • 4The Lyell Centre, Heriot-Watt University, Edinburgh, EH14 4A, UK
  • 5School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK

Abstract. The high latitude oceans are key areas of carbon and heat exchange between the atmosphere and the ocean. As such, they are a focus of both modern oceanographic and palaeoclimate research. However, most palaeoclimate proxies that could provide a long-term perspective are based on calcareous organisms, such as foraminifera, that are scarce or entirely absent in deep-sea sediments south of 50° latitude in the Southern Ocean and north of 40° in the North Pacific. As a result, proxies need to be developed for the opal-based organisms (e.g. diatoms) that are found at these high latitudes, and which dominate the biogenic sediments that are recovered from these regions. Here we present a method for the analysis of the boron (B) content and isotopic composition (δ11B) of diatom opal. We also apply it for the first time to evaluate the relationship between seawater pH and δ11B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured at a range of pCO2/pH. In agreement with existing data, we find that the [B] of the cultured diatom frustules increases with increasing pH (Mejia et al., 2013). δ11B shows a relatively well-defined negative trend with increasing pH; a completely distinct relationship from any other biomineral previously measured. This relationship not only has implications for the magnitude of the isotopic fractionation that occurs during boron incorporation into opal, but also allows us to explore the potential of the boron-based proxies for palaeo-pH and palaeo-CO2 reconstruction in high latitude marine sediments that have, up until now, eluded study due to the lack of suitable carbonate material.

Hannah K. Donald et al.
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Hannah K. Donald et al.
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Short summary
The boron isotope pH proxy is increasingly being used to reconstruct ocean pH in the past. Here we detail a novel analytical methodology for measuring the boron isotopic composition (δ11B) of diatom opal and apply this to the study of the diatom Thalassiosira weissflogii grown in culture over a range of pH. To our knowledge this is the first study of its kind and provides unique insights into the way in which diatoms incorporate boron and their potential as archives of palaeoclimate records.
The boron isotope pH proxy is increasingly being used to reconstruct ocean pH in the past. Here...
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