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

The pH dependency of the boron isotopic composition of diatom opal (Thalassiosira weissflogii)
The pH dependency of the boron isotopic composition of diatom opal (Thalassiosira weissflogii)
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∘ S in the Southern Ocean and north of 40∘ N in the North Pacific. As a result, proxies need to be developed for the opal-based organisms (e.g. diatoms) found at these high latitudes, which dominate the biogenic sediments 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 apply it for the first time to evaluate the relationship between seawater pH, δ11B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured across a range of carbon dioxide partial pressure (pCO2) and pH values. In agreement with existing data, we find that the [B] of the cultured diatom frustules increases with increasing pH (Mejía et al., 2013). δ11B shows a relatively well defined negative trend with increasing pH, completely distinct 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.
1726-4170
2825-2837
Donald, Hannah K.
574e955f-ec6a-49e1-b86a-8bf7d1ab877c
Foster, Gavin L.
fbaa7255-7267-4443-a55e-e2a791213022
Fröhberg, Nico
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Swann, George E. A.
b4d954da-8ae0-49e8-b777-babfdb4344cc
Poulton, Alex J.
14bf64a7-d617-4913-b882-e8495543e717
Moore, C. Mark
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Humphreys, Matthew P.
eda7174e-691f-4a7e-9325-d5609bf64845
Donald, Hannah K.
574e955f-ec6a-49e1-b86a-8bf7d1ab877c
Foster, Gavin L.
fbaa7255-7267-4443-a55e-e2a791213022
Fröhberg, Nico
e8d58174-5007-43d2-9e84-8e55da23b531
Swann, George E. A.
b4d954da-8ae0-49e8-b777-babfdb4344cc
Poulton, Alex J.
14bf64a7-d617-4913-b882-e8495543e717
Moore, C. Mark
7ec80b7b-bedc-4dd5-8924-0f5d01927b12
Humphreys, Matthew P.
eda7174e-691f-4a7e-9325-d5609bf64845

Donald, Hannah K., Foster, Gavin L., Fröhberg, Nico, Swann, George E. A., Poulton, Alex J., Moore, C. Mark and Humphreys, Matthew P. (2020) The pH dependency of the boron isotopic composition of diatom opal (Thalassiosira weissflogii). Biogeosciences, 17 (10), 2825-2837. (doi:10.5194/bg-17-2825-2020).

Record type: Article

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∘ S in the Southern Ocean and north of 40∘ N in the North Pacific. As a result, proxies need to be developed for the opal-based organisms (e.g. diatoms) found at these high latitudes, which dominate the biogenic sediments 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 apply it for the first time to evaluate the relationship between seawater pH, δ11B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured across a range of carbon dioxide partial pressure (pCO2) and pH values. In agreement with existing data, we find that the [B] of the cultured diatom frustules increases with increasing pH (Mejía et al., 2013). δ11B shows a relatively well defined negative trend with increasing pH, completely distinct 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.

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Accepted/In Press date: 21 April 2020
Published date: 27 May 2020

Identifiers

Local EPrints ID: 442302
URI: http://eprints.soton.ac.uk/id/eprint/442302
ISSN: 1726-4170
PURE UUID: 4fc7e7f8-4f20-4518-a178-32427e652da7
ORCID for Gavin L. Foster: ORCID iD orcid.org/0000-0003-3688-9668

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Date deposited: 13 Jul 2020 16:30
Last modified: 18 Feb 2021 17:14

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Contributors

Author: Hannah K. Donald
Author: Gavin L. Foster ORCID iD
Author: Nico Fröhberg
Author: George E. A. Swann
Author: Alex J. Poulton
Author: C. Mark Moore
Author: Matthew P. Humphreys

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