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An improved boron isotope pH proxy calibration for the deep-sea coral Desmophyllum dianthus through sub-sampling of fibrous aragonite

An improved boron isotope pH proxy calibration for the deep-sea coral Desmophyllum dianthus through sub-sampling of fibrous aragonite
An improved boron isotope pH proxy calibration for the deep-sea coral Desmophyllum dianthus through sub-sampling of fibrous aragonite
The isotopic composition of boron (?11B) in marine carbonates is well established as a proxy for past ocean pH, however, its robust application to palaeo-environments relies on the generation of species-specific calibrations. Existing calibrations utilising the deep-sea coral (DSC) Desmophyllum dianthus highlight the potential application of this pervasive species to pH reconstructions of intermediate depth waters. Nevertheless, considerable uncertainty remains regarding the estimation of seawater pH from these bulk skeletal ?11B measurements, likely resulting from microstructural heterogeneities in ?11B of D. dianthus. To circumvent this problem, thus improving the reliability of the D. dianthus ?11B-pH calibration, we present a new ?11B calibration of micro-sampled fibrous aragonite from this species.

Modern coral specimens recovered from the Atlantic, Pacific, and Southern Oceans, micro-sampled using microdrilling, micromilling, and laser cutting extraction, were analysed for trace element (B/Ca, Mg/Ca, Sr/Ca, and U/Ca) and boron isotopic composition. We find the best calibration against the ?11B of borate in local ambient seawater (a function of pH and taken from hydrographic data sets; pH range 7.57 to 8.05) utilises ?11B measurements of fibres with likely slow growth rates and minimal contamination from adjacent microstructures (identified by low Mg/Ca) for each coral specimen. This new calibration exhibits a stronger, and better-defined dependence on ambient seawater pH compared to bulk coral ?11B; ?11Bfibre = (0.93 ± 0.17) × ?11Bborate + (12.02 ± 2.63). We suggest that the majority of the variability in measured ?11B between replicate bands of fibrous aragonite from a D. dianthus specimen can be explained by small incorporation of non-fibrous aragonite and surface impurities during microsampling and growth rate effects. This study confirms the utility of D. dianthus as an archive of precise palaeo-pH (± 0.07 pH units), provided that suitable sampling strategies are applied.
Desmophyllum dianthus, Deep-sea coral, pH, Boron isotopes, Trace element, Fibrous aragonite
0009-2541
148-160
Stewart, Joseph A.
b46385e5-3260-4dae-b5bd-bfaf2a72cac6
Anagnostou, Eleni
4527c274-f765-44ce-89ab-0e437aa3d870
Foster, Gavin L.
fbaa7255-7267-4443-a55e-e2a791213022
Stewart, Joseph A.
b46385e5-3260-4dae-b5bd-bfaf2a72cac6
Anagnostou, Eleni
4527c274-f765-44ce-89ab-0e437aa3d870
Foster, Gavin L.
fbaa7255-7267-4443-a55e-e2a791213022

Stewart, Joseph A., Anagnostou, Eleni and Foster, Gavin L. (2016) An improved boron isotope pH proxy calibration for the deep-sea coral Desmophyllum dianthus through sub-sampling of fibrous aragonite. Chemical Geology, 447, 148-160. (doi:10.1016/j.chemgeo.2016.10.029).

Record type: Article

Abstract

The isotopic composition of boron (?11B) in marine carbonates is well established as a proxy for past ocean pH, however, its robust application to palaeo-environments relies on the generation of species-specific calibrations. Existing calibrations utilising the deep-sea coral (DSC) Desmophyllum dianthus highlight the potential application of this pervasive species to pH reconstructions of intermediate depth waters. Nevertheless, considerable uncertainty remains regarding the estimation of seawater pH from these bulk skeletal ?11B measurements, likely resulting from microstructural heterogeneities in ?11B of D. dianthus. To circumvent this problem, thus improving the reliability of the D. dianthus ?11B-pH calibration, we present a new ?11B calibration of micro-sampled fibrous aragonite from this species.

Modern coral specimens recovered from the Atlantic, Pacific, and Southern Oceans, micro-sampled using microdrilling, micromilling, and laser cutting extraction, were analysed for trace element (B/Ca, Mg/Ca, Sr/Ca, and U/Ca) and boron isotopic composition. We find the best calibration against the ?11B of borate in local ambient seawater (a function of pH and taken from hydrographic data sets; pH range 7.57 to 8.05) utilises ?11B measurements of fibres with likely slow growth rates and minimal contamination from adjacent microstructures (identified by low Mg/Ca) for each coral specimen. This new calibration exhibits a stronger, and better-defined dependence on ambient seawater pH compared to bulk coral ?11B; ?11Bfibre = (0.93 ± 0.17) × ?11Bborate + (12.02 ± 2.63). We suggest that the majority of the variability in measured ?11B between replicate bands of fibrous aragonite from a D. dianthus specimen can be explained by small incorporation of non-fibrous aragonite and surface impurities during microsampling and growth rate effects. This study confirms the utility of D. dianthus as an archive of precise palaeo-pH (± 0.07 pH units), provided that suitable sampling strategies are applied.

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Accepted/In Press date: 22 October 2016
e-pub ahead of print date: 28 October 2016
Published date: 30 December 2016
Keywords: Desmophyllum dianthus, Deep-sea coral, pH, Boron isotopes, Trace element, Fibrous aragonite
Organisations: Geochemistry

Identifiers

Local EPrints ID: 404999
URI: https://eprints.soton.ac.uk/id/eprint/404999
ISSN: 0009-2541
PURE UUID: a7cb1368-f2d2-400e-8cd0-c3b61ec5b857
ORCID for Gavin L. Foster: ORCID iD orcid.org/0000-0003-3688-9668

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Date deposited: 20 Jan 2017 15:01
Last modified: 15 Aug 2019 05:07

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Contributors

Author: Joseph A. Stewart
Author: Eleni Anagnostou
Author: Gavin L. Foster ORCID iD

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