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Publication - Dr Joseph Stewart

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


    Stewart, JA, Anagnostou, E & Foster, GL, 2016, ‘An improved boron isotope pH proxy calibration for the deep-sea coral Desmophyllum dianthus through sub-sampling of fibrous aragonite’. Chemical Geology, vol 447., pp. 148-160


    The isotopic composition of boron (δ11B) 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 δ11B measurements, likely resulting from microstructural heterogeneities in δ11B of D. dianthus. To circumvent this problem, thus improving the reliability of the D. dianthus δ11B-pH calibration, we present a new δ11B calibration of micro-sampled fibrous aragonite from this species.

    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 δ11B of borate in local ambient seawater (a function of pH and taken from hydrographic data sets; pH range 7.57 to 8.05) utilises δ11B
    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 δ11B; δ11Bfibre = (0.93 ± 0.17) × δ11Bborate + (12.02 ± 2.63). We suggest that the majority of the variability in measured δ11B 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.

    Full details in the University publications repository