Precise and accurate analysis of deep and surface seawater Sr stable isotopic composition by double-spike thermal ionization mass spectrometry

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Author(s)

    • Wakaki Shigeyuki
    • Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
    • Obata Hajime
    • Atmosphere and Ocean Research Institute, The University of Tokyo
    • Tazoe Hirofumi
    • Department of Radiation Chemistry, Institute of Radiation Emergency Medicine, Hirosaki University
    • Ishikawa Tsuyoshi
    • Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

Abstract

<p>This paper describes an analytical technique for highly precise and accurate determination of radiogenic and stable Sr isotopic composition using double-spike thermal ionization mass spectrometry. Repeated analyses of the in-house isotopic reference Wako-9999 against NIST SRM-987 gave an average δ<sup>88</sup>Sr of +0.32‰, with a long-term 2 SD external reproducibility of ±0.02‰ (<i>n</i> = 15). Detailed evaluation of the Sr isotope fractionation behavior during a column chromatographic Sr separation process using Sr spec resin showed systematic variation of δ<sup>88</sup>Sr in the eluate, from +1.05‰ to –0.64‰, as the Sr elution progressed. The Sr isotope fractionation factor between the Sr spec resin and 0.05M HNO<sub>3</sub> was estimated as 0.999947 ± 0.000001. During the chemical separation procedure, a very small amount of Sr with a highly fractionated isotopic composition was found to be lost in the sample loading and purification stage prior to Sr collection. This may cause a small but significant systematic mass bias in high-precision non-double-spike analyses. The analysis of 11 seawater samples from the Pacific and Atlantic oceans, including four deep seawater samples taken below the carbonate compensation depth (CCD) in the North Pacific Ocean, gave consistent results, with an average δ<sup>88</sup>Sr of 0.407 ± 0.012‰ (2 SD). All analyzed data agreed with literature values, demonstrating the accuracy of the stable Sr analysis in this study. The δ<sup>88</sup>Sr values of deep seawater, which were consistent with those of surface water, showed that seawater stable Sr isotopic composition is homogeneous to depths below the CCD.</p>

Journal

  • GEOCHEMICAL JOURNAL

    GEOCHEMICAL JOURNAL 51(3), 227-239, 2017

    GEOCHEMICAL SOCIETY OF JAPAN

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