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Accurate and precise microscale measurements of boron isotope ratios in calcium carbonates using Laser Ablation Multicollector-ICPMS

Abstract

The boron isotope compositions (δ11B) of biogenic carbonates have proven to be invaluable tool for investigating changes in ocean carbonate chemistry, especially the impacts of declining seawater pH due to rising levels of atmospheric CO2. Accurate, high precision boron isotopic measurements, however, remain analytically challenging with conventional approaches being extremely time consuming and labour intensive. Here we present a new analytical approach for high precision accurate measurements of boron isotope compositions in biogenic carbonates. We show that Laser Ablation MC-ICPMS equipped with 1013 Ω amplifiers provides a powerful tool for determining micro-scale δ11B values with an internal precision of ≤0.15‰ (2SD) and total long-term accuracy of better than 0.22‰, which is equivalent or better than conventional solution-based methods. This accuracy is achieved by careful subtraction of baseline interferences around the masses of 10B and 11B, which arise from matrix-specific scattered Ca, C, and O ions. We also report detailed protocols for time-response corrections of transient signals in biogenic carbonates, essential for high precision boron isotopic measurements when using 1013 Ω amplifiers. The overall veracity of our Laser Ablation MC-ICPMS method is further demonstrated by the isotopic variability observed at macro- (mm) and micro-scales (μm) in both calcite and aragonite biogenic minerals. An aragonite deep-sea coral Desmophyllum dianthus is shown to preserve large (up to 6‰) variations in δ11B accompanied by near synchronous changes in B/Ca. This variability is excessive compared to that expected for the relatively stable carbonate ion system of deep-ocean waters and is therefore attributed to isotopic and elemental fractionation of B during coral biomineralisation. In contrast, the cyclic 2.5‰ variability is found in the sclerosponge Ceratoporella nicholsoni, which can be directly linked to seasonal changes in seawater pH. These examples highlight the power of Laser Ablation MC-ICPMS and its ability to accurately and precisely recover small environmental signals in boron isotopes. We thus show that these significant improvements in both the internal precision and now long-term accuracy and reproducibility of Laser Ablation MC-ICPMS, together with its microscale analytical capabilities and relatively rapid ease of operation, have the potential to revolutionise the study of boron isotope systematics in biogenic carbonates.

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Publication details

The article was accepted on 03 Jan 2019 and first published on 03 Jan 2019


Article type: Paper
DOI: 10.1039/C8JA00444G
Citation: J. Anal. At. Spectrom., 2019, Accepted Manuscript
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    Accurate and precise microscale measurements of boron isotope ratios in calcium carbonates using Laser Ablation Multicollector-ICPMS

    A. Y. Sadekov, N. S. Lloyd, S. Misra , J. Trotter, J. D'Olivo and M. McCulloch, J. Anal. At. Spectrom., 2019, Accepted Manuscript , DOI: 10.1039/C8JA00444G

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