A new boron isotope-pH calibration for Orbulina universa, with implications for understanding and accounting for ‘vital effects’
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Boron isotope ratios, as measured in planktic foraminifera, can be a useful tracer of past ocean pH, and hence help to discern the concentration of CO2 in the ancient atmosphere. However, different species of planktic foraminifera demonstrate different patterns of boron isotope variation with ambient seawater pH. Therefore when applying the proxy to questions in the geological past, species-specific calibrations are preferable. Beyond the evolutionary history of a calibrated species, we must rely on our understanding of the causes of the observed “vital effects” in the modern ocean, and the applicability of that understanding to extinct species. Here we present a new open-ocean calibration of the planktic foraminifera Orbulina universa, measured via Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS). Unlike other symbiont-bearing foraminifera, O. universa record a δ11B (and hence pH) that is lower than its surrounding seawater, but with a pH-sensitivity roughly equal to that of aqueous borate ion. We discuss the significance of this for application of the boron isotope proxy in deep time, with recommendations for best practice and future research directions.
Henehan , M J , Foster , G L , Bostock , H C , Greenop , R , Marshall , B J & Wilson , P A 2016 , ' A new boron isotope-pH calibration for Orbulina universa , with implications for understanding and accounting for ‘vital effects’ ' Earth and Planetary Science Letters , vol 454 , pp. 282-292 . DOI: 10.1016/j.epsl.2016.09.024
Earth and Planetary Science Letters
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
DescriptionThis work was aided by NERC PhD studentships granted to MH and RG, and NERC grant NE/D00876X/2 awarded to GF. Voyage TAN1106 was funded by the New Zealand government through NIWA core funding, and Cariaco sampling was funded through NSF award 1258991.
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