Abstract
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.
Highlights
IntroductionThe boron isotope-pH proxy (as applied to biogenic carbonates) is widely used for reconstructing palaeo-pH and past CO2 concentrations
The boron isotope-pH proxy is widely used for reconstructing palaeo-pH and past CO2 concentrations
Because a known isotopic fractionation is associated with this speciation (Klochko et al, 2006), and because it is thought that the charged borate ion is the species incorporated into foraminiferal CaCO3 (e.g. Branson et al, 2015; Hemming and Hanson, 1992; Rae et al, 2011), it is possible to infer in situ pH from the boron isotopic composition of foraminiferal calcite
Summary
The boron isotope-pH proxy (as applied to biogenic carbonates) is widely used for reconstructing palaeo-pH and past CO2 concentrations. Foster, 2008; Henehan et al, 2013; Hönisch et al, 2003; Martínez-Botí et al, 2015; Sanyal et al, 2001, 1996), with the magnitude and directionality (i.e. toward higher or lower δ11B) of these effects appearing to be species-specific. These vital effects are commonly explained as manifestations of an altered microenvironment around the foraminifera (Zeebe et al, 2003, 1999a).
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