A 20 million year record of planktic foraminiferal B/Ca ratios: Systematics and uncertainties in pCO2 reconstructions

We use new and published data representing a 20 million long record to discuss the systematics of interpreting planktic foraminiferal B/Ca ratios. B/Ca-based reconstructions of seawater carbonate chemistry and atmospheric pCO2 assume that the incorporation of boron into foraminiferal tests can be empirically described by an apparent partition coefficient, KD 1⁄4 B=CaCaCO3 B OH ð Þ 4 1⁄2 = HCO 3 1⁄2 seawater (Hemming and Hanson, 1992). It has also been proposed that there is a species-specific relationship between KD and temperature (Yu et al., 2007). As we discuss, although these relationships may be robust, there remain significant uncertainties over the controls on boron incorporation into foraminifera. It is difficult to be certain that the empirically defined correlation between temperature and KD is not simply a result of covariance of temperature and other hydrographic variables in the ocean, including carbonate system parameters. There is also some evidence that KD may be affected by solution 1⁄2HCO 3 =1⁄2CO 2 3 ratios (i.e., pH), or by 1⁄2CO 2 3 . In addition, the theoretical basis for the definition of KD and for a temperature control on KD is of debate. We also discuss the sensitivity of pCO2 reconstructions to different KD–temperature calibrations and seawater B/Ca. If a KD–temperature calibration is estimated using ice core pCO2 values between 0 and 200 ka, B/Ca ratios can be used to reasonably approximate atmospheric pCO2 between 200 and 800 ka; however, the absolute values of pCO2 calculated are sensitive to the choice of KD–temperature relationship. For older time periods, the absolute values of pCO2 are also dependent on the evolution of seawater B concentrations. However, we find that over the last 20 Ma, reconstructed changes in declining pCO2 across the Mid-Pleistocene Transition, Pliocene glacial intensification, and the Middle Miocene Climate Transition are supported by the B/Ca record even if a constant coretop KD is used, or different KD–temperature calibrations and models of seawater B evolution are applied to the data. The inferred influence of temperature on KD from coretop data therefore cannot itself explain the structure of a published pCO2 reconstruction (Tripati et al., 2009). We conclude the raw B/Ca data supports a coupling between pCO2 and climate over the past 20 Ma. Finally, we explore possible implications of B/Ca-based pCO2 estimates for the interpretation of other marine pCO2 proxies. 2011 Elsevier Ltd. All rights reserved. 0016-7037/$ see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.01.018 ⇑ Corresponding author at: Department of Earth and Space Sciences, Institute of Geophysics and Planetary Physics, University of California, 595 Charles Young Drive East, Los Angeles, CA 90095, USA. Tel.: +1 626 376 1308. E-mail address: [email protected] (A.K. Tripati). 1 Present address: Met Office, Hadley Centre, Exeter, UK.