Comment on “A critical evaluation of the boron isotope-pH proxy: The accuracy of ancient ocean pH estimates” by M. Pagani, D. Lemarchand, A. Spivack and J. Gaillardet

Pagani et al. [Pagani M., Lemarchand D., Spivack A., and Gaillardet J. (2005). A critical evaluation of the boron isotope-pH proxy: the accuracy of ancient ocean pH estimates. Geochim. Cosmochim. Acta 69(4), 953–961] use data from previous boron isotope studies to suggest that the fractionation between boric acid and borate in seawater as well as the history of δ 11B in seawater are poorly understood, thus limiting our ability to capture realistic ocean pH with this proxy. Although we agree with the authors that the long recognized uncertainty in the secular variation of δ 11B seawater imposes a temporal limit on paleo-pH reconstructions, their evaluation of the δ 11B/pH relationship in carbonates is flawed. Potential complications from vital, temperature and dissolution effects reported in that paper are based on studies that are experimentally and/or analytically poorly constrained. Using published validation studies we will demonstrate that many of the problems outlined by Pagani et al. have already been addressed, or are based on misinterpretations of previous work. Most importantly, statistical evaluation suggests empirical data are best described by a fractionation of ∼20‰. Recent paleoreconstructions confirm that the boron isotope proxy can be used with confidence, if sample selection and analyses are done carefully.