121 Paleoclimatology/Paleoceanography (Posters) I

A CRITICAL REVIEW OF THE BORON ISOTOPE-PH PROXY

Paper number: 121-19
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

Published applications of the boron-pH technique are founded on a theoretical model of carbonate d11B variation with pH that assumes the boron isotopic compositions of carbonates mirror the boron isotopic composition of the tetrahedrally-coordinated boron species in solution. The well constrained reconstruction of pH based on this model requires a precise understanding of the fractionation factor for the equilibrium isotope exchange between boric acid (trigonally-coordinated) and borate (tetrahedrally-coordinated) in solution (a4-3), the equilibrium constant for the dissociation of boric acid (K*B), and the seawater d11B value through time (d11Bsw).  A review of the available data suggests the assumption that carbonate d11B is identical to the d11B of borate is not certain and that both the equilibrium fractionation factor and the history of d11Bsw are poorly constrained. These conclusions reflect on the accuracy and precision of reconstructed pH values over long-time scales.

Evaluation of carbonate d11B from inorganic precipitation experiments, assuming carbonate d11B is isotopically identical to borate in solution, indicate that the most appropriate value of a4-3 is ~0.974 rather than the theoretical value of 0.981 from Kakihana et al. [1977] as has been assumed in some publications.  Further, a compilation of inorganic and culture experiments suggest that either the value of a4-3 is not constant with pH or the assumption that carbonate d11B systematically reflects the isotopic composition of borate is faulty.   Nevertheless, if the fundamentals of the boron-pH technique are assumed correct and the empirical value of a4-3 is applied to foraminifera d11B values measured for the past 60 million years, revised pH reconstructions indicate little change in ocean pH over the Cenozoic.  Ambiguity in pH increases as vital effects, as well as various models for the evolution of d11Bsw are considered.  Given the current understanding of boron systematics, pH values estimated by this technique likely have considerable uncertainty, particularly as reconstructions exceed the residence time of boron in the ocean.