EMPIRICAL DETERMINATION OF THE BORON ISOTOPE EQUILIBRIUM CONSTANT

The boron isotopic composition of marine carbonates is believed to be a useful tracer of seawater pH, which may then be used to reconstruct atmospheric pCO₂ through time.  Use of this proxy requires an intimate understanding of chemical kinetics and thermodynamic isotope exchange reactions between the two dominant boron-bearing species in seawater: boric acid B(OH)₃ and borate ion B(OH)₄^-, which is preferentially incorporated into the carbonate lattice.  However, due to our inability to quantitatively isolate these species from seawater, the magnitude of boron isotope fractionation at different temperatures and salinities has not previously been empirically measured.  All paleo-pH studies have relied on the boron isotope equilibrium constant (K_eq = 1.0194 at 25^oC) estimated theoretically in 1977 by Kakihana and colleagues. Here we present a method for empirical determination of the boron isotope equilibrium constant at different temperatures and ionic strengths. The determinations are based on titration of isotopically labeled solutions, containing either ¹⁰B(OH)₃ or ¹¹B(OH)_3, with NaOH. The pH of the titrated solutions is precisely measured using thymol blue indicator absorbance ratios. Differences in solution pH or, equivalently, borate/boric acid pK values between the isotopically substituted solutions, provides the desired equilibrium constant for the reaction:

¹⁰B(OH)₃ + ¹¹B(OH)₄^- <=> ¹¹B(OH)₃ + ¹⁰B(OH)₄^-.

We have performed experiments to assess the influence of the temperature (25 and 40^oC), ionic strength (0.05 and 0.7 molar) and medium composition (pure water, 0.6 M KCl, and synthetic seawater) on the isotopic equilibrium constant.  Within experimental uncertainty (± 0.002, 1s), our results show only a weak dependence of the equilibrium constant on the above factors. The boron isotope equilibrium constant in seawater (S = 35) was determined to be 1.0265 ± 0.0015 at 25^oC (1s, n=10), which is in poor agreement with the theoretical basis for all previous paleo-pH estimates. Application of the new empirically derived equilibrium constant to previously published results may help to explain systematic offsets from expected d¹¹B values in studies of modern and ancient carbonates.