Experimental measurement of boron isotope fractionation in seawater

The boron isotopic composition of marine carbonates is considered to be a tracer of seawater pH. Use of this proxy benefits from 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−. However, because of our inability to quantitatively separate these species in solution, the degree of boron isotope exchange has only been known through theoretical estimates. In this study, we present results of a spectrophotometric procedure wherein the boron isotope equilibrium constant (11–10KB) is determined empirically from the difference in the dissociation constants of 11B(OH)3 and 10B(OH)3 in pure water, 0.6 mol kg− 1 H2O KCl and artificial seawater. Within experimental uncertainty, our results show no dependence of 11–10KB on temperature, but 11–10KB at 25 °C in pure water was statistically different than results obtained in solutions at high ionic strength. 11–10KB of the seawater (S = 35, BT = 0.01 mol kg− 1 H2O) at 25 °C is 1.0272 ± 0.0006. This result is significantly larger than the theoretical value used in numerous paleo-pH studies (11–10KB = 1.0194).