Stable boron isotope fractionation between dissolved B(OH)3 and B(OH)4

The stable boron isotope ratio (B/B) in marine carbonates is used as a paleo-pH recorder and is one of the most promising paleo-carbonate chemistry proxies. Understanding the thermodynamic basis of the proxy is of fundamental importance, including knowledge on the equilibrium fractionation factor between dissolved boric acid, B(OH)3, and borate ion, B(OH)4 – ( B OH 3 B OH 4 , hereafter (B3–B4)). However, this factor has hitherto not been determined experimentally and a theoretically calculated value (Kakihana and Kotaka, 1977, hereafter KK77) has therefore been widely used. I examine the calculations underlying this value. Using the same spectroscopic data and methods as KK77, I calculate the same B3 B4 1.0193 at 300 K. Unfortunately, it turns out that in general the result is sensitive to the experimentally determined vibrational frequencies and the theoretical methods used to calculate the molecular forces. Using analytical techniques and ab initio molecular orbital theory, the outcome for (B3–B4) varies between 1.020 and 1.050 at 300 K. However, several arguments suggest that (B3–B4) 1.030. Measured isotopic shifts in various B-, D-, and O-labeled isotopomers do not provide a constraint on stable boron isotope fractionation. I conclude that in order to anchor the fundamentals of the boron pH proxy, experimental work is required. The critics of the boron pH proxy should note, however, that uncertainties in do not bias pH reconstructions provided 0016-7037/05 $30.00 .00 (B3–B4) that organism-specific calibrations are used. Copyright © 2005 Elsevier Ltd