Direct measurement of the boron isotope fractionation factor and implications for paleo-pH studies

English Abstract:

The boron isotopic composition of calcium carbonate skeletons is a promising proxy method for reconstructing paleo-ocean pH and atmospheric CO₂ from the geological record. Although the boron isotope methodology has been used extensively over the past two decades to determine ancient ocean-pH, the actual value of the boron isotope fractionation factor between the two main dissolved boron species, ¹¹B(OH)₃ and ¹⁰B(OH)₄^-, has remained uncertain. We developed a new methodology for direct measurement of the boron isotope fractionation factor by using reverse osmosis membrane for boron species separation. We conducted a series of experiments in which seawater, brackish groundwater and boron-spiked artificial solutions were passed through a reverse osmosis membrane under controlled pressure and pH. This experimental design provides a unique methodology to isolate and exclusively measure the isotopic ratio of boric acid and thus determine the boron isotope fractionation factor between boron species. By using the mass-balance of boron species and their d¹¹B, we derived a mathematical expression in which the fractionation factor is the slope in the coordinations of the d¹¹B of boric acid and the fraction of aquatic boron species. We applied the Pitzer ion-interaction approach, combined with ion-pairing calculations for a more accurate determination of species distribution in aquatic solutions of different chemical compositions. Our experiments yielded an independent empirical fractionation factor (ε_B=26.0±1.0‰; 25⁰C) for all of the tested solutions under a wide range of pH conditions. These results resolve the debate over the magnitude of the boron isotope fractionation and open new potential for improvements in reconstruction of paleo-pH of the ocean and atmospheric CO₂ in the geological record. In particular, the new findings provide improved tools to delineate between selective and exclusive incorporation of borate species in carbonate minerals and biological offsets (i.e. vital effects) from thermodynamic isotopic fractionation of boron species.