Different earth system materials and environmental conditions are characterized by discrete ranges for the boron isotopic ratio. Thus, this parameter is extensively used in geochemical fingerprinting, source tracking, contamination prediction, global carbon cycles, and ocean circulation studies.
Paleoceanography and paleoclimate
Dissolved boron in seawater primarily appears in the form of two mononuclear species, boric acid (H3BO3 or B(OH)3) and borate (BO3-3 or B(OH)4−), with a distinct isotopic fractionation between the two species such that boric acid is enriched in the heavier isotope (11B) by 27.2%. The relative proportion of these species is pH dependent as shown below:
B(OH)3 + H2O ↔ B(OH)4− + H+
High pH⟵ ⟶Low pH
Boron is incorporated into biogenic and inorganic carbonates primarily as the borate species. Thus, the boron isotopic composition of marine biogenic carbonates reflects seawater pH. Typically, a 0.1unit increase in seawater pH results in a ~1‰ increase in δ11B of marine biogenic carbonates. Because seawater pH values decrease as more carbon dioxide (CO2) gas is absorbed, δ11B as proxy that can be used to constrain estimates of past atmospheric CO2 concentration (pCO2) and the oceanic carbonate budget. This information helps to better understand historical climate conditions and variations in the global carbon cycle.
Geochemical fingerprinting and source tracking
Boron is widely used in industry (lubricant, flux, additive), agriculture (micronutrient in fertilizers), and households (bleaching agent). Based on data provided by the US Environmental Protection Agency (EPA, 1976), in 1972 alone, a total of 35.5 kiloton of boron was released into the environment, which 73% of it was directly introduced to the water.
Natural denitrification and mixing processes may extensively alter the isotopic values of nitrate contamination (δ15N and δ18O), making the differentiation of urban and agricultural origins of nitrate is very challenging. As δ11B is not affected by fractionation, the isotopic ratio of boron combined with O- and N-isotopic ratios of nitrate prove to be a very powerful tool for tracing contaminant sources.
Briand, C.; Plagnes, V.; Sebilo, M.; Louvat, P.; Chesnot, T.; Schneider, M.; Ribstein, P. and Marchet, P., 2013: Combination of nitrate (N, O) and boron isotopic ratios with microbiological indicators for the determination of nitrate sources in karstic groundwater. Environ. Chem., vol. 10, pp. 365–369. http://dx.doi.org/10.1071/EN13036.
Chalk, T.B.; Foster, G.L.; Wilson, P.A., 2019: Dynamic storage of CO2 in the Atlantic Ocean revealed by boron [CO32-] and pH record. Earth and Planetary Science Letters, vol. 510, pp. 1-11.
EPA-68-01-3201, 1976: Chemical technology and economics in environmental perspectives. Task II- Removal of Boron from wastewater. Environmental Protection Agency, Office of Toxic Substances, Washington, D.C. 20460
Giri, S.J.; Swart, P.K.; Pourmand, A., 2019: The influence of seawater calcium ions on coral calcification mechanisms: Constraints from boron and carbon isotopes and B/Ca ratios in Pocillopora damicornis. Earth and Planetary Sciences Letters, vol. 519, pp.130-140.