Boron Isotope Geochemistry

Boron has two naturally occurring isotopes 10B (19.9%) and 11B (80.1%). Due to a relatively large mass difference (10%) between the two isotopes and high volatility, the boron isotope ratio ranges from −70 to +60‰ in natural materials and has been used as a tracer for studying continental weathering, plate subduction processes, pH variability in the oceans and anthropogenic pollution. Boron isotopic ratios are conventionally expressed in delta notation (δ11B), which denotes the deviation of a measured 11B/10B ratios from the standard in parts per thousand using the equation below:

Compilation of δ11B ranges for various earth system materials and environmental conditions.

Applications

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 + H2↔ 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.

Correlation between δ11B and pH in marine carbonates in sediment cores from North Atlantic during the past 150 thousand years (Chalk et al., 2019).

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.

Estimated tons of boron released to the environment in the Untitled State in 1972.

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.

Source signature of boron and nitrogen isotopic ratios in water (Briand et al., 2013).

References
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.
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 damicornisEarth and Planetary Sciences Letters, vol. 519, pp.130-140.
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