Boron (B) Geochemistry Overview
Boron has two forms of stable isotopes (10B, 11B) as well as 13 forms of radioactive isotopes (ranging from 7B through to 21B, not including the stable forms). Stable forms of boron are the only naturally occurring isotopes with 10B making up 20% of natural boron and 11B making up 80%.
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.
More information on Sample Types and Selection for boron analysis
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This video excerpt is part of Beta Analytic’s webinar: Boron Isotopic Analysis
Paleoclimatology & Paleoceanography
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. However, the particular species and concentration of boron incorporated into biogenic carbonates (e.g. shells) is pH dependent. Thus, the boron isotopic composition of marine biogenic carbonates reflects seawater pH.
Read more on our blog posts:
Studying Past Climates using Stable Isotopes
Geochemical Fingerprinting and Contaminant Source Tracking
Boron is widely used in industry (lubricant, flux, additive), agriculture (micronutrient in fertilizers), and households (bleaching agent). Boron is an important additive in steel manufacturing to increase the hardenability of a material. For example, boron steels are widely used in the nuclear industry to shield systems and control nuclear reactions given its natural ability to absorb neutrons. In other industries, a boron compound known as sodium borate (“Borax”) is used extensively for detergents to remove stains and mildew as well as in cosmetic products as a preservative and buffering agent.
Given the extensive use of boron in many different products, it is released to the environment on a regular basis. Based on data provided by the US Environmental Protection Agency (EPA, 1976), in 1972 alone, a total of 35.5 kilotons of boron was released into the environment, 73% of which was directly introduced to the water. The release of boron is particularly problematic as boron cannot be destroyed in the environment, but rather it can change its form by separating and becoming attached to other particles in soils, sediments, and water. This boron can eventually make its way into food (vegetables and fruit) given its an essential nutrient for plant growth.
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.
Read more on using boron isotopes to enhance nitrate source tracking.
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. Environmental Chemistry, 10(5), pp.365-369. DOI: 10.1071/EN13036
Chalk, T.B., Foster, G.L. and Wilson, P.A., (2019). Dynamic storage of glacial CO2 in the Atlantic Ocean revealed by boron [CO32−] and pH records. Earth and Planetary Science Letters, 510, pp.1-11. DOI: 10.1016/j.epsl.2018.12.022
EPA (Environmental Protection Agency), (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. and 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 Science Letters, 519, pp.130-140. DOI: 10.1016/j.epsl.2019.05.008
Martínez-Botí, M.A., et al. (2015). Boron isotope evidence for oceanic carbon dioxide leakage during the last deglaciation. Nature, 518(7538), pp.219-222. DOI: 10.1038/nature14155