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%. 

boron isotopes

The two boron isotope species that are commonly used in geochemical studies (left) and the further 13 radioactive boron isotope species (right)

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.

Sample types available for boron analysis: shells, corals, carbonates, and water
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 + H2↔ 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.

Boron and pH in sediments

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

At low pH levels in the ocean, boron is typically integrated into a shell in the form of B(OH)3, whereas B(OH)4− is the species of choice in periods of high 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 is a 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. This relationship between boron and oceanic pH can be analysed at an ocean circulation scale, and has even been used as evidence of oceanic carbon leakage during the last deglaciation (Martínez-Botí et al. 2015).
chemical composition of boron

The relationship between pH and the percentage of two different boron species (https://jpt.spe.org/comparison-methods-boron-removal-flowback-and-produced-waters)

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.

Boron Release New

Estimated tons of boron entering the environment since 1972 (Data from EPA-68-01-3201,1976).

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

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.

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. 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