Boron Isotopes as a tracer for oceanic CO2 and ocean acidification

Boron isotopes (δ11B) are predominately made up of two naturally occurring stable isotopes – 10B and 11B. In the biosphere, boron represents a trace element integrated into organisms, allowing one to reconstruct variability in δ11B over time.

Within the marine system, boron isotopes are of interest to paleoclimatologists given the connection between δ11B and CO2 variability in seawater and marine carbonates, including corals, foraminifera, gastropods and bivalves. Within seawater, boron is dissolved in two main forms: boric acid (B(OH)3) and borate (B(OH)4), the relative proportions of each is dependent on the surrounding pH. In conditions where the pH is relatively low, B(OH)3 is the dominant form of boron, whereas B(OH)4 is the dominant form during high pH conditions. This boron is then incorporated into marine calcifiers’ calcium carbonate (CaCO3) structure, trapping the signature within growth or depositional layers. By measuring δ11B within carbonates, the pH condition can be approximated. Since pH is closely related to dissolved CO2 , variability in oceanic CO2 (and ultimately atmospheric CO2) can be reconstructed through time.

The δ11B-pH proxy has been used to reconstruct long-term climate variability (Anagnostou et al., 2020). By analysing δ11B in planktonic foraminifera, Anagnostou et al., (2020) calculated seawater pH and dissolved CO2 in order to determine the evolution of climate during the Eocene – finding that a doubling of atmospheric CO2 resulted in phases of global mean temperature increases over the last 65 million years.

Boron isotopes have the potential to pinpoint the timing of ocean acidification events. Using brachiopod shells, Muller et al. (2020) analysed the Toarcian Oceanic Anoxic Event which occurred 183 million years ago. In this work, δ11B declines occurred concurrently with reductions in carbonate production followed by drops in pH level and eventually collapse and extinction of marine carbonates.

At Isobar Science, δ11B can be analyzed on shells, corals, carbonates, and water, reported as delta (δ) values in units of parts per thousand (per mill) (‰) relative to NIST SRM 951a – Boric Acid Isotopic Standard. Read more on the boron isotope analysis method and sample recommendations.

References

Anagnostou, E., John, E.H., Babila, T.L., Sexton, P.F., Ridgwell, A., Lunt, D.J., Pearson, P.N., Chalk, T.B., Pancost, R.D. and Foster, G.L., (2020). Proxy evidence for state-dependence of climate sensitivity in the Eocene greenhouse. Nature communications, 11(1), pp.1-9.

Foster, G.L. and Rae, J.W., (2016). Reconstructing ocean pH with boron isotopes in foraminifera. Annual Review of Earth and Planetary Sciences, 44, pp.207-237.

Müller, T., Jurikova, H., Gutjahr, M., Tomašových, A., Schlögl, J., Liebetrau, V., Duarte, L.V., Milovský, R., Suan, G., Mattioli, E. and Pittet, B., (2020). Ocean acidification during the early Toarcian extinction event: Evidence from boron isotopes in brachiopods. Geology, 48(12), pp.1184-1188.