Multi-specific calibration of the B isotope proxy in calcareous red algae for pH reconstruction

Calcareous red algae have calcified cell walls constituted by high-Mg calcite or aragonite (Morse et al., 2006). They are considered suitable paleoclimate archives due to their worldwide distribution and their longevity through indeterminate growth (Kamenos et al., 2008). Boron isotopes (δ¹¹B) measured in their calcified thallus are considered a pH proxy (Hemming and Hanson, 1992). In seawater, boron occurs as boric acid and borate ion. Both species are enriched in ¹¹B as pH increases, with boric acid characterized by an enrichment factor of 27‰ compared to borate. The boron isotope proxy theory states that borate is exclusively incorporated in the mineral lattice (Hemming and Hanson, 1992). Therefore, if we measure δ¹¹B in carbonates, we can derive the seawater pH at the time of precipitation. Literature data on δ¹¹B in calcareous red algae are sparse, and the mechanisms of boron incorporation are still poorly known (Piazza et al., 2022). We tested the boron isotope-pH proxy on calcareous red algae grown at 1 m depth close to CO₂ seeps off the coasts of Ischia (Italy), and Methana (Greece), which are both characterized by a broad range of natural pH in seawater (from 6.80 ± 0.43 to 8.08 ± 0.07 units). Environmental data characterizing the seawater during the algal growth were extracted from CMEMS products (Marine Copernicus Service Information), or provided by literature. The δ¹¹B values in the algae (δ¹¹B_algae) analysed by Multi Collector Inductively-Coupled Plasma Mass Spectrometry ranged from 22.23‰ to 26.59‰, calibrated over a range of δ¹¹B in aqueous borate (δ¹¹B_borate) extending from 12.68‰ to 18.05‰. A crystallographic control over boron incorporation was shown by the difference in the isotopic composition of carbonate polymorphs, with Mg-calcite enriched in ¹¹B compared to aragonite. Values of δ¹¹B_algae higher than δ¹¹B_borate could be attributed to the up-regulation of the calcifying fluid pH exerted by the algae. We proposed a multi-specific calibration using literature data of boron isotopes in cultured coralline algae combined with our new data on wild-grown specimens, widening the range of pH considered for δ¹¹B calibrations so far. The proposed calibration is particularly useful when experimental calibration is not possible, such as in the fossil record and in the case of ambiguous identifications.

References

Hemming N. G. & Hanson G. N. 1992. Boron isotopic composition and concentration in modern marine carbonates. Geochim. Cosmochim. Acta, 56, 537-543.

Kamenos N. A., Cusack M. & Moore P. G. 2008. Coralline algae are global paleothermometers with bi-weekly resolution. Geochim. Cosmochim. Acta, 72, 771-779.

Morse J. W., Andersson A. J. & Mackenzie F. T. 2006. Initial responses of carbonate-rich shelf sediments to rising atmospheric pCO₂ and ‘‘ocean acidification’’: role of high Mg-calcites. Geochim. Cosmochim. Acta, 70, 5814-5830.

Piazza G., Bracchi V. A., Langone A., Meroni A. N. & Basso D. 2022. Growth rate rather than temperature affects the B / Ca ratio in the calcareous red alga Lithothamnion corallioides. Biogeosciences, 19, 1047-1065.