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

<p>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 (&#948;¹¹B) 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&#8240; 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 &#948;¹¹B in carbonates, we can derive the seawater pH at the time of precipitation. Literature data on &#948;¹¹B 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 &#177; 0.43 to 8.08 &#177; 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 &#948;¹¹B values in the algae (&#948;¹¹B_algae) analysed by Multi Collector Inductively-Coupled Plasma Mass Spectrometry ranged from 22.23&#8240; to 26.59&#8240;, calibrated over a range of &#948;¹¹B in aqueous borate (&#948;¹¹B_borate) extending from 12.68&#8240; to 18.05&#8240;. 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 &#948;¹¹B_algae higher than &#948;¹¹B_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 &#948;¹¹B 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.</p> <p><strong>References</strong></p> <p>Hemming N. G. & Hanson G. N. 1992. Boron isotopic composition and concentration in modern marine carbonates. Geochim. Cosmochim. Acta, 56, 537-543.</p> <p>Kamenos N. A., Cusack M. & Moore P. G. 2008. Coralline algae are global paleothermometers with bi-weekly resolution. Geochim. Cosmochim. Acta, 72, 771-779.</p> <p>Morse J. W., Andersson A. J. & Mackenzie F. T. 2006. Initial responses of carbonate-rich shelf sediments to rising atmospheric pCO₂ and &#8216;&#8216;ocean acidification&#8217;&#8217;: role of high Mg-calcites. Geochim. Cosmochim. Acta, 70, 5814-5830.</p> <p>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 <em>Lithothamnion corallioides</em>. Biogeosciences, 19, 1047-1065.</p>