δ¹¹B as monitor of calcification site pH in divergent marine calcifying organisms

Abstract. The boron isotope composition (δ¹¹B) of marine biogenic carbonates has been predominantly studied as a proxy for monitoring past changes in seawater pH and carbonate chemistry. However, a number of assumptions regarding chemical kinetics and thermodynamic isotope exchange reactions are required to derive seawater pH from δ¹¹B biogenic carbonates. It is also probable that δ¹¹B of biogenic carbonate reflects seawater pH at the organism's site of calcification, which may or may not reflect seawater pH. Here, we report the development of methodology for measuring the δ¹¹B of biogenic carbonate samples at the multi-collector inductively coupled mass spectrometry facility at Ifremer (Plouzané, France) and the evaluation of δ¹¹B_CaCO3 in a diverse range of marine calcifying organisms reared for 60 days in isothermal seawater (25 °C) equilibrated with an atmospheric pCO₂ of ca. 409 µatm. Average δ¹¹B_CaCO3 composition for all species evaluated in this study range from 16.27 to 35.09 ‰, including, in decreasing order, coralline red alga Neogoniolithion sp. (35.89 ± 3.71 ‰), temperate coral Oculina arbuscula (24.12 ± 0.19 ‰), serpulid worm Hydroides crucigera (19.26 ± 0.16 ‰), tropical urchin Eucidaris tribuloides (18.71 ± 0.26 ‰), temperate urchin Arbacia punctulata (16.28 ± 0.86 ‰), and temperate oyster Crassostrea virginica (16.03 ‰). These results are discussed in the context of each species' proposed mechanism of biocalcification and other factors that could influence skeletal and shell δ¹¹B, including calcifying site pH, the proposed direct incorporation of isotopically enriched boric acid (instead of borate) into biogenic calcium carbonate, and differences in shell/skeleton polymorph mineralogy. We conclude that the large inter-species variability in δ¹¹B_CaCO3 (ca. 20 ‰) and significant discrepancies between measured δ¹¹B_CaCO3 and δ¹¹B_CaCO3 expected from established relationships between abiogenic δ¹¹B_CaCO3 and seawater pH arise primarily from fundamental differences in calcifying site pH amongst the different species. These results highlight the potential utility of δ¹¹B as a proxy of calcifying site pH for a wide range of calcifying taxa and underscore the importance of using species-specific seawater-pH–δ¹¹B_CaCO3 calibrations when reconstructing seawater pH from δ¹¹B of biogenic carbonates.