ALBERT

Description: The isotope composition of boron (B) in marine biogenic carbonates has been predominantly studied as a proxy for monitoring past changes in seawater pH and carbonate chemistry. In order to derive seawater pH from boron isotope ratio data, a number of assumptions related to chemical kinetics and themodynamic isotope exchange reactions are necessary. Furthermore, the boron isotope composition (δ11B) of biogenic carbonates (δ11BCaCO3) is assumed to reflect the δ11B of dissolved borate (B(OH)4−) in seawater. Here we report the development of methodology for measuring the δ11B in biogenic carbonate samples at the multi-collector inductively coupled mass spectrometry facility at Ifremer (Plouzané, France) and the evaluation of δ11BCaCO3 in a diverse range of marine calcifying organisms. We evaluated the δ11BCaCO3 of 6 species of marine calcifiers (a temperate coral, Oculina arbuscula; a coralline red alga, Neogoniolithion sp.; a tropical urchin, Eucidaris tribuloides; a temperate urchin, Arbacia punctulata; a serpulid worm, Hydroides crucigera; and an American oyster, Crassostrea virginica) that were reared for 60 days in isothermal seawater (25 °C) equilibrated with an atmospheric pCO2 of ca. 409 μatm. We observe large inter-species variability in δ11BCaCO3 (ca. 20 ‰) and significant discrepancies between measured δ11BCaCO3 and δ11BCaCO3 expected from established relationships between δ11BCaCO3 and seawater pH. We discuss these results in the context of various proposed mechanisms of biocalcification, including the potential dominant role that internal calcifying site pH plays in regulating CaCO3 saturation state and borate δ11B at the site of calcification and, thus, the δ11B composition of calcifers’ shells and skeletons.

Description: 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 δ11BCaCO3 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 δ11BCaCO3 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 δ11BCaCO3 (ca. 20 ‰) and significant discrepancies between measured δ11BCaCO3 and δ11BCaCO3 expected from established relationships between abiogenic δ11BCaCO3 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–δ11BCaCO3 calibrations when reconstructing seawater pH from δ11B of biogenic carbonates.