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

The boron isotope composition (delta B-11) 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 delta B-11 biogenic carbonates. It is also probable that delta B-11 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 delta B-11 of biogenic carbonate samples at the multicollector inductively coupled mass spectrometry facility at Ifremer (Plouzane, France) and the evaluation of delta(BCaCO3)-B-11 in a diverse range of marine calcifying organisms reared for 60 days in isothermal seawater (25 degrees C) equilibrated with an atmospheric pCO(2) of ca. 409 mu atm. Average delta(BCaCO3)-B-11 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 delta B-11, 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 delta(BCaCO3)-B-11 (ca. 20%) and significant discrepancies between measured delta(BCaCO3)-B-11 and delta(BCaCO3)-B-11 expected from established relationships between abiogenic delta(BCaCO3)-B-11 and seawater pH arise primarily from fundamental differences in calcifying site pH amongst the different species. These results highlight the potential utility of delta B-11 as a proxy of calcifying site pH for a wide range of calcifying taxa and underscore the importance of using species-specific seawater-pH-delta(BCaCO3)-B-11 calibrations when reconstructing seawater pH from delta B-11 of biogenic carbonates.