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

Jill Sutton (1) , Yi-Wei Liu (1, 2) , Justin B. Ries (3) , Maxence Guillermic (2) , Emmanuel Ponzevera (4) , Robert A. Eagle (1, 5, 6)

1 LEMAR - Laboratoire des Sciences de l'Environnement Marin (LEMAR)
2 LGO - Laboratoire Géosciences Océan
3 Northeastern University [Boston]
4 GM - Unité Géosciences Marines
5 UCLA - University of California [Los Angeles]
6 AOS - Department of Atmospheric and Oceanic Sciences [Los Angeles]

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.

Mots clés: ACL; great-barrier-reef; planktonic-foraminifera; b-11 mas nmr; boron isotopic composition; carbon-dioxide concentrations; co2-induced ocean acidification; mc-icp-ms; paleo-ph; seawater ph; tissue-skeleton interface

Domaines: Biodiversité et Ecologie; Océan, Atmosphère

Jill Sutton, Yi-Wei Liu, Justin B. Ries, Maxence Guillermic, Emmanuel Ponzevera, et al.. δ11B as monitor of calcification site pH in divergent marine calcifying organisms. Biogeosciences, 2018, 15 (5), pp.1447-1467. ⟨10.5194/bg-15-1447-2018⟩. ⟨hal-02651007⟩