Crystallographic control on the boron isotope paleo-pH proxy

J. Noireaux (1) , V. Mavromatis (2) , J. Gaillardet (1) , J. Schott (2) , V. Montouillout (3) , P. Louvat (1) , C. Rollion-Bard (1) , D. R. Neuville (1)

1 IPGP - Institut de Physique du Globe de Paris
2 GET - Géosciences Environnement Toulouse
3 CEMHTI - Conditions Extrêmes et Matériaux : Haute Température et Irradiation

When using the boron isotopic composition (δ¹¹B) of marine carbonates as a seawater pH proxy, it is assumed that only the tetrahedral borate ion is incorporated into the growing carbonate crystals and that no boron isotope fractionation occurs during uptake. However, the δ¹¹B of the calcium carbonate from most modern foraminifera shells or corals skeletons is not the same as the δ¹¹B of seawater borate, which depends on pH, an observation commonly attributed to vital effects. In this study, we combined previously published high-field ¹¹B MAS NMR and new δ¹¹B measurements on the same synthetic calcite and aragonite samples precipitated inorganically under controlled environments to avoid vital effects. Our results indicate that the main controlling factors of δ¹¹B are the solution pH and the mineralogy of the precipitated carbonate mineral, whereas the aqueous boron concentration of the solution, CaCO₃ precipitation rate and the presence or absence of growth seeds all appear to have negligible influence. In aragonite, the NMR data show that boron coordination is tetrahedral (BO₄), in addition, its δ¹¹B is equal to that of aqueous borate, thus confirming the paleo-pH hypothesis. In contrast, both trigonal BO₃ and tetrahedral BO₄ are present in calcite, and its δ¹¹B values are higher than that of aqueous borate and are less sensitive to solution pH variations compared to δ¹¹B in aragonite. These observations are interpreted in calcite as a reflection of the incorporation of decreasing amounts of boric acid with increasing pH. Moreover, the fraction of BO₃ measured by NMR in calcite is higher than that inferred from δ¹¹B which indicates a coordination change from BO₄ to BO₃ upon boron incorporation in the solid. Overall, this study shows that although the observed differences in δ¹¹B between inorganic and biological aragonite are compatible with a pH increase at calcification sites, the B speciation and isotope composition of biological calcites call for a more complex mechanism of boron incorporation.

Mots clés: boron isotope; pH-proxy; synthetic calcite and aragonite; NMR

Domaines: Planète et Univers [physics]

J. Noireaux, V. Mavromatis, J. Gaillardet, J. Schott, V. Montouillout, et al.. Crystallographic control on the boron isotope paleo-pH proxy. Earth and Planetary Science Letters, 2015, 430, pp.398-407. ⟨10.1016/j.epsl.2015.07.063⟩. ⟨insu-03579370⟩