Theoretical isotopic fractionation between structural boron in carbonates and aqueous boric acid and borate ion

Abstract

International audienceThe 11B/10B ratio in calcite and aragonite is an important proxy of oceanic water pH. However, the physico-chemical mechanisms underpinning this approach are still poorly known. In the present study, we theoretically determine the equilibrium isotopic fractionation properties of structural boron species in calcium carbonates, BO33-, BO2(OH)2- and B(OH)4- anions substituted for carbonate groups, as well as those of B(OH)4- and B(OH)3 species in vacuum. Significant variability of equilibrium isotopic fractionation properties is observed among these structural species which is related to their contrasted coordination state, B-O bond lengths and atomic-scale environment. The isotopic composition of structural boron does not only depend on its coordination number but also on its medium range environment, i.e. farther than its first coordination shell. The isotopic fractionation between aqueous species and their counterparts in vacuum are assessed using previous investigations based on similar quantum-mechanical modeling approaches. At 300K, the equilibrium isotope composition of structural trigonal species is 7 to 15 ‰ lighter than that of aqueous boric acid molecules, whereas substituted tetrahedral borate ions are heavier than their aqueous counterparts by 10 to 13 ‰. Although significant uncertainties are known to affect the theoretical prediction of fractionation factors between solids and solutions, the usually assumed lack of isotopic fractionation during borate incorporation in carbonates is challenged by these theoretical results. The present theoretical equilibrium fractionation factors between structural boron and aqueous species differ from those inferred from experiments which may indicate that isotopic equilibrium, unlike chemical equilibrium, was not reached in most experiments. Further research into the isotopic fractionation processes at the interface between calcium carbonates and aqueous solution as well as long duration experiments aimed at investigating the kinetics of equilibration of boron environment and isotopic composition are therefore required to refine our understanding of boron coprecipitation in carbonates and thus the theory behind the use of boron isotopes as an ocean pH proxy

Experimental and theoretical modelling of kinetic and equilibrium Ba isotope fractionation during calcite and aragonite precipitation

“…This structural change must occur without further exchange with Ba in solution as the experiments do not document significant Ba isotope fractionation between calcite and the fluid. Such a mechanism has already been invoked to explain the isotopic composition of boron (Sen et al, 1994;Noireaux et al, 2015;Balan et al, 2018) and zinc (Mavromatis et al, 2019) coprecipitated with calcite.…”

First-principles modeling of chlorine isotope fractionation between chloride-bearing molecules and minerals

“…Previous studies have shown that theoretical fractionation factors computed from first-principles within density functional theory (DFT) provide useful information on isotopic systems that are difficult to address on a purely experimental basis because they involve, e.g., uncommon isotopic effects (Schauble et al, 2006), slow chemical reactions (Méheut et al, 2007), weakly fractionating isotopes (Blanchard et al, 2009;Moynier et al, 2011;Blanchard et al, 2017), incorporation of minor or trace elements in minerals (Rustad and Zarzycki, 2008;Balan et al, 2018) or high-temperature reactions between solids and dilute gas (Javoy et al, 2012). In the present work, we apply this approach to chlorine isotopes by providing a set of theoretical fractionation factors between selected molecules (Table 1) and crystalline solids (Table 2) in which chlorine occurs as a major or a trace element, all systems being treated at the same theoretical level.…”

Reviews and syntheses: Revisiting the boron systematics of aragonite and their application to coral calcification

“…Quantifying the carbonate chemistry of the fluid from which corals accrete their skeletons is essential for understanding the mechanisms of skeletal growth and the sensitivity of skeletal composition to environmental variability. It is generally thought that corals precipitate aragonite (CaCO 3 ) crystals within an extracellular fluid-filled space between the living polyp and the skeleton (Barnes, 1970). Evidence from skeletal geochemistry and fluorescent dye experiments suggests that while seawater is the initial source of the calcifying fluid (McConnaughey, 1989;Adkins et al, 2003;Cohen and McConnaughey, 2003;Gagnon et al, 2012;Tambutté et al, 2012), the carbonate chemistry of the calcifying fluid is subject to substantial modifications (i.e., pH and dissolved inorganic carbon, or DIC) that enhance the rapid nucleation and growth of aragonite crystals (Al-Horani et al, 2003;Venn et al, 2011).…”