Boron isotopic fractionation in laboratory inorganic carbonate precipitation: evidence for the incorporation of B(OH)3 into carbonate

Abstract

A laboratory inorganic carbonate precipitation experiment at high pH of 8.96 to 9.34 was conducted, and the boron isotopic fractionations of the precipitated carbonate were measured. The data show that boron isotopic fractionation factors (α carb-3 ) between carbonate and B(OH) 3 in seawater range 0.937 and 0.965, with an average value of 0.953. Our results together with those reported by Sanyal and collaborators show that the α carb-3 values between carbonate and B(OH) 3 in solution are not constant but are negatively correlated with the pH of seawater. The measured boron isotopic compositions of carbonate precipitation (δ 11 B carb ) do not exactly lie on the best-fit theoretical δ 11 B 4 -pH curves and neither do they exactly parallel any theoretical δ 11 B 4 -pH curves. Therefore, it is reasonable to argue that a changeable proportion of B(OH) 3 with pH of seawater should also be incorporated into carbonate except for the dominant incorporation of B(OH) 4 − in carbonate . Hence, in the reconstruction of the paleo-pH of seawater from boron isotopes in marine biogenic carbonates, the use of theoretical boron isotopic fractionation factor (α 4-3 ) between B(OH) 4 − and B(OH) 3 is not suitable. Instead, an empirical equation should be established. boron, isotopic fractionation, biogenic carbonate, B(OH) 3 incorporation Naturally occurring boron has two stable isotopes, 11 B and 10 B, which have average relative abundances of 80.17% and 19.83%, respectively. Boron is dissolvable element and can participate widely in various chemical, physical and geological processes. The isotopic fractionation occurs in these processes. Briefly, there are two dominant species of dissolved boron, B(OH) 3 and B(OH) 4 -, in solution and their relative concentrations are controlled by pH as following relation: B(OH) 3 (B 3 )+ H 2 O→ B(OH) 4 − (B 4 )+ H + (1) Boron has no variety of valences and isotopic fractionation of boron takes place between B 3 and B 4 − , which have different reduced partition function ratios. The light isotope 10 B is enriched in B(OH) 4 − compared to B(OH) 3 [1-3]. The characteristic of boron isotopic fractionation has been proverbially applied in crust-mantle evolution and subduction-related processes, sedimentary environments

Boron isotopes as pH proxy: A new look at boron speciation in deep-sea corals using 11B MAS NMR and EELS

“…Indeed, one of the key assumptions when using d 11 B as a pH proxy is that only BðOHÞ À 4 is incorporated in carbonates (Hemming and Hanson, 1992). The presence of B3 has already been observed in other biogenic carbonates using the same technique (Klochko et al, 2009) or has been suggested from inorganic precipitation of carbonate in laboratory experiments (Xiao et al, 2008). Nevertheless, none of these earlier studies have d 11 B values associated with the B3/B4 ratio, and furthermore in this study we are able to associate these two parameters with the microstructure of the coral.…”

Factors affecting B/Ca ratios in synthetic aragonite

“…In addition, there are suggestions that B(OH) 3 may also be incorporated, especially in calcite (e.g. Xiao et al, 2008;Klochko et al, 2009;Rollion-Bard et al, 2011;Mavromatis et al, 2015;Uchikawa et al, 2015), thus potentially complicating the interpretation of B/Ca ratios.…”

“…Laboratory studies on the incorporation of B into calcium carbonate remain limited, and few potentially controlling factors (e.g. temperature, carbonate chemistry, growth rate etc) have been tested (Kitano et al, 1978;Sen et al 1994;Hemming et al, 1995;Hobbs and Reardon, 1999;Sanyal et al, 2000;Xiao et al, 2008;He et al, 2013;Gabitov et al, 2014;Mavromatis et al, 2015;Uchikawa et al, 2015). Critically only one study has characterized the carbonate chemistry during aragonite precipitation (Mavromatis et al, 2015).…”

Characterization of boron incorporation and speciation in calcite and aragonite from co-precipitation experiments under controlled pH, temperature and precipitation rate

“…Until now, very little experimental work has been done to evaluate boron partition in calcium carbonates (Kitano et al, 1978;Hemming et al, 1995;Sanyal et al, 2000;Xiao et al, 2006Xiao et al, , 2008He et al, 2013). Most of these studies have not examined the effect of the solution pH and the carbonate precipitation rate.…”