An unusual isotopic fractionation of boron in synthetic calcium carbonate precipitated from seawater and saline water

Abstract

Inorganic calcium carbonate precipitation from natural seawater and saline water at various pH values was carried out experimentally. The results show the clear positive relationships between boron concentration and δ 11 B of inorganic calcium carbonate with the pH of natural seawater and saline water. However, the variations of boron isotopic fractionation between inorganic calcite and seawater/saline water with pH are inconsistent with the hypothesis that B(OH) 4 − is the dominant species incorporated into the biogenic calcite structure. The isotopic fractionation factors α between synthetic calcium carbonate precipitate and parent solutions increase systematically as pH increases, from 0.9884 at pH 7.60 to 1.0072 at pH 8.60 for seawater and from 0.9826 at pH 7.60 to 1.0178 at pH 8.75 for saline water. An unusual boron isotopic fractionation factor of larger than 1 in synthetic calcium carbonate precipitated from seawater/saline water at higher pH is observed, which implies that a substantial amount of the isotopically heavier B(OH) 3 species must be incorporated preferentially into synthetic inorganic carbonate. The results propose that the incorporation of B(OH) 3 is attributed to the formation of Mg(OH) 2 at higher pH of calcifying microenvironment during the synthetic calcium carbonate precipitation. The preliminary experiment of Mg(OH) 2 precipitated from artificial seawater shows that heavier 11 B is enriched in Mg(OH) 2 precipitation, which suggests that isotopically heavier B(OH) 3 species incorporated preferentially into Mg(O This result cannot be applied to explain the boron isotopic fractionation of marine bio-carbonate because of the possibility that the unusual environment in this study appears in formation of marine bio-carbonate is infinitesimal. We, however, must pay more attention to this phenomenon observed in this study, which accidentally appears in especially natural environment.Keywords: boron, isotopic fractionation, calcium carbonate precipitate, seawater, saline water.Many investigations focused on boron incorporation into marine organisms by co-precipitation with biogenic carbonate minerals, attempting to reconstruct the past oceanic pH using the isotopic composition of boron in coral and foraminifera [1][2][3][4][5][6][7][8][9][10] . Spivack et al. [3] reported that 21 Ma ago, the surface ocean pH was only 7.4 ± 0.2, which then increased to 8.2 ± 0.2 about 7.5 Ma ago. Sanyal et al. [5] estimated that the deep An unusual isotopic fractionation of boron in synthetic calcium carbonate precipitated from seawater & saline water 455 Atlantic and Pacific oceans had a pH 0.3±0.1 unit higher during the last glaciation. Pearson and Palmer [9] found that the pH was significantly lower during early Palaeogene than in the Neogene period. The application of boron isotope studies in marine biogenic carbonate to the reconstruction of paleo-pH of seawater relies on three hypotheses as follows: (i) A large isotopic fractionation between the two aqueous species, B(OH) 3 and B(OH) 4 − of boron in seawate...

Boron isotope fractionation during brucite deposition from artificial seawater

“…They reasoned that the presence of Mg 2+ or other microelements was the main reason for this observation and concluded that the heavier B(OH) 3 species was incorporated preferentially into Mg(OH) 2 . The results of our study have further confirmed the conclusion of Xiao et al (2006a).…”

“…The length of the resin bed was 1.5 cm. About 200 mg of brucite was first dissolved using a slightly excessive amount of boron-free HCl solution, passed through a column filled with Amberlite IRA 743 resin, and then eluted using approximately 5 ml of 0.1 M HCl at 75 • C. An amount of mannitol approximately equimolar to the boron content was added to the eluate, which was then desiccated to near dryness by partial evaporation in a super clean oven with laminar flow at 60 • C (Xiao et al, 2003). The solution was then loaded again into the column filled with a mixed resin composed of 0.5 ml cation-exchange resin (H + form) and 0.5 ml anion-exchange resin (ion-exchange II, HCO − 3 form).…”

“…Our results indicate that B(OH) 3 is preferentially incorporated into the solid phase during the deposition of brucite. In a precipitated experiment of calcium carbonates from artificial seawater, Xiao et al (2006a) obtained the result that the boron isotopic fractionation factors between calcium carbonates and seawaters were higher than 1. They reasoned that the presence of Mg 2+ or other microelements was the main reason for this observation and concluded that the heavier B(OH) 3 species was incorporated preferentially into Mg(OH) 2 .…”

“…They suggested that careful sampling is necessary before performing boron isotopic measurements in deep-sea corals (Rollion-Bard et al, 2011). Inorganic calcite precipitation experiment has been carried out by Xiao et al (2006a), indicating that the δ 11 B of inorganic calcium carbonate was not parallel with the calculated curve of B(OH) − 4 , but deviated increasingly from the parallel trend as pH increased. When the pH was increased to a certain value, the isotopic fractionation factor of boron between precipitation and solution was greater than 1.…”

“…When the pH was increased to a certain value, the isotopic fractionation factor of boron between precipitation and solution was greater than 1. Xiao et al (2006a) reasoned that the presence of Mg 2+ or other microelements was the main reason for this observation and concluded that B(OH) 3 incorporated preferentially into brucite. If this is true, the isotopic compositions of boron in corals can be affected by the existence of brucite in corals and the preferential incorporation of B(OH) 3 into brucite.…”