Experiments of boron incorporated into Mg(OH)2 from magnesium-free synthetic seawater were carried out at various pH values, in order to investigate the adsorption species and the variation of isotopic fractionation of boron on Mg(OH)2. The results showed that the incorporation of boron into Mg(OH)2 was very rapid and reached the equilibrium after 4 h. The [B]s and the partition coefficient Kd between Mg(OH)2 and final solution decreased with the increasing pH. The maximum values of [B]s and Kd were much higher than that of boron adsorbed on metal oxide or clay minerals, indicating that the incorporation capability of boron into Mg(OH)2 was very strong. When the adsorption reached the equilibrium, the δ 11Bfsw was lower than δ 11Bisw. The boron isotopic fractionation αs-fsw was between 1.0186 and 1.0220 with an average of 1.0203. All these indicated that 11B incorporated into Mg(OH)2 preferentially due to B(OH)3 incorporation into Mg(OH)2 preferentially. The deposition reaction of B(OH)3 with Mg(OH)2 was the direct reason for B(OH)3 incorporation into Mg(OH)2. During the boron incorporation into Mg(OH)2, the isotopic fractionation characteristic of boron was decided by the simultaneous existence of adsorption of boron on Mg(OH)2 and the deposition reaction of H3BO3 with Mg(OH)2. Different from the fact that only B(OH)4-species incorporated into bio-carbonate, B(OH)3 and B(OH)4 incorporated into Mg(OH)2 simultaneously, and B(OH)3 incorporated into it preferentially. The lower pH is, the more incorporated fraction of B(OH)3 will be. Mg(OH)2 exists widely in madrepore, which influences the quantitative correspondence of the boron isotopic composition δ 11Bcarb of corals on the pH of the seawater badly, and brings serious uncertainty to the δ 11Bcarb as the indicator of the ancient seawater pH.
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 (αcarb-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 collabo-rators show that the αcarb-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 (δ11Bcarb) do not exactly lie on the best-fit theoretical δ 11B4-pH curves and neither do they exactly parallel any theoretical δ 11B4-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 sea-water from boron isotopes in marine biogenic carbonates, the use of theoretical boron isotopic frac-tionation factor (α4-3) between B(OH)4- and B(OH)3 is not suitable. Instead, an empirical equation should be established.
XIAO YingKai1,2, LI HuaLing1,3, LIU WeiGuo4, WANG XiuFang1 & JIANG ShaoYong5 1 Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China
