| 其他摘要 | In recent years, studies on reconstructing the ancient seawater pH using the isotopic composition of boron in bio-carbonates, calculating the content of CO2, and then conferring the change of ancient climate, have already become the hotspot issues in the international isotope geochemistry community, and is called as the δ11B-pH proxy. Whether the δ11Bsw of ancient seawater is constant, what is the theoretic fractionation factor between B(OH)4- and B(OH)3,and whether the δ11Bcarb of carbonate is equal to that of B(OH)4- are the three key problems of above technique. Aimed at these problems, the issues increase gradually.
A series of inorganic calcite precipitation experiments were carried out in order to determine the dependence of boron isotopic composition of calcite on the pH of seawater. The results of them are consistent with their hypothesis, and support the fundamental assumption that the B(OH)4- is the dominant species incorporated into calcite. However Pagani et al. (2005) evaluated the boron isotope-pH proxy synthetically, and pointed out the B(OH)4- may not be the only species incorporated into carbonate, butB(OH)3 may also incorporates into carbonate.Inorganic calcium carbonate precipitation was carried out experimentally and an unusual isotopic fractionation of boron was found by Xiao ea al. (2006). Xiao estimated that the existence of Mg or other microelements were the main reasons of it, and concluded that the B(OH)3 may incorporate into carbonates, and B(OH)3 enriched in Mg(OH)2.
When the former synthetic inorganic calcium carbonate precipitation experiments were carried out, only calcium carbonate was considered and the results did support the fundamental assumption that the B(OH)4- is the only or dominant species incorporated into calcium carbonate. But the natural marine bio-carbonates are not pure calsium carbonate, and Mg, Sr, Fe elements exist in them. The existence of the trace elements may change the behavior of boron and has influence to the boron isotopic fractionation. But the studies of this are very little.
The Mg(OH)2 was found in the most of the scleractinian corals, and whether this can influence the boron isotopic composition of corals and then leads to the error of the δ11B-pH proxy is uncertain.
Aimed at these issues, the species and fractionation mechanism of boron incorporated into Mg(OH)2, the relationship of Mg, Sr concentration with boron concentration and boron isotopic in modern corals have been studied in this paper. At the same time, the deposition environment of terrestrial foraminifer was studied by using boron isotopic of bio-carbonates.
The main conclusions have been summarized as follows:
1. During the deposition of Mg(OH)2 from synthetic seawater at different pH values (9.5～13.0), the 11B of Mg(OH)2 deposit ranges from -1.20‰ to 28.26‰, higher than those of seawater (-7.00±0.07‰). The fractionation factors d/s of them are between 1.0177 and 1.0569 with a mean of 1.0329. This is because H3BO3 incorporates preferentially into Mg(OH)2,and makes 11B be enriched in Mg(OH)2 deposit.
2. The boron concentration of Mg(OH)2 deposit and the partition coefficient Kd between Mg(OH)2 deposit and solution range from 228.61 g/g to 937.79 g/g and from 9.31 to 494.20, respectively. Adsorption is still important to the amount of boron incorporated into Mg(OH)2 at high pH。
3. The incorporation of boron into Mg(OH)2 was very rapid and after 4 h it can reach the equilibrium. The [B]s and the partition coefficient Kd between Mg(OH)2 and final solution decreased with the increasing pH and the decreasing solid to solution ratio. The maximum of [B]s and Kd were much higher than that on metal oxide or clay minerals, indicating that the incorporation capability of boron on Mg(OH)2 was much stronger.
4. When the adsorption reached the equilibrium, all the 11B of solution (-19.2‰～-17.8‰) was lower than that of initial solution (-7.00±0.07‰); The boron isotopic fractionation s-fsw between Mg(OH)2 and final solution was between 1.0186 and 1.0220, with an average of 1.0203. All these indicated that 11B was incorporated into Mg(OH)2 preferentially, caused by B(OH)3 preferential incorporation into Mg(OH)2.
5. 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.
6. The existence of adsorption of boron on Mg(OH)2 and the deposition reaction of H3BO3 with Mg(OH)2 at the same time decide the boron concentration and isotopic fractionation character of it.
7. Ca, Sr, B and Na enriched in the corals while Mg is poor in corals. The boron concentration in corals is not mainly decided by these elements.
8. The δ11Bcarb value of corals ranges from 22.8‰ to 27.9‰, with an average value of 25.2‰. Except boron concentration, the δ11Bcarb has weak correlation with other four elements. The fractionation coefficient carb-sw between corals in Beihai, Dengloujiao and Sanya and seawater is 0.9839, 0.9847and 0.9850, respectively. The fractionation coefficient carb-3 between corals and B(OH)3 of seawater ranges from 0.9772 to 0.9800, with an average value 0.9788. The carb-3 decreases with the increasing pH, which indicates B(OH)3 also incorporates into corals with B(OH)4-. When =0.9772 is used, the average δ11Bcarb basically located above the theretical δ11B4 curve, and lower than the δ11Bcarb,indicating that B(OH)3 and B(OH)4- incorporate into corals simultaneously, and B(OH)4- incorpatates into it preferentially.
9. Because B(OH)4- and B(OH)3 incorpatates into corals simultaneously, the hypothesis of d11Bcarb=d11B4 and the pH forum calculated by the δ11Bcarb will be changed, and the δ11B-pH proxy becomes much more complex.
10. Analogue experiment is diffent from natural case, and can not be used to subtitued for natural case completely.
11. The early Quaternary foraminifers in the Yanghuzhuang section inhabited a nonmarine environment and these foraminifers were not the result of a transgression or sea flooding. The foraminifers were not special halobios and can survive in the terrestrial condition. |
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