[B-CG06] 地球史解読：冥王代から現代まで

[BCG06-09] Estimated fractionation factors for microbial sulfate reduction throughout the Proterozoic using multiple sulfur isotope record

★Invited Papers

*栗原 在¹、佐藤 友彦²、澤木 佑介³、Moussavou Mathieu⁴、上野 雄一郎^1,5,6 (1.東京工業大学大学院地球惑星科学専攻、2.岡山理科大学基盤教育センター、3.東京大学大学院総合文化研究科、4.マスク科学技術大学、5.東京工業大学地球生命研究所、6.海洋研究開発機構)

キーワード：微生物硫酸還元、硫黄同位体、硫黄循環、原生代

Sulfur isotopes have been used to study the sulfur cycle throughout Earth's history. Significant isotopic fractionation occurs during microbial sulfate reduction (MSR) into sulfide. Microbial sulfur isotope fractionation is known to increase when grown under higher sulfate concentration. Therefore, it is believed that the microbial fractionation has increased through the Earth’s history in response to global oxygenation. However, the actual fractionation factors for MSR in the Precambrian period has not been estimated precisely due to the lack of geological records of sulfate and the ambiguous interpretation of the large isotopic variations observed in sedimentary sulfides. Here, we report multiple sulfur isotope analysis for various forms of pyrites from the Paleoproterozoic Francevillian succession in Gabon, and re-evaluated previously-reported data for the other Proterozoic strata from 2300 Ma to 635 Ma. Most of the samples exhibit a clear negative correlation between δ³⁴S and Δ³³S values. The observed δ³⁴S-Δ³³S trends in each sample can be explained by the Rayleigh distillation model, which indicates that the pyrites formed within a sediment via MSR. Using the observed δ³⁴S-Δ³³S relation, we provide a new method to estimate fractionation factors without sulfate isotopes (³⁴ε= ³⁴R_sufide/³⁴R_sufate -1 and ³³λ = ln(³³R_sufide/³³R_sufate) / ln(³⁴R_sufide/³⁴R_sufate)). This new method also allows us to estimate δ³⁴S value of initial sulfate. Using the method, we could calculate fractionation factors throughout the Proterozoic even if sulfate are not available. As a result, the fractionation factor was small (³⁴ε > -20‰; ³³λ < 0.512) from 2300 Ma to 700 Ma, while larger after the Ediacaran (³⁴ε < -35‰; ³³λ > 0.512). The change in ³⁴e may imply the transition from a sulfate-limited environment to an electron donor-limited environment. After the Ediacaran, supply of electron donor may have been limited due to the quantitative consumption of organics by aerobic respirations. Our new approach using multiple sulfur isotopes from various types of pyrites provides key information constraining the sulfur cycle in the Precambrian period.