[B-CG07] 地球史解読：冥王代から現代まで

[BCG07-P05] Redox change in Paleoproterozoic estimated from multiple sulfur isotope analysis of Francevillian successions in Gabon

*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、2.岡山理科大学、3.東京大学、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：古原生代、大酸化イベント、ロマグンジイベント、硫黄同位体

Immediately after the Great Oxidation Event (GOE), carbonate carbon isotope shows exceptionally high value (d¹³C_carb > +5‰) from 2.2 and 2.0 billion years ago. This isotope excursion was called as Lomagundi-Jatuli Event (LJE). The high d¹³C_carb value during the LJE may reflect an enhanced organic carbon burial, and thus imply high atmospheric oxygen level at that time (Becker and Holland, 2012). However, the ¹³C-enrichment of carbonate may also be originated from local activity of microbial methanogenesis (Cadeau et al., 2020). It is still controversial whether the oxygen level actually increased during the LJE. Here, we report a new multiple sulfur isotope geochemistry of Paleoproterozoic pyrites in >2.0 Ga Francevillian successions, Gabon and discuss redox change at the end of LJE. In particular, marine carbonate occurs in Lastoursville basin, which shows large positive carbonate carbon isotopes anomaly (Bakakas Mayika et al. 2020). Based on petrological observation, the pyrites in the black shales were classified into five types (Type 1: Early nodule, Type 2: Laminated fine grained pyrite, Type 3: Disseminated pyrite, Type 4: Coarse euhedral pyrite, and Type 5: Late nodule), all of which crystalized within sediments at different timings. The sulfur isotope analysis revealed that these pyrites exhibit large variation of d³⁴S values from -17‰ to +42‰. Type 1 showed the lowest d³⁴S values, followed by Type 2 < Type 3 < Type 4 < Type 5. Therefore, it is suggested that pyrite formed early in the diagenesis has low d³⁴S, while those formed later have high d³⁴S. The large isotopic fractionation observed in organic-rich sediments is generally attributed to microbial sulfate reduction bacteria (MSR). When sulfate is reduced by MSR in sediments, the lighter isotopes are selectively converted to sulfides, and thus the remaining sulfate and late-stage pyrites evolve heavier. Furthermore, different types of pyrites showed a clear negative correlation between their d³⁴S and D³³S values within a single rock sample. The observed relationship was analyzed by the Rayleigh distillation model. As a result, the isotope fractionation factor (34-alpha), the mass-dependent exponent (33-lambda) and initial d³⁴S value of porewater sulfate were estimated as 0.978 - 0.992; 0.5065 - 0.5118, and 0‰ - +13‰, respectively. The calculated 34-alpha and 33-lambda values were consistent with previous incubation experiments of MSR (Johnston et al., 2007; Sim et al., 2011a, b). Based on these results, we estimated the temporal change in d³⁴S of pyrite formed in the earliest stage and that of seawater sulfate. Both the d³⁴S values increased by about 5‰ after the end of LJE. This is consistent with the expected change of sulfur isotope ratios when sulfate reservoir is diminished. In other words, the results suggest that oxygen concentrations in the atmosphere-ocean system decreased at the end of the LJE.