Japan Geoscience Union Meeting 2023

Presentation information

[J] Online Poster

B (Biogeosciences ) » B-CG Complex & General

[B-CG07] Decoding the history of Earth: From Hadean to the present

Fri. May 26, 2023 10:45 AM - 12:15 PM Online Poster Zoom Room (20) (Online Poster)

convener:Tsuyoshi Komiya(Department of Earth Science & Astronomy Graduate School of Arts and Sciences The University of Tokyo), Yasuhiro Kato(Department of Systems Innovation, Graduate School of Engineering, University of Tokyo), Katsuhiko Suzuki(Submarine Resources Research Center, Japan Agency for Marine-Earth Science and Technology), Kentaro Nakamura(Department of Systems Innovation, School of Engineering, University of Tokyo)


On-site poster schedule(2023/5/25 17:15-18:45)

10:45 AM - 12:15 PM

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

*Aru Kurihara1, Tomohiko Sato2, Yusuke Sawaki3, Mathieu Moussavou4, Yuichiro Ueno1,5,6 (1.Tokyo Institute of Technology, 2.Okayama University of Science, 3.The University of Tokyo, 4.University of Sciences and Technology in Masuku, 5.Earth-Life Science Institute, 6.Japan Agency for Marine-Earth Science and Technology)


Keywords:Paleoproterozoic, GOE, LJE, sulfur isotope

Immediately after the Great Oxidation Event (GOE), carbonate carbon isotope shows exceptionally high value (d13Ccarb > +5‰) from 2.2 and 2.0 billion years ago. This isotope excursion was called as Lomagundi-Jatuli Event (LJE). The high d13Ccarb 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.