[B-CG04] Decoding the history of Earth: From Hadean to the present

[BCG04-12] The micro-scale speciation analyses of sulfur in the Ediacaran limestones: Implications for diagenetic mobilization of sulfur species

★Invited Papers

*Fumihiro Matsuura^1,2, Ryoichi Nakada³, Yusuke Sawaki⁴, Yuichiro Ueno^2,5,6, Masafumi Saitoh⁷, Iori Kajitani¹, Tomohiro Usui^1,2 (1.Institute of Space and Astronautical Science, JAXA, 2.Earth-Life Science Institute, Tokyo Tech, 3.Kochi Institute for Core Sample Research, JAMSTEC, 4.Department of Earth Science and Astronomy Graduate School of Arts and Sciences, The University of Tokyo, 5.Institute for Extra-cutting-edge Science and Technology Avant-garde Research (X-star), JAMSTEC, 6.Department of Earth and Planetary Sciences, School of Science, Tokyo Tech, 7.Institut des sciences de la Terre, University of Lausanne)

Keywords:carbonate associated sulfate, organic sulfur, diagenesis, sulfur cycling

The Ediacaran period was characterized by an increase in atmospheric oxygen concentrations that was critical for the evolution of life. Oceanic sulfate concentration increases with the increase of atmospheric oxygen concentrations, and the oceanic sulfate concentration in paleo-ocean is inferred from sulfur isotopic compositions of sedimentary sulfates and pyrite, and sulfur concentrations of pyrite and carbonate associated sulfate (CAS). Sulfur isotopic composition of sedimentary pyrite has been known to express microbial activities which involve large sulfur isotope fractionation; however, in addition to the microbial processes, environmental factors such as sedimentation rate, redox condition, and iron availability have come to be known to have strong impacts on the isotopic composition and concentration of sedimentary pyrite. Besides, elucidating diagenetic impacts on the isotopic composition and concentration of CAS is necessary to reconstruct the paleo-oceanic sulfate concentration. Here, we conducted μ-XRF and S K-edge XANES analyses of limestones of the Ediacaran Doushantuo and Dengying formations using a drilling core samples collected from Three Gorges area, south China. The results show maximum 1237 ± 315 ppm of abundant reduced organic sulfur species such as thiophenes and thiols were exclusively existing in the Dengying limestones. Given the low pyrite content in the Dengying limestones, lower than 100 ppm, the high organic sulfur contents indicate organic matter outcompeted iron to scavenge sulfide expelled from sulfate reducing microbes in sediments, which force us considering environmental factors as well as microbial process to interpret the oceanic sulfur cycling based on the isotopic composition of pyrite. The diagenetic impact on CAS was quantified by comparing contents of sulfur species in micrite and in sparite. High strontium content (>1%) and existence of celestine (SrSO₄) in sparite indicate the Dengying limestones used in this study were only suffered from marine diagenesis. Our results show the CAS content decreased from 277 ± 288 ppm in micrite to 229 ± 138 ppm in sparite, which suggests CAS concentration decreased even during marine diagenesis in which porewater sulfate was mM level. Thus, we conclude the isotope and concentration of CAS in micrite are the most appropriate to extract the information of paleo-oceanic sulfate. Our micro-scale mapping of sulfur species revealed how oceanic sulfate in paleo-ocean was preserved and subsequently altered in the carbonate sediments, which provided us the ways to extract the most primary information of paleo-ocean from carbonate and interpret the paleo-oceanic sulfur cycling.