Iori Kajitani

The Ediacaran period was characterized by numerous events, including the emergence of large multi-cellular metazoans and surface environmental perturbations. This period was also characterized by an increase in atmospheric oxygen concentrations that was critical for the evolution of life. Oceanic sulfate concentrations varied in association with atmospheric oxygen concentrations, which have been constrained by sulfur isotopic compositions of sedimentary sulfates and pyrite, and sulfur concentrations of pyrite and carbonate associated sulfate (CAS). However, other parameters such as sedimentation rate or iron availability have a strong impact on the abundance and isotopic composition of pyrite. In addition, recent studies have demonstrated that some Phanerozoic limestones include a mix of various carbonate minerals, the compositions and diagenetic histories of which differ at the mu m scale. Thus, the mu m scale description of sulfur species is necessary to accurately extract information preserved in carbonate rocks. In this study, we investigated the speciation and concentrations of sulfur in the matrix of the Ediacaran Doushantuo and Dengying limestones exposed in South China using mu-X-ray fluorescence (XRF) mapping and S K-edge X-ray absorption near edge structure (XANES) analyses. In addition to pyrite, the XANES spectra of the Doushantuo limestone indicate that sulfur occurs as CAS, while the Dengying limestone contains CAS and abundant organic sulfur. Pyrite oxidation and re-mineralization of organic sulfur had little influence on CAS content in the samples, whereas sparitization produced decreases in CAS and organic sulfur concentrations. The CAS content of the Dengying sparite was lower than that of the Dengying micrite, indicating that the CAS content decreased even during marine diagenesis. Thus, the micrite is more appropriate for extracting paleo-oceanic information. On the other hand, variations in the CAS concentrations of the limestone matrix in the Dengying Formation were larger than those in the Doushantuo Formation, regardless of grain size. The large variations in the Dengying limestone resulted from local alkalinity fluctuations caused by temporal changes in microbial activity within microbial mats. Existence of abundant organic sulfur in the Dengying limestones has another implication to ancient sedimentary environment. The low pyrite content of the Dengying limestone is likely due to a deficient supply of reactive iron to the sediment-water interface, because the supply of organic matter was likely sufficient. (C) 2021 Elsevier Ltd. All rights reserved.

The aqueous environment of ancient Mars is of significant interest because of evidence suggesting the presence of a large body of liquid water on the surface at ~4 Ga, which differs significantly from the modern dry and oxic Martian environment. In this study, we examined the Fe-bearing minerals in the 4 Ga Martian meteorite, Alan Hills (ALH) 84001, to reveal the ancient aqueous environment present during the formation of this meteorite. Extended X-ray absorption fine structure (EXAFS) analysis was conducted to determine the Fe species in ALH carbonate and silica glass with a high spatial resolution (~1–2 μm). The μ-EXAFS analysis of ALH carbonate showed that the Fe species in the carbonate were dominated by a magnesite-siderite solid solution. Our analysis suggests the presence of smectite group clay in the carbonate, which is consistent with the results of previous thermochemical modeling. We also found serpentine in the silica glass, indicating the decrease of water after the formation of carbonate, at least locally. The possible allochthonous origin of the hematite in the carbonate suggests a patchy redox environment on the ancient Martian surface.

Abstract Understanding the origin of organic material on Mars is a major issue in modern planetary science. Recent robotic exploration of Martian sedimentary rocks and laboratory analyses of Martian meteorites have both reported plausible indigenous organic components. However, little is known about their origin, evolution, and preservation. Here we report that 4-billion-year-old (Ga) carbonates in Martian meteorite, Allan Hills 84001, preserve indigenous nitrogen(N)-bearing organics by developing a new technique for high-spatial resolution in situ N-chemical speciation. The organic materials were synthesized locally and/or delivered meteoritically on Mars during Noachian age. The carbonates, alteration minerals from the Martian near-surface aqueous fluid, trapped and kept the organic materials intact over long geological times. This presence of N-bearing compounds requires abiotic or possibly biotic N-fixation and ammonia storage, suggesting that early Mars had a less oxidizing environment than today.