Geochemical results from upper Ediacaran organic-rich shales / mudstones, Yangtze Craton, South China
These data comprise geochemical results from upper Ediacaran organic-rich shales / mudstones from the Yangtze Craton (South China) from the Doushantuo Formation (member IV) and overlying lower Dengying Formation (= Miaohe Member). Ages range from about 570 to 550 Ma. The data consist of iron partitioning (Fe-speciation), phosphorus partitioning (P-speciation), bulk rock metal concentrations, total organic content and pyrite sulfur isotopes. Major and trace element concentrations of all samples were measured by inductively coupled plasma optical emission spectrometry and mass spectrometry, respectively, after quantitative HNO3-HFHClO4 digestion. Fe speciation was performed after the established methodology of Poulton and Canfield to extract operationally defined Fe phases, including Fe associated with carbonates (Fecarb), ferric oxides (Feox), magnetite (Femag), and pyrite (Fepy). The sum of these Fe pools constitutes the proportion of Fe that is considered highly reactive (FeHR) toward dissolved sulfide. Ratios of FeHR/FeT > 0.38 support FeHR enrichment and deposition under anoxic bottom water conditions, whereas values of <0.22 indicate deposition from oxic bottom waters. The intermediate range of 0.22 to 0.38 is regarded as equivocal because of the possibility for rapid sediment deposition or early diagenetic transformation of unsulfidized FeHR to poorly reactive sheet silicate minerals. For anoxic samples (FeHR/FeT > 0.38), the degree of sulfidation of the FeHR pool can be used to distinguish ferruginous (Fepy/FeHR <0.7) from euxinic (Fepy/FeHR > 0.8) conditions, with an intermediate zone ascribed to “possible euxinia”. Recent analyses of Holocene sapropels and the euxinic Lake Cadagno indicate that Fepy/FeHR > 0.6 may be a more suitable threshold for distinguishing ferruginous from possible euxinic conditions. Selected shale samples were analyzed for pyrite sulfur isotopes (δ34Spy), TOC, organic carbon isotopes (δ13Corg), and P phase associations. The P measurements use a sequential extraction method to distinguish the proportion of total P (PTot) associated with detrital apatite (Pdet) relative to potentially bioavailable and reactive (Preac) minerals, including Fe (oxyhydr)oxides (PFe),organic matter (Porg), and authigenic carbonate fluorapatite, biogenic apatite, and CaCO3-bound P (Pauth). For methods, see: S. W. Poulton, D. E. Canfield, Development of a sequential extraction procedure for iron: Implications for iron partitioning in continentally derived particulates. Chem. Geol. 214, 209–221 (2005). Fred T. Bowyer, Alexander J. Krause, Yafang Song, Kang-Jun Huang, Yong Fu, Bing Shen, Jin Li, Xiang-Kun Zhu, Michael A. Kipp, Lennart M. van Maldegem, Jochen J. Brocks, Graham A. Shields, Guillaume Le Hir, Benjamin J. W. Mills, and Simon W. Poulton Biological diversification linked to environmental stabilization following the Sturtian Snowball glaciation. Sci. Adv. 9 (34), eadf9999. DOI: 10.1126/sciadv.adf9999 (2023)