Amongst multiple geochemical redox indexes, iron-speciation chemistry (FeHR/FeT, Fepy/FeHR) in shales constitutes a reliable proxy to reconstruct the first-order redox framework of the ocean basin. However, iron-speciation is a local marine redox proxy, and only compiled data collected from multiple different sedimentary facies record statistically significant changes of oxidation state within a depositional basin.
Recently, Master's degree student SONG Chenran, Senior Engineers GUAN Chengguo, and Prof. WANG Wei, from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS), and other co-authors, compiled iron speciation data from three newly-reported sections (two outcrops and one drill-core) from the Lower Yangtze block, as well as eleven previously reported sections from the Upper and Middle Yangtze blocks. This study results have been published on line in Precambrian Research.
The compiled sections display iron speciation composition in black shales of the Ediacaran Doushantuo Formation from shallow-, slope-, and deep-water facies. Distribution of RSEs (Mo, V and U) and TOC (total organic carbon) whose enrichments are closely linked to the redox states is also reported in present study. The obtained data provide a robust compilation of change in redox states in the early to middle Ediacaran ocean (635？551 Ma).
Three new sections including one drill-core and two outcrops are reported in the lower Yangtze block. High-resolution iron speciation analysis on the fossiliferous Lantian drill-core in the slope facies demonstrates staged euxinic and oxic conditions. Outcrop samples yield relatively high total iron but low pyrite contents, showing ferruginous conditions. Pervasive anoxia is dominated in Lower Yangtze block of the early Ediacaran period, basically showing overall ferruginous conditions sandwiched by an unstable euxinic zone.
Iron speciation data of fourteen Ediacaran sections from shallow-, slope-, and deep-water facies of the Yangtze block are compiled to depict overall distribution of redox conditions in the early to middle Ediacaran ocean. The statistical results show a widespread basin-scale ferruginous state but a dominated euxinic zone on lower slope facies in the early Ediacaran ocean. On the contrary, extensive euxinic conditions in both the slope and basinal facies are identified in the middle Ediacaran ocean, probably due to increased continental oxidant inputs associated with a more oxidative Earth surface environment.
Compilation of RSEs data including new data from the Lower Yangtze block displays generally low concentrations that approach their upper crust contents in Member II shales during the early Ediacaran period, but relatively high levels in Member IV shales during the middle Ediacaran period. The change of the RSEs enrichment patterns may indicate the rising seawater RSEs reservoir associated with propulsive oxygenation of the Earth surface during early to middle Ediacaran period. Spatial distribution of Mo and V contents show apparent correlation with the distribution of euxinic environment.
Distribution pattern of TOC contents based on the compiled data demonstrates their significant relationship with spatial distribution of the euxinic environment. Particularly, the Member IV shales with more euxinic states contain higher TOC contents. Sulfurization process likely plays a favorable role in burial and preservation of the organic matters in the Doushantuo shales. The euxinic states probably facilitates the exceptional preservation of the soft-bodied Ediacaran fossils.
Reference:: Song Chenran, Guan Chengguo, Wang Wei*, Claeys Philippe, Zhou Chuanming, Wan Bin, Xue Naihua, Hu Yongliang, Pang Ke, Chen Zhe, Yuan Xunlai, 2024. Statistical estimation of the early to middle Ediacaran ocean redox architecture in the Yangtze block of South China. Precambrian Research. 410: 107483. https://doi.org/10.1016/j.precamres.2024.107483.
Integrated iron speciation profiles of the studied sections
Integrated redox-sensitive elements (RSEs) profiles of the studied section
Planar spatial variation of the average data of the iron speciation, RSEs and TOC contents in the shales of the Doushantuo Formation
Contact:
LIU Yun, Propagandist
Email: yunliu@nigpas.ac.cn
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences
Nanjing, Jiangsu 210008, China
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