Refining the early Cambrian marine redox profile by using pyrite sulfur and iron isotopes
Liu, Yarong1,2,3; Ding, Weiming2; Lang, Xianguo4; Xing, Chaochao2; Wang, Ruimin2; Huang, Kangjun5; Fu, Bin6; Ma, Haoran7; Peng, Yongbo7; Shen, Bing2

It is widely accepted that the emergence of metazoans in the Ediacaran-Cambrian transition might be triggered by oxygenation of the atmosphere-ocean system. However, some Ediacaran and early Cambrian macroscopic fossils (such as multicellular algae) were preserved under sulfidic conditions, challenging the conventional view of the prohibition of eukaryotes in the presence of H2S. It was proposed that organisms might have lived in some episodic non-sulfidic intervals although the ocean was predominantly sulfidic. However, the hypothesized shortlived non-sulfidic intervals were hardly identified by traditional bulk sample analyses, which cannot provide sufficiently high temporal resolution to resolve the short-term oxygenation events. In this study, we analyzed pyritized sponge spicules and disseminated pyrites from black shales of the early Cambrian Shuijingtuo Formation in South China. We conducted in situ pyrite sulfur (delta S-34(py)) and iron (delta 56Fepy) isotopes as well as bulk sample pyrite sulfur isotope analyses. The intra-crystal or intra-spicule delta S-34(py) variations reflect the evolution of seawater and porewater geochemistry during pyrite formation, providing a higher temporal resolution of redox fluctuation in the early Cambrian ocean. Our study indicates: (1) The bulk sample pyrite sulfur isotope and pyrite content data suggest diagenetic pyrite precipitation in sediment porewater with H2S diffusion from the overlying sulfidic seawater; (2) Limited intra-crystal and intra-spicule delta S-34(py) variations imply that both spicule and disseminate pyrites had a homogenous S source from sulfidic seawater, and dissimilatory sulfate reduction (DSR) in sediment porewater was negligible; (3) The in situ iron isotope analyses indicate active dissimilatory iron reduction (DIR) in sediment that generated excessive ferrous iron (Fe2+) to compensate for Fe2+ consumption by pyrite precipitation, allowing Fe2+ to accumulate in ferruginous porewater. Therefore, the early Cambrian ocean was characterized by active DSR in sulfidic seawater and DIR in ferruginous sediment porewater. In addition, our study demonstrates that the seawater might be persistently sulfidic, and does not support the presence of intermittent non-sulfidic intervals for the survival of organisms. If correct, it is hypothesized that some primitive sponges might have survived in sulfidic seawater, although the seawater H2S concentration remains unknown.

KeywordDissimilatory sulfate reduction (DSR) Dissimilatory iron reduction (DIR) Sponge spicule South China Shuijingtuo Formation
Indexed BySCI
Funding ProjectNational Natural Science Foundation of China[42072334] ; National Natural Science Foundation of China[41973008] ; National Natural Science Foundation of China[41772359]
WOS Research AreaPhysical Geography ; Geology
WOS SubjectGeography, Physical ; Geosciences, Multidisciplinary
WOS IDWOS:000799367300002
Funding OrganizationNational Natural Science Foundation of China
Document Type期刊论文
Corresponding AuthorShen, Bing
Affiliation1.Chinese Acad Sci, Nanjing Inst Geol & Palaeontol, Ctr Excellence Life & Palaeoenvironment, State Key Lab Palaeobiol & Stratig, Nanjing 210008, Peoples R China
2.Peking Univ, Sch Earth & Space Sci, Key Lab Orogen Belts & Crustal Evolut, MOE, Beijing 100871, Peoples R China
3.Univ Chinese Acad Sci, Beijing 100049, Peoples R China
4.Chengdu Univ Technol, Inst Sedimentary Geol, Chengdu 610059, Peoples R China
5.Northwest Univ, Dept Geol, State Key Lab Continental Dynam, Shanxi Key Lab Early Life & Environm, Xi'an, Peoples R China
6.Australian Natl Univ, Res Sch Earth Sci, Canberra, Australia
7.Nanjing Univ, Int Ctr Isotope Effect Res, Nanjing 210023, Peoples R China
Recommended Citation
GB/T 7714
Liu, Yarong,Ding, Weiming,Lang, Xianguo,et al. Refining the early Cambrian marine redox profile by using pyrite sulfur and iron isotopes[J]. GLOBAL AND PLANETARY CHANGE,2022,213:13.
APA Liu, Yarong.,Ding, Weiming.,Lang, Xianguo.,Xing, Chaochao.,Wang, Ruimin.,...&Shen, Bing.(2022).Refining the early Cambrian marine redox profile by using pyrite sulfur and iron isotopes.GLOBAL AND PLANETARY CHANGE,213,13.
MLA Liu, Yarong,et al."Refining the early Cambrian marine redox profile by using pyrite sulfur and iron isotopes".GLOBAL AND PLANETARY CHANGE 213(2022):13.
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