Sulfur geochemistry of the Salitre Formation phosphorites: Implications for the role of microbial ecology and sulfur cycling in phosphogenesis on an Ediacaran carbonate platform

Creators: Sanders, Cecilia¹; Present, Theodore²; Marroquin, Selva²; Grotzinger, John P.²

1. Johns Hopkins University
2. California Institute of Technology

Abstract

The distribution and activity of polyphosphate-accumulating sulfide oxidizing bacteria within marine sediments control the spatial distribution of sedimentary phosphorite formation in the modern ocean. In modern phosphogenetic settings, the concerted effect of microbial sulfide oxidation and microbial sulfate reduction in sediment pore waters is preserved in the sulfur isotope composition of trace sulfate in authigenic and early-stage diagenetic carbonate cements and phosphatic cements, as well as that of authigenic pyrite. If such variations in microbial sulfur metabolism controlled the spatial distribution of early phosphate mineralization in the geologic past, then one would expect to find differences between the sulfur isotope composition of cement-forming minerals in phosphatic and non-phosphatic facies of ancient sedimentary phosphate deposits. Here, we present paired measurements of the sulfur isotope composition of structural sulfate and structural sulfide in pore cements from co-occurring phosphatic and non-phosphatic facies of the Ediacaran Salitre Formation (Northeastern Brazil). The difference between δ³⁴S of trace structural sulfate in carbonate cements (CAS) or phosphate cements (PAS) and δ³⁴S of structural sulfide in pyrite or chromium-reducible sulfur (CRS), defined as Δ³⁴S, provides a constraint on sulfur cycling within the pore waters from which the cements precipitated. In carbonate-cemented textures of the Salitre Formation, Δ³⁴S = 12 to 31 ‰. In phosphate-cemented textures, however, PAS and pyrite crystals had more similar sulfur isotope compositions: Δ³⁴S = −5–12 ‰. These values support the hypothesis that microbial sulfide oxidation was more prevalent in pore waters where phosphate mineralization occurred compared to pore waters where carbonate mineralization occurred – suggesting that microbial sulfide oxidation may have enabled the formation of sedimentary phosphorite on an Ediacaran carbonate platform, just as in modern phosphogenetic sites.

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Acknowledgement

The authors extend their gratitude to Wu Fenfang, Guan Yunbin, Grecia “Grace” Ames, and Antoine Cremiere of Caltech’s Division of Geological and Planetary Sciences, for the gift of their expertise, and their patience, as we learned new techniques on new analytical instruments.

We also express our gratitude for the friendship and collaboration with the students, professors, and researchers of the Laboratório de Análises de Minerais e Rochas (LAMIR) e Laboratório de Análises de Bacias (LABAP) da Universidade Federal do Paraná, as well as our collaborators from the Universidade Federal do Bahia, without whom we could not have collected our samples and immersed ourselves in the geology and culture of the studied region. We are also wholly indebted to the farmers, miners, educators, and conservationists of the Chapada Diamantina and Cavernas do Peruaçu regions for their guidance and expertise.

We also wish to thank the reviewers and editors for helpful, insightful comments. We also acknowledge that this work was supported primarily by the National Science Foundation Graduate Research Fellowship Program (NSF GRFP), and the Simons Collaboration on the Origins of Life (SCOL).

Lastly, we humbly acknowledge the Xakriabá, the communities of and surrounding the historic Quilombo da Gruta dos Brejões, as well as other indigenous peoples whose names and identities may have been lost to colonialism and occupation, on whose land this work relies.

Contributions

Cecilia Sanders: Writing – review & editing, Writing – original draft, Visualization, Project administration, Methodology, Investigation, Formal analysis, Conceptualization. Theodore Present: Writing – review & editing, Supervision, Methodology, Formal analysis, Data curation. Selva Marroquin: Writing – review & editing, Supervision. John Grotzinger: Supervision, Resources, Funding acquisition.

Data Availability

Data are available through Mendeley Data at https://doi.org/10.17632/2bf8vmrgw8.1. Raw, unprocessed data are available upon request submitted to the corresponding author.

Supplemental Material

Supplementary data (PDF)

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