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

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.