Redox fluctuation and δ¹³C_org-δ³⁴S perturbations recorded in the 1.9 Ga Nuvilik Formation of the Cape Smith belt, Canada

Earth's surface environment underwent the first major step-change in atmospheric oxygen levels during the Paleoproterozoic Great Oxidation and Lomagundi–Jatuli events. A decrease in atmospheric oxygen levels after the Lomagundi–Jatuli Event has previously been suggested, but details of the environmental transitions at 2.0–1.8 Ga are still debated. Here we report major- and trace-element compositions and δ¹³C_org and δ³⁴S values for shale samples from a 50-m-thick section of drill-core from the ca. 1.9 Ga Nuvilik Formation of the uppermost Povungnituk Group, Cape Smith belt, Canada. The section comprises a turbiditic sedimentary sequence of well-bedded sandstone–shale alternations accumulated on the continental margin of the Archean Superior Craton. Stratigraphically, the section comprises 4-m-thick graded beds, a fining-upward sequence of predominantly coarse-sandstone to black-shale alternations, and a fine-grained sandstone-dominated sequence. Shale samples in the studied section contain quartz, white mica, pyrite, and pyrrhotite. Mo and U enrichment factors (Mo_EF and U_EF) display increasing trends in the lower part of the section, reaching values of 11.9 and 2.4, respectively, suggesting development of sulfidic (euxinic) benthic water. Subsequent decreases in Mo_EF and U_EF values represent restoration to suboxic states. This redox fluctuation is accompanied by δ¹³C_org and δ³⁴S perturbations. Concurrent decreases of δ¹³C_org (−30.6‰ to −33.2‰) and δ³⁴S values (+11‰ to −5‰) suggest isotopically light inorganic carbon and sulfur were supplied to the ocean–atmosphere system. δ³⁴S values increased to +14‰ after the decrease, interpreted as being due to isotopic distillation through enhanced microbial sulfate reduction. The δ¹³C_org and δ³⁴S perturbations and the overlying ultramafic volcanic sequence of the Chukotat Group (part of the Circum-Superior large igneous province; LIP) suggest that the redox fluctuation recorded in the Nuvilik Formation might be related to the onset of LIP magmatism.