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 δ13Corg 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 (MoEF and UEF) 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 MoEF and UEF values represent restoration to suboxic states. This redox fluctuation is accompanied by δ13Corg and δ34S perturbations. Concurrent decreases of δ13Corg (−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 δ13Corg 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.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology