Thirteen carbonate concretions from the Rove, Thomson and Gunflint Formations of the early Proterozoic (∼ - 2.0 Ga) Animikie Group in the Lake Superior region were analyzed for variations in carbon and oxygen isotopes. The isotope data of ferroan dolomite concretions from the Rove Formation yield trends comparable to the isotope systematics displayed by authigenic carbonate in modern, organic-rich, marine burial environments. The isotope systematics of the ferroan dolomite concretions suggest that carbonate initially precipitated in a zone of bacterial methane production, and later, at greater burial depths, in a zone of thermal decarboxylation. Recrystallization of calcite concretions in the Rove and Thomson Formations has homogenized the oxygen isotopic composition. However, because the carbon system was apparently rock dominated, the primary σ13C signateres have been preserved. Calcite concretions from the Rove Formation display a strong correlation between σ13C weight percent carbonate, and position in the concretion. The data suggest that these concretions formed progressively during burial in the zone of bacterial sulfate reduction. Sulfur isotopic analysis of pyrite disseminated in the concretions yield high, positive σ34S values. Isotopic data for the pyrite and the host concretion are consistent with models that suggest that high positive σ34S values of early Proterozic pyrite result from bacterial reduction of a limited supply of seawater sulfate. The isotopic composition of concretions in the Gunflint Formation suggest early formation in a system open with respect to seawater bicarbonate. The detailed isotopic study of Precambrian carbonate concretions is a useful approach for investigating Precambrian organic matter diagenesis and the evolution of bacteria.
ASJC Scopus subject areas
- Geochemistry and Petrology