The Paleoproterozoic Animikie Basin in the Lake Superior region of North America is the benchmark for understanding one of the most significant changes in seawater chemistry in Earth history—the transition to a sulfidic ocean (ca. 1.84 Ga). This transition marks a dramatic increase in ocean sulfide produced by bacterial reduction of sulfate formed through the oxic chemical weathering of pyrite during the Great Oxidation Event. Such a change is thought to have caused the cessation of widespread iron formation deposition and marks the beginning of a long lull in the evolution of eukaryotes. The feasibility of using the Animikie Basin to understand this transition is re-evaluated by comparing new sulfur isotope data (δ34Spyrite) from the Michigamme Formation in northern Michigan, United States, to published results from the correlative Rove Formation in Ontario, Canada. These data suggest that microbial sulfate reduction was restricted to organic-rich environments away from the influence of deltas, which delivered low-sulfate river water to the coast. Sulfur isotope data and the patchiness of sulfate reduction within the Animikie Basin suggest strong lateral and vertical gradients in seawater sulfate concentrations. When viewed in the context of the most recent tectonic and sedimentologic models, these observations are best interpreted as reflecting an evolving basin with restricted water circulation, not open circulation with the global ocean as previously postulated. Thus, the Animikie Basin does not contain an accurate record of open-ocean structure and composition during the onset of sulfidic conditions. What is clear is that marginal seas must be carefully evaluated before they are used to model the global ocean, especially given the rarity of preserved Precambrian deep-sea sediments.

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