The ~1880 Ma Gunflint Formation has played a critical role in shaping our view of the evolution of the Precambrian biosphere and the redox state of Earth's early oceans.Herein we present a study of the petrology and geochemistry of calcic grainstones and stromatolitic limestones present at the very top of the Gunflint Formation and compare them with underlying ankeritic grainstones that dominate the shore-proximal portion of the basin. Meteoric calcite cements in the upper limestones formed prior to the unit being brecciated and/or buried by debris carried by the blast cloud from the 1850 Ma Sudbury impact event. The intraclastic grains and cements in the limestone are highly enriched in V, Cr, U and REEs, with REE patterns similar to those of modern groundwater. Cr isotopes have a distinctly positive signature in most samples indicating formation of oxidized, hexavalent Cr during subaerial weathering. These redox-sensitive metals were transported in oxygenated groundwater and precipitated where the fluid encountered reducing conditions. In contrast, the underlying ankeritic marine grainstones have REE patterns similar to those of some modern-day venting hydrothermal fluids and earliest Paleoproterozoic seawater. This work suggests that significant levels of oxygen existed in the atmosphere at the time, whereas, even shallow areas of the world ocean remained very oxygen-deficient. The contrast in redox state of anoxic shallow-marine water verses oxic groundwater was probably linked to strong sub-seafloor hydrothermal circulation and extensive upwelling onto shallow shelves, which could have sustained widespread marine anoxia. More broadly, our study highlights the importance of investigating and understanding lateral geochemical gradients, and the flux rates that control them, in shallow paleomarine settings.

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