TRACKING THE CHEMOCLINE IN ANCIENT OCEANS: AN EXAMPLE FROM THE MESOPROTEROZOIC OF MAURITANIA
Here we present iron speciation and pyrite sulfur isotope data from the ~1.1 Ga Atar and El Mreiti groups, Mauritania, which were deposited within an epeiric sea that flooded much of the West African craton. We examined black shale from both epicratonic and pericratonic environments, permitting us to capture the transition from oxic surface waters to persistently euxinic deeper-marine waters. In particular, black shale of the Touirist Formation (El Mreiti Group) records dynamic redox variability through time, characteristic of deposition at or near the chemocline. Iron speciation data suggest that pyrite formation was intermittently limited by Fe2+ availability, perhaps resulting from inefficient iron shuttling across the epeiric sea. In addition, a greater degree of pyrite sulfur isotopic variability is observed at the chemocline (values ranging up to 40‰) than in shale deposited both above and below the chemocline, suggesting that a dynamic oxidative-reductive sulfur cycle operated at this redox boundary. Ultimately, this study identifies a dynamic redox boundary in the shallowest parts of the Mesoproterozoic water column and provides rare insight into the redox structure of the Mesoproterozoic oceans.