TRACKING THE CHEMOCLINE IN ANCIENT OCEANS: AN EXAMPLE FROM THE MESOPROTEROZOIC OF MAURITANIA

Globally, deep ocean environments appear to have remained persistently anoxic through much of Earth’s Proterozoic history, reflecting generally low levels of atmospheric oxygen. Deep ocean conditions may have also remained largely ferruginous until the latter Proterozoic, when variably ferruginous to sulfidic conditions are interpreted to reflect increased atmospheric oxygenation and subtle changes in the size of the marine sulfur reservoir. Low atmospheric oxygen levels in the Mesoproterozoic (1.6-1.0 Ga) were further coupled with greenhouse climate and globally high sea level that resulted in expansion of shallow-marine, epicratonic depositional environments. Current geological and geochemical data suggest that epeiric settings contained both oxic, sulfate-rich conditions and anoxic, variably sulfidic environments, suggesting that the chemocline resided in shallow waters and may have been controlled by the depth of wave mixing in Mesoproterozoic oceans.

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.