Molybdenum isotope constraints on the extent of late Paleoproterozoic ocean euxinia

The expansion of anoxic and sulfidic (euxinic) deep oceans may explain the termination of global large iron formation (IF) deposition at ~1.85Ga, but the areal extent of late Paleoproterozoic ocean euxinia is poorly constrained. Here, we use the molybdenum (Mo) isotope composition (δ^98/95Mo) of black shales from the <1.84Ga Rove Formation (Animikie Group, Lake Superior region) to place constraints on the extent of late Paleoproterozoic ocean euxinia. Black shales from the lower Rove Formation have ratios of highly reactive Fe to total Fe (Fe_HR/Fe_T typically >0.38) and ratios of pyrite Fe to highly reactive Fe (Fe_PY/Fe_HR typically <0.8) that are consistent with a water column that was generally anoxic and Fe²⁺-rich. Because the Fe_PY/Fe_HR ratios do not imply sustained water column euxinia, the conversion of molybdate (MoO₄^2-) to tetrathiomolybdate (MoS₄^2-) in bottom waters and sediments was probably not quantitative and thus the δ^98/95Mo (±0.1 to +0.8‰) from these shales provides only a minimum estimate for coeval seawater. By contrast, black shales from the middle Rove Formation have high Fe_PY/Fe_HR (>0.8) and Fe_HR/Fe_T (>0.38), which implies persistent water column euxinia. Concentrations of free sulfide may have been sufficient to result in near-quantitative formation of tetrathiomolybdate and its removal from bottom waters. The observed narrow range in δ^98/95Mo of 1.19±0.28‰ (2SD) defined by most euxinic shales is thus inferred to represent the Mo isotope composition of late Paleoproterozoic global seawater. Mass-balance modeling suggests that organic-rich sediments deposited beneath euxinic bottom waters represented a substantial fraction of the oceanic Mo sink at 1.8Ga. The modeling results suggest that euxinic marine environments were common along late Paleoproterozoic ocean margins. These environments could have served as efficient sinks for Fe²⁺_(aq) upwelling from anoxic deeper waters. Hence, the Mo isotope data are consistent with Fe speciation and other geochemical evidence that suggest an expansion of euxinic environments led to the cessation of global large IF deposition after ~1.85Ga.