When do black shales tell molybdenum isotope tales?

Molybdenum (Mo) isotopes in ancient sediments are promising recorders of global ocean paleoredox conditions. Organic-rich black shales can be used to reconstruct ancient ocean Mo isotope compositions if these sediments record the isotopic composition of contemporaneous seawater. Comparison of delta(98/95)Mo in two Devonian shale sequences of similar age, the New York Oatka Creek and Geneseo Formations, reveals that this assumption cannot be applied to all organic-rich shales. Although both sequences contain laminated intervals, elevated organic carbon, and enrichments of redox-sensitive metals, the mean delta(98/95)Mo differs systematically between the formations by similar to 0.59%. Independent paleoredox indicators reveal that portions of the Oatka Creek Formation were deposited under pervasively euxinic (anoxic and sulfidic) conditions, whereas conditions during deposition of the Geneseo Formation were intermittently euxinic to suboxic (oxygen deficient but not sulfidic in the water column). We infer that reconstruction of ancient ocean delta(98/95)Mo from organic-rich shales requires independent verification of persistent local euxinia. With these considerations in mind, our data point to delta(98/95)Mo in the Devonian oceans similar to 0.6% lighter than in today's oceans, consistent with expanded anoxia.