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 δ98/95Mo 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 δ 98/95Mo differs systematically between the formations by ∼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 δ98/95Mo from organic-rich shales requires independent verification of persistent local euxinia. With these considerations in mind, our data point to δ98/95Mo in the Devonian oceans ∼0.6‰ lighter than in today's oceans, consistent with expanded anoxia.
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