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Geology; June 2009; v. 37; no. 6; p. 535-538; DOI: 10.1130/G25186A.1
© 2009 Geological Society of America
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When do black shales tell molybdenum isotope tales?

G.W. Gordon1,*, T.W. Lyons2, G.L. Arnold1, J. Roe3, B.B. Sageman4 and A.D. Anbar1,5

1 School of Earth & Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA
2 Department of Earth Sciences, University of California–Riverside, Riverside, California 92521-0423, USA
3 Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA
4 Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208, USA
5 Department of Chemistry & Biochemistry, Arizona State University, Tempe, Arizona 85287-1404, USA

Correspondence: *E-mail: Gwyneth.Gordon{at}asu.edu.

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}98/95Mo 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 {delta}98/95Mo differs systematically between the formations by ~0.59{per thousand}. 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}98/95Mo from organic-rich shales requires independent verification of persistent local euxinia. With these considerations in mind, our data point to {delta}98/95Mo in the Devonian oceans ~0.6{per thousand} lighter than in today's oceans, consistent with expanded anoxia.






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