Molybdenum isotope signatures in continental margin marine sediments Christopher Siebert a, * , James McManus a , Angela Bice a , Rebecca Poulson a , William M. Berelson b a College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin. Bldg., Corvallis, OR 97331-5503, United States b Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, United States Received 18 April 2005; received in revised form 4 November 2005; accepted 4 November 2005 Available online 10 January 2006 Editor: H. Elderfield Abstract We present molybdenum isotope data for four sediment profiles from continental margin settings. Each profile has a distinctive average isotope composition ranging from d 98 / 95 Mo À 0.5x to 1.3x (relative to J and M laboratory standard). This range lies between the modern ocean water value (2.3x) and the values typical of Mo adsorbed onto Mn oxides (À 0.7x F 0.1x). An important finding of this study is the apparent co-variation between the Mo isotope composition and the accumulation rate of authigenic Mo under reducing conditions. This relationship suggests that the chemical processes responsible for Mo accumulation under reducing conditions produce an isotope signature in marine sediments. In addition to the relationship between Mo accumulation and the Mo isotope signature there is also a relationship between these parameters and the rate of organic carbon oxidation and burial. These relationships suggest that the Mo isotope signature of reducing sediments may serve as a tracer for the cycling of organic carbon in continental margin sediments; however, additional data will be required to refine any such relationships. D 2005 Elsevier B.V. All rights reserved. Keywords: Molybdenum; Isotopes; Diagenesis; Sediments 1. Introduction Under the well oxygenated conditions that prevail over much of the Earth’s surface, molybdenum (Mo) is a highly soluble element, resulting in its being the most concentrated (~100 nM) of the dissolved trace metals in the ocean and Mo having an exceptionally long resi- dence time (~0.8 Ma) (e.g., [1,2]). In contrast to its high solubility in ocean water, Mo is relatively insoluble under the reducing conditions that exist in sediments that line the ocean basin boundaries or marginal basins ([3–14], and others). The resulting sedimentary Mo enrichments thus represent a potential signature of past reducing conditions. In addition to Mo uptake under reducing conditions, Mo is also adsorbed to Mn oxides, and this particular removal pathway may scav- enge up to 70% of the Mo from the oceans with the remainder being precipitated in reducing sediments (e.g., [13–19]). The Mo isotope signature for each of the above sinks is thought to be distinct, with oxic sediments having a 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.11.010 * Corresponding author. Current address: Department of Geological Sciences, Arizona State University, P.O. Box 871404, PSF 686 Tempe, AZ-85287-1404, United States. E-mail address: csiebert@coas.oregonstate.edu (C. Siebert). Earth and Planetary Science Letters 241 (2006) 723 – 733 www.elsevier.com/locate/epsl