Oceanic molybdenum isotope fractionation: Diagenesis and hydrothermal ridge-flank alteration James McManus Large Lakes Observatory, University of Minnesota, Duluth, Minnesota 55812, USA Now at Oregon State University, College of Oceanic and Atmospheric Sciences, 104 Ocean Admin Bldg., Corvallis, Oregon 97331, USA (jmcmanus@coas.oregonstate.edu) Thomas F. Na ¨gler and Christopher Siebert Universita ˚t Bern, Institut fu ¨r Geologie, CH-3012 Bern, Erlachstrasse 9a, Switzerland (naegler@geo.unibe.ch; siebert@geo.unibe.ch) C. Geoffrey Wheat Global Undersea Research Unit, P.O. Box 475, Moss Landing, California 95039, USA (wheat@mbari.org) Douglas E. Hammond University of Southern California, Department of Earth Sciences, Los Angeles, California 90089, USA (dhammond@usc.edu) [1] Isotopic analyses of dissolved molybdenum are presented for sediment pore waters from a reducing sedimentary basin and for fluids from a low-temperature ridge flank hydrothermal system. d 98/95 Mo in these fluids range from 0.8 to 3.5% (relative to a laboratory standard), demonstrating that marine sedimentary reactions significantly fractionate Mo isotopes. Within the upper 3 cm of sediment, manganese oxide dissolution produces an isotopically light fluid relative to seawater (mean of four analyses = 2.1 ± 0.1% versus seawater = 2.3 ± 0.1%). Below 6 cm depth, authigenic Mo uptake results in an isotopically heavier fluid (up to 3.5%) indicating that reducing sediments are likely to be a net sink for isotopically light dissolved Mo. In contrast, fluid circulation within a low-temperature ridge-flank hydrothermal system is a source of isotopically light Mo to the ocean having an end-member fluid of 0.8%. Components: 4917 words, 2 figures. Keywords: Molybdenum; molybdenum isotopes; sediment diagenesis; isotope geochemistry. Index Terms: 1030 Geochemistry: Geochemical cycles (0330); 1050 Geochemistry: Marine geochemistry (4835, 4850); 1045 Geochemistry: Low-temperature geochemistry; 1040 Geochemistry: Isotopic composition/chemistry. Received 5 April 2002; Revised 20 June 2002; Accepted 3 July 2002; Published 19 December 2002. McManus, J., T. Na ¨gler, C. Siebert, C. G. Wheat, and D. Hammond, Oceanic molybdenum isotope fractionation: Diagenesis and hydrothermal ridge-flank alteration, Geochem. Geophys. Geosyst., 3(12), 1078, doi:10.1029/2002GC000356, 2002. G 3 G 3 Geochemistry Geophysics Geosystems Published by AGU and the Geochemical Society AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Geochemistry Geophysics Geosystems Research Letter Volume 3, Number 12 19 December 2002 1078, doi:10.1029/2002GC000356 ISSN: 1525-2027 Copyright 2002 by the American Geophysical Union 1 of 9