HR: 16:00h
AN: PP34A-01 INVITED     [Abstracts]
TI: The Molybdenum Isotope Paleoproxy: Defining Signatures of Reducing Conditions in Marine Sediments
AU: * McManus, J
EM: mcmanus@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences Oregon State University, 104 Ocean Admin Bldg, Corvallis, OR 97330 United States
AU: Poulson, R L
EM: rpoulson@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences Oregon State University, 104 Ocean Admin Bldg, Corvallis, OR 97330 United States
AU: Siebert, C
EM: csiebert@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences Oregon State University, 104 Ocean Admin Bldg, Corvallis, OR 97330 United States
AU: Berelson, W M
EM: berelson@usc.edu
AF: Department of Earth Sciences University of Southern California, 3651 Trousdale Ave., Los Angeles, CA 90089 United States
AB: Molybdenum enrichments in reducing marine sediments have been employed in paleochemical reconstructions, and the Mo isotope system has recently received attention for its potential as an additional paleoproxy. We present new Mo isotope data from several open-ocean sites that, in conjunction with previous data, allow us to propose a mechanistic description of the Mo isotope system in marine sediments. This description purports that there are three unique Mo isotope signatures that fall along a single mixing line, and that the isotope signature in geologic deposits is a mixture of these primary signatures. Manganese-rich (oxic) sediments have measured Mo isotope values that are more negative (relative to seawater) than any other sediment samples analyzed to date and this isotope value is consistent with previous experimental and theoretical work. The euxinic end-member represents syngenetic formation of a Mo sulfide that has an isotope value similar to the modern seawater value; this value is typified by samples from the highly-sulfidic Black Sea. Open-ocean anoxic sites exhibit a unique and invariant Mo isotope signature that lies between the oxic and euxinic values. Anoxic is defined as having dissolved oxygen and sulfide concentrations near zero in the overlying bottom water (sites along the Mexican and Peru margins typify this catagory), whereas euxinic implies detectable sulfide in the water column (typical of isolated basins). In environments where diagenetic processes include a mixture of aerobic and anaerobic respiration, measured Mo isotope compositions fall between the oxic and anoxic signatures. The observed anoxic and euxinic Mo isotope signatures represent two extremes in authigenic Mo isotope values for sediments dominated by anaerobic diagenesis. We suggest that isotope values between the anoxic and euxinic signatures reflect the relative ratio of diagenetic (anoxic) to syngenetic (euxinic) molybdenum sulfide.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1051 Sedimentary geochemistry
DE: 1065 Major and trace element geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005