HR: 1340h
AN: OS33A-0986    [Abstracts]
TI: Molybdenum Isotopes in Oxic Sources and Sinks
AU: * Siebert, C
EM: christopher.siebert@earth.ox.ac.uk
AF: Department of Earth Sciences University of Oxford, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Georg, B
EM: bastian.georg@earth.ox.ac.uk
AF: Department of Earth Sciences University of Oxford, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Halliday, A N
EM: alex.halliday@earth.ox.ac.uk
AF: Department of Earth Sciences University of Oxford, Parks Road, Oxford, OX1 3PR, United Kingdom
AB: Authigenic enrichment of molybdenum (Mo) in marine sediments has been used for some time as an indicator for reducing conditions during sediment deposition. More recently Mo isotopes in marine sediments and seawater have received attention for their potential to quantify regional or global-scale ocean (paleo-) redox conditions. Dissolved molybdate shows a conservative profile in the water column. Under oxidizing conditions Mo is removed from seawater primarily by adsorption to Mn oxides. In reducing, sulfide-rich settings it is scavenged by formation of particle-reactive oxythiomolybdates. These processes are thought to produce a strong fractionation of Mo isotopes in the modern oceans that depends on the relative proportions of oxidized and sulfide-rich settings: the Mo isotope composition of seawater is heavier than that of ferromanganese crusts and oxidized pelagic sediments by ~ 1 permil/amu. In contrast, black shales from reducing environments have intermediate values that approach that of seawater in some settings. As a consequence of isotope mass balance, the Mo isotopic composition of seawater should vary with changes in the relative proportions of reducing and oxic sedimentation in the oceans through time, and this variation may be recorded in marine oxic and anoxic sediments. However, to make quantitative statements about changes in ocean redox over geological time scales several variables in the Mo isotope budget have to be better understood. A recent study by Archer et al. has shown that the dissolved Mo isotope composition of rivers is heavier than previously assumed. In order to develop quantitative models for paleo-ocean redox it is essential to understand what controls the Mo isotope composition of the riverine input and if this process is constant over time. We are studying Mo concentrations and Mo isotopes in weathering profiles of granite. Preliminary data show that Mo is gradually leached from the source rock thus providing a base for possible isotope fractionation during the process. Another important prerequisite for the use of Mo as paleo-proxy is the availability of archives that preserve the Mo isotope signal. Towards that end we are analyzing Mo isotopes in Fe-Mn crusts during known sequences of changes in ocean chemistry. (Geochim. Cosmochim. Acta, 71, 15, Suppl. 1, A33)
DE: 4835 Marine inorganic chemistry (1050)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting