Morford Jennifer L.

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Jennifer L.

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  • Preprint
    A model for uranium, rhenium, and molybdenum diagenesis in marine sediments based on results from coastal locations
    ( 2008-12-31) Morford, Jennifer L. ; Martin, William R. ; Francois, Roger ; Carney, Caitlin M.
    The purpose of this research is to characterize the mobilization and immobilization processes that control the authigenic accumulation of uranium (U), rhenium (Re) and molybdenum (Mo) in marine sediments. We analyzed these redox– sensitive metals (RSM) in benthic chamber, pore water and solid phase samples at a site in Buzzards Bay, Massachusetts, U.S.A., which has high bottom water oxygen concentrations (230–300 mol/L) and high organic matter oxidation rates (390 mol C/cm2/y). The oxygen penetration depth varies from 2–9 mm below the sediment–water interface, but pore water sulfide is below detection (< 2M). The RSM pore water profiles are modeled with a steady–state diagenetic model that includes irrigation, which extends 10–20 cm below the sediment–water interface. To present a consistent description of trace metal diagenesis in marine sediments, RSM results from sediments in Buzzards Bay are compared with previous research from sulfidic sediments (Morford et al., GCA 71). Release of RSM to pore waters during the remineralization of solid phases occurs near the sediment–water interface at depths above the zone of authigenic RSM formation. This release occurs consistently for Mo at both sites, but only in the winter for Re in Buzzards Bay and intermittently for U. At the Buzzards Bay site, Re removal to the solid phase extends to the bottom of the profile, while the zone of removal is restricted to ~2–9 cm for U and Mo. Authigenic Re formation is independent of the anoxic remineralization rate, which is consistent with an abiotic removal mechanism. The rate of authigenic U formation and its modeled removal rate constant increase with increasing anoxic remineralization rates, and is consistent with U reduction being microbially mediated. Authigenic Mo formation is related to the formation of sulfidic microenvironments. The depth and extent of Mo removal from pore water is closely associated with the balance between iron and sulfate reduction and the consumption of pore water sulfide via iron sulfide formation. Pore water RSM reach constant asymptotic concentrations in sulfidic sediments, but only pore water Re is constant at depth in Buzzards Bay. The increases in pore water U at the Buzzards Bay site are consistent with addition via irrigation and subsequent upward diffusion to the removal zone. Deep pore water Mo concentrations exceed its bottom water concentration due to irrigation–induced oxidation and remobilization from the solid phase. In sulfidic sediments, there is no evidence for higher pore water U or Mo concentrations at depth due to the absence of irrigation and/or the presence of more stable authigenic RSM phases. There are good correlations between benthic fluxes and authigenic accumulation rates for U and Mo in sulfidic sediments. However, results from Buzzards Bay suggest irrigation ultimately results in the partial loss of U and Mo from the solid phase, with accumulation rates that are 20–30% of the modeled flux. Irrigation can augment (Re, possibly U) or compromise (U, Mo) authigenic accumulation in sediments, and is important when determining burial rates in continental margin sediments.
  • Preprint
    Insights on geochemical cycling of U, Re and Mo from seasonal sampling in Boston Harbor, Massachusetts, USA
    ( 2006-10-23) Morford, Jennifer L. ; Martin, William R. ; Kalnejais, Linda H. ; Francois, Roger ; Bothner, Michael H. ; Karle, Ida-Maja
    This study examined the removal of U, Mo, and Re from seawater by sedimentary processes at a shallow-water site with near-saturation bottom water O2 levels (240-380 μmol O2/L), very high organic matter oxidation rates (annually averaged rate is 870 μmol C/cm2/y), and shallow oxygen penetration depths (4 mm or less throughout the year). Under these conditions, U, Mo, and Re were removed rapidly to asymptotic pore water concentrations of 2.2–3.3 nmol/kg (U), 7–13 nmol/kg (Mo), and 11–14 pmol/kg (Re). The order in which the three metals were removed, determined by fitting a diffusion-reaction model to measured profiles, was Re < U < Mo. Model fits also suggest that the Mo profiles clearly showed the presence of a near-interface layer in which Mo was added to pore waters by remineralization of a solid phase. The importance of this solid phase source of pore water Mo increased from January to October as the organic matter oxidation rate increased, bottom water O2 decreased, and the O2 penetration depth decreased. Experiments with in situ benthic flux chambers generally showed fluxes of U and Mo into the sediments. However, when the overlying water O2 concentration in the chambers was allowed to drop to very low levels, Mn and Fe were released to the overlying water along with the simultaneous release of Mo and U. These experiments suggest that remineralization of Mn and/or Fe oxides may be a source of Mo and perhaps U to pore waters, and may complicate the accumulation of U and Mo in bioturbated sediments with high organic matter oxidation rates and shallow O2 penetration depths. Benthic chamber experiments including the nonreactive solute tracer, Br-, indicated that sediment irrigation was very important to solute exchange at the study site. The enhancement of sediment-seawater exchange due to irrigation was determined for the nonreactive tracer (Br-), TCO2, NH4 +, U and Mo. The comparisons between these solutes showed that reactions within and around the burrows were very important for modulating the Mo flux, but less important for U. The effect of these reactions on Mo exchange was highly variable, enhancing Mo (and, to a lesser extent, U) uptake at times of relatively modest irrigation, but inhibiting exchange when irrigation rates were faster. These results reinforce the observation that Mo can be released to and removed from pore waters via sedimentary reactions. The removal rate of U and Mo from seawater by sedimentary reactions was found to agree with the rate of accumulation of authigenic U and Mo in the solid phase. The fluxes of U and Mo determined by in situ benthic flux chamber measurements were the largest that have been measured to date. These results confirm that removal of redoxsensitive metals from continental margin sediments underlying oxic bottom water is important, and suggest that continental margin sediments play a key role in the marine budgets of these metals.