Morford Jennifer L.

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Jennifer L.

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  • Preprint
    Geochemical cycling of silver in marine sediments along an offshore transect
    ( 2008-02-08) Morford, Jennifer L. ; Kalnejais, Linda H. ; Helman, Peter ; Yen, Gloria ; Reinard, Melissa
    Although there have been many surface water and water column silver (Ag) analyses in the ocean, the absence of high resolution pore water and solid phase Ag profiles has hampered our understanding of its oceanic geochemical cycling. This manuscript presents pore water and solid phase profiles of Ag along an offshore transect in the northeast Pacific off the coasts of Washington/Oregon states, U.S.A.. Pore water Ag concentrations are uniformly low (< 0.3 nmol kg-1) in profiles from sediments that have low bottom water oxygen concentrations, have shallow oxygen penetration depths (O2,pen < 1 cm) and underlie short water columns (< 500 m water depth). The solid phase Ag concentrations at these sites are also low (< 1 μmol kg-1). This is in contrast to sediments from intermediate water depths (~2000 m) that have similar oxygen penetration depths (O2,pen < 1 cm), but have elevated pore water Ag concentrations (0.7 nmol kg-1) at the sediment–water interface and higher solid phase Ag concentrations (4– 8 μmol kg-1). At sites from ~3000–4000 m water depth, where O2,pen > 1 cm, pore water Ag concentrations reach extremely high concentrations in the top 5 cm (8–24 nmol kg-1). High concentrations in pore waters provide evidence for a flux of Ag from ocean sediments, but the more oxidizing nature of these sediments precludes appreciable solid phase Ag accumulation in the top 30 cm (< 2 μmol kg-1). The accumulation of Ag in sediments is not simply dependent on redox conditions; more oxidizing sediments do not accumulate solid phase Ag, and neither do more reducing sediments from shallow water depths. Only a sufficiently long water column will result in additional delivery of Ag to sediments by scavenging onto settling particles, and result in Ag accumulation in sediments where O2,pen < 1 cm. Although upward Ag fluxes from sediments underlying shorter water columns are small (0.02–0.07 nmol cm-2 y-1), calculated fluxes increase for sediments underlying longer water columns and are largest for the more oxidizing sediments (2–5 nmol cm-2 y- 1). Calculated fluxes of pore water Ag to the solid phase at these more oxidizing stations are inconsistent with measured solid phase Ag concentrations and suggest that the pore water profiles represent non–steady state conditions. Clearly, the early diagenesis of Ag is a highly dynamic process and more research is required to fully understand Ag cycling in sediments in continental margin locations.
  • 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.