Geochemical cycling of silver in marine sediments along an offshore transect

Abstract

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.