Trace metal diagenesis in sulfidic sediments: Insights from Chesapeake Bay

Sediments were recovered from the mid-bay portion of the Chesapeake Bay when bottom water conditions were sulfidic. Metal concentrations (Fe, Mn, Ba, V, Ni, Cu, Co, Cd, Mo, U, and Re) were determined in both pore waters and solid phase samples, which were determined using total and nitric acid digestion (EPA 3051a) methods. V, Cr, and Ni were preferentially retained in pore waters even under sulfidic conditions that would normally result in authigenic accumulation, likely due to complexation with dissolved organic matter (DOM). Mo was efficiently removed from pore waters in the zone in which FeS phases and polysufides are available. Re was not delivered to the sediments through the Mn/Fe shuttle, so that its diffusive flux into sediments solely depends on the extent of reducing conditions. A non-steady state oxygenation event was recorded in the solid phase, with coincident solid phase enrichments in Fe, Mn, Ni, Cu and Co. Deeper enrichments in U and Mo likely reflect remobilization from the relict Fe and Mn peaks, downward diffusion into more reducing conditions, and subsequent immobilization. The absence of peaks in solid phase Re, Cr and Cd was indicative of their lack of association with Mn and Fe oxides and/or resistance of their respective authigenic phases to oxidation on these seasonal to yearly timescales. Relative changes within the solid phase profiles were captured using the acid digestion method for Fe, Ni and Co, and acid digestion concentrations were within 90% of the total concentration for Mn and Cu. The presence of excess pore water sulfide may have promoted the removal of trace metals from the detrital fraction resulting in the formation of authigenic sulfides that might become mobile during oxygenation events.