Effect of estuarine sediment resuspension on early diagenesis, sulfide oxidation and dissolved molybdenum and uranium distribution in the Gironde estuary, France

Abstract

Vertical distributions of dissolved major redox parameters, Mo and U from the water to the sediment were sampled in the salinity gradient of the Gironde estuary. In the water, sulfate and Mo are conservatively transported throughout the salinity gradient. In contrast, U shows a significant removal rate of 85 μmol m − 2 a − 1 related to reductive sequestration of U in the anoxic sediment. Early diagenesis is highly transient and characterized by a three-zone regime related to three density layers functioning at different timescales. Excess of sulfate (up to 1 mM) is observed in the soft mud layer and is assigned to sulfide oxidation by both abiotic Mn- and Fe-oxide reduction and bacterial nitrate reduction. During early diagenesis, Mo and U are released in the sediment porewater from Mn- and Fe-oxides. In not-dredged anoxic sediments, authigenic precipitation of Mo and U with iron sulfides is proposed to explain their removal from solution. In contrast, in dredged sediments, authigenic precipitation of U is inhibited probably owing to the less effective reduction of soluble U(VI) to insoluble U(IV) due to oxidizing transient conditions. Mixing of anoxic sediment porewater with oxygenated water, due to (i) cyclical resuspension of the soft mud layer and (ii) dredging operations in the navigation channel, induces addition of dissolved sulfate (produced from diagenetic sulfide oxidation), Mo and U to the water. This addition is equivalent to 2% (sulfate), 5.5% (Mo) and 0.5% (U) of the dissolved fluvial inputs into the estuary for the natural resuspension, and 0.4% (Mo), less than 0.2% (sulfate, U) for dredge-induced resuspension. Addition of dissolved Mo and U, produced by oxidation of sulfide phases in the dredged sediments redeposited in dynamic environments, are equivalent to 12% (Mo) and 0.8% (U) of their respective fluvial inputs. Additional Mo and U inputs into the water originate from diffusive outflow from muddy sediments and are equivalent to 2% (Mo) and 0.3% (U) of their respective fluvial inputs. However, we show that all these dissolved inputs are scavenged by solid phases.