The distribution of Mo, U, and Cd in relation to major redox species in muddy sediments of the Bay of Biscay

In order to understand the relationship between authigenic precipitation of U, Mo, Cd, and the redox properties of the marine environment, we have studied the vertical distribution of these metals and of the major redox species (oxygen, nitrate, manganese, reactive iron, sulfate, carbon, and sulfur) in modern sediment. We have sampled four sites at 150 to 2800 m depth in the Bay of Biscay. At the shallowest station, where sediment is highly bioturbated, organic carbon levels are above 2% and particulate sulfides are abundant. At the deepest stations, sediment is much less bioturbated, and organic carbon levels are lower. At all sites, early diagenesis follows a well-established depth sequence of redox reactions, based on the bacterially mediated oxidation of organic matter. Manganese-oxides and authigenic U and Mo were extracted by an ascorbate solution. Molybdenum is associated with these oxides in the oxic part of cores. In the anoxic layer of the sediment, Mo precipitates as a detectable authigenic phase only when sulfide minerals are present, i.e. when sulfate reduction becomes important. In anoxic sediments, Cd enrichment is a good indicator of sulfide production from sulfate reduction even if the production is weak. Uranium precipitates at the depth of reactive iron (III) reduction. No relationship was observed between U and S. The U concentration at the depth of precipitation agrees with values calculated using estimated accumulation rates and a downward diffusive flux of dissolved U from the bottom seawater into the sediment. However, authigenic U concentrations continue to increase in the anoxic part of the cores. Therefore, it is difficult to establish a direct relationship between the process of U accumulation and the flux of Corg to the sediment.