Sedimentary redox conditions in continental margin sediments (N.E. Pacific) — Influence on the accumulation of redox-sensitive trace metals

Redox conditions in near-surface sediments from the continental margin off western Canada were characterized to determine how they influence the accumulation of redox-sensitive trace metals. Despite the development of suboxic conditions within millimeters of the sediment–water interface there is little enrichment of Re, U, Cd or Mo in the upper 10 to 15 cm of the sediment column. This observation is consistent with the low extractible solid Mn concentrations but high I/Corg ratios that imply that near-surface sediment redox conditions are poised between Mn and I reduction. In contrast, below ∼ 15 cm Re, and to a lesser extent Cd and U, are enriched (e.g., up to 65 ng/g Re) but only in those cores collected from within and below the oxygen minimum zone. Molybdenum concentrations remain low (< 1.5 μg/g) in all cores indicating that these near-surface sediments never became fully anoxic.A combination of low sedimentation rates on the slope (< 1 to 6 cm/kyr) and/or intense bioturbation at all locations, including on the shelf where the sedimentation rate is substantially higher (40 cm/kyr), has maintained suboxic conditions in these sediments. Suboxic conditions exist despite a high organic carbon flux to the sediment and low bottom water oxygen concentrations, both of which should favour the development of anoxic conditions close to the sediment–water interface. The enrichment of Re, U and Cd that is observed occurs well below the sediment–water interface, overprinting and thus potentially compromising any paleo-signal. Accordingly, caution is necessary when using redox-sensitive trace metal concentrations as paleo-proxies in environments characterized by low sedimentation rates and/or deep bioturbation.