Mobility of metals over the redox boundary in Peru Basin sediments

Downcore variations and phase distribution of metals in the Quaternary sediments from six areas of the Peru Basin were investigated. The sediments are characterised by a 5–20 cm thick, oxic layer at the water/sediment interface formed under the influence of dissolved oxygen in the bottom water. In deeper sediment sections, suboxic conditions are found due to the degradation of organic matter. Variations in the downcore abundance of metals (e.g., Fe, Mn, Co, Ni, Cu, Zn, V, Mo, Cr) were determined by bulk chemical methods. A significant accumulation of manganese is observed at the depth of the manganese redox boundary, caused by diagenetic remobilisation under suboxic conditions and reprecipitation under oxic conditions. To investigate their phase distributions, partitioning of elements in selected samples over the sediment column was carried out in sequential leaching experiments. A comparison between the surface layer and deeper sediment layers shows a distinct change in the element distribution between operationally defined host phases (carbonates, oxy-hydroxides, detrital components). Above the redox boundary at the sediment surface, transition metals are enriched in the reducible Mn-oxide fraction. When the horizontal position of the Mn-redox boundary moves upward, influenced by naturally occurring enhancement of surface-water productivity, these elements are mobilised and available to the manganese nodule accretion process if the redox boundary reaches the level of growth of the nodules. In contrast to the naturally induced mobilisation of metals, a technical impact (e.g., mining of manganese nodules) would be a rapid event occurring under completely different redox conditions. Metals would be released into the bottom water, an oxic environment, where scavenging of dissolved metals and resedimentation takes place in relatively short periods of time. Presumably, a long-distance transport of contaminants will not happen.