Early diagenesis in sediments from the eastern equatorial Pacific, II. Pore water metal results

Abstract

Concentrations of Mn, Fe, Ni and Cu were measured in pore waters extracted from pelagic sediments at two sites in the eastern equatorial Pacific and at two locations in the eastern equatorial Atlantic. The results from these four sites suggest that the pore water profiles of these metals are controlled by processes involving the major oxidants (O 2, NO 3 −, Mn and Fe oxides). Dissolved copper concentrations in the top 2 cm of sediment are 10 times the bottom water value. This shallow regeneration is an important factor in the oceanic budget of copper. Manganese, nickel and iron concentrations near the interface are indistinguishable from bottom water values. At depth in the sediment, Mn oxides are reduced and manganese diffuses upwards to be reoxidized and trapped within the sediment column. The highest nickel concentrations in the Mn reduction zone are 40 times that of bottom water. This nickel gradient drives a flux into the Mn oxidation zone which is large enough to influence the nickel concentration of the bulk sediment. Dissolved Mn concentrations range from 10 nmol/kg in the oxygenated zone to 100 μmol/kg in the reduction zone.