Diagenetic separation of cadmium and manganese in suboxic continental margin sediments

The vertical distribution of Cd differs from that of Mn in sediment cores taken from the continental margins of the Arctic and Atlantic oceans: the oxic surface sediment is depleted in Cd and enriched in Mn, whereas the subsurface sediment is enriched in Cd and depleted in Mn. In some cores the distributions of solid-phase and dissolved Mn are offset: the porewater profiles indicate that Mn is being precipitated relatively deep in the sediment, whereas the solid-phase distributions indicate that Mn has been precipitated very close to the sediment surface in the recent past. At one deep site, located near the base of the continental slope (3000 m), the offset may be caused by a relatively recent decrease in ocean productivity, a corresponding decrease in the organic carbon flux to the sediment, and a subsequent migration of the redox boundary deeper into the sediment. However, at a site where the upper sediment layers have accumulated during the last few decades, this explanation cannot account for the offset distributions. We propose that a fluctuating input of organic matter to the sediment on seasonal or decadal timescales causes the position of the redox boundary to fluctuate. The fluctuating redox boundary pumps dissolved Mn upward and concentrates it as Mn oxide in a distinct layer at the upper limit of the odor excursion; at the same time, it pumps Cd downward and concentrates it in a distinct layer at the lower limit of the redox excursion. The net result of a fluctuating redox boundary is to increase the spatial separation of Cd and Mn during the early diagenesis of suboxic sediments. Copyright (C) 1997 Elsevier Science Ltd.