Authigenic cadmium enrichments in suboxic sediments: Precipitation and postdepositional mobility

The postdepositional mobility of Cd and U has been investigated in North Atlantic turbidites in order to validate their utility as proxies of paleo-organic carbon fluxes. In this region pelagic sediments are interspersed with relatively organic-rich turbidites. Following instantaneous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite, thereby leading to redistribution of authigenic phases according to their affinity to different redox conditions. Authigenic Cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portion than in the deeper, unoxidized part of the turbidite. This distribution indicates that Cd has been continuously remobilized by the oxidation front thereby suggesting high sensitivity of the Cd species to the redox state and an efficient immobilization mechanism for Cd under reducing conditions. We propose the precipitation of CdS as a plausible mechanism for removal of Cd in reducing sediments, rather than adsorption onto mineral surfaces. This proposal is consistent with porewater data indicating diffusion of Cd toward, and uptake by, suboxic sediments. Solubility calculations suggest that precipitation of CdS may occur in suboxic sediments in the presence of trace levels of H2S (below routine detection limits). There is no evidence for release of Cd or U back into seawater upon oxidation of reducing sediments. The strong coupling between precipitation of authigenic Cd and U and the early diagenetic degradation of organic carbon offers the potential of reconstructing paleo-productivity using records of Cd and U. However, postdepositional redistribution of authigenic Cd and U may significantly alter records of authigenic Cd and U thereby rendering paleo-productivity reconstructions uncertain. We propose that the(Cd/U)authigenic ratio may provide a qualitative indicator to assess the relative role of steady-state incorporation relative to transient ‘burn-down’ enrichments.