AUTHIGENIC CADMIUM ENRICHMENTS IN SUBOXIC SEDIMENTS - PRECIPITATION AND POSTDEPOSITIONAL MOBILITY

The postdepositional mobility of Cd and U has been investigated in Nor th Atlantic turbidites in order to validate their utility as proxies o f pales-organic carbon fluxes. In this region pelagic sediments are in terspersed with relatively organic-rich turbidites. Following instanta neous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite, thereby leading to redi stribution of authigenic phases according to their affinity to differe nt redox conditions. Authigenic Cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portio n than in the deeper, unoxidized part of the turbidite. This distribut ion indicates that Cd has been continuously remobilized by the oxidati on front thereby suggesting high sensitivity of the Cd species to the redox state and an efficient immobilization mechanism for Cd under red ucing conditions. We propose the precipitation of CdS as a plausible m echanism for removal of Cd in reducing sediments, rather than adsorpti on onto mineral surfaces. This proposal is consistent with porewater d ata indicating diffusion of Cd toward, and uptake by, suboxic sediment s. Solubility calculations suggest that precipitation of CdS may occur in suboxic sediments in the presence of trace levels of H2S (below ro utine detection limits). There is no evidence for release of Cd or U b ack into seawater upon oxidation of reducing sediments. The strong cou pling between precipitation of authigenic Cd and U and the early diage netic degradation of organic carbon offers the potential of reconstruc ting paleo-productivity using records of Cd and U. However, postdeposi tional redistribution of authigenic Cd and U may significantly alter r ecords of authigenic Cd and U thereby rendering paleo-productivity rec onstructions uncertain. We propose that the (Cd/U)(authigenic) ratio m ay provide a qualitative indicator to assess the relative role of stea dy-state incorporation relative to transient 'burn-down' enrichments.