EARLY DIAGENESIS OF TRACE-METALS USED AS AN INDICATOR OF PAST PRODUCTIVITY CHANGES IN COASTAL SEDIMENTS

In sediments of Kiel Bight, which differ significantly in their redox- states and the rates of C(org) degradation, depth profiles of both dis solved and solid-phase Fe, Mn, Cd, Cu, Ni, and Co were measured. Porew ater fluxes of Cd, Cu, and Ni were significantly higher at shallower ( more oxic) stations as compared to highly reducing deeper subthermocli ne sediments while Mn fluxes behaved the opposite way. When normalized to Fe for grain size correction the measured solid-phase metal conten ts revealed that the high porewater flux has led to a near surface dep letion of Mn from more anoxic sediments and to a reduced mte of accumu lation of anthropogenic Cd in more oxic sediments. This difference in early diagenetic behaviour suggests the use of these metals as a proxy for the redox state and the intensity of carbon recycling. Contrary t o the recent sediment layers the normalized metal contents of preindus trially deposited sediment layers were similar for both metals at all stations. Thus, at stations having highly anoxic sediments today the p reindustrial benthic flux must have been lower for Mn and higher for C d reflecting a shift from more oxic to more reducing conditions in sub thermocline sediments during the last 100 years. This change of the re dox state in subthermocline sediments reflects an increase of primary production and of subsequent C(org) input to the sediment due to eutro phication which accompanied industrialization.