IRON AND MANGANESE CYCLING IN DIFFERENT SEDIMENTARY ENVIRONMENTS ON THE NORTH-SEA CONTINENTAL-MARGIN

Pore water O-2, NO3-, Fe2+ and Mn2+ and solid phase Fe and Mn profiles were measured in sediments located in four different types of sedimen tary environments in the southern and eastern North Sea in August 1991 and February 1992. A steady-state diagenetic model describing solid p hase and pure water metal profiles was developed and applied to Mn and Fe data for 11 and 3 stations, respectively. The quality and quantity of the organic matter deposited in each sedimentary environment are s hown to determine whether sediments become sufficiently depleted of O- 2 and NO3- to allow for (1) Fe and Mn reduction and (2) escape of diss olved Fe2+ and Mn2+ to the overlying water, thus determining whether t hese metal cycles extend into the water column. Reversible sorption in combination with sediment mixing is shown to enhance diffusive transp ort of dissolved metals. Precipitation of Fe2+ and Mn2+ in the form of reduced authigenic minerals is suggested to be responsible for the re versal in gradient of pore water Fe2+ and Mn2+ at depth at many statio ns. Most North Sea sediments are relatively poor in Fe and Mn oxides. High surface concentrations of Fe and Mn oxides (up to 245 and 13 mu m ol g(-1), respectively) were only found in the areas receiving signifi cant amounts of terrigenous material, i.e, the German Eight and Skager rak. Comparison of model calculated rates of Mn and Fe reduction to O- 2 uptake rates indicates that Fe and Mn oxides do not play an importan t role as redox intermediates in organic C oxidation (accounting for < 4%) in most North Sea sediments. Only in the depositional environment of the Skagerrak do model results suggest that metal oxide reduction m ay contribute substantially to organic C oxidation (similar to 20%). ( C) 1997 Elsevier Science Ltd.