Iron and manganese cycling in different sedimentary environments on the North Sea continental margin

Pore water O2, NO3−, Fe2+ and Mn2+ and solid phase Fe and Mn profiles were measured in sediments located in four different types of sedimentary environments in the southern and eastern North Sea in August 1991 and February 1992. A steady-state diagenetic model describing solid phase and pore 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 shown to determine whether sediments become sufficiently depleted of O2 and NO3− to allow for (1) Fe and Mn reduction and (2) escape of dissolved Fe2+ and Mn2+ to the overlying water, thus determining whether these metal cycles extend into the water column. Reversible sorption in combination with sediment mixing is shown to enhance diffusive transport of dissolved metals. Precipitation of Fe2+ and Mn2+ in the form of reduced authigenic minerals is suggested to be responsible for the reversal in gradient of pore water Fe2+ and Mn2+ at depth at many stations. 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 μmol g−1, respectively) were only found in the areas receiving significant amounts of terrigenous material, i.e. the German Bight and Skagerrak. Comparison of model calculated rates of Mn and Fe reduction to O2 uptake rates indicates that Fe and Mn oxides do not play an important 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 may contribute substantially to organic C oxidation (∼ 20%).