Flocculation of dissolved organic matter controls the distribution of iron in boreal estuarine sediments

Abstract

Iron (Fe) plays a key role in sedimentary diagenetic processes in coastal systems, participating in various redox reactions and influencing the burial of organic carbon. Large amounts of Fe enter the marine environment from boreal river catchments associated with dissolved organic matter (DOM). However, the fate of this Fe pool in estuarine sediments has not been extensively studied. Here we show that flocculation of DOM along salinity gradients in an estuary of the northern Baltic Sea efficiently transfers Fe from the dissolved phase into particulate material that accumulates in the sediments. Consequently, we observe a decline with distance offshore in both the Fe content of the sediments and proportion of terrestrial material in the sedimentary organic matter pool. Mössbauer spectroscopy and sequential extractions suggest that large amounts of Fe in sediments of the upper estuarine zone are associated with organic matter as unsulfidized Fe (II) complexes, or present in the form of ferrihydrite, implying a direct transfer of flocculated material to the sediments. Accordingly, the contribution of these components to the total sedimentary Fe declines with distance offshore while other Fe phases become proportionally more important. Sediment core records show that the observed lateral distribution of Fe minerals has remained similar over recent decades, despite variable Fe inputs from anthropogenic sources and eutrophication of the coastal zone. Pore water data suggest that the vertical zonation of diagenetic processes in the sediments is influenced by both the availability of Fe and by bottom water salinity, which controls the availability of sulfate (SO42−).