THE ANAEROBIC DEGRADATION OF ORGANIC-MATTER IN DANISH COASTAL SEDIMENTS - IRON REDUCTION, MANGANESE REDUCTION, AND SULFATE REDUCTION

We used a combination of porewater and solid phase analysis as well as a series of sediment incubations, to quantify organic carbon oxidatio n by dissimilatory Fe reduction, Mn reduction, and sulfate reduction, in sediments from the Skagerrak (located off the northeast coast of Ju tland, Denmark). In the deep portion of the basin, surface Mn enrichme nts reached 3.5 wt%, and Mn reduction was the only important anaerobic carbon oxidation process in the upper 10 cm of the sediment. In the l ess Mn-rich sediments from intermediate depths in the basin, Fe reduct ion ranged from somewhat less, to far more important than sulfate redu ction. Most of the Mn reduction in these sediments may have been coupl ed to the oxidation of acid volatile sulfides (AVS), rather than to di ssimilatory reduction. High rates of metal oxide reduction at all site s were driven by active recycling of both Fe and Mn, encouraged by bio turbation. Recycling was so rapid that the residence time of Fe and Mn oxides, with respect to reduction, ranged from 70-250 days. These res ults require that, on average, an atom of Fe or Mn is oxidized and red uced between 100-300 times before ultimate burial into the sediment. W e observed that dissolved Mn2+ was completely removed onto fully oxidi zed Mn oxides until the oxidation level of the oxides was reduced to a bout 3.8, presumably reflecting the saturation by Mn2+ of highly react ive surface adsorption sites. Fully oxidized Mn oxides in sediments, t hen, may act as a cap preventing Mn2+ escape. We speculate that in sha llow sediments of the Skagerrak, surface Mn oxides are present in a so mewhat reduced oxidation level (<3.8) allowing Mn2+ to escape, and per haps providing the Mn2+ which enriches sediments of the deep basin.