We used a combination of porewater and solid phase analysis, as well as a series of
sediment incubations, to quantify organic carbon oxidation by dissimilatory Fe reduction,
Mn reduction, and sulfate reduction, in sediments from the Skagerrak (located off
the northeast coast of Jutland, Denmark). In the deep portion of the basin, surface
Mn enrichments 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 less Mn-rich sediments
from intermediate depths in the basin, Fe reduction ranged from somewhat less, to
far more important than sulfate reduction. Most of the Mn reduction in these sediments
may have been coupled to the oxidation of acid volatile sulfides (AVS), rather than
to dissimilatory reduction. High rates of metal oxide reduction at all sites were
driven by active recycling of both Fe and Mn, encouraged by bioturbation. Recycling
was so rapid that the residence time of Fe and Mn oxides, with respect to reduction,
ranged from 70-250 days. These results require that, on average, an atom of Fe or
Mn is oxidized and reduced between 100-300 times before ultimate burial into the sediment.
We observed that dissolved Mn2+ was completely removed onto fully oxidized Mn oxides
until the oxidation level of the oxides was reduced to about 3.8, presumably reflecting
the saturation by Mn2+ of highly reactive surface adsorption sites. Fully oxidized
Mn oxides in sediments, then, may act as a cap preventing Mn2+ escape. We speculate
that in shallow sediments of the Skagerrak, surface Mn oxides are present in a somewhat
reduced oxidation level (< 3.8) allowing Mn2+ to escape, and perhaps providing the
Mn2+ which enriches sediments of the deep basin.