A cryptic sulfur cycle driven by iron below the sulfate-methane transition zone

Abstract

My Ph.D thesis deals with biogeochemical cycling of carbon, sulfur and iron compounds in marine sediments on the continental shelves in relation to the activity and distribution of sulfate-reducing bacteria below the sulfate-methane transition. The aim of the first report was to study sulfate reduction and sulfur-iron geochemistry in deep gravity cores of Holocene mud collected from Bay of Aarhus (Denmark). It was the goal to understand if sulfate is generated by reoxidation of sulfide throughout the sulfate and methane zones and try to explain the abundance of active sulfate-reducers deep below the main sulfate zone from geochemical processes (i.e. oxidation of sulfide with iron oxides). We did potential sulfate reduction rate experiments, where extra sulfate and organic substrates were added to sediment sub-samples that were incubated in time series experiments. Sulfate reduction rates in the sulfate-rich sediment layers were high due to the high concentration of reactive organic matter. In the methane zone, sulfate remained at background concentrations of <0.5 mM down to the sulfidization front and sulfate reduction decreased steeply to rates, which at 300-500 cm were 0.2-1 pmol SO42- cm-3 d-1, i.e. four to five orders of magnitude lower than rates near the sediment surface. The potential sulfate reduction rates were found to be 10-40-fold higher than the sulfate reduction rates estimated without extra electron donor-and acceptors added to the sediment. This demonstrated that a physiologically intact community of sulfate-reducing bacteria was present deep below the sulfate-methane transition zone. The background sulfate concentration appears to be generated from the reaction of downwards diffusing sulfide with deeply buried Fe(III) species, such as poorly-reactive iron oxides or iron bound in sheet silicates. The oxidation of sulfide to sulfate in the sulfidic sediment may involve the formation of elemental sulfur or perhaps thiosulfate and the further disproportionation of a small fraction to sulfate. The net production of sulfate from the reaction of sulfide and Fe(III) to form pyrite requires an additional oxidant. This could be CO2 which is reduced to methane and subsequently becomes re-oxidized at the sulfate-methane transition and thereby removes excess reducing power. The second report describes the sediment and pore water geochemistry of long sediment cores collected in the Arkona Basin of the south-western Baltic Sea. We observed an unusual sulfate profile deep within the limnic deposits, by which high concentrations of sulfate were present in the pore water. The study indicated that the high sulfate concentrations within the sub-surface sediment layers of the Arkona Basin were not due to oxidation of reduced sulfur species as previously assumed, but rather due to downward diffusion of sulfate during the early Holocene history of the Baltic Sea. The third report presents data from sediment collected in the western part of the Black Sea. The study demonstrated that sulfate-reducing bacteria were active also several meters below the sulfate-methane transition in Black Sea sediments. The cryptic sulfate reduction below the sulfate-methane transition may be driven by sulfate produced from reoxidation of sulfur compounds in pore water and sediment with oxidized iron minerals. The fourth report aimed to investigate the association between phosphate release, organic phosphorus mineralization, and dense communities of the filamentous sulfur bacteria, Thioploca spp., on the continental shelf off central Chile. We found that the pore water was super-saturated with respect to hydroxyapatite but the concentration of authigenic apatite in the sediment was only a minor P-component in the sediment and most solid-phase phosphate was bound to iron. The large phosphate release was not directly related to the presence of Thioploca but rather the result of a high deposition and mineralization rate of fresh organic detritus.