Importance of microbial iron reduction in deep sediments of river-dominated continental-margins

International audienceRemineralization of organic carbon in deep-sea sediments is thought to proceed primarily via aerobic respiration and sulfate reduction because the supply of nitrate and metal oxides is not usually significant in deep-sea sediments. Dissimilatory metal reduction, on the other hand, may represent a dominant pathway in coastal and continental shelf sediments where delivery of terrigenous Fe(III) and Mn(IV/III) oxides is sufficiently high or where physical mixing processes near the sediment–water interface can result in the reoxidation of Fe2 + or Mn2 +. Passive continental margin sediments receiving outflow from large rivers are well-known deposition centers for organic carbon and may also be hotspots for metal-reducing microbial activity, considering the simultaneous rapid deposition of unconsolidated metal oxides of terrigenous origin. Despite its potential, only a few studies have examined the role of microbial metal reduction in Corg remineralization in these environments. To investigate carbon remineralization processes in continental margin sediments, shallow cores across channels and levees in the Congo River fan (~ 5000 m) and Louisiana slope (< 1800 m) were profiled for the main redox species involved in early diagenesis using a combination of voltammetric gold mercury (Au/Hg) microelectrodes and conventional analyses. Interestingly, metal reduction dominated carbon remineralization processes in the top ~ 20 cm of sediment subject to high deposition, while evidence for sulfate reduction was lacking. These findings suggest that dissimilatory Fe(III) reduction may be more significant than previously thought in continental slope sediments, which may have important implications on carbon cycling in marine environments. In addition, these findings may have geological implications in controlling atmospheric oxygen levels over geological time and the evolution of microbial life on Earth

Early diagenesis in the sediments of the Congo deep-sea fan dominated by massive terrigenous deposits: Part II - Iron-sulfur coupling

WOS:000408783500011International audienceDeep-sea fans are well known depot centers for organic carbon that should promote sulfate reduction. At the same time, the high rates of deposition of unconsolidated metal oxides from terrigenous origin may also promote metal-reducing microbial activity. To investigate the eventual coupling between the iron and sulfur cycles in these environments, shallow sediment cores ( \textless 50 cm) across various channels and levees in the Congo River deep-sea fan (similar to 5000 m) were profiled using a combination of geochemical methods. Interestingly, metal reduction dominated suboxic carbon remineralization processes in most of these sediments, while dissolved sulfide was absent. In some 'hotspot' patches, however, sulfate reduction produced large sulfide concentrations which supported chemosynthetic-based benthic megafauna. These environments were characterized by sharp geochemical boundaries compared to the iron-rich background environment, suggesting that FeS precipitation efficiently titrated iron and sulfide from the pore waters. A companion study demonstrated that methanogenesis was active in the deep sediment layers of these patchy ecosystems, suggesting that sulfate reduction was promoted by alternative anaerobic processes. These highly reduced habitats could be fueled by discrete, excess inputs of highly labile natural organic matter from Congo River turbidites or by exhumation of buried sulfide during channel flank erosion and slumping. Sulfidic conditions may be maintained by the mineralization of decomposition products from local benthic macrofauna or bacterial symbionts or by the production of more crystalline Fe(III) oxide phases that are less thermodynamically favorable than sulfate reduction in these bio-turbated sediments. Overall, the iron and sulfur biogeochemical cycling in this environment is unique and much more similar to a coastal ecosystem than a deep-sea environment

Differential manganese and iron recycling and transport in continental margin sediments of the Northern Gulf of Mexico

International audiencePore water and solid phase geochemical profiles of sediment cores collected along two transects on the western and eastern sides of the Mississippi River mouth in the northern Gulf of Mexico were incorporated into a reactive transport model to determine the role of manganese and iron in the remineralization of carbon. Reactive transport model calculations indicate that sedimentation rates control the intensity of anaerobic carbon remineralization and select for the dominant anaerobic carbon remineralization pathways. Although sulfate reduction dominates the shelf station (65 m water depth), denitrification and microbial manganese reduction appear equally significant anaerobic respiration processes along the continental slope the closest to the Mississippi River, whereas microbial iron reduction does not represent an important process in these sediments. These findings suggest that the differential kinetics of manganese and iron redox transformations influence carbon remineralization processes on the continental slope. The fast kinetics of Fe2+ oxidation near the sediment-water interface and high sedimentation rates maintain Fe under the form of Fe(III) oxides and thermodynamically prevent sulfate reduction from dominating carbon remineralization processes on the slope, whereas the much slower Mn2+ oxygenation kinetics allows diffusion of Mn2+ across the sediment-water interface of the shelf station closest to the river mouth. Exposure to oxygenated bottom waters and entrainment within mobile muds typical of deltaic sediments during high riverine discharge likely promote the formation and downslope transport of Mn(III/IV) oxides within the nepheloid layer. This phenomenon appears to form a manganese ‘conveyor belt’ that selectively enriches Mn(III/IV) oxides relative to Fe(III) oxides in the deep sediment. In contrast, the intensity of anaerobic carbon remineralization processes along the eastern continental slope the farthest from the Mississippi River plume is much lower due to the low organic and lithogenic inputs, and denitrification dominates anaerobic respiration. Overall, these findings suggest that manganese cycling and its role in carbon remineralization processes in continental slope sediments exposed to large riverine inputs may be more important than previously considered

Variations in sediment production of dissolved iron across a continental margin not dominated by major upwelling or riverine inputs

Despite the undeniable effect of iron on shaping patterns of ocean productivity, the relative importance of the different sources of this limiting nutrient to the ocean is still under debate. Although global estimates indicate that the benthic input of iron to the oceans is significant, most studies have investigated continental margins exposed to either upwelling or large riverine inputs, environments that are not representative of the majority of the oceans. Additionally, the number of studies that report dissolved iron concentrations in continental slope sediments is limited, despite the fact that these regions between the shelf edge and the continental rise make up >5% of the sedimentary surface area of the global ocean. The sedimentary flux of iron has traditionally been considered negligible due to the rapid oxidation of Fe²⁺ in oxic waters and poor solubility of the Fe(III) product. The recent realization that ferric iron may be stabilized in solution by organic ligands during oxidation near the sediment-water interface suggests that a significant fraction of the dissolved iron pool may be present under the form organic-Fe(III) complexes that could eventually reach the overlying waters. In this study, the speciation and biogeochemical importance of iron was determined in intact sediment cores along a transect across the entire continental margin near Cape Lookout, North Carolina, a region not dominated by upwelling or riverine inputs that is representative of most passive continental margins. Rates of diffusive oxygen uptake (DOU) and maximum diffusive fluxes of both dissolved Fe²⁺ and organic-Fe(III) complexes decreased from the coastal zone to the continental shelf, remained low on the shelf and the upper continental slope, but rebounded to reach a maximum in mid-slope sediments where concentrations of Fe(III) oxides were the highest along the transect. In turn, DOU decreased and dissolved iron was below detection in lower-slope sediments, indicating that mid-slope sediments represent depocenters where Fe(III) oxides and organic matter may accumulate. Pore water sulfate and sulfide concentrations as well as separate sediment incubations confirmed that sulfate reduction does not greatly influence the cycling of iron in these sediments. The production of dissolved organic-Fe(III) in these continental margin sediments is likely regulated by a combination of aerobic oxidation in the presence of natural organic ligands near the sediment-water interface, dissimilatory iron reduction, or chemical oxidation of Fe(II) complexed to natural organic ligands. Fluxes of Fe²⁺ and organic-Fe(III) complexes across the sediment-water interface were not observed. However, diffusive fluxes of Fe²⁺ and organic-Fe(III) complexes into the oxic zone of these sediments (<1 cm from the sediment-water interface) and production of dissolved Fe(III) in sediment slurry incubations suggest that complexation of Fe(III) in these sediments may contribute to the stabilization and potential transport of dissolved iron into oxygenated deep ocean waters. Extrapolation to the global ocean suggests that mid-slope depocenters contribute considerably to the iron inventory of the ocean, thus warranting the need for measurement of benthic iron fluxes and dissolved iron speciation in these environments