Early diagenesis of iron and sulfur in Bornholm Basin sediments: the role of near-surface pyrite formation

Pyrite formation in marine sedimentary environments plays a key role in the global biogeochemical cycles of carbon, sulfur and iron, regulating Earth’s surface redox balance over geological time scales. The sulfur isotopic composition of pyrite is one of the major geochemical tools for investigating early diagenetic processes in modern marine sediments and substantive changes to the Earth’s surface environment in ancient sedimentary rocks. We studied sulfur–iron diagenesis and the sulfur isotopic evolution in sediments of the Bornholm Basin, southwestern Baltic Sea, to track the formation of pyrite in the near-surface sediments. Pyrite accumulation is observed with depth over the uppermost 100 cm before the extent of pyritization of the highly reactive iron pool (Fepy/FeHR) stays constant at ca. 0.9, suggesting that the use of a single iron-speciation parameter as a proxy for anoxic and sulfidic conditions needs to be supported by other independent indicators in sedimentary records. Stable sulfur isotopic analysis demonstrates that the bulk pools of elemental sulfur and iron monosulfide do not exchange isotopes completely with aqueous sulfide. We suggest that the reactions with polysulfide and aqueous sulfide are probably restricted to the surface of the solid-phase sulfur and iron-sulfur aggregates. Although pyrite is growing throughout the uppermost sediment column, the pyrite at depth has a sulfur isotopic composition similar to that of pyrite that formed near the sediment surface. To understand the isotopic discrepancy between pyrite and aqueous sulfide in the deeper sediments, we developed a simple diagenetic model, which reproduces the observed sulfur isotopic composition of pyrite well. Our results suggest that much of the pyrite is rapidly formed near the sediment–water interface, and its δ34S is not as influenced by the 34S-enriched pool of aqueous sulfide in the deeper part of the sediment, allowing 32S-enriched pyrite to be preserved in deeper sediments. This near-surface diagenesis and the associated isotopic pattern are possibly of relevance for many marine sediments with high organic matter content, and high aqueous sulfide but low reactive iron availability. Moreover, our sulfur isotopic data demonstrate that extremely slow pyritization is ongoing in the deep lacustrine clay sediments. These results have implications for the interpretation of sulfur–iron geochemical data in both modern and ancient settings as well as for improving reconstructions of ancient depositional environments and a better understanding of the marine sulfur cycle throughout Earth’s history

Iron and sulfate reduction structure microbial communities in (sub-)Antarctic sediments

Permanently cold marine sediments are heavily influenced by increased input of iron as a result of accelerated glacial melt, weathering, and erosion. The impact of such environmental changes on microbial communities in coastal sediments is poorly understood. We investigated geochemical parameters that shape microbial community compositions in anoxic surface sediments of four geochemically differing sites (Annenkov Trough, Church Trough, Cumberland Bay, Drygalski Trough) around South Georgia, Southern Ocean. Sulfate reduction prevails in Church Trough and iron reduction at the other sites, correlating with differing local microbial communities. Within the order Desulfuromonadales, the family Sva1033, not previously recognized for being capable of dissimilatory iron reduction, was detected at rather high relative abundances (up to 5%) while other members of Desulfuromonadales were less abundant (<0.6%). We propose that Sva1033 is capable of performing dissimilatory iron reduction in sediment incubations based on RNA stable isotope probing. Sulfate reducers, who maintain a high relative abundance of up to 30% of bacterial 16S rRNA genes at the iron reduction sites, were also active during iron reduction in the incubations. Thus, concurrent sulfate reduction is possibly masked by cryptic sulfur cycling, i.e., reoxidation or precipitation of produced sulfide at a small or undetectable pool size. Our results show the importance of iron and sulfate reduction, indicated by ferrous iron and sulfide, as processes that shape microbial communities and provide evidence for one of Sva1033’s metabolic capabilities in permanently cold marine sediments

Local and temporal variability of biogeochemical trace metal cycling in marine surface sediments

Trace metals such as Mo, U, and V are useful paleo-redox proxies because of their sensitivity to redox conditions and unique incorporation pathways into marine sediments. However, microbial processes can change the primary signal of specific trace metals that can record the environment at the time of deposition. To investigate the impact of microbial activity on trace metals during early diagenesis, geochemical analysis was performed via bag-incubations on samples collected from two giant box corers during RV SONNE Expedition SO260, funded by the MARUM-Center for Marine Environmental Sciences at the University of Bremen. The first core was taken at the head of the Mar del Plata Canyon, consists of mud to fine sand, and was sampled at five depths. The second core was taken on a coral mound, contains larger grains and abundant coral fragments, and was sampled at four depths. Four splits were taken at each depth; the first split was processed on-board, while splits 2-4 were stored in argon flushed aluminum bags at 4°C for processing and analysis every 4 months on-shore. Our data show strong changes in the pore-water trace metal concentrations in the first box core samples, with Mo increasing more than 5000 nM within 8 months. In samples from the second box core, pore-water Mo increases by more than 1000 nM in the first 4 months before decreasing likely due to the onset of sulfate reduction and, consequently, the formation of hydrogen sulfide leading to the (co-)precipitation of Mo. Our data indicate that the dissolution of iron and manganese oxides leads to the release of associated trace metals at different time points for each site. With comparable amounts of organic carbon at both sites, the observed changes are likely related to sediment composition and physical properties. The sediments sampled at the coral mound have a higher overall porosity and thus provide more space for fluid circulation and host microbial communities. This could explain the faster cycling of iron minerals, potential onset of sulfate reduction, and thus changes in the concentration of relevant trace metals. Therefore, our data suggest that trace metal cycling is not only related to the overall sediment composition, but also to physical properties including pore space, permeability, and grain size which affect how much area is available for microbial communities

Impact of physical properties on biogeochemical trace metal cycling in modern marine surface sediments of the Argentine Basin

Specific trace metal contents that can record the environment at the time of deposition are commonly applied as tracers (proxies) to reconstruct ancient oceanic conditions. However, microbial processes can alter the primary trace metal signal of the sediments during sediment burial. To investigate trace metal cycling during early diagenesis, geochemical analyses were performed via bag-incubations on samples collected from two giant box corers retrieved during RV SONNE Expedition SO260, funded by the MARUM-Center for Marine Environmental Sciences at the University of Bremen. The cores were retrieved off-shore Argentina, one from the head of the Mar del Plata Canyon and the other from a coral mound. Collected sediments are dominantly silt to fine grained sand but include dropstones and coral fragments as well. Our data show strong changes in the pore-water trace metal concentrations in the samples from the Mar del Plata Canyon. For example, molybdenum (Mo) increases by more than 5000 nM within 8 months. In samples from the coral mound box core, pore-water Mo increases by more than 1000 nM in the first 4 months before decreasing again likely due to the onset of sulfate reduction and, consequently, the formation of hydrogen sulfide leading to the (co-)precipitation of Mo. Our data indicate that the reductive dissolution of iron and manganese oxides leads to the release of associated trace metals at different time points for each site. The observed changes are likely related to sediment composition and physical properties. Sediments sampled at the coral mound site include coral fragments, increasing overall porosity and permeability providing more space for fluid circulation and to host microbial communities. Therefore, our data suggest that trace metal cycling is closely related to physical properties including pore space, permeability, and grain size that affect how much area is available for microbial communities