Redox Geochemistry Research Articles

Microbial diversity and oil biodegradation potential of northern Barents Sea sediments

The Arctic, an essential ecosystem on Earth, is subject to pronounced anthropogenic pressures, most notable being the climate change and risks of crude oil pollution. As crucial elements of Arctic environments, benthic microbiomes are involved in climate-relevant biogeochemical cycles and hold the potential to remediate upcoming contamination. Yet, the Arctic benthic microbiomes are among the least explored biomes on the planet. Here we combined geochemical analyses, incubation experiments, and microbial community profiling to detail the biogeography and biodegradation potential of Arctic sedimentary microbiomes in the northern Barents Sea. The results revealed a predominance of bacterial and archaea phyla typically found in the deep marine biosphere, such as Chloroflexi, Atribacteria, and Bathyarcheaota. The topmost benthic communities were spatially structured by sedimentary organic carbon, lacking a clear distinction among geographic regions. With increasing sediment depth, the community structure exhibited stratigraphic variability that could be correlated to redox geochemistry of sediments. The benthic microbiomes harbored multiple taxa capable of oxidizing hydrocarbons using aerobic and anaerobic pathways. Incubation of surface sediments with crude oil led to proliferation of several genera from the so-called rare biosphere. These include Alkalimarinus and Halioglobus, previously unrecognized as hydrocarbon-degrading genera, both harboring the full genetic potential for aerobic alkane oxidation. These findings increase our understanding of the taxonomic inventory and functional potential of unstudied benthic microbiomes in the Arctic.

Cyanobacterial blooms increase the release of vanadium through iron reduction and dissolved organic matter complexation in the sediment of eutrophic lakes.

Vanadium (V), a hazardous environmental contaminant, can be highly toxic to aquatic or even human life. Nonetheless, knowledge of its redox geochemistry and mobility in sediments, especially those of eutrophic lakes, remains limited. In this study, we combined in situ high-resolution sampling and laboratory simulation experiments for monitoring soluble and labile V to reveal the mobilization mechanism of V in the sediment of Lake Taihu. The results showed that the concentration of soluble V (1.18-5.22µg L-1) exceeded the long-term ecotoxicology limitation proposed by the government of the Netherlands. The highest value appeared in summer (July to September), with an average concentration of 3.87µg L-1, which exceeded the short-term exposure limit. The remobilization of V in summer was caused by the combined effect of the reduction of Fe(hydr)oxides and dissolved organic matter (DOM) complexation, which accelerated the release of associated Fe-bound V and increased the solubility of DOM-V. Additionally, V showed high mobility in winter, owing to the species of V(Ⅲ)/V(Ⅳ) being oxidized to V(Ⅴ) with higher solubility. It is noteworthy that the elevated remobilization of V in sediments increases the risk of V release from sediments, which poses the threat of water V pollution in Lake Taihu.

Ordovician of the conterminous United States

AbstractThe Ordovician rocks of the conterminous United States (US) have a complex history, spanning multiple ancient basins, shifting palaeoclimate and evolving tectonic regimes. The US portion of the palaeocontinent of Laurentia occupied a relatively stable and isolated position around the southern tropics during the Ordovician. In general, Lower Ordovician rocks form a vast autochthonous blanket of fine-grained (tropical) carbonates that covered much of Laurentia, named the ‘Great American Carbonate Bank’. Outboard, ribbon carbonates and graptolitic shales are found in allochthonous fragments of the ancient continental margin. Middle Ordovician strata are more lithologically diverse, including the addition of several regionally distributed sandstones of the inner detrital belt, mostly overlying the Sauk–Tippecanoe unconformity. Upper Ordovician strata show the greatest lithologic and faunal diversity, reflecting steepening topography resulting from regional compression along the south Laurentian (Appalachian) margin. Recent advances in the interpretation of the US Ordovician come primarily from studies of carbon and oxygen stable isotopes, sequence stratigraphy, palaeoecology, tephrochronology, redox geochemistry, strontium isotopes and geochronology.

Elevated manganese concentrations in groundwater wells after longtime abstraction with bank filtration: developing and testing of a sorption model for Ringerike waterworks, Norway

Groundwater abstraction wells located in aquifers hydraulically connected with rivers or lakes will induce bank filtration if the water level in the aquifer is lowered below the surface-water level. Many groundwater works utilize this induced infiltration to increase the abstraction capacity of their wells. Unfortunately, infiltration of organic-rich surface water may alter the aquifer’s redox geochemistry, and mobilize iron and/or manganese. Ringerike groundwater works in southeastern Norway operates with bank filtration. Manganese concentrations in the abstracted water began to increase 19 years after the start of operations, and stabilized at an elevated level 8 years later. The estimated manganese retardation (Rsorp = 45±18) from a simplified advection-sorption model is comparable to the observed manganese retardation range 36 < Robs < 50, assuming a groundwater residence time tgw ~200 days. The advection-sorption model is based on limited sediment composition and hydrochemical data, which introduces significant uncertainty to Rsorp. However, Rsorp and Robs in the same range, and the good fit between PHREEQC simulated and observed data, indicates that sorption is a possible explanation for the delayed breakthrough of elevated manganese concentrations in Ringerike waterworks’ production wells. Comparable bank filtration schemes are advised to establish observation well(s) between the bank filtration and production well field for early detection of sorption fronts. This would facilitate (1) longer time available for remediation measures and their evaluation, and (2) less manganese to be desorbed during the remediation stage.

Bioavailability, (im)mobilization kinetics, and spatiotemporal patterns of arsenic and cadmium in surficial sediments of a river–estuary–coast system

• High-resolution distributions of DGT-labile As, Cd, Fe, Mn and S are determined. • Fe cycling dominates As mobility, while aerobic OM oxidation controls Cd mobility. • Terrigenous input and redox condition trigger the contrasting seasonal availability of As and Cd. • Cd bioavailability evaluated by DGT clearly increases with salinity gradient. The mobility and bioavailability of toxic arsenic (As) and cadmium (Cd) in sediments are crucial to aquatic ecosystems, while their geochemical transitions and spatiotemporal patterns associated with manganese (Mn), iron (Fe), sulfur (S) and salinity in a river–estuary–coast system are yet largely unexplained. In this study, seasonal field investigation and salinity-dependent indoor experiments were combined using the diffusive gradients in thin films (DGT) technique to measure DGT-labile species from the simultaneous river-estuary-coast interface. The results revealed that As and Fe presented a strong coupling for both DGT species and solid phases, suggesting that As mobility was exclusively regulated under the redox transition of Fe. Unlike other chemicals, the relative enrichment of DGT-labile Cd in surface layers and the close affiliation between sediment Cd fractions and organic carbon indicated that Cd remobilization was largely associated with the aerobic decomposition of organic matter. In addition, the seasonal availability of As and Cd tended to be influenced by terrigenous input and redox condition. The indoor experiment results of an inconspicuous increase in DGT-labile As, and a steady increase in DGT-labile Cd with rising salinity confirm that the bioavailability of Cd evidently increased with salinity gradient, however, the effect of salinity on As remained further debatable. Nevertheless, a model simulation validated that this salinity-dependent Cd mobilization was suboptimal for sustaining a continuous Cd resupply from the sediment to porewater, particularly under limited labile Cd pools. Therefore, with expanding global marine hypoxia and sea level rising, this study highlights that As and Cd redox geochemistry in dynamic estuarine sediments should receive more attention, considering their distinctly contrasting (im)mobilization kinetics.

Crab bioturbation drives coupled iron-phosphate-sulfide cycling in mangrove and salt marsh soils

Coastal wetlands sequestering abundant blue carbon in soils are biogeochemical hotspots and critical habitats for benthic animals like invertebrate fiddler crabs. Here, we reveal how crab bioturbation (i.e., burrowing activity) drives the redox geochemistry of ferrous iron (Fe(II)), phosphate (PO43-), and sulfide (S(-II)) under contrasting vegetation types and hydrological conditions. We used in-situ approaches of diffusive gradients in thin films to perform detailed mm-scale burrow observations in two subtropical wetlands with a vegetation gradient of mudflat-salt marsh-mangrove. Burrow flushing caused a top-down hydrologic connectivity through the crab burrows and thus created a deep depth for the occurrence of Fe(III) reduction and sulfate reduction which were accompanied by P mobilization. The burrow oxidation zone, indicated by lower concentrations of Fe(II), PO43- and S(-II), were shallower in mudflats and salt marshes than in mangroves due to the unique respiratory roots of mangrove plants. The redox of Fe(II), PO43- and S(-II) in crab burrow was insensitive to the convection flow-induced input of dissolved oxygen through the surrounding soil matrix, indicating the burrow soil is an independent microenvironment. Crab burrowing activities favored Fe-S coupling which is conductive the formation of pyrite and alkalinity generation. Overall, our in-situ high-resolution observations and porewater hydraulic dynamics revealed spatially variable soil geochemistry, active coupled cycling of Fe-P-S in crab burrows, and mm-scale hotspots of redox cycling within burrows.

Kinetic characteristics of mobile Mo associated with Mn, Fe and S redox geochemistry in estuarine sediments

Estuarine sediments are crucial repositories and incubators of molybdenum (Mo) during its transport from rivers to the ocean. Here, Mo mobility and related processes in estuarine sediments were explored using high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques. Better correlations were observed between dissolved Mn and Mo than between dissolved Fe and Mo, implying that Mn geochemistry plays a key role in dissolved Mo mobility via molybdate adsorption onto abundant Mn oxides and its substantial release upon intense Mn reduction. As a result, oxic intertidal sediments functioned as Mo sinks, and anoxic subtidal sediments functioned as Mo sources. The opposite vertical distributions between DGT-Labile S and DGT-Labile Mo indicated that the availability of labile Mo can be blocked by aqueous sulfide. However, the corresponding high concentrations of DGT-Labile S and dissolved Mo at subtidal sites demonstrated that the abundant dissolved Mo remobilized via Mn reduction was not effectively solidified by sulfide. Simulation with the DIFS model further verified that redox conditions and induced physicochemical processes are crucial factors controlling Mo mobility, with relatively low dissolved Mo concentrations but an adequate and steady resupply capacity of the bioavailable molybdate in intertidal sediments.

A high-TOC shale in a low productivity world: The late Mesoproterozoic Arctic Bay Formation, Nunavut

The latest Mesoproterozoic Arctic Bay Formation (Borden Basin, Nunavut, Canada) is up to ∼1130 m-thick and contains a significant proportion of unusually organic-rich black shale (up to 12.3 wt% total organic carbon). Insofar as increased biological productivity is related to organic matter burial, this organic-rich succession is seemingly incongruent with the low biological productivity world hypothesised for much of the Proterozoic. To better understand the conditions leading to development of this organic-rich unit, we explore the redox geochemistry of the Arctic Bay Formation using a multi-proxy approach (nitrogen isotopes, iron speciation, total organic carbon, total sulphur, and trace metal abundances). Redox proxy data support a stratified water column, with oxic surface waters underlain by intermittently euxinic waters, which are in turn underlain by persistently ferruginous deeper waters. The highly alkaline, restricted marine basin in which the Arctic Bay Formation was deposited may have allowed for rapid sequestration of highly reactive iron in carbonate minerals, resulting in an ‘excess’ of sulphur that resulted in sulphurisation of organic matter. Estimates for organic matter burial rates during deposition of the Arctic Bay Formation suggest that they were perhaps ∼5–6 times mid-Proterozoic average values (although there are permissible scenarios in which it was extremely productive), underscoring that such organic-rich sedimentary rocks could be produced in a low productivity world.

Magmas are the Largest Repositories and Carriers of Earth’s Redox Processes

Magma is the most important chemical transport agent throughout our planet. This paper provides an overview of the interplay between magma redox, major element chemistry, and crystal and volatile content, and of the influence of redox on the factors that drive igneous system dynamics. Given the almost infinite combinations of temperature, pressure, and chemical compositions relevant to igneous petrology, we focus on the concepts and methods that redox geochemistry provides to understand magma formation, ascent, evolution and crystallization. Particular attention is paid to the strong and complex interplay between melt structure and chemistry, and to the influence that redox conditions have on melt properties, crystallization mechanisms and the solubility of volatile components.

Kinetic Exchange of Remobilized Phosphorus Related to Phosphorus‐Iron‐Sulfur Biogeochemical Coupling in Coastal Sediment

AbstractPhosphorus (P) cycling in coastal sediment can be strongly coupled to iron (Fe) and sulfur (S) geochemical transformations. In this study, the high‐resolution dialysis (HR‐Peeper) and diffusive gradients in thin films (DGT) techniques were employed to synchronously characterize the in situ P‐Fe‐S biogeochemistry in coastal sediments around Xiamen Bay, China. The results show that P remobilization is mainly controlled by dissimilatory Fe reduction in the Jiulong River Estuary (JE) and the eastern coast of Xiamen Bay (EX), while it is dominated by sulfate reduction in the Jiuxi River Estuary. The confined oxidizing depth and the higher DGT‐labile S and soluble Fe (II) concentrations in October indicate that the hypoxia induced by tides and river runoff has a greater effect on sulfate reduction and Fe reduction than high temperatures in July, which accelerate P remobilization and exchange flux. Model simulations of the P resupply and adsorption/desorption kinetics demonstrate that the enhancement of Fe reduction and sulfate reduction triggered by hypoxia promotes the continuous release of P. Both DGT‐labile P and S presented distinct distribution patterns in crab burrows during flood tide and ebb tide. A negative P exchange flux to the overlying water controlled by Fe (III) retention at JE and a positive P exchange flux controlled by intense sulfate reduction at EX were observed, both of which were significantly higher than the molecular diffusion flux. We suggest that internal P loading related to Fe and S redox geochemistry should receive more attention, especially in the context of expanding global marine hypoxia.

Diagenesis and iron paleo-redox proxies: New perspectives from magnetic and iron speciation analyses in the Santa Barbara Basin

Sedimentary redox proxies are usually employed to reconstruct the paleo-redox conditions of bottom water environments, assuming that porewater and bottom water dissolved oxygen concentrations are similar. Using a combination of geochemical and magnetic techniques, we investigate the relationship between iron speciation and mineralogy in recent (~1760–2009 CE) sediments retrieved from the Santa Barbara Basin (SBB) – a modern silled basin with low-oxygen (dissolved O2 < 10 μmol/kg) and sporadically anoxic (no O2 detected) bottom waters. Magnetic analyses reveal that biogenic magnetite is preserved in SBB sulfidic porewaters on at least decadal to centennial time scales. Highly-reactive Fe (oxyhydr)oxides remain preserved despite observed sulfidic porewaters, indicating incomplete pyrite conversion, and producing low Fepy [pyrite Fe]/FeHR ratios. We attribute this observation to restricted porewater reaction kinetics under high sedimentation rates. Our results also reveal non-steady state diagenesis caused by instantaneous depositional events (e.g., turbidites and flood layers). The most reducing water column suggested by Fe speciation coincided with the Macoma layer, where in situ colonization of hypoxia-intolerant bivalve shells argues for the most oxygenated bottom water in the 250 year record. A turbidite potentially introduced fresh unsulfidized FeHR that buffered upward-diffusing sulfide from underlying sediments. Subsequent pyrite precipitation following re-establishment of sulfidic porewaters could have facilitated a “false positive” interpretation. In comparison, redox-sensitive metal enrichments (MoEF, UEF, and ReEF) were not obscured by post-depositional diagenesis and appear to accurately record redox geochemistry at the sediment-water interface.

Arsenic concentrations after drinking water well installation: Time-varying effects on arsenic mobilization

Chronic exposure to geogenic arsenic via drinking water is a worldwide health concern. However, effects of well installation and operation on arsenic concentrations and mobilization are not well understood. This knowledge gap impacts both reliable detection of arsenic in drinking water and effective public health recommendations to reduce exposure to arsenic. This study examines changes in arsenic and redox geochemistry over one year following installation of 254 new domestic water wells in three regions of the north-central USA that commonly have elevated arsenic concentrations. Our regions' geologic settings share some important characteristics with other high-arsenic aquifers: igneous bedrock aquifers; or late Pleistocene-age glacial sand and gravel aquifers interbedded with aquitards. Over the study, arsenic concentrations increased by 16% or more in 25% of wells in glacial aquifer regions, and the redox conditions changed towards more reducing. In wells in the bedrock region, there was no significant change in arsenic concentrations, and redox conditions changed towards more oxidizing. Our findings illustrate the importance of understanding short- to moderate-term impacts of well installation and operation on arsenic and aqueous chemistry, as it relates to human exposure. Our study informs water quality sampling requirements, which currently do not consider the implications sampling timing with respect to well installation. Evaluating arsenic concentrations in samples from new wells in the context of general regional pH and redox conditions can provide information regarding the degree of disequilibrium created by well drilling. Our analysis approach may be transferable and scalable to similar aquifer settings across the globe.

Photoelectric conversion on Earth’s surface via widespread Fe- and Mn-mineral coatings

Sunlight drives photosynthesis and associated biological processes, and also influences inorganic processes that shape Earth's climate and geochemistry. Bacterial solar-to-chemical energy conversion on this planet evolved to use an intricate intracellular process of phototrophy. However, a natural nonbiological counterpart to phototrophy has yet to be recognized. In this work, we reveal the inherent "phototrophic-like" behavior of vast expanses of natural rock/soil surfaces from deserts, red soils, and karst environments, all of which can drive photon-to-electron conversions. Using scanning electron microscopy, transmission electron microscopy, micro-Raman spectroscopy, and X-ray absorption spectroscopy, Fe and Mn (oxyhydr)oxide-rich coatings were found in rock varnishes, as were Fe (oxyhydr)oxides on red soil surfaces and minute amounts of Mn oxides on karst rock surfaces. By directly fabricating a photoelectric detection device on the thin section of a rock varnish sample, we have recorded an in situ photocurrent micromapping of the coatings, which behave as highly sensitive and stable photoelectric systems. Additional measurements of red soil and powder separated from the outermost surface of karst rocks yielded photocurrents that are also sensitive to irradiation. The prominent solar-responsive capability of the phototrophic-like rocks/soils is ascribed to the semiconducting Fe- and Mn (oxyhydr)oxide-mineral coatings. The native semiconducting Fe/Mn-rich coatings may play a role similar, in part, to photosynthetic systems and thus provide a distinctive driving force for redox (bio)geochemistry on Earth's surfaces.

Influence of different redox conditions and dissolved organic matter on pesticide biodegradation in simulated groundwater systems

Insights into the influence of redox conditions, that is the availability of electron acceptors, and dissolved organic matter (DOM) on pesticide biodegradation in groundwater are key to understanding the environmental fate of pesticides in natural groundwater systems. Here, the influence of redox conditions and supplemental DOM addition on biodegradation of pesticides, 2,4-dichlorophenoxyacetic acid (2,4-D), 2,6-dichlorobenzamide (BAM), mecoprop-p (MCPP) and bentazone, was tested in microcosm and subsequent column experiments. Pesticide degradation, functional genes and changes in specific fractions and quantity of DOM were systematically quantified. In aerobic microcosm experiments, the highest 2,4-D degradation rate was obtained with the presence of more assimilable DOM. In column experiments, minimal pesticide degradation (≤33.77%) in any anaerobic redox conditions was observed in the absence of DOM. However, in the presence of DOM, 2,4-D biodegradation was considerably enhanced under nitrate-reducing conditions (from 23.5 ± 10.2% to 82.3 ± 11.6%) and in a column without external electron acceptor amendment (from −6.3 ± 12.6% to 31.1 ± 36.3%). Observed preferential depletion of the fulvic acid fraction of DOM provides indications for specific functional DOM properties. The qPCR results show an increase in microbial biomass and functional genes (tfdA) in liquid phase after DOM addition. The results of this work provide insights into the interplays among DOM, redox geochemistry, and pesticide biodegradation, and show the potential of a novel approach – DOM addition to groundwater systems – for in situ biostimulation technology to remove pesticides from groundwater systems.

Microbial assemblage and palaeoenvironmental reconstruction of the 1.38Ga Velkerri Formation, McArthur Basin, northern Australia.

The ca. 1.38 billion years (Ga) old Roper Group of the McArthur Basin, northern Australia, is one of the most extensive Proterozoic hydrocarbon‐bearing units. Organic‐rich black siltstones from the Velkerri Formation were deposited in a deep‐water sequence and were analysed to determine their organic geochemical (biomarker) signatures, which were used to interpret the microbial diversity and palaeoenvironment of the Roper Seaway. The indigenous hydrocarbon biomarker assemblages describe a water column dominated by bacteria with large‐scale heterotrophic reworking of the organic matter in the water column or bottom sediment. Possible evidence for microbial reworking includes a large unresolved complex mixture (UCM), high ratios of mid‐chained and terminally branched monomethyl alkanes relative to n‐alkanes—features characteristic of indigenous Proterozoic bitumen. Steranes, biomarkers for single‐celled and multicellular eukaryotes, were below detection limits in all extracts analysed, despite eukaryotic microfossils having been previously identified in the Roper Group, albeit largely in organically lean shallower water facies. These data suggest that eukaryotes, while present in the Roper Seaway, were ecologically restricted and contributed little to export production. The 2,3,4‐ and 2,3,6‐trimethyl aryl isoprenoids (TMAI) were absent or in very low concentration in the Velkerri Formation. The low abundance is primary and not caused by thermal destruction. The combination of increased dibenzothiophene in the Amungee Member of the Velkerri Formation and trace metal redox geochemistry suggests that degradation of carotenoids occurred during intermittent oxygen exposure at the sediment–water interface and/or the water column was rarely euxinic in the photic zone and likely only transiently euxinic at depth. A comparison of this work with recently published biomarker and trace elemental studies from other mid‐Proterozoic basins demonstrates that microbial environments, water column geochemistry and basin redox were heterogeneous.

Controls on O2Production in Cyanobacterial Mats and Implications for Earth's Oxygenation

Cyanobacterial mats are widely assumed to have been globally significant hot spots of biogeochemistry and evolution during the Archean and Proterozoic, but little is known about their quantitative contributions to global primary productivity or Earth's oxygenation. Modern systems show that mat biogeochemistry is the outcome of concerted activities and intimate interactions between various microbial metabolisms. Emerging knowledge of the regulation of oxygenic and sulfide-driven anoxygenic photosynthesis by versatile cyanobacteria, and their interactions with sulfur-oxidizing bacteria and sulfate-reducing bacteria, highlights how ecological and geochemical processes can control O2production in cyanobacterial mats in unexpected ways. This review explores such biological controls on O2production. We argue that the intertwined effects of light availability, redox geochemistry, regulation and competition of microbial metabolisms, and biogeochemical feedbacks result in emergent properties of cyanobacterial mat communities that are all critical yet largely overlooked mechanisms to potentially explain the protracted nature of Earth's oxygenation.

Archean phosphorus liberation induced by iron redox geochemistry

The element phosphorus (P) is central to ecosystem growth and is proposed to be a limiting nutrient for life. The Archean ocean may have been strongly phosphorus-limited due to the selective binding of phosphate to iron oxyhydroxide. Here we report a new route to solubilizing phosphorus in the ancient oceans: reduction of phosphate to phosphite by iron(II) at low (<200 °C) diagenetic temperatures. Reduction of phosphate to phosphite was likely widespread in the Archean, as the reaction occurs rapidly and is demonstrated from thermochemical modeling, experimental analogs, and detection of phosphite in early Archean rocks. We further demonstrate that the higher solubility of phosphite compared to phosphate results in the liberation of phosphorus from ferruginous sediments. This phosphite is relatively stable after its formation, allowing its accumulation in the early oceans. As such, phosphorus, not as phosphate but as phosphite, could have been a major nutrient in early pre-oxygenated oceans.

Arsenic, vanadium, iron, and manganese biogeochemistry in a deltaic wetland, southern Louisiana, USA

Abstract Geochemical cycling of the redox-sensitive trace elements arsenic (As) and vanadium (V) was examined in shallow pore waters from a marsh in an interdistributary embayment of the lower Mississippi River Delta. In particular, we explore how redox changes with depth and distance from the Mississippi River affect As and V cycling in the marsh pore waters. Previous geophysical surveys and radon mass balance calculations suggested that Myrtle Grove Canal and the bordering marsh receive fresh groundwater, derived in large part from seepage of the Mississippi River, which subsequently mixes with brackish waters of Barataria Bay. Additionally, the redox geochemistry of pore waters in the wetlands is affected by Fe and S cycling in the shallow subsurface (0–20 cm). Sediments with high organic matter content undergo SO 4 2 − reduction, a process ubiquitous in the shallow subsurface but largely absent at greater depths (~ 3 m). Instead, at depth, in the absence of organic-rich sediments, Fe concentrations are elevated, suggesting that reduction of Fe(III) oxides/oxyhydroxides buffers redox conditions. Arsenic and V cycling in the shallow subsurface are decoupled from their behavior at depth, where both V and As appear to be removed from solution by either diffusion or adsorption onto, or co-precipitation with, authigenic minerals within the deeper aquifer sediments. Pore water As concentrations are greatest in the shallow subsurface (e.g., up to 315 nmol kg − 1 in the top ~ 20 cm of the sediment) but decrease with depth, reaching values − 1 at depths between 3 and 4 m. Vanadium concentrations appear to be tightly coupled to Fe cycling in the shallow subsurface, but at depth, V may be adsorbed to clay or sedimentary organic matter (SOM). Diffusive fluxes are calculated to examine the export of trace elements from the shallow marsh pore waters to the overlying canal water that floods the marsh. The computed fluxes suggest that the shallow sediment serves as a source of Fe, Mn, and As to the surface waters, whereas the sediments act as a sink for V. Iron and Mn fluxes are substantial, ranging from 50 to 30,000 and 770 to 4300 nmol cm − 2 yr − 1 , respectively, whereas As fluxes are much less, ranging from 2.1 to 17 nmol cm − 2 yr − 1 . Vanadium fluxes range from 3.0 nmol cm − 2 yr − 1 directed into the sediment to 1.7 nmol cm − 2 yr − 1 directed out of the sediment

Redox chemistry of nickel in soils and sediments: A review

Knowledge on the redox geochemistry of Ni is behind in comparison to other heavy metals. Hence, this article reviews the direct and indirect impact of redox potential (EH) on mobilization and release dynamics of Ni in soils and sediments across the world. Nickel can show a different behavior in response to EH. Mobilization of Ni increased at low EH in various soils; however, oxic conditions can lead to an increased mobilization of Ni in other soils. Those differences occur because the mobilization of Ni is often indirectly affected by EH, e.g. through EH-dependent pH changes, co-precipitation with iron (Fe) and manganese (Mn) (hydr)oxides, complexation with soil organic carbon, similar position of Ni and magnesium (Mg) in the soil solid phase, and/or precipitation as sulphides. Dissolved concentrations of Ni showed a similar pattern like Fe and increased at low EH in many soils, which might be explained by the reductive dissolution of Fe (hydr)oxides and the release of the co-precipitated/sorbed Ni. Few other studies indicated that Ni might be associated with Mn oxides rather than with Fe oxides. Additionally, the formation of soluble complexes with dissolved organic carbon may contribute to a mobilization of Ni at low EH. Nickel and Mg are similarly affected by redox changes especially in serpentine soils. This review summarizes the recent knowledge about the redox chemistry of Ni and contributes thus to a better understanding of the potential mobilization, hazard, and eco-toxicity of Ni in frequently flooded soils and sediments as agricultural ecosystems.

Role of oxbow lakes in controlling redox geochemistry of shallow groundwater under a heterogeneous fluvial sedimentary environment in an agricultural field: Coexistence of iron and sulfate reduction

This study aimed to extend the knowledge of the vertical distribution of redox conditions of shallow groundwater in heterogeneous fluvial sediments near oxbow lakes. For this study, we revisited the study area of Kim et al. (2009) to examine the redox zoning in details. Three multi-level samplers were installed along a flow path near two oxbow lakes to obtain vertical profiles of the subsurface geology and hydrochemical and isotopic data (δ18O and δD of water, δ15N and δ18O of nitrate, and δ34S of sulfate) of groundwater. Geologic logging showed that characteristics of the heterogeneous subsurface geology are closely related to the pattern of vertical redox zoning. Hydrochemical data in conjunction with nitrogen and sulfur isotope data show that the redox status of groundwater near oxbow lakes is controlled by denitrification, iron reduction, and sulfate reduction. The oxidizing condition of groundwater occurs in the sand-dominant alluvium located in the up-gradient of oxbow lakes, whereas the reducing condition accompanying denitrification, iron reduction, and local sulfate reduction is developed in silt-rich alluvium in and the downgradient of oxbow lakes. The occurrence of sulfate reduction was newly found in this study. However, the vertical profiles of redox-sensitive parameters show that iron reduction and sulfate reduction occur concurrently near oxbow lakes, although the measured redox potentials suggest that thermodynamic conditions are controlled by the stability of Fe2+/Fe-oxides. Therefore, this study shows that the redox condition of groundwater in the iron-rich zone should be carefully interpreted. For this purpose, depth-specific sampling and careful examination of sulfur isotope data will be very useful for identifying the redox processes occurring in the zone with overlapping iron reduction and sulfate reduction in heterogeneous fluvial sediments.