Iron Reduction Zone Research Articles

Migration and transformation mechanisms of iron and manganese during river infiltration affected by the changes in riverbed sediment thickness

Riverbed scouring and siltation, affected by variations in river flow and sediment concentration, can cause changes in the sediment thickness, leading to the complex feedback between riverbed permeability and nutrient–reactive transport processes. Currently, the migration and transformation of iron and manganese under changed riverbed sediment thickness have not been taken into account. Based on indoor one-dimensional soil column simulation experiments, along with in-situ monitoring, Rhizon pore water sampling, and 16 s rRNA high-throughput sequencing technologies, this study revealed the migration and transformation patterns of iron and manganese in the river infiltration zone and their contribution rates under different sediment thickness conditions. The results demonstrated that as the riverbed sediment thickness increased, the river infiltration rate and sediment permeability coefficient demonstrated a significant decrease over time. For instance, a 5 cm sediment thickness can decrease the sediment permeability coefficient by 30 % within 32 d of infiltration, expand the disconnection zone to 25 cm, narrow the oxidation–reduction zone, and cause the iron and manganese reduction zone to evolve from a single peak to a double peak pattern. Additionally, as the sediment thickness increased, the contribution of organic matter bound iron-manganese oxidation and iron-manganese oxide reduction to Mn2+ and Fe2+ concentrations in sediment pore water decreased, while the contribution of adsorption and complexation-precipitation increased. This study holds great significance for ensuring the safety of drinking water supply and promoting the sustainable utilization of groundwater resources.

Formation of ammonite concretions through organic decomposition in the iron-reduction zone

ABSTRACT The ammonites in spherical carbonate concretions often preserve their original three-dimensional (3-D) shell shapes and detailed fragile structures. However, the formation process of spherical ammonite concretions is not fully understood. Herein, the ammonite concretions identified in the Cretaceous (Campanian) Osoushinai Formation, Yezo Group, Japan, are examined to understand their formation process during soft-tissue decomposition after burial in marine sediments. In the Osoushinai Formation, almost all observed ammonites in concretions preserve their 3-D form without phragmocone deformation. The calcite filling in the remaining body chamber of ammonites (BC1) shows that shells were buried with soft tissues. These occurrences, negative δ13C values, and the near-zero δ18O values of BC1 as well as the concretions indicate that both BC1 and concretions rapidly formed from dissolved inorganic carbon derived from organic matter, including the soft tissue of dead organisms, in the shallow part of the sediments. The increasing Fe concentration in BC1 shows that BC1 formed in the iron-reduction (FeR) zone, where organic matter was decomposed owing to the activity of iron-reducing microorganisms. The similarity of the elemental and isotopic compositions of BC1 and concretions show that they concurrently formed in the FeR zone. In the Osoushinai Formation, an abundant influx of Fe(III) and intense bioturbation during the deposition of the formation promoted organic decomposition in the FeR zone, causing rapid formation of BC1 and concretions. Such rapidly formed calcite fillings and concretions protected fossils from deformation and dissolution during diagenesis to preserve their 3-D form. Overall, the findings of this study provide new insight into the relation between sedimentary environments and the fossil preservation process via rapid concretion formation.

Iron Reduction in Profundal Sediments of Ultraoligotrophic Lake Tahoe under Oxygen-Limited Conditions.

Increased periods of bottom water anoxia in deep temperate lakes due to decreasing frequency and depth of water column mixing in a warming climate may result in the reductive dissolution of iron minerals and increased flux of nutrients from the sediment into the water column. Here, we assessed the sediment properties and reactivities under depleted oxygen concentrations of Lake Tahoe, a deep ultraoligotrophic lake in the Sierra Nevada mountain range. Using whole-core incubation experiments, we found that a decrease in dissolved oxygen concentration in the top 2 cm of the sediment resulted in an extension of the microbial iron reduction zone from below 4.5 to below 1.5 cm depth. Concentrations of reactive iron generally decreased with sediment depth, and microbial iron reduction seemingly ceased as concentrations of Fe(II) approximated concentrations of reactive iron. These findings suggest that microorganisms preferentially utilized reactive iron and/or iron minerals became less reactive due to mineral transformation and surface passivation. The estimated release of iron mineral-associated phosphorus is not expected to change Lake Tahoe's trophic state but will likely contribute to increased phytoplankton productivity if mixed into surface waters.

A study of iron carbonates and clay minerals for understanding the origin of marine ooidal ironstone deposits

A detailed study of cement and matrix of ooidal ironstones of the Bakchar deposit (Western Siberia) was carried out to assess the factors influencing ore-forming processes. This research focusses on a debated issue about iron sources for Phanerozoic ooidal ironstone. Siderite, the main cementing mineral in the ironstones, is of two varieties that differ in morphology, chemical and isotopic composition, and types of fluid inclusions. The first variety of siderite occurs either as veinlet or as anhedral cement, consisting of isotopically light carbon and oxygen (δ13C from −39.5 to −29.7‰ and δ18O from −15.2 to −5.1‰). Fluid inclusions show a relatively high homogenization temperature (170–320 °C) and methane in the gas phase, corresponding to methane-bearing hydrothermal fluids. The second variety of siderite occurs as aggregates of small crystals in the ironstone matrix and is relatively enriched in heavier stable carbon and oxygen isotopes (δ13C from −28.1 to −12.2‰ and δ18O from −28.4 to −2.1‰), suggesting its formation on the seafloor under the combined influence of methane flux and seawater. The zone of the ooidal ironstone most proximal to the most intense ore-mineralization, an area of intense diffusion of metals and methane through marine sediments, is marked by the first variety of siderite, with rare authigenic minerals, a low clay content (kaolinite, chlorite, nontronite) and a high Fe content of the smectites. The authigenic mineral associations reflect the transition from methanic (sulfate-methane transition zone) to ferruginous (iron reduction zone) through sulfidic environment. The intermediate zone is dominated by montmorillonite-illite-saponite-nontronite and the second variety of siderite. The distal zone of the ore deposit is relatively unaltered, and is marked by illite-montmorillonite (illite-smectite), with rare second variety of siderite. The ratio of 1 M phyllosilicates (kaolinite) to the main swelling 2 M phyllosilicates in the clay fraction (K/M mineral index) decreases from the proximal zone of the intense iron-mineralization to the distal zone. The trend of clay mineral changes reflects the decreasing alteration of sediment matrix away from the main iron-rich and/or carbonate-bearing fluid influx. This study shows the expected lateral and vertical variation in authigenic minerals associated with marine Phanerozoic ironstone deposits, related to variable flux of metal- and methane-bearing fluids.

Early Diagenetic Dolomite as a Potential Archive of Paleo‐redox Fluctuations in an Early Tonian Marine Basin?

AbstractThe Tonian Period witnessed important environmental changes and critical evolutionary innovations. Published iron speciation data suggest a global redox transition of mid‐depth seawaters from euxinic to ferruginous in early Tonian, but details of this transition remain unknown. This study explores Tonian stromatolitic carbonates as a possible archive of paleoenvironmental changes, through the investigation of dolomitic limestones and dolostones associated with stromatolites of the Weiji Formation in the Huaibei region of North China. Three types of dolomitization are recognized on the basis of petrographic and geochemical data. Type I and II dolomitization resulted in dolomitic limestones characterized by LREE depletions, MREE enrichments, positive yttrium anomalies, and a lack of europium anomalies, indicating early diagenetic dolomitization, possibly in the iron reduction zone and under the influence of bottom seawater. The lack of cerium anomalies in these carbonates suggests anoxia in shallow marine environments. The coexistence of ferroan/non‐ferroan dolomite crystals and overgrowth bands is interpreted as possible evidence for rapid fluctuations between iron‐rich and iron‐depleted conditions in pore‐waters or seawaters. In contrast, type III dolomitization resulted in pervasively dolomitized stromatolitic carbonates and likely represents late diagenetic processes. This study highlights the potential of early diagenetic dolomite as an archive for paleoenvironmental investigations.

Microbially induced chromium isotope fractionation and trace elements behavior in lower Cambrian microbialites from the Jaíba Member, Bambuí Basin, Brazil.

In east-central Brazil, the Ediacaran-Cambrian Bambuí Basin has the potential to provide a record of unique geochemical responses of Earth's ocean and atmosphere evolution during this key time interval. From this perspective, we studied an interval of the upper Bambuí Basin using sedimentologic, stratigraphic, and chemostratigraphic tools. The lower Cambrian Jaíba Member of the uppermost Serra da Saudade Formation is an interval of up to 60m-thick of carbonate rocks disposed into two shallowing upward trends. Inner to outer ramp and high-energy shoal deposits are described, in which laminated microbialites are the prevailing sedimentary facies. REE+Y data suggest contamination by iron (oxy)hydroxides that are dissociated from the riverine detritic flux. Sedimentary iron enrichment may be related to the settling of iron nanoparticles in coastal environments, diagenetic iron mobilization, or both. MREE enrichment is caused by microbial degradation of organic matter in the iron reduction zone during the anoxic early-diagenetic stage. Chromium isotopes yielded negatively fractionated values (δ53 Cr=-0.69 to -0.27‰), probably resulting from biotic and abiotic reduction of dissolved Cr(VI) to light and less toxic Cr(III) within pores of microbial mats. The δ53 Cr data of the Jaíba microbialite are thus a product of metabolic reactions in microbial mats and do not reflect seawater signal. The isotopic offset from seawater is feasible from molecular diffusion of Cr into pore water and reduction reactions occurring deep inside the mat, although the exact mechanism and consequences are not yet fully understood due to the poor preservation of metabolic reactions in the geological record. Our study suggests that Cr isotopes can be used to reconstruct Cr and other metals cycling within ancient microbial mats, and that caution should be taken when using past microbialites to infer seawater Cr records and redox state of the atmosphere and ocean.

Can Primary Ferroan Dolomite and Ankerite Be Precipitated? Its Implications for Formation of Submarine Methane-Derived Authigenic Carbonate (MDAC) Chimney

Microbes can mediate the precipitation of primary dolomite under surface conditions. Meanwhile, primary dolomite mediated by microbes often contains more Fe2+ than standard dolomite in modern microbial culture experiments. Ferroan dolomite and ankerite have been regarded as secondary products. This paper reviews the process and possible mechanisms of microbial mediated precipitation of primary ferroan dolomite and/or ankerite. In the microbial geochemical Fe cycle, many dissimilatory iron-reducing bacteria (DIRB), sulfate-reducing bacteria (SRB), and methanogens can reduce Fe3+ to Fe2+, while SRB and methanogens can also promote the precipitation of primary dolomite. There are an oxygen respiration zone (ORZ), an iron reduction zone (IRZ), a sulfate reduction zone (SRZ), and a methanogenesis zone (MZ) from top to bottom in the muddy sediment diagenesis zone. DIRB in IRZ provide the lower section with Fe2+, which composes many enzymes and proteins to participate in metabolic processes of SRB and methanogens. Lastly, heterogeneous nucleation of ferroan dolomite on extracellular polymeric substances (EPS) and cell surfaces is mediated by SRB and methanogens. Exploring the origin of microbial ferroan dolomite may help to solve the “dolomite problem”.

14C-Free Carbon Is a Major Contributor to Cellular Biomass in Geochemically Distinct Groundwater of Shallow Sedimentary Bedrock Aquifers.

Despite the global significance of the subsurface biosphere, the degree to which it depends on surface organic carbon (OC) is still poorly understood. Here, we compare stable and radiogenic carbon isotope compositions of microbial phospholipid fatty acids (PLFAs) with those of in situ potential microbial C sources to assess the major C sources for subsurface microorganisms in biogeochemical distinct shallow aquifers (Critical Zone Exploratory, Thuringia Germany). Despite the presence of younger OC, the microbes assimilated 14C‐free OC to varying degrees; ~31% in groundwater within the oxic zone, ~47% in an iron reduction zone, and ~70% in a sulfate reduction/anammox zone. The persistence of trace amounts of mature and partially biodegraded hydrocarbons suggested that autochthonous petroleum‐derived hydrocarbons were a potential 14C‐free C source for heterotrophs in the oxic zone. In this zone, Δ14C values of dissolved inorganic carbon (−366 ± 18‰) and 11MeC16:0 (−283 ± 32‰), an important component in autotrophic nitrite oxidizers, were similar enough to indicate that autotrophy is an important additional C fixation pathway. In anoxic zones, methane as an important C source was unlikely since the 13C‐fractionations between the PLFAs and CH4 were inconsistent with kinetic isotope effects associated with methanotrophy. In the sulfate reduction/anammox zone, the strong 14C‐depletion of 10MeC16:0 (−942 ± 22‰), a PLFA common in sulfate reducers, indicated that those bacteria were likely to play a critical part in 14C‐free sedimentary OC cycling. Results indicated that the 14C‐content of microbial biomass in shallow sedimentary aquifers results from complex interactions between abundance and bioavailability of naturally occurring OC, hydrogeology, and specific microbial metabolisms.

Identification of groundwater redox process induced by landfill leachate based on sensitive factor method.

Landfill site is a significant source of groundwater pollution. To ensure that the groundwater contamination of landfills can be controlled and repaired scientifically, the identification of groundwater pollution process is needed. On the basis of biogeochemical process of leachate pollutants in the groundwater environment, a sensitive factor method for the identification of groundwater redox process from landfills was established in this research. The method encompasses four phases, including sensitive factors selection, redox zone characterization, weight calculation, and redox zone identification. In the sensitive factor index system employed here, five indicators involving dissolved oxygen (DO), nitrite, Fe2+, sulfide, and CO2 were selected. The boundary of each redox zones was determined by the quantitative method, and the weight of each indicator was calculated by combined weight method. This method was applied to a landfill site in the northeast of China. The result showed that there were five redox zones that appeared in pollution plume, including methanogenic zone (MGZ), sulfate reduction zone (SRZ), iron reduction zone (IRZ), nitrate reduction zone (NRZ), and oxygen reduction zone (ORZ). The results were consistent with the actual situation of the site. The sensitive factor method was scientific and effective to identify the groundwater redox process in landfill and can provide reference data related to investigation and remediation of groundwater pollution in landfill sites.

Microbial sulfur metabolism evidenced from pore fluid isotope geochemistry at Site U1385

At Site U1385, drilled during IODP Expedition 339 off the coast of Portugal on the continental slope, high-resolution sulfate concentration measurements in the pore fluids display non-steady-state behavior. At this site there is a zone of sulfate reduction in the uppermost seven meters of sediment, followed by a 38-meter interval where sulfate concentrations do not change, and finally sulfate concentrations are depleted to zero between 45 and 55meters below seafloor. Below the sulfate minimum zone, there is abundant methane, suggesting that the lower sulfate consumption zone is coupled to anaerobic methane oxidation. We analyze pore water samples from IODP Site U1385 for sulfur and oxygen isotope ratios of dissolved sulfate, as well as the sulfur isotope composition of sedimentary pyrite. The sulfur isotopes in pore fluid sulfate display similar non-steady-state behavior similar to that of the sulfate concentrations, increasing over the uppermost zone of sulfate reduction and again over the lower zone of sulfate-driven anaerobic methane oxidation. The oxygen isotopes in sulfate increase to the ‘apparent equilibrium’ value in the uppermost zone of sulfate reduction and do not change further. Our calculations support the idea that sulfite to sulfide reduction is the limiting step in microbial sulfate reduction, and that the isotope fractionation expressed in the residual pore water sulfate pool is inversely proportional to the net sulfate reduction rate. The sulfur isotope composition of pyrite acquires one value in the uppermost sediments, which may be overprinted by a second value in the deeper sediments, possibly due to iron release during anaerobic methane oxidation or iron diffusion from a higher zone of bacterial iron reduction. Our results have implications for modeling the sulfur isotope composition of the pyrite burial flux in the global biogeochemical sulfur cycle.

Methanogens rapidly transition from methane production to iron reduction.

Methanogenesis, the microbial methane (CH4 ) production, is traditionally thought to anchor the mineralization of organic matter as the ultimate respiratory process in deep sediments, despite the presence of oxidized mineral phases, such as iron oxides. This process is carried out by archaea that have also been shown to be capable of reducing iron in high levels of electron donors such as hydrogen. The current pure culture study demonstrates that methanogenic archaea (Methanosarcina barkeri) rapidly switch from methanogenesis to iron-oxide reduction close to natural conditions, with nitrogen atmosphere, even when faced with substrate limitations. Intensive, biotic iron reduction was observed following the addition of poorly crystalline ferrihydrite and complex organic matter and was accompanied by inhibition of methane production. The reaction rate of this process was of the first order and was dependent only on the initial iron concentrations. Ferrous iron production did not accelerate significantly with the addition of 9,10-anthraquinone-2,6-disulfonate (AQDS) but increased by 11-28% with the addition of phenazine-1-carboxylate (PCA), suggesting the possible role of methanophenazines in the electron transport. The coupling between ferrous iron and methane production has important global implications. The rapid transition from methanogenesis to reduction of iron-oxides close to the natural conditions in sediments may help to explain the globally-distributed phenomena of increasing ferrous concentrations below the traditional iron reduction zone in the deep 'methanogenic' sediment horizon, with implications for metabolic networking in these subsurface ecosystems and in past geological settings.

Redox zones stratification and the microbial community characteristics in a periphyton bioreactor

Bioremediation techniques based on microorganisms have been widely applied to treat polluted surface water, but the efficiencies have been limited, especially in deep and static waters. Microbial aggregates, known as periphyton, were introduced into a tank bioreactor to improve pollutants removal and a periphyton bioreactor with an 84cm column was built to investigate microbe–wastewater interactions. Periphyton greatly improved water quality and produced a distinct stratification in the water column into five redox zones with slight overlaps. From top to bottom these were: oxygen reduction, nitrate reduction, iron reduction, sulfate reduction and methanogenic zone. Periphyton communities had high species diversities (767–947 OTUs) with the facultative zone (middle layer) having higher species richness and functional diversity than the aerobic (top layer) and anaerobic zones (bottom layer). A good knowledge of interactions between periphyton and water column stratification could benefit from integration of periphyton to improve bioremediation of deep and static water.

Origin of dolomites in the Lower Cambrian Xiaoerbulak Formation in the Tarim Basin, NW China: Implications for porosity development

Dolomites occur pervasively in the Cambrian strata in the Tarim Basin, NW China. Although the Cambrian strata have been deeply buried and affected by multiple phases of dolomitization, some intervals in the upper part of the Lower Cambrian Xiaoerbulak Formation developed high porosity. The goal of this study is to understand the origin of different types of dolomites and the formation mechanism of the porosity in the Xiaoerbulak Formation. The geochemistry of matrix dolomites suggests that they formed from middle rare earth element (MREE)-enriched anoxic pore fluids, close to or within the zone of iron reduction. The similar REE+Y patterns and overlapping δ13C values between pore-filling and matrix dolomites indicate that the fluids that were responsible for the precipitation of pore-filling dolomite apparently inherited the signatures of the formation waters that were stored in the host strata. Low δ18O values coupled with high Ba, Zn, and rare earth element (REE) content of pore-filling dolomites indicate that pore-filling dolomites were formed at elevated temperatures. The precipitation of authigenic quartz and saddle dolomites and high Mn content in pore-filling dolomites indicate that hydrothermal fluids that mostly originated from Cambrian basinal clastic units or basement rocks were involved. The mixture of formation water and external hydrothermal fluids is the most likely explanation for the formation of significant porosity and precipitation of pore-filling dolomites at depth. Breccia dolomite, zebra dolomite, and saddle dolomite occur mostly in areas that are close to faults, which suggests that hydrothermal fluids passed through strike-slip faults in this area when these faults were activated. The development of permeable layers in the upper part of the Xiaoerbulak Formation overlain by impermeable layers of the Wusongger Formation suggests a possible potential diagenetic trap. When the faults were activated, high-pressure and high-temperature fluids flowed up through faults and hit low-permeability beds at the base of the Wusongger Formation. Then, the hydrothermal fluids flowed laterally into permeable dolomite strata at the top of the Xiaoerbulak Formation, and encountered the formation brines. Large volumes of secondary porosity formed when host dolomite was leached by the mixing fluids, and pore-filling dolomites and other minerals formed soon afterward. Both the faults and original host facies exerted important influences on the lateral extent of the dissolution.

Impact of anaerobic oxidation of methane on the geochemical cycle of redox-sensitive elements at cold-seep sites of the northern South China Sea

Cold hydrocarbon seepage is a frequently observed phenomenon along continental margins worldwide. However, little is known about the impact of seeping fluids on the geochemical cycle of redox-sensitive elements. Pore waters from four gravity cores (D-8, D-5, D-7, and D-F) collected from cold-seep sites of the northern South China Sea were analyzed for SO42−, Mg2+, Ca2+, Sr2+, dissolved inorganic carbon (DIC), δ13CDIC, dissolved Fe, Mn, and trace elements (e.g. Mo, U). The sulfate concentration–depth profiles, δ13CDIC values and (ΔDIC+ΔCa2++ΔMg2+)/ΔSO42− ratios suggest that organoclastic sulfate reduction (OSR) is the dominant process in D-8 core. Besides OSR, anaerobic oxidation of methane (AOM) is partially responsible for depletion of sulfate at D-5 and D-7 cores. The sulfate consumption at D-F core is predominantly caused by AOM. The depth of sulfate–methane interface (SMI) and methane diffusive flux of D-F core are calculated to be ~7m and 0.035molm−2yr−1, respectively. The relatively shallow SMI and high methane flux at D-F core suggest the activity of gas seepage in this region. The concentrations of dissolved uranium (U) were inferred to decrease significantly within the iron reduction zone. It seems that AOM has limited influence on the U geochemical cycling. In contrast, a good correlation between the consumption of sulfate and the removal of molybdenum (Mo) suggests that AOM has a significantly influence on the geochemical cycle of Mo at cold seeps. Accordingly, cold seep environments may serve as an important potential sink in the marine geochemical cycle of Mo.

Diagenetic alteration of magnetic minerals in Labrador Sea sediments (IODP Sites U1305, U1306, and U1307)

In order to reveal the potential effects of early diagenesis on magnetic minerals in deep‐sea sediments, we studied early diagenetic zones and magnetic mineral characteristics of Lower Pliocene hemipelagic sediment samples from IODP Sites U1305, U1306, and U1307 on Eirik Drift, Labrador Sea. All samples analyzed were unlithified silty clay sediments recovered by a piston corer from depths down to 200 meters composite depth (mcd). Based on shipboard interstitial‐water geochemistry, we divided the sediment column from each site into six early diagenetic zones. Magnetite (Fe3O4) was present at all analyzed depths, whereas maghemite (γFe2O3) was found only above the iron reduction zone. We attribute this to associated changes in interstitial redox conditions, which induced preferential dissolution of maghemitized surfaces on magnetite grains. Mineral magnetic results indicate a general down‐hole change in mean grain size of magnetic minerals. At Site U1307, which has relatively low organic carbon contents, the diagenetic zones occur at greater depths than at the other studies sites. This suggests that interstitial oxygen levels at this site remained high enough to degrade organic matter through oxic bacterial activity, and that detrital magnetic minerals have been preserved even at depth.

Hierarchical Bayesian estimation improves the spatial resolution of diffuse optical tomography: A test with head phantom experiment

Bioremediation techniques based on microorganisms have been widely applied to treat polluted surface water, but the efficiencies have been limited, especially in deep and static waters. Microbial aggregates, known as periphyton, were introduced into a tank bioreactor to improve pollutants removal and a periphyton bioreactor with an 84 cm column was built to investigate microbe–wastewater interactions. Periphyton greatly improved water quality and produced a distinct stratification in the water column into five redox zones with slight overlaps. From top to bottom these were: oxygen reduction, nitrate reduction, iron reduction, sulfate reduction and methanogenic zone. Periphyton communities had high species diversities (767–947 OTUs) with the facultative zone (middle layer) having higher species richness and functional diversity than the aerobic (top layer) and anaerobic zones (bottom layer). A good knowledge of interactions between periphyton and water column stratification could benefit from integration of periphyton to improve bioremediation of deep and static water.

Diagenetic sensitivity of paleoenvironmental proxies: A rock magnetic study of Australian continental margin sediments

A rock magnetic study of the upper 40 m of the Ocean Drilling Program Site 133‐820A recovered at the outer edge of the northeastern Australian continental margin shows that downcore variations in magnetic parameters are diagenetically driven and correlate with the changes in global sea level. We identified intervals enriched in single‐domain (SD) magnetite in the studied section. Unlike previous studies that postulated a detrital source, we show that the bulk of the SD fraction is biologically produced and is likely to be authigenic. The abundance of SD magnetite thus cannot be used as an indicator for provenance or sediment transport mechanisms. The biogenic magnetite was preserved during the high sedimentation rate periods, likely due to a short residence time in the corrosive zone of active iron reduction. The presence of the biogenic magnetite thus can be used as an indicator for a low degree of reductive dissolution. More advanced dissolution during the periods of slow sedimentation, coincident with sea level highstands, resulted in significant changes in the composition of the detrital assemblage, particularly in relative abundances of different mineral phases. The overall stability of the detrital magnetic minerals toward dissolution varies in the studied section as hematite > magnetite > goethite. Due to the higher stability of hematite, reductive diagenesis in general will lead to the lowering of the goethite/hematite (G/H) ratio, which can be mistaken for an increased aridity in the sedimentary source areas in the conventional interpretation for this proxy. The sensitivity of the G/H proxy to diagenesis should be taken into account in paleoenvironmental studies.

Phosphorus cycling in marine sediments from the continental margin off Namibia

In this study we investigate benthic phosphorus cycling in recent continental margin sediments at three sites off the Namibian coastal upwelling area. Examination of the sediments reveals that organic and biogenic phosphorus are the major P-containing phases preserved. High C org/P org ratios just at the sediment surface suggest that the preferential regeneration of phosphorus relative to that of organic carbon has either already occurred on the suspension load or that the organic matter deposited at these sites is already rather refractory. Release of phosphate in the course of benthic microbial organic matter degradation cannot be identified as the dominating process within the observed internal benthic phosphorus cycle. Dissolved phosphate and iron in the pore water are closely coupled, showing high concentrations below the oxygenated surface layer of the sediments and low concentrations at the sediment–water interface. The abundant presence of Fe(III)-bound phosphorus in the sediments document the co-precipitation of both constituents as P-containing iron (oxyhydr)oxides. However, highly dissolved phosphate concentrations in pore waters cannot be explained, neither by simple mass balance calculations nor by the application of an established computer model. Under the assumption of steady state conditions, phosphate release rates are too high as to be balanced with a solid phase reservoir. This discrepancy points to an apparent lack of solid phase phosphorus at sediment depth were suboxic conditions prevail. We assume that the known, active, fast and episodic particle mixing by burrowing macrobenthic organisms could repeatedly provide the microbially catalyzed processes of iron reduction with authigenic iron (oxyhydro)oxides from the oxic surface sediments. Accordingly, a multiple internal cycling of phosphate and iron would result before both elements are buried below the iron reduction zone.

English

The Greens Creek Mine's 3.5M tonne dry-stack tailings pile receives about 800 tonnes of filter-pressed tailings daily. The silt-sized tailings are placed in thin, compacted lifts using a bulldozer and vibratory roller. Carbonate minerals produce a long lag time to acid generation despite a net-neutralization potential of -210 tonnes CaCO3/ktonne. Key influences on pore water compositions include near-surface pyrite and thiosulfate oxidation, vadose zone iron and manganese reduction and saturated zone sulfate reduction. Annual precipitation in excess of 1450 mm poses the largest challenge to achieving design placement densities in this seismically active region. Despite higher operational costs relative to slurry-tailings disposal, the decision to produce dewatered tailings provided economic and environmental advantages. It reduced the ultimate footprint of the facility, lowered closure and water treatment costs, improved pile stability and reduced environmental liability by allowing half of the tailings to be returned underground for use as structural

Transport and fate of nitrate in a glacial outwash aquifer in relation to ground water age, land use practices, and redox processes.

A combination of ground water modeling, chemical and dissolved gas analyses, and chlorofluorocarbon age dating of water was used to determine the relation between changes in agricultural practices, and NO3- concentrations in ground water of a glacial outwash aquifer in west-central Minnesota. The results revealed a redox zonation throughout the saturated zone with oxygen reduction occurring near the water table, NO3- reduction immediately below it, and then a large zone of ferric iron reduction, with a small area of sulfate (SO4(2-)) reduction and methanogenesis (CH4) near the end of the transsect. Analytical and NETPATH modeling results supported the hypothesis that organic carbon served as the electron donor for the redox reactions. Denitrification rates were quite small, 0.005 to 0.047 mmol NO3- yr(-1), and were limited by the small amounts of organic carbon, 0.01 to 1.45%. In spite of the organic carbon limitation, denitrification was virtually complete because residence time is sufficient to allow even slow processes to reach completion. Ground water sample ages showed that maximum residence times were on the order of 50 to 70 yr. Reconstructed NO3- concentrations, estimated from measured NO3- and dissolved N gas showed that NO3- concentrations have been increasing in the aquifer since the 1940s, and have been above the 714 micromol L(-1) maximum contaminant level at most sites since the mid- to late-1960s. This increase in NO3- has been accompanied by a corresponding increase in agricultural use of fertilizer, identified as the major source of NO3- to the aquifer.