Fluctuations In Redox Conditions Research Articles

Detrital zircon REE and tectonic settings

Earth's long-term history is preserved in the rock archive, yet this record is incomplete, hindering our understanding of how tectonic processes have evolved and shaped our planet's evolution. The physio-chemical resilience of zircon and its ability to be rapidly analyzed for age, isotopic, and elemental data has made it a key phase in unraveling Earth's long-term evolution. The europium anomaly (Eu/Eu*) and the differentiation degree between light and heavy rare earth elements (LREEN¯/HREEN¯) of zircons positively correlate with the pressure of magma crystallization because high pressure inhibits plagioclase crystallization and promotes garnet growth. However, this correlation can be influenced by fluctuations in redox conditions, protolith composition, and water content of magma, whose stability varies in different tectonic settings. Therefore, the correlation coefficient between Eu/Eu* and LREEN¯/HREEN¯ of detrital zircons (rDz) provides a new proxy to help distinguish tectonic settings. We evaluate this new proxy using available global detrital zircon REE data from different tectonic settings in modern and deep times. The data from well-constrained simple tectonic settings suggest that rDz is higher in convergent settings (0.53–0.85) than in collisional settings (0.12–0.51). Convergent settings can also yield relatively low rDz (0.28–0.55) when there is a contribution of a mantle plume lying below the overriding plate of a subduction zone.When studying a detrital zircon archive derived from a large distributive province that includes sources from a variety of settings, known provenance information should be used to disentangle mixed source signals. Also, only considering detrital zircon grains that formed a short time (e.g., 50 or 100 Myr) prior to deposition can reduce the likelihood of multi-cycle distal mixing of contemporaneous but tectonically distinct zircons in a sample. Additionally, greater confidence in interpretations of changes in tectonic settings may be derived from evolution patterns of rDz instead of isolated values. A decreasing trend can likely be interpreted as a transition from convergent to collisional or mantle plume settings, whereas an increasing trend implies an opposite tectonic transition.

Electrofermentation increases concentration of poly γ-glutamic acid in Bacillus subtilis biofilms.

Fluctuations in redox conditions in bioprocesses can alter the end-products, reduce their concentration, and lengthen the process time. Electrofermentation enables rapid metabolic modulation of biosynthesis and allows control of redox imbalances in biofilm-based fermentation processes. In this study, electrofermentation is used to boost the production of the bacterial biopolymer poly-γ-glutamic acid (γ-PGA) from Bacillus subtilis ATCC 6051. When compared to control experiments (3.3 ± 0.99 g L-1 ), the application of an electrode potential E = 0.4 V versus Ag/AgCl results in a more than two-fold increase in the production of γ-PGA (9.13 ± 1.4 g L-1 ). Using an engineered B. subtilis strain, in which γ-PGA production is driven by isopropyl β-d-1-thiogalactopyranoside, electrofermentation improves polymer concentrations from 15.4 ± 1.5 to 23.1 ± 1.6 versus g L-1 . These results confirm that electrofermentation conditions can be adopted to increase the concentration of γ-PGA and perhaps other extracellular biopolymers in industrial strains.

Large Igneous Province Emplacement Triggered an Oxygenation Event at ∼1.4 Ga: Evidence From Mercury and Paleo‐Productivity Proxies

AbstractThe mid‐Proterozoic (∼1.8–0.8 Ga) ocean‐atmosphere system is hypothesized to have experienced fluctuations in redox conditions with transient oxygenation events. One of these happened at ∼1.4 Ga, and it is speculated that this event may link to the emplacement of large igneous province (LIP) at this time. However, direct evidence for this relationship remains to be proved. Here, we report Hg/TOC, P, and trace element concentrations across the ∼1.4 Ga oxygenation event in the Xiamaling Formation of North China. A prominent increase in Hg/TOC is slightly earlier than that of nutrient contents (especially P), pyrite and TOC abundances, suggesting that this distinct oxygenation event was likely the result of LIP activity at ∼1.4 Ga, which increased nutrient and sulfate supply from continental weathering to the ocean, sustaining elevated primary productivity, organic carbon and pyrite burial. This study indicates that LIP weathering could trigger transient oxygenation events during the mid‐Proterozoic.

Long-term change in uranium migration processes in highly eroded granite, demonstrated by uranium series disequilibrium in fracture-filling materials

The mineralization and mobilization of uranium (U) are strongly associated with subsurface redox processes, which can affect the U inventory of deep geological repositories of nuclear waste and are to be considered in safety assessments. The Korea Atomic Energy Research Institute (KAERI) underground research tunnel (KURT) is located in an active groundwater recharge area in granitic bedrock, where oxidizing meteoric water infiltrates the subsurface through permeable zones of highly weathered granite, forming multiple redox fronts. Fracture-filling materials (FFMs) are U-enriched, with concentrations of up to ∼50 mg/kg. We examined long-lived radioactive isotopes of 230Th, 232Th, 234U, 235U, and 238U and other redox-sensitive elements and isotopes, e.g., Fe, δ56Fe, and Ce/Ce*, in the FFMs to understand fluctuations in redox conditions in the granitic fracture system. The primary focus was on U migration processes and to decipher the depths of meteoric water infiltration over geological timescales. Enrichment of uranium and other redox-sensitive elements occurred in FFMs at relatively shallow depths (∼180 m below ground level, bgl), implying that the U distribution was associated with redox interactions of the intruding groundwater that precipitated the secondary minerals. Interaction of FFMs with groundwater caused activity ratios (AR) of the FFMs to deviate moderately from secular equilibrium (AR = 1); 234U/238U AR of 0.87–1.25, and 230Th/238U AR of 0.76–1.22. A dynamic redox transition zone, witnessed by uranium series disequilibrium, shows both deposition and complex redistribution in the 120–205 Ka time frame, is confined to the upper ∼180 m bgl of the fractured rock. The results provide new insights into the long-term paleo-redox processes in the complex granitic fracture systems, which are analogs of potential deep-seated geological repositories for high-level radioactive waste.

Unexpected mechanism for glucose-primed soil organic carbon mineralization under an anaerobic–aerobic transition

Redox reactions are important for cycling of carbon (C) in soils frequently subject to fluctuations in redox conditions, such as wetland soils, which contribute around one third of the global terrestrial C reservoir. Priming effects (PE), induction of changes in C mineralization due to additions of energy-rich organic carbon (OC) substrates, have been largely ignored during redox transitions in current modelling and experimental efforts for evaluating C cycles. In this study, we investigated the effects of glucose input on the mineralization of soil OC during an anaerobic–aerobic transition. Substantially more soil OC was mineralized under aerobic conditions in samples that received glucose under preceding anaerobic conditions compared to controls that did not receive glucose and to samples that received glucose under aerobic-only conditions. Our results reveal a novel PE by which glucose indirectly primes the mineralization of soil OC by increasing the reductive release of iron oxide-associated OC under anaerobic conditions, followed by a dramatic positive PE when aerobic conditions are reestablished. Inclusion of this novel indirect priming mechanism is crucial for accurately predicting the contribution of atmospheric greenhouse gases from soils experiencing redox fluctuations (e.g., wetlands).

Dissolved organic carbon retention by coprecipitation during the oxidation of ferrous iron

The adsorption of dissolved organic carbon (DOC) on iron (Fe) (hydr)oxides represents an important stabilization mechanism for soil organic matter (OM) and contributes to soil C accumulation. However, in soils that experience periodic fluctuations in redox conditions the interaction between DOC and Fe (hydr)oxides may not only involve organic coatings on mineral surfaces, but also Fe-DOC coprecipitates that form during the oxidation of soil solutions containing important amounts of DOC and Fe2+. The aim of this work is to provide new insights into the mechanisms involved, and the amount and selectivity of C retained during the coprecipitation process. A series of Fe-OM associations with increasing C loading was synthesized at pH6 by surface adsorption or coprecipitation (oxidation of ferrous iron) utilizing rice-straw derived dissolved organic matter. The kinetics of Fe2+ oxidation and complexation, and the total and selective retention of DOC during the coprecipitation process were evaluated. Moreover, synthesized associations, as well as a field coprecipitate collected in situ from a paddy soil, were studied by X-ray diffraction, N2 gas adsorption-desorption isotherms, electrophoretic mobility measurements and thermogravimetric analyses. Coprecipitation resulted in higher organic C contents (49–213mgg−1) with respect to adsorbed systems (18–47mgg−1), and favoured the inclusion of OM within highly aggregated associations having particularly low BET specific surface areas. Although coprecipitation led to a strong, selective retention of aromatic constituents, the initial complexation of Fe2+ by aliphatic carboxylic moieties and precipitation as C-rich Fe-OM associations contributed to the total C retention, particularly at higher solution C/Fe ratios. These aliphatic complexes formed during coprecipitation may play an important, though often underestimated, role in C stabilization in soils experiencing frequent redox fluctuations and often characterized by elevated soluble Fe2+ and DOC concentrations.

High-frequency fluctuations in redox conditions during the latest Permian mass extinction

New high-resolution geochemical and sedimentological data from Fiskegrav, East Greenland, reveal fluctuations in marine redox conditions associated with the final disappearance of bioturbating organisms during the latest Permian mass extinction (LPME). Sedimentological observations imply a transgressive episode, and associated geochemical evidence for decreasing oxygen availability and the establishment of persistently ferruginous (Fe2+-rich) conditions implies the shoreward migration of oxygen deficient waters. The long-term decline in dissolved oxygen (DO) availability could have been exacerbated by increasing water temperatures, reducing the solubility of oxygen and promoting thermal stratification. Mixing of the water column could have been further inhibited by freshwater influxes that could have generated salinity contrasts that reinforced thermal stratification. Enhanced runoff could also have increased the delivery of nutrients to the marine shelf, stimulating biological oxygen demand (BOD). During the transition to persistently ferruginous conditions we identify intervals of intermittent benthic meiofaunal recolonization, events that we attribute to small transient increases in DO availability. The mechanism controlling these fluctuations remains speculative, but given the possible centennial- to millennial-scale frequency of these changes, we hypothesise that the mid-latitude setting of Fiskegrav during the Late Permian was sensitive to changes in atmospheric circulation patterns, which may have influenced local precipitation and intermittently modulated some of the processes promoting anoxia.

Paleoredox reconstruction of bituminous shales from the Miocene Hançili Formation, Çankırı-Çorum Basin, Turkey: Evaluating the role of anoxia in accumulation of organic-rich shales

The Early-Middle Miocene succession in the Çankırı-Çorum Basin (ÇÇB) represents a potentially large bituminous shale resource in Turkey. In order to elucidate paleoredox conditions during deposition of the Hançili Formation and to evaluate the role of anoxia in accumulation of the organic-rich shales, a total of 70 samples were collected using channel-profile sampling strategy. Rock Eval pyrolysis data indicates that the majority of the samples have excellent generative potential. Total organic carbon (TOC) content of the Hançili ranges from 0.61 to 13.91 wt% (5.59 wt% on average). Pyrolysis data as well as microscopic and isotopic studies of the samples indicate that the organic matter in the Hançili bituminous shales is immature to early mature, predominantly oil prone (Types I and II), and algal in origin. Moreover, the isotopic (δ13Corg) and geochemical studies suggest that the studied samples were deposited in a lacustrine setting which is consistent with the previous sedimentological inferences. Based on average enrichment factors (EF) of redox-sensitive elements, Ni (∼2.18), V (∼1.58), Cu (∼1.47) and Mo (∼1.1) are only slightly enriched relative to the average shale, whereas U (∼0.76), Pb (∼0.71) and Zn (∼0.73) are depleted. Such an enrichment pattern for redox sensitive trace elements reflects an environment with an oxic-dysoxic water column. Well-known redox-sensitive geochemical ratios (e.g., V/(V+Ni), Ni/Co) also show small-scale fluctuations in redox conditions during deposition of the Hançili Formation. TOC-S-Fe relationships suggest that the Hançili Formation were deposited under oxic-dysoxic water conditions. However, high TOC and HI along with low OI values of the samples suggest that the samples have not weathered under oxic-dysoxic regime. To investigate the role of anoxia in accumulation of the Hançili organic-rich shales, TOC content of 41 samples were plotted against the paleoredox ratios. Lack of correlation between the TOC content and redox indicators suggests that the redox conditions were not a controlling factor in accumulation of the organic-rich shales of the Hançili Formation. Other factors such as high sedimentation rate and periodic eutrophication (e.g., algal bloom) may have influenced accumulation of organic matter (OM) and compensated for the lack of preservative anoxic conditions.

Fluctuation of redox conditions in radioactive waste disposal cell: characterisation of corrosion layers formed on archaeological analogues

Oxygen trapped during the operational phase in disposal cells of an underground radioactive waste repository is often considered to be quickly consumed, notably by corrosion of metallic materials or reducing microorganisms. This would lead to anoxic conditions in most of each disposal cell. In addition to this, a shift from anoxic to oxic conditions could not be excluded locally in disposal cells after their closure due to the ventilation in handling drifts that could contribute to the regeneration of oxygen at the head of each disposal cell. The impact of these transient phases on corrosion processes may affect the confinement properties of metallic components and should thus be assessed. To this end, ferrous archaeological analogues are studied. The present paper focuses on the characterisation of nails that have undergone such transient phases. First, a sample exposed for several hundred years to an aerated environment and then to an anoxic carbonated environment in laboratory for 3 years has been analysed. Corrosion layers after the oxic phase contain mostly oxihydroxides (such as goethite) and some layers perpendicular or parallel to the interface, which is in good agreement with an aerated corrosion. After 3 years under anoxic conditions, the main phase of the corrosion layer is siderite. Second, a sample exposed to an anoxic environment for several hundred years has been immersed in an aerated solution for 5 weeks. The main phases of the corrosion layer after the anoxic phase are iron carbonates and a goethite layer formed on the outer part of the corrosion layer after the immersion in an aerated solution. In both cases, the formation of new phases in the corrosion layer is in good agreement with predictions of thermodynamic modelling based on the respective environmental conditions.

Litho-, bio- and chemostratigraphy across the Cenomanian/Turonian boundary (OAE 2) in the Vocontian Basin of southeastern France

The Oceanic Anoxic Event 2 (OAE 2) was one of at least two large oceanic anoxic events during the Cretaceous Period, and it is characterized in the stratigraphic record by double positive peaks in the δ 13C curves of carbonate and organic carbon. Since the positive δ 13C shift is believed to have resulted from massive burial of isotopically depleted organic carbon generated globally in anoxic oceanic environments, the positive δ 13C shift tend to coincide with widespread deposition of organic-rich sediments. In the case of OAE 2, however, the organic-rich horizons do not accord exactly with the two intervals with positive δ 13C shifts at many sites such as Pueblo, Tarfaya, Eastbourne, and Gubbio (e.g., Tsikos, H., Jenkyns, H.C., Walsworth-Bell, B., Petrizzo, M.R., Forster, A., Kolonic, S., Erba, E., Premoli Silva, I., Baas, M., Wagner, T., Sinninghe Damesté, J.S., 2004. Carbon-isotope stratigraphy recorded by the Cenomanian–Turonian Oceanic anoxic event: correlation and implications based on three key localities. Journal of the Geological Society, London 161, 711–719). The Thomel Level of the Cenomanian/Turonian (C/T) boundary in the Vocontian Basin (southeastern France) accumulated during the OAE 2, and it is marked by frequent intercalations of black shale and organic-rich marl. Detailed litho-, bio- and chemostratigraphy analyses across the C/T boundary in the Vocontian Basin indicate that the δ 13C carbonate profile fluctuation across the Thomel Level corresponds well with changes in lithofacies, total organic carbon (TOC), and calcium carbonate (CaCO 3) abundance. Positive δ 13C excursions within the OAE 2 correlate with basal and middle parts of the Thomel Level that are characterized by high TOC, low CaCO 3, and predominantly contain organic-rich sediments such as black shale and dark gray marl. On the other hand, negative δ 13C intervals within the OAE 2 correspond with lower and upper parts of the Thomel Level that consist of marly limestone and limy marl with low TOC and high CaCO 3 values. Since the fluctuation of redox conditions during Thomel Level deposition in the Vocontian Basin coincides well with the global carbon burial rate indicated by its δ 13C profile, the Thomel Level could be an ideal candidate for the reference section of the OAE 2.

Regeneration of Palladium Subsequent to Solid Solution and Segregation in a Perovskite Catalyst: An Intelligent Catalyst

The object of this study was to provide a function for self-regeneration of precious metals in a usage ambience without auxiliary treatment. The strategy was to control the catalytic active site of those crystalline ceramics known as perovskite-type oxides at the atomic level in order to create the new, needed function. Three series of Pd-containing perovskite catalyst systems were prepared by coprecipitation of Pd with La, Fe, and Co using the alkoxide method. It was confirmed that Pd formed a solid solution of the perovskite-type oxide. And Pd in the perovskite crystal structure exhibited an abnormal oxidation number or higher binding energy than the normal bivalence, and it was presumed to be the reason for increasing the catalytic activity. The results of dissolution analysis for the aged Pd-perovskite catalyst suggested that Pd was not only dispersed on the surface of the perovskite crystal, but was present also in the solid solution of the perovskite crystal. The formation of a solid solution in this Pd-perovskite crystal was affected by the B site elements. And Pd in LaFe0.54Co0.36Pd0.10O3 system was more durable than in LaCo0.90Pd0.10O3 or in LaFe0.90Pd0.10O3. Furthermore, the formation of Pd solid solution into these perovskite crystals also depended on atmospheres and temperatures. It appeared that a high state of dispersion was maintained as Pd repeatedly forms solid solutions in the perovskite crystal or segregates out from the crystal depending on the fluctuation of redox conditions and temperatures in automotive catalyst ambience. We named such a catalyst, wherein a precious metal regenerates itself while in operation and remains highly active without requiring any auxiliary treatment, “an intelligent catalyst”.

Sulfur isotopes and paragenesis of sulfide minerals in the Silurian Waldron Shale, southern Indiana

A diverse assemblage of sulfide morphologies and a wide range of sulfur-isotope compositions indicate that precipitation of sulfide minerals was an ongoing process during both early and later stages of diagenesis in the Silurian Waldron Shale in Indiana. Five distinct forms of sulfide minerals have been identified in samples from seven locations in southern Indiana: (1) pyritized brachiopod shells, (2) pyritized burrows, (3) pyrite cubes, (4) prismatic marcasite, and (5) cockscomb marcasite. Petrographic evidence is used to determine a probable sequence of sulfide precipitation. Isotope compositions range from about -40‰ to +40‰, and each sulfide morphology exhibits a distinctive range of δ34S values. These results are consistent with a model in which sulfate reduction and subsequent precipitation of sulfide minerals occurred in a system that became progressively more closed with respect to sulfate. The sequence of sulfide mineralization reflects the chemical evolution of pore waters during diagenesis of the Waldron Shale. Redox conditions in the sediment were initially aerobic to dysaerobic, sulfate reduction occurring only in reducing microniches where brachiopod shells and then organic-walled burrows were pyritized. Sediments became more fully reducing with burial, when euhedral sulfides were formed. The transition from pyrite to marcasite precipitation suggests a decrease in pore-water pH during later diagenesis and may also indicate fluctuations in redox conditions.

Arsenic geochemistry in geothermal systems

Arsenic is an important trace constituent in geothermal fluids, ranging in concentration from less than 0.1 to nearly 50 ppm. An evaluation of published fluid analyses from geothermal systems indicates that the As content of the reservoir fluids varies inversely with P H 2S and directly with temperature. Aqueous As species occur in two oxidation states, As III and As V. As III predominates in the reservoir fluids, where H 3AsO 3 is inferred to be the dominant aqueous species. Both AS III and As V occur in hot spring fluids. The concentration of As in high-temperature reservoir fluids is regulated by reactions involving pyrite. Concentrations up to 3.8 weight percent As have been measured in pyrite from two systems. The erratic distribution of As in the samples studied is interpreted as resulting from local fluctuations in redox conditions. Arsenopyrite and other As minerals are undersaturated with respect to the high temperature fluids found in most reservoirs. In contrast, orpiment, realgar, As-rich stibnite and marcasite, and iron oxides control the deposition of As in hot spring environments. These minerals become stable in response to decreasing temperature and pH and increasing P O 2 .