Redox Oscillations Research Articles

Rhythmic glucose metabolism regulates the redox circadian clockwork in human red blood cells

Circadian clocks coordinate mammalian behavior and physiology enabling organisms to anticipate 24-hour cycles. Transcription-translation feedback loops are thought to drive these clocks in most of mammalian cells. However, red blood cells (RBCs), which do not contain a nucleus, and cannot perform transcription or translation, nonetheless exhibit circadian redox rhythms. Here we show human RBCs display circadian regulation of glucose metabolism, which is required to sustain daily redox oscillations. We found daily rhythms of metabolite levels and flux through glycolysis and the pentose phosphate pathway (PPP). We show that inhibition of critical enzymes in either pathway abolished 24-hour rhythms in metabolic flux and redox oscillations, and determined that metabolic oscillations are necessary for redox rhythmicity. Furthermore, metabolic flux rhythms also occur in nucleated cells, and persist when the core transcriptional circadian clockwork is absent in Bmal1 knockouts. Thus, we propose that rhythmic glucose metabolism is an integral process in circadian rhythms.

Manganese: The overlooked contaminant in the world largest mine tailings dam collapse

Manganese (Mn) is an abundant element in terrestrial and coastal ecosystems and an essential micronutrient in the metabolic processes of plants and animals. Mn is generally not considered a potentially toxic element due to its low content in both soil and water. However, in coastal ecosystems, the Mn dynamic (commonly associated with the Fe cycle) is mostly controlled by redox processes. Here, we assessed the potential contamination of the Rio Doce estuary (SE Brazil) by Mn after the world’s largest mine tailings dam collapse, potentially resulting in chronic exposure to local wildlife and humans. Estuarine soils, water, and fish were collected and analyzed seven days after the arrival of the tailings in 2015 and again two years after the dam collapse in 2017. Using a suite of solid-phase analyses including X-ray absorption spectroscopy and sequential extractions, our results indicated that a large quantity of MnII arrived in the estuary in 2015 bound to Fe oxyhydroxides. Over time, dissolved Mn and Fe were released from soils when FeIII oxyhydroxides underwent reductive dissolution. Due to seasonal redox oscillations, both Fe and Mn were then re-oxidized to FeIII, MnIII, and MnIV and re-precipitated as poorly crystalline Fe oxyhydroxides and poorly crystalline Mn oxides. In 2017, redox conditions (Eh: −47 ± 83 mV; pH: 6.7 ± 0.5) favorable to both Fe and Mn reduction led to an increase (~880%) of dissolved Mn (average for 2015: 66 ± 130 µg L-1; 2017: 582 ± 626 µg L-1) in water and a decrease (~75%, 2015: 547 ± 498 mg kg−1; 2017: 135 ± 80 mg kg−1) in the total Mn content in soils. The crystalline Fe oxyhydroxides content significantly decreased while the fraction of poorly ordered Fe oxides increased in the soils limiting the role of Fe in Mn retention. The high concentration of dissolved Mn found within the estuary two years after the arrival of mine tailings indicates a possible chronic contamination scenario, which is supported by the high levels of Mn in two species of fish living in the estuary. Our work suggests a high risk to estuarine biota and human health due to the rapid Fe and Mn biogeochemical dynamic within the impacted estuary.

Impact of Fe(III) (Oxyhydr)oxides Mineralogy on Iron Solubilization and Associated Microbial Communities.

Iron-reducing bacteria (IRB) are strongly involved in Fe cycling in surface environments. Transformation of Fe and associated trace elements is strongly linked to the reactivity of various iron minerals. Mechanisms of Fe (oxyhydr)oxides bio-reduction have been mostly elucidated with pure bacterial strains belonging to Geobacter or Shewanella genera, whereas studies involving mixed IRB populations remain scarce. The present study aimed to evaluate the iron reducing rates of IRB enriched consortia originating from complex environmental samples, when grown in presence of Fe (oxyhydr)oxides of different mineralogy. The abundances of Geobacter and Shewanella were assessed in order to acquire knowledge about the abundance of these two genera in relation to the effects of mixed IRB populations on kinetic control of mineralogical Fe (oxyhydr)oxides reductive dissolution. Laboratory experiments were carried out with two freshly synthetized Fe (oxyhydr)oxides presenting contrasting specific surfaces, and two defined Fe-oxides, i.e., goethite and hematite. Three IRB consortia were enriched from environmental samples from a riverbank subjected to cyclic redox oscillations related to flooding periods (Decize, France): an unsaturated surface soil, a flooded surface soil and an aquatic sediment, with a mixture of organic compounds provided as electron donors. The consortia could all reduce iron-nitrilotriacetate acid (Fe(III)-NTA) in 1–2 days. When grown on Fe (oxyhydr)oxides, Fe solubilization rates decreased as follows: fresh Fe (oxyhydr)oxides > goethite > hematite. Based on a bacterial rrs gene fingerprinting approach (CE-SSCP), bacterial community structure in presence of Fe(III)-minerals was similar to those of the site sample communities from which they originated but differed from that of the Fe(III)-NTA enrichments. Shewanella was more abundant than Geobacter in all cultures. Its abundance was higher in presence of the most efficiently reduced Fe (oxyhydr)oxide than with other Fe(III)-minerals. Geobacter as a proportion of the total community was highest in the presence of the least easily solubilized Fe (oxyhydr)oxides. This study highlights the influence of Fe mineralogy on the abundance of Geobacter and Shewanella in relation to Fe bio-reduction kinetics in presence of a complex mixture of electron donors.

Accumulation of Methylmercury in the High-Altitude Lake Uru Uru (3686 m a.s.l, Bolivia) Controlled by Sediment Efflux and Photodegradation

In shallow aquatic environments, sediment is a significant source of monomethylmercury (MMHg) for surface water (SW). High-altitude aquatic ecosystems are characterized by extreme hydro-climatic constraints (e.g., low oxygen and high UV radiation). We studied, during two seasons, the diel cycles of MMHg in SW and sediment porewaters (PW) of Lake Uru Uru (3686 m a.s.l, Bolivia) contaminated by urban and mining activities. Our results show that diel changes in SW MMHg concentrations (up to 1.8 ng L−1) overwhelm seasonal ones, with higher MMHg accumulation during the night-time and the dry season. The calculation of MMHg diffusive fluxes demonstrates that the sediment compartment was the primary source of MMHg to the SW. Most MMHg efflux occurred during the dry season (35.7 ± 17.4 ng m−2 day−1), when the lake was relatively shallow, more eutrophicated, and with the redoxcline located above the sediment–water interface (SWI). Changes in MMHg accumulation in the PWs were attributed to diel redox oscillations around the SWI driving both the bacterial sulfate reduction and bio-methylation. Finally, we highlight that although MMHg loading from the PW to the SW is large, MMHg photodegradation and demethylation by microorganisms control the net MMHg accumulation in the water column.

From sinks to sources: The role of Fe oxyhydroxide transformations on phosphorus dynamics in estuarine soils

The availability of phosphorus (P) in estuarine ecosystems is ultimately controlled by the nature of interactions between dissolved P and the soil components (e.g., soil minerals), especially iron (Fe) oxyhydroxides. P retention on Fe oxyhydroxides and its subsequent availability depends on mineral crystallinity and susceptibility to dissolution. However, in estuarine soils, geochemical conditions (e.g., redox oscillation and high soil organic matter content) may alter the fate of P and decrease the environmental quality of estuarine waters. The large input of Fe-rich tailings into the Rio Doce Estuary in Brazil in 2015 after a rupture of a Fe ore tailings dam (i.e., “Mariana mine disaster”) offers a unique framework to evaluate the Fe oxyhydroxides role in P availability in estuarine soils, their potential effects on the cycling of P and eutrophication. We observed a significant correlation between Fe minerals and the P content in the estuary soils, suggesting that P enrichment was promoted by the deposited Fe-rich tailings. Adsorption isotherm curves indicated that mine tailings had a strong affinity for P due to presence of crystalline Fe oxyhydroxides in the tailings. Significant losses of Fe (62%) and P (56%) from the estuarine soil was observed two years after the initial impact and in response to redox conditions oscillations. Additionally, the content of high crystallinity Fe oxyhydroxides decreased significantly, whereas that of low crystallinity Fe oxyhydroxides showed an increase over time. These changes were associated with the dissimilatory Fe reduction, which led an increase in the concentrations of readily available P (2015: 2.30 ± 0.41 mg kg−1; 2017: 3.83 ± 1.82 mg kg−1; p < 0.001) in the studied soils. Moreover, in 2017, the dissolved P content exceeded the recommended environmental safety limits by five times. Our results indicate that Fe oxyhydroxides are a continuous source of dissolved P for the ecosystem, and Fe-rich tailings deposited in the estuarine ecosystem may be linked to a potential eutrophication.

Anaerobic and aerobic biodegradation of soil-extracted dissolved organic matter from the water-level-fluctuation zone of the Three Gorges Reservoir region, China

The biodegradation of dissolved organic matter (DOM) in natural environments is determined by its molecular composition and reactivity. Redox oscillations are common in the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR). As a consequence, the soil DOM released is degraded under both anaerobic and aerobic conditions. The DOM compounds available for degradation under contrasting redox conditions and the resulting DOM composition still need to be elucidated. By combining laboratory experiments with an in-depth characterization of DOM optical properties, we show that different pathways controlled the depletion and enrichment of the DOM optical components under different oxygen regimes. In particular, 28-day dark biodegradation assays showed that up to 39.5 ± 4% DOM was degraded under anaerobic conditions, while 55.5 ± 6% DOM was biodegraded under aerobic conditions. Aerobic biodegradation resulted in a higher aromaticity and degree of humification of the DOM compared to anaerobic degradation. The specific UV absorbance at a wavelength of 254 (SUVA254) and biological index (BIX) could be used to track DOM biodegradation under anaerobic conditions. Under aerobic conditions, the SUVA254, BIX and concentration of coloured DOM (CDOM, reflected by a (355)) could track DOM biodegradation, and significant amounts of CDOM could be aerobically biodegraded.

In situ observation of oscillatory redox dynamics of copper

How a catalyst behaves microscopically under reaction conditions, and what kinds of active sites transiently exist on its surface, is still very much a mystery to the scientific community. Here we present an in situ study on the red-ox behaviour of copper in the model reaction of hydrogen oxidation. Direct imaging combined with on-line mass spectroscopy shows that activity emerges near a phase boundary, where complex spatio-temporal dynamics are induced by the competing action of simultaneously present oxidizing and reducing agents. Using a combination of in situ imaging with in situ X-ray absorption spectroscopy and scanning photoemission microscopy, we reveal the relation between chemical and morphological dynamics and demonstrate that a static picture of active sites is insufficient to describe catalytic function of redox-active metal catalysts. The observed oscillatory redox dynamics provide a unique insight on phase-cooperation and a convenient and general mechanism for constant re-generation of transient active sites.

The Belousov-Zhabotinsky Reaction in Thermoresponsive Core-Shell Hydrogel Microspheres with a Tris(2,2'-bipyridyl)ruthenium Catalyst in the Core.

The Belousov-Zhabotinsky (BZ) reaction shows temporal or spatiotemporal structures such as redox oscillation of the catalyst, [ruthenium(II)tris(2,2'-bipyridine)][PF6]2 ([Ru(bpy)3][PF6]2). In this study, autonomously oscillating hydrogel microspheres (microgels) were investigated, which show swelling/deswelling oscillation induced by the redox oscillation of the BZ reaction inside the gel. Despite the periodically and autonomously induced oscillation that does not require an external stimulus, it has not been possible to perform any manipulation of the oscillatory behavior over time. The results of the present study show that it is possible to reversibly switch the microgel oscillations from an "on" active state of the BZ reaction to an "off" inactive state by changing the temperature in combination with thermoresponsive microgels. To realize on-demand switching, the construction of double-shell structures is crucial; the thermoresponsive first shell allows the microgels to modulate the diffusion of the substrates or intermediates in the BZ reaction, while the second shell maintains colloidal stability under high temperatures and high ion concentrations. The functionalized double-shell microgels were prepared via multistep seeded precipitation polymerization. The oscillatory switching behavior of the BZ reaction was observed directly and evaluated by ultraviolet-visible (UV-vis) spectroscopy. The central concept of this study, i.e., "on-off switching" can be expected to benefit the development of advanced bioinspired materials.

Origin and stability of uranium accumulation-layers in an Alpine histosol

Uranium (U) accumulation in organic soils is a common phenomenon that can lead to high U concentration in montane wetlands. The stability of the immobilized U in natural wetlands following redox fluctuations and re-oxidation events, however, is not currently known. In this study, we investigated a saturated histosol that had accumulated up to 6000 ppm of U at 30 cm below ground level (bgl). Uranium in the waters feeding the wetland originates from the weathering of surrounding gneiss rocks, a process releasing trace amounts (<3 ppb) of soluble U into nearby streams. Redox oscillations in the first 20 cm bgl led to the accumulation of U, Ca, S in low permeability layers at 30 and 45 cm bgl. XRF measurements along the core showed that U strongly correlates with sulfur (S) and calcium (Ca), but not iron (Fe). We tested the stability of uranium in the histosol over a nine-month laboratory amendment of a large core of the histosol (∅ 30 cm; length 55 cm) with up to 500 ppm nitrate. Nitrate addition was followed by complete nitrate reduction and re-generation of oxidizing Eh conditions in the top 25 cm of the soil without U release to the soil pore waters above background levels (1–2 ppb). Our results demonstrate that, fast reduction of nitrate, sulfate, and Fe(III) occur in the soil without U release. The remarkable stability of sorbed U in the histosol may result from buffering by sulfide and Sn° and/or strong U(IV)-OM or U(VI)-OM enhanced by organic S moieties or bridging complexation by Ca. That U in the soil was immobile under nitrate addition for up to 9 months can inform remediation strategies based on the use of artificial wetlands to limit U mobility in contaminated sites.

Quantification and molecular characterization of organo-mineral associations as influenced by redox oscillations

Organo-mineral association is one of the most important stabilization mechanisms of soil organic matter. However, few studies have been conducted to assess the retention, transformation, and transportation of colloids (1–1000 nm) and associated organic carbon (OC) in soil. Given the particularly significant role that wetland soils play in carbon storage and cycling, we quantified the dynamics of organo-mineral association within colloidal size range by conducting three consecutive 35-day redox (reduction-oxidation) oscillation experiments using a wetland soil. Molecular compositions of natural nanoparticle (NNP, 2.3–100 nm), fine colloid (100–450 nm), and particulate (450–1000 nm) fractions were measured using isotope ratio mass spectrometry (IRMS) and x-ray photoelectron spectroscopy (XPS). Results showed that NNP and fine colloids constituted up to 8.94 ± 0.50% and 22.19 ± 7.52% of bulk C concentration (2.3–1000 nm), respectively; indicating substantial contributions of these two fractions to the operationally defined “dissolved” (<450 nm) fraction. There was significant enrichment in heavier δ13C isotopes (p < 0.001) with size: NNP (−29.64 ± 0.32‰) < fine colloid (−28.81 ± 0.31‰) < particulate (−28.34 ± 0.25‰) fractions. NNP had the highest percentages of carbonyl/carboxyl C (C=O); while fine colloid and particulate fractions contained more reduced aromatic or aliphatic C (C–C, C=C, C–H). OC became more enriched (‰) in microbial-derived C (higher δ13C) with increasing particle size as well as with repeated redox oscillations. Our findings clearly demonstrate limitations of using the operationally defined “dissolved” fraction (<450 nm) to assess C cycling in ecosystems such as wetlands. Increase in colloid and OC concentrations and presence of more microbial-derived C in larger size fractions additionally imply that redox oscillations promote the formation of molecularly diverse sub-colloid sized organo-mineral associations. Being a composite unit of soil microaggregates, organic-mineral associations can thus influence the overall stability of OC in wetland soils that undergo frequent redox oscillations.

Organic matter provenance and depositional environment of marine-to-continental mudstones and coals in eastern Ordos Basin, China—Evidence from molecular geochemistry and petrology

Cyclothems, composed of interbedded mudstone, coal and sandstone layers, make up the Taiyuan and Shanxi Formations in the Late Carboniferous to Early Permian in North China under a marine-to-continental depositional environment. The cyclothems act as important fossil energy hosts, such as coalbeds, hydrocarbon source rocks and unconventional natural gas reservoirs. Organic geochemistry and petrology of mudstones and coals in the Taiyuan and Shanxi Formations in the eastern Ordos Basin were studied to reveal the organic matter sources and paleoenvironments. Total organic carbon (TOC) contents vary from 1.1 wt% (mudstone) to 72.6 wt% (coal). The samples are mainly within the oil window, with the Tmax values ranging from 433 to 469 °C. Organic petrology and source biomarkers indicate that the mudstones were sourced from a mixed organic matter input, and terrigenous organic matter predominates over aquatic organic matter. The coals are mostly sourced by terrigenous organic matter inputs. High concentrations of hopanes argue for a strong bacterial input. Some m/z 217 mass chromatograms have peaks at the hopanes' retention times as a result of high hopane to sterane ratios. These hopane-derived peaks do not interfere the identification of the steranes because the hopanes and the steranes have different retention times. Maturity-dependent biomarkers demonstrate that the samples have been thermally mature, which agree with the Tmax values. Anomalously low C29 20S/(20S + 20R) and C29 ββ/(ββ + αα) sterane ratios are present in all the samples, and are interpreted as due to the terrigenous organic matter input or the coal-related depositional environment. In addition, biomarkers and iron sulfide morphology indicate that the organic matter of the mudstones deposited in a proximal setting with shallow, brackish/fresh water bodies. With consideration of preservation of organic matter, the redox conditions are dysoxic. Redox oscillations resulted in the records of oxic conditions in some samples. Finally, the coals and the mudstones mainly generate gas and have poor oil generative potential.

A Mississippian black shale record of redox oscillation in the Craven Basin, UK

Early diagenetic redox oscillation processes have been rarely recognised in the ancient rock record but potentially exert an important control on mineral authigenesis, hydrocarbon prospectivity and supply of metals and/or reduced S as part of associated mineral systems. The upper unit of the Mississippian Bowland Shale Formation is a candidate record of diagenetic redox oscillation processes because it was deposited under a relatively high sediment accumulation rate linked to a large delta system, and under dominantly anoxic and intermittently sulphidic bottom-water conditions. In order to characterise the syngenetic and early diagenetic processes, sedimentological and geochemical data were integrated through the Upper Bowland Shale at three sites in the Craven Basin (Lancashire, UK). Organic matter (OM) comprises a mixture of Type II, II-S, II/III and III OM. ‘Redox zones’ are defined by patterns of Fe-speciation and redox-sensitive trace element enrichment and split into two groups. ‘Sulphidic’ zones (EUX, AN-III, AN-I and AN-IT) represent sediments deposited under conditions of at least intermittently active sulphate-reduction in bottom-waters. ‘Non-sulphidic’ zones (OX-RX, OX-F and OX) represent sediments deposited under non-sulphidic (oxic to ferruginous anoxic) bottom-waters. Operation of a shelf-to-basin ‘reactive Fe’ (FeHR) shuttle, moderated by sea level fluctuation and delta proximity, controlled the position and stability of redoxclines between zones of Fe and sulphate reduction, and methanogenesis. Early diagenetic redoxclines were capable of migration through the shallow sediment column relatively quickly, in response to sea level fluctuation. Preservation of syngenetic and early diagenetic geochemical signals shows redoxclines between Fe and sulphate reduction, and the upper boundary of sulphate-methane transition zone, were positioned within decimetres (i.e., 10 s cm) of seabed. Falling sea level and increasing FeHR supply is recognised as a switch from zones EUX (high sea level), AN-III and ultimately AN-I and AN-IT (low sea level). Zone AN-I defines the operation of ‘redox oscillation’, between zones of Fe and sulphate reduction in shallow porewaters, associated with enhanced degradation of OM and complete dissolution of primary carbonate. Preservation of OM and carbonate, in this system, was a function of changing bottom and pore water redox processes. Redox oscillation operated in a siliciclastic, prodeltaic environment associated with a relatively high sediment accumulation rate and high loadings of labile organic matter and metal oxides. These findings are important for understanding Late Palaeozoic black shales in the context of hydrocarbon and mineral systems.

Differential temporal susceptibility to chemotherapeutic treatment in proliferative glioblastoma cancer cells displaying redox and metabolic oscillations

Differential temporal susceptibility to chemotherapeutic treatment in proliferative glioblastoma cancer cells displaying redox and metabolic oscillations

Mercury spikes at the Devonian-Carboniferous boundary in the eastern part of the Rhenohercynian Zone (central Europe) and in the South China Block

Mercury (Hg) chemostratigraphy is an efficient tool for tracing distal volcanism, as suggested in many recent studies. In our research, we focused on Hg and other trace element contents in two Devonian-Carboniferous boundary sections from two different and spatially distant areas. The Lesní lom section (Czech Republic) was located in the south-eastern part of the Laurussia plate and the Duli (Guangxi, China) section in the South China Plate. In both studied sections, the highest Hg, Hg/Fexs or Hg/Al values occurred at the Devonian-Carboniferous boundary. In Lesní lom, organic matter remineralization was connected with origin of pyrite which sequestered a part of the Hg content. The decoupling of Hg and TOC within the Duli section strata and the significant correlation with Al and Fe may suggest that Hg is hosted primarily by clay particles, derived from the landmass into the marine realm. Mercury was also incorporated into pyrites but the poor correlation between Hg and TS (total sulfur) indicates a possible loss of S during oxygenation events. In the Lesní lom section, and partly in the Duli section, Fe and Mn redox cycling may have played an important role in sediment Hg content. Both sections were influenced by widespread hydrothermal and volcanic activity, indicated by positive Eu anomalies, MSI, Fe/Ti, Zr/Al2O3 and Ti/Al2O3 values. It remains to be clarified whether, besides the interplay of local magmatic activity and redox oscillations, there may have been some additional influence of the multi-phase Viluy Large Igneous Province, although its younger phase (364.4 ± 1.7 Ma) slightly precedes the Hg anomalies obtained.

A Robust Model for Circadian Redox Oscillations.

The circadian clock is an endogenous oscillator that controls daily rhythms in metabolism, physiology, and behavior. Although the timekeeping components differ among species, a common design principle is a transcription-translation negative feedback loop. However, it is becoming clear that other mechanisms can contribute to the generation of 24 h rhythms. Peroxiredoxins (Prxs) exhibit 24 h rhythms in their redox state in all kingdoms of life. In mammalian adrenal gland, heart and brown adipose tissue, such rhythms are generated as a result of an inactivating hyperoxidation reaction that is reduced by coordinated import of sulfiredoxin (Srx) into the mitochondria. However, a quantitative description of the Prx/Srx oscillating system is still missing. We investigate the basic principles that generate mitochondrial Prx/Srx rhythms using computational modeling. We observe that the previously described delay in mitochondrial Srx import, in combination with an appropriate separation of fast and slow reactions, is sufficient to generate robust self-sustained relaxation-like oscillations. We find that our conceptual model can be regarded as a series of three consecutive phases and two temporal switches, highlighting the importance of delayed negative feedback and switches in the generation of oscillations.

Biogenic matter characteristics, deposition flux correction, and internal phosphorus transformation in Jiaozhou Bay, North China

Abstract Sediment resuspension in coastal environments is associated with both nutrient repartitioning and with the biological carbon pump, which are further linked to climate change, eutrophication, and marine pollution. To explore these relationships, a resuspension framework, including surficial/core sediments and settling trap-collected particles (TCPs), was assessed in the Jiaozhou Bay (JZB) to examine the behavior and fate of particulate biogenic elements (C, N, P and Si). Resuspension-mediated differences in the concentrations of numerous biogenic proxies were significant and mainly corresponded to selective upwelling of fine-sized, nutrient-enriched particles, to autochthonous organic matter degradation below the trap-deployed level comprising water column transportation and long-term retention at the sediment-water interface. This is further reinforced by smaller inorganic carbon concentrations in sediments and origin-indicating stoichiometric ratio records (OC:ON, OC:OP). Nevertheless, biogenic silica (BSi) exhibited a near homologous concentration and had the largest preservation efficiency. Three principal components, representing production and preservation, remineralisation, and a particle-size effect, accounted for 55.5%, 14.0%, and 9.1% of the total variance, respectively. After calibration, the primary sinking flux was two orders of magnitude smaller than the measured absolute sedimenting flux. Also, the limited bottom achieving contribution with a maximum of 28.3% for freshly biogenic particles produced in autumn/winter circumstances with low primary productivity was inferred. The recurrent resuspension processes in JZB demonstrated that 5–12 cycles of similar or higher intensity resuspension was required before freshly-formed biogenic particles could ultimately be integrated into the sediments. Regarding the internal P sink-switching mechanism, the previously transformed Fe-bound fraction across the estuarine salinity gradient has limited application under offshore redox oscillations, whereas near-quantitative transfer of dissolved P from remineralized organic matter to authigenic carbonate fluorapatite could substantially reduce its bio-availability in resuspended particulate P.

Controls on amorphous organic matter type and sulphurization in a Mississippian black shale

Paleoredox proxies (Fe speciation, trace element and δ34Spy) integrated with sedimentological and palynological observations link the distribution and type of particulate organic matter (OM) preserved to hydrocarbon source rock potential. In the Mississippian Bowland Shale Formation (Lancashire, UK), particulate OM is dominated by “heterogeneous” amorphous OM (AOM), primarily “sharp-edged, pellet-like” (AOMpel) and “heterogeneous, granular” (AOMgr) types. AOMpel is abundant in muds deposited under anoxic and moderately to highly sulphidic conditions and most likely represents the fecal minipellets of zooplankton and/or pellets of macro-zooplankters. We recognize two intervals, “A” and “B,” which exhibit Sorg/TOC > 0.04, suggesting a bulk Type II-S kerogen composition. The Interval A palynofacies is typified by pyritized AOMpel (AOMpyr) particles that contain high-relief organic spheres surrounding individual pyrite framboids, within each AOMpyr particle. These textures are interpreted as sulphurized OM local to pyrite framboids (Sorg-PF). Sorg-PF is rarely observed in Interval B, and absent in all other samples. Redox oscillation between ferruginous and euxinic conditions during early diagenesis of Interval A likely promoted S cycling in microenvironments surrounding pyrite framboids, which generated reactive S species and reactive OM required for sulphurization. Early diagenetic redox oscillation processes were apparently triggered by relative sea level fall, associated with an increased supply of FeHR from adjacent shelves into the basin. Interval B represents deposition during the late stages of basin infill and transition from anoxic to (sub)oxic bottom waters, where AOMpel is replaced by AOMgr as the dominant type of AOM. A large particle diameter at the limit of the mesh size (500 μm), sheet-like, fragmented character, and presence of candidate organic sheaths suggests AOMgr at least partially represent fragments of benthic microbial mats, probably as sulphide-oxidizers. A ternary plot of AOMpel + AOMpyr versus AOMgr versus spores + phytoclasts links the observed palynofacies to bottom and pore water redox conditions, water column productivity and proximity to fluvial (deltaic) supply of spores and phytoclasts. These variables were moderated by changing basin accommodation, driven primarily by eustatic sea level fluctuation. A sequence-stratigraphic control on AOM type and sulphurization is important for understanding the link between source rock heterogeneity and the timing of hydrocarbon generation and expulsion from this source rock.

In vitro simulation of oscillatory redox conditions in intertidal sediments: N, Mn, Fe, and P coupling

In coastal environments, oscillating redox conditions represent a functional state affecting organic matter mineralization. Such transient diagenetic processes remain difficult to study in situ, and we therefore designed a specific reactor to provide experimental results that are environmentally relevant in this context. Here, we present the results of two independent experiments carried out with sediment having contrasting Fe, Mn contents, collected from a coastal tidal lagoon (the Arcachon bay) and a mesotidal estuary (Adour river). Sediment and overlying water were mixed to form slurries that were submitted to redox oscillations to assess the diagenetic mechanisms that affect N, P, Fe, Mn, and S. Changing from anoxic to oxic conditions, we observed a rapid oxidation of dissolved Fe(II) and dissolved inorganic phosphorus (DIP) was apparently trapped by the newly formed Fe-oxyhydroxides (Fe-ox). DIP was totally titrated in the coastal lagoon sediment, but not in estuarine sediment, where the initial amount of Fe available was lower. In both experiments, Mn(II) was only slowly oxidized during the oxidation events and a major part of Mn(II) was adsorbed on new Fe-ox. In coastal lagoon sediment, ammonium remained constant in oxic conditions while nitrate was produced from organic-N mineralization. On the contrary, in estuarine sediment, ammonium was quantitatively oxidized to nitrate. When the conditions became anoxic again, direct reduction of nitrate to ammonium occurred in coastal lagoon sediment. Anaerobic production of nitrate occurred in estuarine sediment, probably because Mn-oxides (Mn-ox), which had a high concentration, acted as an oxidant for ammonium. Consequently, nitrate production prevented Fe(II) accumulation. The Mn-N-Fe coupling outlined here is an apparent indirect oxidation of Fe(II) by Mn-ox through anaerobic nitrification (with Mn-ox) and denitrification (with Fe-ox). This coupling also implied P availability because of the strong control of P by Fe. These experimental results show that nutrient dynamics in oscillatory redox environments such as the estuarine turbidity zone, bioturbated sediment, or tidal permeable sediments highly depends on Mn-and Fe-ox availability.

Marine anoxia and sedimentary mercury enrichments during the Late Cambrian SPICE event in northern Scotland

Elevated mercury concentrations in ancient sedimentary rocks are used as a fingerprint for large igneous province (LIP) volcanism because there is a tight association between known LIPs and coeval sedimentary Hg anomalies. While nonvolcanic processes of sedimentary Hg enrichment, including redox variations, have been demonstrated in modern settings, interpretations of ancient sedimentary Hg records have focused on LIP volcanism. Here, we document a link between sedimentary Hg enrichment and marine redox changes during the late Cambrian Steptoean positive isotopic carbon excursion (SPICE) event, a time with no known LIP. We report a new occurrence of the SPICE event from the Eilean Dubh Formation of northern Scotland, which preserves a series of coeval Hg enrichments. Abundant glauconite, a redox-sensitive iron-bearing mineral, co-occurs stratigraphically with the SPICE and Hg enrichments but is rare to absent from the rest of the section, and bioturbation is low in strata spanning the SPICE. We suggest that local Hg enrichments were driven by changing marine redox conditions during the SPICE event, rather than emplacement of a LIP. Redox oscillations should be considered as an additional control on Hg enrichments in the geologic record.

Wetland methane emissions dominated by plant‐mediated fluxes: Contrasting emissions pathways and seasons within a shallow freshwater subtropical wetland

AbstractWetlands represent the largest natural source of methane; however, very few studies have simultaneously quantified the three main atmospheric flux pathways (i.e., diffusive, ebullition, and plant‐mediated). Unlike better‐studied northern hemisphere systems, many Australian subtropical wetlands undergo extreme wet/dry oscillations, which may strongly impact methane dynamics. We assessed diurnal methane emissions of multiple pathways during three distinct seasonal events within an Australian freshwater wetland. Six‐fold higher methane emissions occurred during summer compared to autumn floods (which followed an extensive dry‐period) and winter/cool conditions. Over three seasons, diffusion represented the highest average areal fluxes (25.9 ± 73.2 mmol m−2 d−1) but were within range of fluxes through water lily aerenchyma (20.8 ± 41.5 mmol m−2 d−1). Average ebullition rates were 5.5 ± 9.7 mmol m−2 d−1. Water column CH4 displayed high spatiotemporal variability, ranging from 55.0 to 253.5 μmol L−1. Time series δ13C–CH4 isotope measurements revealed an oxidation fraction of ~ 15% at night‐time and ~ 36% during day‐time, and night‐time diffusive fluxes were consistently ~ three‐fold higher than day‐time fluxes. By aggregating seasons and weighting for changes in lily coverage, plant‐mediated fluxes accounted for ~ 59% of the annual methane emissions, whereas ebullition and diffusion each accounted for ~ 20%. The up‐scaled annual area‐weighted wetland methane flux (combined pathways) was 27.3 ± 36.7 mmol m−2 d−1. We contend that water lilies (Nymphaea sp.) are the significant carbon source, mediator, and conduit for methane fluxes in this system, and the extremely large seasonal variability of methane emissions reflect dynamic redox oscillations driven by oscillating wet and dry conditions.