Reduction Of Mn Oxides Research Articles

Microbial contribution to the formation of the Carboniferous sedimentary manganese deposits in northwestern China

There is an increasing consensus that microbes play a critical role in the genesis of sedimentary manganese (Mn) carbonate deposits. However, compelling evidence for the microbial involvement during sedimentary Mn metallogenesis remains insufficient, and the specific effects of microbial mediation need to be further elucidated. To advance our understanding of the microbial Mn metallogenesis, multi-scale petrographic observations and high-resolution mineralogical analyses (mainly based on Fourier transform infrared spectrometer and Raman spectroscopy) were conducted on the Carboniferous black shale-hosted Malkansu and limestone-hosted Zhaosu Mn ore deposits in northwestern China. Results show that the Mn ore samples are characterized by finely laminated texture with μm-scale cyclic variations of microbe-associated minerals including rhodochrosite, apatite, quartz, and organic matter as well. These features indicate the ubiquity of microbial mats during Mn mineralization. Microbial involvement is also supported by Mn carbonate aggregates with variable morphologies (filamentous with pearl necklace-like inner forms resembling vermiform morphologies and spheroidal, coccoid-like forms), as well as the annular organic matter that resembles bacterial cells. The cyclic occurrence of trace amounts of remnant Mn oxides verify that primary Mn accumulation took place under oxic bottom waters. The negative co-variation between Mn carbonates and organic matter on Raman profiles suggests that the former was formed during diagenesis via organic matter decomposition coupled with Mn oxide reduction. Combined with previous experimental simulations and natural observations, a two-step microbially mediated Mn metallogenic model is tentatively proposed for the studied Mn deposits: (1) Mn(II)-oxidizing bacteria effectively catalyzed the deposition of considerable Mn oxides under oxygenated bottom water columns, a process that was simultaneously sustained by oxygenic microbial mats such as cyanobacteria; (2) Buried Mn oxides were reduced by highly reactive organic matter sourced from dead biomats of Mn(II)-oxidizing bacteria and benthic phototrophs, a process mediated by Mn(IV)-reducing bacteria which not only promoted the reaction rate but also served as nucleation sites for Mn carbonate minerals. This model can also help to explain the occurrence of diverse authigenic accessory minerals (e.g., quartz, apatite, and talc) and remarkable enrichments of several specific elements (e.g., Co, Ce, and Mo) in the studied Mn ores, which were closely linked to the decomposed cells and extracellular polymeric substance (EPS). By comparing with previous microbial investigations on Mn deposits over geological history, we further highlight the fundamental role of microorganisms in regulating the formation of economic Mn carbonate deposits.

Role of manganese oxides and oxyhydroxides in phosphate sequestration in natural aquatic environments

Excessive inputs of phosphorus (P) to aquatic environments can lead to eutrophication. Adsorption of phosphate on Fe oxides is one of the main process that can limit P availability. Oxidized forms of Mn have also been suggested to play a role in P trapping. However, although a considerable number of studies have shown the relationships between the geochemistry of Fe and that of phosphates, few studies have attempted to show the direct links between Mn and P. In the present study, we studied the adsorption of phosphate on synthetized Mn(III) and Mn(IV) oxides placed under natural conditions. The aim was to compare the role of Fe and Mn oxides in phosphate adsorption. Two muddy sediments were collected in a river bed at the edge of a large agricultural area. A sandy sediment was collected downstream. A muddy and a sandy sample were taken in a coastal environment. The experiments on phosphate adsorption by sediment and Mn oxides were carried out in slurries containing in situ waters spiked with 10 μM phosphate. Control experiments without Mn-oxide addition showed that the natural sediments tested still had the capacity to adsorb phosphate, in particular due to the presence of reactive Fe(III) oxides, extractable by an ascorbate solution. The addition of Mn(III) and Mn(IV) oxides in much larger quantities than the initial quantity of Fe oxides had little impact on the rate of phosphate adsorption. For both Mn(III) and Mn(IV) oxides, the Mn/P ratio between added particulate Mn and adsorbed P was very high, with values between 260 and 1000. The Fe/P ratio of the iron oxides contained in the slurries was between 6 and 20. On average, the P adsorption capacity of the Fe oxides was 50 times greater than that of the Mn oxides. Manganese oxides are generally less abundant than iron oxides in natural environments. Mn oxides therefore play a minor role in P sequestration compared with Fe oxides. Even if there are environments where Mn oxides can concentrate, the reduction of Mn oxides and subsequent liberation of adsorbed P does not represent a major risk for eutrophication of aquatic ecosystems.

Regeneration of sedimentary manganese in coastal sediments deciphered by 224Ra/228Th disequilibria

Manganese (Mn) is a vital micronutrient and participates in multiple biochemical reactions and enzyme catalytic activities. Its cycling is tightly connected with iron (Fe) and nitrogen (N). Although coastal sediments are recognized as an important source of dissolved Mn to marine waters, this contribution remains inadequately quantified. In the summer of 2019 and 2020, we investigated benthic fluxes of dissolved Mn, Fe and ammonia (NH4+) in the Changjiang Estuary and East China Sea shelf using the 224Ra/228Th disequilibrium approach. Our results showed that the availability of reactive Mn oxides (MnD) played a crucial role in sedimentary Mn regeneration, as revealed by the positive correlation (r = 0.75, P < 0.05) between Mn fluxes and MnD contents. In addition, the positive correlation (r = 0.80, P < 0.01) between the decomposition rates of sedimentary organic matter (NH4+ flux) and Mn fluxes suggested that the reduction of MnD was mainly driven by the organic carbon oxidation. Furthermore, NH4+ and Mn fluxes exhibited an exponential increase against the product of dissolved oxygen concentration (DO) and the amplification factor of sediment surface area (ξ). In this context, ξ represents the rate of bottom water DO pumped into the sediment via physical reworking and bio-irrigation. In contrast to the most efficient Fe released from sediment overlain by hypoxic waters (DO <62.5 μM), the maximum Mn flux (63.5 ± 9.4 mmol m−2 d−1) was observed at sediment with oxygenated bottom waters (DO = 158 μM). This implies that the regeneration of Mn was associated with a more permissive redox state compared to that of Fe. We further demonstrated that Mn flux was 1–2 orders of magnitude higher than those estimated through traditional methods. Therefore, coastal sediments may contribute more Mn to ocean waters than previously thought. The precise estimation of Mn release from coastal sediments holds critical significance for research on the global Mn budget.

Substantially excess uranium in the waters of an intertidal salt marsh: A case study of eastern Chongming Beach

Knowledge of the geochemical behavior of uranium is key to an accurate tracing of the decay chain of the uranium series nuclides for material transportation processes on different temporal scales. However, direct observations for understanding dissolved uranium input in tidal water are very limited. Therefore, dissolved uranium was measured in tidal creek water, overlying water, and pore water collected from the central cross section of eastern Chongming Beach, Changjiang Estuary, from November 2012 to October 2013, to observe the spatiotemporal distribution of uranium. The results indicated that the behavior of dissolved uranium is conservative in the tidal creek waters during the flooding process, and an excess of dissolved uranium is observed during the ebbing process; excess uranium phenomena are dominant in the overlying and pore waters throughout the year, particularly at the middle and high tidal flats. The element concentration profiles in pore water showed that uranium release follows Mn oxide reduction in the subsurface suboxic zone. Hence, the exchange process between tidal water and pore water, coupled with ebb tide, represents a potential path for uranium release into the estuary. The three-end-member (Changjiang River, seawater, and pore water) model estimates that 35% of the amount of uranium in ebbing water is derived from pore water. The exchange rate between tidal water and pore water is 196.4 nmol/(cm2·d), with an export flux of 2.34 × 104 mol/a. We suggest that the release of uranium from intertidal salt marshes is an important uranium source in estuarine and coastal regions.

Manganese mobilization from sediment to groundwater in alluvial-lacustrine aquifer system along the lower reaches of Han River

Geogenic contaminants in groundwater limit the drinking water safety and sustainable water supply for domestic purposes throughout the world. Elevated concentrations of manganese (Mn) in groundwater have been one of the major concerns since its adverse health effects to humans, whereas the underlying enrichment mechanism within aquifers is poorly understood. This research aims to connect the spatial distributions of groundwater Mn in different units of the aquifer and the variations of sediment geochemistry to provide comprehensive understanding of Mn provenance and its mobilization through sediment pore-networks into groundwater. Along the lower reaches of Han River within the central Yangtze River Basin, significant heterogenous Mn distribution has been observed with over 60 % of groundwater exceeding the World Health Organization drinking water guideline. Groundwater chemistry and sequential extraction results indicated that the oxidation of organic matter and reduction of Mn oxides and Fe (hydr)oxides in sediments could drive groundwater Mn enrichment. Herein, we propose a burial-dissolution type as the major hydrogeochemical process of Mn mobilization in the region. These findings are of great importance in understanding the geogenic sources of Mn and the controlling processes for its spatial heterogeneity in alluvial-lacustrine aquifer system, which might be applicable to similar Mn-affected geological settings.

Geochemistry of Cryogenian Datangpo manganese deposits in the southeastern Yangtze Platform of South China: implications for the origin of metallogenesis and depositional environment

ABSTRACT Sedimentary manganese (Mn) deposits in the southeastern Yangtze Platform of China have been newly discovered with considerable potential for resource exploration. Such deposits formed due to dynamic changes in the global environment, including the break-up of the supercontinent Rodinia, Snowball Earth glaciation, and the Neoproterozoic Oxygenation Event. The Mn-bearing sequences are hosted in the basal Datangpo Formation and are interbedded with black carbonaceous shale in a series of graben sub-basins in the Nanhua Rift Basin. We investigated the origin of these Mn deposits by analysing the major and trace elements, total organic carbon, and stable carbon and oxygen isotopes. Positive Eu anomalies were observed, indicating that the Mn was derived from a hydrothermal system. Redox-related elements, including Ce, Mo, U, V, S, and P, indicated that the depositional environment of the Mn layers was oxidative and that Mn2+ was initially oxidized to Mn oxides. However, the host rock (i.e. black carbonaceous shale) was deposited under dysoxic – anoxic and even sulfidic water conditions. The alternating distribution of Mn ore and host rock suggested alternating redox conditions in the sub-basins. A negative correlation between the Mn content and δ 13Ccarb values (−10‰–−7‰) and a deficiency of sulphide minerals indicate that the diagenetic degradation of organic matter (δ 13Corg from − 34‰ to − 31.9‰) took place through Mn oxide reduction rather than microbial sulphate reduction and can contribute to a considerable source of bicarbonates. The approximately homogeneous δ 13Ccarb depletion and their apparent discrepancy with δ 13Corg indicate that the post-depositional genesis of the Mn carbonate ore occurred in a relatively open early diagenetic pore water system with a considerable contribution from seawater-derived carbon sources.

Sedimentary Mn Metallogenesis and Coupling among Major Geo-Environmental Events during the Sturtian Glacial–Interglacial Transition

The Sturtian (720–670 Ma) glacial–interglacial transition period was an important interval for sedimentary manganese metallogenesis, including the Mn oxide deposit in the Otjosondu region in Namibia and Mn carbonate deposits in the Datangpo Formation in the south-eastern Yangtze Platform, South China. During this period, Earth experienced the breakup of Rodinia, the Sturtian glaciation, and the Neoproterozoic oxygenation event. In this study, we investigate scenarios that might have provided geologically and geochemically favorable conditions for Mn metallogenesis. In these scenarios, the global recovery of microorganisms enhanced marine primary productivity and O2 levels of the hydrosphere and atmosphere during the Sturtian glacial–interglacial transition. However, the water column was not completely oxidized, maintaining redox stratification. Transgression–regression cycles or O2-rich downwelling drove the exchange of oxygenated topwater and anoxic deep water in rift-related basins that developed due to Rodinia’s breakup. The coupling of these processes precipitated existing dissolved Mn(II) at the margins of basins (Otjosondu region) or at their centers (Yangtze Platform). In the latter case, precursor Mn oxides were further converted into Mn carbonates via the reduction of Mn oxides coupled with organic matter oxidation during early diagenesis. A brief review of Mn metallogenesis in the geological record reveals that Mn metallogenic processes typically occur under geo-environmental conditions that, in concert, produce favorable conditions for Mn sourcing, concentration, and sedimentation.

Particle- and Light-Mediated Processes Control Seasonal Manganese Oxide Cycling in a Meromictic Pond

Manganese (Mn) oxides are strong oxidants and sorbents of nutrients and contaminants, thus their formation mechanisms impact multiple biogeochemical cycles. Manganese in oxic surface waters is often found as dissolved Mn species, such as Mn(II) and Mn(III)-ligand complexes, rather than Mn oxides. This is believed to be a result of Mn oxide reduction by hydrogen peroxide associated with photolysis of organic matter and direct organic matter-mediated reduction within sunlit waters. Nevertheless, Mn oxides can persist in some surface environments, which indicates an incomplete understanding of controls on Mn oxide distributions. Here, we couple field- and lab-based analyses to explore Mn oxide distributions, Mn oxidation rates, and underlying controls on Mn oxide formation within Siders Pond, a brackish and meromictic pond on Cape Cod (Massachusetts, USA) during the summer and fall of 2020. Manganese oxides were observed consistently in sunlit surface waters with concentrations declining to undetectable at the base of the chemocline. Surface Mn oxide concentrations were highest in late summer, reaching concentrations of ∼1 μM, and lowest in late fall, reaching only ∼50 nM. Minerals identified using synchrotron-based absorbance measurements were structurally similar to δ-MnO2 and feitknechtite. Substantial light-mediated Mn oxidation only took place in live incubations of Siders Pond water while net Mn reduction proceeded in killed incubations. Thus, total particle-mediated oxidation by microbes and minerals combined outpaced photoreduction, leading to net accumulation of Mn oxides within Siders Pond. Our results identify important roles for microbial- and mineral-mediated oxidation in determining Mn oxide distributions within surface waters of a natural setting, a finding that may help explain comparable distributions in other locations.

Mineralization of organic matter in microbialites and implications for microbialite reservoirs in Member IV of the Leikoupo Formation, Sichuan Basin, China

Microbialites are important reservoirs for oil and gas. The mineralization of organic matter in microbialites during early diagenesis can produce acidic fluids that dissolve carbonate grains, and can also result in an alkaline pore water that precipitates cement. The mineralization of organic matter in microbialites and its effect on microbialite reservoirs have not yet been studied in detail. In this study, quantitative statistical analysis of the two-dimensional spatial occurrence of pores and microbial fabrics, in situ geochemical analysis of specific components (microbial, transitional zone, and fine spar fabrics), and qualitative evaluation of the implications for microbialite reservoirs were undertaken on microbialites from Member IV of the Leikoupo Formation, Sichuan Basin, China. The quantitative statistical analysis shows that pores are spatially associated with microbial fabrics, but porosity has a poor correlation with microbial fabric content. In situ geochemical data indicate that microbialites with different porosities experienced different processes of organic matter mineralization. The processes of organic matter mineralization such as oxidation and nitrate reduction can provide more dissolution micropores than the process related with sulfate reduction, whereas the process of organic matter mineralization related with Fe–Mn oxide reduction results in cementation. Micropores created by organic matter mineralization can act as fluid channels for later dissolution and are important in the development of microbialite reservoirs.

Evolution of diagenetic fluids in the deeply buried, upper Ediacaran Dengying Formation, Central Sichuan Basin, China

Deeply buried Precambrian carbonate reservoirs host economic petroleum reserves. The upper Ediacaran Dengying Formation (maximum burial depth >8 km) is predominantly composed of dolostone with porosity ranging from 2 to 4%, and minor limestone with a porosity of <2%. Detailed petrological observations and high-resolution geochemical analyses show the evolution from depositional fluids to late diagenetic fluids. Dolostone is subdivided into dolo-mudstone, dolo-laminite, stromatolitic dolostone, thrombolitic dolostone and oncoidal dolostone. These dolostones were precipitated from the Ediacaran seawater and yield rare earth elements and yttrium (REY) patterns similar to those of modern seawater. In the early diagenetic regimes, fibrous dolomite cement, characterised by superchondritic Y/Ho ratios, was first precipitated from marine porewaters, and then fine-crystalline dolomite cement, characterised by low Y/Ho ratios, was formed during meteoric diagenesis. Bladed dolomite cement was subsequently precipitated from (modified) seawater-dominated porewaters. In the intermediate burial regimes, medium- to coarse-crystalline dolomite cement was formed during reduction of Mn oxides. In the deep-burial regimes, saddle dolomite was precipitated from hydrothermal fluids and yields REY patterns characterised by MREE enrichment. Its superchondritic Y/Ho ratios were likely caused by the fractionation between Y and Ho during the co-precipitation of fluorite. Calcite cement has similar parent fluids to those of saddle dolomite. These dolomite phases have similar δ13C values, while their δ18O values gradually decrease during the paragenetic sequence. Multiphase dolomitisation led to the increase in Ca/Mg ratios of formation water and reservoir performance. This study is significant for those concerned with the deeply buried dolostone reservoirs. KEY POINTS Microbialites and five types of dolomite cements have different REY patterns. Co-precipitation of fluorite led to the superchondritic Y/Ho ratios of dolomite. Dolomite and calcite grains show micro-scale geochemical variations. This dolostone reservoir underwent multiphase fluids, including meteoric water, seawater and hydrothermal fluid.

Abundant Fe(III) Oxide‐Bound Arsenic and Depleted Mn Oxides Facilitate Arsenic Enrichment in Groundwater From a Sand‐Gravel Confined Aquifer

AbstractEnrichment mechanisms of high‐arsenic (&gt;10 μg/L) groundwater in confined aquifers with sand‐gravel sediments were rarely documented. Both groundwater and sediment were sampled to investigate mechanisms of groundwater arsenic mobility in sand‐gravel confined aquifers from the Songnen Basin, China. The results show that, although high arsenic groundwater was present in both shallow sandy (&lt;60 m) and deep sand‐gravel aquifers (60–100 m), relatively higher arsenic groundwater was hosted in the deep confined aquifers. The significantly higher proportion of biologically degradable organic carbon (BDOC) in deep confined groundwater than those in shallow unconfined ones (p = 0.03) suggests that higher arsenic concentrations were associated with higher DOM biodegradability. High‐arsenic groundwater was the result of Fe(III) oxide reduction facilitated by BDOC degradation. Arsenic‐rich pyrite framboids were observed in shallow aquifers, which would lower groundwater arsenic concentrations. Sand‐gravel sediments in deep aquifers had remarkably higher contents of extracted Fe(III) oxides and the bound arsenic, HCl‐extractable Fe(II) minerals, and adsorbed arsenic, but considerably lower contents of Mn oxides and Mn(II) minerals than the shallow sandy sediments. Siderite, which is not as efficient as pyrite in sequestering arsenic, was the dominant Fe(II) mineral in the deep aquifer. In the deep aquifer, Mn oxide reduction was almost completed and Fe(III) oxide reduction was predominated, which resulted in high concentrations of groundwater arsenic due to high arsenic/iron molar ratio in the Fe(III) oxides. This study proposed that geogenic high‐arsenic groundwater can occur in sand‐gravel aquifers being high in Fe(III) oxide‐bound arsenic but poor in Mn oxides under reducing conditions.

The dual role of microbes in the formation of the Malkantu manganese carbonate deposit, NW China: Petrographic, geochemical, and experimental evidence

Microbial activity has been proposed to play an important role in the genesis of manganese (Mn) carbonate deposition in natural sediments, with microbial reduction of Mn(IV) oxides as a crucial factor that enhances the saturation state of Mn carbonates in pore waters. Recently, the nucleation of Mn carbonates on cellular templates has been suggested to represent a further microbial control. However, microstructural evidence for this model remains tentative and the Mn carbonate nucleation pathway is still unconstrained. Here we report petrographic and in-situ chemistry that supports the direct role(s) of microbes in the formation of Mn carbonates in the giant Malkansu Mn metallogenic belt at West Kunlun, Northwest China. The Mn carbonate ores exhibit negative δ13C values (−10.2 to −12.7‰) that indicate the microbial reduction of Mn oxide. Microscopic data show that the Mn carbonates exhibit spheroidal rod and dumbbell-like shaped morphologies with a well-defined core-shell structure. These carbonates are intimately associated with net-like organic matter. An incubation experiment was also carried out showing that biogenic Mn carbonate minerals share a strikingly similar crystal form with the natural Mn ore samples. Together, these results provide further evidence for a dual role of microbes in Mn‑carbonate ore formation: Mn(II) and carbonate produced by the microbial reduction of Mn(IV) oxides, and the nucleation of Mn carbonate minerals on cellular surfaces and/or organic matrices. Potentially, these Mn carbonates may have self-assembled to form rod- and dumbbell-like shaped morphologies, analogous to other carbonate systems. The unique core-shell structure observed herein also indicates that the formation of Mn‑carbonates mediated by microbes follows a heterogeneous nucleation pathway.

Insights into hydrothermal controls and processes leading to the formation of the Late Ediacaran Gaoyan stratiform manganese-carbonate deposit, Southwest China

The stratiform Mn-carbonate ore from the Gaoyan deposit occurs within terrigenous-sedimentary rocks of the upper part of the Ediacaran Doushantuo Formation and is characterized by rhodochrosite spherulites. Here we examine the effect of depositional environment and hydrothermal contribution based on new petrographical, mineralogical and geochemical data to shed light on major controls on formation of sedimentary Mn mineralization. The significant geochemical characteristics such as high Mn/Fe ratios (average 32.07), high Ba content (average 1940.9 ppm), positive Eu anomalies (average 1.38) and low REE contents (average 51 ppm) reveal a hydrothermal source. Meanwhile, 87Sr/86Sr ratios of Mn ores are substantially higher than those of carbonate wall rocks and contemporaneous seawater, indicating mixing of radiogenic Sr from the Mn-rich hydrothermal extraction from underlying sediments. This explanation is supported by a positive linear correlation between 87Sr/86Sr ratios and Eu anomalies. The slightly positive Ce anomaly of most Mn carbonate ore samples is mirrored in the negative Ce anomaly of black shale, while different Ce anomalies provide evidence that Mn redox cycling took place across a sub-oxic to anoxic/euxinic water interface. The euxinic conditions, as evidenced by the co-occurrence of fine-grained disseminated pyrite to framboidal aggregates (average diameters <5 μm) with rhodochrosite, can account for the high Mn/Fe ratios via ongoing Fe-sulfide precipitation. Calcite introduction to metal-rich hydrothermal waters with a lower pH relative to surface waters, together with Mn-oxide reduction and sulfate reduction, triggers substantial increases in rhodochrosite saturation and precipitation near the water–sediment interface. Subsequently, increasing Ca2+ in early diagenesis resulted in the precipitation of kutnohorite rims. Altogether, we develop a hydrothermal sedimentary model of the Gaoyan Mn mineralization with rhodochrosite spherulites as the hydrothermal sedimentary mineral phases, and kutnohorite forming during early diagenesis. Slightly light carbon isotope compositions (∼–6.1‰) of the manganese carbonate ores than those of normal dolostones (∼–4.8‰) suggest a limited carbon source from oxidation of organic material by methanogenesis processes during burial diagenesis. This model supports the model for direct Mn-carbonate precipitation from redox-stratified water columns and means that sedimentary Mn mineralization may be mainly controlled by hydrothermal contribution, rather than seawater redox conditions. This means exploration strategies that focus on oxic zone alone as an indication of prospectivity may be ineffective.

The IRIS Imager: A freeware program for quantification of paint removal on IRIS films

AbstractSoils with seasonal or continuous water saturation are characterized by unique redox‐related processes including Fe and Mn oxide reduction. Indicators of reduction in soils (IRIS) devices were created as low‐cost, direct sensors of such reduced chemistry. Such IRIS devices are painted with oxides of Fe or Mn, inserted into the soil, and then removed after a period of time; once removed, the paint lost due to reductive dissolution of these oxides is used to indicate the presence, location, and/or intensity of reducing conditions. However, quantifying the paint removal using existing methods can be subjective and time consuming. Here, we describe the use of the IRIS Imager, an image analysis program that calculates removal of paint from IRIS films in L*a*b* color space (where L* is lightness, a* is red–green value, and b* is blue–yellow value) by comparing the change in lightness between initial and final IRIS film images. Paint removal from films deployed in flooded rice (Oryza sativa L.) paddy soils were quantified using the IRIS Imager, the grid method, and chemical extractions of IRIS films. All three methods were suitable for quantification of paint removal, but the IRIS Imager provided additional statistics to assess heterogeneity in paint removal on individual films and a less subjective approach to quantifying Mn oxide paint removal when Fe oxidation on Mn films was present. This free software can be used with IRIS devices to reproducibly measure paint removal from Fe oxide and Mn oxide IRIS and Fe oxide precipitation on Mn oxide IRIS.

A New Model for the Genesis of Carboniferous Mn Ores, Longtou Deposit, South China Block

AbstractThe lower Carboniferous Luzhai and Baping Formations (ca. 359 Ma) of the South China block, Guangxi Province, comprise an ca. 170-m-thick clastic-carbonate succession capped by Mn ore horizons near the town of Longtou. Excellent exposure of the stratigraphic succession provides an unparalleled opportunity to investigate the origin of carbonate-hosted Mn deposits, which are generally understudied. Lithofacies associations suggest inner and middle shelf clastic rocks accumulated with deposition of carbonates on a mesotrophic middle to outer shelf. In the Longtou region, carbonate deposition during marine transgression culminated with the precipitation of high-grade Mn deposits during maximum flooding. Mn ore horizons are composed of amalgamated alabandite-bearing rhodochrosite, Mn calcite, and braunite laminae. Mn carbonates have been largely interpreted as forming in oxic water columns via reduction of Mn oxides by organic matter. However, paragenetic relationships and δ13C values (similar to those of seawater) indicate the Mn carbonates of Longtou were formed during authigenesis by the emplacement of anoxic, Mn-rich water masses on the distal to middle shelf. Such anoxia is interpreted to have shut down the carbonate factory and diminished sedimentation, a prerequisite for the concentration and precipitation of Mn carbonates in pore water. This research supports the notion that areas of the Paleozoic deep ocean were persistently anoxic and periodically tapped by coastal upwelling to produce Mn- and Fe-rich deposits. Application of this emerging ore deposit model to other economically important carbonate-hosted Mn deposits may improve resource exploration.

Geochemistry and molybdenum isotopes of the basal Datangpo Formation: Implications for ocean-redox conditions and organic matter accumulation during the Cryogenian interglaciation

The basal Datangpo Formation in South China deposited in the Cryogenian interglacial period has gained increasing interest with the well development of organic-rich black shale and its hosted manganese (Mn) deposits (Mn carbonate), which contain important clues to the paleoclimate and ocean redox of this critical interval in Earth's history. The depositional environment of Mn deposits is still uncertain, as suggested by different suites of redox-indicating parameters. In addition, the Mn deposits are notable for their enrichment in organic matter (OM), despite the fact that the diagenetic origin for the Mn deposits would result in large consumption of OM. To gain insight into these issues, we present the geochemical and molybdenum (Mo) isotopic data of black shale and Mn carbonate samples from a new section. The Datangpo Mn carbonates are characterized by low Mo content (mostly <3.2 ppm) and slightly light Mo isotopic composition (mostly from +0.07‰ to +0.52‰), suggesting that Mo, once adsorbed in Mn-oxide particles, was released in suboxic or anoxic (non-sulfidic) pore water due to Mn-oxide reduction and then diffused back into the water column. Hence the extremely light Mo isotopic signatures might be rarely preserved in sediments. The Mn carbonate formation accompanied by OM re-mineralization within sediments may have caused lower total organic carbon (TOC) content relative to that in black shales. However, elevated nutrient input related with hydrothermal fluid activity probably enhanced primary productivity during the Mn deposition period. It is evidenced by: (1) the Mn-TOC-total inorganic carbon covariation displayed by the Datangpo section and (2) a positive correlation between TOC and Mn content when Mn content is higher than 10% in a large compiled dataset. A compilation of Mo concentration and Mo isotope data suggests that widespread anoxia prevailed during the Cryogenian interglaciation. Instead, Neoproterozoic ocean oxygenation was postponed to the Ediacaran.

Severe Coastal Hypoxia Interchange with Ocean Acidification: An Experimental Perturbation Study on Carbon and Nutrient Biogeochemistry

Normally atmospheric CO2 is the major driver of ocean acidification (OA); however, local discharge/degradation of organic matter (OM) and redox reactions can exacerbate OA in coastal areas. In this work we study the response of nutrient and carbon systems to pH decrease in relation to hydrographically induced intermittent characteristics and examine scenarios for future ocean acidification in a coastal system. Laboratory microcosm experiments were conducted using seawater and surface sediment collected from the deepest part of Elefsis Bay; the pH was constantly being monitored while CO2 gas addition was adjusted automatically. In Elefsis Bay surface pCO2 is already higher than global present atmospheric values, while near the bottom pCO2 reaches 1538 μatm and carbonate saturation states were calculated to be around 1.5. During the experiment, in more acidified conditions, limited alkalinity increase was observed and was correlated with the addition of bicarbonates and OM. Ammonium oxidation was decelerated and a nitrification mechanism was noticed, despite oxygen deficiency, paralleled by reduction of Mn-oxides. Phosphate was found significantly elevated for the first time in lower pH values, without reprecipitating after reoxygenation; this was linked with Fe(II) oxidation and Fe(III) reprecipitation without phosphate adsorption affecting both available dissolved phosphate and (dissolved inorganic nitrogen) DIN:DIP (dissolved inorganic phosphate)ratio.

Positive cerium anomalies imply pre-GOE redox stratification and manganese oxidation in Paleoproterozoic shallow marine environments

The Paleoproterozoic Koegas Subgroup (Transvaal Supergroup, South Africa) was deposited in the immediate prelude to the Great Oxidation Event (GOE), and can therefore shed light on oceanic paleoredox conditions just before atmospheric oxidation. Manganese enrichments of ~16 wt% in diagenetic kutnahorite horizons suggest that Mn2+ oxidation occurred, either by free O2 or by an ancient photosystem. Iron and molybdenum isotope trends also support the existence of a Mn4+ oxide sediment flux, suggesting that the Koegas basin may have been redox stratified. Evidence from detrital and authigenic pyrite with mass-independently fractionated sulfur isotopes, however, suggests that the atmosphere was devoid of oxygen. To resolve this contradiction, this paper presents new constraints on pathways of Mn2+ oxidation from field, petrographic, stable isotope, and rare earth element and yttrium (REYSN) analysis of stromatolitic carbonates from the upper Koegas Subgroup. Ferroan dolostones and limestones preserve marine REYSN arrays with positive CeSN anomalies. These differences are explained by a redox stratified basin, whereby Mn2+ and Ce3+ are oxidized at a redoxcline and Ce is adsorped onto sinking Mn oxide particles. Mn oxide particles and a negative Ce anomaly from the oxidized upper water column are transferred into carbonates accumulating above the redoxcline. Diagenetic fluids later reduce the Mn oxides to kutnahorite. Below the redoxcline, reduction of Mn oxides enriches carbonates in Mn and a positive Ce anomaly. This contribution adds evidence for the development of oxygen oases and redox-stratified basins before the GOE. Redox stratification was best developed during transgressions. During regressions, a deltaic system prograded into the Koegas Basin. High sedimentation rates likely allowed for preservation of detrital pyrite only in the deltaic sandstones, thus explaining the contradictory geochemical evidence. No previously unknown ancient photosystem of Mn oxidation is required to explain Mn oxidation.

Evaluating a primary carbonate pathway for manganese enrichments in reducing environments

Most manganese (Mn) enrichments in the sedimentary rock record are hosted in carbonate minerals, which are assumed to have formed by diagenetic reduction of precursor Mn-oxides, and are considered diagnostic of strongly oxidizing conditions. Here we explore an alternative model where Mn-carbonates form in redox-stratified water columns linked to calcium carbonate dissolution. In ferruginous Brownie Lake in Minnesota, USA, we document Mn-carbonates as an HCl-extractable phase present in sediment traps and in reducing portions of the water column. Mn-carbonate becomes supersaturated in the Brownie Lake chemocline where dissolved oxygen concentrations fall below 5 μM, and Mn-oxide reduction increases the dissolved Mn concentration. Supersaturation is enhanced when calcite originating from surface waters dissolves in more acidic waters at the chemocline. In the same zone, sulfate reduction and microaerobic methane oxidation add dissolved inorganic carbon (DIC) with negative δ13C. These observations demonstrate that sedimentary Mn enrichments may 1) develop from primary carbonate phases, and 2) can occur in environments with dissolved oxygen concentrations <5 μM. Primary Mn-carbonates are likely to originate in environments with high concentrations of dissolved Mn (>200 μM), and where Mn and Fe are partitioned by S cycling, photoferrotrophy, or microaerophilic Fe-oxidation. A shallow lysocline enhances Mn-carbonate production by providing additional DIC and nucleation sites for crystal growth. This carbonate model for Mn-enrichments is expected to be viable in both euxinic and ferruginous environments, and provides a more nuanced view of the relationships between Mn and carbon cycling, with applications throughout the rock record.

Microbial Diversity Responding to Changes in Depositional Conditions during the Last Glacial and Interglacial Period: NE Ulleung Basin, East Sea (Sea of Japan)

Microbial interaction with minerals are significantly linked with depositional conditions during glacial and interglacial periods, providing a unique redox condition in the sedimentary process. Abiotic geophysical and geochemical properties, including sedimentary facies, magnetic susceptibility, grain size, clay mineralogy, and distribution of elemental compositions in the sediments, have been widely used to understand paleo-depositional environments. In this study, microbial abundance and diversity in the core sediments (6.7 m long) from the northeastern slope of Dokdo Island were adapted to characterize the conventionally defined sedimentary depositional units and conditions in light of microbial habitats. The units of interglacial (Unit 1, &lt;11.5 ka) and late glacial (Unit 2, 11.5–14.5 ka) periods in contrast to the glacial period (Unit 3, &gt;14.5 ka) were distinctively identified in the core, showing a sharp boundary marked by the laminated Mn-carbonate (CaM) mud between bioturbated (Unit 1 and 2) and laminated mud (Unit 3). Based on the marker beds and the occurrence of sedimentary facies, core sediments were divided into three units, Unit 1 (&lt;11.5 ka, interglacial), Unit 2 (11.5–14.5 ka, late glacial), and Unit 3 (&gt;14.5 ka, glacial), in descending order. The sedimentation rate (0.073 cm/year), which was three times higher than the average value for the East Sea (Sea of Japan) was measured in the late glacial period (Unit 2), indicating the settlement of suspended sediments from volcanic clay in the East Sea (Sea of Japan), including Doldo Island. The Fe and Mg-rich smectite groups in Unit 2 can be transported from volcanic sediments, such as from the volcanic island in the East Sea or the east side of Korea, while the significant appearance of the Al-rich smectite group in Unit 1 was likely transported from East China by the Tsushima Warm Current (TWC). The appearance of CaM indicates a redox condition in the sedimentary process because the formation of CaM is associated with an oxidation of Mn2+ forming Mn-oxide in the ocean, and a subsequent reduction of Mn-oxide occurred, likely due to Mn-reducing bacteria resulting in the local supersaturation of Mn2+ and the precipitation of CaM. The low sea level (−120 m) in the glacial period (Unit 3) may restrict water circulation, causing anoxic conditions compared to the late glacial period (Unit 2), inducing favorable redox conditions for the formation of CaM in the boundary of the two units. Indeed, Planctomycetaceae, including anaerobic ammonium oxidation (ANAMMOX) bacteria capable of oxidizing ammonium coupled with Mn-reduction, was identified in the CaM layer by Next Generation Sequencing (NGS). Furthermore, the appearance of aerobic bacteria, such as Alphaproteobacteria, Gammaproteobacteria, and Methylophaga, tightly coupled with the abundance of phytoplankton was significantly identified in Unit 1, suggesting open marine condition in the interglacial period. Bacterial species for each unit displayed a unique grouping in the phylogenetic tree, indicating the different paleo-depositional environments favorable for the microbial habitats during the glacial and interglacial periods.