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1.
Pore water profiles of total-CO2, pH, PO3?4, NO?3 plus NO?2, SO2?4, S2?, Fe2+ and Mn2+ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized (O2 > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO2 concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.  相似文献   

2.
The control of cathodoluminescence in dolomite by iron and manganese   总被引：10，自引：0，他引：10  
Variations in the cathodoluminescent properties of carbonates are usually attributed to differing proportions of manganese (Mn2+) as the most important activator, and iron (Fe2+) as the main inhibitor of luminescence. Interactions between manganese and iron concentrations and the luminescent properties of dolomite are demonstrated by petrographic and chemical analyses of 86 samples of dolomite representing a range of depositional environments and ages (Cambrian to Cretaceous) and a wide geographical distribution (North America and Europe). Iron and manganese are positively correlated in the dolomites, with the former showing a greater range of variation. Very small amounts of manganese are sufficient to activate the luminescence and as little as 100 ppm Mn2+ is present in highly luminescing samples. The intensity of luminescence is not proportional to the manganese concentration. Iron begins to quench luminescence as its concentration reaches 10,000 ppm. Above that level, luminescence is rapidly lost and total extinction occurs among samples containing more than 15,000 ppm Fe2+, regardless of the manganese concentration.  相似文献   

3.
The formation of authigenic manganese minerals and ores in the pelagic regions of the ocean is related to oxidation of Mn2+ extracted from basalts and other rocks with heated seawater. For littoral parts of the ocean and lakes mobilization of Mn2+ and Fe2+ is admitted finding its way to the bottom sediments (along with the organic substances) from land in the form of Mn4+. The main manganese mineral of oceanic and continental basins is vernadite. Its deposition is considered a result of the activity of microorganisms.  相似文献   

4.
Incubation experiments were adopted to characterize the rates and pathways of iron reduction and the contributions to anaerobic organic matter mineralization in the upper 0–5 cm of sediments along a landscape-scale inundation gradient in tidal marsh sediments in the Min River Estuary, Southeast China. Similar sediment characteristics, single-species vegetation, varied biomass and bioturbation, distinct porewater pH, redox potential, and electrical conductivity values have resulted in a unique ecogeochemical zonation along the inundation gradient. Decreases in solid-phase Fe(III) and increases in nonsulfidic Fe(II) and iron sulfide were observed in a seaward direction. Porewater Fe2+ was only detected in the upland area. High rates of iron reduction were observed in incubation jars, with significant accumulations of nonsulfidic Fe(II), moderate accumulations of iron sulfides, and negligible accumulations of porewater Fe2+. Most of the iron reduction was microbially mediated rather than coupled to reduced sulfides. Microbial iron reduction accounted for 20–89 % of the anaerobic organic matter mineralization along the inundation gradient. The rate and dominance of microbial iron reduction generally decreased in a seaward direction. The contributions of microbial iron reduction to anaerobic organic matter mineralization depended on the concentrations of bioavailable Fe(III), the spatial distribution of which was significantly related to tidal inundation. Our results clearly showed that microbial iron reduction in the upper sediments along the gradient is highly dependent on spatial scales controlled primarily by tidal inundation.  相似文献   

5.
Iron and manganese solubility at the sediment/water interface has been studied at a water depth of 20 m in Kiel Bight, Western Baltic. By means of an in situ bell jar system enclosing 3.14 m2 sediment surface and 2094 l water a complete redox turn-over in the bottom water was simulated in an experiment lasting 99 days. The concentration of dissolved Fe in the bell jar water never exceeded 0.041 μmol · dm?3during the first 50 days of the experiment and then rose abruptly as the Eh fell from +600 to ?200 mV. The concentration of dissolved Fe under oxic and anoxic conditions seems to be limited by equilibria with solid Fe-phases (hydroxides and amorphous sulphide, respectively). In contrast to Fe, manganese was released continuously from the bottom during the first 50 days of the experiment leading to exponentially increasing manganese concentrations in the bell jar water. During this time dissolved O2 had become ready depleted and pH had dropped from 8.3 to 7.5. Contrary to iron, manganese being solubilized in reduced sediment layers can penetrate oxic strata in metastable form due to slow oxidation kinetics; when the redoxcline moves upwards Mn2+ is enriched in bottom waters. The maximum concentration of dissolved Mn under anoxic conditions is controlled by a solid phase with solubility properties similar to MnCO3 (rhodochrosite). Bottom water enrichment in dissolved Mn2+ could be traced to originate from excess solid manganese within the top 3 cm of the sediment.  相似文献   

6.
Sunlight-induced reduction and dissolution of colloidal Fe-Mn (hydr)oxide minerals yields elevated concentrations of Fe2+ and Mn2+ in natural waters. Since these elements may be biolimiting micronutrients, photochemical reactions might play a significant role in biogeochemical cycles. Reductive photodissolution of Fe (hydr)oxide minerals may also release sorbed metals. The reactivity of Fe-Mn (hydr)oxide minerals to sunlight-induced photochemical dissolution is determined by the electronic structure of the mineral-water interface. In this work, oxygen K-edge absorption and emission spectra were used to determine the electronic structures of iron(III) (hydr)oxides (hematite, goethite, lepidocrocite, akaganeite and schwertmannite) and manganese(IV) oxides (pyrolusite, birnessite, cryptomelane). The band gaps in the iron(III) (hydr)oxide minerals are near 2.0-2.5 eV; the band gaps in the manganese (IV) oxide phases are 1.0-1.8 eV. Using published values for the electrochemical flat-band potential for hematite together with experimental pHpzc values for the (hydr)oxides, it is possible to predict the electrochemical potentials of the conduction and valence bands in aqueous solutions as a function of pH. The band potentials enable semiquantitative predictions of the susceptibilities of these minerals to photochemical dissolution in aqueous solutions. At pH 2 (e.g., acid-mine waters), photoreduction of iron(III) (hydr)oxides could yield millimolal concentrations of aqueous Fe2+ (assuming surface detachment of Fe2+ is not rate limiting). In seawater (pH 8.3), however, the direct photo-reduction of colloidal iron(III) (hydr)oxides to give nanomolal concentrations of dissolved, uncomplexed, Fe2+ is not thermodynamically feasible. This supports the hypothesis that the apparent photodissolution of iron(III) (hydr)oxides in marines systems results from Fe3+ reduction by photochemically produced superoxide. In contrast, the direct photoreduction of manganese oxides should be energetically feasible at pH 2 and 8.3.  相似文献   

7.
Equilibria between different valence states of Fe and Mn have been studied in a microcline-plagioclase-quartz gneiss which locally contains ferromagnesian minerals unusually high in Mn+3 and Fe+3 and low in Fe+2. The compositions of coexistent minerals have been determined by chemical and microprobe analyses. The minerals in some layers were formed under highly-oxidizing conditions, as indicated by extremely low Fe+2/Fe+3 ratios in the silicates, by the presence of hematite, and by the occurrence of piemontite, which requires Mn+3 for its formation. The minerals in other layers were formed under less-oxidizing conditions, as indicated by the fact that epidote, rather than piemontite, crystallized with Mn-rich garnet and by the presence of biotite rather than phlogopite. In the less-oxidized layers Mn+3 appears to be absent. The differences in oxidation of Fe and Mn occur between adjacent layers and probably reflect sedimentary differences preserved despite the metamorphism.Iron and manganese with different valences are sharply partitioned between the coexisting phases. In highly-oxidized layers, muscovite contains more iron (as Fe+3) than coexistent phlogopite; in piemontite most of the manganese is Mn+3, while in coexistent garnet most of the manganese is Mn+2. In less-oxidized layers, epidote contains no Mn+3 and contains less Mn+2 than coexistent garnet, biotite, or amphibole. Analytical data, crystal-chemical arguments, and characteristics of Fe and Mn L-spectra indicate that in coexistent garnet and piemontite, Fe+2, Fe+3, Mn+2, and Mn+3 are present, in spite of the fact that trivalent manganese strongly oxidizes divalent iron in aqueous solution under normal conditions.Contribution No. 1468.  相似文献   

8.
In depressions of the Baltic Sea, where the bottom is periodically marked by stagnation, silt contains as much as 5% Mn (up to 17% in some layers) and 9–10% Corg. Silt in such depressions is laminated. The marine sediment sequence is stratified due to the influx of oceanic water into sea: the upper layers are oxic, while the lower (near-bottom) layers are hydrosulfuric. Boundary between them is represented by the transitional O2-H2S layer. This zone (redox barrier) is marked by drastic variation in Eh. Zone below this barrier is characterized by the accumulation of huge amounts of the dissolved manganese (Mn2+) and iron (Fe2+), which diffuse from the hydrosulfuric layer into the oxic layer under the influence of gradient and precipitated as suspeusion with as much as 15% Fe and 45% Mn. When fresh oxygenated saline water is transported to depressions, the hydrosulfuric setting gives way to oxic one and the dissolved elements are transformed into the particulate phases as hydroxides and geologically instantly precipitated at the bottom. After 5–10 yr, the setting changes; hydrogen sulfide is again delivered to water column from the bottom. This is accompanied by supply of the dissolved Mn2+ and Fe2+ previously accumulated as gel-type sediment at the bottom. Thus, the cycle of elements is repeated. The latter, however, is not completely dissolved. Some portion remains at the bottom as black rhodochrosite microlayers (laminas) that contain as much as 29% Mn. The black laminas accumulated during aeration include remains of bottom foraminifers. In addition, the bottom comprises pale diatom laminas and brownish gray varieties composed of clayey and organic substances. Bulk samples of the laminated silt contain as much as 12.9% Mn or 26.9% MnCO3. Depressions in the Baltic Sea represent a unique site of the Earth marked by accumulation of the carbonate-manganiferous sediments at present. We believe that Oligocene manganese carbonate-oxide ores described by N.M. Strakhov and coauthors were accumulated in the same manner. Compositions of manganiferous sediments in the Baltic region and some ancient ores in Europe are compared. The author studied five stages of Mn accumulation and sediment transformation into ores.  相似文献   

9.
Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mössbauer spectroscopy. Both methods showed good agreement ( <6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1?N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24?h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe2+. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.  相似文献   

10.
Four cores of anoxic sediments were collected from the Seine estuary to assess the early diagenesis pathways leading to the formation of previously reactive phase. Pore waters were analyzed for dissolved iron (Fe) and manganese (Mn) and different ligands (e.g., sulfate, chloride, total inorganic carbon). The anoxic zone is present up to the first centimeter depth, in these conditions the reduction of Mn and Fe oxides and SO4 2− was verified. The sulfate reduction was well established with a subsequent carbon mineralization in the NORMAI94 core. The chemical speciation of Mn and Fe in the dissolved and solid phases was determined. For the dissolved phase, thermodynamic calculations were used to characterize and illustrate the importance of carbonate and phosphate phases as sinks for Fe and Mn. The ion activity product (IAP) of Fe and Mn species was compared to the solubility products (Ks) of these species. In the solid phase, the presence of higher concentration of calcium carbonate in the Seine sediments is an important factor controlling Mn cycle. The carbonate-bound Mn can reach more than 75% of the total concentration. This result is confirmed by the use of electron spin resonance (ESR) spectroscopy. The reduction of Fe is closely coupled to the sulfate reduction by the formation of new solid phases such as FeS and FeS2, which can be regarded as temporal sinks for sulfides. These forms were quantified in all cores as acid volatile sulfide (AVS: FeS+ free sulfide) and chromium reducible sulfide (CRS: FeS2+elemental sulfur S0).  相似文献   

11.
Biogeochemical processes induced by the deposition of gravity layer in marine sediment were studied in a 295-day experiment. Combining voltammetric microelectrode measurements and conventional analytical techniques, the concentrations of C, O2, N-species, Mn and Fe have been determined in porewaters and sediments of experimental units. Dynamics of the major diagenetic species following the sudden sediment deposition of few cm-thick layer was explained by alternative diagenetic pathways whose relative importance in marine sediments is still a matter of debate. Time-series results indicated that the diffusion of O2 from overlying waters to sediments was efficient after the deposition event: anoxic conditions prevailed during the sedimentation. After a few days, a permanent oxic horizon was formed in the top few millimetres. At the same time, the oxidation of Mn2+ and then Fe2+, which diffused from anoxic sediments, contributed to the surficial enrichment of fresh Mn(III/IV)- and Fe(III)-oxides. Vertical diffusive fluxes and mass balance calculations indicated that a steady-state model described the dynamic of Mn despite the transitory nature of the system. This model was not adequate to describe Fe dynamics because of the multiple sources and phases of Fe2+. No significant transfer of Mn and Fe was observed between the underlying sediment and the new deposit: Mn- and Fe-oxides buried at the original interface acted as an oxidative barrier to reduced species that diffused from below. Nitrification processes led to the formation of a NO3−/NO2− rich horizon at the new oxic horizon. Over the experiment period, NO3− concentrations were also measured in the anoxic sediment suggesting anaerobic nitrate production.  相似文献   

12.
In iron-manganese nodules from the floor of Pacific ocean, Baltic, White Sea and Kara Sea, iron bydroxide '-FeOOH was analysed in the laboratory. In buried ooze, reduction processes generate Fe(HCO3)2 which migrates into the upper part of the bottom ooze and into near bottom sea water where Fe(OH)2 is formed. The oxidation process of Fe2+ to Fe3+, without participation of iron bacteria, leads to the topotactic transformation of Fe(OH)2 to '-FeOOH. Marine water does not contain Fe2+ and cannot be a direct source of iron deposited in the nodules. Discovery of '-FeOOH in marine nodules permits the consideration that both iron and manganese were derived from the buried bottom mud, which during diagenetic processes led to the transfer of these metals in solutions and their upward migration.  相似文献   

13.
Nontronite, limonite (with opal) and vivianite are forming at present in the aereated, shallower parts (water depths <250 m) of Lake Malawi. They overlie diatomite or coarse grained clastic sediments. Our investigations indicate a precipitation of nontronite and limonite (and opal) at the sediment/oxic water interface from geothermal solutions rich in SiO2 percolating through the sedimentary fill of the basin. Under reducing conditions and a pH less than 7 (as occurring in the deeper parts of our sediment cores) iron and manganese are leached and discharged into the lake. In the anoxic parts of the lake (water depth greater than 250 m) Fe3+- and Mn3+-hydroxide are precipitated within the lake's water in the mixing zone of the aerobic with the anaerobic water bodies. High dispersion and a strong supply of detrital material, however, prevent a stronger enrichment of the hydroxides in the sediment. The formation of vivianite can be explained by a dissolution of Ca-phosphate (fish debris) within the sediment and a re-deposition as Fe-phosphate in the uppermost sediment layers under reducing but slightly alkaline conditions. The results of our investigations on Recent iron formation in Lake Malawi offer an explanation for the genesis of certain sedimentary iron ores in the geological past. They indicate that a formation of iron-rich sediments — including the silicate facies — is not restricted to the marine environment.
Zusammenfassung Nontronit, Limonit (und Opal) sowie Vivianit werden an zahlreichen Stellen des südlichen Malawi-Sees in Wassertiefen bis 250 m (aerober Bereich) abgeschieden und bilden dort die jüngste sedimentäre Einheit über Diatomit oder grobklastischem Sediment. Es muß angenommen werden, daß die Bildung der Eisenmineralien aus geothermalen Lösungen erfolgt, die reich an gelöster Kieselsäure und Fe2+ (untergeordnet Mn2+) sind, wobei Eisen und Mangan aus dem Sediment selber stammen, da heiße Quellen außerhalb des Malawi-Sees diese Elemente nicht enthalten. Die Auslösung von Eisen und Mangan erfolgt durch saure Lösungen im reduzierenden Milieu unter Bedingungen, wie sie im tieferen Teil der Sediment-Kerne angetroffen wurden (pH bis 3,6!). Beim Zusammentreffen dieser Lösungen mit dem sauerstoffhaltigen, schwach alkalischen Wasser des Malawi-Sees kommt es zur Ausfällung von Nontronit oder — bei wahrscheinlich höherem Redox-Potential — von Limonit und Opal. Beim Austritt der Fe2+, Mn2+- und SiO2-reichen Lösungen in den anaeroben Teil des Malawi-Sees (Wassertiefen größer als 250 m) unterbleibt zunächst eine Ausfällung. Erst in der Mischzone des anoxischen mit dem oxischen Wasserkörper können Fe3+- und Mn3+-Hydroxide ausgeschieden werden, die jedoch infolge hoher Dispergierung und starker Zufuhr von klastischem Material eine starke Verdünnung erfahren. Für die Vivianit-Bildung kann angenommen werden, daß ein in den Sedimenten häufig vorhandener Apatit-Anteil (Fischreste) durch die sauren Lösungen aufgelöst und in den obersten Sedimentschichten (reduzierendes Milieu, pH jedoch >7) zusammen mit Fe2+ der Lösung als Vivianit abgeschieden wird. Die vorliegenden Untersuchungen geben Hinweise auf die Bildung bestimmter fossiler sedimentärer Eisenerze und zeigen gleichzeitig, daß die Genese eisenreicher Sedimente — auch von silikatischem Fazies-Typ — nicht an den marinen Ablagerungsraum gebunden ist.
  相似文献   

14.
The environmental conditions that prevailed during the formation of the Rohtas carbonates have been delineated on the basis of the Eh-pH diagrams for V, Mn, Fe+2 and Fe+3 compounds. The high content of vanadium in the insoluble residue is indicative of the prevalence of reducing environment. During early-diagenesis manganese seems to have been mobilised from the soft sediments. Higher manganese content in the carbonates is a result of late-diagenesis. Prior to late diagenesis, ferric iron appears to have been precipitated from the waters while manganese remained in solution, and this process accounts for the low iron content of the carbonates.  相似文献   

15.
Manganese mineralisation in the Oakover Basin is associated with Mesoproterozoic extension, basin formation and deposition of the Manganese Group. The underlying basement architecture of the Oakover Basin (a local half-graben geometry), inherited from the Neoarchean rifting event, plays an important role on the distribution, style and timing of manganese deposits. Fault-hosted manganese deposits are dominant along the ‘active’ faulted eastern margin, whereas flat-lying sedimentary deposits are dominant along the western ‘passive’ margin reflecting differences in ore-forming processes. The large number of significant manganese deposits in the Oakover Basin, previously thought to reflect a spatial association with Carawine Dolomite, more likely reflects the restricted nature of the Mesoproterozoic basin and development of a large reservoir of Mn2+ and Fe2+ in an anoxic zone of a stratified basin. Low O2 conditions in the basin were caused by a paleotopographic high forming a barrier to open ocean circulation. The western margin sedimentary deposits formed later than the fault-hosted hydrothermal deposits along the eastern margin, once a significant reservoir of Mn2+ and Fe2+ had developed, and when there was sufficient subsidence to allow migration of the redox front onto the shallow shelf, with Mn precipitation on and within the seafloor sediments. The sedimentary manganese deposits are not uniformly distributed along the western edge of the basin; instead they are concentrated into discrete areas (e.g. Mt Cooke–Utah–Mt Rove, Bee Hill, Skull Springs and the Ripon Hills districts), suggesting a degree of structural control on their distribution. Fault-hosted manganese is observed beneath and adjacent to many of the sedimentary deposits. Marked geochemical differences are observed between the Woodie Woodie hydrothermal deposits and the sedimentary deposits. Woodie Woodie deposits display higher Ba, U, Mo, As, Sn, Bi, Pb, S and Cu than the sedimentary deposits, reflecting the composition of the hydrothermal fluids. The Al2O3 values of the Ripon Hills and Mt Cooke deposits are much higher than the Woodie Woodie deposits, reflecting the composition of the dominant host rock, as Al2O3 is typically <5 wt% in the Carawine Dolomite, but is >10 wt% in basal shale units of the Manganese Group. Highly variable Mn:Fe ratios (?5:1) in the hydrothermal manganese at Woodie Woodie reflects rapid deposition of Mn in and around fault zones. In contrast, slower accumulation of Mn oxides on and within the seafloor to form the large sedimentary deposits results in Mn:Fe ratios closer to 1:1 and elevated Co + Ni and REE values.  相似文献   

16.
Sediment cores were sampled from Xiamen Western Bay at five sites during the summer and winter of 2006 and Hg–Au microelectrodes were used to make on board measurements of the concentration gradients of dissolved oxygen, Mn2+, and Fe2+ within the sediments. The O2 concentrations decreased sharply from about 200 μmol L−1 in the bottom seawater to zero within a depth of a few millimeters into the sediment. Dissolved Mn2+ was detected below the oxic zones with peak concentrations up to 600 μmol L−1, whereas dissolved Fe2+ had peak concentrations up to 1,000 μmol L−1 in deeper layers. The elemental contents of organic carbon and nitrogen within the sediments were analyzed and their C/N ratios were in the range of 9.0 to 10.1, indicative of heavy terrestrial origin. Sediments from two sites near municipal wastewater discharge outlets had higher organic contents than those from the other sites. These high organic contents corresponded to shallow O2 penetration depths, high dissolved Mn2+ and Fe2+ concentrations, and negative redox potentials within the sediments. This indicated that the high organic matter content had promoted microbial respiration within the sediments. Overall, the organic content did not show any appreciable decrease with increasing sediment depths, so a quadratic polynomial function was used to fit the curve of O2 profiles within the sediments. Based on the O2 profiles, O2 fluxes across the seawater and sediment interface were estimated to be in the range 6.07 to 14.9 mmol m−2 day−1, and organic carbon consumption rates within the surface sediments were estimated to be in the range 3.3 to 20.8 mgC cm−3 a−1. The case demonstrated that biogeochemistry within the sediments of the bay was very sensitive to human activities such as sewage discharge.  相似文献   

17.
The influence of bottom water anoxia on manganese (Mn), iron (Fe), and sulfur (S) biogeochemistry was examined in defaunated sandy sediment from Kærby Fed, Denmark, under controlled laboratory incubations. The initial narrow peaks and steep gradients in solid Mn(IV) and Fe(III) as well as porewater Mn2+ and Fe2+ observed in the upper 2–5 cm of the sediment indicate rapid metal reduction-oxidation cycles under oxic conditions in the overlying water. The fe zones were generally displaced about 0.5 cm downward compared with the Mn zones due to differences in reactivity. Mn(IV) was reduced and gradually disappeared first (within 10 d) when the sediment was exposed to anoxia followed by reduction and disappearance of Fe(III) (day 7 to 18). The associated loss of Mn2+ to the overlying water was most rapid during the first 15 d, whereas the Fe2+ efflux initiated around day 10, and after a few days with modest rates the efflux peaked around day 20. A considerable portion of the total Mn (26%) and Fe (23%) inventory initially present in the sediment was lost by efflux after about 1 mo of anoxia. The ability of the sediment to retain upward diffusion of H2S gradually disappeared in a temporal pattern closely related to the changes in pool size of the reactive Mn and Fe present. The total metal pool in Kærby Fed sediment prevented H2S release to the overlying water for at least a month of anoxia. It is speculated that external supplies from the overlying water allows a rapid refuelling of surface Mn and Fe oxides in the field when oxic conditions returns between periods of anoxia.  相似文献   

18.
Fe cycling at two sites in the Mediterranean Sea (southwest of Rhodes and in the North Aegean) has been studied, combining the pore water determination of nutrients, manganese, and iron, citrate-bicarbonate-dithionite (CDB) and total sediment extractions, X-ray diffraction, and 57Fe Mössbauer spectroscopy (MBS). At the Rhodes site, double peaks in the CDB-extractable Mn and Fe profiles indicate non-steady-state diagenesis. The crystalline iron oxide hematite, identified at both sites by room temperature (RT) MBS, appears to contribute little to the overall Fe reduction. MBS at liquid helium temperature (LHT) revealed that the reactive sedimentary Fe oxide phase was nanophase goethite, not ferrihydrite as is usually assumed. The pore water data at both sites indicates that upon reductive dissolution of nanophase goethite, the upward diffusing dissolved Fe2+ is oxidized by Mn oxides, rather than by nitrate or oxygen. The observed oxidation of Fe2+ by Mn oxides may be more common than previously thought but not obvious in sediments where the nitrate penetration depth coincides with the Mn oxide peak. At the Rhodes site, the solid-phase Fe(II) increase occurred at a shallower depth than the accumulation of dissolved Fe2+ in the pore water. The deeper relict Mn oxide peak acts as an oxidation barrier for the upward diffusing dissolved Fe2+, thereby keeping the pore water Fe2+ at depth. At the North Aegean site, the solid-phase Fe(II) increase occurs at approximately the same depth as the increase in dissolved Fe2+ in the pore water. Overall, the use of RT and cryogenic MBS provided insight into the solid-phase Fe(II) gradient and allowed identification of the sedimentary Fe oxides: hematite, maghemite, and nanophase goethite.  相似文献   

19.
Strong enrichments of cobalt occur in marine manganese nodules, soils, wads, and natural and synthetic minerals such as hollandite, cryptomelane, psilomelane, lithiophorite, birnessite, and δ-MnO2. Previously, it was suggested that Co3+ ions in these minerals replace either Mn3+ or substitute for Fe3+ in incipient goethite epitaxially intergrown with δ-MnO2. Neither of these interpretations is now considered to be satisfactory on account of the large discrepancy of ionic radius between octahedrally coordinated low-spin Co3+ and high-spin Mn3+ or Fe3+ in oxide structures. The close agreement between the ionic radii of Co3+ and Mn4+ suggests that some cobalt substitutes for Mn4+ ions in edge-shared [MnO6] octahedra in many manganese(IV) oxide mineral structures. It is proposed that hydrated cations, including Co2+ ions, are initially adsorbed on to the surfaces of certain Mn(IV) oxides in the vicinity of essential vacancies found in the chains or sheets of edge-shared [MnO6] octahedra. Subsequently, fixation of cobalt takes place as a result of oxidation of adsorbed Co2+ ions by Mn4+ and replacement of the displaced manganese by low-spin Co3+ ions in the [MnO6] octahedra or vacancies.  相似文献   

20.
Electron paramagnetic resonance (EPR) measurements on dolomites from 9 different localities revealed contents of Mn2+ on two axial sites in all of them. The center with largerzero-field splitting (ZFS) was always present in much higher concentrations, except for a sample from Oberdorf it amounted to 95 percent or more of the total. This dolomite was the only one with a considerable content of Fe3+ on one axial site, almost certainly substituting for Mg2+. With X-ray irradiation the concentration of Fe3+ increased by about 30 percent showing that at least some of the divalent iron also substitutes for Mg. The ZFSs for Fe3+ and Mn2+ with larger ZFS increase with decreasing temperature in the same manner. The previous assignment of this Mn2+ to Mg sites is thus confirmed. An almost regular increase of the trigonal distortions at the divalent ions in different carbonates with increasing ionic radius is indicated by their crystal structure data. The very small ZFS for Mn2+ on Ca sites in dolomite must thus result from a strong local relaxation in the direction of a more regular octahedral arrangement. It is difficult to explain the different distribution ratios of Mn2+ on Ca and Mg sites with differences in growth and/or annealing temperatures alone. Thus different supply of Mg2+ and Ca2+ in the growth solutions may also contribute.  相似文献   

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