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1.
Measured pore-water concentrations of iron in interbedded pelagic and turbiditic sediments from the Nares Abyssal Plain are in excellent agreement with sediment colour and measured redox potential. The organic carbon content of these sediments appears to define the redox conditions and consequently the porewater and solid-phase concentration of constituents that are involved in early diagenetic reactions. In the turbiditic sediments the concentration of NO3− generally goes to zero within a sediment depth of 1 m, whereas at 8 m in a pelagic core from the same area the concentration of NO3− is still higher than it is in the bottom water. The pore-water concentration of Mn2+ in the turbiditic sediments increases sharply down to a depth of approximately 3 m and from thereon remains nearly constant due to saturation with respect to Mn, Ca-CO3. The pore water of the turbiditic sediments is also saturated with respect to calcite. The few “diagenetic spikes” in the pore-water concentration of NO3− and Mn2+ and the concentration/depth profile of dissolved iron, H4SiO4 and phosphate all clearly demonstrate the inhomogeneous nature of interbedded pelagic and turbiditic sediments. The simultaneous occurrence of peaks of dissolved iron/silica and of sediment intervals with a relatively high organic carbon content is attributed to enhanced early diagenetic reactions associated with the decomposition of organic matter in these specific intervals. Linked with these reactions is the irregular pore-water concentration of phosphate, which is shown to originate partly from the oxidation of organic matter, but mainly from the desorption of phosphate from iron oxide. Potential concentrations of phosphate are calculated from the stoichiometric early diagenetic reactions and compared with measured concentrations. Due to the unique combination of low porosity and relatively high sedimentation rates, the sediments from the Nares Abyssal Plain are an ideal basis for the study of such interbedded sequences of pelagic and turbiditic deposits.  相似文献   

2.
We measured the Fe isotope fractionation during the reactions of Fe(II) with goethite in the presence and absence of a strong Fe(III) chelator (desferrioxamine mesylate, DFAM). All experiments were completed in an O2-free glove box. The concentrations of aqueous Fe(II) ([Fe(II)aq]) decreased below the initial total dissolved Fe concentrations ([Fe(II)total], 2.15 mM) due to fast adsorption within 0.2 day. The concentration of adsorbed Fe(II) ([Fe(II)ads]) was determined as the difference between [Fe(II)aq] and the concentration of extracted Fe(II) in 0.5 M HCl ([Fe(II)extr]) (i.e., [Fe(II)ads] = [Fe(II)extr] − [Fe(II)aq]). [Fe(II)ads] also decreased with time in experiments with and without DFAM, documenting that fast adsorption was accompanied by a second, slower reaction. Interestingly, [Fe(II)extr] was always smaller than [Fe(II)total], indicating that some Fe(II) was sequestered into a pool that is not HCl-extractable. The difference was attributed to Fe(II) incorporated into goethite structure (i.e., [Fe(II)inc] = [Fe(II)total] −[Fe(II)extr]). More Fe(II) was incorporated in the presence of DFAM than in its absence at all time steps. Regardless of the presence of DFAM, both aqueous and extracted Fe(II) (δ56/54Fe(II)aq and δ56/54Fe(II)extr) became isotopically lighter than or similar to goethite (− 0.27‰) at day 7, implying that the isotope exchange occurred between bulk goethite and aqueous Fe. Consistently, the mass balance indicated that the incorporated Fe is isotopically heavier than extracted Fe. These observations suggested that (i) co-adsorption of Fe(II) with DFAM resulted in more pervasive electron transfer, (ii) the electron transfer from heavy Fe(II) in the adsorbed Fe(II) to light Fe(III) in goethite results in the fixation of heavy adsorbed Fe(III) on the surface and accumulation of Fe(II) within the goethite, and (iii) desorption of the reduced, light Fe from goethite does not necessarily occur at the same surface sites where adsorption occurred.  相似文献   

3.
The Bemidji aquifer in Minnesota, USA is a well-studied site of subsurface petroleum contamination. The site contains an anoxic groundwater plume where soluble petroleum constituents serve as an energy source for a region of methanogenesis near the source and bacterial Fe(III) reduction further down gradient. Methanogenesis apparently begins when bioavailable Fe(III) is exhausted within the sediment. Past studies indicate that Geobacter species and Geothrix fermentens-like organisms are the primary dissimilatory Fe-reducing bacteria at this site. The Fe mineralogy of the pristine aquifer sediments and samples from the methanogenic (source) and Fe(III) reducing zones were characterized in this study to identify microbiologic changes to Fe valence and mineral distribution, and to identify whether new biogenic mineral phases had formed. Methods applied included X-ray diffraction; X-ray fluorescence (XRF); and chemical extraction; optical, transmission, and scanning electron microscopy; and Mössbauer spectroscopy.All of the sediments were low in total Fe content (≈ 1%) and exhibited complex Fe-mineralogy. The bulk pristine sediment and its sand, silt, and clay-sized fractions were studied in detail. The pristine sediments contained Fe(II) and Fe(III) mineral phases. Ferrous iron represented approximately 50% of FeTOT. The relative Fe(II) concentration increased in the sand fraction, and its primary mineralogic residence was clinochlore with minor concentrations found as a ferroan calcite grain cement in carbonate lithic fragments. Fe(III) existed in silicates (epidote, clinochlore, muscovite) and Fe(III) oxides of detrital and authigenic origin. The detrital Fe(III) oxides included hematite and goethite in the form of mm-sized nodular concretions and smaller-sized dispersed crystallites, and euhedral magnetite grains. Authigenic Fe(III) oxides increased in concentration with decreasing particle size through the silt and clay fraction. Chemical extraction and Mössbauer analysis indicated that this was a ferrihydrite like-phase. Quantitative mineralogic and Fe(II/III) ratio comparisons between the pristine and contaminated sediments were not possible because of textural differences. However, comparisons between the texturally-similar source (where bioavailable Fe(III) had been exhausted) and Fe(III) reducing zone sediments (where bioavailable Fe(III) remained) indicated that dispersed detrital, crystalline Fe(III) oxides and a portion of the authigenic, poorly crystalline Fe(III) oxide fraction had been depleted from the source zone sediment by microbiologic activity. Little or no effect of microbiologic activity was observed on silicate Fe(III). The presence of residual “ferrihydrite” in the most bioreduced, anoxic plume sediment (source) implied that a portion of the authigenic Fe(III) oxides were biologically inaccessible in weathered, lithic fragment interiors. Little evidence was found for the modern biogenesis of authigenic ferrous-containing mineral phases, perhaps with the exception of thin siderite or ferroan calcite surface precipitates on carbonate lithic fragments within source zone sediments.  相似文献   

4.
In low-temperature aqueous solutions (< 100°C, pH 2–4.5), chalcopyrite (CuFeS2) does not form through direct precipitation from solution. The pathway is exclusively via precursor iron sulphides and dissolved Cu salts. The reaction of dissolved Cu (II) salts with natural hexagonal pyrrhotite (Fe0.9S) is diffusion controlled. The initial stage has an apparent activation energy of 11.4 ± 1.8 kJ mol−1 and the rate (in units of mol dm−3s−1 cm−2) is independent of the solid reactant surface area. The reaction proceeds through a series of metastable Cu-Fe-sulphide intermediaries. These phases form a series of ephemeral layers penetrating into the pyrrhotite surface. The first phase formed has the stoichiometry Cu0.1Fe0.9S. No Fe is released into the solution during its formation and this, together with the extremely low apparent activation energy and the stoichiometry, suggest that it is formed by stuffing of electron holes in the pyrrhotite structure with Cu ions. The transformation from the hexagonal close-packed arrangement of the pyrrhotite structure to the essentially cubic packing in chalcopyrite proceeds through a series of intermediaries, approximating in composition to members of the cubanite group. The rate of formation of these phases is controlled by the coupled diffusion of Fe (II), Fe (III), Cu (I) and Cu (II) species through the surface reaction zone, although the process as a whole can be approximated by steady-state diffusion of total Cu into a semi-infinite medium. Experiments with metastable precursor iron monosulphide phases, including amorphous FeS and synthetic mackinawite indicate similar reaction pathways.

The results suggest that chalcopyrite formation in low-temperature natural systems may be significantly constrained by kinetic factors. Chalcopyrite is, at least, a diagenetic mineral since its formation requires the prior formation of iron sulphides. However, at ambient temperatures its formation is probably limited to very early diagenesis.  相似文献   


5.
《Applied Geochemistry》1999,14(5):655-667
Sediments sampled at a hydrocarbon-contaminated, glacial-outwash, sandy aquifer near Bemidji, Minnesota, were analyzed for sediment-associated Fe with several techniques. Extraction with 0.5 M HCl dissolved poorly crystalline Fe oxides and small amounts of Fe in crystalline Fe oxides, and extracted Fe from phyllosilicates. Use of Ti-citrate-EDTA-bicarbonate results in more complete removal of crystalline Fe oxides. The average HCl-extractable Fe(III) concentration in the sediments closest to the crude-oil contamination (16.2 μmol/g) has been reduced by up to 30% from background values (23.8 μmol/g) as a result of Fe(III) reduction in contaminated anoxic groundwater. Iron(II) concentrations are elevated in sediments within an anoxic plume in the aquifer. Iron(II) values under the oil body (19.2 μmol/g) are as much as 4 times those in the background sediments (4.6 μmol/g), indicating incorporation of reduced Fe in the contaminated sediments. A 70% increase in total extractable Fe at the anoxic/oxic transition zone indicates reoxidation and precipitation of Fe mobilized from sediment in the anoxic plume. Scanning electron microscopy detected authigenic ferroan calcite in the anoxic sediments and confirmed abundant Fe(III) oxyhydroxides at the anoxic/oxic boundary. The redox biogeochemistry of Fe in this system is coupled to contaminant degradation and is important in predicting processes of hydrocarbon degradation.  相似文献   

6.
The ion activity product of Fe and phosphate in interstitial waters from four sediment cores taken from Greifensee, Switzerland indicate the presence of vivianite [Fe3(PO4)2 · 8 H2O] in the solid phase. Analysis of the sediment using an electron microprobe and by electron microscopy revealed P-rich grains to be also enriched in Fe. The combined methods provide strong evidence that vivianite is forming authigenically in the sediments. Thermodynamic stability calculations demonstrate that the most stable Fe and phosphate minerals (pyrite, siderite and apatite) are not the ones controlling the pore water chemistry. The results emphasize the importance of rate processes of mineral formation in early diagenesis.Calculations based on the sediment phosphate concentration, and the degree of supersaturation of Fe and phosphate in the upper portion (0–15 cm) of the pore waters, indicate that the rate of vivianite mineral growth is controlled by a surface reaction rather than a diffusion mechanism. The response time of dissolved phosphate in the sediment pore waters with respect to mineral precipitation is on the order of 1–20 days. Less than 15% of the phosphate released by organic matter degradation at the sediment-water interface and below is retained in the sediments.  相似文献   

7.
Authigenesis of iron-rich phosphate nodules occurs in iron-rich cold-seep sediments (MD052911 core) at Yung-An Ridge offshore southwestern Taiwan. Raman, FTIR, and quantitative X-ray energy-dispersive spectroscopic analyses indicate that the phosphate mineral is vivianite (or barićite) and shows Fe/Mg molar ratios spanning from ca. 0.6 to 4.0 and a general down core trend of increasing Fe/Mg ratios. The formation of vivianite is limited to a depth interval of 13–17 mbsf (meters below seafloor) and is most prominent at ∼16 mbsf in association with high dissolved iron concentrations and depleted dissolved sulfide below a peak sulfidization zone (enriched in mackinawite and greigite). Alternate growths of vivianite and iron monosulfides and compositional zoning with Mg enriched towards the peripheries of individual nodules occur in the transition from the zone of vivianite mineralization to the sulfidization zone. The crystallization of vivianite below the sulfidization front could have been favored by scavenging of downward diffusive dissolved sulfide from pore waters in the sulfidization zone. Alternate growths and overlapping of the zones of iron monosulfides and vivianite can be attributed to fluctuations of the sulfidization front and methane flux. The discovery of vivianite in the Yung-An Ridge sediments implies that authigenic vivianite can be an important sink for phosphorus burial in cold-seep sediments that have high reactive-iron contents and high sedimentation rates.  相似文献   

8.
Solid and colloidal iron oxides are commonly involved in early diagenesis. More readily available soluble Fe(III) should accelerate the cycling of iron (Fe) and sulfur (S) in sediments. Experiments with synthetic solutions (Taillefert et al. 2000) showed that soluble Fe(III) (i.e., <50 nm diameter) reacts at a mercury voltammetric electrode at circumneutral pH if it is complexed by an organic ligand. The reactivity of soluble organic-Fe(III) with sulfide is greatly increased compared to its solid equivalent (e.g., amorphous hydrous iron oxides or goethite). We report here data from two different creeks of the Hackensack Meadowlands District (New Jersey) collected with solid state Au/Hg voltammetric microelectrodes and other conventional techniques, which confirm the existence of soluble organic-Fe(III) in sediments and its interaction with sulfide. Chemical profiles in these two anoxic sediments show the interaction between iron and sulfur during early diagenesis. Soluble organic-Fe(III) and Fe(II) are dominant in a creek where sulfide is negligible. This dominance suggests that the reductive dissolution of iron oxides goes through the dissolution of solid Fe(III), then reduction to Fe(II), or that soluble organic-Fe(III) is formed by chemical or microbial oxidation of organic-Fe(II) complexes. In a creek sediment where sulfide occurs in significant concentration, the reductive dissolution of Fe(III) is followed by formation of FeS(aq), which further precipitates. Dissolved sulfide may influence the fate of soluble organic-Fe(III), but the pH may be the key variable behind this process. The high reactivity of soluble organic-Fe(III) and its mobility may result in the shifting of local reactions, at depths where other electron acceptors are used. These data also suggest that estuarine and coastal sediments may not always be at steady state.  相似文献   

9.
Incubation studies were carried out using 5 freshly collected sediments from shallow aquifers of the Hetao Basin, Inner Mongolia. The aquifer sediments covering a range of redox conditions, as indicated by their deep grey to yellow color were mixed with degassed artificial As solution or degassed deionized water at a ratio of solid to water of about 1:10 (wt./wt.). Suspensions which were either amended with glucose or autoclaved, were incubated in parallel with unamended suspensions. Five microcosm cultures of unamended sediments gradually release the equivalent of 0.03–0.30 μg/g As to the dissolved phase. The addition of glucose as a potential electron donor results in a marked stimulation in the mobilization of As (0.71–3.81 μg/g) in the amended incubations for all sediments. The quantity of As released accounts for 60–70% of As bound to Fe/Mn oxides in the original sediments. The microbially mediated mobilization of As with the organic nutrient as an electron donor is strongly associated with the As bound to Fe/Mn oxides, as well as the exchangeable As. During the incubations amended with glucose, 2–4% of the sediment Fe is released. The results suggest that the introduction of labile dissolved organic C into the yellowish sediment aquifers with As-free groundwater would reduce a significant proportion of the Fe(III) oxyhydroxides mediated by anaerobic bacteria respiration and increase groundwater As concentrations.  相似文献   

10.
Extended X-ray absorption fine structure (EXAFS) spectroscopy and chemical analyses were combined to determine the Fe bearing minerals in recent lake sediments from Baldeggersee (Switzerland). The upper section of a laminated sediment core, deposited under eutrophic conditions, was compared to the lower part from an oligotrophic period. Qualitative analysis of FeK EXAFS agreed well with chemical data: In the oligotrophic section Fe(II)–O and Fe(III)–O specieswere present, whereas a significant fraction of Fe(II)–S sulfides was strongly indicated in the eutrophic part. A statistical analysis was performed by least square fitting of normalized reference spectra. The set of reference minerals included Fe(III) oxides and Fe(II) sulfides, carbonates and phosphates. In the oligotrophic regime no satisfying fit was obtained using the set of reference spectra, indicating that siderite (FeCO3) was not present in a significant amount in these carbonate-rich sediments. Simulated EXAFS spectra for a(Cax, Fe1-x)CO3solid solution allowed reconstructing the specificfeatures of the experimental spectra, suggesting that this phase was the dominant Fe carrier in the oligotrophic section of the core. In the eutrophic part, mackinawite was positively identified and represented the dominant Fe(II) sulfide phase. This finding agreed with chemical extraction, which indicated that18–40 mol% of Fe was contained in the acid volatile iron sulfide fraction. EXAFS spectra of the eutrophic section were best fitted by considering the admixture of mackinawite and the Fe–Ca carbonate phase inferred to be predominant in the oligotrophic regime.  相似文献   

11.
The first precipitate formed through the reaction between aqueous Fe(II) salts and dissolved sulphide at ambient temperatures and pH < 9, appears to be a highly disordered gel approaching the composition Fe(HS)2 on a water-free basis. After 0.4 s this precipitate loses sulphide and amorphous FeS begins to appear. The mackinawite structure begins to develop after several hours.

The rate of formation of the initial precipitate can be approximated by a pseudo first-order reaction, directly dependent on total sulphide concentration and with an apparent pseudo first-order rate constant of 48 ± 9 s−1. Dissolved Fe concentration does not appear to be rate limiting.

The estimated solubility of the initial phase is variable but consistently one to two orders of magnitude greater than the measured solubilities for amorphous FeS. In natural systems this may lead to variable Fe(II) solubilities in sulphidic environments. This initial material may play a more central role in iron sulphide reaction pathways than either mackinawite or amorphous FeS.  相似文献   


12.
云南星云湖中蓝铁矿结核的发现及环境意义   总被引：1，自引：0，他引：1  
赵永胜  赵霞飞 《沉积学报》1991,9(3):116-122
本文从矿物学、物理性质和化学组成等方面确定了首次在云南星云湖沉积物中发现的蓝铁矿结核.提供了蓝铁矿的X射线的峰值特征和红外光谱图;在讨论铁质和磷质来源及还原环境产生的基础上,提出了蓝铁矿的形成机理。认为蓝铁矿结核可以反映原始沉积环境,是-种内陆淡水湖泊的沉积标志,仅存在于淡水浅湖及湖湾沉积物中,反映-种浅湖及湖湾的低能环境。  相似文献   

13.
I. Th. Rosenqvist 《Lithos》1970,3(4):327-334
Concretions of vivianite attaining 0.5 g occur in the upper 4 m of the bottom sediments of the Åsrum lake, Vestfold, S. Norway. The vivianite-bearing zone is of lacustrine origin. Vivianite is absent in the underlying lagunal and marine sediments. Electro-chemical and chemical data are given for the interstitial water and the clay.

Solubility product of vivianite is determined. The equilibrium constant is K = 10−40 for the reaction 3 Fe++ + 2 HPO4− + 2 OH− = vivianite. The concretions grew by diffusion from a slightly supersaturated interstitial water phase. The degree of supersaturation is calculated. The stability relations of vivianite and hydroxyl apatite are discussed.  相似文献   


14.
This study reexamines the notion that extensive As mobilization in anoxic groundwater of Bangladesh is intimately linked to the dissolution of Fe oxyhydroxides on the basis of analyses performed on a suite of freshly collected samples of aquifer material. Detailed sediment profiles extending to 40 to 70 m depth below the surface were obtained at six sites where local groundwater As concentrations were known to span a wide range. The sediment properties that were measured include (1) the proportion of Fe(II) in the Fe fraction leached in hot 1.2 N HCl, (2) diffuse spectral reflectance, and (3) magnetic susceptibility.In parallel with local concentrations of dissolved As ranging from <5 to 600 μg/L, Fe(II)/Fe ratios in shallow (gray) Holocene sands tended to gradually increase with depth from values of 0.3 to 0.5 to up to 0.9. In deeper (orange) aquifers of presumed Pleistocene age that were separated from shallow sands by a clay layer and contained <5 μg/L dissolved As, leachable Fe(II)/Fe ratios averaged ∼0.2. There was no consistent relation between sediment Fe(II)/Fe and dissolved Fe concentrations in groundwater in nearby wells. The reflectance measurements indicate a systematic linear relation (R2 of 0.66; n = 151) between the first derivative transform of the reflectance at 520 nm and Fe(II)/Fe. The magnetic susceptibility of the shallow aquifer sands ranged from 200 to 3600 (x 10−9 m3/kg SI) and was linearly related (R2 of 0.75; n = 29) to the concentrations of minerals that could be magnetically separated (0.03 to 0.79% dry weight). No systematic depth trends in magnetic susceptibility were observed within the shallow sands, although the susceptibility of deeper low-As aquifers was low (up to ∼200 × 10−9 m3/kg SI).This set of observations, complemented by incubation results described in a companion paper by van Geen et al. (this volume), suggests that the release of As is linked to the transformation of predominantly Fe (III) oxyhydroxide coatings on sand particles to Fe(II) or mixed Fe(II/III) solid phases with a flatter reflectance spectrum such as siderite, vivianite, or magnetite, without necessarily resulting in the release of Fe to groundwater. The very low As/Fe ratio of magnetically separated minerals compared to the As/Fe of bulk acid leachate (2 vs. 40 10−6, respectively) suggests that such a transformation could be accompanied by a significant redistribution of As to a mobilizable phase on the surface of aquifer particles.  相似文献   

15.
Based on data on the composition of ore-bearing hydrothermal solutions and parameters of ore-forming processes at various antimony and antimony-bearing deposits, which were obtained in studies of fluid inclusions in ore minerals, we investigated the behavior of Sb(III) in the system Sb–Cl–H2S–H2O describing the formation of these deposits.

We also performed thermodynamic modeling of native-antimony and stibnite dissolution in sulfide (mHS− = 0.0001−0.1) and chloride (mCl− = 0.1−5) solutions and the joint dissolution of Sb(s)0 and Sb2S3(s) in sulfide-chloride solution (mHS− = 0.01; mCl− = 1) depending on Eh, pH, and temperature. All thermodynamic calculations were carried out using the Chiller computer program. Under the above conditions, stibnite precipitates in acid, weakly acid to neutral, and medium redox solutions, whereas native antimony precipitates before stibnite under more reducing conditions in neutral to alkaline solutions.

The metal-bearing capacity of hydrothermal solutions (200–250 °C) of different compositions and origins has been predicted. We have established that the highest capacity is specific for acid (pH = 2–3) high-chloride solutions poor in sulfide sulfur and alkaline (pH = 7–8) low-chloride low-sulfide solutions.  相似文献   


16.
Previous studies have shown that saltmarsh macrophytes have a significant influence on sediment biogeochemistry, both through radial release of oxygen from roots and also via primary production and release of labile organic exudates from roots. To assess the seasonal influence of the needle rush, Juncus roemarianus, on saltmarsh sediment geochemistry, pore waters and sediments were collected from the upper 50 cm of two adjacent sites, one unvegetated and the other vegetated by Juncus roemarianus, in a Georgia saltmarsh during winter and summer. Pore waters collected at 1- to 2-cm intervals were analyzed for pH, alkalinity, dissolved phosphate, ammonium, Fe(II), Fe(III), Mn(II), sulfide, sulfate, and organic carbon. Sediments were collected at 5-cm intervals and analyzed for iron distribution in the solid phase using a two-step sequential extraction. The upper 50 cm of the sediment pore waters are mostly sulfidic during both winter and summer. The pore water and sediment geochemistry suggest organic matter degradation is coupled mostly to Fe(III) and sulfate reduction. In summer, there is greater accumulation of alkalinity, sulfide, ammonium, and phosphate in the pore waters and lower levels of ascorbate extractable Fe, which is presumed to be comprised primarily of readily reducible Fe(III) oxides, in the sediments, consistent with higher organic matter degradation rates in summer compared to winter. Lower pH, alkalinity, ammonium, and sulfide concentrations in sediments with Juncus, compared to nearby unvegetated sediments, is consistent with release of oxygen into the Juncus rhizosphere, especially during summer.  相似文献   

17.
Subterranean estuaries are characterized by the mixing of terrestrially derived groundwater and seawater in a coastal aquifer. Subterranean estuaries, like their river water-seawater counterparts on the surface of the earth, represent a major, but less visible, hydrological and geochemical interface between the continents and the ocean. This article is the first in a two-part series on the biogeochemistry of the subterranean estuary at the head of Waquoit Bay (Cape Cod, MA, USA). The pore-water distributions of salinity, Fe and Mn establish the salt and redox framework of this subterranean estuary. The biogeochemistry of Fe, Mn, P, Ba, U and Th will be addressed from the perspective of the sediment composition. A second article will focus on the groundwater and pore-water chemistries of Fe, Mn, U and Ba.Three sediment cores were collected from the head of Waquoit Bay where the coastal aquifer consists of permeable sandy sediment. A selective dissolution method was used to measure the concentrations of P, Ba, U and Th that are associated with “amorphous (hydr)oxides of iron and manganese” and “crystalline Fe and Mn (hydr)oxides.” The deeper sections of the cores are characterized by large amounts of iron (hydr)oxides that are precipitated onto organic C-poor quartz sand from high-salinity pore waters rich in dissolved ferrous iron. Unlike Fe (hydr)oxides, which increase with depth, the Mn (hydr)oxides display midcore maxima. This type of vertical stratification is consistent with redox-controlled diagenesis in which Mn (hydr)oxides are formed at shallower depths than iron (hydr)oxides. P and Th are enriched in the deep sections of the cores, consistent with their well-documented affinity for Fe (hydr)oxides. In contrast, the downcore distribution of Ba, especially in core 3, more closely tracks the concentration of Mn (hydr)oxides. Even though Mn (hydr)oxides are 200-300 times less abundant than Fe (hydr)oxides in the cores, Mn (hydr)oxides are known to have an affinity for Ba which is many orders of magnitude greater than iron (hydr)oxides. Hence, the downcore distribution of Ba in Fe (hydr)oxide rich sediments is most probably controlled by the presence of Mn (hydr)oxides. U is enriched in the upper zones of the cores, consistent with the formation of highly reducing near-surface sediments in the intertidal zone at the head of the Bay. Hence, the recirculation of seawater through this type of subterranean estuary, coupled with the abiotic and/or biotic reduction of soluble U(VI) to insoluble U(IV), leads to the sediments acting as a oceanic net sink of U. These results highlight the importance of permeable sediments as hosts to a wide range of biogeochemical reactions, which may be impacting geochemical budgets on scales ranging from coastal aquifers to the continental shelf.  相似文献   

18.

We examined the chemical reactions influencing dissolved concentrations, speciation, and transport of naturally occurring arsenic (As) in a shallow, sand and gravel aquifer with distinct geochemical zones resulting from land disposal of dilute sewage effluent. The principal geochemical zones were: (1) the uncontaminated zone above the sewage plume [350 μM dissolved oxygen (DO), pH 5.9]; (2) the suboxic zone (5 μM DO, pH 6.2, elevated concentrations of sewage-derived phosphate and nitrate); and (3) the anoxic zone [dissolved iron(II) 100–300 μM, pH 6.5–6.9, elevated concentrations of sewage-derived phosphate]. Sediments are comprised of greater than 90% quartz but the surfaces of quartz and other mineral grains are coated with nanometer-size iron (Fe) and aluminum (Al) oxides and/or silicates, which control the adsorption properties of the sediments. Uncontaminated groundwater with added phosphate (620 μM) was pumped into the uncontaminated zone while samples were collected 0.3 m above the injection point. Concentrations of As(V) increased from below detection (0.005 μM) to a maximum of 0.07 μM during breakthrough of phosphate at the sampling port; As(III) concentrations remained below detection. These results are consistent with the hypothesis that naturally occurring As(V) adsorbed to constituents of the coatings on grain surfaces was desorbed by phosphate in the injected groundwater. Also consistent with this hypothesis, vertical profiles of groundwater chemistry measured prior to the tracer test showed that dissolved As(V) concentrations increased along with dissolved phosphate from below detection in the uncontaminated zone to approximately 0.07 and 70 μM, respectively, in the suboxic zone. Concentrations of As(III) were below detection in both zones. The anoxic zone had approximately 0.07 μM As(V) but also had As(III) concentrations of 0.07–0.14 μM, suggesting that release of As bound to sediment grains occurred by desorption by phosphate, reductive dissolution of Fe oxides, and reduction of As(V) to As(III), which adsorbs only weakly to the Fe-oxide-depleted material in the coatings. Results of reductive extractions of the sediments suggest that As associated with the coatings was relatively uniformly distributed at approximately 1 nmol/g of sediment (equivalent to 0.075 ppm As) and comprised 20%-50% of the total As in the sediments, determined from oxidative extractions. Quartz sand aquifers provide high-quality drinking water but can become contaminated when naturally occurring arsenic bound to Fe and Al oxides or silicates on sediment surfaces is released by desorption and dissolution of Fe oxides in response to changing chemical conditions.

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19.
This study explores the fractionation of iron isotopes (57Fe/54Fe) in an organic-rich mudstone succession, focusing on core and outcrop material sampled from the Upper Jurassic Kimmeridge Clay Formation type locality in south Dorset, UK. The organic-rich environments recorded by the succession provide an excellent setting for an investigation of the mechanisms by which iron isotopes are partitioned among mineral phases during biogeochemical sedimentary processes.Two main types of iron-bearing assemblage are defined in the core material: mudstones with calcite ± pyrite ± siderite mineralogy, and ferroan dolomite (dolostone) bands. A cyclic data distribution is apparent, which reflects variations in isotopic composition from a lower range of δ57Fe values associated with the pyrite/siderite mudstone samples to the generally higher values of the adjacent dolostone samples. Most pyrite/siderite mudstones vary between −0.4 and 0.1‰ while dolostones range between −0.1 and 0.5‰, although in very organic-rich shale samples below 360 m core depth higher δ57Fe values are noted. Pyrite nodules and pyritized ammonites from the type exposure yield δ57Fe values of −0.3 to −0.45‰. A fractionation model consistent with the δ57Fe variations relates the lower δ57Fe pyrite and siderite ± pyrite mudstones values to the production of isotopically depleted Fe(II) during biogenic reduction of the isotopically heavier lithogenic Fe(III) oxides. A consequence of this reductive dissolution is that a 57Fe-enriched iron species must be produced that potentially becomes available for the formation of the higher δ57Fe dolostones. An isotopic profile across a dolostone band reveals distinct zonal variations in δ57Fe, characterized by two peaks, respectively located above and below the central part of the band, and decoupling of the isotopic composition from the iron content. This form of isotopic zoning is shown to be consistent with a one-dimensional model of diffusional-chromatographic Fe-isotope exchange between dolomite and isotopically enriched pore water. An alternative mechanism envisages the infiltration of dissolved ferrous iron from variable (high and low) δ57Fe sources during coprecipitation of Fe(II) ion with dolomite. The study provides clear evidence that iron isotopes are cycled during the formation and diagenesis of organic carbon-rich sediments.  相似文献   

20.
The distribution and partitioning of dissolved andparticulate arsenic and phosphorus in the water columnand sediments of the Saguenay Fjord in Quebec, Canada,are compared. In addition, selective and/or sequentialextractions were carried out on the suspendedparticulate matter (SPM) and solid sediments tocontrast their geochemical behaviors in this naturalaquatic system.Results of our analyses show that both arsenic andsoluble reactive phosphate are actively scavenged fromthe water column by settling particles. Upon theiraccumulation at the sediment-water interface some Asand P may be released to porewaters following thedegradation of organic matter to which they areassociated. The porewater concentrations are, however,limited by their strong affinity for authigenic,amorphous iron oxyhydroxides which accumulate in theoxic sediments near the sediment-water interface.The geochemical behavior of arsenic and phosphorusdiverge most strikingly upon the development of anoxicconditions in the sediments. Following their burial inthe anoxic zone, amorphous iron oxyhydroxides arereduced and dissolved, releasing phosphate and arsenicto the porewaters. We observed, however, thatporewater arsenic concentrations increase at shallowerdepths than phosphate in the sediments. The reductionof arsenate, As(V), to arsenite, As(III), and itsdesorption prior to the reductive dissolution of thecarrier phase(s) may explain this observation.Driven by the strong concentration gradientestablished in the suboxic zone, phosphate diffuses uptowards the oxic layer where it is readsorbed byauthigenic iron oxyhydroxides. In the organic-rich andrapidly accumulating sediments at the head of theFjord, porewater sulfate depletion and the resultingabsence of a sulfide sink for Fe(II), may lead to theformation of vivianite in the fermentation zone, apotential sink for phosphate. Arsenite released to theporewaters in the suboxic and anoxic zones of thesediments diffuses either down, where it is adsorbedto or incorporated with authigenic iron sulfides, orup towards the oxic boundary. Arsenite appears tomigrate well into the oxic zone where it may beoxidized by authigenic manganese oxides before beingadsorbed by iron oxyhydroxides present at the samedepth. Whereas, in the absence of authigenic carbonatefluorapatite precipitation, the ability of oxicsediments to retain mineralized phosphate is afunction of their amorphous iron oxyhydroxide content,arsenic retention may depend on the availability ofmanganese oxides, the thickness of the oxic layer and,its co-precipitation with iron sulfides at depth.  相似文献   

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