Geochemistry of Diagenesis of Sediments in the Barents Sea: Forms of Iron and Sulfur

The chemical composition of sediments of cores taken from different areas of the Barents Sea was studied. The study involves reactive iron (Fe_react) and reduced sulfur. The surficial oxidized layer with a thickness of 0.5–25 cm is characterized by increased content of Fe(III) in the form of oxyhydroxides and possibly hydroxophosphates. This layer is locally enriched in Mn. Reactive iron in sediments of horizons, which underlie the oxidized layer, is probably present mainly in the composition of silicates. The role of sulfides (acid-soluble phases and pyrite) among Fe_react forms is predominantly subordinate. The core of pre-Holocene sediments is characterized by a strong correlation between the total content of reduced sulfur and its isotopic composition. This testifies to authigenic origin of pyrite in the pre-Holocene mud.     

Sulfur biogeochemistry and isotopic fractionation in shallow groundwater and sediments of Owens Dry Lake, California

Groundwater and sediment samples (∼ 1 m depth) at sites representative of different groundwater pathways were collected to determine the aqueous speciation of sulfur and the fractionation of sulfur isotopes in aqueous and solid phases. In addition, selected sediment samples at 5 depths (from oxic to anoxic layers) were collected to investigate the processes controlling sulfur biogeochemistry in sedimentary layers. Pyrite was the dominant sulfur-bearing phase in the capillary fringe and groundwater zones where anoxic conditions are found. Low concentrations of pyrite (< 5.9 g kg^− 1) coupled with high concentrations of dissolved sulfide (4.81 to 134.7 mg L^− 1) and low concentrations of dissolved Fe (generally < 1 mg L^− 1) and reducible solid-phase Fe indicate that availability of reactive Fe limits pyrite formation. The relative uniformity of down-core isotopic trends for sulfur-bearing mineral phases in the sedimentary layers suggests that sulfate reduction does not result in significant sulfate depletion in the sediment. Sulfate availability in the deeper sediments may be enhanced by convective vertical mixing between upper and lower sedimentary layers due to evaporative concentration. The large isotope fractionation between dissolved sulfate and sedimentary sulfides at Owens Lake provides evidence for initial fractionation from bacterial sulfate reduction and additional fractionation generated by sulfide oxidation followed by disproportionation of intermediate oxidation state sulfur compounds. The high salinity in the Owens Lake brines may be a factor controlling sulfate reduction and disproportionation in hypersaline conditions and results in relatively constant values for isotope fractionation between dissolved sulfate and total reduced sulfur.     

The role of carbonate ions in pyrite oxidation in aqueous systems

The mechanism of pyrite oxidation in carbonate-containing alkaline solutions at 80 °C was investigated with the help of rate experiments, thermodynamic modeling and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrite oxidation rate increased with pH and was enhanced by addition of bicarbonate/carbonate ions. The carbonate effect was found to be limited to moderately alkaline conditions (pH 8-11). Metastable Eh-pH diagrams, at 25 °C, indicate that soluble iron-carbonate complexes (FeHCO₃⁻, FeCO₃⁰, Fe(CO₃)(OH)⁻ and FeCO₃²⁻) may coexist with pyrite in the pH range of 6-12.5. Above pH 11 and 13, the Fe(II) and Fe(III) hydroxocomplexes, respectively, become stable, even in the presence of carbonate/bicarbonate ions. Surface-bound carbonate complexes on iron were also identified with DRIFTS as products of pyrite oxidation in addition to iron oxyhydroxides and soluble sulfate species. The conditions under which thermodynamic and DRIFTS analyses indicate the presence of carbonate compounds also correspond to those in which the fastest rate of pyrite oxidation in carbonate solutions was observed. Following the Singer-Stumm model for pyrite oxidation in acidic solutions, it is assumed that Fe(III) is the preferred pyrite oxidant under alkaline conditions. We propose that carbonate ions facilitate the electron transfer from soluble iron(II)-carbonate to O₂, increase the iron solubility, and provide buffered, favorable alkaline conditions at the reaction front, which in turn favors the overall kinetics of pyrite oxidation. Therefore, the electron transfer from sulfur atoms to O₂ is facilitated by the formation of the cycle of Fe(II)-pyrite/Fe(III)-carbonate redox couple at the pyrite surface.     

Phosphorus recycling and burial in Baltic Sea sediments with contrasting redox conditions

In this study, redox-dependent phosphorus (P) recycling and burial at 6 sites in the Baltic Sea is investigated using a combination of porewater and sediment analyses and sediment age dating (²¹⁰Pb and ¹³⁷Cs). We focus on sites in the Kattegat, Danish Straits and Baltic Proper where present-day bottom water redox conditions range from fully oxygenated and seasonally hypoxic to almost permanently anoxic and sulfidic. Strong surface enrichments of Fe-oxide bound P are observed at oxic and seasonally hypoxic sites but not in the anoxic basins. Reductive dissolution of Fe-oxides and release of the associated P supports higher sediment-water exchange of PO₄ at hypoxic sites (up to ∼800 μmol P m⁻² d⁻¹) than in the anoxic basins. This confirms that Fe-bound P in surface sediments in the Baltic acts as a major internal source of P during seasonal hypoxia, as suggested previously from water column studies. Most burial of P takes place as organic P. We find no evidence for significant authigenic Ca-P formation or biogenic Ca-P burial. The lack of major inorganic P burial sinks makes the Baltic Sea very sensitive to the feedback loop between increased hypoxia, enhanced regeneration of P and increased primary productivity. Historical records of bottom water oxygen at two sites (Bornholm, Northern Gotland) show a decline over the past century and are accompanied by a rise in values for typical sediment proxies for anoxia (total sulfur, molybdenum and organic C/P ratios). While sediment reactive P concentrations in anoxic basins are equal to or higher than at oxic sites, burial rates of P at hypoxic and anoxic sites are up to 20 times lower because of lower sedimentation rates. Nevertheless, burial of reactive P in both hypoxic and anoxic areas is significant because of their large surface area and should be accounted for in budgets and models for the Baltic Sea.     

The Differential Geochemical Behavior of Arsenic and Phosphorus in the Water Column and Sediments of the Saguenay Fjord Estuary, Canada

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.     

Pyrite oxidation in the tertiary sands of the London Basin aquifer

Possible groundwater quality changes related to pyrite oxidation during artificial groundwater recharge and its storage in the Tertiary sands of the London Basin are investigated. Pyrite textures in the Tertiary sands are examined by scanning electron microscopy while an experimental approach is used to study mechanisms of pyrite oxidation and of some associated chemical reactions. In the Tertiary sands of the London Basin aquifer, pyrite occurs as aggregates made of discrete individual crystals 0.5–5 μm in size or, in a cryptocrystalline form, often as pseudomorphs of biogenic debris. It can expose a very large specific surface area to porefluids. Although ferric iron, which can be an oxidising agent of pyrite, is abundant in the solid phase of the Tertiary sands, it does not appear to take a significant part in this case. Pyrite oxidation seems to rely on a supply of oxygen. Leaching experiments using a 0.001 M H₂SO₄ solution were carried out to examine interactions between mildly acidic groundwater resulting from pyrite oxidation at a moderate rate and the host-sediment. In the presence of CaCO₃ in the solid phase, H⁺ is rapidly buffered by CaCO₃ dissolution. Oscillations of this reaction around equilibrium appear to trigger cation-exchange reactions on clay mineral surfaces, resulting in the release of major cations (e.g. K and Mg) into solution. In the absence of CaCO₃ in the solid phase, H⁺ buffering occurs less efficiently solely through exchange of cations for H⁺ on clay minerals surfaces. If the rate of pyrite oxidation in the Tertiary sands becomes high enough for the buffering capacity of the system to be exceeded, the groundwater pH begins to fall. Interactions between low pH (2) groundwaters and the host sediments were examined by leaching solid material in 0.01 M and 0.1 M H₂SO₄ solutions. Concentrations of Fe, Mg and K increase in solution throughout the experiment, indicating partial dissolution of clay minerals. The composition of the porefluid thus depends on the geochemical composition and surface area of the different clay minerals present.     

Formation of syngenetic and early diagenetic iron minerals in the late Archean Mt. McRae Shale, Hamersley Basin, Australia: New insights on the patterns, controls and paleoenvironmental implications of authigenic mineral formation

A section through the late Archean Mt. McRae Shale comprising, in ascending order, a lower shale interval (LSI), a banded iron formation (BIF), an upper shale (USI) and a carbonate (C1) has been analyzed for total Fe and Al contents and authigenic Fe present as carbonate, oxide, sulfide and silicate phases. The authigenic mineralogy is controlled by the episodic addition of Fe from hydrothermal activity and removal of Fe by sulfide, relative to rates of clastic sedimentation. The LSI and BIF have mean Fe_T/Al values of 2 and 5, respectively, that record iron enrichment from hydrothermal sources. Iron was precipitated primarily as siderite accompanied by Fe-rich chlorite from anoxic bottom waters rich in dissolved Fe. Pyrite formation was probably limited by the availability of sulfate, which was present at low concentrations and became rapidly depleted. The USI has generally lower Fe_T/Al values (0.6-1.3), similar to those found in Paleozoic shales, with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle. This interval contains authigenic Fe predominantly as pyrite, where high values for DOP (>0.8) indicate the existence of a water column that became rich in dissolved sulfide (euxinic) when sulfate concentrations increased due to a transient or secular increase in ocean/atmosphere oxygenation. High concentrations of dissolved sulfide maintained low concentrations of dissolved Fe, which allowed only minor amounts of Fe to be precipitated as carbonates and silicates. The USI also has elevated concentrations of organic matter that most probably reflect increased productivity and likely limited euxinia to midportions of the water column on the basin margin. The carbonate C1 represents a basinal chemistry where sulfide has been removed and Fe_T/Al values are ∼1 indicating that hydrothermal activity again produced dissolved Fe-rich bottom waters. Detailed iron speciation of the Mt. McRae Shale can be used to recognize spatial and temporal variations in iron and sulfur inputs to the late Archean Hamersley Basin, just prior to the Paleoproterozoic rise in atmospheric oxygenation, and our refined methods have relevance to all Fe-rich deposits.     

Diagenesis of oxyanions (V, U, Re, and Mo) in pore waters and sediments from a continental margin

This research tests the hypothesis that trace metals respond to the extent of reducing conditions in a predictable way. We describe pore water and sediment measurements of iron (Fe), manganese (Mn), vanadium (V), uranium (U), rhenium (Re), and molybdenum (Mo) along a transect off Washington State (USA). Sediments become less reducing away from the continent, and the stations have a range of oxygen penetration depths (depth to unmeasurable O₂ concentration) varying from a few millimeters to five centimeters. When oxygen penetrates ∼1 cm or less, Fe is reduced in the pore waters but reoxidized near the sediment-water interface, preventing a flux of Fe²⁺ to overlying waters, whereas Mn oxides are reduced and Mn²⁺ diffuses to overlying waters. Both Re and U authigenically accumulate in sediments. Only at the most reducing location, where the oxygen penetrates 0.3 cm below the sediment-water interface, does the surface 30 cm of sediments become reducing enough to authigenically accumulate Mo.Stations in close proximity to the Juan de Fuca Ridge crest are enriched in Mn and Fe from hydrothermal plume processes. Both V and Mo clearly associate with Mn cycling, whereas U may be associating with either Mn oxides and/or Fe oxyhydroxides. Rhenium is uncomplicated by adsorption to Mn oxides and/or Fe oxyhydroxides, and Re accumulation in sediments appears to be due solely to the extent of reducing conditions. Therefore, authigenic sediment Re enrichment appears to be the best indicator for intermediate reducing conditions, where oxygen penetrates less than ∼1 cm below the sediment-water interface, when coupled with negligible authigenic Mo enrichment.     

Arsenic in framboidal pyrite from recent sediments of a shallow water lagoon of the Baltic Sea

Arsenic is a redox‐sensitive element of environmental relevance and often enriched in iron sulphides. Because sediments from the Achterwasser lagoon, a part of the estuarine system of the river Oder, south‐west Baltic Sea, show unexpectedly high pyrite concentrations of up to 7·5 wt% they were used to investigate the influence of authigenic pyrite on the mobility and burial of As in the coastal environment. Micro‐X‐ray‐fluorescence measurements of 106 micrometre‐sized pyrite framboids from the anoxic sediments show highly variable As concentrations ranging from 6 to 1142 μg g^?1. Even within a 1 cm thick layer, the As concentration of different framboids varies greatly and no clear depth trend is visible throughout the 50 cm long sediment core. Pyrite can account for 9 to 55% (average 22%) of the total As budget of the sediments and the degree of trace metalloid pyritization for As ranges from 26 to 61%, indicating that authigenic pyrite formation is an important process in the geochemical cycling of As in coastal sediments. High‐resolution micro‐X‐ray fluorescence mapping of single pyrite grains shows that As is distributed inhomogeneously within larger framboids, suggesting changing pore water composition during pyrite growth. X‐ray absorption near edge structure spectra indicate that As is usually present as As(‐I) substituting S in the pyrite lattice. However, in samples close to the sediment/water interface a considerable part of As is in higher valence states (+III/+V). This can be explained by frequent re‐suspension of the surficial sediments to the oxic water column due to wave action and subsequent re‐deposition, leading to the adsorption of As oxyanions onto pyrite. Although reduced As(‐I) becomes more important in the deeper samples, reflecting decreasing redox potential and a longer time since deposition, the occurrence of oxidized As species (AsIII/AsV) in pyrite in the anoxic part of the sediment suggests formation under dysoxic conditions.     

Voltammetric evidence for soluble FeS complexes in anoxic estuarine muds

Voltammetric methods using direct insertion of a gold-amalgam microelectrode with a sensitive, computercontrolled voltammeter detected soluble iron(II) sulfide, [FeS]_aq, in the porewaters of anoxic, sulfidic, fine-grained sediments from the Loughor Estuary, Wales. The voltammetric results are reproducible. Studies of cores stored in sealed, refrigerated containers for up to 21 d reveal no measurable oxidation. [FeS]_aq forms in this estuarine environment as a result of the dissolution of amorphous FeS, and appears to be involved in the formation of pyrite. [FeS]_aq makes no significant contribution to the total sulfide and iron contents of the sediment but could constitute an important component of the dissolved Fe(II) and S(−II) contents of the porewater. Mass balance calculations show pyrite forms in this system by the addition of sulfur to FeS rather than by the loss of iron from FeS. The overall process appears to involve [FeS]_aq as an intermediary. Although the porewaters of the Loughor Estuary sediments are iron-rich relative to seawater, the iron sulfide-forming process is iron-limited rather than sulfide-limited. Reactive iron is bound to sulfide rapidly in the sediment. After the reactive iron is bound to sulfide, additional sulfide produced is fixed as pyrite.     

Distribution and partitioning of major and trace elements in pyrite-bearing sediments of a Mediterranean coastal lagoon

The formation of iron sulphide minerals exerts significant control on the behaviour of trace elements in sediments. In this study, three short sediment cores, retrieved from the remote Antinioti lagoon (N. Kerkyra Island, NW Greece), are investigated concerning the solid phase composition, distribution, and partitioning of major (Al, Fe) and trace elements (Cd, Cu, Mn, Pb, and Zn). According to ²¹⁰Pb, the sediments sampled correspond to depositions of the last 120 years. The high amounts of organic carbon (4.1–27.5%) result in the formation of Fe sulphides, predominantly pyrite, already at the surface sediment layers. Pyrite morphologies include monocrystals, polyframboids, and complex FeS–FeS₂ aggregates. According to synchrotron-generated micro X-ray fluorescence and X-ray absorption near-edge structure spectra, authigenically formed, Mn-containing, Fe(III) oxyhydroxides (goethite type) co-exist with pyrite in the sediments studied. Microscopic techniques evidence the formation of galena, sphalerite and CuS, whereas sequential extractions show that carbonates are important hosts for Mn, Cd, and Zn. However, significant percentages of non-lattice held elements are bound to Fe/Mn oxyhydroxides that resist reductive dissolution (on average 60% of Pb, 46% of Cd, 43% of Zn and 9% of Cu). The partitioning pattern changes drastically in the deeper part of the core that is influenced by freshwater inputs. In these sediments, the post-depositional pyritization mechanism, illustrated by overgrowths of Fe monosulphides on pre-existing pyrite grains, results in relatively high degree of pyritization that reaches 49% for Cd, 66% for Cu, 32% for Zn and 7% for Pb.     

三水盆地古近系(土布)心组黑色页岩中黄铁矿的形成及其控制因素

黄铁矿是富有机质沉积的特征矿物。根据TOC/S、TOC/DOP、S/Fe关系以及S TOC Fe多重线性回归分析结果对三水盆地古近系〖HT5”,6”〗土〖KG-*3〗布〖HT5”SS〗心组红岗段黑色页岩中沉积黄铁矿的形成及其控制因素进行了分析。土布心组红岗段黑色页岩的黄铁矿有成岩黄铁矿和同生黄铁矿两种成因组分。红岗段下部（亚段A）有机碳含量普遍较低,底部水体以弱氧化条件为主,硫酸盐还原作用发生于沉积物/水界面以下,黄铁矿为成岩成因,其形成主要受有机质的限制。红岗段中上部（亚段B和C）的沉积条件变化频繁,其有机碳含量变化幅度大。富有机质（TOC>4%）岩层形成于缺氧的底部水体条件下。水体中可含H2S,碎屑铁矿物在埋藏之前即与之在水体中反应形成同生黄铁矿。这一过程不受有机质的限制,而是受活性铁与H2S接触时间的限制。同时,由于大量淡水输入导致硫酸盐浓度的降低,从而对硫化物形成有一定的限制作用。对于低有机质（TOC<4%）样品,黄铁矿由同生和成岩组分组成。其中以成岩黄铁矿为主,其形成过程主要受有机质限制,而同生黄铁矿受铁矿物与H2S接触时间的限制。     

Oxidation of pyrite during extraction of carbonate associated sulfate

The sulfur isotopic composition of carbonate associated sulfate (CAS) has been used to investigate the geochemistry of ancient seawater sulfate. However, few studies have quantified the reliability of δ³⁴S of CAS as a seawater sulfate proxy, especially with respect to later diagenetic overprinting. Pyrite, which typically has depleted δ³⁴S values due to authigenic fractionation associated with bacterial sulfate reduction, is a common constituent of marine sedimentary rocks. The oxidation of pyrite, whether during diagenesis or sample preparation, could thus adversely influence the sulfur isotopic composition of CAS. Here, we report the results of CAS extractions using HCl and acetic acid with samples spiked with varying amounts of pyrite. The results show a very strong linear relationship between the abundance of fine-grained pyrite added to the sample and the resultant abundance and δ³⁴S value of CAS. This data represents the first unequivocal evidence that pyrite is oxidized during the CAS extraction process. Our mixing models indicate that in samples with much less than 1 wt.% pyrite and relatively high δ³⁴S_pyrite values, the isotopic offset imparted by oxidation of pyrite should be much less than ? 4‰. A wealth of literature exists on the oxidation of pyrite by Fe³⁺ and we believe this mechanism drives the oxidation of pyrite during CAS extraction, during which the oxygen used to form sulfate is taken from H₂O, not O₂. Consequently, extracting CAS under anaerobic conditions would only slow, but not halt, the oxidation of pyrite. Future studies of CAS should attempt to quantify pyrite abundance and isotopic composition.     

Geochemistry and specific features of manganese ore formation in sediments of oceanic bioproductive zones

Processes of authigenic manganese ore formation in sediments of the northern equatorial Pacific are considered on the basis of study of the surface layer (<2 mm) of ferromanganese nodule and four micronodule size fractions from the associated surface sediment (0–7 cm). Inhomogeneity of the nodule composition is shown. The Mn/Fe ratio is maximal in samples taken from the lateral sectors of nodule at the water-sediment interface. Compositional differences of nodules are related to the preferential accumulation of microelements in iron oxyhydroxides (P, Sr, Pb, U, Bi, Th, Y, and REE), manganese hydroxides (Co, Ni, Cu, Zn, Cd, Mo, Tl, W), and lithogenous component trapped during nodule growth (Ga, Rb, Ba, and Cs). The Ce accumulation in the REE composition is maximal in the upper and lower parts of the nodule characterized by the minimal Mn/Fe values. The compositional comparison of manganese micronodules and surface layers of the nodule demonstrated that the micronodule material was subjected to a more intense reworking during the diagenesis of sediments. The micronodules are characterized by higher Mn/Fe and P/Fe ratios but lower Ni/Cu and Co/Ni ratios. The micronodules and nodules do not differ in terms of contents of Ce and Th that are least mobile elements during the diagenesis of elements. Differences in the chemical composition of micronodules and nodules are related not only to the additional input of Mn in the process of diagenesis, but also to the transformation of iron oxyhydroxides after the removal of Mn from the close association with Fe formed in the suspended matter at the stage of sedimentation.     

Abundance and Chemical Speciation of Phosphorus in Sediments of the Mackenzie River Delta, the Chukchi Sea and the Bering Sea: Importance of Detrital Apatite

Utilizing a sequential extraction technique this study provides the first quantitative analysis on the abundance of sedimentary phosphorus and its partitioning between chemically distinguishable phases in sediments of the Bering Sea, the Chukchi Sea and the Mackenzie River Delta in the western Arctic Ocean. Total sedimentary phosphorus (TSP) was fractionated into five operationally defined phases: (1) adsorbed inorganic and exchangeable organic phosphorus, (2) Fe-bound inorganic phosphorus, (3) authigenic carbonate fluorapatite, biogenic apatite and calcium carbonate-bound inorganic and organic phosphorus, (4) detrital apatite, and (5) refractory organic phosphorus. TSP concentrations in surface sediments increased from the Chukchi Sea (18 μmol g⁻¹ of dried sediments) to the Bering Sea (22 μmol g⁻¹) and to the Mackenzie River Delta (29 μmol g⁻¹). Among the five pools, detrital apatite phosphorus of igneous or metamorphic origin represents the largest fraction (~43%) of TSP. The second largest pool is the authigenic carbonate fluorapatite, biogenic apatite as well as CaCO₃ associated phosphorus (~24% of TSP), followed by the Fe-bound inorganic phosphorus, representing ~20% of TSP. The refractory organic P accounts for ~10% of TSP and the readily exchangeable adsorbed P accounts for only 3.5% of TSP. Inorganic phosphorus dominates all of phosphorus pools, accounting for an average of 87% of the TSP. Relatively high sedimentary organic carbon and total nitrogen contents and low δ¹³C values in the Mackenzie River Delta together with the dominance of detrital apatite in the TSP demonstrate the importance of riverine inputs in governing the abundance and speciation of sedimentary phosphorus in the Arctic coastal sediments.     

In situ adsorption of mercury,methylmercury and other elements by iron oxyhydroxides and organic matter in lake sediments

Samples of authigenic material, sediment overlying water and oxic surface sediment (0–0.5-cm depth) from a perennially oxygenated lacustrine basin were analysed to investigate which solid phases are important for binding a suite of trace elements (Ag, As, Ca, Cd, Cu, Hg, In, methylmercury (MeHg), Mg, Mo, Pb, Sb and Zn). The authigenic material, which was collected with inert Teflon sheets deployed for several years across the sediment–water interface, contained mainly poorly crystallized Fe oxyhydroxides and natural organic matter, presumably humic substances derived from the watershed. Manganese oxyhydroxides were not present in the collected authigenic material due to the slightly acidic condition (pH = 5.6) of the lake that prevents the formation and recycling of these compounds. Conditional equilibrium constants for the adsorption of cationic (K_Fe–M) and anionic (K_Fe–A) trace elements onto the authigenic Fe oxyhydroxides were estimated from their concentrations in the authigenic material and in bottom water samples. These field-derived values of K_Fe–M and K_Fe–A were compared with those predicted by the surface complexation model, using laboratory-derived intrinsic adsorption constants and the water composition at the study site. Equilibrium constants (K_POM–M) were also calculated for the adsorption of the cationic trace elements onto the humic substances contained in the diagenetic material. The field-derived values of K_POM–M were compared to those predicted by the speciation code WHAM 6 for the complexation of the trace elements by dissolved humic substances in the lake. Combining the results of the present study with those on the distributions of trace elements in the porewater and solid-phase sediments reported in previous studies at the same site, it was determined whether the trace elements bind preferentially to Fe oxyhydroxides or natural organic matter in oxic sediments. The main inferences are that the anionic trace elements As, Mo and Sb, as well as the cationic metal Pb are preferentially bound to the authigenic Fe oxyhydroxides whereas the other trace elements, and especially Hg and MeHg, are preferentially bound to the humic substances.     

Geochemical and anthropogenic enrichments of Mo in sediments from perennially oxic and seasonally anoxic lakes in Eastern Canada

We measured the vertical distributions of Mo, Fe, Mn, sulfide, sulfate, organic carbon, major ions, and pH in sediment porewater from one perennially oxic and three seasonally anoxic lacustrine basins in Eastern Canada, as well as those of Mo, acid volatile sulfide, Fe, Mn, Al, organic C, ²¹⁰Pb and ¹³⁷Cs in sediment cores from the same sites. The only input of anthropogenic Mo to these lakes comes from atmospheric deposition.The relatively monotonous distribution of Mo in the porewater of three seasonally anoxic basins suggests that Mo is not redistributed in the sedimentary column during periods of anoxia. In contrast, porewater Mo profiles obtained at three sampling dates in a perennially oxic basin display sharp Mo peaks below the sediment-water interface, indicating redistribution subsequent to deposition. Modeling of these latter porewater Mo profiles with a diagenetic reaction-transport equation coupled to comparisons among the various porewater and solid phase profiles reveal that Mo is released at 1-2 cm depth as a consequence of the reductive dissolution of Fe oxyhydroxides and scavenged both at the vicinity of the sediment-water interface, by re-adsorption onto authigenic Fe oxyhydroxides, and deeper in the sediments where dissolved sulfide concentrations are higher. The estimated rate constant for the adsorption of Mo onto Fe oxyhydroxides is 36 ± 45 cm³ mol⁻¹ s⁻¹.Diagenetic modeling indicates that authigenic Mo in sediments of the perennially oxic basin represents about one-third of the total solid phase Mo in the first cm below the sediment-water interface and only one tenth below this horizon. If we assume that no authigenic Mo is accumulated in the seasonally anoxic lake sediments we conclude that the sediment Mo concentrations, which are up to 3-16 times higher than the average lithogenic composition, depending on the lake, are mainly due to atmospheric deposition of anthropogenic Mo and not to the formation of authigenic Mo phases. Reconstructed historical records of the atmospheric Mo deposition indicate maximum values in the 1970s and 1980s and significant decreases since then. Emissions to the atmosphere associated with the smelting of non-ferrous ores and coal combustion appear to be the most important sources of anthropogenic Mo.     

Co-diagenesis of iron and phosphorus in hydrothermal sediments from the southern East Pacific Rise: Implications for the evaluation of paleoseawater phosphate concentrations

We present a detailed study of the co-diagenesis of Fe and P in hydrothermal plume fallout sediments from ∼19°S on the southern East Pacific Rise. Three distal sediment cores from 340-1130 km from the ridge crest, collected during DSDP Leg 92, were analysed for solid phase Fe and P associations using sequential chemical extraction techniques. The sediments at all sites are enriched in hydrothermal Fe (oxyhydr)oxides, but during diagenesis a large proportion of the primary ferrihydrite precipitates are transformed to the more stable mineral form of goethite and to a lesser extent to clay minerals, resulting in the release to solution of scavenged P. However, a significant proportion of this P is retained within the sediment, by incorporation into secondary goethite, by precipitation as authigenic apatite, and by readsorption to Fe (oxyhydr)oxides. Molar P/Fe ratios for these sediments are significantly lower than those measured in plume particles from more northern localities along the southern East Pacific Rise, and show a distinct downcore decrease to a depth of ∼12 m. Molar P/Fe ratios are then relatively constant to a depth of ∼35 m. The Fe and P speciation data indicate that diagenetic modification of the sediments is largely complete by a depth of 2.5 m, and thus depth trends in molar P/Fe ratios can not solely be explained by losses of P from the sediment by diffusion to the overlying water column during early diagenesis. Instead, these sediments are likely recording changes in dissolved P concentrations off the SEPR, possibly as a result of redistribution of nutrients in response to changes in oceanic circulation over the last 10 million years. Furthermore, the relatively low molar P/Fe ratios observed throughout these sediments are not necessarily solely due to losses of scavenged P by diffusion to the overlying water column during diagenesis, but may also reflect post-depositional oxidation of pyrite originating from the volatile-rich vents of the southern East Pacific Rise. This study suggests that the molar P/Fe ratio of oxic Fe-rich sediments may serve as a proxy of relative changes in paleoseawater phosphate concentrations, particularly if Fe sulfide minerals are not an important component during transport and deposition.     

Controls on iron-isotope fractionation in organic-rich sediments (Kimmeridge Clay, Upper Jurassic, Southern England)

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.     

Pyrite oxidation by Acidithiobacillus ferrooxidans at various concentrations of dissolved oxygen

Pyrite oxidation rates were examined at various concentrations of dissolved oxygen (DO) in the presence of the sulfur and iron oxidizer Acidithiobacillus ferrooxidans. Five different batch experiments were performed at room temperature for 75 days under various DO levels (273, 129, 64.8, 13.2, and ≤ 0.006 μM), containing pyrite grains (particle size 63–250 μm) and a modified 9K nutrient medium at pH 3. The reactors were inoculated with A. ferrooxidans. In all experiments, pH decreased with time and sulfur and iron were released to the solution, indicating pyrite oxidation at all DO levels. Pyrite oxidation rates (ca. 5 × 10^− 10 mol m^− 2 s^− 1 at 273 μM DO) from all experiments showed positive correlation with DO, Fe(III), and bacterial concentration. These rates were significantly slower than rates presented in other published studies, but this is probably due to the significantly greater Fe(III) concentration at lower pH in these previous studies. The results obtained in this study suggest that ferric iron reduction at the pyrite surface is the primarily mechanism for microbial pyrite oxidation in the presence of DO. The results from our study support the indirect mechanism of sulfide oxidation, where A. ferrooxidans oxidizes ferrous iron in the presence of DO, which then oxidizes pyrite.