Formation Of Iron Sulfide Minerals Research Articles

Laboratory and initial field testing of the Min‐Trap™ for tracking reactive iron sulfide mineral formation during in situ remediation

AbstractThe Mineral Trap, or Min‐Trap™, is a monitoring well‐based sampler designed to collect direct physical evidence of reactive mineral formation in situ without collecting soil or rock core samples. The Min‐Trap consists of a nonreactive granular medium (e.g., silica sand) within water‐permeable mesh pillows that are supported inside a slotted polyvinyl chloride housing that is incubated within a conventional monitoring well. The primary objective of the Min‐Trap in this application is to collect reactive minerals that are forming in the aquifer in a retrievable format that can be submitted for laboratory analysis. To evaluate the capability of Min‐Traps to capture reactive iron minerals, both a laboratory tank test and a field test were conducted. Both tests confirmed that iron sulfide minerals form in the Min‐Trap under sulfate reducing conditions within several weeks. Analysis of the precipitated minerals via the AMIBA analysis suite showed that they almost entirely consisted of weak acid soluble (biogenic, microcrystalline) ferrous iron‐based minerals, and at least two thirds of the sulfur‐containing minerals were monosulfides (i.e., mackinawite) at the end of each test. Scanning electron microscopy confirmed the colocation of iron and sulfur in the mineral masses. The dominance of the ferrous iron and reduced sulfur verifies that little to no oxidation of the captured minerals occurred between sample collection and analysis. A subsurface soil core was collected during the field test next to the Min‐Trap‐containing well. AMIBA results were consistent between the native soil and the Min‐Trap except for much higher strong acid soluble (crystalline) ferric iron in the native soil when compared to the silica sand of the Min‐Trap, as expected. This work shows that Min‐Traps are useful for documenting the formation of reactive iron sulfides (FeSx) that can form during in situ anaerobic biostimulation and can drive complementary abiotic treatment of chlorinated volatile organic compounds. Mineralogical data obtained from Min‐Traps can be applied to assess remedial objectives at several stages of the remedial program, including initial characterization, alternatives evaluation, feasibility testing, remedy optimization, and transition from active treatment to passive remedial methods.

Iron cycling in Arctic methane seeps

Anoxic marine sediments contribute a significant amount of dissolved iron (Fe2+) to the ocean which is crucial for the global carbon cycle. Here, we investigate iron cycling in four Arctic cold seeps where sediments are anoxic and sulfidic due to the high rates of methane-fueled sulfate reduction. We estimated Fe2+ diffusive fluxes towards the oxic sediment layer to be in the range of 0.8 to 138.7 μmole/m2/day and Fe2+ fluxes across the sediment-water interface to be in the range of 0.3 to 102.2 μmole/m2/day. Such variable fluxes cannot be explained by Fe2+ production from organic matter–coupled dissimilatory reduction alone. We propose that the reduction of dissolved and complexed Fe3+ as well as the rapid formation of iron sulfide minerals are the most important reactions regulating the fluxes of Fe2+ in these cold seeps. By comparing seafloor visual observations with subsurface pore fluid composition, we demonstrate how the joint cycling of iron and sulfur determines the distribution of chemosynthesis-based biota.

Authigenic metastable iron sulfide minerals preserve microbial organic carbon in anoxic environments

The burial of organic carbon (OC) in sedimentary environments promotes long-term carbon sequestration, which allows the release of oxygen in the atmosphere. Organo-mineral interactions that form between terrigenous minerals and OC during transport to and deposition on the seabed enhance OC preservation. Here, we propose an authigenic mechanism for the coupled preservation of labile OC and metastable iron sulfide minerals under anoxic conditions. Sulfate-reducing microorganisms (SRM) are ubiquitous in anoxic environments and produce the majority of free sulfide in marine sediments, leading to the formation of iron sulfide minerals in situ. Using high spatial resolution microscopy, spectroscopy and spectro-microscopy, we show that iron sulfide biominerals precipitated in the presence of SRM incorporate and adsorb organic molecules, leading to the formation of stable organo-mineral aggregates that could persist for years in anoxic environments. OC/iron sulfide assemblages consist of the metastable iron sulfide mineral phases mackinawite and/or greigite, along with labile organic compounds derived from microbial biomass or from organic molecules released extracellularly by SRM. Together these results underscore the role that a major group of anoxic microbes play in OC preservation and illustrate the value of the resulting authigenic metastable iron sulfide minerals mackinawite and greigite in protecting labile organic molecules from degradation over time.

Reaction pathways of iron-sulfide mineral formation: an in situ X-ray diffraction study

Iron sulfides were synthesized via a co-precipitation method. In addition, synchrotron-radiation experiments were performed under a range of pH and temperature conditions (up to 100 °C) to compare the results of in situ and ex situ crystal growth investigation of iron sulfides. In acidic environments, H 2 S acts as an oxidant, oxidizing Fe 2+ to Fe 3+ and allowing formation of greigite from mackinawite. However, under neutral conditions, due to very low H 2 S concentrations, the oxidant may be S (instead of H 2 S), allowing mackinawite to transform into greigite. Both mackinawite and magnetite were present under alkaline conditions, with possible transitions of Fe 2+ +2OH − →Fe(OH) 2 , followed by 3Fe(OH) 2 →Fe 3 O 4 +2H 2 O+H 2 . In situ X-ray diffraction results indicate that the mineral transformation rate under acidic conditions is faster than under neutral and alkaline conditions. This means that acid environments can enhance rapid phase transformation of iron sulfides. The results under different experimental conditions suggest that there is a variety of formation pathways for iron-sulfide minerals owing to the presence of different oxidants in different geochemical environments.

Sulfate-reducing bacteria influence the nucleation and growth of mackinawite and greigite

Sedimentary iron sulfide minerals play a key role in maintaining the oxygenation of Earth’s atmosphere over geological timescales; they also record critical geochemical information that can be used to reconstruct paleo-environments. On modern Earth, sedimentary iron sulfide mineral formation takes places in low-temperature environments and requires the production of free sulfide by sulfate-reducing microorganisms (SRM) under anoxic conditions. Yet, most of our knowledge on the properties and formation pathways of iron sulfide minerals, including pyrite, derives from experimental studies performed in abiotic conditions, and as such the role of biotic processes in the formation of sedimentary iron sulfide minerals is poorly understood. Here we investigate the role of SRM in the nucleation and growth of iron sulfide minerals in laboratory experiments. We set out to test the hypothesis that SRM can influence Fe-S mineralization in ways other than providing sulfide through the comparison of the physical properties of iron sulfide minerals precipitated in the presence and in the absence of the sulfate-reducing bacterium Desulfovibrio hydrothermalis AM13 under well-controlled conditions. X-ray diffraction and microscopy analyses reveal that iron sulfide minerals produced in the presence of SRM exhibit unique morphology and aggregate differently than abiotic minerals formed in media without cells. Specifically, mackinawite growth is favored in the presence of both live and dead SRM, when compared to the abiotic treatments tested. The cell surface of live and dead SRM, and the extracellular polymers produced by live cells, provide templates for the nucleation of mackinawite and favor mineral growth. The morphology of minerals is however different when live and dead cells are provided. The transformation of greigite from mackinawite occurred after several months of incubation only in the presence of live SRM, suggesting that SRM might accelerate the kinetics of greigite formation under strict anoxic conditions. Pyrite formation was not observed in any experiments. While SRM provide nearly all the sulfide to the Fe-S system at low temperatures, we also posit that SRM play an additional formative role in the size, morphology and potentially the mineralogy of iron sulfide minerals in sedimentary environments, therefore potentially influencing their reactivity. Attempting to reconstruct modern and ancient biogeochemical cycles based on the geochemistry of iron sulfide minerals formed under purely abiotic conditions should be therefore done with caution.

Sulfate reduction and formation of iron sulfide minerals in nearshore sediments from Qi'ao Island, Pearl River Estuary, Southern China

Sulfate reduction usually leads to the formation of iron sulfide minerals in marine sediments and strongly influences the global coupled carbon-iron-sulfur biogeochemical cycles in marine sediments. Here, we report sulfate reduction and pyrite formation from the coastal sediments near Qi'ao Island, Pearl River Estuary using a combination of sedimentary pore water and solid sediment geochemical analyses. Strong enrichments in reactive iron (276.8–358.2 μmol/g) were observed, especially at the sites located near the river outlets. High contents of reactive iron and low values of the degree of iron pyritization (DOP) indicate that the limiting effect of reactive iron on pyrite formation can be excluded at these sites. In the upper sedimentary layer, conservative mixing of river water and marine water reinforces the fact that sulfate reduction is not appreciable. The limited sulfate supply and low sulfate reduction rates lead to not enough dissolved sulfide available for formation of iron sulfide minerals. However, below this dilution-mixing layer, pore water geochemical profiles indicate that a fraction of sulfate consumption is mediated by upward-diffusing methane. An additional sulfide is produced from anaerobic oxidation of methane (AOM) and later reacts with reactive iron, resulting in enhancement of pyrite formation in the sediments. By contrast, it appears that sulfate concentration is a primary factor controlling for pyrite formation in these estuarine environments. Our study highlights the need in future work for an integrated analysis of the hydrologically driven change in the bottom seawater sulfate concentrations to better understand the regulation factors of global sulfur reservoirs in coastal sediments of estuarine environments, as sulfur burial is an important factor in determining the past atmospheric oxygen level.

What Do We Really Know about the Role of Microorganisms in Iron Sulfide Mineral Formation?

Iron sulfide mineralization in low-temperature systems is a result of biotic and abiotic processes, though the delineation between these two modes of formation is not always straightforward. Here we review the role of microorganisms in the precipitation of extracellular iron sulfide minerals. We summarize the evidence that links sulfur-metabolizing microorganisms and sulfide minerals in nature and we present a critical overview of laboratory-based studies of the nucleation and growth of iron sulfide minerals in microbial cultures. We discuss whether biologically derived minerals are distinguishable from abiotic minerals, possessing attributes that are uniquely diagnostic of biomineralization. These inquiries have revealed the need for additional thorough, mechanistic and high-resolution studies to understand microbially mediated formation of a variety of sulfide minerals across a range of natural environments.

Molecular assays advance understanding of sulfate reduction despite cryptic cycles

Abiotic and secondary biotic reactions can contribute to the formation of cryptic biogeochemical cycles, resulting in an underestimation of carbon and nutrient budgets. This Texas coastal estuary sediment study provided a unique opportunity to use multidisciplinary RNA-based molecular and geochemical approaches to identify cryptic cycles associated with sulfate reduction, a commonly measured biogeochemical process considered to be the predominant anoxic terminal electron accepting process in shallow marine environments. Active sulfate reduction within an environment is typically determined by the detection of sulfides. However, a biologically driven cryptic cycle was determined by identifying metabolically active sulfate reducing and sulfur oxidizing lineages co-locating within the sediments, effectively masking sulfide production through re-oxidation back to sulfate. Similar co-location of sulfate and iron reducing lineages prevented the detection of sulfides through the formation of iron sulfide minerals, producing a geochemically driven cryptic cycle. We also showed that sulfate reduction rates determined by 35SO42− incubation analysis can be positively correlated with dsrA transcript abundance, and thus this molecular technique may be a proxy for the prohibitive radioactive method. Based on these results, support is given to a synergistic geochemical and molecular biological strategy to better provide an understanding of marine sulfur cycle.

Carbon mineralization and carbonate preservation in modern cold-water coral reef sediments on the Norwegian shelf

Abstract. Cold-water coral ecosystems are considered hot-spots of biodiversity and biomass production and may be a regionally important contributor to carbonate production. The impact of these ecosystems on biogeochemical processes and carbonate preservation in associated sediments were studied at Røst Reef and Traenadjupet Reef, two modern (post-glacial) cold-water coral reefs on the Mid-Norwegian shelf. Sulfate and iron reduction as well as carbonate dissolution and precipitation were investigated by combining pore-water geochemical profiles, steady state modeling, as well as solid phase analyses and sulfate reduction rate measurements on gravity cores of up to 3.25 m length. Low extents of sulfate depletion and dissolved inorganic carbon (DIC) production, combined with sulfate reduction rates not exceeding 3 nmol S cm−3 d−1, suggested that overall anaerobic carbon mineralization in the sediments was low. These data showed that the coral fragment-bearing siliciclastic sediments were effectively decoupled from the productive pelagic ecosystem by the complex reef surface framework. Organic matter being mineralized by sulfate reduction was calculated to consist of 57% carbon bound in CH2O groups and 43% carbon in -CH2- groups. Methane concentrations were below 1 μM, and failed to support the hypothesis of a linkage between the distribution of cold-water coral reefs and the presence of hydrocarbon seepage. Reductive iron oxide dissolution linked to microbial sulfate reduction buffered the pore-water carbonate system and inhibited acid-driven coral skeleton dissolution. A large pool of reactive iron was available leading to the formation of iron sulfide minerals. Constant pore-water Ca2+, Mg2+ and Sr2+ concentrations in most cores and decreasing Ca2+ and Sr2+ concentrations with depth in core 23–18 GC indicated diagenetic carbonate precipitation. This was consistent with the excellent preservation of buried coral fragments.

Factors Controlling Sulfide Geochemistry in Sub-tropical Estuarine and Bay Sediments

The primary factors that control the concentration of total reduced (inorganic) sulfide in coastal sediments are believed to be the availability of reactive iron, dissolved sulfate and metabolizable organic carbon. We selected nine sites in shallow (<3 m), close to sub-tropical, estuaries and bays along the central Texas coast that represented a range in sediment grain size (a proxy for reactive iron), salinity (a proxy for dissolved sulfate), and total organic carbon (a proxy for metabolizable organic carbon). Based on these parameters a prediction was made of which factor was likely to control total reduced sulfide at each site and what the relative total reduced sulfide concentration was likely to be. To test the prediction, the sediments were analyzed for total reduced sulfide, acid volatile sulfide, and citrate dithionate-extractable, HCl-extractable and total Fe in the solid phase. Using solid-state gold–mercury amalgam microelectrodes and voltammetry, we determined pore water depth profiles of Fe(II) and ΣH2S and presence or absence of FeS(aq). At five of the nine sites the calculated degree of sufildization of citrate dithionite-reactive-iron was close to or greater than 1 indicating that rapidly reactive iron was probably the limiting factor for iron sulfide mineral formation. At one site (salinity = 0.9) dissolved Fe(II) was high, ΣH2S was undetectable and the total reduced sulfide concentration was low indicating sulfate limitation. At the last three sites a low degree of sulfidization and modest total reduced (inorganic) sulfide concentrations appeared to be the result of a limited supply of metabolizable organic carbon. Fe(II)–S(-II) clusters (FeS(aq)) were undetectable in 10 out of 12 bay sediment profiles where ΣH2S was close to or below detection limits, but was observed in all other porewater profiles. Acid volatile sulfide, but not total reduced sulfide, was well correlated with total organic carbon and ranged from being undetectable in some cores to representing a major portion of total reduced sulfide in other cores. Although predicted controls on total reduced sulfide were good for very low salinity water or sandy sediments, they were only right about half the time for the other sediments. The likely reasons for the wrong predictions are the poor correlation of total organic carbon with grain size and differing fractions of metabolizable organic carbon in different sedimentary environments. Differences in sediment accumulation rates may also play a role, but these are difficult to determine in this region where hurricanes often resuspend and move sediments. This study demonstrates the need to examine more complex and often difficult to determine parameters in anoxic “normal marine” sediments if we are to understand what controls the concentration and distribution of sulfides.

Sulfate reduction and iron sulfide mineral formation in the southern East China Sea continental slope sediment

Sulfate reduction rate, organic carbon and sulfide burial rate; organic carbon, carbonate carbon, and reactive iron contents; grain size; and sedimentation rate were determined in sediments of the southern East China Sea continental slope. The results show high sulfate reduction and pyrite sulfur burial rates in slope areas with high organic carbon and sedimentation rates. Unusually high rates of organic carbon deposition enhance sulfate reduction and pyrite sulfide burial in the region. Both sulfate reduction rates and pyrite sulfur burial rates increased linearly with increasing organic carbon burial rate, indicating that deposition of organic carbon on the slope is the primary controlling factor for pyrite formation. Abundant reactive iron indicated that iron is not limiting pyrite formation. Pyrite is the predominant sulfide mineral; however, acid volatile sulfide constituted up to 50% of total sulfide at some stations. Up to 240 μmol/g of pyrite sulfur and 5 mmol/m 2/day of sulfate reduction rates were found in the slope sediment. Sulfate reduction rate and pyrite sulfur did not decrease with increasing overlying water depth. High organic carbon burial rates enhanced the sulfate reduction rate and subsequently the rate of pyrite sulfur burial in the slope region. As a result, the southern East China Sea continental slope environment is an efficient pyrite sulfur burial environment.

Benthic sulfate reduction along the Chesapeake Bay central channel. I. Spatial trends and controls

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 168:213-228 (1998) - doi:10.3354/meps168213 Benthic sulfate reduction along the Chesapeake Bay central channel. I. Spatial trends and controls M. C. Marvin-DiPasquale*, D. G. Capone University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, PO Box 38, Solomons, Maryland 20688, USA *Present address: U.S. Geological Survey, Water Resources Division, MS 480, 345 Middlefield Rd, Menlo Park, California 94025, USA. E-mail: mmarvin@usgs.gov ABSTRACT: Factors controlling the spatial distribution of benthic sulfate reduction (SR) were investigated at 3 stations [upper (UB), mid (MB) and lower bay (LB)] along the Chesapeake Bay (eastern USA) central channel from early spring through late fall, 1989 to 1994. Annual rates of 0 to 12 cm depthintegrated SR were 0.96, 9.62 and 6.33 mol S m-2 yr-1 for UB, MB and LB, respectively, as calculated from 35SO42- incubations. SR was carbon limited at UB, LB, and at the sediment surface at MB, and SO42- limited at depth at MB. Temperature explained 33 to 68% of the variability in annual rates, with an apparent influence on SR which increased in the seaward direction in surface sediments. We speculate that the enhanced response of SR to temperature in LB surface sediments was linked to seasonal variations in macrofaunal activity associated with temperature. Estimates of reduced-S burial indicated that only 4 to 8% of sulfur reduced annually was buried as Fe-S minerals at MB and LB, with the remainder presumably being reoxidized. In contrast, >50% of the sulfur reduced annually was buried at UB, due to comparatively low SR rates and the high concentration of reactive iron in the oligohaline region. SR mineralized 18 to 32% of the annual primary production. Our results indicate that organic quality may be more important than the absolute quantity of organic loading in dictating the magnitude of benthic SR rates along an estuarine gradient. Spatial trends in SR reflected the combined influence of deposited organic matter quality and quantity, SO42- availability, the presence or absence of benthic macrofauna, overlying water dissolved O2 conditions, reduced-S reoxidation dynamics, and iron-sulfide mineral formation. KEY WORDS: Sulfate reduction · Anaerobic metabolism · Sediment · Estuary · Chesapeake Bay Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 168. Publication date: July 09, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Phosphate mobilization in iron-rich anaerobic sediments: microbial Fe (III) oxide reduction versus iron-sulfide formation

Mechanisms of phosphate (PO4 3- ) mobilization and retention were examined in iron-rich anaerobic freshwater wetland, lake, and coastal marine sediments. Direct microbial Fe(III) oxide reduction solubilized only 3-25% of initial solid-phase PO 4 3- during sulfate-free sediment incubation experiments. Experiments with reduced, non-sulfidic solid-phase Fe(II)-rich sediment demonstrated PO4 3- sorption by the solid-phase, and chemical equilibrium calculations indicated that conditions were favorable for precipitation of Fe(II)-P04 minerals [e.g. Fe 3 (PO 4 ) 2 ] in such sediments. These results suggested that much of the PO 4 3- released from Fe(III) oxides during microbial Fe(III) reduction was captured by solid-phase reduced iron compounds (Fe(II) hydroxide-PO4 complexes and/or Fe(II)-PO4 minerals). Enhanced liberation of PO 4 3- to sediment porewaters (33-100% of initial solid-phase PO 4 3- ) occurred during anaerobic incubation in the presence of abundant sulfate and was directly correlated with sulfate reduction and iron-sulfide mineral formation. Incubation of PO 4 3- -amended sediment with different amounts of sulfate demonstrated a linear correlation between PO 4 3- release and sulfate reduction. Release of PO 4 3- to sediment porewaters during decomposition of fresh organic matter (freeze-dried cyanobacteria) was more extensive in sulfate-amended (67% of added organic P) than in sulfate-free sediment (17% of added organic P), and the ratio of dissolved PO 4 3- released to organic carbon oxidized was seven-fold higher in sulfate-amended sediment despite a common level of overall organic C and P mineralization in the two treatments. Our results demonstrate that iron-rich anaerobic sediments can immobilize substantial amounts of PO 4 3- under Fe(III) oxide-reducing conditions, but that extensive PO 4 3- release will take place if sediment Fe compounds are converted to iron-sulfides via bacterial sulfate reduction.

Determination of the Electrochemical Properties of a Soluble Aqueous FeS Species Present in Sulfidic Solutions

Field and laboratory data are presented that show a soluble FeS species(FeSaq) exists in sulfidic seawater solutions, and is observedwhen the IAP exceeds the Ksp of amorphous FeS. TheFeSaq yields a discrete signal (double peak) using square-wavevoltammetry and two one-electron waves in sampled DC polarographyexperiments at the Hg electrode. The aqueous FeS species reacts irreversiblyat the electrode as a single FeS subunit and not as a polymeric entity. Thepeak potential of FeSaq occurs at -1.1 V whereas the peakpotential of Fe $$(H_2 O)_6^{2 + } $$ occurs at-1.45 V; the positive shift for Fe2+ reduction inFeSaq indicates a change in geometry for Fe2+from octahedral to tetrahedral. The kinetics of electron transfer at theelectrode are determined to be similar for both Fe2+ andFeSaq. Molecular orbital energy diagrams, further indicatethat Fe(II) does change from octahedral to tetrahedral geometry in solution.First, Fe(II) exists as octahedralFe $$(H_2 O)_6^{2 + } $$ in solution whichundergoes a substitution reaction of bisulfide for water. The resultingcomplex, Fe(H2O)5(HS)+, thentransforms to a tetrahedral complex on further addition of sulfide. Thisgeometry change is consistent with the formation of amorphous FeS thatconverts to mackinawite which has tetrahedral Fe(II). The process is entropydriven because of the water loss that occurs. The overall sequence can berepresented as: $$\begin{gathered} 3Fe(H_2 O)_6^{2 + } + 3HS^ - \to 3Fe(H_2 O)_5 (HS)^ + + 3H_2 O \hfill \\ 3Fe(H_2 O)_5 (HS)^ + + 3HS^ - \to Fe_3 S_3 (H_2 O)_6 + 3H_2 S + 9H_2 O \hfill \\ \end{gathered} $$ Soluble FeS species are important asreactants in the formation of iron-sulfide minerals including pyrite.

Early diagenetic processes, fluxes, and reaction rates in sediments of the South Atlantic

Porewaters recovered from sediment cores (gravity corers, box corers, and multicorers) from various subrogions of the South Atlantic (Amazon River mouth, equatorial upwelling, Congo River mouth, Benguela coastal upwelling area, and Angola Basin) were investigated geochemically. Objectives included determination of Eh, pH, oxygen, nitrate, sulfate, alkalinity, phosphate, ammonium, fluoride, sulfide, Cal, Mg, Sr, Fe, Mn, and Si, in order to quantify organic matter diagenesis and related mineral precipitation and dissolution processes. Porewater profiles from the eastern upwelling areas of the South Atlantic suggest that sulfate reduction in the deeper parts of the sediment may be attributed mainly to methane oxidation, whereas organic matter degradation by sulfate reduction is restricted to the near-surface sediments. Further, a prominent concentration gradient change of sulfate and related mineralization products occurred typically in the upwelling sediments at a depth of 4 to 8 m, far below the zone of bioturbation or bioirrigation. Because other sedimentological reasons seem to fail as explanations, an early diagenetic sulfide oxidation to sulfate within the anoxic environment is discussed. Porewater profiles from the sediments of the Amazon fan area are mainly influenced by reactions with Fe(III)-phases. The remarkable linearity of the concentration gradients of sulfate supports the idea of distinct reaction layers in these sediments. In contrast to the upwelling sediments, the sulfate gradient develops from the sediment surface to a sulfate reduction zone at a depth of 5.3 m, probably because a reoxidation of sulfide is prevented by the reaction with iron oxides and the formation of iron sulfide minerals. A comparison of organic matter degradation rates from the different areas of the South Atlantic show the expected relationship to primary productivity. Oxygen is the dominating oxidant, whereas organic matter degradation by nitrate, Mn(IV)- and probably also Fe(III)-reduction is several orders of magnitude lower. Sulfate reduction is quantitatively of similar importance as oxygen respiration in the organic-rich sediments, but may also include methane oxidation. Reoxidation of ammonium and Mn 2+ by oxygen or nitrate do not alter significantly the estimation of organic matter degradation in the oxic zone, but may contribute to the nitrate reduction in suboxic layers.

Seasonal iron cycling in the salt-marsh sedimentary environment: the importance of ligand complexes with Fe(II) and Fe(III) in the dissolution of Fe(III) minerals and pyrite, respectively

A biogeochemical cycle is proposed for the reactivity of iron in salt-marsh sediments. The main reactions of the iron cycle are: (1) solubilization of Fe(III) by organic ligands; (2) reduction of soluble Fe(III) to Fe(II) by these ligands, soluble reduced sulfur or solid phase reduced sulfur; (3) the oxidation of the resulting Fe(II) (complexed to organic chelates) by Fe(III) minerals; (4) the formation of iron sulfide minerals when dissolved sulfide is in excess. The cycle of iron solubilization will continue as long as bacteria and/or plants produce organic ligands. The cycle will stop when sulfate reduction rates are high and organic ligand production is low. At this point soluble hydrogen sulfide reacts with Fe(II) and Fe(III) to form sulfide minerals. Penetration of O 2 into the surface sediments will also oxidize Fe(II) to Fe(III) with subsequent formation of Fe(III) (oxy)hydroxide minerals. The reactions which represent the iron cycle indicate that the iron mineral system has substantial acid/base buffering capacity. The ligands responsible for the cycling of iron are weak field anionic ligands containing oxygen as the ligating atom. The electron transfer from Fe(II) complexes to Fe(III) minerals is discussed using the molecular orbital approach. An outer sphere electron transfer is possible. Laboratory evidence is presented for the reaction of Fe(III) complexes with pyrite over the pH range of 4–6.5. The Fe(II) production rate and pH decrease are consistent with field data from Great Marsh, Delaware. The direct oxidant for the oxidation of pyrite and other reduced sulfur compounds in salt-marsh sediments is Fe(III) rather than oxygen based on the cycle and data presented. Oxygen is not present in any pore waters sampled in this work. This is consistent with the microelectrode work of other researchers. Manganese oxides are not likely oxidants in salt-marsh sediments in Great Marsh, Delaware because they are not as abundant as Fe(III) minerals. The iron cycle presented may occur in other marine and freshwater sedimentary systems and in aquatic systems with oxic/anoxic interfaces.

Sulfate reduction and iron sulfide mineral formation in Gulf of Mexico anoxic sediments

Sulfate reduction and iron sulfide mineral formation in Gulf of Mexico anoxic sediments