Reactive Iron Phases Research Articles

Storage and Distribution of Organic Carbon and Nutrients in Acidic Soils Developed on Sulfidic Sediments: The Roles of Reactive Iron and Macropores.

In a boreal acidic sulfate-rich subsoil (pH 3-4) developing on sulfidic and organic-rich sediments over the past 70 years, extensive brownish-to-yellowish layers have formed on macropores. Our data reveal that these layers ("macropore surfaces") are strongly enriched in 1 M HCl-extractable reactive iron (2-7% dry weight), largely bound to schwertmannite and 2-line ferrihydrite. These reactive iron phases trap large pools of labile organic matter (OM) and HCl-extractable phosphorus, possibly derived from the cultivated layer. Within soil aggregates, the OM is of a different nature from that on the macropore surfaces but similar to that in the underlying sulfidic sediments (C-horizon). This provides evidence that the sedimentary OM in the bulk subsoil has been largely preserved without significant decomposition and/or fractionation, likely due to physiochemical stabilization by the reactive iron phases that also existed abundantly within the aggregates. These findings not only highlight the important yet underappreciated roles of iron oxyhydroxysulfates in OM/nutrient storage and distribution in acidic sulfate-rich and other similar environments but also suggest that boreal acidic sulfate-rich subsoils and other similar soil systems (existing widely on coastal plains worldwide and being increasingly formed in thawing permafrost) may act as global sinks for OM and nutrients in the short run.

Production and preservation of organic carbon in sub-seafloor tephra layers

The deposition of volcanic ash into the ocean initiates a range of chemical and biological reactions. During diagenesis, these reactions may enhance the preservation of organic carbon (OC) in marine sediments, which ultimately promotes CO2 sequestration from the ocean-atmosphere system. However, this interpretation is reliant on a small number of studies that make a link between tephra and OC burial. Here, we compare organic and inorganic geochemical data from tephra-bearing marine sediments from three sites that differ widely in their location, age, and composition. We show that OC is buried in, and proximal to, tephra layers, in proportions higher than would be expected via simple admixture of surrounding sediment. Our data indicate that this OC is preserved primarily through interactions with reactive iron phases, which act to physically protect the carbon from oxidation. Analysis of the composition of the OC associated with reactive iron indicates it is isotopically (consistently more negative δ13C than sediment) and chemically (comprised of compounds not found in the sediment) distinct from OC in the background sediments. We interpret this signal as indicating a microbial source of OC, with autochthonous OC production resulting from autotrophic microbial exploitation of nutrients supplied from tephra. This finding has implications for our understanding of carbon cycling on Earth, and possibly for the emergence of life in terrestrial and perhaps even extra-terrestrial environments.

Oxygenation of the Earth aided by mineral–organic carbon preservation

Photosynthesis produces molecular oxygen, but it is the burial of organic carbon in sediments that has allowed this O2 to accumulate in Earth’s atmosphere. Yet many direct controls on the preservation and burial of organic carbon have not been explored in detail. For modern Earth, it is known that reactive iron phases are important for organic carbon preservation, suggesting that the availability of particulate iron could be an important factor for the oxygenation of the oceans and atmosphere over Earth history. Here we develop a theoretical model to investigate the effect of mineral–organic preservation on the oxygenation of the Earth, supported by a proxy compilation for terrigenous inputs and the burial of reactive iron phases, and find that changes to the rate of iron input to the global ocean constitute an independent control on atmosphere–ocean O2 and marine sulfate levels. We therefore suggest that increasing continental exposure and denudation may have helped fuel the rise in atmospheric O2 and other oxidants over Earth history. Finally, we show that inclusion of mineral–organic preservation makes the global marine O2 reservoir more resilient to changes in nutrient levels by breaking the link between productivity and organic carbon burial. We conclude that mineral–organic preservation is an important missing process in current assessments of Earth’s long-term carbon cycle.

Does Arctic warming reduce preservation of organic matter in Barents Sea sediments?

Over the last few decades, the Barents Sea experienced substantial warming, an expansion of relatively warm Atlantic water and a reduction in sea ice cover. This environmental change forces the entire Barents Sea ecosystem to adapt and restructure and therefore changes in pelagic-benthic coupling, organic matter sedimentation and long-term carbon sequestration are expected. Here we combine new and existing organic and inorganic geochemical surface sediment data from the western Barents Sea and show a clear link between the modern ecosystem structure, sea ice cover and the organic carbon and CaCO3 contents in Barents Sea surface sediments. Furthermore, we discuss the sources of total and reactive iron phases and evaluate the spatial distribution of organic carbon bound to reactive iron. Consistent with a recent global estimate we find that on average 21.0 ± 8.3 per cent of the total organic carbon is associated to reactive iron (fOC-FeR) in Barents Sea surface sediments. The spatial distribution of fOC-FeR, however, seems to be unrelated to sea ice cover, Atlantic water inflow or proximity to land. Future Arctic warming might, therefore, neither increase nor decrease the burial rates of iron-associated organic carbon. However, our results also imply that ongoing sea ice reduction and the associated alteration of vertical carbon fluxes might cause accompanied shifts in the Barents Sea surface sedimentary organic carbon content, which might result in overall reduced carbon sequestration in the future. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

Spatially Resolved Organomineral Interactions across a Permafrost Chronosequence.

Permafrost contains a large (1700 Pg C) terrestrial pool of organic matter (OM) that is susceptible to degradation as global temperatures increase. Of particular importance is syngenetic Yedoma permafrost containing high OM content. Reactive iron phases promote stabilizing interactions between OM and soil minerals and this stabilization may be of increasing importance in permafrost as the thawed surface region ("active layer") deepens. However, there is limited understanding of Fe and other soil mineral phase associations with OM carbon (C) moieties in permafrost soils. To elucidate the elemental associations involved in organomineral complexation within permafrost systems, soil cores spanning a Pleistocene permafrost chronosequence (19,000, 27,000, and 36,000 years old) were collected from an underground tunnel near Fairbanks, Alaska. Subsamples were analyzed via scanning transmission X-ray microscopy-near edge X-ray absorption fine structure spectroscopy at the nano- to microscale. Amino acid-rich moieties decreased in abundance across the chronosequence. Strong correlations between C and Fe with discrete Fe(III) or Fe(II) regions selectively associated with specific OM moieties were observed. Additionally, Ca coassociated with C through potential cation bridging mechanisms. Results indicate Fe(III), Fe(II), and mixed valence phases associated with OM throughout diverse permafrost environments, suggesting that organomineral complexation is crucial to predict C stability as permafrost systems warm.

Pyrite oxidation in shales: Implications for palaeo-redox proxies based on geochemical and SEM-EDX evidence

Pyrite oxidation in marine shales during weathering has been widely studied, however, the effects of this process on geochemical proxies commonly used to reconstruct ocean redox conditions, or the details of pyrite oxidation at the mineral grain scale, have not received much scientific attention. We conducted a four-week laboratory experiment designed to stimulate pyrite oxidation at shale outcrops, and to assess effects on the chemical phase of iron and sulphur in the samples both in bulk sediments (useful for comparison to palaeo-environmental geochemistry) and at a finer scale (to gain a better understanding of the nature of these changes). Geochemical and scanning electron microscopy (SEM) techniques provide evidence for pyrite oxidation, carbonate dissolution, and iron (oxyhydr)oxide formation during the experiment. The net effect of the experiment on the ratio of highly reactive iron phases (FeHR) to total iron (FeT) is minimal (≪0.03% difference), suggesting that this redox proxy behaves relatively conservatively during weathering. The effect of weathering on the ratio of pyrite-bound iron to highly reactive iron (FePY/FeHR), used to investigate the availability of sulphur, in contrast, is pronounced (up to 32.5% difference) due to the oxidation of pyrite and the precipitation of iron (oxyhydr)oxides in the shale samples. Electron microscopy provides evidence that iron (oxyhydr)oxides precipitated in situ as rims around cores of pyrite particles, “passivating” and protecting them from further oxidation. The quantification of these partly oxidised pyrite particles is now possible using a novel automated particle analysis method coupled to chemical mapping, developed in this study. We conclude that this method can be of wide use, both to quantify pyrite oxidation, and assess the significance of FePY/FeHR at measured in shale outcrop samples.

A biogeochemical survey of the anoxic basin Golfo Dulce, Costa Rica

The waters and surface sediments of a 200 m deep tropical fjord were investigated with respect to chemical zonation, rates of respiratory processes. and benthic fluxes. Oxygen was found to ca. 100 m water depth. but only the upper pan of the oxycline was associated with the pycnocline. which was situated above the sill located at 60 m. Nitrate-rich water entered the bay at sill depth, and denitrification was indicated at anoxic depths. Hydrogen sulfide was only found in low concentrations near the bottom, and nitrate was a possible oxidant for the hydrogen sulfide. The observations reproduced those of a survey in 1969 to a large extent. This suggests that water exchange is frequent enough to prevent the development of strongly reducing conditions in the bottom water, High rates of dark oxygen uptake indicated intense carbon cycling within the euphotic zone. The rates decreased rapidly with depth, and at anoxic depths, rates of denitrification and sulfate reduction were more than 100-fold lower than the surface oxygen uptake. The chemical zonation as well as x-radiographs indicated that sediments underlying oxic bottom water were strongly irrigated and bioturbated while anoxic sediments were laminated and composed of turbidites. The rates of carbon oxidation were 5-10 times higher in the sediments underlying oxic bottom water than in those of the bottom of the basin. Sulfate reduction was a dominating process. accounting for about 50% and 100% of the carbon oxidation at the oxic and anoxic sites respectively. Even at the anoxic sites, hydrogen sulfide did not accumulate in the pore Waler and a high sedimentation of reactive iron phases is suggested as a contributing cause.

Sulfur Cycling in an Iron Oxide-Dominated, Dynamic Marine Depositional System: The Argentine Continental Margin

The interplay between sediment deposition patterns, organic matter type and the quantity and quality of reactive mineral phases determines the accumulation, speciation and isotope composition of pore water and solid phase sulfur constituents in marine sediments. Here, we present the sulfur geochemistry of siliciclastic sediments from two sites along the Argentine continental slope—a system characterized by dynamic deposition and reworking, which result in non-steady state conditions. The two investigated sites have different depositional histories but have in common that reactive iron phases are abundant and that organic matter is refractory—conditions that result in low organoclastic sulfate reduction rates. Deposition of reworked, isotopically light pyrite and sulfurized organic matter appear to be important contributors to the sulfur inventory, with only minor addition of pyrite from organoclastic sulfate reduction above the sulfate-methane transition (SMT). Pore-water sulfide is limited to a narrow zone at the SMT. The core of that zone is dominated by pyrite accumulation. Iron monosulfide and elemental sulfur accumulate above and below this zone. Iron monosulfide precipitation is driven by the reaction of low amounts of hydrogen sulfide with ferrous iron and is in competition with the oxidation of sulfide by iron (oxyhydr)oxides to form elemental sulfur. The intervals marked by precipitation of intermediate sulfur phases at the margin of the zone with free sulfide are bordered by two distinct peaks in total organic sulfur. Organic matter sulfurization appears to precede pyrite formation in the iron-dominated margins of the sulfide zone, potentially linked to the presence of polysulfides formed by reaction between dissolved sulfide and elemental sulfur. Thus, SMTs can be hotspots for organic matter sulfurization in sulfide-limited, reactive iron-rich marine sedimentary systems. Furthermore, existence of elemental sulfur and iron monosulfide phases meters below the SMT demonstrates that in sulfide-limited systems metastable sulfur constituents are not readily converted to pyrite but can be buried to deeper sediment depths. Our data show that in non-steady state systems, redox zones do not occur in sequence but can reappear or proceed in inverse sequence throughout the sediment column, causing similar mineral alteration processes to occur at the same time at different sediment depths.

Soil organic carbon stabilization by iron in permafrost regions of the Qinghai‐Tibet Plateau

AbstractA close relationship exists between soil organic carbon (SOC) and reactive iron; however, little is known about the role of iron in SOC preservation in permafrost regions. We determined the amount of SOC associated with reactive iron phases (OC‐Fe) in the permafrost regions of the Qinghai‐Tibetan Plateau (QTP). The results showed that the percentage of OC‐Fe ranged between 0.9% and 59.5% in the upper 30 cm of soil and that the OC‐Fe represented 19.5 ± 12.3% of the total SOC pool. No clear vertical distribution pattern in OC‐Fe was present in the upper 1 m of soil. Throughout the year, the OC‐Fe accounted for relatively stable proportions of the total SOC pool. This study suggests that approximately 20% of SOC is a potential rusty OC pool in the permafrost regions of the QTP. Biogeochemical processes related to the reaction of iron may play important roles in soil carbon cycles in permafrost regions.

Organic carbon remobilized from thawing permafrost is resequestered by reactive iron on the Eurasian Arctic Shelf

AbstractGiven the potential for permafrost carbon (PF/C)‐climate feedbacks in the Siberian‐Arctic land‐ocean system, there is a need for understanding the fate of thawed‐out PF/C. Here we show that the sequestration of OC by reactive iron (OC‐Fe) ranges between 0.5 and 22% on the Eurasian Arctic Shelf, with higher values in the Kara Sea (KS) (18 ± 6%) and the Laptev Sea (LS) (14 ± 4%). The Δ14C/δ13C signatures of the OC‐Fe are substantially older and more terrestrial than the bulk sediment OC in the LS but younger and more dominated by marine plankton sources in the East Siberian Sea (ESS). Statistical source apportionment modeling reveal that reactive iron phases resequestered 15 ± 5% of thawing PF/C in the LS and 6.4 ± 5% in the ESS, derived from both coastal erosion of ice complex deposit and thawing topsoil. This Fe‐associated trap of PF/C constitutes a reduction of the degradation/outgassing and thus also an attenuation of the PF/C‐climate feedback.

Intermediate sulfur oxidation state compounds in the euxinic surface sediments of the Dvurechenskii mud volcano (Black Sea)

The deep Black Sea is known to be depleted in electron-acceptors for sulfide oxidation. This study on depth distributions of sulfur species (S(II), S(0), Sn2-, S2O32-, SO32-, SO42-) in the Dvurechenskii mud volcano, a cold seep situated in the permanently anoxic eastern Black Sea basin (Sorokin Trough, 2060m water depth), showed remarkable concentrations of sulfide oxidation products. Sulfite concentrations of up to 11μmolL−1, thiosulfate concentrations of up to 22μmolL−1, zero-valent sulfur concentrations of up to 150μmolL−1 and up to five polysulfide species were measured in the upper 20cm of the sediment. Electron-acceptors found to be available in the Dvurechenskii mud volcano (DMV) for the oxidation of hydrogen sulfide to sulfide oxidation intermediates are iron-minerals, and probably also reactive manganese phases. Up to 60μmolg−1 of reactive iron-minerals and up to 170μmolL−1 dissolved iron was present in the central summit with the highest fluid upflow and fresh mud outflow. Thus, the source for the oxidative power in the DMV are reactive iron phases extruded with the mud from an ancient source in the deeply buried sediments, leading to the formation of various sulfur intermediates in comparably high concentrations. Another possible source of sulfide oxidation intermediates in DMV sediments could be the formation of zero-valent sulfur by sulfate dependent anaerobic microbial oxidation of methane followed by disproportionation of zero-valent sulfur. Sulfide oxidation intermediates, which are produced by these processes, do not reach thermodynamic equilibrium with rhombic sulfur, especially close to the active center of the DMV due to a short equilibration time. Thus, mud volcano sediments, such as in the DMV, can provide oxidizing niches even in a highly reduced environment like the abyssal part of the Black Sea.

Modelling Cu(II) adsorption to ferrihydrite and ferrihydrite–bacteria composites: Deviation from additive adsorption in the composite sorption system

Bacterially associated iron (hydr)oxides are widespread in natural environments and are potent scavengers of dissolved metal ions. However, it is unclear whether metal sorption on these composites adheres to the additivity principle, and thus whether metal concentrations in environments where these composites comprise a significant proportion of the reactive iron phases can be modelled assuming component additivity. Here we address this issue for Cu adsorption on ferrihydrite–Bacillus subtilis composites. We precipitated pure ferrihydrite and ferrihydrite composites with different ferrihydrite:bacteria mass ratios, and measured Cu adsorption as a function of pH, Cu adsorbed concentration and composite mass ratio. We develop a molecular-level surface complexation model for Cu adsorption on pure ferrihydrite. We then combine our end-member models for Cu adsorption on B. subtilis (Moon and Peacock, 2011) and ferrihydrite to model the observed Cu adsorption on the composites, adopting a component linear additivity approach. By comparing observed Cu adsorption to that predicted by our composite model, constrained to the exact best fitting end-member stability constants, we find that Cu adsorption behaviour on ferrihydrite–B. subtilis composites deviates from additivity. Specifically, Cu adsorption on composites composed mainly of ferrihydrite is enhanced across the adsorption pH edge (pH ∼3–6), while on our composite composed mainly of bacteria adsorption is enhanced at mid-high pH (pH ∼5–6) but diminished at mid-low pH (pH ∼5–3), compared to additivity. In current surface complexation modelling constructs, Cu adsorption on composites composed mainly of ferrihydrite can be modelled in a component additivity approach, by optimising the stability constants for Cu adsorption on the ferrihydrite and bacteria fractions to values that are within the uncertainty on the end-member stability constant values. The deviation from additivity of these composites, apparent when using the exact best fitting end-member stability constants, is therefore either a modelling artefact due to uncertainties in the surface adsorption properties of the end-member phases, or is relatively minor and cannot be separated from these uncertainties that are inherently present in the parameterisation of the surface adsorption properties in the modelling. In contrast, composites composed mainly of bacteria express significant deviation from Cu adsorption additivity that cannot be modelled in a component additivity approach. We propose that the deviations in Cu adsorption from additivity are the result of physiochemical interactions between the composite fractions that change the surface charge of the ferrihydrite and B. subtilis fractions compared to the isolated end-member phases. The magnitude and direction of the additivity deviations due to these electrostatic effects are then a function of the affinity of Cu for the bacterial fraction and the mass fraction of bacteria in the composite.

Preservation of organic matter in sediments promoted by iron

The biogeochemical cycles of iron and organic carbon are strongly interlinked. In oceanic waters, organic ligands have been shown to control the concentration of dissolved iron. In soils, solid iron phases shelter and preserve organic carbon, but the role of iron in the preservation of organic matter in sediments has not been clearly established. Here we use an iron reduction method previously applied to soils to determine the amount of organic carbon associated with reactive iron phases in sediments of various mineralogies collected from a wide range of depositional environments. Our findings suggest that 21.5 ± 8.6 per cent of the organic carbon in sediments is directly bound to reactive iron phases. We further estimate that a global mass of (19-45) × 10(15) grams of organic carbon is preserved in surface marine sediments as a result of its association with iron. We propose that these associations between organic carbon and iron, which are formed primarily through co-precipitation and/or direct chelation, promote the preservation of organic carbon in sediments. Because reactive iron phases are metastable over geological timescales, we suggest that they serve as an efficient 'rusty sink' for organic carbon, acting as a key factor in the long-term storage of organic carbon and thus contributing to the global cycles of carbon, oxygen and sulphur.

A rusty carbon sink

The finding that reactive iron species may have a role in stabilizing organic matter in ocean sediments underlines the tight coupling between the biogeochemical cycles of carbon and iron. See Letter p.198 It is well known that solid iron phases can preserve organic carbon in soils, but it remains uncertain whether significant amounts of organic carbon can be preserved by iron in sediments. Yves Gelinas et al. study a range of freshwater and marine sediments and find that almost one-quarter of the organic carbon in the sediments tested is directly bound to reactive iron phases. They further estimate that about 22% of the total surface marine sedimentary organic carbon is preserved by its association with iron, which suggests that reactive iron phases are a key factor in the long-term storage of organic carbon. This 'rusty sink' links the global cycles of carbon, oxygen and sulphur.

The imprint of methane seepage on the geochemical record and early diagenetic processes in cold-water coral mounds on Pen Duick Escarpment, Gulf of Cadiz

The diagenetic history and biogeochemical processes in three cold-water coral mounds located in close proximity to each other on Pen Duick Escarpment in the Gulf of Cadiz were examined. The influence of ascending methane-rich fluids from underlying sediment strata delineated two mound groups: Alpha and Beta Mound showed evidence for the presence of a sulfate-methane transition zone (SMTZ) at shallow depth, whereas Gamma Mound appeared to lack a shallow SMTZ. In the methane influenced Alpha and Beta Mound, upward diffusion of hydrogen sulfide from the shallow SMTZ caused extensive pyritization of reactive iron phases as indicated by values for the degree-of-pyritization >0.7. This secondary pyritization overprinted the sulfur isotope composition of sulfides formed during organoclastic sulfate reduction. The almost complete consumption of reactive iron phases by upward diffusing sulfide limited dissimilatory iron reduction to the top layer in these mounds while organic matter in the pyritized zones below was primarily degraded by organoclastic sulfate reduction. Hydrogen sulfide produced during sulfate reduction coupled to the anaerobic oxidation of methane (AOM) diffused upward and induced aragonite dissolution as evidenced in strongly corroded corals in Alpha Mound. This mound has been affected by strong fluctuations in the depth of the SMTZ, as observed by distinct layers with abundant diagenetic high-Mg calcite with a 13C-depleted carbon isotope composition. In the non-methane influenced Gamma Mound low sulfate reduction rates, elevated concentrations of dissolved iron, and solid-phase iron speciation indicated that organic matter mineralization was driven by dissimilatory iron reduction and organoclastic sulfate reduction coupled to oxidative sulfur cycling. The latter process led to 34S-depletion in pyrite of up to 70‰ relative to pore-water sulfate.

Sources and mechanisms for the enrichment of highly reactive iron in euxinic Black Sea sediments

The deep basinal euxinic sediments of the Black Sea are enriched in iron that is highly susceptible to sulfidization compared to oxic/suboxic continental margin sediments in the Black Sea and oxic/dysoxic continental margin and deep-sea sediments worldwide. A mass balance treatment of iron speciation data from three deep basin sediment cores shows that this enrichment is due to a combination of (1) highly reactive iron-bearing phases (sulfides and oxides) whose ultimate source was by diagenetic mobilization from the shelf (Wijsman and others, 2001) and (2) enhanced iron reactivity in the lithogenous component of deep basinal sediments. The cause of the enhanced reactivity of lithogenous iron is problematical. Possible mechanisms include microbial oxidation of non-reactive iron silicates and the preferential deposition of a fine-grained, reactive iron-enriched lithogenous component in the deep basin by the fractionation of the lithogenous flux during transport across the shelf. The application of paleoredox indicators based on iron reactivity (Degree of Pyritization, Indicator of Anoxicity) should take into account that the availability of highly reactive iron, and hence the concentration of reactive iron phases in the sediment, is controlled by a variety of chemical, physical and biological factors, some of which are not related directly to redox conditions in the water column.

Control of sulfate pore-water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments

Gravity driven mass-flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western Argentine Basin. In this study we present geochemical data from a total of 23 gravity cores. Pore-water SO 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane (AOM). The different shapes of SO 4 profiles (concave, kink- and s-type) can be consistently explained by sedimentary slides possibly in combination with changes in the CH 4 flux from below, thus, mostly representing transient pore-water conditions. Since slides may keep their original sedimentary signature, a combined analysis and numerical modeling of geochemical, physical properties, and hydro acoustic data could be applied in order to reconstruct the sedimentary history. We present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day. The results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep-sea, depositing high amounts of reactive compounds. The high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by AOM. For the total region we estimate a SO 4 (or CH 4) flux of 6.6 × 10 10 moles per year into the zone of AOM. Projected to the global continental slope and rise area, this may sum up to about 2.6 × 10 12 moles per year. Provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1.2 × 10 12 moles per year as previously estimated. Thus, AOM obviously contributes very significantly to the regulation of global sulfur reservoirs, which is hitherto not sufficiently recognized. This finding may have implications for global geochemical models, as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time.

Early diagenesis and sulphur speciation in sediments of the Oman Margin, northwestern Arabian Sea

The pore-waters from two boxcores from the Oman Margin, from sites NIOP484 and NIOP487 (respectively in and below the oxygen minimum zone in the water column), were analysed for concentrations of iodide, sulphide, thiosulphate, phosphate, silica, ammonia, nitrate, nitrite, dissolved iron, dissolved manganese and dissolved sulphur. In addition, the solid phase of samples from the same cores was analysed for organic carbon, pyritic sulphur, acid volatile sulphide, elemental sulphur, organic polysulphides, humic sulphur, reactive iron phases and reactive manganese phases. Pore-water profiles show that post-oxic diagenesis dominates in these Holocene sediments. Only minor amounts of reduced sulphur species were found in pore-waters, despite the low oxygen concentration of the bottom-water and the relatively high organic-matter content of the sediments. Significant amounts of solid-phase reduced sulphur were only found in the lower half of the boxcore at station NIOP484. Consequently, sulphate reduction is not as significant in these sediments relative to sediments from other upwelling areas, and thus more sedimentary organic matter is preserved.

Heavy sedimentary sulfur isotopes as indicators of super-anoxic bottom-water conditions

Iron monosulfides and pyrite (total Cr-reducible S—TRS) in the bottom sediments of the exceptionally sulfide-rich euxinic fjord, Framvaren, south Norway, are unusually enriched in 34 S. Most of these TRSs probably form in bottom waters, where reactive detrital iron phases encounter isotopically heavy dissolved sulfide. Enrichment in 34 S in Framvaren is a result of intense bacterial reduction of a sulfate reservoir whose exchange with the open sea is limited by shallow sills and a low-salinity surface-water layer. 34 S-enriched TRS in the deposits of ancient euxinic environments is commonly interpreted as evidence for early diagenetic sulfide formation where residual, less reactive iron phases were in contact with isotopically heavy dissolved sulfide (sulfate-depleted pore waters) for prolonged periods during burial. However, the Framvaren data indicate that isotopically heavy pyrite may form in relatively shallow (80-180 m), super-anoxic bottom-water pools. In contrast, the presence of isotopically light pyrite does not indicate an absence of restricted conditions, but only that most iron sulfide formation was completed before 34 S-enriched sulfide could be encountered in the water column.