The Impact of Sediment and Carbon Fluxes on the Biogeochemistry of Methane and Sulfur in Littoral Baltic Sea Sediments (Himmerfjärden, Sweden)

Three sediment stations in Himmerfjärden estuary (Baltic Sea, Sweden) were sampled in May 2009 and June 2010 to test how low salinity (5–7 ‰), high primary productivity partially induced by nutrient input from an upstream waste water treatment plant, and high overall sedimentation rates impact the sedimentary cycling of methane and sulfur. Rates of sediment accumulation determined using ²¹⁰Pb_excess and ¹³⁷Cs were very high (0.65–0.95 cm?year^?1), as were the corresponding rates of organic matter accumulation (8.9–9.5 mol C?m^?2?year^?1) at all three sites. Dissolved sulfate penetrated <20 cm below the sediment surface. Although measured rates of bicarbonate methanogenesis integrated over 1 m depth were low (0.96–1.09 mol?m^?2?year^?1), methane concentrations increased to >2 mmol?L^?1 below the sulfate–methane transition. A steep gradient of methane through the entire sulfate zone led to upward (diffusive and bio-irrigative) fluxes of 0.32 to 0.78 mol?m^?2?year^?1 methane to the sediment–water interface. Areal rates of sulfate reduction (1.46–1.92 mol?m^?2?year^?1) integrated over the upper 0–14 cm of sediment appeared to be limited by the restricted diffusive supply of sulfate, low bio-irrigation (α?=?2.8–3.1 year^?1), and limited residence time of the sedimentary organic carbon in the sulfate zone. A large fraction of reduced sulfur as pyrite and organic-bound sulfur was buried and thus escaped reoxidation in the surface sediment. The presence of ferrous iron in the pore water (with concentrations up to 110 μM) suggests that iron reduction plays an important role in surface sediments, as well as in sediment layers deep below the sulfate–methane transition. We conclude that high rates of sediment accumulation and shallow sulfate penetration are the master variables for biogeochemistry of methane and sulfur cycling; in particular, they may significantly allow for release of methane into the water column in the Himmerfjärden estuary.     

A cryptic sulfur cycle driven by iron in the methane zone of marine sediment (Aarhus Bay, Denmark)

Sulfate reduction and sulfur-iron geochemistry were studied in 5-6 m deep gravity cores of Holocene mud from Aarhus Bay (Denmark). A goal was to understand whether sulfate is generated by re-oxidation of sulfide throughout the sulfate and methane zones, which might explain the abundance of active sulfate reducers deep below the main sulfate zone. Sulfate penetrated down to 130 cm where methane started to build up and where the concentration of free sulfide peaked at 5.5 mM. Below this sulfate-methane transition, sulfide diffused downwards to a sulfidization front at 520 cm depth, below which dissolved iron, Fe²⁺, accumulated in the pore water. Sulfate reduction rates measured by ³⁵S-tracer incubations in the sulfate zone were high due to high concentrations of reactive organic matter. Within the sulfate-methane transition, sulfate reduction was distinctly stimulated by the anaerobic oxidation of methane. In the methane zone below, sulfate remained at positive “background” concentrations of <0.5 mM down to the sulfidization front. Sulfate reduction decreased steeply to rates which at 300-500 cm depth were 0.2-1 pmol SO₄²⁻ cm⁻³ d⁻¹, i.e., 4-5 orders of magnitude lower than rates measured near the sediment surface. The turn-over time of sulfate increased from 3 years at 12 cm depth to 100-1000 years down in the methane zone. Sulfate reduction in the methane zone accounted for only 0.1% of sulfate reduction in the entire sediment column and was apparently limited by the low pore water concentration of sulfate and the low availability of organic substrates. Amendment of the sediment with both sulfate and organic substrates immediately caused a 10- to 40-fold higher, “potential sulfate reduction” which showed that a physiologically intact community of sulfate reducing bacteria was present. The “background” sulfate concentration appears to be generated from the reaction of downwards diffusing sulfide with deeply buried Fe(III) species, such as poorly-reactive iron oxides or iron bound in reactive silicates. The oxidation of sulfide to sulfate in the sulfidic sediment may involve the formation of elemental sulfur and thiosulfate and their further disproportionation to sulfide and sulfate. The net reaction of sulfide and Fe(III) to form pyrite requires an additional oxidant, irrespective of the formation of sulfate. This could be CO₂ which is reduced with H₂ to methane. The methane subsequently diffuses upwards to become re-oxidized at the sulfate-methane transition and thereby removes excess reducing power and enables the formation of excess sulfate. We show here how the combination of these well-established sulfur-iron-carbon reactions may lead to the deep formation of sulfate and drive a cryptic sulfur cycle. The iron-rich post-glacial sediments underlying Holocene marine mud stimulate the strong sub-surface sulfide reoxidation observed in Aarhus Bay and are a result of the glacial to interglacial history of the Baltic Sea area. Yet, processes similar to the ones described here probably occur widespread in marine sediments, in particular along the ocean margins.     

Sediment accumulation rates and geochronologies measured in the Saguenay Fjord using the Pb-210 dating method

Sediment accumulation rates were estimated from-the vertical distribution of excess Pb-210 measured in sediment cores collected at seven stations in the Saguenay Fjord, Quebec. These rates decrease with increasing water depth and distance from the mouth of the Saguenay River, ranging from 4.0 g cm^?2 yr^?1 (～- 7 cm yr^?1) near the head of the fjord to 0.07 g cm^?2 yr^?1 (～- 0.1 cm yr^?1) in the deep inner basin of the fjord. In one core from the head of the fjord, layered sediment structures, having different physical characteristics and composition, appear related to recent, pulsed inputs of older raised marine deposits displaced by a landslide in 1971. Synchronous depositional anomalies in several cores provide evidence of other large scale sediment redistribution processes in the fjord. Pb-210 geochronologies are generally in good agreement with time-stratigraphic horizons inferred both from Cs-137 activity profiles and from the analysis of pollen assemblages in one core.     

Silica distribution in interstitial waters and sediments from the southeastern pacific

Analyses for silica in the interstitial water of five cores from the southeast Pacific are presented. Silica is enriched in these interstitial waters resulting in a vertical flux of silica of between 10 and 50 μmol cm^?2 yr^?1 from the sediment into the overlaying seawater. This flux is generated by the dissolution of biogenic silica, the dissolution of which is increased in areas of bottom water turbulence. The Si, Al and calculated opal (Leinen, 1977) contents of the bulk sediment of these cores are also presented. Small scale variations over depth intervals of tens of centimetres are present as a result of chaning conditions of sedimentation.     

Seasonal variations in sediment sulfur cycling in the Ballastplaat mudflat, Belgium

Sulfate reduction rate (SRR) and pools of reduced inorganic sulfur, acid volatile sulfide (AVS), chromium reducible sulfur (CRS), and elemental sulfur (S^o), were studied from June 1990 till March 1992 at two locations on the Ballastplaat mudflat in the Scheldt estuary. The sediment composition at station A was mainly sand with low organic content whereas sediments at station B were dominated by silt and clay with high organic content. SRR was positively related to temperature; more pronounced at station B (Ea=190 kJ mol⁻¹) than at station A (Ea=110 kJ mol⁻¹). The maximum SRR values observed equalled 14 μmol cm⁻³ d⁻¹ at station B and 1 μmol cm⁻³ d⁻¹ at station A. AVS was the dominant radiolabelled end product of the sulfate reduction reaction, except in surface sediments where pyrite and S^o were more dominant. However, CRS was the predominant reduced inorganic sulfur pool in the sediments. Both AVS and CRS pools showed temporal variations out of phase with SRR. SRR peaked in summer, while the concentrations of AVS and CRS were highest in fall. The accumulation of AVS and CRS started late summer after depletion of oxidants, which had accumulated during winter and spring. The estimated annual SRR and thus sulfide production in the upper 15 cm of station B was of the order of 100 mol m⁻² yr⁻¹, and at station A of the order of 12 mol m⁻² yr⁻¹. The sulfur mass balance shows that only a very small fraction, if any, of the produced sulfide is retained as reduced inorganic sulfur in the sediment.     

Geochronology and sedimentology of the lower Passaic River,New Jersey

In an attempt to characterize localized rates of sediment accretion, 10 sediment cores were collected from the lower reach of the Passaic River, a major tributary of Newark Bay, New Jersey. Sediments were assayed for ²¹⁰Pb activity at predetermined depths and the rate of sediment accretion (cm yr^?1) was estimated from the least squares regression of the log of unsupported activity versus depth. Sediment accretion rates, derived from ²¹⁰Pb measurements (R_Pb) were used to predict the depth interval within the core containing sediments deposited around 1954; subsequent ¹³⁷Cs analyses were focused on this depth interval. Sediment accretion rates derived from ¹³⁷Cs measurements (R_Cs) were extrapolated from the depth of the 1954 horizon. Lead-210 derived sediment accretion rates in cores collected from a sediment bench extending along the inside bend on the southern shore of a meander in the river, ranged from 4.1 cm yr^?1 to 10.2 cm yr^?1 and averaged 6.8 cm yr^?1. The R_Cs estimates for cores from this area ranged from 3.8 cm yr^?1 to 8.9 cm yr^?1 and averaged 6.6 cm yr^?1. The R_Cs for cores collected in a more hydrologically dynamic reach of the river upstream of the sediment bench, were only 0.41 cm yr^?1 and 0.66 cm yr^?1. The results of this investigation indicate that this reach of the lower Passaic River is an area of high sediment accumulation, retaining much of the sediment load deposited from upstream and downstream sources. The rates of sediment accretion in the lower Passaic River are among the highest reported anywhere in the Newark Bay estuary.     

The geochemistry of salt marshes: Sedimentary ion diffusion,sulfate reduction,and pyritization

A series of seasonal cores was taken in a high marsh near the terminus of Delaware Bay, U.S.A. A seasonal harmonic diffusion model was successfully fit to the concentration profiles of chloride ion in the salt marsh pore waters yielding a calculated sedimentary diffusion coefficient.Virtually all other chemical reactions within salt marsh sediments are directly linked to the rate and stoichiometry of organic decomposition. The rich organic input from the grass Spartina alterniflora is oxidized anaerobically through the process of sulfate reduction. Over 90% of this net decomposition of organic matter takes place in the uppermost 20 cm. The model for sulfate reduction proposed yields an internally consistent set of both pore water (HCO^?₃, NH⁺₄, HPO^2?₄, HS^?, SO^2?₄) and solid phase (FeS₂) distribution profiles for these sediments. Steady state assumptions and the use of mean annual constants can be employed to model the net rates of diagenetic processes in salt marshes. The pore water concentrations of sulfate ion as well as those ions released by sulfate reduction (HCO^?₃, NH⁺₄, HPO^2?₄, HS^?) are modeled by a system composed of an upper zone, where extensive reconsumption of these metabolite ions occurs, and a lower zone where steady state production and no ion reconsumption occurs.A major product of the sulfate reduction is pyrite, whose accumulation rate is greatest between 7 and 9 cm depth, where it equals the net rate of sulfate reduction. Above this zone little pyrite accumulates due to extensive reoxidation. Below 9 cm the rate of pyritization is controlled by the rate of sulfidation of a refractory iron phase.     

Reconstruction of Recent Sedimentary Processes in a Carbonate Platform (Gulf of Batabano,Cuba) Using Environmental Radiotracers

Recent (past 100 years) sedimentary processes in the highly dynamic Gulf of Batabano (Cuba, Caribbean Sea) were investigated through the analyses of environmental radionuclides (e.g., ²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs, ^239,240Pu, and ¹⁴C) in nine sediment cores. We evaluated the mean mass accumulation rates (MARs) and the surface mixed layers (SMLs) in each sediment core. Based on these results, three sedimentary environments were identified in the study region. In the central zone, the sediments were mainly composed of carbonate transported from the southern area and showed elevated mass accumulation rates (MAR, 0.11–0.23 g cm^?2 year^?1) and relatively deep surface mixed layers (SML, 14–16 cm). The southwestern zone was characterized by lower MAR (0.05–0.08 g cm^?2 year^?1) and thinner SML (7–8 cm). In both areas, the long sediment mixing times in the SMLs (of 45–61 years) smoothed out the sedimentary records. The coastline sedimentary environments were characterized by higher MAR (0.30–0.57 g cm^?2 year^?1) and the sedimentary records displayed clear signatures of extreme climatic events such as the intensive rains in 1999 reported for La Coloma and the hurricanes Lili and Isodore in 2002. Our study shows that the application of the ²¹⁰Pb sediment dating method in dynamic costal zones is a challenging task but still may provide important information regarding sedimentation and mixing processes in the ecosystem.     

Sulfate enrichments in estuarine waters of North Carolina

Sulfate concentrations in the mesohaline surface waters of the Pamlico and Neuse River estuaries were enriched, relative to the conservative seawater ratio with Cl^? (0.0517 (M)), by 5 to 43% between late winter and early summer. During this period, sulfate concentrations increased to a maximum excess of 3.5 mM in the bottom waters (0.5 m deep) through intermittent periods of both very low river flow and bottom water anoxia. The calculated net sulfate production rate for this period was 18 mmol per m² per d in the bottom waters. By late summer, the excess sulfate (an average of 12 mol per m²) had been removed from the water column, presumably due to SO₄ ^?2 reduction in anoxic bottom waters and sediments. Qualitative laboratory experiments with slurries of mud exposed to excess O₂ and treated with inhibitors of cytochrome oxidase showed that it was possible to produce the excess SO₄ ^?2 under these conditions via biochemical (not chemical) oxidation of pyride that occurs within the top 5 cm of mud (ca. 100 mmol pyrite-S per g dry mud). Whether the in situ substrate for SO₄ ^?2 production was pyrite, S⁰, or S⁺², is unknown, but the predominance of aqua regia extractable pyrite-Fe and the accumulation of excess SO₄ ^?2 in slurries with insufficient other sources of oxidizable S, indicates that it may be an important substrate for biochemical production of SO₄ ^?2.     

Sulfate reduction and porewater chemistry in a gulf coast<Emphasis Type="Italic">Juncus roemerianus</Emphasis> (Needlerush) marsh

Sulfate reduction rates were measured over the course of a year in the sediments of aJuncus roemerianus marsh located in coastal Alabama. Sulfate reduction rates were typically highest in the surface 0–2 cm and at depths corresponding to peak belowground biomass of the plants. The highest volume-based sulfate reduction rate measured was 1,350 μmol liter-sediment⁻¹ d⁻¹ in September 1995. Areal sulfate reduction rates (integrated to 20 cm depth) were strongly correlated to sediment temperature and varied seasonally from 15.2 mmol SO ₄ ²⁻ m⁻² d⁻¹ in January 1995 to 117 mmol SO ₄ ²⁻ m⁻² d⁻¹ in late August 1995. Despite high sulfate reduction rates porewater dissolved sulfide concentrations were low (<73 μM), indicating rapid sulfide oxidation or precipitation. Sulfate depletion data indicated that net oxidation of sediment sulfides occurred in March through May, following a period of infrequent tidal flooding and during a period of high plant production. Porewater Fe(II) reached very high levels (maximum of 969 μM; mean for all dates was 160 μM), particularly during periods of high sulfate reduction. The annual sulfate reduction rate integrated over the upper 20 cm of sediment was 22.0 mol SO ₄ ²⁻ m⁻² yr⁻¹, which is among the highest rates measured in a wetland ecosystem. Based on literature values of net primary production inJ. roemerianus marshes, we estimate that an amount equivalent to 16% to 90% of the annual belowground production may be remineralized through sulfate reduction.     

The influence of tidal action on porewater nitrate concentration and dynamics in intertidal sediments of the Sado estuary

The change in porewater nitrate (NO₂ ^? + NO₃ ^?) concentrations during exposure of intertidal sediment was studied at a fixed location in the Sado estuary, southwest Portugal, in November 1994. In order to follow nitrate concentration and dynamics from pre-ebb to post flood, during the day, high vertical resolution profiles (0.2 cm) were studied. As a complement, in February 1995, potential nitrification rates in the sediment were measured by laboratory incubations, with high vertical resolution (0.2 cm) up to 3 cm depth. Oxygen penetration was measured with polarographic mini-electrodes. The sediment’s texture as well as the organic matter composition in carbon and nitrogen were studied in deeper (30 cm) cores. In February 1993,²¹⁰Pb activity depth profiles were measured in a core sampled at the beginning of exposure, in order to evaluate the possibility of nonlocal particle exchange. C:N ratios and²¹⁰Pb activity profiles show evidence of nonlocal exchange of solid phase particles between the surface and deeper sediment, most likely due to macrofaunal activity. As a consequence, fresh organic matter is brought from the surface to 7–9 cm depth, causing enhancement of nutrient concentrations. Results of this study suggest nitrate dynamics in intertidal sediments of the Sado estuary are strongly influenced by tidal action. Periodic submersion and exposure allow for the diversification of pathways of oxygen supply to the sediment. Tidal stress at the sediment-water interface during the arrival (flooding) and departure (exposure) of the tidal front at the site has an important bearing on the effective depth of the nitrification zone. A denitrification rate of 2.16 μmol N dm^?5 h^?1 was measured directly from the nitrate inventory in the 1.5–6 cm depth layer. The schematic model of N cycling in these sediments suggests that 20% of the N pool is denitrified during exposure, and that this process is limited by O₂ availability for nitrification.     

Sediment-water oxygen and nutrient fluxes in a river-dominated estuary

Sediment oxygen uptake and net sediment-water fluxes of dissolved inorganic and organic nitrogen and phosphorus were measured at two sites in Fourleague Bay, Louisiana, from August 1981, through May 1982. This estuary is an extension of Atchafalaya Bay which receives high discharge and nutrient loading from the Atchafalaya River. Sediment O₂ uptake averaged 49 mg m^?2 h^?1. On the average, ammonium (NH₄ ⁺) was released from the sediments (mean flux =+129 μmol m^?2 h^?1), and NO₃ ^? was taken up (mean flux =?19 μmol m^?2h^?1). However, very different NO₃ ^? fluxes were observed at the two sites, with sediment uptake at the upper, river-influenced, high NO₃ ^? site (mean flux =?112 μmol m^?2 h^?1) and release at the lower, marine-influenced low NO₃ ^? site (mean flux =+79 μmol m^?2 h^?1). PO₄ ^3? fluxes were low and often negative (mean flux =?8 μmol m^?2 h^?1), while dissolved organic phosphorus fluxes were high and positive (mean flux =+124 μmol m^?2 h^?1). Dissolved organic nitrogen fluxes varied greatly, ranging from a mean of +305 μmol m^?2 h^?1 at the lower bay, to ?710 μmol m^?2 h^?1 at the upper bay. Total dissolved nitrogen and phosphorus fluxes indicated the sediments were a nitrogen (mean flux =+543 μmol m^?2 h^?1) and phosphorus source (mean flux =+30 μmol m^?2 h^?1) at the lower bay, and a nitrogen sink (mean flux =?553 μmol m^?2 h^?1) and phosphorus source (mean flux =+17 μmol m^?2 h^?1) in the upper bay. Mean annual O∶N ration of the positive inorganic sediment fluxes were 27∶1 at the upper bay and 18∶1 at the lower bay. Based on these data we hypothesize that nitrification and denitrification are important sediment processes in the upper bay. We further hypothesize that Atchafalaya River discharge affects sediment-water fluxes through seasonally high nutrient loading which leads to net nutrient uptake by sediments in the upper bay and release in the lower bay, where there is less river influnces.     

Iron, sulfur, and carbon diagenesis in sediments of Tomales Bay, California

Analysis of 3-m sediment cores revealed that profiles of carbon (C), sulfur (S), and iron (Fe) varied with relative distance from marine and terrestrial sediment sources in Tomales Bay California. Despite relatively high sedimentation rates throughout the bay (historically 3–30 mm yr⁻¹), sulfate reduction of deposited organic matter led to free-sulfide accumulation in sediments only at the location farthest from terrestrial runoff, the source of reactive iron. Acid-volatile sulfide concentrations in all sediments (<10 μmol g⁻¹) were low relative to concentrations of chromiumreducible sulfide (up to 400 μmol g⁻¹ farthest from the reactive iron source). A calculated index of iron availability, used to describe sediment resistance to build-up of free sulfide, was lowest at this location. Recent, upward shifts in reactive Fe concentration and in the relative contribution of terrestrial orgnic carbon (measured as a shift in δ¹³C of bulk sediment organic matter) in all cores indicated that erosion and transport of sediments from the watershed surrounding Tomales Bay increased after European settlement in the 1850s.     

Variability of dissolved organic carbon in sediments of a seagrass bed and an unvegetated area within an estuary in Southern Texas

Pore-water dissolved organic carbon (PWDOC) concentrations were examined in vegetated and bare sediments of aHalodule wrightii seagrass bed, and in a mud bottom sediment of a southern Texas estuary. Temporal variability was examined at diel (dawn and noon) and bimonthly time scales. Distribution patterns of PWDOC were compared with physical, chemical, and biological factors thought to exert control on PWDOC. Concentration of PWDOC, bacterial production, and resultant PWDOC turnover times displayed statistically significant spatial and temporal variability. Concentration of PWDOC ranged from 14 mg C 1^?1 to 107 mg C 1^?1 of pore water, or 9–71 μg C cm^?3 wet sediment. PWDOC was more variable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 μg C cm^?3) and high bacterial production rates (mean = 1.92 μg C cm^?3 h^?1) were observed at the mud station, whereas PWDOC concentrations were high (mean = 24.6 μg C cm^?3) and bacterial production rates were low (mean = 0.43 μg C cm^?3 h^?1) at the bare station. PWDOC turnover times (T_t), assuming 50% bacterial growth efficiency (1–840 h) were shortest at the mud station (mean=13 h) and longest at the bare station (mean=180 h). In the overlying water column, T_t values were longer, ranging from 1,000–10,000 h. PWDOC concentrations were 25% higher in vegetated sediments than in neighboring bare sediments. This difference was probably due to inputs of labile photosynthetic excretia, since bacterial production rates in vegetated sediments displayed significant diel variability and were 4 times greater than that of bare sediments. Based upon the entire data set, PWDOC was significantly related to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxygen metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that organism activities and detrital quality are the major determinants controlling variability in PWDOC.     

Sedimentary record of heavy metal pollution in Lake Erie

Three cores, one kilometer apart, from each of seven locations along Lake Erie were analyzed for heavy metals and dated by ²¹⁰Pb techniques. The sedimentary record of anthropogenic inputs of heavy metals parallels the increasing intensity of cultural activity in the lake basin. On the average, pollution sources annually contribute 0.4 μg of Cd, 12 μg of Cu, 12 μg of Pb and 36 μg of Zn deposited per each cm² of the Eastern Basin sediments: 0.5, 8.8, 11 and 31 μg of Cd, Cu, Pb and Zn, respectively, deposited per cm² of Western Basin sediments and 0.7, 1.4, 2.0 and 5.6 μg of Cd, Cu, Pb and Zn, respectively, deposited per cm² of fine-grained sediments in the Central Basin. These anthropogenic flux rates exceed the pre-colonial data by 80–600%. The mean flux rates for ²¹⁰Pb into the Eastern. Central and Western Basins are 0.45, 0.07 and 0.15dpm cm^?2 yr^?1. respectively. From an inventory of sources and sinks of the metals, it is shown that about 2500 × 10³ kg of Cu. 1900 × 10³ kg of Pb and 6750 × 10³kg of Zn are delivered annually into the lake. The calculated retention in the lake sediments of 45%, 65% and 35% of the total annual inputs of Cu. Pb and Zn, respectively, agrees closely with the accumulation of data derived from sediment analyses. Sewage discharges, direct and indirect, are shown to be an important source of metal in the lake. The mean residence times in the water column are inferred to be 104 days for Cu. 180 days for Pb and 152 days for Zn.     

Preferential flow and segregation of porewater solutes in Wetland sediment

Sediment macropores (with effective diameters larger than 100 μm) comprise 11% of the bulk sediment volume in a tidal freshwater wetland vegetated withPeltandra virginica. In order to determine effects of macroporous sediment structure on solute transport, we conducted a solute tracer experiment in the sediment. The effective transport volume (θ_eff, the volume of sediment through which solute was transported normalized to sediment bulk volume) was 0.15 cm³ cm^?3, which is considerably smaller than the total pore space that is potentially available for transport (porosity of sediment is 0.63 cm³ cm^?3). A mean transport time of 13 d was required to flush preferential flow paths inPeltandra hummocks; hydrologic turnover of the volumetrically dominant matrix pores (0.53 cm³ cm^?3) was apparently much slower. Based on porewater sampler design and hydrological principles, we suggest that N₂-purged tension solution samplers and diffusion equilibrators preferentially sample porewater from macropore and matrix domains, respectively. Dissolved ammonium and orthophosphate concentrations were three-fold higher in matrix pores compared to macropores, which is consistent with our finding that more rapid hydrological flushing occurred in macropores compared to matrix pores. Further evaluation of porewater sampler designs in macroporous sediment is needed to improve studies of hydrologic transport and biogeochemical cycling in wetlands.     

Intense pyrite formation under low-sulfate conditions in the Achterwasser lagoon, SW Baltic Sea

In comparison to similar low-sulfate coastal environments with anoxic-sulfidic sediments, the Achterwasser lagoon, which is part of the Oder estuary in the SW Baltic Sea, reveals unexpectedly high pyrite concentrations of up to 7.5 wt%. Pyrite occurs mainly as framboidal grains variable in size with diameters between 1 and 20 μm. Pyritization is not uniform down to the investigated sediment depth of 50 cm. The consumption of reactive-Fe is most efficient in the upper 20 cm of the sediment column, leading to degrees of pyritization (DOP) as high as 80 to 95%.Sediment accumulation in the Achterwasser takes place in high productivity waters. The content of organic carbon reaches values of up to 10 wt%, indicating that pyrite formation is not limited by the availability of organic matter. Although dissolved sulfate concentration is relatively low (<2 mmol/L) in the Achterwasser, the presence of H₂S in the pore water suggests that sulfate is unlikely to limit pyrite authigenesis. The lack of free Fe(II) in the pore waters combined with the possibility of a very efficient transformation of Fe-monosulfides to pyrite near the sediment/water interface suggests that pyrite formation is rather controlled by (i) the availability of reactive-Fe, which limits the FeS formation, and by (ii) the availability of an oxidant, which limits the transformation of FeS into pyrite. The ultimate source for reactive-Fe is the river Oder, which provides a high portion of reactive-Fe (∼65% of the total-Fe) in the form of suspended particulate matter. The surficial sediments of the Achterwasser are reduced, but are subject to oxidation from the overlying water by resuspension. Oxidation of the sediments produces sulfur species with oxidation states intermediate between sulfide and sulfate (e.g., thiosulfate and polysulfides), which transform FeS to FeS₂ at a significant rate. This process of FeS-recycling is suggested to be responsible for the formation of pyrite in high concentrations near the sediment surface, with DOP values between 80 and 95% even under low sulfate conditions.A postdepositional sulfidization takes place in the deeper part of the sediment column, at ∼22 cm depth, where the downward diffusion of H₂S is balanced by the upward migration of Fe(II). The vertical fluctuation of the diffusion front intensifies the pyritization of sediments. We suggest that the processes described may occur preferentially in shallow water lagoons with average net-sedimentation rates close to zero. Such environments are prone to surficial sediment resuspension, initiating oxidation of Fe-sulfides near the sediment/water interface. Subsequent FeS₂ formation as well as postdepositional sulfidization leads to a major pyrite spike at depth within the sediment profile.     

The effect of dissolved barium on biogeochemical processes at cold seeps

A numerical model was applied to investigate and quantify the biogeochemical processes fueled by the expulsion of barium and methane-rich fluids in the sediments of a giant cold-seep area in the Derugin Basin (Sea of Okhotsk). Geochemical profiles of dissolved Ba²⁺, Sr²⁺, Ca²⁺, SO₄²⁻, HS⁻, DIC, I⁻ and of calcium carbonate (CaCO₃) were fitted numerically to constrain the transport processes and the kinetics of biogeochemical reactions. The model results indicate that the anaerobic oxidation of methane (AOM) is the major process proceeding at a depth-integrated rate of 4.9 μmol cm⁻² a⁻¹, followed by calcium carbonate and strontian barite precipitation/dissolution processes having a total depth-integrated rate of 2.1 μmol cm⁻² a⁻¹. At the low seepage rate prevailing at our study site (0.14 cm a⁻¹) all of the rising barium is consumed by precipitation of barite in the sedimentary column and no benthic barium flux is produced. Numerical experiments were run to investigate the response of this diagenetic environment to variations of hydrological and biogeochemical conditions. Our results show that relatively low rates of fluid flow (<∼5 cm a⁻¹) promote the dispersed precipitation of up to 26 wt% of barite and calcium carbonate throughout the uppermost few meters of the sedimentary column. Distinct and persistent events (several hundreds of years long) of more vigorous fluid flow (from 20-110 cm a⁻¹), instead, result in the formation of barite-carbonate crusts near the sediment surface. Competition between barium and methane for sulfate controls the mineralogy of these sediment precipitates such that at low dissolved methane/barium ratios (<4-11) barite precipitation dominates, while at higher methane/barium ratios sulfate availability is limited by AOM and calcium carbonate prevails. When seepage rates exceed 110 cm a⁻¹, barite precipitation occurs at the seafloor and is so rapid that barite chimneys form in the water column. In the Derugin Basin, spectacular barite constructions up to 20 m high, which cover an area of roughly 22 km² and contain in excess of 5 million tons of barite, are built through this process. In these conditions, our model calculates a flux of barium to the water column of at least 20 μmol cm⁻² a⁻¹. We estimate that a minimum of 0.44 × 10⁶ mol a⁻¹ are added to the bottom waters of the Derugin Basin by cold seep processes, likely affecting the barium cycle in the Sea of Okhotsk.     

Denitrification coupled to pyrite oxidation and changes in groundwater quality in a shallow sandy aquifer

This study focuses on denitrification in a sandy aquifer using geochemical analyses of both sediment and groundwater, combined with groundwater age dating (³H/³He). The study sites are located underneath cultivated fields and an adjacent forested area at Oostrum, The Netherlands. Shallow groundwater in the region has high nitrate concentrations (up to 8 mM) due to intense fertilizer application. Nitrate removal from the groundwater below cultivated fields correlates with sulfate production, and the release of dissolved Fe²⁺ and pyrite-associated trace metals (e.g. As, Ni, Co and Zn). These results, and the presence of pyrite in the sediment matrix within the nitrate removal zone, indicate that denitrification coupled to pyrite oxidation is a major process in the aquifer. Significant nitrate loss coupled to sulfate production is further confirmed by comparing historical estimates of regional sulfate and nitrate loadings to age-dated groundwater sulfate and nitrate concentrations, for the period 1950-2000. However, the observed increases in sulfate concentration are about 50% lower than would be expected from complete oxidation of pyrite to sulfate, possibly due to the accumulation of intermediate oxidation state sulfur compounds, such as elemental sulfur. Pollutant concentrations (NO₃, Cl, As, Co and Ni) measured in the groundwater beneath the agricultural areas in 1996 and 2006 show systematic decreases most likely due to declining fertilizer use.     

Estimating selenium removal by sedimentation from the Great Salt Lake,Utah

The mass of Se deposited annually to sediment in the Great Salt Lake (GSL) was estimated to determine the significance of sedimentation as a permanent Se removal mechanism. Lake sediment cores were used to qualitatively delineate sedimentation regions (very high to very low), estimate mass accumulation rates (MARs) and determine sediment Se concentrations. Sedimentation regions were defined by comparison of isopach contours of Holocene sediment thicknesses to linear sedimentation rates determined via analysis of ²¹⁰Pb, ²²⁶Ra, ⁷Be and ¹³⁷Cs activity in 20 short cores (<5 cm), yielding quantifiable results in 13 cores. MARs were developed via analysis of the same radioisotopes in eight long cores (>10 cm). These MARs in the upper 1–2 cm of each long core ranged from 0.019 to 0.105 g_sed/cm²/a. Surface sediment Se concentrations in the upper 1 or 2 cm of each long core ranged from 0.79 to 2.47 mg/kg. Representative MARs and Se concentrations were used to develop mean annual Se removal by sedimentation in the corresponding sedimentation region. The spatially integrated Se sedimentation rate was estimated to be 624 kg/a within a range of uncertainty between 285 and 960 kg/a. Comparison to annual Se loading and other potential removal processes suggests burial by sedimentation is not the primary removal process for Se from the GSL.