Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury

A 1-D biogeochemical reactive transport model with a full set of equilibrium and kinetic biogeochemical reactions was developed to simulate the fate and transport of arsenic and mercury in subaqueous sediment caps. Model simulations (50?years) were performed for freshwater and estuarine scenarios with an anaerobic porewater and either a diffusion-only or a diffusion plus 0.1-m/year upward advective flux through the cap. A biological habitat layer in the top 0.15?m of the cap was simulated with the addition of organic carbon. For arsenic, the generation of sulfate-reducing conditions limits the formation of iron oxide phases available for adsorption. As a result, subaqueous sediment caps may be relatively ineffective for mitigating contaminant arsenic migration when influent concentrations are high and sorption capacity is insufficient. For mercury, sulfate reduction promotes the precipitation of metacinnabar (HgS) below the habitat layer, and associated fluxes across the sediment–water interface are low. As such, cap thickness is a key design parameter that can be adjusted to control the depth below the sediment–water interface at which mercury sulfide precipitates. The highest dissolved methylmercury concentrations occur in the habitat layer in estuarine environments under conditions of advecting porewater, but the highest sediment concentrations are predicted to occur in freshwater environments due to sorption on sediment organic matter. Site-specific reactive transport simulations are a powerful tool for identifying the major controls on sediment- and porewater-contaminant arsenic and mercury concentrations that result from coupling between physical conditions and biologically mediated chemical reactions.     

Sediment Response to 25?Years of Persistent Hypoxia

We investigated the impact of persistent hypoxia on sediment chemistry by comparing total, reactive (extractible with 1?M hydroxylamine?Chydrochloride in 25?% acetic acid), and dissolved forms of the redox-sensitive elements Mn, Fe, and As in cores recovered between 1982 and 2007 at two sites in the Lower St. Lawrence Estuary (LSLE) where the bottom water has been severely hypoxic since the early 1980s. The data reveal that the concentrations and the vertical distributions of total solid-phase and dissolved Mn as well as total solid-phase Fe and As were not significantly affected by persistent hypoxia. In contrast, the composition of solid-phase Fe and As changed significantly as did the pore-water concentrations of both these elements. The relative amounts of solid-phase reactive Fe and As increased, and the abundance of pyrite and pyritic?CAs decreased in the sediment layer that accumulated since 1982. We propose that persistent hypoxic conditions restrict the supply of oxygen to the sediment and increase the relative contribution of alternate electron acceptors??Mn(IV), Fe(III), and sulfate??to microbial oxidation of organic matter. In marine iron-rich environments, such as the LSLE sediment, increased sulfate reduction may promote conversion of less reactive Fe phases to more reactive Fe phases which, in turn, interfere with pyrite formation. Consequently, a chalcophile element such as As, which would normally be sequestered with authigenic pyrite, remains available for recycling across the oxic?Canoxic boundary in the sediment.     

Redox cycling of iron and manganese in sediments of the Kalix River estuary, Northern Sweden

Iron and manganese redox cycling in the sediment — water interface region in the Kalix River estuary was investigated by using sediment trap data, pore-water and solid-phase sediment data. Nondetrital phases (presumably reactive Fe and Mn oxides) form substantial fractions of the total settling flux of Fe and Mn (51% of Fe_total and 84% of Mn_total). A steady-state box model reveals that nondetrital Fe and Mn differ considerably in reactivity during post-depositional redox cycling in the sediment. The production rate of dissolved Mn (1.6 mmol m^–2 d^–1) exceeded the depositional flux of nondetrital Mn (0.27 mmol m^–2 d^–1) by a factor of about 6. In contrast, the production rate of upwardly diffusing pore-water Fe (0.77 mmol m^–2 d^–1) amounted to only 22% of the depositional flux of nondetrital Fe (3.5 mmol m^–2 d^–1). Upwardly diffusing pore-water Fe and Mn are effectively oxidized and trapped in the oxic surface layer of the sediment, resulting in negligible benthic effluxes of Fe and Mn. Consequently, the concentrations of nondetrital Fe and Mn in permanently deposited, anoxic sediment are similar to those in the settling material. Reactive Fe oxides appear to form a substantial fraction of this buried, non-detrital Fe. The in-situ oxidation rates of Fe and Mn are tentatively estimated to be 0.51 and 0.16–1.7 mol cm^–3 d^–1, respectively.     

Surface metal enrichment and partitioning of metals in a dated sediment core from a Norwegian fjord

A 24-cm long sediment core from an oxic fjord basin in Ranafjord, Northern Norway, was sliced in 2 cm sections and analysed for As, Co, Cu, Ni, Hg, Pb, Zn, Mn, Fe, ignition loss and Pb-210. Partitioning of metals between silicate, non-silicate and non-detrital phases was assessed by leaching experiments, in an attempt to understand the mechanisms of surface metal enrichment in sediments. Relative to metal concentrations in sediments deposited in the 19th century, metals in near surface sediments were enriched in the following order: Pb > Mn > Hg > Zn > Cu > As > Fe. Cobalt and Ni showed no enrichment. The non-detrital fraction of Cu, Pb, Mn and Zn was significantly higher in the upper 10 cm than at greater depth in the core. This corresponds to sediments deposited since 1900, when mining activities started in the area. The enrichment of Cu, Pb and Zn is assumed to be mainly a result of mining, while Mn is apparently enriched in the surface due to migration of dissolved Mn and precipitation in the oxic surface layer. Elevated concentrations of As and Fe in the upper 4 cm are presumably due to discharges from a coke plant and an iron works respectively. The excess Hg present in the near surface sediments is tightly bound, either in coal particles or ore dust introduced by local industry, or via long distance transport of atmospheric particles. Calculations of metal flux to the sediments indicate an anthropogenic flux of Zn equal to its natural flux, while the flux of Pb shows a threefold increase above natural input.     

Geochemical Processes Controlling the Formation of As-Rich Waters Within a Tailings Impoundment (Carnoulès, France)

The distribution of arsenic (As(III), As(V)) and iron (Fe(II), Fe(III)) species was monitored during 1 year in a borehole drilled in the Carnoulès tailings impoundment which contains As-rich pyrite. The concentrations of total As and Fe in subsurface waters exhibited strong variations over one year, which were controlled by dissolved oxygen concentrations. At high oxygen levels, extremely high As (up to 162 mM) and Fe (up to 364 mM) concentrations were reached in the borehole, with the oxidised species predominant. As and Fe concentrations decreased 10-fold under oxygen-deficient conditions, as a result of pH increase and subsequent precipitation of As(V) and Fe(III). From drill core sections, it appeared that at low dissolved oxygen levels, As(III) was primarily released into water by the oxidation of As-rich pyrite in the unsaturated zone. Subsequent As and Fe precipitation was promoted during transport to the saturated zone; this reaction resulted in As enrichments in the sediment below the water table compared to the original content in pyrite, together with the formation of As-rich (up to 35 wt% As) ferruginous material in the unsaturated zone. High amounts of As(V) were released from these secondary phases during leaching experiments with oxygenated acid sulfate-rich waters; this process is believed to contribute to As(V) enrichment in the subsurface waters of the Carnoulès tailings during periods of high dissolved oxygen level.     

Diagenetic cycling of arsenic in the sediments of eutrophic Baldeggersee,Switzerland

Sedimentation and benthic release of As was determined in Baldeggersee, a eutrophic lake in central Switzerland. Sediment traps recorded As sedimentation during 1994, including a flood event in spring. Diagenetic processes were studied using porewater profiles at the sediment–water interface and in deeper sediment strata deposited in the mesotrophic lake (before 1885). Sediment cores were used to calculate the accumulation and to construct the balance of sedimentation and remobilisation. The results showed that the lake sediment acts as an efficient sink for As. Only 22% of the particulate As flux reaching the sediment surface was remobilised at the sediment–water interface. The As accumulation in the recent varved section of the eutrophic lake was 40 mg As m⁻² a⁻¹. Iron reduction in older sediment caused a remobilisation of 1.2 mg As m⁻² a⁻¹. This upward flux from the deeper sediment was quantitatively immobilised in the recent sulfidic sediments. The flood event in spring contributed about 34% of the yearly sediment load and led to distinct peak profiles of dissolved As in the porewater. This evidence for rapid remobilisation disappeared within months.     

The contrasting biogeochemistry of iron and manganese in the Pacific Ocean

Vertical and horizontal distributions of dissolved and suspended particulate Fe and Mn, and vertical fluxes of these metals (obtained with sediment traps) were determined throughout the Pacific Ocean. In general, dissolved Fe is low in surface and deep waters (0.1 to 0.7 nmol/kg), with maxima associated with the intermediate depth oxygen minimum zone (2.0 to 6.6 nmol/kg). Vertical distributions of dissolved Mn are similar to previous reports, exhibiting a surface maximum, a subsurface minimum, a Mn maximum layer coincident with the oxygen minimum zone, and lowest values in deep waters.Near-shore removal processes are more intense for dissolved Fe than for dissolved Mn. Dissolved Mn in the surface mixed layer remains elevated much farther offshore than dissolved Fe. Elevated near-surface dissolved Mn concentrations occur in the North Pacific Equatorial Current, suggesting transport from the eastern boundary. Near-surface mixed-layer dissolved Mn concentrations are higher in the North Pacific gyre reflecting enhanced northern hemisphere aeolian sources.Residence time estimates for the settling of refractory paniculate Fe and Mn from the upper water column are 62–220 days (Fe), and 105–235 days (Mn). In contrast, residence times for the scavenging of dissolved Fe and Mn are 2–13 years (Fe) and 3–74 years (Mn). Scavenging residence times for dissolved Mn based on horizontal mixing in the surface mixed layer of the northeast Pacific are 0.4 years (nearshore) to 19 years (1000 km offshore).There is no evidence for in situ Fe redox dissolution within sub-oxic waters in the eastern tropical North Pacific. Dissolved Fe appeared to be controlled by dissolution from sub-oxic sediments, with oxidative scavenging in the water column or upper sediment layers. However, in situ Mn dissolution within the oxygen minimum zone was evident.     

全球海水剖面Fe同位素组成的不均一性及其影响因素

全球海水剖面Fe同位素组成存在显著不均一性.对大西洋洋中脊、大西洋近海岸带、东太平洋和西太平洋弧后扩张中心多个站位的海水剖面溶解Fe浓度和Fe同位素组成进行了综合分析,得出以下主要认识:（1）不同区域的海水剖面溶解Fe浓度和Fe同位素组成呈现不同的变化特征,海水Fe同位素的变化趋势与海水溶解氧浓度变化一致,而与海水溶解Fe浓度呈镜像变化关系;（2）不同深度的海水溶解Fe浓度和Fe同位素组成特征的主要控制因素不同.表层海水受到大气降尘、生物作用影响呈现富重Fe同位素特征,受河流的影响Fe同位素组成偏轻;深层海水主要受到深海沉积和海底热液活动的影响,其中沉积物中的非还原溶解Fe导致海水富集重Fe同位素,而受洋中脊热液流体影响的深部海水显著富集轻Fe同位素;（3）将目前已知海底热液溶解Fe通量最小值（0.5 Gmol/a）作为全球大洋的热液溶解Fe通量,利用不同来源的溶解Fe同位素与其通量间的关系估算海底热液对海洋的Fe循环的贡献为~5.5%.由于海底热液流体的Fe通量可能远大于0.5 Gmol/a,因此,海底热液活动对海洋溶解Fe的贡献可能远超过前人的估算结果（6.0%）.      

Chlor-alkali industrial contamination and riverine transport of mercury: Distribution and partitioning of mercury between water,suspended matter,and bottom sediment of the Thur River,France

Total dissolved and total particulate Hg mass balances were estimated during one hydrological period (July 2001–June 2002) in the Thur River basin, which is heavily polluted by chlor-alkali industrial activity. The seasonal variations of the Hg dynamics in the aquatic environment were assessed using total Hg concentrations in bottom sediment and suspended matter, and total and reactive dissolved Hg concentrations in the water. The impact of the chlor-alkali plant (CAP) remains the largest concern for Hg contamination of this river system. Upstream from the CAP, the Hg partitioning between dissolved and particulate phases was principally controlled by the dissolved fraction due to snow melting during spring high flow, while during low flow, Hg was primarily adsorbed onto particulates. Downstream from the CAP, the Hg partitioning is controlled by the concentration of dissolved organic and inorganic ligands and by the total suspended sediment (TSS) concentrations. Nevertheless, the particulate fluxes were five times higher than the dissolved ones. Most of the total annual flux of Hg supplied by the CAP to the river is transported to the outlet of the catchment (total Hg flux: 70 μg m⁻² a⁻¹). Downstream from the CAP, the bottom sediment, mainly composed of coarse sediment (>63 μm) and depleted in organic matter, has a weak capacity to trap Hg in the river channel and the stock of Hg is low (4 mg m⁻²) showing that the residence time of Hg in this river is short.     

Managanese and iron in the White Sea: Sedimentation and diagenesis

Iron and manganese in bottom sediments studied along the sublatitudinal transect from Kandalaksha to Arkhangelsk are characterized by various contents and forms depending on sedimentation environments, grain size of sediments, and diagenetic processes. The latter include redistribution of reactive forms leading to enrichment in Fe and Mn of the surface sediments, formation of films, incrustations, and ferromanganese nodules. Variations in the total Fe content (2–8%) are accompanied by changes in the concentration of its reactive forms (acid extraction) and the concentration of dissolved Fe in the interstitial water (1–14 μM). Variations in the Mn content in sediments (0.03–3.7%) and the interstitial water (up to 500 μM) correspond to a high diagenetic mobility of this element. Changes in the valence of chemical elements results in the redox stratification of sediment strata with maximum concentrations of Fe, Mn, and sulfides. Organic matter of sediments with a considerable terrestrial constituent is oxidized by bottom water oxygen mainly at the sediment surface or in anaerobic conditions within the sediment strata. The role of inorganic components in organic matter oxidation changes from surface sediments, where manganese oxyhydroxide dominates among oxidants, to deeper layers, where sulfate of interstitial water serves as the main oxidant. Differences in river runoff and hydrodynamics are responsible for geochemical asymmetry of the transect. The deep Kandalaksha Bay serves as a sediment trap for manganese (Mn content in sediments varies within 0.5–0.7%), whereas the sedimentary environment in the Dvina Bay promotes its removal from sediments (Mn 0.05%).     

氧化还原动态变化对沉积物砷和氟释放的影响:以河北白洋淀平原为例

季风性波动引起的降雨、径流和排泄过程会引发浅层地下水系统周期性氧化还原动态变化,从而对地下水系统中有害组分的迁移转化产生影响。为探讨氧化还原动态过程对沉积物中砷(As)和氟(F)释放的影响,本研究选择河北白洋淀地区沉积物样品,利用发酵罐作为反应器,建立氧化还原动态实验体系,并监测动态变化过程中实验体系各组分含量的变化。结果表明,碱性和还原环境均有利于地下水中As、F的富集。还原阶段较高的pH条件有利于溶液中F^-的解吸,且体系中有机物降解会产生大量HC0₃^-和C0₃^2-,与F^-发生竞争吸附而有利于F^-的富集。对于溶液中As的富集,一方面是由于还原条件下体系中的As以As(III)为主,受沉积物的吸附作用较弱,从而有利于As被释放到溶液中;另一方面是因为还原阶段较高的pH也会使反应体系中As和沉积物间的吸附作用被减弱,造成As的解吸附。由于实验所用沉积物砷含量较低,不同S0₄^2-浓度条件对氧化还原动态过程中As、F迁移的影响不明显。总之,氧化还原动态变化过程会强烈影响地下水系统中砷、氟的富集。     

Early diagenesis of trace metals (Cd, Cu, Co, Ni, U, Mo, and V) in the freshwater reaches of a macrotidal estuary

Vertical profiles from the water column, including the maximum turbidity zone (MTZ) to the consolidated sediment were sampled in September 2000 in the freshwater reaches of the Gironde Estuary during a complete neap tide-spring tide cycle. The vertical distributions of dissolved major redox parameters and metals (Mn, Fe, Cd, Cu, V, Co, Ni, Mo, and U) were determined. Reactive particulate metal fractions were also determined from selective leaching. The studied system is characterized by density layers functioning at different time-scales, consisting of two mobile layers, i.e., the liquid (LM) and the soft mud (SM), overlying consolidated sediments (CS). This results in a three-zone diagenetic regime where (1) O₂ dynamics are fast enough to show depletion in the rapidly mixed LM sequence (tidal time-scale), (2) denitrification occurs on the weekly time-scale mixing SM sequence, and (3) the Mn, Fe, and sulfate cycling occurs in the CS layer (annual time-scale). The studied trace metals show differential behavior during early diagenesis: (1) Cd, Cu, and V are released into pore water preferentially from organic matter in the SM, (2) Co, Ni, and U are released in the CS from Mn and Fe oxides during reductive dissolution, and (3) Mo from both processes. Transient conditions (i.e., oscillations of redox fronts and reoxidation processes), due to the dynamics of the mobile layers, strongly influence the trace metal distributions as inducing resolubilization (Cd, Cu, and Mo). In the CS, authigenic metal phases accumulate, either by direct precipitation with sulfides (Cu, Cd) or co-precipitation with Fe-sulfides (Mo). Microbially mediated reduction of Fe oxides is proposed to control U removal from pore water by reduction of U(VI) to U(IV) at depth. However, a significant fraction of the trace metals is trapped in the sediment in exchangeable forms, and therefore is susceptible to be mobilized due to resuspension of estuarine sediment during strong river flood periods and/or dredging activities.     

Seasonal variations of arsenic at the sediment–water interface of Poyang Lake,China

Arsenic species including arsenite, arsenate, and organic arsenic were measured in the porewaters collected from Poyang Lake, the largest freshwater lake of China. The vertical distributions of dissolved arsenic species and some diagenetic constituents [Fe(II), Mn(II), S(−II)] were also obtained in the same porewater samples in summer and winter. In sediments the concentration profiles of total As and As species bound to Fe–Mn oxyhydroxides and to organic matter were also determined along with the concentrations of Fe, Mn and S in different extractable fractions. Results indicate that, in the summer season, the concentrations of total dissolved As varying from 3.9 to 55.8 μg/L in sediments were higher than those (5.3–15.7 μg/L) measured in the winter season, while the concentrations of total As species in the solid phase varied between 10.97 and 25.32 mg/kg and between 7.84 and 30.52 mg/kg on a dry weight basis in summer and winter, respectively. Seasonal profiles of dissolved As suggest downward and upward diffusion, and the flux of dissolved As across the sediment–water interface (SWI) in summer and winter were estimated at 3.88 mg/m² a and 0.79 mg/m² a, respectively. Based on porewater profiles and sediment phase data, the main geochemical behavior of As was controlled by adsorption/desorption, precipitation and molecular diffusion. The solubility and migration of inorganic As are controlled by Fe–Mn oxyhydroxides in summer whereas they appear to be more likely controlled by both amorphous Fe–Mn oxyhydroxides and sulfides in winter. A better knowledge of the cycle of As in Poyang Lake is essential to a better management of its hydrology and for the environmental protection of biota in the lake.     

Importance of terrestrially-derived,particulate phosphorus to phosphorus dynamics in a west coast estuary

Allochthonous inputs of suspended particulate matter from freshwater environments to estuaries influence nutrient cycling and ecosystem metabolism. Contributions of different biogeochemical reactions to phosphorus dynamics in Tomales Bay, California, were determined by measuring dissolved inorganic phosphorus exchange between water and suspended particulate matter in response to changes in salinity, pH, and sediment redox. In serum bottle incubations of suspended particulate matter collected from the major tributary to the bay, dissolved inorganic phosphorus release increased with salinity during the initial 8 h; between 1–3 d, however, rates of release were similar among treatments of 0 psu, 16 psu, 24 psu, and 32 psu. Release was variable over the pH range 4–8.5, but dissolved inorganic phosphorus releases from sediments incubated for 24 h at the pH of fresh water (7.3) and seawater (8.1) were similarly small. Under oxidizing conditions, dissolved inorganic phosphorus release was small or dissolved inorganic phosphorus was taken up by particulate matter with total P content <50 μmoles P g^?1; release was greater from suspended particulate matter with total phosphorus content >50 μmoles P g^?1. In contrast, under reducing conditions maintained by addition of free sulfide (HS^?), dissolved inorganic phosphorus was released from particles at all concentrations of total phosphorus in suspended particulate matter, presumably from the reduction of iron oxides. Since extrapolated dissolved inorganic phosphorus release from this abiotic source can account for only 12.5% of the total dissolved inorganic phosphorus flux from Tomales Bay sediments, we conclude most release from particles is due to organic matter oxidation that occurs after estuarine deposition. The abiotic, sedimentary flux of dissolved inorganic phosphorus, however, could contribute up to 30% of the observed net export of dissolved inorganic phosphorus from the entire estuary.     

Diagenetic cycling of trace elements in the bottom sediments of the Swan River Estuary,Western Australia

Teflon strips were used in-situ in the bottom sediments at two sites in the Swan River Estuary to collect diagenetic Fe–Mn oxyhydroxides and monitor monthly changes in their morphology and trace element geochemistry. This study demonstrates that substantial concentrations of trace elements accumulate at the redox front during the formation of diagenetic Fe–Mn oxyhydroxides. It is likely that the Fe–Mn oxyhydroxides initially nucleate and grow on the Teflon strips via bacterial activity. Trace element geochemistry of the diagenetic Fe–Mn oxyhydroxides is influenced by changes in the supply of trace elements from either the bottom sediments and/or water column or changes in the physico-chemical status of bottom and porewaters. If sufficient diagenetic Fe–Mn oxyhydroxides are preserved in the upper layer(s) of the bottom sediment it is possible that diagenetic (secondary) trace element enrichment profiles may be produced which modify the historical input of natural or anthropogenic trace element sources. Alternatively, partial or complete dissolution of the diagenetic Fe–Mn oxyhydroxides in response to temporal changes in the redox status of the bottom sediment may lead to a substantial underestimate of trace element fluxes in historical bottom sediment profiles. This study highlights that considerable care must be taken when interpreting short- to long-term geochemical profiles in bottom sediments due to the possible occurrence of rapid, seasonally mediated diagenetic processes.     

Interplay of S and As in Mekong Delta sediments during redox oscillations

The cumulative effects of periodic redox cycling on the mobility of As,Fe,and S from alluvial sediment to groundwater were investigated in bioreactor experiments.Two particular sediments from the alluvial floodplain of the Mekong Delta River were investigated:Matrix A(14 m deep)had a higher pyrite concentration than matrix B(7 m deep)sediments.Gypsum was present in matrix B but absent in matrix A.In the reactors,the sediment suspensions were supplemented with As(Ⅲ)and SO_4~(2-),and were subjected to three full-redox cycles entailing phases of nitrogen/CO_2,compressed air sparging,and cellobiose addition.Major differences in As concentration and speciation were observed upon redox cycling.Evidences support the fact that initial sediment composition is the main factor controlling arsenic release and its speciation during the redox cycles.Indeed,a high pyrite content associated with a low SO_4~(2-)content resulted in an increase in dissolved As concentrations,mainly in the form of As(Ⅲ),after anoxic half-cycles;whereas a decrease in As concentrations mainly in the form of As(Ⅴ),was instead observed after oxic half-cycles.In addition,oxic conditions were found to be responsible for pyrite and arsenian pyrite oxidation,increasing the As pool available for mobilization.The same processes seem to occur in sediment with the presence of gypsum,but,in this case,dissolved As were sequestered by biotic or abiotic redox reactions occurring in the Fe—S system,and by specific physico-chemical condition(e.g.pH).The contrasting results obtained for two sediments sampled from the same core show that many complexes and entangled factors are at work,and further refinement is needed to explain the spatial and temporal variability of As release to groundwater of the Mekong River Delta(Vietnam).     

The biogeochemical cycle of iron and associated elements in Lake Kinneret

Iron biogeochemical cycling and distribution between particulate, reactive (colloidal + dissolved, oxine-labile, Fe(II)) fractions were studied in the seasonally stratified, mesotrophic Lake Kinneret. This article presents various aspects of the Fe budget in the lake and relates them to the chemical reactivity of various physicochemical forms of Fe.The budget of Fe in Lake Kinneret is dominated by fluvial Fe load, rather than by internal recycling of Fe from the sediment, as shown by the fact that 75 to 94% of the variance in Fe concentrations in the lake can be explained by the fluctuation in the water discharge of the Jordan River. Iron associated with phytoplankton accounts for 9-16% of the bulk particulate Fe in the lake. However, within patches of the dominate algae, the dinoflagellate Peridinium gatunense, algal Fe accounts for more than 70% of the lake’s particulate Fe. The algal Fe is predominantly intracellular, and the Fe: chlorophyll a ratios were within the range of published values for dinoflagellates and cyanobacteria. Iron associated with particles larger than 0.025 μm (20-300 nM) accounts for 80-95% of total Fe in the epilimnion of Lake Kinneret throughout the year. In contrast, this fraction of Fe is dominant in the hypolimnion only during the period of lake mixing. Iron concentration of different size fractions (<0.025 μm, <0.2 μm and >0.025 μm) in the surface water covaried throughout the research period. These covariations suggest dynamic transformations of Fe between different size fractions, either due to partial dissolution/precipitation or desorption/adsorption. Oxine-labile Fe concentrations, the Fe fraction considered chemically labile and available for phytoplankton, ranged from 15 to 75 nM. In wintertime, the oxine-labile Fe accounts for only 10-20% of the total Fe, while in other seasons most of the Fe is oxine-labile. Oxine-labile Fe concentrations always exceed the 0.025 μm-filtered Fe, implying that some of the larger particles contain oxine labile-Fe, and therefore are reactive. The fraction of reactive particles in Lake Kinneret (10-80%) is high relative to that of the marine environments, and can most likely be attributed to its fluvial source, which contains a significant proportion of reactive Fe oxide and hydrous oxide particles.The annual variability in the epilimnetic concentrations of other trace elements and nutrients, such as Al, Mn, Cd, Zn, Pb, and P were studied and grouped according to their resemblance with river water discharge, redox processes, or biological uptake and scavenging. Distribution patterns of Al, Pb and Cd resembled that of Fe, suggesting that similar processes control the concentrations of these metals, namely significant contribution from the watershed, high affinity to particulate matter and little control of biota on their fate in the lake. Other elements such as Zn and P are more affected by biological uptake, while Mn is more affected by redox cycling within the lake.     

Particulate iron and manganese in the Santa Barbara Basin,California

Particulate Fe and Mn may be important trace metal scavengers in the water column as well as being probable indicators of biologically mediated redox processes. A study has been made of suspended particulate composition in the Santa Barbara Basin, a shallow near-shore basin off southern California with sub-oxic conditions below sill depth. Observations have revealed several interesting phenomena relating to the geochemistry of Fe and Mn. Most striking is a profound enrichment of particulate Fe in samples from the bottom two hundred meters. These particulates have a constant Fe/P mole ratio of about three and may originate at the sediment-water interface or may be transported to the basin from local marshes. For particulate Mn, enrichments are observed both in the sub-sill waters and near the base of the euphotic zone. A consideration of particle removal rates suggests that the sub-photic zone enrichment has a biogenic origin. In the sub-sill waters, enrichment in Mn is apparently due to the precipitation of dissolved Mn diffusing from the anoxic basin sediments. A simple mass balance suggests that most of the Mn lost from the sediments is transported from the Santa Barbara Basin in dissolved form.     

Source/process apportionment of major and trace elements in sinking particles in the Sargasso sea

Elemental composition of the particle flux at the Oceanic Flux Program (OFP) time-series site off Bermuda was measured from January 2002 to March 2005. Eighteen elements (Mg, Al, Si, P, Ca, Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Sr, Cd, Ba and Pb) in sediment trap material from 500, 1500 and 3200 m depths were quantified using fusion-HR-ICPMS. Positive Matrix Factorization (PMF) was used to elucidate sources, elemental associations and processes that affect geochemical behavior in the water column.Results provide evidence for intense elemental cycling between the sinking flux material and the dissolved and suspended pools within mesopelagic and bathypelagic waters. Biological processing and remineralization rapidly deplete the sinking flux material in organic matter and associated elements (N, P, Cd, Zn) between 500 and 1500 m depth. Suspended particle aggregation, authigenic mineral precipitation, and chemical scavenging enriches the flux material in lithogenic minerals, barite and redox sensitive elements (Mn, Co, V, Fe). A large increase in the flux of lithogenic elements is observed with depth and confirms that the northeast Sargasso is a significant sink for advected continental materials, likely supplied via Gulf Stream circulation.PMF resolved major sources that contribute to sinking flux at all depths (carbonate, high-Mg carbonate, opal, organic matter, lithogenic material, and barite) as well as additional depth-specific elemental associations that contribute about half of the compositional variability in the flux. PMF solutions indicate close geochemical associations of barite-opal, Cd-P, Zn-Co, Zn-Pb and redox sensitive elements in the sinking flux material at 500 m depth. Major reorganizations of element associations occur as labile carrier phases break down and elements redistribute among new carrier phases deeper in the water column.Factor scores show strong covariation and similar temporal phasing among the three trap depths and indicate a tight coupling in particle flux compositional variability throughout the water column. Seasonality in flux composition is primarily driven by dilution of the lithogenic component with freshly-produced biogenic material during the late winter primary production maximum. Temporal trends in scores reveal subtle non-seasonal changes in flux composition occurring on month long timescales. This non-seasonal variability may be driven by changes in the biogeochemical properties of intermediate water masses that pass through the region and which affect rates of chemical scavenging and/or aggregation within the water column.     

Deciphering As and Cu cycling in sediment pore waters in a large marina (Port Camargue,southern France) using a multi-tracer (Fe,Mn, U,Mo) approach

The sediments of the Port Camargue marina (South of France) are highly polluted by Cu and As (Briant et al., 2013). The dynamics of these pollutants in pore waters was investigated using redox tracers (sulfides, Fe, Mn, U, Mo) to better constrain the redox conditions.In summer, pore water profiles showed a steep redox gradient in the top 24 cm with the reduction of Fe and Mn oxy-hydroxides at the sediment water interface (SWI) and of sulfate immediately below. Below a depth of 24 cm, the Fe, Mn, Mo and U profiles in pore waters reflected Fe and Mn reducing conditions and, unlike in the overlying levels, sulfidic conditions were not observed. This unusual redox zonation was attributed to the occurrence of two distinct sediment layers: an upper layer comprising muddy organic-rich sediments underlain by a layer of relatively sandy and organic-poor sediments. The sandy sediments were in place before the building of the marina, whereas the muddy layer was deposited later. In the muddy layer, large quantities of Fe and Mo were removed in summer linked to the formation of insoluble sulfide phases. Mn, which can adsorb on Fe-sulfides or precipitate with carbonates, was also removed from pore waters. Uranium was removed probably through reduction and adsorption onto particles. In winter, in the absence of detectable pore water sulfides, removal of Mo was moderate compared to summer.Cu was released into solution at the sediment water interface but was efficiently trapped by the muddy layer, probably by precipitation with sulfides. Due to efficient trapping, today the Cu sediment profile reflects the increase in its use as a biocide in antifouling paints over the last 40 years.In the sandy layer, Fe, Mn, Mo and As were released into solution and diffused toward the top of the profile. They precipitated at the boundary between the muddy and sandy layers. This precipitation accounts for the high (75 μg g⁻¹) As concentrations measured in the sediments at a depth of 24 cm.