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1.
Manganese is a major redox reactive element of benthic metabolism. We have built a database of existing knowledge on the benthic geochemistry of Mn in the Bay of Biscay, in order to comprehensively assess the behaviour of Mn in a variety of environments during early diagenesis. The database contains vertical profiles of particulate and dissolved Mn species of 59 cores collected during 17 cruises between 1997 and 2006 at nine stations positioned between 140 and 4,800 m water depths. At all studied stations, Mn species follow the conventional distribution, where Mn(III,IV) species are enriched in the oxic layer, and dissolved Mn is present in the anoxic sediments. A minor part of Mn-oxides originates from sedimenting particles. The major part is of diagenetic origin, and derives from the oxidation of upward-diffusing dissolved Mn(II). Mn-oxide inventories are higher at the deeper stations than at the shallower ones. This difference cannot be attributed to different sources of sedimenting particles, but it must depend on sedimentation rate and diagenetic processes. At depth, dissolved Mn(II) concentrations are constant. This probably reflects equilibrium with an authigenic Mn(II) phase, which is the ultimate phase into which Mn is fossilized. The Mn content of deeper anoxic sediments is similarly low in all the cores studied, associated with corresponding trends of Mn content in sedimenting particles of the Bay of Biscay. Bioturbation, rather than redox oscillations, can convey Mn(III,IV) species downwards into the anoxic sediments where they are reduced, associated with a peak of dissolved Mn. Because dissolved Mn(II) is re-oxidized when it diffuses towards the oxic layer, the inventory of the diagenetic Mn(III,IV) phase remains at steady state, especially at stations where the oxic layer is thick. It then becomes possible to calculate the residence time of diagenetic Mn(III,IV) particles within the oxic layer, using the upward-directed flux of pore water Mn(II). By applying this residence time to the accumulation of sediments within the oxic layer, we obtain the sediment mass accumulation rate. The values calculated for the sediments of the Bay of Biscay fit well with accumulation rates obtained from radionuclides or sediment traps. The method has also been validated with data collected in other marine sedimentary environments. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

2.
Methylmercury (MeHg) concentration and production rates were studied in bottom sediments along the mainstem of Chesapeake Bay and on the adjoining continental shelf and slope. Our objectives were to 1) observe spatial and temporal changes in total mercury (HgT) and MeHg concentrations in the mid-Atlantic coastal region, 2) investigate biogeochemical factors that affect MeHg production, and 3) examine the potential of these sediments as sources of MeHg to coastal and open waters. Estuarine, shelf and slope sediments contained on average 0.5 to 1.5% Hg as MeHg (% MeHg), which increased significantly with salinity across our study site, with weak seasonal trends. Methylation rate constants (kmeth), estimated using enriched stable mercury isotope spikes to intact cores, showed a similar, but weaker, salinity trend, but strong seasonality, and was highly correlated with % MeHg. Together, these patterns suggest that some fraction of MeHg is preserved thru seasons, as found by others [Orihel, D.M., Paterson, M.J., Blanchfield, P.J., Bodaly, R.A., Gilmour, C.C., Hintelmann, H., 2008. Temporal changes in the distribution, methylation, and bioaccumulation of newly deposited mercury in an aquatic ecosystem. Environmental Pollution 154, 77] Similar to other ecosystems, methylation was most favored in sediment depth horizons where sulfate was available, but sulfide concentrations were low (between 0.1 and 10 μM). MeHg production was maximal at the sediment surface in the organic sediments of the upper and mid Bay where oxygen penetration was small, but was found at increasingly deeper depths, and across a wider vertical range, as salinity increased, where oxygen penetration was deeper. Vertical trends in MeHg production mirrored the deeper, vertically expanded redox boundary layers in these offshore sediments. The organic content of the sediments had a strong impact on the sediment:water partitioning of Hg, and therefore, on methylation rates. However, the HgT distribution coefficient (KD) normalized to organic matter varied by more than an order of magnitude across the study area, suggesting an important role of organic matter quality in Hg sequestration. We hypothesize that the lower sulfur content organic matter of shelf and slope sediments has a lower binding capacity for Hg resulting in higher MeHg production, relative to sediments in the estuary. Substantially higher MeHg concentrations in pore water relative to the water column indicate all sites are sources of MeHg to the water column throughout the seasons studied. Calculated diffusional fluxes for MeHg averaged  1 pmol m− 2 day− 1. It is likely that the total MeHg flux in sediments of the lower Bay and continental margin are significantly higher than their estimated diffusive fluxes due to enhanced MeHg mobilization by biological and/or physical processes. Our flux estimates across the full salinity gradient of Chesapeake Bay and its adjacent slope and shelf strongly suggest that the flux from coastal sediments is of the same order as other sources and contributes substantially to the coastal MeHg budget.  相似文献   

3.
The uppermost 5–6 cm of the sediments (between 8 and 2248 m water depths) were studied to understand the effects of varying redox conditions on the Mn distribution in the recent sediments of the Black Sea. It was found that most Mn concentrations are consistent with the average abundance in crustal and/or sedimentary rocks. There exist no important differences between Mn concentrations in oxic (shallower water; <70 m) and anoxic (deeper water; 120 m) sediments. Previously reported Mn-enrichment above the Black Sea oxic/anoxic interface, due to the peculiar redox cycling, shows no significant contribution of Mn to the bottom sediments. A marked relationship between total Mn concentrations and clay/mud contents at shelf depth along the southern Black Sea margin indicates increased accumulation of Mn in association with the fine-grained particles and eastward water circulation.  相似文献   

4.
Depth profiles of the naturally-occurring radionuclides 238U, 234U, 226Ra, 228Ra and 228Th were obtained in two diverse anoxic marine environments; the permanently anoxic Framvaren Fjord in southern Norway and the intermittently anoxic Saanich Inlet in British Columbia. Concentrations of total H2S were over three orders of magnitude greater in the anoxic bottom waters of Framvaren Fjord compared to those in Saanich Inlet.In Framvaren Fjord, the O2/H2S interface was located at 17 m. While dissolved 238U behaved conservatively throughout the oxic and anoxic water columns, concentrations based on the 238U/salinity ratio in oxic oceanic waters were almost 30% lower. Dissolved 226Ra displayed a sharp maximum just below the O2/H2S interface, coinciding with dissolved Mn (II) and Fe (II) maxima in this zone. It is suggested that reductive dissolution of Fe-Mn oxyhydroxides remobilizes 226Ra in this region.In Saanich Inlet, the O2/H2S interface was located at 175 m. Dissolved 238U displayed a strongly nonconservative distribution. The depth profiles of dissolved 226Ra and 228Th correlated well with the distribution of dissolved Mn (II) in the suboxic waters above the O2/H2S interface, suggesting that reduction of particulate Mn regulates the behavior of 226Ra and 228Th in this region.Removal residence times for dissolved 228Th in the surface oxic waters of both systems are longer than those generally reported for particle-reactive radionuclides in coastal marine environments. In the anoxic waters of Framvaren Fjord and Saanich Inlet, however, the dissolved 228Th removal residence times are quite similar to values reported for dissolved 210Pb in the anoxic waters of the Cariaco Trench and the Orca Basin. This implies that the geochemistries of Th and Pb may be similar in anoxic marine waters.  相似文献   

5.
Marine sediments represent a major carbon reservoir on Earth. Dissolved organic matter(DOM) in pore waters accumulates products and intermediates of carbon cycling in sediments. The application of excitation-emission matrix spectroscopy(EEMs) in the analysis of subseafloor DOM samples is largely unexplored due to the redoxsensitive matrix of anoxic pore water. Therefore, this study aims to investigate the interference caused by the matrix on EEMs and propose a guideline to prepare pore water sam...  相似文献   

6.
Anoxic sulfidic waters provide important media for studying the effect of reducing conditions on the cycling of trace metals. In 1987–1988, dissolved and particulate trace metal (Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb) concentrations were determined in the water column of the anoxic Drammensfjord basins, southeastern Norway. The iminodiacetic acid type chelating resin (Chelex 100) was used for the preconcentration of trace metals. The trace metal concentrations were determined using atomic absorption spectrophotometry (AAS), differential pulse polarography (DPP), and differential pulse-anodic stripping voltammetry (DP-ASV).It was observed that the trace metals Mn and Fe were actively involved in the processes of redox cycling (oxidationreduction and precipitation-dissolution) at the O2/H2S interface. The dissolved concentrations of Mn, Fe and Co showed maxima just below the O2/H2S interface. The seasonal enhancement in the maxima of both dissolved and particulate Mn and Fe at the redox cline is mainly governed by the downward movement of water which carries oxygen. An association of Co with the Mn cycle was observed, while the total dissolved Ni was decreased by only 10–35% in the anoxic waters. The dissolved concentrations of Cu, Zn, Pb and, to a lesser extent, Cd decreased in the anoxic zone.  相似文献   

7.
In order to understand the role of sulfate and Fe(III) reduction processes in the net production of monomethylmercury (MMHg), we amended anoxic sediment slurries collected from the Venice Lagoon, Italy, with inorganic Hg and either potential electron acceptors or metabolic byproducts of sulfate and Fe(III) reduction processes, gradually changing their concentrations. Addition of sulfide (final concentration: 0.2–6.3 mM) resulted in an exponential decrease in the sulfate reduction rate and MMHg concentration with increasing concentrations of sulfide. Based on this result, we argue that the concentration of dissolved sulfide is a critical factor controlling the sulfate reduction rate, and in turn, the net MMHg production at steady state. Addition of either Fe(II) (added concentration: 0–6.1 mM) or Fe(III) (added concentration: 0–3.5 mM) resulted in similar trends in the MMHg concentration, an increase with low levels of Fe additions and a subsequent decrease with high levels of Fe additions. The limited availability of dissolved Hg, associated with sulfide removal by precipitation of FeS, appears to inhibit the net MMHg production in high levels of Fe additions. There was a noticeable reduction in the net MMHg production in Fe(III)-amended slurries as compared to Fe(II)-amended ones, which could be caused by a decrease in the sulfate reduction rate. This agrees with the results of Hg methylation assays using the enrichment cultures of anaerobic bacteria: whereas the enrichment cultures of sulfate reducers showed significant production of MMHg (4.6% of amended Hg), those of Fe(III), Mn(IV), and nitrate reducers showed no production of MMHg. It appears that enhanced Fe(III)-reduction activities suppress the formation of MMHg in high sulfate estuarine sediments.  相似文献   

8.
The objective of this study was to investigate the effects of resuspension on the fate and bioaccumulation of mercury (THg) and methylmercury (MeHg) in shallow estuarine environments, using mesocosms. Two 4-week experiments were conducted in July (Experiment 1) and October (Experiment 2) of 2001 with Baltimore Harbor sediments. Hard clams, Mercenaria mercenaria, were introduced into sediments for Experiment 2. Tidal resuspension (4 h on and 2 h off cycles) was simulated, with 3 replicate tanks for each treatment—resuspension (R) and non-resuspension (NR). Sediment cores were collected during the experiments for THg, MeHg, organic content and AVS analyses, and for the determination of methylation/demethylation using Hg stable isotopes (199Hg(II) and CH3199Hg(II)). Zooplankton samples were collected once a week while clams were taken before and after Experiment 2 for THg and MeHg analyses. Our results suggest that the interplay between Hg methylation and MeHg degradation determines the overall MeHg pool in sediments. Sediment resuspension does not appear to directly impact the Hg transformations but can lead to changes in the association to Hg binding phases, influencing Hg methylation. The bioaccumulation results indicate that sediment resuspension can play an important role in transferring sediment MeHg into organisms.  相似文献   

9.
通过分析长江河口湿地典型植物根际沉积物柱样(0～40 cm)中总汞(THg)、甲基汞(MeHg)及其与粒度、总有机碳(TOC)、还原态硫等环境因子之间的关系,探讨了互花米草(Spartina alterniflora)入侵对沉积物中汞形态特征的影响及主控因子。结果表明:(1)不同植物(互花米草、芦苇(Phragmites communis)、海三棱藨草(Scirpus mariqueter)和水葱(Scirpus tabernaemontani))根际沉积物中THg均值为49.9～100.9 μg/kg,其与粒径小于16 μm颗粒物组分体积百分比及TOC含量之间存在显著正相关关系(r2=0.85,p<0.01;r2=0.58,p<0.01),这意味着沉积物中矿物?有机物复合体细颗粒物的空间分异决定着总汞的空间分异。互花米草入侵促进了细颗粒的沉积,进而间接促进了沉积物中总汞含量的增加。(2)不同植物根际沉积物中MeHg均值为0.3～1.4 μg/kg,MeHg/THg均值为0.4%～1.4%,互花米草、芦苇及海三棱藨草根际沉积物中MeHg含量及MeHg/THg值随深度增加不断减小,但无显著差异,表明了互花米草入侵对沉积物中汞甲基化过程的影响可能有限。Pearson相关分析表明,MeHg/THg与THg、TOC、酸挥发性硫之间不存在显著的正相关关系。硫的K边同步辐射结果进一步表明了硫形态(如有机硫和S2?)变化与MeHg变化关系不大。MeHg/THg值呈表层(0～8 cm)高,底层低的分布规律,表明了表层沉积物中汞的甲基化潜势较大,这可能与表层新鲜有机质(如藻类和植物凋落物)的不断供给及其降解过程密切相关,还需深入研究。  相似文献   

10.
Between 1980 and 1984 extensive studies were carried out in the Baltic Sea on trace metals (Cd, Co, Cu, Fe, Hg, Mn, Ni, Pb and Zn) in water, suspended matter and sediments. The results enabled the influence of different factors on metal distribution patterns to be considered. The vertical profiles of dissolved and particulate metals in waters of the central deep basins reflect influences caused by oxygen deficiency and anoxic conditions in near-bottom water layers. Peculiarities at Station BY15 in the Gotland Deep included high dissolved Fe, Mn and Co concentrations and remarkable enrichment of Zn (0.64%), Cd (51 μg g−1) and Cu (0.15%) in particulate matter from the anoxic zone. Manganese-rich particles were accumulated above this layer.In fine-grained soft sediments below anoxic deep waters, maximum contents of Cd, Cu and Zn were observed, relative to other coring sites, between Bothnian Bay and Lübeck Bight. The Hg content in sediments probably reflects the joint flocculation with organic matter. Land-based sources seem to play the leading part for maximum lead contents.  相似文献   

11.
《Marine Chemistry》2001,73(3-4):215-231
In-situ benthic flux studies were conducted at three stations in Upper Galveston Bay twice during March 1996 to directly measure release rates of dissolved Mn, Fe, Ni and Zn from the sediments. Results showed reproducible increases with time in both replicate light and light–dark benthic chambers, resulting in average fluxes of −1200±780, −17±12, −1.6±0.6 and −2.4±0.79 μmol m−2 day−1 for Mn, Fe, Ni and Zn, respectively. Sediment cores collected during 1994–1996 showed that surficial pore water concentrations were elevated compared to overlying water column concentrations, suggesting diffusive release from the sediments. Diffusive flux estimates of Mn and Zn agreed in direction with chamber fluxes measured on the same date, but only accounted for 5–38% of the measured flux. Diffusive fluxes of Fe agreed with measured fluxes at the near Trinity River station but overestimated actual release in the mid and outer Trinity Bay regions, possibly due to inaccurate determination of the Fe pore water gradients or rapid oxidation processes in the overlying water at these stations.In general, measured fluxes of Mn and Ni were higher in the mid Trinity Bay region and suggested a mechanism for the elevated trace metal concentrations previously reported for this region of Galveston Bay. However, the fluxes of Fe were highest in close proximity to the Trinity River, supporting the elevated Fe concentrations measured in this region during this and other studies, and decreased towards middle and outer Trinity Bay. Trace metal turnover times were between 0.1 and 1.2 days for Mn, between 1.3 and 4.6 days for Fe, and between 27 and 100 days for Ni and 12–20 days Zn, and were considerably shorter than the average Trinity Bay water residence time (1.5 years) for this period. Comparing area averaged benthic inputs to Trinity River inputs shows the sediments to be a significant source of trace metals to Galveston Bay. However, while benthic inputs of trace metals were measured, water column concentrations remained low despite rapid turnover times for Mn and Fe, suggesting removal of these metals from the water column after release from the sediments.  相似文献   

12.
In June 1981, dissolved Zn, Cd, Cu, Ni, Co, Fe, and Mn were determined from two detailed profiles in anoxic Baltic waters (with extra data for Fe and Mn from August 1979). Dramatic changes across the O2H2S interface occur in the abundances of Cu, Co, Fe, and Mn (by factors of ?100). The concentrations of Zn, Cd, and Ni at the redox front decrease by factors between 3 to 5.Equilibrium calculations are presented for varying concentrations of hydrogen sulfide and compared with the field data. The study strongly supports the assumption that the solubility of Zn, Cd, Cu, and Ni is greatly enhanced and controlled by the formation of bisulfide and(or) polysulfide complexes. Differences between predicted and measured concentrations of these elements are mainly evident at lower ΣH2S concentrations.Cobalt proved to be very mobile in anoxic regions, and the results indicate that the concentrations are limited by CoS precipitation. The iron (Fe2+) and manganese (Mn2+) distribution in sulfide-containing waters is controlled by total flux from sediment-water interfaces rather than by equilibrium concentrations of their solid phases (FeS and MnCO3). The concentrations of these metals are therefore expected to increase with prolonged stagnation periods in the basin.  相似文献   

13.
The early diagenesis of trace elements (V, Cr, Co, Cu, Zn, As, Cd, Ba, U) in anoxic sediments of the Achterwasser, a shallow lagoon in the non-tidal Oder estuary in the Baltic Sea, was investigated in the context of pyrite formation. The dissolved major redox parameters show a two-tier distribution with transient signals in the occasionally re-suspended fluid mud layer (FM) and a permanently established diagenetic sequence in the sediment below. Intense microbial respiration leads to rapid depletion of O2 within the uppermost mm of the FM. The reduction zones of Mn, Fe and sulfate overlap in the FM and in the permanently anoxic sediment section which appears to be a typical feature of estuarine sediments, under low-sulfate conditions. Degrees of pyritization (DOP) range from 50% in the FM to remarkably high values > 90% at 50 cm depth. Pyrite formation at the sediment surface is attributed to the reaction of Fe-monosulfides with intermediate sulfur species via the polysulfide pathway. By contrast, intense pyritization in the permanently anoxic sediment below is attributed to mineral growth via adsorption of aqueous Fe-sulfide complexes onto pyrite crystals which had originally formed in the surface layer.The studied trace elements show differential behavior patterns which are closely coupled to the diagenetic processes described above: (i) Zn, Cu and Cd are liberated from organic matter in the thin oxic layer of the sediment and diffuse both upwards across the sediment/water boundary and downwards to be trapped as monosulfides, (ii) V, Cr, Co and As are released during reductive dissolution of Mn- and Fe-oxyhydroxides, (iii) U removal from pore water occurs concomitantly to Fe reduction in the FM and is attributed to reduction of U(VI) to U(IV), (iv) the Ba distribution is controlled by reductive dissolution of authigenic barite in the sulfate reduction zone coupled with upward diffusion and re-precipitation. The incorporation of trace elements into pyrite is most intense for Co, Mn and As, intermediate for Cu and Cr and little to negligible for U, Zn, Cd, V and Ba. The observed trend is largely in agreement with previous studies and may be explained with differing rates for ligand exchange. Slow and fast ligand exchange and thus precipitation kinetics are also displayed by downcore increasing (Mn, Cr, Co and As) or constantly low (Zn, Cu, Cd) pore water concentrations. The downward increasing degrees of trace metal pyritization (DTMP) for Co, Cu, Zn and As are, in analogy to pyrite growth, assigned to adsorption of sulfide complexes or As oxyanions onto preexisting pyrite minerals.  相似文献   

14.
We report a simplified synthesis, and some performance characteristics, for 8-hydroxyquinoline (8-HOQ) covalently bonded to a chemically resistant TosoHaas TSK vinyl polymer resin. The resin was used to concentrate trace metals from stored, acidified seawater samples collected from Jellyfish Lake, an anoxic marine lake in the Palau Islands. The Mn, Fe, and Zn profiles determined from the 8-HOQ resin extraction were similar to those determined using Chelex-100 resin. The Zn and Cd profiles did not exhibit removal by sulfide “stripping” in contrast to other anoxic marine basins. The profiles of Co and Ni also exhibited elevated concentrations in the anoxic hypolimnion. The solution speciation and saturation states for the metals were calculated using revised metal-bisulfide stability constants. The calculations suggest that the MS(HS)− species dominates the solution speciation for Mn, Co, Ni, Zn, Cd, and Pb. Cu(I) is modeled as the CuS− or Cu(HS)2− species, while Fe(II) behaves as the free Fe2+ cation. The Mn, Co, Ni, Cu and Cd concentrations appeared to be at least 10-fold undersaturated, while the Fe(II), Zn, and Pb concentrations were close to saturation with respect to their metal sulfides.  相似文献   

15.
Dissolved and particulate trace metals (Cu, Cd, Pb, Zn, Ni, Fe and Mn) measured at six stations along the Scheldt estuary in October/November 1978 are compared with more recent data. Based on Ca content in the suspended matter, three distinct geochemical regions could be distinguished: the upper estuary (salinity 1–7) dominated by fluvial mud, mid-estuary (salinity 7–17) where the composition of the suspended matter remained relatively constant, and the lower estuary where marine mud prevailed. Re-suspension of sediments is the major factor controlling the composition of the particles in the upstream region. Anoxic conditions prevailed in the upper part of the estuary extending to a salinity of 15 in 1978, while at present the seaward boundary of the anoxic water body is located at less saline waters. Furthermore, the present-day metal load is much lower than in 1978. As a consequence of the changed situation, maxima in dissolved concentrations of redox-sensitive metals in the mid/lower estuary have moved as well, which affects the trace metal re-distribution pattern. In the anoxic zone, exchange processes between dissolved and particulate metal fractions were strongly redox regulated, with Fe and Mn as excellent examples. Iron was removed from the dissolved phase in the early stages of mixing resulting in an increase in the suspended particulate matter of the leachable ‘non-residual' Fe fraction from 2 to 3.5%. Due to its slower kinetics, removal of Mn from solution occurred in mid-estuary where oxygen concentrations increased. Cu, Cd and Zn on the contrary were mobilised from the suspended particles during estuarine mixing. External inputs of Pb, and to a lesser extent of Cu, in the lower estuary resulted in the increase of their particulate and the dissolved concentrations. Calculated Kd (distribution coefficient) values were used to assess the redistribution between the dissolved and particulate phase of the investigated metals. Due to the existence of the anoxic water body in the upper estuary, the importance of redox processes in determining the Kd values could be demonstrated. The sequence of Kd values in the upper estuary (Fe, Cd, Zn, Pb > Cu > Ni, Mn) is significantly different from that in the lower estuary (Fe > Mn > Pb, Ni, Zn, Cu, Cd). Thus, in such a dynamic estuary single metal-specific Kd values cannot be used to describe redistribution processes.  相似文献   

16.
Mercury speciation was performed in excess activated sewage sludge (ASS) and in marine sediments collected at the AAS disposal site off the Mediterranean coast of Israel in order to characterize the spatial and vertical distribution of different mercury species and assess their environmental impact. Total Hg (HgT) concentrations ranged between 0.19 and 1003ng/g at the polluted stations and 5.7 and 72.8ng/g at the background station, while the average concentration in ASS was 1181+/-273ng/g. Only at the polluted stations did HgT concentrations decrease exponentially with sediment depth, reaching background values at 16-20cm, the vertical distribution resulting from mixing of natural sediment with ASS solids and bioturbation by large populations of polycheates. Average Methyl Hg (MeHg) concentration in ASS was 39.7+/-7.1ng/g, ca. 3% of the HgT concentration, while the background concentrations ranged between 0.1 and 0.61ng/g. MeHg concentrations in surficial polluted sediments were 0.7-5.9ng/g (ca. 0.5% of the HgT) and decreased vertically, similar to HgT. A positive correlation between MeHg and Hg only at the polluted stations, higher MeHg concentrations at the surface of the sediment and not below the redoxline, and no seasonality in the concentrations suggest that the MeHg originated from the ASS and not from in situ methylation. By doing selective extractions, we found that ca. 80% of the total Hg in ASS and polluted sediments was strongly bound to amorphous organo-sulfur and to inorganic sulfide species that are not bioavailable. The fractions with potential bioaccessible Hg had maximal concentrations in the range in which biotic effects should be expected. Therefore, although no bioaccumulation was found in the biota in the area, the concentration in the polluted sediments are not negligible and should be carefully monitored.  相似文献   

17.
Iron-rich concretions are frequently found around plant roots in Tagus estuary (Portugal) where radial delivery of O2 takes place. Salt marsh sediments exhibit cracks that are an additional feature to introduce O2 and other solutes in the upper sediments. Metal concentrations in salt marsh sediments are clearly above the background levels reflecting the anthropogenic sources from a large city with 2.5 million inhabitants, and several industrial centres. In order to evaluate how both oxidised structures influences the redistribution of redox sensitive elements in salt marsh sediments, concretions were collected from roots of Halimione portucaloides below the oxygenated zone. These tubular cylindrical structures were analysed for Fe, Al, Mn, As, and P along 1-cm radial transect in a millimetre scale from the inner part to the adjacent anoxic sediment. In addition, oxidised cracks were analysed for the same spatial resolution, from the sediment–water interface to anoxic layers (2-cm transept). The parallelism between Fe, As, and P concentrations at this microscale is the most noticeable aspect. Iron and As presented very high concentrations in the 4-mm concretions (3.4 mmol g−1 and 3.1 μmol g−1, respectively) and decreased sharply to the host sediment. Oxygen released from roots oxidise the solid sulphides, and the reduced Fe and As are transported towards the root by both diffusion and pore water flow associated with the root water uptake. Subsequently, Fe(III) precipitates and As is retained by sorption and/or coprecipitation. These elements are also enriched in the first 2-mm of oxidised cracks, but in lower concentrations (50% and 30%, respectively). Manganese concentrations in concretions were low (11.8 μmol g−1), indicating that Fe dominates the sediment chemistry. Phosphorus and iron concentrations in the ascorbate fraction were higher in the oxidising surfaces of concretions (10.7 μmol g−1 and 1.6 mmol g−1, respectively) and of cracks (5.1 μmol g−1 and 0.47 mmol g−1). The parallelism of Fe and As distributions includes not only their similar redox chemistries, but also that to phosphate, including control by coprecipitation of the host iron phases. The mechanisms involved in the mobilisation of As and P are however different, whereas As comes from the oxidation of iron sulphides; dissolved P derives from reduction of ferri-hydroxide phases.  相似文献   

18.
In situ experiments using isotopically labeled mercury species (199Hg(II) and Me201Hg) are used to investigate mercury transformation mechanisms, such as methylation, demethylation and reduction, in coastal and marine surface waters of the Mediterranean Sea. The aim of this work is to assess the relative contribution of photochemical versus biological processes to Hg transformation mechanisms. For this purpose, potential transformation rates measured under diurnal and dark incubation conditions are compared with major biogeochemical parameters (i.e. hydrological and biological data) in order to obtain the relative contribution of various biotic and abiotic mechanisms in both surface (high light) and bottom (low light) waters of the euphotic zone. The results demonstrate that coastal and marine euphotic zones are significant reactors for all Hg transformations investigated (i.e. methylation, demethylation, reduction). A major outcome demonstrates that Hg methylation is taking place in oxic surface seawater (0.3–6.3% day− 1) and is mainly influenced by pelagic microorganism abundance and activities (phyto- and bacterioplankton). This evidences a new potential MeHg source in the marine water column, especially in oligotrophic deep-sea basins in which biogeochemistry is mostly governed by heterotrophic activity. For coastal and marine surface waters, although MeHg is mainly photochemically degraded (6.4–24.5% day− 1), demethylation yields observed under dark condition may be attributed to microbial or chemical pathways (2.8–10.9% day− 1). Photoreduction and photochemical reactions are the major mechanisms involved in DGM production for surface waters (3.2–16.9% day− 1) but bacterial or phytoplanktonic reduction of Hg(II) cannot be excluded deeper in the euphotic zone (2.2–12.3% day− 1). At the bottom of the euphotic zone, photochemical processes are thus avoided due to the attenuation of UV-visible sunlight radiation allowing biotic processes to be the most significant. These results suggest a new potential route for Hg species cycling in surface seawater and especially at the maximum biomass depth located at the bottom of the euphotic zone (i.e. maximum chlorophyll fluorescence). In this environment, DGM production and demethylation mechanisms are thus probably reduced whereas Hg methylation is enhanced by autotrophic and heterotrophic processes. Experimental results on mercury species uptake during these investigations further evidenced the strong affinity of MeHg for biogenic particles (i.e. microorganisms) that correspond to the first trophic level of the pelagic food web.  相似文献   

19.
A series of high resolution (10 cm) vertical profiles of iron were determined across the oxic/anoxic boundary in the Lower Pond of the Pettaquamscutt Estuary. Selective chemical treatments and multiple analytical methods were used to detemine the oxidation state and lability of iron across the oxic/anoxic boundary. The vertical distributions of dissolved and total iron were determined by atomic absorption spectroscopy, and dissolved Fe(II) and reducible iron were determined using a modified Ferrozine spectrophotometric method. Well-developed maxima of total dissolved iron ≈7·5 μM occurred within the oxic/anoxic transition zone. Analysis of Fe(II) by the FZ method indicates that more than 95% of the dissolved iron determined by atomic absorption spectroscopy within the maximum is in the form of Fe(II). The concentration of dissolved Fe(II) ranged from <4 nM in oxygenated surface waters to between 7 and 8 μM at the total dissolved iron maximum.Both dissolved and total iron samples were treated with ascorbic acid to quantify the fraction of iron that was reducible in this system. Dissolved iron is quantitatively reduced to Fe(II) by 3·5 m depth, and particulate iron was almost completely dissolved by 6 m. Thermodynamic speciation calculations indicate that the dominant species of Fe(II) in the anoxic waters is the Fe(HS)+complex. In addition, the concentration of Fe(II) in the anoxic zone appears to be controlled by precipitation of a sulfide phase, the ion activity product for waters below 7 m is in good agreement with the solubility product of mackinawite.The vertical distribution of oxidation states of the metals indicates non-equilibrium conditions due to microbiological and chemical processes occurring in the redox transition zone. A one-dimensional vertical, eddy diffusion model is presented that incorporates redox reactions of iron, sulfide and oxygen. The modeling suggests the maximum in Fe(II) can be achieved through inorganic oxidation and reduction reactions, however the depth at which the maximum occurs is sensitive to sulfide oxidation, which appears to be dominated by biological oxidation. The magnitude of the Fe(II) maximum depends on the flux of iron into the basin, and reductive dissolution of particulate iron.  相似文献   

20.
This work describes a laboratory experiment designed to unravel mercury species reactivity in superficial coastal sediments oscillating between oxic and anoxic conditions. The experimental set-up has been applied to a sediment slurry from the Arcachon Bay (France) to follow the evolution of both naturally occurring (i.e. endogenous) and isotopically enriched added mercury species (i.e. exogenous, 1??IHg and 2?1MMHg) at environmental levels. The transformation and partition between the different phases (aqueous, solid and gaseous) of the endogenous and exogenous mercury species (inorganic Hg (IHg), monomethyl Hg (MMHg), elemental Hg (Hg?) and dimethyl Hg (DMHg)) have been investigated by isotopic speciation methods throughout the experiment. The results demonstrate that the experimental approach is able to promote sediment redox oscillations and to simultaneously follow the biogeochemical fate of naturally occurring or added mercury species. Experimentally driven redox transition events were found to significantly enhance the aqueous Hg species concentrations, while the MMHg burden is not greatly affected. Indeed, during the anoxic-oxic transition, while aqueous endogenous IHg and MMHg exhibited a two-fold increase, aqueous exogenous IHg and MMHg increased 7 and 4 times, respectively. Transient increases of the net IHg methylation were recorded during the redox transitions with the largest increase of the MMHg contents (factor 1.8) observed during the oxic-anoxic transition. High resolution in situ redox experiments have not been performed up to now, therefore the developed experimental set-up provides novel insights in both the influence of redox conditions on Hg methylation/demethylation and adsorption/desorption processes and kinetics in superficial sediments.  相似文献   

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