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1.
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.  相似文献   

2.
Porewater samples were obtained on five occasions during spring, summer and fall by in situ dialysis from three sites of a large freshwater wetland situated along the St. Lawrence River. These samples were analysed for total dissolved mercury ([Hg]T) and methylmercury ([MeHg]) concentrations and for complementary variables including dissolved sulfate, sulfide and elemental sulfur concentrations. Sediment cores were obtained on three occasions from one of these sites for the determination of total mercury ({Hg}T) and methylmercury ({MeHg}) concentration as well as mercury methyltransferase (HgMT) activity profiles. {MeHg} and HgMT activity varied with time and sediment depth. The porewater [Hg]T and [MeHg] depth profiles varied with time and among sites. Modeling the porewater [MeHg] profiles with a one-dimensional reaction-transport equation allowed identification of the sediment depths where MeHg is produced or consumed, as well as an estimate of the net in situ MeHg production rates in the sediments. The model-predicted depths of MeHg production, as well as the sulfate concentration and the HgMT activity depth distributions are all consistent with the involvement of sulfate reducing bacteria in the production of MeHg.  相似文献   

3.
Potential hydrolysis rates of three different polysaccharides, pullulan, laminarin, and xylan, were measured in intact sediment cores from Cape Lookout Bight, North Carolina, in order to constrain the rates at which a fraction of the high-molecular-weight sedimentary carbon pool may be hydrolyzed to lower molecular weights. Potential hydrolysis rates of pullulan were somewhat higher than those of laminarin and xylan. Highest potential rates were measured in surface sediments; rates at depths of 5–7 and 14–16 cm differed relatively little from one another. Total dissolved carbohydrates, dissolved organic carbon (DOC), sulfate, and sulfate reduction rates were also measured and compared with data previously collected at Cape Lookout Bight in order to investigate carbohydrate dynamics and establish the relative contribution of carbohydrates to the sedimentary carbon budget. Total porewater carbohydrates constitute a disproportionate fraction of DOC, ranging from a maximum of 85% in near-surface intervals to 24% at depths of 14–16 cm. A comparison of potential hydrolysis rates, dissolved carbohydrate concentrations, DOC, and sulfate reduction rates, along with results from a wide range of studies previously conducted at this site suggests that hydrolysis of high-molecular-weight polysaccharides can potentially be very rapid relative to carbon remineralization rates. Dissolved porewater carbohydrates form a dynamic pool that is likely turned over on short timescales in Cape Lookout Bight sediments.  相似文献   

4.
史春潇  雷怀彦  赵晶  张劼  韩超 《沉积学报》2014,32(6):1072-1082
本研究应用微生物16S rRNA-DGGE和T-RFLP技术,结合环境参数,对我国天然气水合物潜在区南海九龙甲烷礁附近973-4柱状样沉积物中3个层位12个不同深度(表层20 cm至382 cm,中层552 cm至796 cm,深层862 cm至1 196 cm)细菌群落结构及其分布进行了对比研究.其中T-RFLP实验表明,细菌丰度、香农指数和均匀度变化趋势相同,由深层到716 cm处先降后升,中层716 cm深度范围处微生物群落丰度、均匀度、香农指数相对较高,716 cm至表层先降后升.DGGE图谱和T-RFLP色谱峰聚类分析表明:表层20 cm至192 cm相似性较高,表层236 cm至382 cm与深层1 082 cm、1 196 cm群落结构相似性较高,但中层沉积物中微生物群落结构与表层及深层均有较大差异.环境参数表明中层甲烷含量较高,推测甲烷是影响微生物群落结构差异的主要因素之一.T-RFLP色谱峰与微生物数据库比对及DGGE条带测序也表明了:本区变形杆菌(Proteobacteria)为优势菌群,其中α-、γ-、δ-变形杆菌(Proteobacteria)为主要的细菌亚群,其他细菌包括放线菌(Actinobacteria)、厚壁菌门(Firmicutes)、绿弯菌门(Chloroflexi)为次优势菌群.甲烷含量较高的中层,甲烷氧化菌(Methanotrophs),硫还原菌(Sulfate-reducing bacteria)等与甲烷密切相关的细菌均有被检测到,表明该区域存在与天然气水合物的分解释放相关的微生物群落.  相似文献   

5.
Methanogen ether lipids have been quantified in sediments from a Florida swamp and the Atlantic ocean. Swamp cores containing acyclic and monocyclic isopranyl ethers are clearly differentiated from deep sea sediments which also contain bicyclic compounds. A concentration maximum near the bottom of the sulfate reducing zone in deep sea sediments may reflect a biogeochemical system in which methanogenesis and sulfate reduction are coupled by the process of methane oxidation. Lipid diagenesis is evident in the deep sea sediments. Species zonation, possibly caused by oxygen sensitivity, is detected in the relative lipid abundances in swamp sediments.  相似文献   

6.
In situ Gibbs energies of reaction (ΔG) for acetate-oxidizing sulfate reduction, acetate-oxidizing iron reduction, and acetoclastic methanogenesis, and sulfate-reducing methanotrophy are consistently negative and relatively constant throughout most of the sediment column at the eastern equatorial Pacific Ocean Drilling Program (ODP) Site 1226. The energy yields (−ΔG) closely match the values (for acetate-oxidizing sulfate reduction and acetoclastic methanogenesis) in published culturing experiments with actively growing cells and, for sulfate-reducing methanotrophy, in other environments.Although microbes mediating these reactions compete for substrates, mutualistic interactions between them appear to sustain their co-existence in deep subseafloor sediments for millions of years (the interval over which the sediments have been deposited). These competing and mutualistic interactions collectively constitute a highly coupled reaction network where relative rates of reaction are regulated by the in situ Gibbs energies of reaction.  相似文献   

7.
A methane-sulfate coupled reaction diffusion model has been developed to describe the inverse relationship commonly observed between methane and sulfate concentrations in the pore waters of anoxic marine sediments. The sediment column was divided into two zones; an upper zone where diagenetic reaction rates are limited by the concentration of oxidizable organic matter and a lower zone in which reaction rates are limited by the concentration of oxidizing agent—sulfate. For each zone differential equations describing the distribution of methane and sulfate were derived. The boundary conditions used to solve these equations resulted in a set of four coupled equations. When fit to data from Saanich Inlet (B.C., Canada) and Skan Bay (Alaska) the model not only reproduces the observed methane and sulfate pore water concentration profiles but also accurately predicts the methane oxidation and sulfate reduction rates. Maximum methane oxidation rates occur at the transition boundary from the upper to the lower layer. In Saanich Inlet sediments from 23 to 40% of the downward sulfate flux is consumed in methane oxidation while in Skan Bay this value is only about 12%.  相似文献   

8.
We used a combination of porewater and solid phase analysis, as well as a series of sediment incubations, to quantify organic carbon oxidation by dissimilatory Fe reduction, Mn reduction, and sulfate reduction, in sediments from the Skagerrak (located off the northeast coast of Jutland, Denmark). In the deep portion of the basin, surface Mn enrichments reached 3.5 wt%, and Mn reduction was the only important anaerobic carbon oxidation process in the upper 10 cm of the sediment. In the less Mn-rich sediments from intermediate depths in the basin, Fe reduction ranged from somewhat less, to far more important than sulfate reduction. Most of the Mn reduction in these sediments may have been coupled to the oxidation of acid volatile sulfides (AVS), rather than to dissimilatory reduction. High rates of metal oxide reduction at all sites were driven by active recycling of both Fe and Mn, encouraged by bioturbation. Recycling was so rapid that the residence time of Fe and Mn oxides, with respect to reduction, ranged from 70-250 days. These results require that, on average, an atom of Fe or Mn is oxidized and reduced between 100-300 times before ultimate burial into the sediment. We observed that dissolved Mn2+ was completely removed onto fully oxidized Mn oxides until the oxidation level of the oxides was reduced to about 3.8, presumably reflecting the saturation by Mn2+ of highly reactive surface adsorption sites. Fully oxidized Mn oxides in sediments, then, may act as a cap preventing Mn2+ escape. We speculate that in shallow sediments of the Skagerrak, surface Mn oxides are present in a somewhat reduced oxidation level (< 3.8) allowing Mn2+ to escape, and perhaps providing the Mn2+ which enriches sediments of the deep basin.  相似文献   

9.
The organic rich sediments of the Skagerrak contain high quantities of shallow gas of mostly biogenic origin that is transported to the sediment surface by diffusion. The sulfate methane transition zone (SMTZ), where anaerobic oxidation of methane (AOM) and sulfate reduction occur, functions as a methane barrier for this upward diffusing methane.To investigate the regulation of AOM and sulfate reduction rates (SRR) and the controls on the efficiency of methane consumption, pore water concentrations, and microbial rates of AOM, sulfate reduction and methanogenesis were determined in three gravity cores collected along the slope of the Norwegian Trench in the Skagerrak. SRR occurred in two distinct peaks, at the sediment surface and the SMTZ, the latter often exceeding the peak AOM rates that occurred at the bottom of the SMTZ. Highest rates of both AOM and SRR were observed in a core from a pockmark, where advective methane transport occurred, generating high methane and sulfate fluxes. But even at this site with a shallow SMTZ, the entire flux of methane was oxidized below the sediment surface. AOM, SRR and methanogenesis seem to be closely associated and strongly regulated by sulfate concentrations, which were, in turn, regulated by the methane flux. Rate measurements of SRR, AOM and methanogenesis revealed a tight coupling of these processes. Bicarbonate-based methanogenesis occurred at moderate sulfate concentrations (>5 mM) above the AOM zone but seemed to be inhibited in the depth where AOM occurred. The unbalanced stoichiometry of AOM and SRR in the SMTZ was more pronounced in rate measurements than in methane and sulfate fluxes, and seemed more likely be related to enhanced SRR in this zone than an underestimation of methane fluxes.  相似文献   

10.
The role of acetate in the biogeochemical cycling of organic matter in contemporary marine anoxic sediments of Skan Bay, Alaska was investigated with inhibition and quasi in situ turnover experiments. The turnover time for acetate oxidation in the upper 30 cm of the sediment column is ca. 1 hr. A molybdate inhibition experiment indicated that sulfate reducing bacteria were responsible for more than 95% of acetate oxidation. However, measured acetate oxidation rates exceeded sulfate reduction rates indicating that acetate oxidation rates are overestimated. Values for acetate concentration calculated from sulfate reduction rates (0.3–3.4 μM) were considerably lower than directly measured acetate concentrations (3.1–10.8 μM). Much of the chemically measured acetate may be microbially unavailable, perhaps in the form of a soluble or colloidal complex. A sorption experiment indicates that 10% to 40% of added acetate associates with Skan Bay sediment particles. Production of methane from acetate was detected only at 2 m depth.  相似文献   

11.
Groundwater and sediment samples (∼ 1 m depth) at sites representative of different groundwater pathways were collected to determine the aqueous speciation of sulfur and the fractionation of sulfur isotopes in aqueous and solid phases. In addition, selected sediment samples at 5 depths (from oxic to anoxic layers) were collected to investigate the processes controlling sulfur biogeochemistry in sedimentary layers. Pyrite was the dominant sulfur-bearing phase in the capillary fringe and groundwater zones where anoxic conditions are found. Low concentrations of pyrite (< 5.9 g kg− 1) coupled with high concentrations of dissolved sulfide (4.81 to 134.7 mg L− 1) and low concentrations of dissolved Fe (generally < 1 mg L− 1) and reducible solid-phase Fe indicate that availability of reactive Fe limits pyrite formation. The relative uniformity of down-core isotopic trends for sulfur-bearing mineral phases in the sedimentary layers suggests that sulfate reduction does not result in significant sulfate depletion in the sediment. Sulfate availability in the deeper sediments may be enhanced by convective vertical mixing between upper and lower sedimentary layers due to evaporative concentration. The large isotope fractionation between dissolved sulfate and sedimentary sulfides at Owens Lake provides evidence for initial fractionation from bacterial sulfate reduction and additional fractionation generated by sulfide oxidation followed by disproportionation of intermediate oxidation state sulfur compounds. The high salinity in the Owens Lake brines may be a factor controlling sulfate reduction and disproportionation in hypersaline conditions and results in relatively constant values for isotope fractionation between dissolved sulfate and total reduced sulfur.  相似文献   

12.
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 210Pbexcess 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.  相似文献   

13.
Oxygen profiles were measured in the sediments of the Gulf of Aqaba (Red Sea), an oligotrophic marine system affected by episodic seasonal flash floods and intense aeolian dry deposition. Sediment cores were retrieved from shallow (15–45 m), intermediate (250–561 m) and deep (700 m) water sites of south–north and east–west transects. Dissolved oxygen concentrations were measured simultaneously by using microelectrodes and microoptodes immediately after sampling and after transportation. Oxygen penetration depths were found to increase from 2 to 5 mm at the shallow water sites with sandy permeable sediments to 10–21 mm at the deeper sites with cohesive muddy sediments. This increase corresponds to decrease in oxygen diffusive fluxes at the sediment–water interface and oxygen consumption rates with depth. Oxygen consumption rates exhibit local maxima at the oxic–anoxic sediment boundary, which may be attributed to oxygen reduction coupled to oxidation of dissolved Fe(II) and Mn(II) at deep and intermediate water sites and of hydrogen sulfide at shallow water sites. Microelectrodes and microoptodes measurements of cohesive sediments from deep and intermediate water sites yielded similar results. By comparison, the microoptodes displayed more robust measurements than microelectrodes in sandy near-shore sediments. This was attributed to their flexible fiber structure that is less likely to break or to abruptly displace sand particles. After transportation of sediment cores from Eilat to Beer Sheva followed by ≤?24-h storage, no changes in oxygen fluxes and consumption rates were detected.  相似文献   

14.
Rates of sulfate reduction, oxygen uptake and carbon dioxide production in sediments from a short Spartina alterniflora zone of Great Sippewissett Marsh were measured simultaneously during late summer. Surface sediments (0–2 cm) were dominated by aerobic metabolism which accounted for about 45% of the total carbon dioxide production over 0–15 cm. Rates of sulfate reduction agreed well with rates of total carbon dioxide production below 2 cm depth indicating that sulfate reduction was the primary pathway for sub-surface carbon metabolism. Sulfate reduction rates were determined using a radiotracer technique coupled with a chromous chloride digestion and carbon disulfide extraction of the sediment to determine the extent of formation of radiolabelled elemental sulfur and pyrite during shortterm (48 hr) incubations. In the surface 10 cm of the marsh sediments investigated, about 50% of the reduced radiosulfur was recovered as dissolved or acid volatile sulfides, 37% as carbon disulfide extractable sulfur, and only about 13% was recovered in a fraction operationally defined as pyrite. Correlations between the extent of sulfate depletion in the marsh sediments and the concentrations of dissolved and acid volatile sulfides supported the results of the radiotracer work. Our data suggest that sulfides and elemental sulfur may be major short-term end-products of sulfate reduction in salt marshes.  相似文献   

15.
对综合大洋钻探计划(IODP)311航次652个岩心沉积物样品进行了自生黄铁矿颗粒筛选、显微形貌特征及其硫稳定同位素组成等初步研究。扫描电镜(SEM)照片显示黄铁矿以微球粒状和立方体状形貌产出,其成因与微生物作用和无机作用有关。黄铁矿的δ34SCDT值变化范围较大,从-35.4‰到+53.6‰,其成因与甲烷厌氧氧化作用(AOM)的关系密切。海水源为主的硫酸盐参与了沉积物上部的AOM过程,黄铁矿硫稳定同位素正偏的原因可能与较强的AOM作用和较多的残余硫酸盐参与有关。冷泉背景站位中黄铁矿的δ34SCDT值随着深度增加而增加,从浅表层的-35.83‰增加到深处的32.49‰,反映深处沉积物内黄铁矿形成过程中曾有过较多的残余硫酸盐参与还原,暗示其背景曾经是更高的甲烷通量和更强的AOM作用。研究结果提供了现代海洋天然气水合物背景下沉积物中自生黄铁矿及其硫稳定同位素特征记录,对于寻找我国海域天然气水合物资源,探索地史时期古海洋沉积物中甲烷事件记录具有重要的意义。  相似文献   

16.
Dissolved organic carbon (DOC) flux dynamics were examined in the context of other biogeochemical cycles in intertidal sediments inhabited by benthic microalgae. In August 2003, gross oxygenic photosynthetic (GOP) rates, oxygen penetration depths, and benthic flux rates were quantified at seven sites along the Duplin River, GA, USA. Sediments contained abundant benthic microalgal (BMA) biomass with a maximum chlorophyll a concentration of 201 mg chl a m?2. Oxygen microelectrodes were used to determine GOP rates and O2 penetration depth, which were tightly correlated with light intensity. Baseline and 15N-nitrate amended benthic flux core incubations were employed to quantify benthic fluxes and to investigate the impact of BMA on sediment water exchange under nitrogen (N)-limited and N-replete conditions. Unamended sediments exhibited tight coupling between GOP and respiration and served as a sink for water column dissolved inorganic nitrogen (DIN) and a source of silicate and dissolved inorganic carbon (DIC). The BMA response to the N addition indicated sequential nutrient limitation, with N limitation followed by silicate limitation. In diel (light–dark) incubations, biological assimilation accounted for 83% to 150% of the nitrate uptake, while denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) accounted for <7%; in contrast, under dark conditions, DNF and DNRA accounted for >40% of the NO3 ? uptake. The N addition shifted the metabolic status of the sediments from a balance of autotrophy and heterotrophy to net autotrophy under diel conditions, and the sediments served as a sink for water column DIN, silicate, and DIC but became a source of DOC, suggesting that the increased BMA production was decoupled from sediment bacterial consumption of DOC.  相似文献   

17.
Rates of anaerobic decomposition of Lake Erie sediments were determined for seven depth intervals at three temperatures. Sealed sediment sections were incubated under anoxic conditions and the interstitial waters were serially sampled over a period of approximately 200 days. Concentration increases of bicarbonate, phosphate, ammonium, Ca, Mg, Fe and Mn in pore water within any given depth interval followed zero order kinetics over the sampling period and exhibited Arrhenius temperature dependency. Rates of release to the pore waters were proportional to the concentrations in the solid phases, indicating first order kinetics overall.The rates and temperature dependencies of these fermentation reactions were only slightly less than those reported from sediments undergoing sulfate reduction. The observed release rates decreased exponentially with depth in the sediment due to a corresponding decrease in the amount of metabolizable organic matter and acid hydrolyzable mineral phases.A stoichiometric model was constructed utilizing the observed release rates and assumed chemical reactions to predict the stoichiometry of the decomposing organic matter and the nature of the hydrogen buffer. The modeling indicates that 60% of the observed bicarbonate release is the direct result of organic decomposition, that 20% of the release is from the dissolution of calcium carbonate mineral phases, and that the remaining 20% of the release is from the dissolution of magnesium, iron und manganese carbonate mineral phases.Kinetic modeling of the observed production rates accurately predicts the vertical profiles of Ca, Mg, Fe and Mn, but cannot quantitatively account for all the concentration differences of the nutrient elements C, N and P. This implies that in addition to decomposition, increased depositional flux also accounts for the significant changes in concentrations of the nutrient elements in the near surface sediments.  相似文献   

18.
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.  相似文献   

19.
We report methane concentrations in the bottom water layer and the upper layer of bottom sediments and the results of acoustic explorations of methane seeps on the shelf bordering the continental slope of the Sea of Japan region, in which electromagnetic Schumann’s resonance oscillations were earlier recorded at continental-slope water depths of 500, 1000, and 2000 m. The occurrence of Schumann’s resonances at such great depths is explained by an increase (a factor of more than 25) in the electrical resistivity of a ~ 2000 m thick sediment layer with the pore space largely filled with free methane. A new method is proposed for determining the depth of the sources of anomalous concentrations of methane in bottom sediments or in the bottom water layer on a deep shelf. The method is based on recording Schumann’s resonances during measurements of the natural electric field at a series of increasing depths in areas bordering the continental slope.  相似文献   

20.
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%).  相似文献   

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