Diagenetic sulfate reduction in marine sediments

The importance of interstitial pore water as a vehicle for transport of dissolved ions between sediment at depth and the overlying ocean reservoir may be overestimated. The major cause of this overestimation is the assumption that the rate of ionic diffusion through the sediment is of a similar magnitude to the rate of reaction occurring within the sediment column. Examples are given to indicate that the above assumption may not be correct, at least for diagenetic sulfate reduction. Closed-system modeling, although approximate, is shown to be consistent, both qualitatively and isotopically, with measured changes that are observed for several sedimentary environments.     

珠江口淇澳岛海岸带沉积物中硫酸盐还原和不同形态硫的分布

对珠江口淇澳岛海岸带3个站位(QA-11,QA-9和QA-14)的沉积物中不同形态的还原硫(酸可挥发性硫,黄铁矿和有机硫)、总有机碳(TOC)和孔隙水中SO42-,甲烷浓度进行了测定,并且利用稳态扩散模型计算其中2个站位(QA-9和QA-14)硫酸盐还原通量[1.74和1.14 mmol/(m2.d)]和甲烷厌氧氧化通量[0.34和0.29 mmol/(m2.d)]。研究结果表明由于潮间带沉积物受到SO42-供给的限制,因此位于潮间带的QA-11站位硫酸盐还原带较浅(约16 cm);在潮下带的QA-9和QA-14站位,随离海岸距离和水深的增加,硫酸盐还原通量呈现减小的趋势,并且硫酸还原逐渐受到可利用活性有机质的限制;甲烷厌氧氧化对硫酸盐还原的贡献表现出增加的趋势,由19.2%增加至25.5%。三个站位沉积物中按不同形态还原硫含量由大到小列出,它们是有机硫(OS)、黄铁矿(DS)、酸可挥发性硫(AVS)。沉积物中AVS的空间分布与硫酸盐还原通量有正相关性。QA-11和QA-14站位的黄铁矿与AVS硫的含量比值大于3,分别为7.9和3.6,表明两个站位的黄铁矿形成可能受硫酸盐还原作用的控制;QA-9站位黄铁矿与AVS硫的含量比值为2.2,暗示AVS向黄铁矿转化受到可利用活性铁的限制。     

In situ permeabilities of selected coastal marine sediments

An in situ permeameter probe was deployed off Fort Walton Beach, Florida in shallow-water coastal sandy sediment. Stations were occupied in a 600 m /spl times/ 600 m area. Intrinsic permeability in sand at 17 stations varied from 0.3 to 6.1 /spl times/ 10/sup -11/ m/sup 2/ to subbottom depths of 50 cm. Permeability decreased with increasing subbottom depth and minimal compaction. The sediment is a well-sorted, medium quartz sand with a mean grain size of approximately 0.34-0.52 mm and with 5%-8% carbonate shells and shell fragments. The probe was tested in a slightly finer-grained sediment near the main study area and revealed permeabilities of 0.1-3.2 /spl times/ 10/sup -11/ m/sup 2/. The permeabilities measured are reasonable for the observed sandy and slightly finer-grained sediments.     

Sulfate reduction and anaerobic methane oxidation in Black Sea sediments

Beyond the shelf break at ca. 150 m water depth, sulfate reduction is the only important process of organic matter oxidation in Black Sea sediments from the surface down to the sulfate–methane transition at 2–4 m depth. Sulfate reduction rates were measured experimentally with ³⁵SO₄²⁻, and the rates were compared with results of two diffusion-reaction models. The results showed that, even in these non-bioirrigated sediments without sulfide reoxidation, modeling strongly underestimated the high reduction rates near the sediment surface. A hybrid modeling approach, in which experimentally measured rates in the upper sediment layers force a model that includes also the deeper layers, probably provides the most realistic estimate of sulfate reduction rates. Areal rates of sulfate reduction were 0.65–1.43 mmol SO₄²⁻ m⁻² d⁻¹, highest in sediments just below the chemocline. Anaerobic methane oxidation accounted for 7–11% of the total sulfate reduction in slope and deep-sea sediments. Although this methane-driven sulfate reduction shaped the entire sulfate gradient, it was only equivalent to the sulfate reduction in the uppermost 1.5 cm of surface sediment. Methane oxidation was complete, yet the process was very sluggish with turnover times of methane within the sulfate–methane transition zone of 20 yr or more.     

Sulfate reduction below the sulfate-methane transition in Black Sea sediments

A sudden increase in salinity about 7000 years ago caused a shift in the deposited sediments of the Black Sea from limnic to brackish-marine. Due to the development of an anoxic deep water basin and a relatively high sulfate concentration, organic matter is mineralized primarily through sulfate reduction in modern Black Sea sediments. Earlier studies showed that sulfate-reducing bacteria are abundant within the limnic sub-surface sediment in spite of extremely low concentrations of sulfate and organic carbon. A main objective of the present study was therefore to understand the depth distribution of sulfate reduction across the different sediment units, even deep below the sulfate-methane transition. Our study combined experimental measurements of sulfate reduction using ³⁵S radiotracer with analyses of sulfur and iron geochemistry in pore water and sediment. Potential sulfate reduction rates were measured with ³⁵S in sediment samples that were amended with sulfate and organic substrates and incubated in time-series up to 48 h. Sulfate reduction could thereby be detected and quantified at depths where concentrations of sulfate were otherwise too low to enable calculation of the rates. The results demonstrate that sulfate-reducing bacteria are active several meters below the sulfate-methane transition in Black Sea sediments. The cryptic sulfate reduction below the sulfate-methane transition may be driven by sulfate produced from re-oxidation of reduced sulfur species with oxidized iron minerals buried in the deep limnic sediment.     

九龙江河口沉积物中硫酸盐还原与甲烷厌氧氧化:同位素地球化学证据

通过沉积物柱孔隙水中甲烷,SO2-4,Cl-,δc(34S-SO2-4)、δc(13 C-CH4)的垂直分布特征,研究了硫酸盐还原和甲烷厌氧氧化(anaerobic oxidation of methane,简称AOM)过程在九龙江河口沉积物中的分布规律.测定结果显示两个站位(J-A和J-E)间隙水中SO2-4浓度随深...     

Sulfate reduction and sulfide deposition in deep-sea sediments from the southwestern Japan Sea

Vertical profiles of total sulfur and organic carbon have been measured in two deep-sea piston cores from the southwestern Japan Sea where sulfate reduction is proceeding within the sediments. The content of total sulfur, most of which is present as pyrite, increases gradually with increasing depth, showing several peaks. The amount of diagenetically deposited sulfide-sulfur is estimated using a steady-state model that considers vertical change in the diffusion coefficient. It is suggested that two-thirds to three-fourths of the observed total sulfur content has been deposited diagenetically.     

Seasonal changes of concentrations of inorganic and organic nitrogen in coastal marine sediments

The seasonal fluctuations of the concentration of nitrogenous compounds in sediments was investigated for three regions of the Seto Inland Sea in Japan; the variation of nitrogenous compounds in sediments was also studied in a laboratory experiment.The amounts of ammonium, dissolved organic nitrogen, nitrite and nitrate, as percentages of the dissolved total nitrogen of the interstitial water, were in the ranges of 47–99%, 10–50%, 0·1–0·6% and 0·3–4·1%, respectively. Ammonium was the major component and organic nitrogen was the next most important. The concentrations of these nitrogenous compounds changed seasonally: dissolved total nitrogen was higher in the warm month of September than in May; ammonium increased in warm months and decreased in cold months, but nitrite and nitrate increased in cold months. It was possible to explain the seasonal fluctuation of nitrogenous compounds in terms of the rates of the metabolic pathways of nitrogen in the sediments.Ammonium was not necessarily correlated with dissolved organic nitrogen. From this, it was considered that ammonium did not occur from solubilization of particulate organic nitrogen followed by mineralization, but from direct mineralization of particulate organic nitrogen in sediments.For the sediments of Suho Nada, Hiuchi Nada and station B-47 in Beppu Bay, the ratio of dissolved ammonium to adsorbed ammonium in the sediments was in the range 10–25%, but the ratio was 60–70% of adsorbed ammonium in the considerably anaerobic sediments at station B-45 in Beppu Bay. The ratio of dissolved ammonium to adsorbed ammonium increased with the increase of the concentration of sulfide in sediments. It was recognized that the anaerobic conditions of the sediments led to the dissolution of adsorbed ammonium.     

Assessment of copper,cadmium and zinc remobilization in Mediterranean marine coastal sediments

The remobilization of copper, cadmium and zinc in sediments of three selected coastal microenvironments of the Aegean Sea (Eastern Mediterranean) is assessed. Various analytical methods and techniques were employed providing concentrations, profiles and forms of metals and organic matter in sediments and pore waters.     

Hydrocarbons in Washington coastal sediments

The sources and distributions of polycyclic aromatic hydrocarbons (PAH) and aliphatic hydrocarbons are characterized in seventeen sediments from a highly river-influenced sedimentary environment off the southwestern coast of Washington. The major hydrocarbons are land-derived, introduced as preformed compounds and display long-term stability in sediment cores. A series of PAH of anthropogenic origin and two naturally derived compounds, retene and perylene, dominate the PAH composition in these sediments. Plantwax n-alkanes are the major aliphatic hydrocarbon constituents. Aliphatic hydrocarbons of marine origin, pristane and a series of four acyclic, multibranched C₂₅ polyolefins, are also observed in many sediments. The concentrations of these marine-derived hydrocarbons decrease to negligible levels rapidly with sediment depth from the sea-sediment interface, suggesting degradation.In general, the major land-derived hydrocarbons are concentrated in the midshelf silt deposit which extends northwestward along the continental shelf from the Columbia River mouth. A quantitatively more minor, natural series of phenanthrene homologs, also of terrestrial origin, is preferentially advected further offshore and deposited in continental slope sediments. These distributions are consistent with recognized particle associations for these compounds and sediment dispersal processes in this coastal environment. Sediment core records suggest the present pattern of dispersal has persisted for at least the past century and possibly since the Late Pleistocene.     

Scoping the effective marine environmental assessment of dredging and ocean disposal of coastal sediments in Korea

The application of scoping is reviewed to improve the efficiency and rationality of the marine environmental impact assessment system applied to the dredging of coastal sediments and the oceanic disposal of dredged material in Korea. Both the impact levels and those items requiring assessment vary in response to differing goals of the coastal activities involved and also to changes in the characteristics of different marine environments. As a consequence, different assessment fields need to be emphasized for evaluation of the impact that both dredging and dumping might have on the sites involved. The status of current assessment procedures is examined and its problems are diagnosed. Following a survey of interested parties and a review of case studies of the oceanic disposal of dredged sediment in other countries, checklists of core assessment items are proposed as part of a revamped review process, along with improvements to the assessment system.     

Molecular weight and trace metal distributions in fulvic and humic acid fractions of coastal marine sediments

Molecular weight and trace metal distributions of fulvic and humic acid fractions of marine sediments from the Seto Inland Sea were investigated by using a gel filtration technique in combination with atomic absorption and emission spectrometries. A binary molecular weight distribution was found both in the fulvic acid fraction and in the humic acid fraction. The fulvic acid fractions with molecular weights of less than 1.5×10³ and of 5×10³?10⁴ accounted for 38–57 % and 25–41 % of the total fulvic acids, respectively, and the humic acid fractions with molecular weights of 10²?10⁴ and of over 2×10⁵ accounted for 58–73 % and 16–27 % of the total humic acids, respectively. The components with molecular weights of over 1.5×10³ contained 54 %, on average, of the Fe, Zn and Cu in the fulvic acid fraction, and the components with molecular weights of over 10⁴ contained 58 %, on average, of these metals in the humic acid fraction.     

A comparison of non-hydrolytic methods for extracting amino acids and proteins from coastal marine sediments

Advances in analytical techniques now allow for the potential analysis of intact peptides and proteins isolated from marine sediments. However, there is no established technique for the extraction of macromolecular materials from marine sediments. Six different methods for extracting the amino acid component from coastal marine sediments were compared to the standard hot acid hydrolysis technique for their percent recovery and amino acid composition. The standard hot acid hydrolysis on dried, whole sediments released the greatest concentration of total amino acids (PS-THAA; 3.52 mg gdwt^− 1 ± 10% (SMD)), yet this only accounted for 22% of the total nitrogen in Puget Sound sediments (Washington, USA). Repeated hydrolysis of the same samples did not improve the recovery of nitrogen by more than an additional 10%. Base extraction (0.5 N NaOH) was the second best method for recovering amino acid nitrogen, releasing 60% of the Puget Sound total hydrolyzable amino acids (PS-THAA) (corresponding to 13% of the total sedimentary nitrogen), and has the advantage that it does not rely on peptide hydrolysis to free the nitrogenous component from the sediment matrix. The amino acid distribution of the 0.5 N NaOH extract was not significantly different than the initial THAA. Other non-hydrolyzing methods released lower yields of amino acids (Triton X-100 ≥ hot water > 50 mM NH₄HCO₃ > HF), but might prove to be of use to investigators interested in specific fractions of sedimentary organic nitrogen because these four methods had distinctly different amino acid compositions (enrichments in basic amino acids and depletions in acidic amino acids). Treatments with HF both before and after traditional hydrolysis and/or extractions with base did not release any more of the sedimentary nitrogen. Our results are consistent with the hypothesis that a large fraction of the sedimentary nitrogen (TN) is protected within an organic matrix.     

Rates of nitrification and nitrate reduction in nearshore marine sediments at near ambient substrate concentrations

Nitrification and nitrate reduction were measured simultaneously by a ¹⁵N-isotope dilution technique in the top 2 cm of sandy sediments in Great South Bay, Long Island, New York. Experiments were done at three times, under three different sets of environmental conditions. Nitrification rates remained between 0.010 and 0.015 μg-at N (g dry wt)⁻¹ (24 h)⁻¹ despite decreasing temperature. Nitrate reduction ranged from 0.02 to 0.11 μg-at N (g dry wt)⁻¹ (24 h)⁻¹. Nitrate reduction exceeded nitrification in two experiments. In the third, at low temperature and apparently high oxygen levels, rates of nitrification and nitrate reduction were comparable. We conclude that there is not a constant relationship between nitrification and nitrate reduction in this environment. Attempts to measure rates of nitrification by using the inhibitor chlorate were not successful.     

~(35)SO_4~(2-)示踪法测定九龙江河口沉积中硫酸盐还原速率

在2011年7月利用35SO2-4培养示踪法测定九龙江河口两个站位(A站位位于咸淡混合区,盐度3~5;B站位位于海相区,盐度20~25)沉积柱中硫酸盐还原速率的垂直分布。结果显示A站位沉积柱中硫酸盐还原速率变化范围为54~2 345nmol/(cm3·d),从表层到底部先增大后减小,最大值出现在20cm深度附近;B站位硫酸盐还原速率在24~987nmol/(cm3·d)之间,分别在10cm和78cm深度附近出现两个峰值,分别为876nmol/(cm3·d)和987nmol/(cm3·d)。综合分析两个站位孔隙水中SO2-4、甲烷浓度和沉积物中总有机碳、温度和氧化还原电位的垂直变化趋势与其硫酸盐还原速率的分布规律,表明A站位沉积物中硫酸盐还原以有机矿化为主;B站位受到有机质矿化和甲烷厌氧氧化的共同作用;两个站位硫酸盐还原速率及垂直分布趋势受孔隙水中SO2-4浓度、有机质活性和温度的共同影响;根据各个层位硫酸盐还原速率估算两个站位硫酸盐还原通量(以硫计)分别为527.9mmol/(m2·d)和357.1mmol/(m2·d),表明硫酸盐还原是九龙江河口有机质厌氧矿化的重要路径。     

Analysis of the experimental compressibility of deep water marine sediments from the Gulf of Guinea

This study strives to analyze the compressibility of deep water marine sediments originating from the Gulf of Guinea. Our approach is multi-scaled and, on the macroscopic level, based on two equations which link the overburden effective stress with the liquidity index, the Biarez & Favre equation on the one hand, and the Burland equation on the other hand (after calculation of the ICL expression as a function of the I_L parameter). On the local level, the evolution of the microstructures is analysed and compared with GoG remoulded clay. Two additional observation techniques have been used, the scanning electron microscope and the mercury intrusion porosimetry.     

Evaluation of the indophenol method to measure NH4 in extracts from coastal marine sediments

It has been found that the measurement of NH₄⁺ in extracts of marine sediments was influenced by factors which inhibit the indophenol colour development, and through gel chromatography, diffusion, dilution and internal standard assays, it was possible to determine the extent of this interference. The measurement of NH₄⁺ in KCl extracts and porewater, using internal standards, indicated that colour development was inhibited 13% and 51%, respectively. Dilution of samples alleviated this inhibition, but more than five-fold dilution was necessary for some extracts. Diffusion of NH₃, collection and measurement generally gave concentrations comparable with those derived from the use of internal standards. These data suggest that the indophenol reagents should be added to sufficient diluted sediment extracts or to diffusates from extracts, to prevent interference to the colour development of NH₄⁺.     

Mercury methylation,demethylation and reduction rates in coastal and marine surface waters of the Mediterranean Sea

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.     

Acoustic impedance of marine sediments

A heavy walled steel tube has been used as an impedance tube for measuring the acoustic impedance of marine sediments. The acoustic characteristics of the water filled tube were found to be satisfactory for the measurements. Values of acoustic velocity and attenuation in the sediments were determined from the acoustic impedance measurements and found to agree with previously published data.