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1.
Dolomites in thick sections of Miocene Monterey Shale and related formations in the Temblor Range of California acquired their isotopic compositions as they formed at shallow depth in the original sediment rich in organic matter, and retained the composition against the vicissitudes of burial diagenesis. The oxygen isotopes of dolomites of successive beds record changes in temperature of bottom water while the carbon isotopes of the same samples indicate changes in the kind of microbial activity (sulfate reduction vs carbohydrate fermentation) that prevailed at shallow depths in the sediment. In an auxiliary study, two samples of dolomite from sediments of Cariaco Basin off Venezuela (DSDP site 147) were found to have δ5C13 of ?14.1 and ?9.8 per ml PDB, although they occur in a heavy-carbon zone containing bicarbonate as heavy as +8.4 per ml. These dolomites probably originated at shallow depth in the light-carbon zone of microbial sulfate reducers and were buried under later sediments down into the heavy-carbon zone of microbial fermenters of carbohydrates without losing their original light-carbon composition.  相似文献   

2.
Numerous studies of marine environments show that dissolved organic carbon (DOC) concentrations in sediments are typically tenfold higher than in the overlying water. Large concentration gradients near the sediment–water interface suggest that there may be a significant flux of organic carbon from sediments to the water column. Furthermore, accumulation of DOC in the porewater may influence the burial and preservation of organic matter by promoting geopolymerization and/or adsorption reactions. We measured DOC concentration profiles (for porewater collected by centrifugation and “sipping”) and benthic fluxes (with in situ and shipboard chambers) at two sites on the North Carolina continental slope to better understand the controls on porewater DOC concentrations and quantify sediment–water exchange rates. We also measured a suite of sediment properties (e.g., sediment accumulation and bioturbation rates, organic carbon content, and mineral surface area) that allow us to examine the relationship between porewater DOC concentrations and organic carbon preservation. Sediment depth-distributions of DOC from a downslope transect (300–1000 m water depth) follow a trend consistent with other porewater constituents (ΣCO2 and SO42−) and a tracer of modern, fine-grained sediment (fallout Pu), suggesting that DOC levels are regulated by organic matter remineralization. However, remineralization rates appear to be relatively uniform across the sediment transect. A simple diagenetic model illustrates that variations in DOC profiles at this site may be due to differences in the depth of the active remineralization zone, which in turn is largely controlled by the intensity of bioturbation. Comparison of porewater DOC concentrations, organic carbon burial efficiency, and organic matter sorption suggest that DOC levels are not a major factor in promoting organic matter preservation or loading on grain surfaces. The DOC benthic fluxes are difficult to detect, but suggest that only 2% of the dissolved organic carbon escapes remineralization in the sediments by transport across the sediment-water interface.  相似文献   

3.
Measured pore-water concentrations of iron in interbedded pelagic and turbiditic sediments from the Nares Abyssal Plain are in excellent agreement with sediment colour and measured redox potential. The organic carbon content of these sediments appears to define the redox conditions and consequently the porewater and solid-phase concentration of constituents that are involved in early diagenetic reactions. In the turbiditic sediments the concentration of NO3− generally goes to zero within a sediment depth of 1 m, whereas at 8 m in a pelagic core from the same area the concentration of NO3− is still higher than it is in the bottom water. The pore-water concentration of Mn2+ in the turbiditic sediments increases sharply down to a depth of approximately 3 m and from thereon remains nearly constant due to saturation with respect to Mn, Ca-CO3. The pore water of the turbiditic sediments is also saturated with respect to calcite. The few “diagenetic spikes” in the pore-water concentration of NO3− and Mn2+ and the concentration/depth profile of dissolved iron, H4SiO4 and phosphate all clearly demonstrate the inhomogeneous nature of interbedded pelagic and turbiditic sediments. The simultaneous occurrence of peaks of dissolved iron/silica and of sediment intervals with a relatively high organic carbon content is attributed to enhanced early diagenetic reactions associated with the decomposition of organic matter in these specific intervals. Linked with these reactions is the irregular pore-water concentration of phosphate, which is shown to originate partly from the oxidation of organic matter, but mainly from the desorption of phosphate from iron oxide. Potential concentrations of phosphate are calculated from the stoichiometric early diagenetic reactions and compared with measured concentrations. Due to the unique combination of low porosity and relatively high sedimentation rates, the sediments from the Nares Abyssal Plain are an ideal basis for the study of such interbedded sequences of pelagic and turbiditic deposits.  相似文献   

4.
The quantitative significance of organic matter degradation in bringing about the early diagenetic mobility of anthropogenic trace metals (Cu, Zn, Pb) is assessed specifically in relation to the use of estuarine sediments as historical records of pollution. A 1,500 mm salt-marsh sediment depth profile from Tites Point, Severn Estuary, England, was sampled at 10-mm intervals. Organic carbon determinations were carried out by a wet oxidation technique, and ‘organic fraction’ metals were separated by sequential leaching. Results demonstrated that organic phase metals are quantitatively significant in Severn Estuary sediments, particularly Cu and Zn (Cu>Zn), and that metals are probably released from this fraction during early diagenesis. The degree of release, and the apparent loss of the released trace metals from the sediment, would suggest that the use of estuarine sediments as historical records of pollution requires qualification.  相似文献   

5.
Aliphatic hydrocarbon compositions were quantitatively characterized in plankton, sediment trap-collected particulate materials and sediments from Dabob Bay using high resolution glass capillary gas chromatography. The average net accumulation of individual hydrocarbons measured in a 1-yr series of sediment traps was compared with the net accumulation of corresponding compounds measured in three depth intervals of 210Pb-dated bottom sediments. Systematic and rapid decreases in the net accumulation of individual hydrocarbons were observed from the sediment traps to the sediments. Most pronounced decreases were measured for planktonically derived hydrocarbon constituents (e.g. pristane and two unsaturated compounds) which are rapidly remineralized at or near the sediment-water interface. Consequently, the amount of each compound measured in deposited sediments is not necessarily a quantitative indication of its initial flux to the sediments. The n-alkanes (C25,27,29,31). characteristic of terrestrial plant waxes, are the predominant hydrocarbons measured by 4–6 cm depth in these sediments and show reasonably constant net accumulation below this interval.Significant diagenetic alteration of the bulk organic matter contained in the average sediment trap particulate material is also noted through comparison with bottom sediments on the basis of organic C/N and δ13C measurements. Organic matter elementally similar to marine plankton is preferentially remineralized upon deposition of the sedimentary particulates. The residual organic matter remaining and buried in the bottom sediments closely resembles terrestrial organic matter.  相似文献   

6.
Concretions from the Kimmeridge Clay Formation are of three types: calcareous concretions, septarian calcareous concretions and pyrite/calcite concretions and nodules, which occur within different mudstone facies. Isotopic and chemical analysis of the concretionary carbonates indicate growth in the Fe-reduction, sulphate-reduction and decarboxylation zones. The septarian concretions show a long and complex history, with early initiation of growth and development spanning several phases of burial, each often resulting in the formation of septaria. Growth apparently ceased in the transitional zone between the sulphate-reduction and the methanogenesis zones. Very early growth in the Fe-reduction zones is also seen in one sample. The non-septarian concretions began growth later within the sulphate-reduction zone and have had a simpler burial history while the pyrite/calcite concretions show carbonate cementation in the sulphate-reduction-methanogenesis transition zone. A ferroan dolomite/calcite septarian nodule with decarboxylation zone characteristics also occurs. Development of concretions appears to be indirectly controlled by the sedimentation rate and depositional environment, the latter determining the organic matter input to the sediments. Calcareous concretions predominate in swell areas and during periods of low sedimentation rate in the basins with poor organic matter preservation and deposition of calcareous mudstones. Pyrite/calcite concretions occur in organic-rich mudstones deposited under higher sedimentation rates in the basins, while the ferroan dolomite nodule grew under very high sedimentation rates.  相似文献   

7.
冲绳海槽中段西陆坡下缘天然气水合物存在的可能性分析   总被引：28，自引：0，他引：28  
孟宪伟  刘保华 《沉积学报》2000,18(4):629-633
海洋中的天然气水合物主要发育在有机质供应充分、沉积速率快、热流值较高、水深大于300m的大陆斜坡和活动边缘的增生楔发育区;沉积物类型主要以泥质砂岩、砂质泥岩和浊积岩为主。似海底反射层(BSR)和极性反转是识别天然气水合物层的关键标志。冲绳海槽中段西陆坡下缘水深大于1000m;沉积物类型主要为粉砂质泥和泥质粉砂,在部分层位见浊积层。与东海陆架相比,西陆坡下缘的有机质含量、沉积速率的热流值都较高,其范围分别为0.75%～1.25%、10～40cm/ka和70～437mw/m2;单道地震剖面具有明显的似海底反射层(BSR)和极性反转特征,因此,推断冲绳海槽中段西陆坡下缘可能存在天然气水合物层。  相似文献   

8.
Recent geochemical studies provide evidence that changes in vertical distributions of nutrients in lake sediments are driven by anthropogenic activities, based primarily on trends of increasing concentrations in upper sediment layers. However, the present study shows that vertical concentration profiles of C, N and P in lake sediments can be higher in the upper, most recently deposited sediment strata, driven largely by natural diagenetic processes and not eutrophication alone. Sediment cores from 14 different lakes in New Zealand and China were examined ranging from oligotrophic to highly eutrophic and shallow to deep, and it was found that the shape of vertical profiles of total P, a key nutrient for lake productivity, can be similar in sediments across gradients of widely differing trophic status. Empirical and mechanistic diagenesis steady state profile models were derived and applied to describe the vertical distribution of C, N and P in the sediments. These models, which focus on large scale temporal (decades) and spatial (up to 35 cm in the vertical) processes, revealed that density-differentiated burial and biodiffusive mixing, were strongly correlated with vertical concentration gradients of sediment C, N and P content, whereas lake trophic status was not. A sensitivity analysis of parameters included in the diagenetic model further showed that the processes including flux of organic matter to the sediment–water interface, burial (net sedimentation), breakdown of organic matter and biodiffusion all can significantly influence the vertical distribution of sediment P content. It was concluded that geochemical studies attempting to evaluate drivers of the vertical distribution of sediment C, N and P content in lake sediments should also account for the natural diagenetic drivers of vertical concentration gradients, assisted with application of similar models to those presented in this study. This would include quantification of key sediment diagenesis model parameters to separate out the influence of anthropogenic activities.  相似文献   

9.
The behavior of stable carbon and oxygen isotopes in carbonates during the deposition and diagenesis of sediments in the bioproductive Upper Famennian Pripyat Trough (southern Belarus) is discussed. Limestones and clayey limestones (Corg 0.92 ± 0.11%) are characterized by very low δ13C values (–9.6 ± 0.3‰). Parental sediments of these rocks were deposited in the shallow-water zone during slow downwarping episodes of the seafloor. Lithification of the sediments took place in oxidative conditions of the diagenesis zone. Organic matter was actively oxidized by free oxygen. Carbon dioxide with isotopically light organic carbon formed in this process was used during the crystallization of diagenetic carbonates that are visually indiscernible from the sedimentational variety. Marls, clayey marls, and carbonate-bearing clays (Corg 6.02 ± 0.80%) are characterized by δ13C values as high as –3.5 ± 0.6‰. In combustible shales (Corg >10%), δ13C value is ‒1.2 ± 0.6‰). The clayey rocks mark the episodes of sedimentation in relatively deep-water conditions that appear during the uncompensated sagging of the basin floor. The diagenetic zone with free oxygen was significantly decreased or absent at all. Here, oxygen of marine sulfates was the main or single oxidizer of organic matter (sulfate reduction). The sulfate oxygen is a weaker oxidizing agent than free oxygen. Therefore, much more organic matter was retained and fossilized in clayey rocks than in carbonate rocks. Organic carbon released during the sulfate reduction and mobilized later for the diagenetic carbonate formation was insufficient for the significant decrease of δ13C values relative to the marine carbonate standard. Isotopic composition of carbonate oxygen in the studied rocks is invariable and does not depend on the content of clay and organic matter. In these rocks, δ18O values are at the level (approximately –5‰) shown for the Famennian in the Global Chemostratigraphic Chart. This fact is consistent with the supposition that oxygen isotope composition of atmospheric precipitates, which influenced the rock formation via continental flow, is close to that of sea water in low latitudes where the territory of Belarus was located in the Late Devonian.  相似文献   

10.
Sedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried Upper Jurassic Fulmar Formation, South Central Graben, UKCS. As a result of the bioturbated nature of the shallow marine strata, variations in ichnofabrics and cross-cutting relationships between trace fossils are important in identifying and correlating stratal surfaces. Two transgressive and two regressive surfaces/sequence boundaries were studied, using petrographical, mineralogical and geochemical techniques. Cements, including early diagenetic pyrite framboids, grain-rimming apatite, microcrystalline dolomite and ankerite, occur at, above and immediately below both transgressive and forced regressive surfaces/sequence boundaries. Breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time, enhancing diagenetic reactions at and below the surfaces. The distribution of ankerite, despite being a deep-burial cement, was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces. The diagenetic character of the forced regressive surfaces/sequence boundaries is more complex than that of the transgressive surfaces, both in terms of mineralogy and paragenesis. This is attributed to the superimposition of diagenetic assemblages as a result of erosion, re-sedimentation and sediment by-passing. Although the diagenetic signature of the surfaces has been modified by dissolution/replacement reactions during deep burial, early diagenetic signatures can still be distinguished.  相似文献   

11.
通过中上扬子地区寒武系野外露头实测、踏勘、前人研究成果整理以及盆地腹地露头缺乏地区的钻井资料的分析,研究了中上扬子地区中寒武世的古地理背景。白云岩的成因、平面上以及垂向上的分布特征都受控于古地理。中寒武统整个台地区为局限台地,在台地发育咸化澙湖、潮坪、局限潮下、浅滩等次一级的古地理单元。中寒武统的膏岩与白云岩不同程度互层。向台地内部,滩相发育愈少,咸化澙湖、潮坪以及局限潮下越发育;越往台地的边缘,滩相越发育。研究区中寒武统白云岩按照晶体大小可以分为泥粉晶白云岩和砂糖状白云岩。泥粉晶白云岩为潮坪准同生白云岩,机理为蒸发泵作用,白云化流体来自于澙湖浓缩的海水。砂糖状白云岩绝大多数为回流渗透白云化成因,白云化流体主要来自于蒸发泵机理富余的高镁钙比流体。回流渗透白云化过程缓慢,白云石结晶较好。寒武系岩性在垂向上有很好的叠置关系,反映古地貌随地质时代的变迁。  相似文献   

12.
Pore water studies enable (1) the detection of diagenetic reactions actively occurring in the sediment at the time of sampling,(2) the distinction between principal modes of solute transport, i.e., between advection (convection) and diffusion, and (3) the assessment of mineral-solution equilibria, Pore waters are, therefore, preferred diagnostic objects in the study of diagenesis, particularly early diagenesis. The single most important factor for pore-fluid evolution in modern offshore basins is sedimentation rate which is closely correlated with organtic matter content of the sediment. Organic matter represents the most reactive sediment constituent which, through bacterial decomposition, provides some of the main solutes involved in early diagenetic mineralization reactions. On the basis of sedimentation rate and organic matter content, it is convenient to distinguish two end-members of basins (environments) with respect to early diagenesis: I. Low to intermediate-sedimentation rate basins with convection or diffusion-controlled pore-water profiles and II. high-sedimentation rate basins with reaction-controlled pore-water profiles. The first group of basins or environments, which is the subject of this paper, is typically represented by pelagic sediments. Three principally different trends of pore-water evolution occur: 1. convection-controlled profiles with no pronounced vertical gradients for the dissolved species; 2. diffusion-controlled profiles with vertical gradients but linear correlations between major ions, especially Ca and Mg, and 3. profiles with gradients but no linear correlation between the major ions. The later are transitional to the trends seen in high-sedimentation rate basins. With respect to redox-potentials, the diagenetic environments of low-sedimentation rate basins are generally oxic to suboxic.  相似文献   

13.
The Parnok deposit is made up of stratiform lodes of iron (magnetite) and manganese (oxide-carbonate, carbonate, and carbonate-silicate) ores localized among terrigenous-carbonate sediments (black shales) on the western slope of the Polar Urals. The lithological study showed that ore-bearing sediments were accumulated in a calm hydrodynamic setting within a relatively closed seafloor area (trap depressions). Periodic development of anaerobic conditions in the near-bottom seawater was favorable for the accumulation of dispersed organic matter in the terrigenous-carbonate sediments. Carbon required to form calcium carbonates in the ore-bearing sediments was derived from carbon dioxide dissolved in seawater. In the organic-rich sediments, carbonates were formed with the participation of carbon dioxide released by the destruction of organic matter. However, δ13C values (from 0.5 to ?4.4‰ PDB) suggest a relatively low fraction of the isotopically light biogenic carbon in the host calcite. The most probable sources of Fe and Mn were hydrothermal seepages at the seafloor. The Eh-pH conditions during stagnation were favorable for the precipitation of Fe and accumulation of Mn in a dissolved state. Transition from the stagnation regime to the concentration of oxygen in near-bottom waters was accompanied by oxidation of the dissolved Mn and its precipitation. Thus, fluctuations in Eh-pH parameters of water led to the differentiation of Fe and Mn. Initially, these elements were likely precipitated as oxides and hydroxides. During the subsequent lithification, Fe and Mn were reduced to form magnetite and rhodochrosite. The texture and structure of rhodochrosite aggregates indicate that manganese carbonates already began to form at the diagenetic stage and were recrystallized during the subsequent lithogenetic stages. Isotope data (δ13C from ?8.9 to ?17.1‰ PDB) definitely indicate that the oxidized organic matter of sediment served as the main source of carbon dioxide required to form manganese carbonates. Carbonates from host rocks and manganese ores have principally different carbon isotopic compositions. Unlike carbonates of host rocks, manganese carbonates were formed with an active participation of biogeochemical processes. Further processes of metagenesis (T ≈ 250–300°C, P ≈ 2 kbar) resulted in the transformation of textures, structures, and mineral composition of all rocks of the deposit. In particular, increase in temperature and pressure provided the formation of numerous silicates in manganese ores.  相似文献   

14.
In the modern hypersaline carbonate lagoon and sabkha sedimentary environments of Abu Dhabi (United Arab Emirates), three types of organic matter originate respectively from microbial mat, Avicennia mangrove, and Halodule lagoonal seagrass. The study of recent sedimentary processes and cross sections through the sabkha sediments lead to the definition of organo-sedimentary facies based on geochemical and sedimentological criteria. This permits the construction of an organo-sedimentary sequence which expresses the Holocene sedimentary record involving a transgressive and a regressive sequence. The various organic facies occur in both sequences.Heterogeneity within the individual organic facies reflects several factors, including sedimentation dynamics, mineral matrix, oxidation and reduction, and selective organic and mineral diagenesis. These parameters are discussed in terms of depositional environment and location within the organo-sedimentary sequence. Changes in distribution, quantity, and preservation potential of the buried organic matter are discussed in terms of sea level changes and sedimentary accretion rates.  相似文献   

15.
We analyzed 77 surface sediment samples collected in the southwestern East/Japan Sea from the Korea Strait through the Ulleung Basin and the Korea Plateau for grain size, calcium carbonate, organic carbon, and major (Na, Mg, Al, Fe, K, Ca, and Ti) and trace elements (P, Mn, Sr, Li, Sc, V, Cr, Co, Ni, Zn, Cu, and Pb).The chemical composition of the surface sediments was found to be highly variable spatially. Cluster analysis of surface sediment chemical compositions indicated five major geochemical sedimentary environments: basin, lower slope, coast and upper slope, inner shelf, and outer shelf. Continental-shelf sediments were rich in shell fragments and had relict and coarse-grained characteristics. Recent fine-grained sediments were only distributed in coastal, slope, and basin areas. Concentrations of Al, K, Ca, Ti, Cr, and Sc were highest in the coastal and upper slope areas and decreased with water depth. Elemental ratios using major and trace elements indicated that coastal and upper slope detrital sediments were mixtures of sediments derived from the Changjiang (Yangtze) and Nakdong Rivers. Although the concentrations of organic carbon, P, Mn, V, Co, Ni, Cu, and Pb increased with water depth, their distribution patterns indicated authigenic (V, Cu, and Pb) and diagenetic (Fe, P, Mn, Co, and Ni) origins. The distribution pattern with water depth suggested that the chemical composition of surface sediment was determined by sedimentologic and geochemical processes, such as the supply of detrital and biogenic materials, and authigenic and post-depositional diagenetic processes in sediments.  相似文献   

16.
The Cow Head Group is an Early Palaeozoic base-of-slope sediment apron composed of carbonate and shale. Whereas coarse-grained conglomerate and calcarenite are readily interpreted as debris-flow and turbidite deposits, calcilutite (lime mudstone), calcisiltite, and shale combine to form three distinct lithofacies whose present attributes are a function of both sedimentation and early diagenesis. Shale is the most common lithology. Black, green, and red shale colour variations reflect the abundance of organic matter in the source area and oxygenation conditions of the sea bottom. In black and green shale, millimetre- to centimetre-thick, alternating dark and light laminations represent terrigenous mud turbidites and hemipelagites, respectively. The calcisiltite/shale facies is uncommon and is composed of numerous graded carbonate-shale sequences (GCSS) deposited from waning carbonate turbidites and fall-out of terrigenous muds. Some of the characteristics of ribbon and parted lime mudstones in the calcilutite/shale facies can be explained by deposition of carbonate mud from dilute turbidity currents or hemipelagic settling. Other features are diagenetic in origin. The lack of micrite in GCSS and in the interbedded shales of the calcilutite/shale facies is interpreted to reflect early dissolution of the finer carbonate from these sediments. This remobilized carbonate was precipitated locally to: lithify lime mudstone turbidites or hemipelagites; form diagenetic lime mudstone beds and nodules; cement calcisiltites; and form dolomite. Many of the calcisiltites and calcilutites were, therefore, carbonate enriched at the expense of adjacent argillaceous sediments. These attributes characterize not only fine-grained sediments of the Cow Head Group but many other Early Palaeozoic slope carbonates as well, suggesting that the model proposed here for depositionl diagenesis has wider application.  相似文献   

17.
The Vempalle Formation of the Proterozoic Cuddapah basin has a well developed sequence of carbonate rocks, which are interbedded with shales, siltstones and chert. The stromatolitic carbonates are conspicuous at many places but the oolitic carbonates are less prominent and are present only in some areas. All the carbonates are pervasively dolomitized. Petrographic examination of these carbonates revealed that they are predominantly made up of fine grained micrite with patchy development of sparite and chert/quartz. The stromatolitic carbonates show distinct banding of alternate carbonate and cherty layers. The latter are rich in organic matter indicating prevalence of profuse biogenic activity. The oolitic carbonates comprise of ooids showing both concentric and radial patterns and made up of carbonate/chert and cemented by micro/mega quartz or carbonate itself. Diagenetic and post depositional features are reflected in cementation, recrystallization, compaction, stylolite formation and silicification processes. Various stages of cementing material are observed. Secondary vein fillings of carbonate or quartz traverse the carbonate/cherty groundmass. Intraclasts present suggest occasional erosional destruction of associated sediments, short lived transport and local redeposition. Accessory silicate minerals represent terrigenous influx during deposition. Dolomitization of the carbonates was fabric retentive and early diagenetic. The environmental conditions were characterised by low energy, within a shallow water zone, in occasional higher energy events and turbulence. The carbonates appear to have been deposited on a shallow water ramp within a tidal regime.  相似文献   

18.
Research concerning the fate and biogeochemical cycling of mercury (Hg) within coastal ecosystems has suggested that microbially mediated diagenetic processes control Hg mobilization and that ligands with strong affinity for Hg, such as dissolved inorganic sulfide (S(-II)) and dissolved organic matter (DOM), control Hg partitioning between the dissolved and particulate phases. We have studied total Hg cycling in the sediments of the Penobscot River estuary using a combination of equilibrium porewater samplers and kinetic modeling. The Penobscot estuary has been subject to Hg contamination from multiple industries including a recently closed chlor-alkali production facility. The Hg concentration within the estuary surface sediments ranges from 1.25 to 27.5 nmol Hg g−1 sediment and displays an association with sediment organic matter and a concentration maximum within 3 cm of the sediment-water interface (SWI). Porewater profiles for the Penobscot estuary are divisible into three kinetically discrete intervals with respect to Hg dynamics. Beginning at depth in the sediment and moving upward toward the SWI we have defined: (1) a zone of net Hg solubilization at depth, with a zero-order net Hg production rate , (2) a zone of net Hg consumption within the zone dominated by FeS(s) precipitation with , and (3) a zone of net diffusive transfer within the vicinity of the SWI. Zone 1 is characterized by dissolved S(-II) concentrations ranging from 400 to 500 μM. Equilibrium modeling in this zone suggests that inorganic S(-II) plays the dominant role in both mobilization of sediment-bound Hg and complexation of dissolved Hg. In zone 2, FeS(s) precipitation occurs concomitant with Hg consumption. Net transfer within zone 3 is consistent with the potential for ligand-mediated Hg efflux across the SWI. S(-II)-mediated Hg mobilization at depth in Penobscot estuary sediments suggests a broadening of the depth interval over which biogeochemical Hg cycling must be examined. Our results also show that, while estuary sediments act as a net sink for particulate Hg inputs, they may also function for a considerable time interval as a source of dissolved Hg.  相似文献   

19.
The geochemistry data show that the total organic carbon （TOC） contents are high in the surface sediments in the eastern part of the Moknine＇ s Sebkha. Low decreasing of organic matter （ OM ） with increasing depth indicates the good preservation of OM in modern sediments. ＂Lignite levels＂ inserted in the sand sequence and deposited at the edge and in the intermediate zone are considered as lithologic and organic markers. It is characterized by high TOC and poor hydrogen index （HI） , indicating a higher plant origin and good preservation of OM in thin beds under anoxic condition. In all samples low values of HI are typical for strong terrigenous input in the Sebkha. Gas chromatography （GC） of saturate fraction showed that OM in the border zone is provided from plants but in the central zone OM is a mixture of terrestrial fraction and little fraction from microalgae. The study of free lipids indicated that this environment was influenced by intense bacterial and microbial activities, as evidenced by the abundance of n-alkanes and nC18-nC22.  相似文献   

20.
A peculiar facies of the Norian–Rhaetian Dachstein‐type platform carbonates, which contains large amounts of blackened bioclasts and dissolutional cavities filled by cements and internal sediments, occurs in the Zlatibor Mountains, Serbia. Microfacies investigations revealed that the blackened bioclasts are predominantly Solenoporaceae, with a finely crystalline, originally aragonite skeleton of fine cellular structure. Blackening of other bioclasts also occurs subordinately. Solenoporacean‐dominated reefs, developed behind the platform margin patch‐reef tract, were the main source of sand‐sized detritus. The blackened and other non‐blackened bioclasts are incorporated in automicrite cement. Radiaxial fibrous calcite cements in the dissolutional cavities are also black, dark grey or white. Reworked black pebbles were reported from many occurrences of peritidal deposits; in those cases, the blackening took place under pedogenic, meteoric diagenetic conditions. In contrast, in the inner platform deposits of the Ilid?a Limestone, the blackening of bioclasts occurred in a marine–meteoric mixing‐zone, as indicated by petrographic features and geochemical data of the skeleton‐replacing calcite crystals. Attributes of mixing‐zone pore waters were controlled by mixing corrosion, different solubility of carbonate minerals and microbial decomposition of organic matter. In the moderate‐energy inner platform environment, large amounts of microbial organic tissue were accumulated and subsequently decomposed, triggering selective blackening in the course of early, shallow burial diagenesis. The δ18O and δ13C values of the mixing‐zone precipitates and replacive calcite do not produce a linear mixing trend. Variation mainly resulted from microbial decomposition of organic matter that occurred under mixing‐zone conditions. The paragenetic sequence implies cyclic diagenetic conditions that were determined by marine, meteoric and mixing‐zone pore fluids. The diagenetic cycles were controlled by sea‐level fluctuations of moderate amplitude under a semi‐arid to semi‐humid climate.  相似文献   

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