The geochemistry of salt marshes: Sedimentary ion diffusion,sulfate reduction,and pyritization

A series of seasonal cores was taken in a high marsh near the terminus of Delaware Bay, U.S.A. A seasonal harmonic diffusion model was successfully fit to the concentration profiles of chloride ion in the salt marsh pore waters yielding a calculated sedimentary diffusion coefficient.Virtually all other chemical reactions within salt marsh sediments are directly linked to the rate and stoichiometry of organic decomposition. The rich organic input from the grass Spartina alterniflora is oxidized anaerobically through the process of sulfate reduction. Over 90% of this net decomposition of organic matter takes place in the uppermost 20 cm. The model for sulfate reduction proposed yields an internally consistent set of both pore water (HCO^?₃, NH⁺₄, HPO^2?₄, HS^?, SO^2?₄) and solid phase (FeS₂) distribution profiles for these sediments. Steady state assumptions and the use of mean annual constants can be employed to model the net rates of diagenetic processes in salt marshes. The pore water concentrations of sulfate ion as well as those ions released by sulfate reduction (HCO^?₃, NH⁺₄, HPO^2?₄, HS^?) are modeled by a system composed of an upper zone, where extensive reconsumption of these metabolite ions occurs, and a lower zone where steady state production and no ion reconsumption occurs.A major product of the sulfate reduction is pyrite, whose accumulation rate is greatest between 7 and 9 cm depth, where it equals the net rate of sulfate reduction. Above this zone little pyrite accumulates due to extensive reoxidation. Below 9 cm the rate of pyritization is controlled by the rate of sulfidation of a refractory iron phase.     

Tidal and seasonal variations of sulfate ion in a New Jersey marsh system

Sulfate variations during a tidal cycle were investigated at three sites in a highly polluted tidal marsh. Sulfate- chlorinity relationships were determined in the light of fluctuations in temperature, dissolved oxygen and pH. The relationships were found to be seasonal in nature, being affected by temperature and rainfall effects on biologic productivity. Both reduction of sulfate ion to a sulfide species and oxidation of sulfide species to the sulfate ion were found to occur. Consideration of the sulfide oxidation process suggests the possibility that metals precipitated as sulfides may be mobilized and redistributed in the marsh system.     

Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides,elemental sulfur,and pyrite

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.     

The dependence of bacterial sulfate reduction on sulfate concentration in marine sediments

The effect of dissolved sulfate concentration on the rate of bacterial sulfate reduction in marine sediment from Long Island Sound was examined using a radio-sulfur technique. The experimental results show that the rate is independent of the dissolved sulfate concentration until low levels are reached (<3 mM), and that, when interpreted using a Monod-type rate law, a saturation constant, K_s, of 1.62 ± 0.16 M results. This weak dependence implies that the dissolved sulfate exerts only a limited influence on the rate of sulfate reduction in marine sediments. Given such a weak dependence, dissolved sulfate profiles in marine sediments must resemble profiles generated by models with sulfate independent kinetics. Initially, this would suggest that currently used sulfate-independent diagenetic models are appropriate in modelling sulfate profiles. However, comparison of these models with those containing weak sulfate-dependent kinetic terms shows that there exists considerable disagreement between these models when the parameter grouping $\text{(D}{}_{\mathrm{s}}\text{k)}\text{1}\text{2}\text{/w}$ is larger than ~0.2 and smaller than ~3.0. (Here D_s is the SO^;₄ diffusion coefficient, k the organic matter decay constant and w the sediment burial velocity.) When the currently used models are corrected by employing physically meaningful boundary conditions, this divergence disappears. The modelling results, therefore, confirm the conclusion that any sulfate dependence inherent to the reduction kinetics does not appreciably affect sulfate pore water profiles, and that previous diagenetic studies using strong sulfate dependent models are erroneous.     

潮汐作用下盐沼孔隙水流动及溶质运移模拟

滨海盐沼是重要的陆地-海洋交界带生态系统。目前国际上存在关于盐沼的两大假设:盐沼系统输出养分和盐沼植物带状分布。为验证这两大假设,增强对盐沼湿地的了解,盐沼孔隙水流动及溶质运移研究至关重要。为模拟复杂盐沼系统孔隙水流动及溶质运移,改进了美国地质勘测局编制的SUTRA程序。基于假定的潮沟横断面物理条件,对孔隙水流动及溶质运移过程进行了模拟分析。结果表明潮沟附近孔隙水及溶质交换较快,潮水浸淹会减缓潮沟附近出现物质集结。落潮时潮沟附近有明显垂向流和水平流,远潮沟地带主要为水平流。潮沟附近土壤通气条件较好。这些模拟结果较好的吻合了潮沟附近较盐沼内部盐沼植物长势较好的现象。     

Regulation of bacterial sulfate reduction and hydrogen sulfide fluxes in the central namibian coastal upwelling zone

The coastal upwelling system off central Namibia is one of the most productive regions of the oceans and is characterized by frequently occurring shelf anoxia with severe effects for the benthic life and fisheries. We present data on water column dissolved oxygen, sulfide, nitrate and nitrite, pore water profiles for dissolved sulfide and sulfate,³⁵S-sulfate reduction rates, as well as bacterial counts of large sulfur bacteria from 20 stations across the continental shelf and slope. The stations covered two transects and included the inner shelf with its anoxic and extremely oxygen-depleted bottom waters, the oxygen minimum zone on the continental slope, and the lower continental slope below the oxygen minimum zone. High concentrations of dissolved sulfide, up to 22 mM, in the near-surface sediments of the inner shelf result from extremely high rates of bacterial sulfate reduction and the low capacity to oxidize and trap sulfide. The inner shelf break marks the seaward border of sulfidic bottom waters, and separates two different regimes of bacterial sulfate reduction. In the sulfidic bottom waters on the shelf, up to 55% of sulfide oxidation is mediated by the large nitrate-storing sulfur bacteria, Thiomargarita spp. The filamentous relatives Beggiatoa spp. occupy low-O₂ bottom waters on the outer shelf. Sulfide oxidation on the slope is apparently not mediated by the large sulfur bacteria. The data demonstrate the importance of large sulfur bacteria, which live close to the sediment-water interface and reduce the hydrogen sulfide flux to the water column. Modeling of pore water sulfide concentration profiles indicates that sulfide produced by bacterial sulfate reduction in the uppermost 16 cm of sediment is sufficient to account for the total flux of hydrogen sulfide to the water column. However, the total pool of hydrogen sulfide in the water column is too large to be explained by steady state diffusion across the sediment-water interface. Episodic advection of hydrogen sulfide, possibly triggered by methane eruptions, may contribute to hydrogen sulfide in the water column.     

Biogeochemical Modelling of a Seasonally Anoxic Lake: Calibration of Successive and Competitive Pathways and Processes in Lake Aydat,France

A reactive transport model was developed to describe seasonal variations of biogeochemical and physical processes in Lake Aydat. The model includes physical processes such as vertical mixing, sedimentation and advection related to inflows into the lake and biogeochemical conversion processes in the water column and in the sediment surface layer. The reactions described in the model include primary redox reactions such as primary production, aerobic and anaerobic respiration, methanogenesis and secondary reactions established between oxidants and reducers produced by the primary reactions. After adjusting various kinetic constants, the model reasonably reproduced the main features of seasonal variations of dissolved oxygen and nitrate depth profiles and pH. The reactive transport model was also used to quantify the relative importance of different biogeochemical pathways. For instance, ferrous denitrification seems to play an important role when stratification is increasing.     

Temporal and spatial variation in intertidal sedimentation rates

Sediment deposition and erosion rates are reported for an intertidal zone in the Burry Inlet, South Wales. Measured deposition rates over the salt marsh are compared with deposition rates calculated from observed suspended sediment concentrations. Notably, it is concluded that residual turbulence at slack water should not be discounted when calculating deposition rates. Grain-size distributions of suspended sediments over the marsh surface, during flood and ebb tides, contrast with the grain-size distribution of deposited marsh sediments, the latter being significantly coarser. These data in conjunction with mass budget calculations are used to relate total annual deposition and sediment supply by tidal action during settled meteorological periods. The analysis suggests that episodic storm-induced sediment transport is probably an important mechanism for introducing coarse sediment on to the marsh surface. Finally, it is noted that seasonal reworking of the sandy non-cohesive sediments may be related to variations in the intensity of wave-breaking throughout the year.     

Quantification of co-occurring reaction rates in deep subseafloor sediments

Net rates of biogeochemical reactions in subseafloor sediments can be quantified from concentration profiles of dissolved reactants or products and physical properties of the sediment. To study net rates of microbial activities in deep sediments, we developed a robust approach that is well suited to use over a broad range of sediment depths. Our approach is based on a finite-difference solution to a continuity equation that considers molecular diffusion, sediment burial, fluid advection, and reaction under the assumption of steady state. Numerical procedures are adopted to identify the maximum number of depth intervals with statistically different reaction rates. The approach explicitly considers downcore variation in physical properties and sample spacing. Uncertainties in the rate estimates are quantified using a Monte Carlo technique. We tested our approach using synthetic concentration profiles generated from analytical solutions to the continuity equation. We then applied the approach to concentration profiles of dissolved sulfate, sulfide, methane, and manganese in the 420-m thick sediment column of eastern equatorial Pacific Ocean Drilling Program Site 1226. Our results indicate that (i) sulfate reduction and iron reduction occur at most sediment depths, (ii) net methane production occurs in discrete depth intervals and (iii) manganese reduction occurs near the seafloor and deep in the sediments. These results provide quantitative evidence that multiple respiration pathways co-exist in the same depth intervals of these deep subseafloor sediments.     

Sedimentation rates in flow-restricted and restored salt marshes in Long Island Sound

Many salt marshes in densely populated areas have been subjected to a reduction in tidal flow. In order to assess the impact of tidal flow restriction on marsh sedimentation processes, sediment cores were collected from flow-restricted restricted salt marshes along the Connecticut coast of Long Island Sound. Cores were also collected from unrestricted reference marshes and from a marsh that had been previously restricted but was restored to fuller tidal flushing in the 1970's. High bulk densities and low C and N concentrations were found at depth in the restricted marsh cores, which we attribute to a period of organic matter oxidation, sediment compaction, and marsh surface subsidence upon installation of flow restrictions (between 100 and 200 years before the present, depending on the marsh). Recent sedimentation rates at the restricted marshes (as determined by¹³⁷Cs and²¹⁰Pb dating) were positive and averaged 78% (¹³⁷Cs) and 50% (²¹⁰Pb) of reference marsh sedimentation rates. The accumulation of inorganic sediment was similar at the restricted and reference marshes, perhaps because of the seasonal operation of the tide gates, while organic sediment accretion (and pore space) was significantly lower in the restricted marshes, perhaps because of higher decomposition rates. Sedimentation rates at the restored marsh were significantly higher than at the reference marshes. This marsh has responded to the higher water levels resulting from restoration by a rapid increase in marsh surface elevation.     

Sediment community metabolism associated with continental shelf hypoxia, Northern Gulf of Mexico

Net fluxes of respiratory metabolites (O₂, dissolved inorganic carbon (DIC), NH₄ ⁺, NO₃ ^?, and NO₂ ^?) across the sediment-water interface were measured using in-situ benthic incubation chambers in the area of intermittent seasonal hypoxia associated with the Mississippi River plume. Sulfate reduction was measured in sediments incubated with trace levels of³⁵S-labeled sulfate. Heterotrophic remineralization, measured as nutrient regeneration, sediment community oxygen consumption (SOC), sulfate reduction, or DIC production, varied positively as a function of temperature. SOC was inversely related to oxygen concentration of the bottom water. The DIC fluxes were more than 2 times higher than SOC alone, under hypoxic conditions, suggesting that oxygen uptake alone cannot be used to estimate total community remineralization under conditions of low oxygen concentration in the water column. A carbon budget is constructed that compares sources, stocks, transformations, and sinks of carbon in the top meter of sediment. A comparison of remineralization processes within the sediments implicates sulfate reduction as most important, followed by aerobic respiration and denitrification. Bacteria accounted for more than 90% of the total community biomass, compared to the metazoan invertebrates, due presumably to hypoxic stress.     

Drying effects on sorption capacity of coastal sediment: The importance of architecture and polarity of organic matter

Investigations on how desiccation changes sorption of organic compounds by salt marsh sediments provide insight into the physical and chemical properties of these wide-spread coastal sediments. We measured sorption of compounds with different polarities (lysine, tyrosine, naphthalene and aniline) onto natural sediments and sediments that were dried and rewetted. Sorption of lysine by marsh sediment decreased significantly when the sediment was dried using a freeze-drier, oven, or desiccator, and sorption capacity was not restored when sediments were rewetted. In contrast to lysine, the sorption capacity of more hydrophobic compounds (tyrosine, aniline and naphthalene) increased significantly after salt marsh sediment was dried. These results suggest that drying greatly increased sediment hydrophobicity. Consistently, water drop penetration time, an index of hydrophobicity, was significantly lower for combusted sediments than for those that were simply dried. Sediments treated with EDTA, or boiled in seawater, exhibited a similar or even greater reduction in lysine sorption capacity compared with sediments that were dried. Water retention capacity of salt marsh sediment decreased 50% after sediment was dried. The effects of pH and salinity on lysine sorption in wet and dry sediments suggest that carboxyl groups play a major role in lysine sorption through cation ion exchange, and drying may reduce access to carboxyl groups. We hypothesize that the three-dimensional (3D) structure of organic matter, originating mainly from Spartina alterniflora, is an important factor controlling sorption capacity in salt marsh sediment. The drying process makes sedimentary organic matter change conformation, shrink in volume, and expose hydrophobic groups, thus becoming more hydrophobic. In environments with wet and dry cycles, the distribution of hydrophobic or hydrophilic compounds between solution and particulate phases could thus be influenced by the 3D structure and polarity of organic matter.     

Biogeochemical cycling in an organic-rich coastal marine basin—3. Dissolved gas transport in methane-saturated sediments

The transport of dissolved gases in the anoxic sediments of Cape Lookout Bight, North Carolina, is controlled by diffusion and bubble ebullition and exhibits a distinct seasonal cycle. Detailed seasonal profiles of CH₄, N₂ and ²²²Rn and direct gas flux measurements indicate that ebullition dominates the flux of all dissolved gases across the sediment-water interface during summer months, and is of major importance on an annual basis. Transport within the upper 6–8 cm of sediment appears to be controlled by molecular diffusion of gases. Transport at greater depths is controlled by diffusion in winter and ebullition in summer. Rn-222 profiles were used to estimate the rate of stripping of dissolved gases within the CH₄ production zone (10–30 cm); rates averaged 3–5 percent per day and agreed with estimates derived from N₂ profiles. As a result of summer ebullition, the sediments of the bight are never at saturation with respect to the major gases present in seawater. Evidence from other sites suggests that ebullition may be an important transport process in many coastal sediments, and may provide mechanism for the transport of volatile reduced compounds between anoxic sediments and the atmosphere. ²²²Rn is a useful tracer for quantifying this transport.     

Pyrite accumulation and sulfate depletion as affected by root distribution in aJuncus (needle rush) salt marsh

This study was undertaken to investigate patterns of pyrite accumulation found in aJuncus roemerianus tidal marsh of north Florida. We speculate that the pattern of pyrite accumulation was caused mainly by the distribution of roots. Sediment cores from living stands (LS), dead stands (DS), and recently killed stands (RKS) were collected and analyzed for dissolved organic carbon (DOC), sulfate, chloride, Cr(II)-reducible sulfide, and biomass of roots and rhizomes. Living roots were distributed mainly in the upper 16 cm and concentrated 4–10 cm below the sediment surface. Pyrite accumulations were significantly different among the three types of sediment cores in the upper 16 cm sediment and follow the descending order of DS (615 μmol cm^?3)>RKS (547 μmol cm^?3)>LS (368 μmol cm^?3). Between 20 cm and 30 cm, pyrite contents approached values of 40 μmol cm^?3 and 55 μmol cm^?3 in all sediment cores. The degree of pyritization approaches 92–94% between 14 cm and 20 cm where pyrite accumulation was probably limited by available iron. Root distribution also affected the redistribution of iron through iron sulfide formation. Sulfate depletion in the upper 16 cm was higher in RKS (79.3 μmol cm^?3) than DS (49.1 μmol cm^?3). No significant sulfate depletion was found in LS. Sulfate reduction under DS was likely limited by the readily available substrates. Root distribution had a major influence on pyrite accumulation and sulfate reduction of the marsh, and its effects need to be addressed in studies of wetland sulfur dynamics.     

Weathering and Geochemical Processes Controlling Solute Acquisition in Ganga Headwater–Bhagirathi River, Garhwal Himalaya, India

Water and suspended sediment samples were collected along a longitudinal transect of the Bhagirathi – a headwater stream of the river Ganga, during the premonsoon and postmonsoon seasons, in order to assess the solute acquisition processes and sediment transfer in a high elevation river basin. Study results show that surface waters were dominated by HCO₃ and SO₄ in anionic abundance and Ca in cationic concentrations. A high concentration of sulphate in the source region indicates oxidative weathering of sulphide bearing minerals in the drainage basin. The combination of high concentrations of calcium, bicarbonate and sulphate in river water suggests that coupled reaction involving sulphide oxidation and carbonate dissolution are mainly controlling the solute acquisition processes in the drainage basin. The sediment transfer reveals that glacial weathering and erosion is the major influence on sediment production and transfer. The seasonal and spatial variation in ionic concentration, in general, is related to discharge and lithology. The sediment mineralogy and water mineral equilibrium indicate that water composition is in equilibrium with kaolinite. The river Bhagirathi annually delivers 0.74 M.tons of dissolved and 7.88 M.tons of suspended load to the river Ganga at Devprayag. The chemical and physical denudation rate of the Bhagirathi is 95 and 1010 tons/km²/yr, higher than the Indian and global average.     

The concentration and distribution of different mercury species in the water columns and sediment of Aha Lake

In order to find out whether Aha Lake was polluted by the acid mining waste water or not, the concentration and distribution of different mercuryspecies in the water columns and sediment profile collected from Aha Lake were investigated. It was found that discernible seasonal variation of different mercury species in water body were obtained in the Aha Reservoir. With regards to the whole sampling periods, the concentrations of HgP in the Aha Reservoir water body were evidently correlated to the concentrations of total mercury, showing that total mercury was mostly associated with particle mercury. The concentrations of methylmercury in water body were also evidently correlated to the concentrations of dissolved mercury. The dissolved mercury evidently affects the distribution and transportation of methylmercury. However, there is no correlation between methylmercury and total mercury. The dissolved mercury, reactive mercury, dissolved methylmercury levels in the water body of high flow period were much higher than those in low flow period. The distribution, speciation and levels of mercury within the Aha Reservoir water body were governed by several factors, such as the output of river, the release of sediment . Discernible seasonal variation of total mercury and methylmercury in porewater was described during the sampling periods, with the concentrations in high flow period generally higher than those in low flow period. The methylmercury in pore water column was evidently correlated to that of the sediment. The results indicated that highly elevated MeHgD concentrations in the porewater were produced at the depths from 2 to 5 cm in the sediment profile, and decreased sharply with depth. A positive correlation has been found between MeHgD formation and sulfate reducing bacterial activity. These highly elevated concentrations of MeHgD at the intersurface between waters and sediments suggest a favorable methylation condition. Moreover,     

Transport of dissolved methoxy-nonafluorobutane through a saturated column

Solute transport experiments were conducted in a one-dimensional saturated column using dissolved methoxy-nonafluorobutane (HFE-7100), a Novec engineered fluid developed by the 3M Corporation, as the solute. Novec engineered fluids are considered dense non-aqueous phase liquids (DNAPLs) because they are immiscible with water and have a specific gravity greater than one. The HFE-7100 fluid is safer and environmentally friendlier than common DNAPL contaminants such as tetrachloroethylene (PCE) or trichloroethylene (TCE); thus, it is an ideal substitute DNAPL for laboratory groundwater contamination research. Three sets of solute transport experiments were conducted. The first set of experiments was conducted in a glass-bead-packed column using dissolved HFE-7100 as the solute. The second set of experiments was conducted in a sand-packed column using dissolved HFE-7100 as the solute. The third set of experiments was conducted in a sand-packed column using dissolved PCE as the solute. The dissolved HFE-7100 column breakthrough concentrations were compared with dissolved PCE breakthrough concentrations. Results show that the one-dimensional solute transport equation was successful in describing the transport behavior of dissolved HFE-7100. This study demonstrates that the HFE-7100 fluid can be used as a safer substitute DNAPL for groundwater contaminant dissolution and transport research.     

Patterns of sediment deposition in subsiding coastal salt marshes,Terrebonne Bay,Louisiana: The role of winter storms

High rates of wetland loss in the Mississippi deltaic plain have been attributed to a combination of insufficient marsh sedimentation and relative sea-level rise rates of over 1.2 cm yr^?1. This study examines contemporary patterns of sediment delivery to the marsh surface by evaluating the contribution of individual marsh flooding events. Strong meteorological effects on water level in Terrebonne Bay often mask the usual microtidal fluctuations in water level and cause flood events to be of unpredictable frequency and duration. Sediment deposited on the marsh surface was collected weekly at two sites. Preliminary results allow the relative contributions of tidal and storm inundations to be calculated. Maximum sedimentation is associated with strong southerly winds both causing increased flooding and mobilizing sediment from open bay areas. Sediment deposition is limited by the availability of suspended sediment and the opportunity for its transport onto the marsh surface.     

Microbial nitrogen removal in a developing suburban estuary along the South Carolina coast

The conversion of undisturbed coastal regions to commercial and suburban developments may pose a threat to surface and groundwater quality by introducing nitrate-nitrogen (NO₃ ^?-N) from runoff of land-applied wastewater and fertilizers. Microbial denitrification is an important NO₃ ^?-N removal mechanism in coastal sediments. The objective of this study was to compare denitrification and nitrate conversion rates in coastal sediments from a golf course, suburban site, undeveloped marsh, and nonmarsh area near rapidly developing Hilton Head Island, South Carolina. Nitrous oxide was measured using gas chromatography and nitrate and ammonium concentrations were measured using a flow injection autoanalyzer in microcosms spiked, with 50 μg NO₃ ^?-N gdw^?1. The two marsh sites had the greatest ammonium production, which was correlated with fine sediment particle size and higher background sediment nitrate and surface water sulfate concentrations. The golf course swale had greatest denitrification rates, which were correlated with higher total carbon and organic nitrogen in sediments. Nitrate was consumed in golf course sediments to a greater extent than in the undeveloped marsh and upland freshwater sites, suggesting that the undeveloped sites and receiving estuaries may be more susceptible to nitrate contamination than the golf course swale and marsh under nonstorm conditions. Construction of swales and vegetated buffers using sediments with high organic carbon content as best management practices may aid in removing nitrate and other contaminants from runoff prior to its transport to the receiving marsh and estuary.     

Hydrochemistry,origin and evolution of sedimentary subsurface fluids: I. Early diagenetic pore-water evolution in low-sedimentation rate offshore basins

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.