Anoxic Pore Waters Research Articles

Sedimentological, geochemical and microfaunal responses to environmental changes around the Cenomanian–Turonian boundary in the Outer Carpathian Basin; a record from the Subsilesian Nappe, Poland

Sedimentological, micropalaeontological and geochemical data from the upper Cenomanian–lower Turonian Zasań section (Outer Carpathians, Subsilesian Nappe, Poland) provide new insights into the sedimentary environment and its changes in a basin at the northern margin of the Western Tethys. A global oceanic event (OAE2) has been identified on the basis of lithology and radiolarian stratigraphic data. This event is marked by a series of organic-rich (TOC up to 5%) non-calcareous shales, practically devoid of benthic microfossils, indicative of bottom water anoxia. The marine type of organic matter and the occurrence of radiolarian-rich layers, with abundant small opportunistic forms, suggest that coastal upwelling was probably the main driving force for the accumulation of organic matter during the latest Cenomanian. The early diagenetic ferromanganese nodules followed by a 30 cm-thick series of manganese-rich non-calcareous variegated shales and bentonites, directly overlie the black facies of OAE2. The Fe–Mn nodules formed in two stages reflecting changes in bottom water dynamics across the C–T boundary. During the first stage, the Fe–Mn carbonate nodules were precipitated in presence of anoxic pore and/or bottom water that could be episodically oxygenated. Later, an increase in deep-water circulation led to strong oxygenation of the bottom and pore water, causing diagenetic modification of the nodule carbonate into ferromanganese oxides and hydroxides. The manganiferous sediments recorded in the Zasań section represent an isochronous horizon within the Outer Carpathian Basin, probably concurrent with manganese enrichments in many other epicontinental seas and deep-water basins, coinciding with the end of the carbon isotope excursion close to the base of the Turonian. The manganese event may have been related to increased deep water circulation associated with the maximum flooding of the late Cenomanian–early Turonian transgression. The Mn event may have also been enhanced as a result of remobilization of Mn 2+ dissolved in anoxic bottom and pore waters in the organic-rich sediments directly underlying the manganese sediments. The lower Turonian succession records increasing, but not completed, oxygenation of bottom water. The oxygenation was interrupted by periods of anoxia, marked by a few intervals of dark laminated black shales, strongly enriched in organic matter (TOC up to 10%). Their marine origin and the occurrence of radiolarian-rich layers demonstrate enhanced primary productivity and a low clastic supply during the sea-level high-stand. Frequency of radiolarian-rich layers increases up section, reflecting an increase in upwelling circulation at the northern margin of the Carpathian Basin during the Early Turonian.

Irrigation patterns in permeable sediments induced by burrow ventilation: a case study of Arenicola marina

In sandy sediments, a strong connection exists between the physics of flow and the ecol- ogy of burrow-ventilating macrofauna. We developed a general modelling procedure that quantifies this link involving 3 steps. (1) Burrow-ventilating organisms can be described as mechanical pumps. (2) The pumping of burrow water into blind-ending tubes induces advective flow in the sediment. (3) The resistance to pore water flow is governed by the friction between solid and fluid, i.e. Darcy's law. This analysis allows the determination of the operation point of an 'organism pump' under in situ conditions, and we applied it in a detailed modelling study of the lugworm Arenicola marina. A 3-dimensional finite element model encompasses the lugworm's J-shaped burrow and represents a typical lugworm territory at in situ density. We simulated the associated flow patterns in the sediment and analysed the factors that influence the lugworm's ventilation rate. Since the lugworm's oxygen supply critically depends on the burrow ventilation rate, we advance the following 2 ecological hypotheses: (1) decreasing the permeability of the burrow lining greatly increases the efficiency of oxygen supply, as it prevents the re-entry of anoxic pore water; and (2) the permeability of the bulk sediment constrains the lugworm's habitat. When permeability falls below a critical threshold, the sediment's resistance becomes too high, thus resulting in an insufficient oxygen supply. Overall, we show that permeability exerts an important control on ventilation activity, and hence resource avail- ability, in sandy sediment ecosystems. From an evolutionary point of view, we anticipate biological feedbacks on this physical control — in particular, behavioural adaptations that increase the perme- ability in the bulk sediment but decrease the permeability near the burrow wall.

Reactive iron enrichment in sediments deposited beneath euxinic bottom waters: constraints on supply by shelf recycling

Abstract Modern and ancient euxinic sediments are often enriched in iron that is highly reactive towards dissolved sulphide, compared to continental margin and deep-sea sediments. It is proposed that iron enrichment results from the mobilization of dissolved iron from anoxic porewaters into overlying seawater, followed by transport into deep-basin environments, precipitation as iron sulphides, and deposition into sediments. A diagenetic model shows that diffusive iron fluxes are controlled mainly by porewater dissolved iron concentrations, the thickness of the surface oxygenated layer of sediment and to a lesser extent by pH and temperature. Under typical diagenetic conditions (pH < 8, porewater Fe 2+ = 10 −6 g cm −3 ) iron can diffuse from the porewaters in continental margin sediments to the oxygenated overlying seawater at fluxes of 100–1000 μg cm −2 a −1 . The addition of reactive iron to deep-basin sediments is determined by the magnitude of this diffusive flux, the export efficiency (ɛ) of recycled iron from the shelf, the ratio of source area ( S ) to basin sink area ( B ) and the trapping of reactive iron in the deep basin. Values of ɛ are poorly constrained but modern enclosed or semi-enclosed sedimentary basins show a wide variation in S/B ratios (0.25–13) where the shelf source area is defined as sediments at less than 200 m water depth. Diffusive fluxes in the range 100–1000 μg cm −2 a −1 are able to produce the observed reactive iron enrichments in the Black Sea, the Cariaco Basin and the Gotland Deep for values of ɛ × S/B from 0.1–5. Transported reactive iron can be trapped physically and/or chemically in deep basins. Physical trapping is controlled by basin geometry and chemical capture by the presence of euxinic bottom water. The S/B ratios in modern basins may not be representative of those in ancient euxinic/semi-euxinic sediments but preliminary data suggest that ɛ × S/B in ancient euxinic sediments has a similar range as in modern euxinic sediments.

Factors affecting dissolved organic matter dynamics in mixed-redox to anoxic coastal sediments

Mixed-redox (suboxic, or oscillating between oxidizing and reducing conditions) to anoxic marine sediments from the Raritan -New York Bay complex and the Inner New York Bight of the eastern U.S. were studied to investigate the factors controlling the accumulation of pore-water dissolved organic carbon (DOC). DOC increased with depth at each of four study sites, but accumulation was generally limited in the mixed-redox zone relative to the anoxic zone. Humic-like fluorescence intensity also differed between mixed-redox and anoxic zones of the sediment, such that anoxic pore waters were relatively enriched in fluorescent, humic-like compounds. A pore-water DOC model was tested for its capacity to explain these geochemical patterns. Model results for a heavily irrigated and a non-bioturbated site both suggest that, excluding the uppermost few cm of the sediment column, pore-water DOC is predominantly comprised of poorly-reactive material with estimated degradation rate constants on the order of 10 −3 yr −1. Model results are also consistent with the suggestion that DOC accumulation is suppressed in the mixed-redox compared to the anoxic zones of the sediment due to rapid oxidation of high-molecular-weight DOC, and limited production and enhanced oxidation of the less reactive polymeric low-molecular-weight component of the DOC pool. An assessment of sorptive behavior of DOC in the surface sediments of the study area suggests that sorption can influence pore-water DOC distributions under conditions of high sedimentation and low bioirrigation.

Oxygen dynamics in permeable sediments with wave‐driven pore water exchange

The effects of advective pore water exchange driven by shallow water waves on the oxygen distribution in a permeable (k = 3.3 x 10−12 to 4.9 x 10−11 m2) natural sediment were studied with a planar oxygen optode in a wave tank. Our experiments demonstrate that pore water flow driven by the interaction of sediment topography and oscillating boundary flow changes the spatial and temporal oxygen distribution in the upper sediment layer. Oxygenated water intruding in the ripple troughs and deep anoxic pore water drawn to the surface under the ripple crests create an undulating oxic‐anoxic boundary within the upper sediment layer, mirroring the topographical features of the sediment bed. Anoxic upwelling zones under ripple crests can separate the oxic sediment areas of neighboring ripple troughs with steep horizontal oxygen concentration gradients. The optode showed that migrating wave ripples are trailed by their pore water flow field, alternately exposing sediment volumes to oxic and anoxic pore water, which can be a mechanism for remobilizing particulate oxidized metal precipitates and for promoting coupled nitrification‐denitrification. More rapid ripple migration (experimental threshold ~20 cm h−1) produces a continuous oxic surface layer that inhibits the release of reduced substances from the bed, which under slowly moving ripples is possible through the anoxic vertical upwelling zones. Swift, dramatic changes in oxygen concentration in the upper layers of permeable seabeds because of surface gravity waves require that sediment‐dwelling organisms are tolerant to anoxia or highly mobile and enhance organic matter mineralization.

Rare earth elements in pore waters of marine sediments

The rare earth elements (REEs) were measured in pore waters of the upper ∼25 cm of sediment from one site off Peru and three sites on the California margin. The pore water REE concentrations are higher than sea water and show systematic down core variations in both concentration and normalized pattern. From these analyses and from comparison to other chemical species measured (dissolved Fe, Mn, Ba, oxygen, nitrate, phosphate), it is suggested that pore water REEs can be grouped into three categories: those that are from an Fe-source, those that are from a POC-source, and cerium oxide. REEs from the Fe-source appear where anoxia is reached; they have a distinctive “middle-REE (MREE) enriched” pattern. The concentrations in this source are so elevated that they dominate REE trends in the Fe-oxide reduction zone. The net result of flux from the POC-source is relative enrichment of heavy-REEs (HREEs) over light-REEs (LREEs), reflecting remineralizing POC and complexation with DOC. A common “linear” REE pattern, seen in both oxic and anoxic sediments, is associated with this POC-source, as well as a “HREE enriched” pattern that is seen in surficial sediments at the Peru site. Overall, the pore water results indicate that Mn-oxides are not an important carrier of REEs in the oceans.A REE biogeochemical model is presented which attempts to reconcile REE behavior in the water and sediment columns of the oceans. The model proposes that POC, Fe-oxide and Ce-oxide sources can explain the REE concentration profiles and relative abundance patterns in environments ranging from oxic sea water to anoxic pore water. The model is also consistent with our observation that the “Ce-anomaly” of pore water does not exceed unity under any redox condition.

The role of aqueous iron(II) and manganese(II) in sub-aqueous active barrier systems containing natural clinoptilolite

Increasing attention is being placed on capping as a relatively new option in managing both contaminated sediments and dredged materials, due to its economic and environmental benefits. Capping denotes the placement of a cover onto potentially hazardous sediments or dredged material dumps to inhibit the transfer of contaminants into the water column. Retention of divalent iron and manganese cations using sandy capping layers containing natural zeolites as a reactive additive (active barrier systems, ABS) is evaluated in this study. Three different natural zeolite (clinoptilolite) rocks, two from deposits in Australia and one from a North-American deposit, were investigated and compared with respect to their mineralogical, physical and chemical properties. In particular, results from batch and column experiments show that ABS based on these materials can efficiently demobilise iron and manganese from percolating, anoxic pore water by cation exchange under favourable conditions. The retention, however, may be reduced strongly where competitive exchange with divalent cations such as calcium prevails or where mobile colloidal pore water constituents such as clay minerals or humic substances bind fractions of the dissolved iron or manganese. Therefore, the potential of ABS as a means for in situ remediation has to be evaluated diligently with particular regard to the pore water composition of the sediment to be capped.

Analytical and thermodynamic approaches to the mineralogical and compositional studies on anoxic sediments

The identification of certain minerals directly in the raw sediment has proved to be difficult, if not impossible, because of their instability and/or low contents. This explains why the characterization/composition/crystalline nature of multiple (co)precipitates and solid solutions often necessitate the combined use of density separation methods and macro and microanalytical techniques, and in some cases the possible existence of certain mineral solids is only sustained from thermodynamic considerations. In this context, the comparison of porewater concentration profiles with thermodynamic calculations recently proved to be a convenient way of obtaining clues relative to the potential occurrence of natural minerals. Porewaters and sedimentary-solid phases were extracted from sliced sediment samples collected in the Seine estuary (northern France), and studied as a function of sediment depth. Porewater concentration profiles were determined for Ca, Fe, Mg, Mn, Na, P and Sr using inductively coupled plasma atomic emission spectroscopy, and for dissolved sulfur using square wave, cathodic stripping voltammetry. To obtain information about sediment mineralogy, sedimentary solid phases were analysed directly and after density separation with a heavy liquid (CHBr3) by means of several techniques: X-ray diffraction; electron spin resonance and micro-Raman spectroscopies. Furthermore, using sequential extraction procedures, the chemical speciation versus depth of several elements (Al, Ca, Fe, Mg, Mn, P, Pb, Sr, Ti, and Zn) and particularly sulfur [i.e. acid volatile sulfides (AVS) and chromium reducible sulfurs (CRS)] were undertaken. From these analytical data, some thermodynamic calculations [using ion activity products (LAP)] were attempted for the anoxic porewaters where most of the ionic complexing species were measured to support the involvement of relevant geochemical equilibria between these ions and some metals and the existence of any discrete solid phases (calcite, dolomite, greigite and probably vivianite, apatite and siderite), as well as coprecipitates and solid solutions in calcium carbonate. Thermodynamic equilibria in sedimentary media are rarely achieved because many chemical processes in these systems are established in long periods. Nevertheless, these calculations remain useful to increase our insight into the considered system. They help to support our view about the possible existence of certain minerals (iron sulfides, calcite, dolomite...). They also help account for the real power of ESR for indicating the presence of hypothesised solid solutions, MnxCa1-xC03. The critical investigations of the authors, however, reveal some weaknesses of XRD and Raman microscopy for identifying minor minerals/precipitates, which result from combinations between the inorganic anions P04 3-, C03 2- and S2- and the metallic cations Fe2+, Mn2+, Mg2+, and Sr2+.

Influence of aeration on hydrophobic organic contaminant distribution and diffusive flux in estuarine sediments.

Dredging operations, resuspension events during storms, and bioturbation alter the oxic state of estuarine sediments and induce changes in the composition of dissolved and particle-associated natural organic matter. These changes may alter the distribution of hydrophobic organic chemicals (HOCs) in sediments and their diffusive flux across the sediment-water interface. In this study, the impact of aerating anoxic sediments on the distribution and diffusive flux of a model HOC, 2,2',4,4'-tetrachlorobiphenyl (TeCB), was investigated. Anoxic estuarine sediments collected from three sites along a salinity gradient were used to determine site-specific apparent sorption coefficients for porewater dissolved organic carbon (Kpwdoc) and sediment organic carbon (Koc) under anoxic and oxic conditions. A two-compartment sediment flux model was employed to examine the diffusive flux of TeCB under both oxic states. Aeration of anoxic porewaters resulted in significant decreases in porewater dissolved organic matter (DOMpw) aromaticity as indicated by declines in molar absorptivity at 254 nm (p < 0.005). Aeration also resulted in a 9-13% decrease in DOMpw concentration (p < 0.005) at the two sites exhibiting lower ionic strengths; the high ionic strength site did not exhibit a significant change in DOMpw concentration (p > 0.10). The impact of aeration on TeCB distribution and diffusive flux appeared to be site-specific. Aeration of anoxic sediments induced a significant 1.4 log unit reduction in Kpwdoc at the lowest ionic strength site (p < 0.0005), while sediments from the intermediate ionic strength site exhibited a significant 0.6 log unit increase (p < 0.005). No significant change in sorption to DOMpw was observed for the high ionic strength site (p > 0.10). The sediment displaying the drop in Kpwdoc also exhibited a significant 0.4 log unit drop in Koc (p < 0.01), while the other two sites did not exhibit significant aeration-induced changes in sorption to particle-associated organic matter (p > 0.10). No significant change in diffusive flux was observed for two sites (p > 0.10), while a significant 89-110 mg m(-2) yr(-1) increase in diffusive flux was observed at the low ionic strength site (p < 0.10). This latter result represented approximately a doubling in diffusive flux. In the systems studied, facilitation of TeCB transport across the sediment-water interface by organic colloids did not appear important.

Zinc and lead sequestration in an impacted wetland system

The fate and bioavailability of pollutant metals is important to the long-term health of freshwater wetland systems, particularly in areas with elevated background levels of trace metals. Precipitation of some authigenic mineral phases in wetland sediments can potentially limit metal bioavailability through sequestration in low-solubility compounds, such as metal sulfides. Zinc and Pb concentrations were analyzed in water, plant and sediment samples taken from a metal-contaminated wetland in Chicago, IL, USA in order to assess metal fate and bioavailability, while X-ray absorption spectroscopy (XAS) studies were conducted to determine zinc speciation in the wetland sediments. The results showed sequestration of metal into the pond sediments, most likely in iron precipitate phases. In surface sediments, re-release of Zn and Pb into pore waters was correlated with microbial iron reduction, while the presence of sulfide in anoxic pore waters corresponded to decreased dissolved concentrations of both elements. Analysis of extended X-ray absorption fine structure (EXAFS) data confirmed that sulfide compounds dominated zinc speciation throughout the sediment. Uptake of trace metals in Phragmites plants was limited primarily to plant roots, while concentrations of both Pb and Zn in aquatic vegetation were significantly elevated, representing a potential bioaccumulation hazard.

Chemical stratigraphy of recent sediments from a depth gradient in a meromictic lake, Nordbytjernet, SE Norway, in relation to variable external loading and sedimentary fluxes

Cores of recent sediments were sampled along a depth gradient in a 23 m deep kettle lake with stagnant deep waters containing exceptionally high concentrations of dissolved iron and manganese. Sediment cores were taken on two occasions, in 1978 and 1997, before and after an incidence of full circulation. The aims of this study are to see how oxic and anoxic conditions in the water column influence stratigraphy and sediment focusing and, to compare cores from 1979 and 1998 to see how measured element fluxes and external events are reflected in the chemical stratigraphy. Element analyses show characteristic stratigraphic patterns that depend on the ability to undergo redox transformations, sorptive properties and chemical equilibria in the anoxic deep waters and porewaters. In sediments from the oxic part of the lake Al, Cr, Co, Ni, Zn Cu, Cd, and Pb were well correlated. Positive correlations are seen between elements associated with primary production and sulphur. In the anoxic part of the lake most metals were positively correlated with carbonate. Phosphorus correlated positively with iron in sediments from oxic waters and negatively with manganese and iron deep-water sediments. Porewater analyses indicate that recycling from the deep-water sediments was negligible. The stratigraphy of lead agrees with the historic variation in atmospheric input and is used as a chronological marker. Assessed deposition rates agree with measurements in sediment traps. Most elements more than double their rates of deposition towards the deepest point of the lake, while sulphur, manganese and carbonate had maxima around the depth of the redoxcline in the water. Variations in the external loading and variable redox conditions in the deep waters explain variations in the chemical composition of recent sediments.

Depth Variability and some Geochemical Characteristics of Fe, Mn, Ca, Mg, Sr, S, P, Cd and Zn in Anoxic Sediments from Authie Bay (Northern France)

A study of an anoxic sediment core from Authie Bay in northern France has been undertaken; porewaters and sedimentary solid phases have been analysed by several techniques (inductively coupled plasma atomic emission spectroscopy; graphite-furnace atomic absorption spectroscopy; X-ray diffraction; and electron spin resonance spectroscopy). The chemical speciation versus depth of the major elements (Ca, Fe, Mg, Mn, P and Sr), some trace elements (Cd and Zn) and especially sulfur [i.e., dissolved sulfur as Acid Volatile Sulfide (AVS) and Chromium Reducible Sulfur (CRS)] have also been performed using sequential extraction procedures. Together with these analytical data, thermodynamic calculations have been used in order to gain information about the effects of microbial degradation of organic matter in these recent sediments on these elements during early diagenesis and/or the possible generation of new characteristic assemblages of authigenic mineral precipitates of carbonates, phosphates and/or sulfides. Thus, these calculations have shown that iron sulfide precipitates in interstitial waters to give the crystalline form greigite and, to a lesser extent, mackinawite, whereas different divalent metals (noted Me) as Mg, Mn, Sr, Cd, Zn and even Fe interact favourably with Ca 2+ and CO2−3 [through a removal of these elements from anoxic porewaters by (co)precipitation and/or adsorption on to the sedimentary carbonate phase (calcite)] to give (1) a solid mineral: dolomite [CaMg(CO 3) 2]; and (2) solid solutions as Me xCa 1−xCO 3. The distribution coefficients for these metals in calcite have been calculated and compared to those reported in the literature.

Early diagenetic processes during Mn-carbonate formation: evidence from the isotopic composition of authigenic Ca-rhodochrosites of the Baltic Sea

The formation of authigenic Ca-rich rhodochrosite (ACR) in sapropelic sediments of the Gotland Basin, Baltic Sea, is governed by deepwater renewal processes whereby saline water from the North Atlantic flushes the brackish anoxic Baltic Deeps. The carbon and oxygen isotopic compositions of these Mn-carbonates suggest that ACR formation takes place just below the sediment surface and that dissolved compounds from the deepwater column, such as water and bicarbonate molecules, were incorporated in ACR during authigenesis. Porewaters near the sediment surface display δ18O values of −5.4‰ (VSMOW) and are generally depleted in 18O, compared to the oxygen isotopic composition of water in equilibrium with Mn-carbonate solid solutions (ACR δ18O values are −4.6‰). This suggests that early burial diagenetic processes significantly modify the initial isotopic composition of water during Mn-carbonate formation. The reduction of sulfate having δ18O values of +8.4‰ accounts for a permanent enrichment of porewater 18O and observed δ18O values at depth equal to −4.6‰. However, this process does not explain the observed disequilibrium in the oxygen isotopic composition between water and ACR close to the sediment surface where Mn-carbonate formation takes place. Based on isotopic mass balance calculations, we suggest that MnO2 with δ18O values of +8.9‰ released oxygen enriched in 18O into the anoxic porewaters close below the sediment surface. This process should occur after oxygenation events during deepwater renewal when MnO2 accumulates at the surface of anoxic sediments. Manganese carbonates formed in these waters display δ18O values of ∼1.0‰ heavier than values expected solely from the initial deepwater composition. This quantitatively explains the discrepancy between paleosalinities calculated from ACR δ18O based on Mn-carbonate/water isotopic equilibrium fractionation and direct observations for the same period. Our results emphasize the important role of microbial MnO2 reduction during rhodochrosite authigenesis and suggest that Mn(II) activity, rather than alkalinity, is the limiting component for sedimentary Mn-carbonate formation.

Bioirrigation modeling in experimental benthic mesocosms

Burrow irrigation by benthic infauna affects chemical mass transfer regimes in marine and estuarine sediments. The bioirrigation facilitates rapid exchange of solutes between oxygenated overlying water and anoxic pore water, and thus promotes biogeochemical reactions that include degradation of sedimentary organic matter and reoxidation of reduced species. A comprehensive understanding of chemical mass transfer processes in aquatic sediments thus requires a proper treatment of bioirrigation. We investigated bioirrigation processes during early diagenesis using laboratory benthic mesocosms. Bioirrigation was carried out in the mesocosms by Schizocardium sp., a funnel-feeding enteropneust hemichordate that builds and ventilates a U-shaped burrow. Interpretation of the laboratory results was aided by a two-dimensional multicomponent model for transport and reactions that explicitly accounts for the depth-dependent distribution of burrows as well as the chemical mass transfers in the immediate vicinity of burrow walls. Our study shows that bioirrigation significantly affects the spatial distributions of pore water solutes. Moreover, bioirrigation promotes burrow walls to be the site of steep geochemical gradients and rapid chemical mass transfer. Our results also indicate that the exchange function, a, widely used in one-dimensional bioirrigation modeling, can accurately describe the bioirrigation regimes if its depth attenuation is coupled to the depth-dependent distribution of burrows. In addition, this study shows that the multicomponent 2D reaction-transport model is a useful research tool that can be used to critically evaluate common biogeochemical assumptions such as the prescribed depth dependencies of organic matter degradation rate and C/N ratio, as well as the lack of macrofaunal contribution of metabolites to the pore water.

Diurnal changes in pore water sulfide concentrations in the seagrass Thalassia testudinum beds: the effects of seagrasses on sulfide dynamics.

The dynamics of the seagrass-sulfide interaction were examined in relation to diel changes in sediment pore water sulfide concentrations in Thalassia testudinum beds and adjacent bare areas in Corpus Christi Bay and lower Laguna Madre, Texas, USA, during July 1996. Pore water sulfide concentrations in seagrass beds were significantly higher than in adjacent bare areas and showed strong diurnal variations; levels significantly decreased during mid-day at shallow sediment depths (0-10 cm) containing high below-ground tissue biomass and surface area. In contrast, diurnal variations in sediment sulfide concentrations were absent in adjacent bare patches, and at deeper (>10 cm) sediment depths characterized by low below-ground plant biomass or when the grasses were experimentally shaded. These observations suggest that the mid-day depressions in sulfide levels are linked to the transport of photosynthetically produced oxygen to seagrass below-ground tissues that fuels sediment sulfide oxidation. Lower sulfide concentrations in bare areas are likely a result of low sulfate reduction rates due to low organic matter available for remineralization. Further, high reoxidation rates due to rapid exchange between anoxic pore water and oxic overlying water are probably stimulated in bare areas by higher current velocity on the sediment surface than in seagrass beds. The dynamics of pore water sulfides in seagrass beds suggest no toxic sulfide intrusion into below-ground tissues during photosynthetic periods and demonstrate that the sediment chemical environment is considerably modified by seagrasses. The reduced sediment sulfide levels in seagrass beds during photosynthetic periods will enhance seagrass production through reduced sulfide toxicity to seagrasses and sediment microorganisms related to the nutrient cycling.

Biogeochemistry of sulfur in a sediment core from the west-central Baltic Sea: Evidence from stable isotopes and pyrite textures

The biogeochemistry of the sulfur cycle in a ca. 5-m-long sediment core from the eastern slope (221 m water depth) of the Landsort Deep in the west-central Baltic Sea was investigated by analyzing the solid phase records of sulfur isotopes and pyrite textures, besides selected main and minor elements. The sediments were deposited during post-glacial history of the Baltic Sea when the basin experienced alteration of brackish (Yoldia Sea, Littorina Sea) and freshwater (Baltic Ice Lake, Ancylus Lake) conditions. The stable isotopic composition of total sulfur was analyzed as a function of depth. In selected samples pyrite (FeS 2), greigite (Fe 3S 4), and barite (BaSO 4) fractions were separated for isotope analyses. Pyrite textures were analyzed by SEM and optical microscopy. Microbial reactions associated with the oxidation of organic matter resulted in assemblages of authigenic sulfide minerals which for the post-Ancylus Lake brackish water environment are dominated by pyrite and for freshwater environments by Fe-monosulfides. The sulfur isotopic composition of the brackish water Littorina Sea sediments ( δ 34S between −40 and −27‰ vs. V-CDT) is believed to be determined by cellular sulfate reduction rates and reactions involving intermediate sulfur species. The availability of reactive iron and decomposable organic matter as well as sedimentation rate and the chemocline position are important variables upon the δ 34S values of sulfides in brackish water environment. The syn-depositional abundance of sulfur and organic matter, and transport of dissolved sulfur species vs. rates of microbial reactions determine δ 34S in the freshwater sediments. The upper part of the Ancylus Lake sediments is sulfidized by downward diffusing H 2S and/or sulfate from overlying brackish water sediments. Minor concretionary barite formation in the freshwater sediments is most likely due to the reaction of pore water sulfate diffusing downward from brackish water sediments with barium desorbed from freshwater sediments. The size distribution of pyrite framboids in the brackish sediments indicates that the formation mainly occurred from anoxic pore waters, although some pyrite formation in an anoxic water column cannot be excluded.

Dolomitization of Holocene Shallow-Marine Deposits Mediated by Sulfate Reduction and Methanogenesis in Normal-Salinity Seawater, Northern Belize

ABSTRACT Dolomite constitutes an average of 12% of the Holocene organic-rich sediments over a 15 km2 area of the Cangrejo Shoals mudbank in northern Belize. Although it defines a laterally persistent stratiform body that averages 3 m thick, it is present throughout the 7.6-m-thick sediment section. These transgressive sediments are less than 6400 years old and were deposited in shallow-marine environments of normal salinity. The dolomite is dominantly cement, and average crystal size is 7 m. There are no significant correlations among amount of dolomite vs. sediment texture, mineralogy, porosity, or mole % MgCO3 in associated particulate high-Mg calcite, depth, or location on the shoals. The dolomites are poorly ordered and calcic (39.5-44.5 mole % MgCO3), with low mean Mn (210 ppm) and relatively high mean Sr (1034 ppm) concentrations. There is no evidence of recrystallization or geochemical alteration of the dolomite. 18O values of the dolomites range from 0.5 to 2.8o/ooPDB, and the mean value (2.1o/oo) suggests that the dolomite precipitated from normal-salinity pore water. Dolomite 13C values range from -5.2o/oo to +11.6o/ooPDB (mean seawater 13C = 0.5o/oo), which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water. Dolomitization attending these organodiagenetic reactions apparently was reversible over time, and episodic rather than continual precipitation is indicated. Requisite Mg and Ca were provided by seawater and by some dissolution of host sediments. The most rapid period of dolomitization may have been during early transgression, when relatively high sedimentation rates sustained high levels of organodiagenesis and pore-water alkalinities.

Application of Gibbs Energy Minimization to ModelEarly-Diagenetic Solid-Solution Aqueous-Solution EquilibriaInvolving Authigenic Rhodochrosites in Anoxic BalticSea Sediments

The natural early-diagenetic environment "anoxic porewater - authigenic mineral phases" has been characterized in sediment of the Gotland Deep, Baltic Sea, by a closed-system model. Occurrence of carbonate precipitates as thin almost pure white laminae was considered as a natural experiment for long-term equilibration between these phases and porewater. Plots of distribution coefficients indicate that metastable equilibrium exists between porewater and the authigenic Ca-rich rhodochrosite phases below 7 cm depth. A thermodynamic model of porewater geochemistry at in situP = 25 bar andT =5 C was developed using the Gibbs energy minimization (GEM) approach. The values of isobaric-isothermal potentials of Mn, Ca, Fe, Mg, Sr, Ba, C, and O, calculated from the porewater composition, were used in a new "dual thermodynamic" calculation approach to estimate solid activity coefficients of the end-members in the non-ideal solid solution (Mn, Ca, Mg, Sr, Ba, Fe)CO3, i.e., at full major and minor multi-component complexity. The regular Margules interaction parameters for the composing binaries estimated by this model wereMn-Ca =1 .9 0.5,Mn-Mg = 0.6, Ca-Mg =3 .7,Mn-Fe =0 .2,Ca-Fe =2 .8,Mn-Sr =9 .7,Ca-Sr = 2.15, Mn-Ba =4 .0,Ca-Ba = 1.4, validating the theoretical predictions given by Lippmann in his pioneering 1980's paper. The strictly thermodynamic equilibrium model is not only able to match both the measured porewater and carbonate solid-solution composition, but also to predict that the porewater pH, pe, alkalinity, and dissolved Mn, Fe, and S concentrations are controlled by the authigenic mineral buffering assemblage mackinawite-greigite-rhodochrosite. Our model is only compatible with the idea of ACR formation with typical composition (XMn between 70-75%) in the topmost sediment layer which, however, needs a major source of Mn II . This is provided by reduction of particulate Mn oxides precipitated in significant amounts in the water column upon major inflow events in the Baltic Sea. The model enables also to set up scenarios of changing environmental conditions, e.g., to predict the non-linear response of the carbonate solid-solution composition to changes in Mn loading, alkalinity and salin- ity of the sediment-water system. The results suggest that the major and especially minor element contents (Sr, Mg, Ba) in authigenic carbonates can be applied as an environmental paleoproxy.

Benthic foraminifera of dysoxic sediments: chloroplast sequestration and functional morphology

Our recent surveys of dysoxic and anoxic sites reveal that many of the common foraminiferal inhabitants sequester chloroplasts. Such species include: Nonionella stella, which dominates the laminated sediments of the silled Santa Barbara Basin and comprises up to 82% of the living assemblage when [O 2] is below 2 μM (∼0.04 ml/l); the closely related species Nonionellina labradorica, which occurs in oxygen-depleted, silled fjords of Sweden; Stainforthia fusiformis, which dominates dysoxic sediments of Norwegian fjords; and Bulimina elegantissima, which is abundant in a shallow-water oil seep site supporting the filamentous, sulfide-oxidizing bacteria Beggiatoa. The literature contains examples of at least eight Elphidium species and one species from each of three other foraminiferal genera (i.e., Haynesina, Nonion, Reophax) that are known to sequester chloroplasts. These foraminifera are typically infaunal and/or may live under dysoxic conditions. Photosynthetic activity of the sequestered chloroplasts might provide oxygen to the host foraminifera, thereby enabling them to inhabit anoxic pore waters. However, given that most of the surveyed sites occur in the aphotic zone where light levels are too low to fuel photosynthesis, it is more likely that the host employs an as yet unidentified biochemical pathway associated with the sequestered chloroplasts. Additionally, these foraminifera have external test ornamentations that may facilitate separation of the chloroplasts from their algal prey. We discuss potential uses for these morphological features in interpreting the fossil record.

Aeration Effects on the Partitioning of a PCB to Anoxic Estuarine Sediment Pore Water Dissolved Organic Matter

Pore water dissolved organic matter (DOM) plays an important role in the distribution, mobility, and bioavailability of hydrophobic organic chemicals (HOCs) in sediment environments. The effect of aeration on the partitioning of 2,2{prime},4,4{prime}-tetrachlorobiphenyl (TeCB) to anoxic pore water DOM from three estuarine sites was investigated. Pore water DOM was fractionated into molecular size and polarity fractions by ultrafiltration and XAD-8 resin chromatography. Total organic carbon analysis was utilized to determine shifts in molecular size and polarity distributions. Changes in functional groups and aromaticity were evaluated for whole and fractionated pore waters by specific UV absorbance at 254 nm (SUVA{sub 254}). The solubility enhancement method was used to determine the partitioning of TeCB to whole and fractionated pore water DOM. At two sites, the overall TeCB-DOM distribution coefficient decreased by an order of magnitude after aeration. The higher molecular size and all polarity fractions exhibited a decrease in partitioning behavior upon aeration. The aromaticity and TeCB-DOM distribution coefficient of the lowest molecular size fraction decreased upon aeration. The highest and lowest molecular size fractions contributed the most to overall partitioning. The observed aeration effects in anoxic estuarine sediment pore waters differed significantly from those previously reported in freshwater systems.