An Experimental Apparatus for Laboratory and Field-Based Perfusion of Sediment Porewater with Dissolved Tracers

The water–sediment interface is a dynamic zone where the benthic and pelagic environments are linked through exchange and recycling of organic matter and nutrients. However, it is often difficult to measure rate processes in this zone. To that end, we designed an experimental apparatus for continuous and homogeneous perfusion of sediment porewater with dissolved conservative (SF₆, Rhodamine WT dye) and isotopic (H¹³CO₃⁻ and ¹⁵NH₄⁺) tracers to study nitrogen and carbon cycling by the sediment microbial community of shallow illuminated sediments. The perfusionator consists of a 60-cm ID × 60-cm high cylinder that includes a reservoir for porewater at the base of the sediment column. Porewater amended with conservative and stable isotopic tracers was pumped through a mixing reservoir and upward through the overlying sediments. We tested the perfusionator in a laboratory setting, as part of an outdoor mesocosm array, and buried in coastal sediments. Conservative and isotopic tracers demonstrated that the porewater tracers were distributed homogeneously through the sediment column in all settings. The perfusionator was designed to introduce dissolved stable isotope tracers but is capable of delivering any dissolved ionic, organic, or gaseous constituent. We see a potentially wide application of this technique in the aquatic and marine sciences in laboratory and field settings.     

Dissolved Fe2+ and ∑H2S Behavior in Sediments Seasonally Overlain by Hypoxic-to-anoxic Waters as Determined by CSV Microelectrodes

Variability of dissolved Fe²⁺ and ∑H₂S concentrations in porewaters were studied, using cathodic stripping voltammetry (CSV) microelectrodes, in sediments overlain by hypoxic waters in the summer from the southeastern region of Corpus Christi Bay, Texas (CCB) and the Mississippi River Bight (MRB), Louisiana. These measurements were complimented by sediment microcosm studies where oxygen concentrations in the overlying water were manipulated. Sulfate reduction rates, benthic oxygen demand, total reduced sulfide, porewater sulfate, and total organic carbon were also determined. Fe²⁺ and ∑H₂S were the major dissolved redox-reactive dissolved species in these sediments. During hypoxic conditions, an upward migration of porewater Fe²⁺ and ∑H₂S occurred, with Fe²⁺ reaching much higher maximum concentrations than ∑H₂S. Statistically significant (p < 0.05) differences in both Fe²⁺ and ∑H₂S occurred between sediments at the CCB and MRB study sites. Although both sites were Fe-dominated, reactive and dissolved iron were higher while ∑H₂S was lower at the MRB site. However, there were no statistically significant (p > 0.05) difference in regard to ∑H₂S between microcosm and field monitoring studies. Results indicated that, for Fe²⁺ and ∑H₂S, relatively large and rapid changes occurred in both the concentrations and distributions of these important porewater constituents in response to relatively short-term changes in overlying water oxygen content. Model calculations indicated that conditions in the sediments can be responsible for the induction of hypoxic conditions in the formation of hypoxic overlying waters in <6 days at CCB and ~20 days at MRB.     

Carbon Cycling and the Coupling Between Proton and Electron Transfer Reactions in Aquatic Sediments in Lake Champlain

We used fine-scale porewater profiles and rate measurements together with a multiple component transport–reaction model to investigate carbon degradation pathways and the coupling between electron and proton transfer reactions in Lake Champlain sediments. We measured porewater profiles of O₂, Mn²⁺, Fe²⁺, HS⁻, pH and pCO₂ at mm resolution by microelectrodes, and profiles of NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺, total inorganic carbon (DIC) and total alkalinity (TA) at cm resolution using standard wet chemical techniques. In addition, sediment–water fluxes of oxygen, DIC, nitrate, ammonium and N₂ were measured. Rates of gross and net sulfate reduction were also measured in the sediments. It is shown that organic matter (OM) decomposes via six pathways: oxic respiration (35.2%), denitrification (10.4%), MnO₂ reduction (3.6%), FeOOH reduction (9.6%), sulfate reduction (14.9%), and methanogenesis (26.4%). In the lake sediments, about half of the benthic O₂ flux is used for aerobic respiration, and the rest is used for the regeneration of other electron acceptors produced during the above diagenetic reactions. There is a strong coupling between O₂ usage and Mn²⁺ oxidation. MnO₂ is also an important player in Fe and S cycles and in pH and TA balance. Although nitrate concentrations in the overlying water were low, denitrification becomes a quantitatively important pathway for OM decomposition due to the oxidation of NH₄ ⁺ to NO₃ ⁻. Finally, despite its low concentration in freshwater, sulfate is an important electron acceptor due to its high efficiency of internal cycling. This paper also discusses quantitatively the relationship between redox reactions and the porewater pH values. It is demonstrated here that pH and pCO₂ are sensitive variables that reflect various oxidation and precipitation reactions in porewater, while DIC and TA profiles provide effective constraints on the rates of various diagenetic reactions.     

Benthic metabolism and nutrient cycling along an estuarine salinity gradient

Benthic metabolism and nutrient exchange across the sediment-water interface were examined over an annual cycle at four sites along a freshwater to marine transect in the Parker River-Plum Island Sound estuary in northeastern Massachusetts, U.S. Sediment organic carbon content was highest at the freshwater site (10.3%) and decreased along the salinity gradient to 0.2% in the sandy sediments at the marine end of the estuary. C:N ratios were highest in the mid estuary (23:1) and lowest near the sea (11:1). Chlorophyll a in the surface sediments was high along the entire length of the estuary (39–57 mg chlorophyll a m⁻²) but especially so in the sandy marine sediments (172 mg chlorophyll a m⁻²). Chlorophyll a to phaeophytin ratios suggested most chlorophyll is detrital, except at the sandy marine site. Porewater sulfide values varied seasonally and between sites, reflecting both changes in sulfate availability as overlying water salinity changed and sediment metabolism. Patterns of sediment redox potential followed those of sulfide. Porewater profiles of inorganic N and P reflected strong seasonal patterns in remineralization, accumulation, and release. Highest porewater NH₄ ⁺ values were found in upper and mid estuarine sediments, occasionally exceeding 1 mM N. Porewater nitrate was frequently absent, except in the sandy marine sediments where concentrations of 8 μM were often observed. Annual average respiration was lowest at the marine site (13 mmol O₂ m⁻² d⁻¹ and 21 mmol TCO₂ m⁻² d⁻¹) and highest in the mid estuary (130 mmol O₂ m⁻² d⁻¹ and 170 mmol TCO₂ m⁻² d⁻¹) where clam densities were also high. N₂O and CH₄ fluxes were low at all stations throughout the year: Over the course, of a year, sediments varied from being sources to sinks of dissolved organic C and N, with the overall spatial pattern related closely to sediment organic content. There was little correlation between PO₄ ³⁻ flux and metabolism, which we attribute to geochemical processes. At the two sites having the lowest salinities, PO₄ ³⁻ flux was directed into the sediments. On average, between 22% and 32% of total system metabolism was attributable to the benthos. The mid estuary site was an exception, as benthic metabolism accounted for 95% of the total, which is attributable to high densities of filter-feeding clams. Benthic remineralization supplied from less than 1% to over 190% of the N requirements and 0% to 21% of the P requirements of primary producers in this system. Estimates of denitrification calculated from stoichiometry of C and N fluxes ranged from 0% for the upper and mid estuary site to 35% for the freshwater site to 100% of sediment organic N remineralization at the marine site. We hypothesize that low values in the upper and mid estuary are attributable to enhanced NH₄ ⁺ fluxes during summer due to desorption of exchangeable ammonium from rising porewater salinity. NH₄ ⁺ desorption during summer may be a mechanism that maintains high rates of pelagic primary production at a time of low inorganic N inputs from the watershed.     

Sequestration mechanisms and anthropogenic inputs of rhenium in sediments from Eastern Canada lakes

The concentrations of Re, as well as those of several other geochemical variables, were measured in dated sediment cores and in porewater samples from four lacustrine basins in Eastern Canada: one, perennially oxic, located 40 km from Québec City and three, seasonally anoxic, located within 25 km of non-ferrous metal smelters. The drainage basins of these lakes are uninhabited and have not been affected by human activity or wildfires. All of the depth profiles of dissolved Re indicate: higher Re concentrations in the water overlying the sediment than in the porewater; diffusion of Re across the sediment-water interface; a progressive decrease in porewater Re concentrations to reach minimum values of ∼0.5 pM within a 10-cm sediment depth interval. Modeling of these Re porewater profiles with a one-dimensional transport-reaction equation indicates that Re is removed from porewater within this depth interval. Based on thermodynamic predictions of Re speciation and of saturation states and on comparison of these predictions with sulfide porewater profiles, we infer that Re is removed from porewater by precipitation of rheniite (ReS_2(s)). The rate constant for the formation of ReS_2(s) in sediments is estimated from the modeling exercise to be 0.51 ± 0.64 × 10⁻²¹ mol cm⁻³ s⁻¹. Accumulation of sedimentary Re shows a strong authigenic component, as in anoxic marine sediments. Sharp increases in solid-phase Re during the last century are attributed to atmospheric deposition of anthropogenic Re deriving from coal burning and nearby smelter emissions.     

A study of celestine equilibrium in marine sediments using the entire ODP/IODP porewater data base

The stability of celestine (SrSO₄) in marine sediments has been investigated through the calculation of its saturation index at the in situ temperature and pressure, using the entire ODP/IODP porewater composition data base (14,416 samples recovered from sediments collected during 95 ODP and IODP Legs) that has been thoroughly corrected for missing data and inconsistencies. The porewater in situ pressure has been obtained from depth data, and the in situ sample temperature has been calculated from the bottom seawater temperature and from the measured thermal gradient. When the latter is unavailable, a default value of 35 °C/km or an assumed gradient similar to that of a site located close to the study area has been used. Molarities have been converted to molalities through the calculation of the porewater densities, which have represented by an empirical function of the total dissolved solids load up to 150 g/L.Ocean bottom waters are largely undersaturated with respect to celestine. Yet sediment porewater saturation is not uncommon: it is reached in 83 boreholes (i.e. about 10% of all boreholes) drilled during 22 ODP/IODP Legs. Celestine equilibrium can be reached through two different non-exclusive causes: a strontium increase in porewater linked to the dissolution of Sr-rich aragonite or biogenic calcite and precipitation of Sr-poor calcite (carbonate recrystallization), or an increase in the strontium and sulfate porewater concentrations linked to a salinity increase due to the presence of brines.Sediments at most of the sites exhibiting celestine equilibrium share common characteristics such as a carbonate-rich lithology (typically higher than 80 wt.%) and a low organic carbon content (generally below 1 wt.%). These results indicate that modification of porewaters during burial diagenesis can easily lead to celestine saturation, especially in carbonate-rich sediments. We then briefly discuss former interpretations of the presence of celestine in ancient and recent marine sediments, as well as the consequences of the incorporation of celestine formation in diagenetic models using the porewater Sr content and Sr isotopic composition.     

Lucinid clam influence on the biogeochemistry of the seagrass<Emphasis Type="Italic">Thalassia testudinum</Emphasis> sediments

Lucinid bivalves dominate the infauna of tropical seagrass sediments. While the effect of seagrass on lucinids has been studied, the reverse effect has largely been ignored. Lucinids can alter porewater chemistry (i.e., increase porewater nutrients by suspension feeding and decrease porewater sulfides by oxygen introduction and bacterial oxidation), which can potentially change seagrass productivity and growth morphology. To observe correlations between porewater chemistry and lucinid presence, a field survey and laboratory microcosm experiment were conducted. Survey sampling sites with lucinids had significantly lower sulfide and higher ammonium concentrations than sampling sites without lucinids. There was no difference in phosphate concentration among sampling sites. Both lucinid species used in the microcosm experiment (Ctena orbiculata andLucinesca nassula) significantly lowered sulfide concentrations in the sediment porewater. Microcosm and field survey results were incorporated into a sulfide budget. In seagrass sediments, lucinids remove 2–16% of the total sulfide produced. Sulfide is a major stressor to both plants and animals in Florida Bay sediments; this removal may be important to maintaining seagrass productivity and health. Oxygen introduction into sediments byC. orbiculata was estimated in a dye experiment.C. orbiculata were added to small tubes containing sieved mud and incubated in a bath of seawater with a Rhodamine WT. Rhodamine WT accumulation in the sediment was measured. A first order estimate showed that oxygen introduction can account for less than 5% ofC. orbiculata sulfide removal.     

Iron, sulfur, and carbon diagenesis in sediments of Tomales Bay, California

Analysis of 3-m sediment cores revealed that profiles of carbon (C), sulfur (S), and iron (Fe) varied with relative distance from marine and terrestrial sediment sources in Tomales Bay California. Despite relatively high sedimentation rates throughout the bay (historically 3–30 mm yr⁻¹), sulfate reduction of deposited organic matter led to free-sulfide accumulation in sediments only at the location farthest from terrestrial runoff, the source of reactive iron. Acid-volatile sulfide concentrations in all sediments (<10 μmol g⁻¹) were low relative to concentrations of chromiumreducible sulfide (up to 400 μmol g⁻¹ farthest from the reactive iron source). A calculated index of iron availability, used to describe sediment resistance to build-up of free sulfide, was lowest at this location. Recent, upward shifts in reactive Fe concentration and in the relative contribution of terrestrial orgnic carbon (measured as a shift in δ¹³C of bulk sediment organic matter) in all cores indicated that erosion and transport of sediments from the watershed surrounding Tomales Bay increased after European settlement in the 1850s.     

Porewater sulphur geochemistry and fossil preservation during phosphate diagenesis in a Lower Cretaceous shelf mudstone

Lower Cretaceous mudstones exposed at Speeton in North Yorkshire, UK, contain lobsters and burrows preserved in diagenetic phosphate concretions. Isotopic compositions of sulphur in both diagenetic sulphide and structural sulphate in diagenetic phosphate have been measured in an attempt to constrain diagenetic porewater chemistry. The occurrence of phosphatized and pyritized lobsters and similarly preserved burrows, allows a detailed comparison of these microenvironments with the host sediments. Host sediments are extensively bioturbated and characterized by very light sulphide isotopic compositions (mean sulphide δ³⁴S = –48·3 ± 3‰ (1σ, n = 19)) and sulphate isotopic compositions that are lighter than Lower Cretaceous seawater sulphate (mean sulphate δ³⁴S = +8·7 ± 3·2‰ (1σ, n = 19)). These isotopic values can be explained by the action of bioturbating macrofauna; the introduction of oxygen in ventilating seawater results in the oxidation of early formed isotopically light pyrite, resulting in porewater sulphate that is enriched in ³²S. Subsequent pyrite formation via bacteriogenic reduction of isotopically light porewater sulphate leads to the formation of isotopically light pyrite, whilst residual porewater sulphate apparently remains relatively enriched in the ³²S isotope. Sulphur isotopic values for the burrows are very different; sulphide isotopic compositions average –34·4 ± 0·4‰ (1σ, n = 3) and sulphate isotopic compositions average –14·4 ± 6·6‰ (1σ, n = 4). These isotopic compositions are the result of rapid development of closed system conditions in burrows, resulting in the build-up of acidity necessary for phosphate precipitation and coprecipitation of isotopically light sulphate (formed by oxidation of isotopically light sulphide surrounding the burrows). Lobster shell isotopic compositions fall between these two groups. On the basis of their isotopic compositions, some lobsters appear to have died close to the sediment–water interface, whilst others appear to have been buried (in burrows) prior to death.     

Selenium Behavior in San Francisco Bay Sediments

Sediment and porewater samples (1997–1999) were collected in the Northern Reach of the San Francisco Bay and Sacramento–San Joaquin Delta for determinations of sedimentary selenium and its chemical speciation. Total sedimentary selenium increased with depth, with approximately 50% of the sedimentary selenium as elemental selenium and 35% as organic selenide. Porewater total dissolved selenium increased with depth in the estuary and Delta, and fluxes out of the sediments were calculated at 0.01 and 0.06 nmol cm⁻² year⁻¹ for the estuary and Delta, respectively. Present-day sediment–water exchange of dissolved selenium and internal transformations cannot explain the observed increase in total sedimentary selenium with depth. However, mass balance calculations demonstrate that the increase in total selenium with depth may be linked to higher dissolved selenium concentrations in the water column in the 1980s, suggesting that the sediments could be used as historical recorders of selenium in the estuary.     

The Influence of Element Diffusion at the Sediment—Water Interface on the Basic Chemical Composition of Overlying Water Mass in Lake Lugu,China

Little work has been done on the influence of seiments on the basic chemical composition of overlying water mass.This paper deals with the vertical profile of the basic constituents such as Ca^ ,K^ ,Na^ ,and HCO3^-,as well as of pH in the overlying water mass and sediment porewater of Lake Lugu-a semi-closed,deep lake in Yunnan Province.The reand sediment porewater of Lake Lugu- a semi-closed,deep lake in Yunnan Province.The results revealed that those basic constituents may diffuse and transport from bottom sediments to overlying water mass through porewater.In the paper are also quantitatively evaluated the diffusive fluxes and the extent of their influence on overlying water mass,indicating that the lake sediment-water interface diffusion plays an important role in controlling the basic chemical composition of water in the whole lake.     

Trends in phosphatase activity along a successional gradient of tidal freshwater marshes on the Cooper River South Carolina

Phosphatase activity was measured in sediments from tidal freshwater habitats adjacent to the Cooper River in South Carolina representing different stages of ecological succession. It was found that sediment (0–5 cm) acid phosphatase activity, alkaline phosphatase activity and phosphodiesterase activity increased with increasing successional stage and phytomass. Acid phosphatase activity in creased from 7.5±1.2 (±1 SD) in subtidal sediment from a shallow open water habitat without vegetation to 61.2±4.9 μmol g⁻¹ hr⁻¹ (μmol of p-nitrophenol released per gram of dry sediment per hour) in intertidal sediments colonized by emergent macrophytes, while alkaline phosphatase activity increased from 2.1±0.1 to 19.01±1.5 μmol g⁻¹ hr⁻¹. Phosphodiesterase activity increased from 1.8±0.1 to 20.2±2.0 μmol g⁻¹ hr⁻¹ along the same gradient. Acid phosphatase activity was highly correlated (R²=0.92, P<0.001) with the organic matter content of the sediment. A study of phosphatase kinetics showed that V_max of all phosphatases also increased along the successional gradient. Trends in phosphatase activity and V_max correlated positively with plant biomass and negatively with concentrations of soluble reactive phosphorus in porewater, sediment extractable phosphorus, and total phosphourus. The porewater N∶P atom ratio decreased along the succession gradient from 25.3 in an early stage, open water community to 13.0 in a community dominated by emergent vegetation. The data also show that the distribution of the forms of phosphorus changed with successional stage. The change in distribution and the increased biological demand for phosphorus that paralleled succession were mediated by the activity of phosphatase enzymes.     

Transport of terrestrial organic carbon to the oceans by rivers: re-estimating flux- and burial rates

 This study re-estimates one important component in the global carbon cycle: the modern global fluviatile organic carbon discharge- and burial rates. According to these results, approximately 430×10¹² g of terrestrial organic carbon are transported to the ocean in modern times. This amount is higher than the latest estimates but takes into account new data from Oceania not previously considered in global flux studies. However, only the minor amount of 10% or approximately 43×10¹² gC year^–1 is most likely buried in marine sediments. This amount is similar to the burial of marine organic carbon in the coastal ocean (55×10¹² gC year^–1). Adding both estimates gives approximately 100×10¹² gC year^–1, which is the value calculated by Berner (1982) for "terrestrial" deltaic-shelf sediments. However, the results in this study suggest that on a global scale the organic carbon content in coastal ocean sediments is not solely of terrestrial origin but a mixture of nearly equal amounts of marine and terrestrial organic carbon. The major part of the terrestrial organic carbon that enters the ocean by rivers (approximately 400×10¹² gC year^–1) seems to be either (a) remineralised in the ocean, whereas the mechanism by which the terrestrial organic carbon is oxidised in the ocean are unknown; or (b) is dispersed throughout the oceans and accumulates in pelagic sediments. Received: 9 November 1998 / Accepted: 25 May 1999     

Laboratory and field evidence in support of the electrogeochemically enhanced migration of ions through glaciolacustrine sediment

Ionic aureoles, overlying or contiguous to massive sulphide occurrences, are postulated to have been emplaced as a result of natural galvanic forces. The occurrences of these galvanic forces, most commonly referred to as the self-potential or spontaneous polarization phenomena, are well documented, but ionic migration in response to these electrical forces remained to be proven conclusively.The purpose of this study was to attempt to document ionic migration attributable solely to electrogeochemical transport. Pleistocene glaciolacustrine sediments rich in clay-size particles was the geological substrate chosen to minimize migration processes other than those of electrogeochemical transport.Laboratory experiments which simulated a field situation and utilized the radioisotope ⁶⁵Zn as a tracer, revealed a readily detectable diffusion of Zn²⁺ into glaciolacustrine clay; the distance of penetration of the Zn²⁺ being significantly increased by the application of a DC electrical potential, similar in magnitude to a natural self-potential. The rate of diffusion was greatest around the margins of the cathodic electrode located at the base of the clay layers; the areas shown to have the greatest current density. The resulting pattern of Zn penetration into the clay, when viewed in a vertical section, showed a double peaked increase in Zn concentration on either side, and a distinct decrease in Zn penetration directly above this cathode. Extrapolation of the average diffusion coefficient found for Zn²⁺ (D₀ = 6.3 × 10⁻⁹cm s⁻¹) to other ions commonly associated with base metal deposits, revealed that only H⁺ could be expected to move through appreciable thicknesses (>20 m) of clay in the 8000 years since sediment deposition.The hypothesis that H⁺ would diffuse through varved clay, and that the resulting pattern would be predictably modified by a self-potential field generated by an oxidizing sulphide body, was tested over the Magusi River volcanogenic massive sulphide deposit near Noranda, Quebec. The organic soil horizon (H) and top of the clay soil layer (C) were sampled and analyzed for conductivity, pH, and other major and minor elements. The mineral horizon revealed distinct changes in conductivity, pH, Ca, Mg, and Fe above the contacts of the hanging wall and footwall of the sulphide horizon, thus producing a double-peaked “rabbit-ear” anomalous pattern. The H soil horizon, when corrections for variable “total” element and carbon concentrations were applied, showed clear anomalies in Fe and other pH sensitive elements. A ratio of EDTA Fe/TOTAL Fe to total organic carbon revealed the best “rabbit-ear” anomaly above the sulphide horizoon. The use of organic horizon geochemistry as a method of evaluating EM conductors overlain by water-saturated lacustrine sediments is recommended.     

Anaerobic Carbon Mineralisation Through Sulphate Reduction in the Inner Shelf Sediments of Eastern Arabian Sea

Monsoon-induced coastal upwelling, land run-off, benthic and atmospheric inputs make the western Indian shelf waters biologically productive that is expected to lead to high rates of mineralisation of organic matter (OM) in the sediments. Dissimilatory sulphate reduction (SR) is a major pathway of OM mineralisation in near-shore marine sediments owing to depletion of other energetically more profitable electron acceptors (O₂, NO₃ ^?, Mn and Fe oxides) within few millimetres of the sediment-water interface. We carried out first ever study to quantify SR rates in the inner shelf sediments off Goa (central west coast of India) using the ³⁵S radiotracer technique. The highest rates were recorded in the upper 10 cm of the sediment cores and decreased gradually thereafter below detection. Despite significant SR activity in the upper ～12 to 21 cm at most of the sites, pore water sulphate concentrations generally did not show much variation with depth. The depth integrated SR rate (0.066–0.46 mol m^?2 year^?1) decreased with increasing water depth. Free sulphide was present in low concentrations (0–3 μM) in pore waters at shallow stations (depth <30 m). However, high build-up of sulphide (100–600 μM) in pore waters was observed at two deeper stations (depths 39 and 48 m), 7–11 cm below the sediment-water interface. The total iron content of the sediment decreased from ～7 to 5 % from the shallowest to the deepest station. The high pyrite content indicates that the shelf sediments act as a sink for sulphide accounting for the low free sulphide levels in pore water. In the moderately organic rich (2–3.5 %) sediments off Goa, the measured SR rates are much lower than those reported from other upwelling areas, especially off Namibia and Peru. The amount of organic carbon remineralised via sulphate reduction was ～0.52 mol m^?2 year^?1. With an estimated average organic carbon accumulation rate of ～5.6 (±0.5) mol m^?2 year^?1, it appears that the bulk of organic matter gets preserved in sediments in the study region.     

Images of concentrations of dissolved sulphide in the sediment of a lake and implications for internal sulphur cycling

The high-resolution mapping of dissolved sulphide in modern sediment of a permanently stratified (meromictic) lake is possible using a novel sulphide imaging technique (called a 'sulphur print'). The technique is simple, easy to use and can map a relatively large area (up to several dm²). In situ sulphur prints from anoxic fine-grained sediments are able to reveal internal structures in apparently homogeneous materials. Sulphur prints have been used to examine the formation and distribution of dissolved sulphide in the sediment and water column of the meromictic Lake Cadagno (southern Switzerland). The prints show clearly that a previously unknown laminar convective flow of sulphide-free porewater occurs across the sediment–water interface. Such convective flow out of the sediment must be accompanied by convective flow of sulphide- and sulphate-rich lake water into the sediment, and may be an important mechanism for the accumulation of sulphur in the sediment.     

Advection and diffusion determine vertical distribution of microbial communities in intertidal sediments as revealed by combined biogeochemical and molecular biological analysis

Porewater advection stimulates nutrient exchange and microbial activity in shallow marine sediments, whereas element cycling in deeper diffusion-dominated sediments is comparatively slow due to limited nutrient supply. We studied the vertical distribution of microbial communities and organic matter (OM) cycling in these contrasting porewater regimes down to 5 m depth at an intertidal flat of the southern North Sea. Archaea, Bacteria and Eukarya were targeted, combining intact polar lipid (IPL) analysis with qualitative and quantitative molecular biological techniques. The largely sandy section 1 of the core (<75 cm) is characterized by rapid burial of fresh marine OM and intense porewater advection. This supply fuels heterotrophic microbes, as evident from the ¹³C isotopic composition of total organic carbon and IPL derivatives. Major sources of OM are algae and cyanobacteria, as suggested by the elevated amount of eukaryotic 18S rRNA gene copies and phosphate-free IPLs. The relative abundance of most phospholipids remained largely constant over the entire core, except for diphosphatidylglycerol, which represented about half of total IPL abundance in the lower part of section 1 (>50 cm) and the diffusion-dominated section 2 (75–490 cm). This suggests bacteria adapting their membranes in response to increasing physicochemical stress and starvation in the nutrient limited, fine grained sediments of section 2 with less bioavailable, predominantly terrestrial, OM. Relative amounts of bacterial acyl ether and diether phospholipids increased in this lower section and were assigned to sulfate reducers and yet uncultured myxobacteria. Archaea were an order of magnitude less abundant than Bacteria, and were affiliated mainly with Methanosarcinales and Methanomicrobiales. Accordingly, the archaeal IPL composition was typical for a methanogenic community. IPLs not exclusively derived from in situ microbial production emphasize that these biomarkers have to be interpreted with caution in sediments with complex hydrogeology. Our results demonstrate that contrasting subsurface flow regimes significantly impact on the vertical zonation of biogeochemical properties and microorganisms in marine sediments.     

Predicting New Hampshire indoor radon concentrations from geologic information and other covariates

 Generalized geologic province information and data on house construction were used to predict indoor radon concentrations in New Hampshire (NH). A mixed-effects regression model was used to predict the geometric mean (GM) short-term radon concentrations in 259 NH towns. Bayesian methods were used to avoid over-fitting and to minimize the effects of small sample variation within towns. Data from a random survey of short-term radon measurements, individual residence building characteristics, along with geologic unit information, and average surface radium concentration by town, were variables used in the model. Predicted town GM short-term indoor radon concentrations for detached houses with usable basements range from 34 Bq/m³ (1 pCi/l) to 558 Bq/m³ (15 pCi/l), with uncertainties of about 30%. A geologic province consisting of glacial deposits and marine sediments was associated with significantly elevated radon levels, after adjustment for radium concentration and building type. Validation and interpretation of results are discussed. Received: 20 October 1997 · Accepted: 18 May 1998     

Sediment trapping and transport in the ACE Basin,South Carolina

A study took place during May 1998 and May 1999 to examine the processes controlling localized accumulation of fine-grained sediments in the lower Ashepoo River. This region, referred to as the Mud Reach, is an area of muddy bottom sediments bounded by fine sands. The Mud Reach is located downstream of the landward extent of the salt intrusion where an estuarine turbidity maximum commonly occurs. Tidal time-series measurements made in the Mud Reach during May 1998, when river discharge was at a 10-yr high, showed high concentrations of suspended sediment (0.05–1 g I⁻¹) during maximum tidal current velocity with concentrations in the bottom 30 cm exceeding 70 g I⁻¹ (fluid mud). A correlation between salinity stratification and increased suspended sediment concentration suggests that inhibited vertical mixing enhances the settling of flocculated sediments to the bed. Measurements made during May 1999 show a two-order-of-magnitude decrease in the concentration of near-bed sediments. A decrease in river discharge during the 1999 observation period of more than 100 m³ s⁻¹ suggests that changes in the hydrography and in the supply of sediments to the system both may be important factors in the trapping of fine-grained sediments in the region. The source of sediments is likely from muddy deposits in the Fenwick Cut, a man-made section of the Atlantic Intracoastal Waterway about 2 km north of the Mud Reach that connects the Ashepoo and Edisto Rivers. The Fenwick Cut appears to be an effective area for trapping sediments where shoaling has increased by 130% in the last decade. Current measurements show that flow velocities decrease through the Cut, likely allowing for the settling of suspended particles that form the thick deposits of unconsolidated mud observed during both years.     

Geochemistry and genesis of the Lengenbach Pb-Zn-As-Tl-Ba-mineralisation,Binn Valley,Switzerland

 The Lengenbach Pb-Zn-As-Tl-Ba mineralisation is located in Triassic dolostones of the Penninic zone in the Swiss Alps where Alpine metamorphism reached upper greenschist to lower amphibolite grade. Geochemical data are used to constrain the origin of this unique occurrence. Two metamorphic redox environments are present: the As(III)-rich zone is controlled by barite-pyrite while the reduced zone contains graphite or pyrrhotite-pyrite and formally zerovalent As. The As(III)-rich zone is characterised by a mineral assemblage consistent with fO₂ in the stability field of barite+pyrite. An As-(Pb, Tl)-rich sulphide melt coexisted with a hydrothermal fluid at >kk300 °C in this zone. Mineralised dolostones are anomalous in As, Pb, Ag, Tl, Hg, Zn, Ba, Cd, Fe, Cu, Mo, U, V, B, Ga, Cr and possibly Sn and Au (in order of decreasing enrichment). As, Pb and Zn are present in the 0.1 to 1% range, Tl and Ag reach several hundred ppm. Uraninite is concentrated in silicate-rich bands and yields a late Alpine U-Pb age of 18.5±0.5 Ma. Pb- and S isotopic variations are interpreted by metamorphic overprinting and re-equilibration within an isochemically metamorphosed mineralisation. Hydrothermal sulphides are more strongly affected by uranogenic Pb than massive Pb-As-sulphides representing a former sulphide melt. The least overprinted mineralisation is characterised by ²⁰⁶Pb/²⁰⁴Pb U003U=18.44−18.56, ²⁰⁷Pb/²⁰⁴Pb=15.60−15.75, ²⁰⁸Pb/²⁰⁴Pb =38.44−38.84 and δ³⁴S (sulphide)=−25±2‰. S isotopic variations are largely a result of sulphide-sulphate re-equilibration yielding temperatures of 450± 30 °C. ⁸⁷Sr/⁸⁶Sr ratios of mineralised samples are lower than or equal to host dolostones, precluding major infiltration of basement-derived fluids during Alpine metamorphism. The Sr source (⁸⁷Sr/⁸⁶Sr close to 0.708) probably was seawater with a radiogenic, detrital mineral component. The genesis of the unique Lengenbach mineralisation is interpreted as the result of isochemical metamorphic overprinting of a carbonate hosted stratiform sulphide mineralisation. Well-crystallised sulphide minerals in fissures and druses formed during retrograde cooling of a sulphide melt in equilibrium with a hydrothermal fluid. The primary mineralisation was probably formed at or close below the sea floor and fed by sulphide-poor hydrothermal fluids. Sulphide was largely derived from seawater by open system bacterial sulphate reduction. U, V and Mo may be seawater-derived. Received: 1 February 1995/Accepted: 10 January 1996