Sediment-Water Interface

Sediment oxygen demand (SOD) is controlled by oxygen transfer in a diffusive boundary layer and by oxygen consumption due to microbial and chemical processes inside the sediment. SOD is a function of flow velocity above a sediment/water interface, i.e. shear velocity, and biochemical activity rates. A model was presented for the unsteady response of SOD and the dissolved oxygen (DO) profile near the sediment/water interface. The time required to reach a steady state DO profile and SOD for zero DO inside the sediment initially gets faster as biochemical activity becomes strong. The values of SOD estimated by the model were compared with experimental data. The model can reproduce observed diffusive boundary layer thicknesses. It is also found that values of SOD estimated by the model were of the same magnitude as the observed data.

A broad sampling program was carried out in the spring of 2019 on the Brittany coast to assess how the surface sediment characteristics drive the benthic effluxes of ammonium (NH + 4 ) and phosphate (PO 4 ) from intertidal mudflats. A total of 200 sediment samples were characterized by their porosity, grain-size, elemental composition and pigment contents, as well as the benthic fluxes of NH + 4 and PO 4 determined by core incubations. The results showed that (1) a high phaeopigment and iron-bound phosphorus content (Fe-P) and a low porosity were significantly related to the high PO 4 flux, and (2) a high porosity and the TN:Org-P ratio in the sediment organic matter (SOM) were related to the high NH + 4 flux. This indicated that PO 4 fluxes would be more driven by the redox status of the sediment through the desorption of Fe-P under specific anoxic conditions during the algal decomposition. NH + 4 fluxes would be more driven by high NH + 4 recycling rates from SOM mineralization and high sediment-water exchanges, enhancing the diffusion of NH + 4 to the overlying water. The present study allowed to highlight the large variability in the benthic nutrient fluxes at the regional scale, as a result of the connections between microbial (SOM mineralization), chemical (adsorption-desorption) and physical (diffusion) processes.

The early diagenesis of the major carrier phases (Fe and Mn minerals), trace elements (As, Co, Cr, Hg, MeHg, Ni) and nutrients (ΣNO 3 , NH 4 + , ΣPO 4 ) and their exchange at the sediment water/interface were studied in the Berre Lagoon, a Mediterranean lagoon in France, at one site under two contrasting oxygenation conditions (strictly anoxic and slightly oxic) and at an adjacent site with perennially welloxygenated water. From the concentration profiles of the primary biogeochemical constituents and trace elements of the pore and bottom waters, as well as the total and reactive particulate phases, we were able to locate and identify the diagenetic reactions controlling the mobility of trace elements in the sediments and quantify their rates by coupling one-dimensional steady-state transport-reaction modelling and thermodynamic speciation calculations. Under oxic conditions and in the absence of benthic organisms, the main redox reactions were well identified vertically in the surface sediments and followed the theoretical sequence of oxidant consumption: O 2 > NO 3 /MnO 2 > Fe(OH) 3 > SO 4 2-. However, under anoxic conditions, only MnO 2 , Fe(OH) 3 and SO 4 2-reduction were present, and they all occurred at the interface. We identified the main biogeochemical controls on the mobility of As, Cr, Hg, MeHg and Ni in the surface sediments as the adsorption/desorption and/or coprecipitation/codissolution with Fe oxy-hydroxides. In contrast, Co mobility was primarily controlled by its reactivity towards Mn oxy-hydroxides. In sulphidic sediments, As, Hg and MeHg were sequestered along with Fe sulphides, whereas Co and Ni precipitated directly as metallic sulphides and Cr mobility was enhanced by complexation with dissolved organic ligands. The fluxes of trace elements at the sediment-water interface are essentially dependent on the localisation of their remobilisation and immobilisation reactions under the interface, which in turn is governed by the benthic water oxygenation conditions and kinetic competition among those reaction and diffusion processes. Under oxic conditions, the precipitation of Fe or Mn oxy-hydroxides in the surface sediments constitutes the most efficient mechanism to sequester most of the trace elements studied, thus preventing their diffusion to the water column. Under anoxic conditions the export of trace elements to the water column is dependent on the kinetic competition among the remobilisation during the reductive dissolution of Fe and/or Mn oxy-hydroxides, diffusion and immobilisation with sulphides. We also show that benthic organisms in the perennially oxygenated site have a clear impact on this general pattern. Based on our extensive dataset and geochemical modelling, we predict that the planned reoxygenation of the entire lagoon basin, if complete, will most likely limit or reduce the export of the trace elements from the sediments to the water column and therefore limit the impact of the contaminated sediment. ► Main constituents and trace elements concentration profiles in contaminated sediments. ► Contrasting oxygen concentrations in the overlying water column. ► Combination of thermodynamic calculations and inverse transport-reactions modelling. ► Identification and quantification of diagenetic reactions controlling trace elements mobility. ► Assessment of trace elements and nutrients fluxes at the sediment-water interface.

We investigated the nutrient uptake capacity of three lakes (Uzlina, Matita and Rosu) within the Danube Delta during high water level in June and low water level in September 1999. Special emphasis was placed on nutrient cycling at the sediment-water interface and on the self-purification capacity of the lakes in the Danube Delta. In order to estimate the nutrient uptake of selected lakes we present in this paper the results of water analyses, benthic flux chamber experiments and deck incubation experiments of 15 N-labeled sediment cores at the inflow and the outlet of the lakes. The external input of dissolved inorganic nitrogen and silica into the lakes decreases with increasing distance to the main Danube branches whereas the total dissolved phosphorus input is independent of the hydrological distance to the main branches. The nutrient loading is highest in the inflow channels, and decreases towards the outflow of the lakes. In June, the uptake of NO 3 -, TDP and Si(OH) 4 in the lakes was higher than in September. In contrast, NH 4 + uptake was more intense in September, when benthic release was more intense as well. On average, about 76% of the external plus internal nitrogen and phosphorus input into the lakes was taken up by macrophytes and phytoplankton during the growing season, whereas the uptake of external nutrient input amounted to about 43%. The benthic release of ammonia and silica increases from June to September and indicates, that part of the nutrients taken up during the growing season might be released during winter. We estimate the net impact of the Delta on the nutrient reduction of the Danube during the growing season is about 4.3%, assuming 10% of the Danube water is flowing through the Delta.

The currents in the lowest few millinleters of the bottom boundary layer of lakes are highly important for the dissipation of kinetic energy and for chemical processes such as oxygen transfer into the sediment. So far, no highresolution flow velocity profiles close to the sediment-water interface have been reported for such systems because a suitable flow meter was lacking. This article introduces a novel sensor for the measurement of extremely lowflow velocities. The sensor is based on a gas transducer surrounded by a gas reservoir. It measures the change in the partial pressure of a tracer gas outside the reservoir tip due to advective transport. The sensor is suitable for measurements of velocities smaller than 1 mm s-1 , with a spatial resolution of 100 to 250 µm. The flow measurements prove to be insensitive to temperature changes between 5 and 15 °C. The sensor is robust against relative pressure changes, and angular differences in the sensitivity can be calibrated. We present high-resolution in situ measurements at the bottom of a pre-alpine lake with shear velocities as low as 0.13 ± 0.02 cm s-1 • The velocity profile nicely resolves the transition zone between the viscous and the logarithmic boundary layer.

A new approach to predict biogenic particle fluxes to the seafloor is presented, which is based on diffusive oxygen uptake and, in particular, opal fluxes to the seafloor. For this purpose, we used a recently published empirical equation coupling benthic silica to oxygen fluxes, and showing a clear negative correlation between Si and O 2 fluxes. Dissolution of biogenic silica mediated by aerobic microbial activity has been inferred at 24 sites along the African and South American continental margins. Based on the assumption that these findings hold essentially for the entire Southern Atlantic Ocean, we applied the silica to oxygen flux ratio to a basin-wide grid of diffusive oxygen uptake extracted from the literature. Assuming that the silica release across the sediment-water interface equals the particulate flux of biogenic opal to the seafloor, we estimated minimum opal rain rates. We combined these calculations with published relationships between aerobic organic carbon mineralization and dissolution rates of calcite above the hydrographical lysocline, thereby assessing the calcite rain rate and particulate organic carbon flux to the seafloor. The addition of the buried fraction completes our budget of biogenic particulate rain fluxes. The combination of such empirical equations provides a powerful and convenient tool which greatly facilitates future investigations of biogenic particle fluxes to the seafloor. K. Seiter (*) GDfB-

A new approach to predict biogenic particle fluxes to the seafloor is presented, which is based on diffusive oxygen uptake and, in particular, opal fluxes to the seafloor. For this purpose, we used a recently published empirical equation coupling benthic silica to oxygen fluxes, and showing a clear negative correlation between Si and O 2 fluxes. Dissolution of biogenic silica mediated by aerobic microbial activity has been inferred at 24 sites along the African and South American continental margins. Based on the assumption that these findings hold essentially for the entire Southern Atlantic Ocean, we applied the silica to oxygen flux ratio to a basin-wide grid of diffusive oxygen uptake extracted from the literature. Assuming that the silica release across the sediment-water interface equals the particulate flux of biogenic opal to the seafloor, we estimated minimum opal rain rates. We combined these calculations with published relationships between aerobic organic carbon mineralization and dissolution rates of calcite above the hydrographical lysocline, thereby assessing the calcite rain rate and particulate organic carbon flux to the seafloor. The addition of the buried fraction completes our budget of biogenic particulate rain fluxes. The combination of such empirical equations provides a powerful and convenient tool which greatly facilitates future investigations of biogenic particle fluxes to the seafloor. K. Seiter (*) GDfB-

Burrowing mayflies (Hexagenia limbata and H. rigida) recolonized sediments of the western basin of Lake Erie in the 1990s following decades of pollution abatement. We predicted that Hexagenia would also disperse eastward or expand from existing localized populations and colonize large regions of the other basins. We sampled zoobenthos in parts of the western and central basins yearly from 1997-2005, along the north shore of the eastern basin in 2001-2002, and throughout the lake in 2004. In the island area of the western basin, Hexagenia was present at densities ≤ 1,278 nymphs/m 2 and exhibited higher densities in odd years than even years. By contrast, Hexagenia became more widespread in the central basin from 1997-2000 at densities ≤ 48 nymphs/m 2 but was mostly absent from 2001-2005. Nymphs were found along an eastern basin transect at densities ≤ 382/m 2 in 2001 and 2002. During the 2004 lake-wide survey, Hexagenia was found at 63 of 89 stations situated throughout the western basin (≤ 1,636 nymphs/m 2 , mean = 195 nymphs/m 2 , SE = 32, N = 89) but at only 7 of 112 central basin stations, all near the western edge of the basin (≤ 708 nymphs/m 2 ), and was not found in the eastern basin. Hexagenia was found at 2 of 62 stations (≤ 91 nymphs/m 2 ) in harbors, marinas, and tributaries along the south shore of the central basin in 2005. Oxygen depletion at the sediment-water interface and cool temperatures in the hypolimnion are probably the primary factors preventing successful establishment throughout much of the central basin. Hexagenia can be a useful indicator of lake quality where its distribution and abundance are limited by anthropogenic causes.

Colemanite deposits have been known from northern Ventura County since the late 1890s. Prospecting for colemanite over an area of as much as 20 square miles in the late 1890s and early 1900s resulted in the discovery of three mines. The Frazier, Columbus and Russell mines produced hand-sorted colemanite ore between 1899 and 1913. Estimated production of crude ore for the Frazier and Columbus mines is about 34,000 short tons. There are no published estimates of production for the Russell mine. These three mines are located in Sections 14 and 23, T. 8 N., R. 21 W. SMB. (see Fig. I). Over 4,276 acres of placer mining claims, and 25 acres of lode and millsite claims were patented between 1904 and 1949 in the township. These claims cover some ground which has not proven productive.

The quality and interpretability of the paleobiological record depends on the preservation of morphological and geochemical fossils. Siliceous microfossils and sedimentary pigments are often cornerstones in paleoecology, although the microbial and geochemical processes conducive to their preservation remain poorly constrained. We examined sediments from an alpine lake in Banff National Park (Alberta, Canada) where diatom frustules are completely dissolved within 50 years of deposition. Diatom dissolution, silica recycling, and diagenetic alteration of algal pigments were investigated, in conjunction with porewater geochemistry and microelectrode profiling of the sediment-water interface. Analysis of sediment trap material showed *90% of biogenic silica (BSi) production is lost prior to burial. Silica flux calculations, based on dissolved silica (as H 4 SiO 4 ) in pore-waters, show a further *6% of total BSi is returned to the water column from the upper 4 cm of sediments, implying that only *4% of total BSi is permanently archived in sediments. In situ sediment pH and O 2 profiles reveal that aerobic respiration by bacteria fully consumes oxygen by a depth of 4 mm into the sediment, with associated strong pH and redox gradients. During sedimentation and early diagenesis, diatoms undergo loss of extracellular polymeric substances that coat their frustules, promoting silica dissolution and leading to the loss of the microfossil record by a depth of 3.25 cm. Sedimentary pigments similarly undergo rapid degradation, but diatom-related carotenoids persist below the depth of silica dissolution. This work provides new insights on diagenetic processes in lakes, with broad implications for the interpretation of sedimentary proxies for algal production.

Spatial variations in the concentration and nature of colored dissolved organic matter (CDOM) in the western Arctic Ocean were examined by three-dimensional excitation/emission matrix (3-D EEM) spectroscopy. CDOM profiles showed distinctive features well correlated with hydrographic characteristics. CDOM fluorescence was particularly high at depths between 40 and 200 m (up to 3 fluorescence units (Fl.U.)) in both Chukchi Sea and Beaufort Sea transects. Penetration of the high CDOM signal, formed on the shelves, into the Canada Basin was confined to the upper halocline layer (salinity of $33.1). This layer had distinctive 3-D EEM fingerprints in fluorescence spectra, showing a marked terrestrial humic signature. The presence of CDOM in the halocline layer likely resulted from two main processes: the brine rejection during sea ice formation and transport across the sediment-water interface during early diagenesis. Despite the high primary productivity in the Chukchi shelf, CDOM contribution from in situ production seemed to have little influence on the overall CDOM distributions in the study area.

Irrigation by benthic macrofauna has a major influence on the biogeochemistry and microbial community structure of sediments. Existing quantitative models of bioirrigation rely primarily on chemical, rather than ecological, information and the depth-dependence of bioirrigation intensity is either imposed or constrained through a data fitting procedure. In this study, stochastic simulations of 3D burrow networks are used to calculate mean densities, volumes and wall surface areas of burrows, as well as their variabilities, as a function of sediment depth. Burrow networks of the following model organisms are considered: the polychaete worms Nereis diversicolor and Schizocardium sp., the shrimp Callianassa subterranea, the echiuran worm Maxmuelleria lankesteri, the fiddler crabs Uca minax, U. pugnax and U. pugilator, and the mud crabs Sesarma reticulatum and Eurytium limosum. Consortia of these model organisms are then used to predict burrow networks in a shallow water carbonate sediment at Dry Tortugas, FL, and in two intertidal saltmarsh sites at Sapelo Island, GA. Solute-specific nonlocal bioirrigation coefficients are calculated from the depth-dependent burrow surface areas and the radial diffusive length scale around the burrows. Bioirrigation coefficients for sulfate obtained from network simulations, with the diffusive length scales constrained by sulfate reduction rate profiles, agree with independent estimates of bioirrigation coefficients based on pore water chemistry. Bioirrigation coefficients for O 2 derived from the stochastic model, with the diffusion length scales constrained by O 2 microprofiles measured at the sediment/water interface, are larger than irrigation coefficients based on vertical pore water chemical profiles. This reflects, in part, the rapid attenuation with depth of the O 2 concentration within the burrows, which reduces the driving force for chemical transfer across the burrow walls. Correction for the depletion of O 2 in the burrows results in closer agreement between stochastically-derived and chemicallyderived irrigation coefficient profiles.

Underground seepage and the sediment-water interface are important pathways for transporting concentrated pore water solutes through permeable sediments. In this study, we utilized radium-224/thorium-228 disequilibria to examine solute transport throughout shallow sediments (depth < 30 cm) of a tidal marshland, North Inlet, South Carolina, over a seasonal cycle. Distinct 224 Ra-228 Th disequilibria patterns indicate that significant pore water exchange occurred both directly across the sediment-water interface and at depth as underground seepage. Temporal variations in phosphorus (P), iron (Fe), and manganese (Mn) pore water concentrations also support this two-layer transport model. Solute transport across the interface was influenced by seasonal bioirrigation, with highest exchange rates occurring in the late spring. Interfacial flux of dissolved Fe ranged over two orders of magnitude, from 0.5 mmol m -2 d -1 to 134 mmol m -2 d -1 , and fluxes of soluble reactive phosphate (SRP) and dissolved Mn varied from 0.01 mmol m -2 d -1 to 0.72 mmol m -2 d -1 and from 0.04 mmol m -2 d -1 to 2.7 mmol m -2 d -1 , respectively. Seepage occurs year-round, with fluid exchange rates related to the distance from the tidal creek. SRP transport via seepage ranged from 0.98 mmol m -2 d -1 to 5.0 mmol m -2 d -1 , one order of magnitude higher than across the sediment-water interface. Reduction/oxidation reactions at depth diminished trace metal concentrations in deep pore water, and lowered the transport efficiency of Fe and Mn along seepage paths. Although an appreciable fraction of exported solutes likely returns to marsh sediments during tidal circulation, our results indicate that organic-rich sediments provide a potentially large source of nutrients and trace elements to the coastal ecosystem.

Limited use of substrate ecospace by a meager infauna in streams and lakes is indicated by analysis of more than 10 000 observations of bioturbation in Permian through Jurassic freshwater deposits. In contrast, marine substrates have been inhabited and bioturbated since the Cambrian; colonization of freshwater substrates differed fundamentally from that of marine substrates. Bioturbation-enhanced flux of dissolved materials across the sediment-water interface was not operating until after the Paleozoic in freshwater ecosystems.

The #ux and taxonomic composition of coccolithophores has been determined for sediment trap samples collected over a two-year period from Guaymas Basin, central Gulf of California. The Gulf of California is a subtropical marginal sea with exceptionally high rates of primary productivity. The annual biogenic sediment #uxes are largely dominated by silica, with diatoms as the major contributor to the opal #ux. For the July 1990 through November 1992 study period, the total coccolithophore export production in Guaymas varied in response to the strength of El Nin o/Southern Oscillation (ENSO). Coccolithophore production follows the general surface primary productivity pattern with relatively high #uxes from late fall to late winter, and low production in the summer when the water column is highly strati"ed and nutrient depleted. High #uxes of up to 10 coccoliths m\ day\ and 3;10 coccospheres m\ day\ indicate that coccolithophores are an important component of the phytoplankton population in Guaymas Basin. The intensi"cation of ENSO conditions during 1992 altered the typical hydrography of the Gulf of California by increasing SST and reducing the strength of spring upwelling. This is re#ected in reduced opal and diatom #uxes during this period. Conversely, these more oligotrophic conditions resulted in higher total coccolithophore #uxes during spring and summer 1992. Gephyrocapsa oceanica is present in the Gulf throughout the study period, but is dominant during weak or non-ENSO times. Emiliania huxleyi and, to a lesser degree, Florisphaera profunda are abundant during strong ENSO periods. Two major coccolithophore signals are indicative of strong ENSO conditions and could be used as paleo-ENSO indicators in the Gulf of California: (a) the switch in dominance between * Corresponding author.

The accuracy of Arctic lake chronologies has been assessed by measuring the 14C activities of modern carbon sources and applying these isotopic mass balances to dating fossil lake materials. Small (&lt;1 km2) shallow (&lt;25 m) Arctic lakes with watersheds &lt;12 km2have soil and peat stratigraphic sections with14C activities ranging from 98 to 51% Modern. The14C activity of particulate organic carbon, dissolved organic carbon, and dissolved inorganic carbon from lake and stream waters ranges from 121 to 95% Modern. The sediment–water interface of the studied lakes shows consistent14C ages of ∼100014C yr, although the14C activity of living aquatic vegetation is 115% Modern. Radiocarbon measurements of components of the lacustrine carbon pool imply that the ∼100014C yr age of the sediment–water interface results from deposition of14C-depleted organic matter derived from the watershed.

Benthic fluxes of phosphate and phosphorus distribution in sediments from the southern Baltic Sea were investigated in spring and autumn in 2005 and 2007–2010. Strong spatial variability of phosphate fluxes was observed across the sediment-water interface. The highest values of phosphate flux from sediment (up to 37 μmol m−2 h−1), resulting from the high mineralization rate of organic matter and rapid phosphorus turnover due to macrofaunal activity and hydrodynamic conditions, were observed in the shallow area at depths ranging from 50 to 69 m. The rate of phosphate exchange in the transportation and accumulation bottom area with the water depth ≥72 m was several times lower (2.12–6.22 μmol m−2 h−1). In continuously hypoxic or anoxic sediments, phosphorus was preserved in the refractory organic form, and sediments were depleted of redox-dependent phosphorus forms. In shallow area with well oxygenated near-bottom water, phosphorus was present mainly in the calcium-bound form.

. PAH concentrations were determined in a dated sediment core collected from Esthwaite Water EW , a seasonally anoxic lake in the English Lake District. The most dramatic variations are associated with increased PAH fluxes from Ž y2 y1 . ; 1900 and a sub-surface maximum ; 29 mg m year in the late 1960s to early 1970s, followed by a fivefold decrease in fluxes to the sediment-water interface. This trend is believed to reflect enhanced fossil fuel burning, followed by general improvements in combustion technologies, shifts in the fuels used for domestic space heating and the implementation of various emission controls on releases from certain known PAH sources. When the relative contributions from individual compounds to the ÝPAH were plotted as vertical profiles, coherent time trends emerged. Perylene dominated the pre-1900 sedimentary PAH composition, contributing ) 75% to the ÝPAH mixture. The perylene profile provides good evidence for both natural and anthropogenic sources to EW. The ratios of annual sediment trap fluxes to surficial sediment accumulation rates are substantially greater than one for the low molecular weight compounds, suggesting release at or near the sediment water interface and subsequent recycling of these compounds. Recycling was found to increase with increasing solubility and decreasing log K . ᮊ 1998 ow Elsevier Science B.V.

Over 300 surface sediment samples from the Central and South Atlantic Ocean and the Caribbean Sea were investigated for the preservation state of the aragonitic test of Limacina inyata. Results are displayed in spatial distribution maps and are plotted against cross-sections of vertical water mass con"gurations, illustrating the relationship between preservation state, saturation state of the overlying waters, and overall water mass distribution. The microscopic investigation of L. inyata (adults) yielded the Limacina dissolution index (LDX), and revealed three regional dissolution patterns. In the western Atlantic Ocean, sedimentary preservation states correspond to saturation states in the overlying waters. Poor preservation is found within intermediate water masses of southern origin (i.e. Antarctic intermediate water (AAIW), upper circumpolar water (UCDW)), which are distinctly aragonite-corrosive, whereas good preservation is observed within the surface waters above and within the upper North Atlantic deep water (UNADW) beneath the AAIW. In the eastern Atlantic Ocean, in particular along the African continental margin, the LDX fails in most cases (i.e. less than 10 tests of L. inyata per sample were found). This is most probably due to extensive `metabolica aragonite dissolution at the sediment-water interface combined with a reduced abundance of L. inyata in the surface waters. In the Caribbean Sea, a more complex preservation pattern is observed because of the interaction between di!erent water masses, which invade the Caribbean basins through several channels, and varying input of bank-derived "ne aragonite and magnesian calcite material. The solubility of aragonite increases with increasing pressure, but aragonite dissolution in the sediments does not simply increase with water depth. Worse preservation is found in intermediate water depths following an S-shaped curve. As a result, two aragonite lysoclines are observed, one above the other. In four depth transects, we show that the western Atlantic and Caribbean LDX records resemble sur"cial calcium carbonate data and C and carbonate ion concentration pro"les in the water column. Moreover, preservation of L. inyata within AAIW and UCDW improves signi"cantly to the north, whereas carbonate corrosiveness diminishes due to

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Mechanisms for fluid flow such as compactive dewatering, gravity-driven hydrologic flow, and seismic pumping are not geologically plausible for Mississippi Valley-type (MVT) mineralization in the Lennard Shelf, Canning Basin, northwestern Australia. Sulfides at Cadjebut and the other Lennard Shelf MVT deposits precipitated from overpressured, hydrocarbon-rich brines. The deposits lie below a major mid-Carboniferous unconformity that is linked to a global sea-level fall. Independent evidence at Cadjebut also constrains the time of mineralization to the mid-Carboniferous. A sea-level fall at this time could have raised the sediment-water interface above the thermocline, thereby increasing the temperature of the basin by -16 °C. Model calculations show that a rise in basin temperature would induce sufficient hydrocarbon gas maturation to expel 1005 km 3 of brine. This brine volume would be more than sufficient to transport and deposit the metal budget of the known MVT deposits on the Lennard Shelf. The existence worldwide of MVT deposits below unconformities suggests that sea-level-induced hydrocarbon generation could have assisted MVT mineralization in other provinces.

Inorganic nitrogen and soluble reactive phosphate (o-P) concentrations were measured in the water of a marsh and in its interstitial water at two sites, and in the river water of a floodplain marsh of the Lower Paraná River. These values were compared with the N and P concentration in sediments and macrophyte biomass in order to assess nutrient availability, fate

Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists.

The results are reported of a geochemical study of sediment cores and surface waters taken over an annual cycle from the compost-based constructed wetland at Quaking Houses, NE England. The wetland was built to treat acidic and metalliferous waters emanating from colliery spoil. The influent waters contain up to 10 mM SO 2À 4 , total Fe around 100 lM, and a mean pH of 6.2. The organic-rich sediments sustain a coupled redox cycle of Fe and S which occurs throughout the year but which is more intense in the summer months. Throughout the sediments, reduction of Fe(III) and SO 2À 4 apparently occur within the same macroscopic volume of sediment, along with oxidation of sulfide and Fe(III). In the top 2 cm of the sediment, pore water Fe concentrations reach a maximum of $1 mM in the presence of high concentrations of Fe oxides and the occurrence of SO 4 reduction. Partial re-oxidation of sulfide is indicated by the presence of significant elemental S. Total Fe in the surface 10 cm has a mean value of 7-9% of sediment dry weight, of which sulfide-Fe represents 7-12% and reactive amorphous Fe 20-45%; around 90% of the solid phase sedimentary Fe occurs as oxides or oxyhydroxides. It is suggested that the downwards diffusion of dissolved Fe from the near-surface maximum is sustained by the precipitation of Fe oxides as a result of radial O 2 loss from roots in the dense rhizosphere. Pore water pH is between 7.2 and 7.8 and alkalinity increases downwards, coupled to microbial SO 4 and Fe reduction. Transport processes occurring at and across the sediment-water interface are sufficiently rapid in the $20 h residence time of the waters to: (a) remove 70-90% of influent Fe and 15-25% of influent SO 4 into surface sediments and (b) increase both the pH and alkalinity of effluent waters. Coupling of the Fe and S cycles is fundamental to effective remediation in terms of both alkalinity generation and the retention of metals.

Approximately 600 km of high-resolution seismic reflection data were collected to investigate the late-Quaternary stratigraphic development of Lake Titicaca. The focus of this report is on two seismic sequence boundaries, which are interpreted as erosional surfaces formed at times of low lake level. The younger erosional surface occurs as much as 90 m below the present lake level and up to 8 m below the present sediment-water interface. This erosional surface is interpreted to be coeval with a well-documented early-to mid-Holocene lowstand, dated between ~8,000 and 3,600 cal yr BP. An earlier and previously unknown erosional surface occurs at a sub-bottom depth of approximately 30 m, and as much as 240 m below the present lake level, which implies a major late-Pleistocene lowstand of Lake Titicaca. By extrapolation of sedimentation rates from the upper ~14 m of sediment, we estimate the age of this older lowstand at >90,000 cal yr BP. Both lowstands of Lake Titicaca indicated by the seismic data are likely to have been a response to climatic change in the region.

Pyrite as a proxy for the redox states of bottom waters was evaluated in the context of modern salt marsh sediments. Two sites, 40 m apart, with very different physicochemical properties were sampled seasonally. The sites differed from each other in (1) the amount of tidal flushing, (2) vegetation, (3) redox condition below the sediment-water interface, (4) carbon content and (5) solid phase iron oxyhydroxide and iron sulfide content of the sediment. The calculated degree of pyritization (DOP) varies between 0.44 and 1.0, depending on the site and the season. Different DOP values were obtained for the same samples when different techniques were applied for extraction of reactive iron. Framboidal pyrite was dominant; however, other textural forms of pyrite were also observed. The size distribution of the individual pyrite crystals and the framboids was fairly narrow; with the majority of them having diameters <10 lm. Despite deposition taking place in aerobic waters (fully oxygenated waters) at the sampled sites, both the DOP and pyrite textures indicated dysoxic (waters with low oxygen concentrations) to euxinic (waters with no oxygen present, and H 2 S may be continually or intermittently present) bottom waters. These results indicate that caution must be applied when using DOP and pyrite morphologies as proxies for interpreting the redox conditions of different depositional environments.

The time‐dependent flow and dispersion of suspended sediments in the western basin of Lake Erie are being studied by means of numerical models utilizing data from remote‐sensing studies and flume experiments. Mechanisms of sediment dispersion included in the models are convection and turbulent diffusion, river loading, gravitational settling, and physical resuspension and deposition at the sediment‐water interface. The time‐dependent currents are computed by means of a free surface hydrodynamic model. A wave‐hindcasting model is used to compute the wave parameters needed for estimation of shear stress generated at the sediment‐water interface under given wind conditions. The rate of sediment resuspension as a function of bottom shear stress and sediment properties is based on data from flume experiments using lake sediments. A series of numerical calculations with the models were performed on a two‐dimensional lake with a variable bottom representing a transverse cross section of La...

Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists.

Benthic organic matter microbial oxidation is the basic process by which oxidants such as oxygen, nitrate and sulphate are consumed in sediments, while ammonium and phosphate are released. Although these processes play a crucial role in river biogeochemistry, their modelling remains a difficult challenge. Thouvenot et al.

The deterioration of reservoirs in southern China due to the kinetics of Iron (Fe), Phosphorus (P) and sulphide (S) at the sediment-water interface (SWI) is a major problem that needs urgent attention. Studies on the biogeochemistry of Fe, P, and S using high-resolution profile techniques in reservoirs in this region are limited. The diffusive gradient in thin films (DGT) technique, high-resolution dialysis, DGT-computer imaging densitometry (CID), DGT-induced fluxes in sediments (DIFS) and planar optode (PO) device were used to describe the dynamics Fe-P-S in SWI during hypoxia. The results showed the release of Fe-P-S in SWI was due to sulfate reduction and iron reduction influenced greatly by hypoxia. Positive apparent fluxes were recorded indicating that the sediments release Fe-P-S to the overlying water. High positive correlations (r 2 N 0.7) for DGT-labile Fe and DGT-labile P in sediments revealed that iron-bound P controlled the release of P at SWI during reductive dissolution. The low correlation between DGT-labile Fe and DGT-labile S (r 2 b 0.4) disclosed the combative nature between sulfate reduction and iron reduction process. The low correlation occurred because of the co-precipitation between Fe and S, forming black materials such as monosulfide (FeS) and pyrite (FeS 2 ) in a hypoxic environment. The DIFS model Science of the Total Environment 724 (2020) 138204

The deterioration of reservoirs in southern China due to the kinetics of Iron (Fe), Phosphorus (P) and sulphide (S) at the sediment-water interface (SWI) is a major problem that needs urgent attention. Studies on the biogeochemistry of Fe, P, and S using high-resolution profile techniques in reservoirs in this region are limited. The diffusive gradient in thin films (DGT) technique, high-resolution dialysis, DGT-computer imaging densitometry (CID), DGT-induced fluxes in sediments (DIFS) and planar optode (PO) device were used to describe the dynamics Fe-P-S in SWI during hypoxia. The results showed the release of Fe-P-S in SWI was due to sulfate reduction and iron reduction influenced greatly by hypoxia. Positive apparent fluxes were recorded indicating that the sediments release Fe-P-S to the overlying water. High positive correlations (r 2 N 0.7) for DGT-labile Fe and DGT-labile P in sediments revealed that iron-bound P controlled the release of P at SWI during reductive dissolution. The low correlation between DGT-labile Fe and DGT-labile S (r 2 b 0.4) disclosed the combative nature between sulfate reduction and iron reduction process. The low correlation occurred because of the co-precipitation between Fe and S, forming black materials such as monosulfide (FeS) and pyrite (FeS 2 ) in a hypoxic environment. The DIFS model Science of the Total Environment 724 (2020) 138204

The groundwater contribution into Par Pond, a former cooling reservoir for two nuclear reactors located on the Department of Energy's Savannah River Site (South Carolina), was estimated using a standard hydrologic budget as well as one augmented by a natural tracer approach. We determined a geochemical budget for ***Rn, normally found at much higher concentrations in groundwater than surface waters, to assist in constraining the hydrologic estimates. The radon budget accounted for ail quantifiable surface sources and sinks including the flux across the sediment-water interface which was determined by application of an advection-diffusion model. All hydrologic parameters and radon concentrations were monitored seasonally from February 1994 to August 1995. Using the water balance approach alone, the average groundwater discharge entering the lake was estimated to have an upper limit of approximately 0.95 + 0.13 m3 s-'. The groundwater contribution obtained using the combined hydrologic/***Rn approach ranged from 0.17 to 0.76 m3 s-' with a best estimate of 0.35 ? 0.16 m3 s-'. Lake profiles show enhanced "*Rn concentrations in some areas indicating that groundwater enters Par Pond mostly through a small region in the northern portion of the lake, probably via small seeps or springs. Estimates show that groundwater plays a significant role in the overall water budget of the lake, accounting for IO%-33% of the total estimated inflow from all measured sources. Our results show that supplementing a standard hydrological water balance with radon budget considerations helps to constrain estimated groundwater flow into surface reservoirs. 0 1997 Elsevier Science B.V.

X-ray Fluorescence Analysis with Synchrotron Radiation (SR XFA) was used to study, at a high-resolution ($200 yr), the behavior of elements in an 80 kyr core record from the Okhotsk Sea. We measured biogenic Ba (Ba Bio ), biogenic carbonate, and organic carbon as bio production indicators, elements that record the Amur terrigenous input, and Mn as signature of water ventilation and redox conditions at the sediment/water interface. The Okhotsk Sea in last glacial time responded to abrupt warm spells in the Atlantic and Greenland. Bio production in the Okhotsk Sea increased dramatically during warm D/O events and decreased during cold and dry H-events.

Few studies have investigated capabilities or ecological implications of chemical sensing in deposit feeders. We tested three species of spionid polychaetes (Boccardia proboscidea Hartman, Polydora cornuta Bosc, and Pseudopolydora kempi japonica Imajima and Hartman) for responses to a variety of compounds: amino acids, carbohydrates, organic acids, nucleotides, and plant and animal extracts that might have been used as general indicators of food quality or of the presence of specific foods. Five individuals of each species were exposed to 44 different dissolved chemical cues introduced in slow, laminar flow to control location and dose (1 mM for most cues) of delivery. Cues that altered feeding behaviors of one or more species were then tested against ten additional individuals of each species, in part to overcome dangers of multiple testing in the original screening. We observed differences among the three species both in their background behavioral patterns and in their responsiveness to dissolved cues applied at palp level just above the sediment-water interface, although significant changes in feeding behavior occurred only in Boccardia and Pseudopolydora. Taurine, threonine and valine acted as strong phagodepressants, whereas proline was significantly phagostimulatory to Boccardia. No hexoses elicited significant responses. Natural extracts were more stimulatory than the low-molecular-weight monomers tested.

for the Thesis Presented In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy (in Civil Engineering) August, 2007 Research concerning the fate and biogeochemical cycling of mercury (Hg) within coastal ecosystems has suggested that microbially-mediated diagenetic processes control Hg mobilization and that ligands with strong affinity for Hg control Hg Finally and most significantly of all-thank you, Elliot, for absolutely everything. When 1 woke up that day and announced that it was time to go back to graduate school you rearranged many things to make it possible. I want you to know that I saw all that you did and I appreciate it beyond measure. As Joe Strummer said: without your love I won't make it through. in TABLE OF CONTENTS ACKNOWLEDGEMENTS ii LIST OF TABLES viii LIST OF FIGURES ix PREFACE x GLOSSARY OF TERMS xiv CHAPTER 1. Mercury dynamics in sulfide-rich sediments: geochemical influence on mercury mobilization within the Penobscot River Estuary,

The absorption, or uptake, of anthropogenic CO 2 by the oceans results in a decrease in pH and carbonate ion concentration, [CO 3 2-]; as a consequence, the saturation state of seawater with respect to CaCO 3 minerals (calcite, aragonite) falls, leading to a shallowing of their saturation depths and triggering an increase in their dissolution at the seafloor. Nearly one-third of the seafloor is covered by CaCO 3-rich sediments, and their dissolution is the ultimate sink of anthropogenic CO 2. Despite numerous past studies, the kinetics of CaCO 3 dissolution in seawater are still poorly quantified, and the mechanisms controlling the reaction are scarcely understood. Results from in situ studies disagree with laboratory studies, most of which have been carried out under conditions, e.g., mineral suspensions, that are not representative of processes at the seafloor. Herein we report measurements of the dissolution rate of calcite, formed into synthetic sediment disks by mixing various amounts of this mineral with montmorillonite. These disks were placed in a stirred-flow reactor and exposed to a range of saturation states and shear stress conditions to simulate conditions encountered at the seafloor. The dissolution rates, normalized to the interfacial area of the sediment disks, were linearly dependent on the undersaturation state of the bulk seawater in the reactor; in addition, the rate displayed a square-root dependence on the calcite content under quiescent conditions. The rate of release of reaction products from the sediment increased with stirring rate, i.e., shear stress, until it became invariant at higher stirring rates. This latter result argues that calcite dissolution at the seafloor is transport (water-side) controlled at the shear stresses known to exist at the seafloor, which advises a simpler kinetic description of calcite dissolution at the seafloor.

Marine sediments typically exhibit steep porosity gradients in their uppermost centimeters. Although the decrease in porosity with depth below the sediment-water interface is primarily due to compression arising from the accumulation of overlying sediment, early diagenetic mineral dissolution and precipitation reactions may potentially also affect the porosity gradient. Here, we present a steady state compaction model, based on the mass and momentum conservation of total fluid and solid phases, in order to quantify the relative contributions of mineral reactions and physical compaction on porosity changes. The compaction model is applied to estimate hydraulic conductivity and compressive response coefficients of deep-sea sediments from measured porosity depth profiles. The results suggest an inverse relation between the compressive response coefficient and the lithogenic content of marine sediments. For deep-sea sediments exhibiting high rates of dissolution of siliceous shell fragments, the compaction model ignoring mineral reactions overestimates the hydraulic conductivity and compressive response coefficients. In contrast to non-compacting porous media, mineral dissolution in surficial sediments can lead to lower porosity. However, as illustrated for a deep-sea sediment in the equatorial Atlantic characterized by extensive dissolution of calcareous shell fragments, the effect of mineral dissolution and precipitation reactions on porosity gradients is, in most cases, negligible.

Major biogeochemical processes in the newly created, shallow Lake Agmon (Hula Valley, northern Israel) were investigated from 1994 to 1996. Sediment cores, lake water and porewater were analyzed for chemical composition and spatial distribution. Sediment analyses revealed that Lake Agmon has two different sediment types: peat sediments in the northern and central parts, and marls in the southern part. The

Extensive measurements of nitrous oxide (N O) were made in the central and eastern Arabian Sea during the northeast monsoon (February-March), intermonsoon (April-May) and southwest monsoon (July-August) seasons. The latitudinal and longitudinal variations, along with seasonal changes with respect to winter convection and coastal upwelling, are clearly discernible. Vertical profiles collected down to 1000 m show that the Arabian Sea water column is supersaturated with N O at all depths. However, N O consumption at intermediate depths, coincident with the oxygen minimum and associated with sediment-water interfaces, and in the denitrifying zone, coincident with NO\ secondary maxima, are also apparent. The N O concentration varies from &10 nM near the surface to about 80 nM in the secondary peak region (+800 m). Interrelationships with chemical parameters suggest nitrification to be the main process for the production of N O in the oceanic water. Plots of apparent oxygen utilization vs production of N O indicate a consistent linear relationship for AOU between 0 and 200 M.

Parts of the sandy littoral sediment of a Precambrian shield lake (Lake 302S) were intermittently covered with a layer of flocculent organic‐rich material 1–20 mm thick. Sandy sediments with flocculent surface sediment had higher rates of respiration (31–105%) and photosynthesis (37– 224%) than those without. Densities of invertebrate macrofauna were 10 times higher in the flocculent surface sediment than in the underlying sandy sediment, accounting for 17 vs. 5% of the total respiration. Microprofiles of oxygen concentration showed that almost all of the dissolved oxygen in the overlying water was consumed as it diffused through 5–7 mm of the flocculent surface sediment, so that the underlying sandy sediment was almost completely anaerobic in the presence of flocculent surface material. The presence of flocculent surface sediment also decreased the depth of penetration of sulfate into the sandy sediment since the zone of sulfate reduction was located closer to the sediment‐water in...

Sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice cover of 190 days per year. In situ, transparent and dark flux chambers were used for direct measurements of benthic fluxes of dissolved oxygen, nutrients, silica and certain metals, taking into account primary production and mineral precipitation. The range of benthic flux observed for dissolved oxygen ͑DO͒, dissolved inorganic carbon ͑DIC͒, ammonium, ortho-P, silica, calcium, and magnesium was-45.89 to 187.03,-99.32 to 50.96,-1.30 to 1.27,-0.51 to 0.39,-62.3 to 9.3,-33.82 to 16.83, and-23.93 to 7.52 mmol m Ϫ 2 d Ϫ 1 , respectively ͑negative value indicating flux towards the lake bottom͒. Low benthic NH 4 ϩ and ortho-P fluxes were likely related to benthic algal production, and aerobic bottom water. Ortho-P fluxes could also be controlled by the dissolution/precipitation of ferrihydrite, calcite, and perhaps hydroxyapatite. The negative silica fluxes were caused by diatom frustule synthesis. Benthic calcium and magnesium fluxes could be related to algal production and dissolution/ precipitation of calcium and/or Ca,Mg-carbonates. Fluxes of DO, DIC, pH and alkalinity were related to benthic biological processes. It is likely that some of the carbon precipitates as calcite at the high pH in the summer and dissolves at neutral pH in the winter. Mean of the ratio of gross benthic DIC consumption and gross benthic DO production was 0.94 Ϯ 0.18, consistent with algal production using NH 4 ϩ as N source. During the summer weeks the water column pH remains above 10. This high pH is caused by direct and indirect utilisation of CO 2 , HCO 3 Ϫ , CO 3-2 , H 4 SiO 4°a nd H 3 SiO 4 by primary producers. This study shows that in shallow lakes at high latitudes, where summer days are long and the primary production is mostly by diatoms, the pH is forced to very high values. The high pH could lead to a positive feedback for the Si flux, but negative feedback for the NH 4 ϩ flux.

In situ paired light and dark-stirred benthic flux chambers were used to estimate dissolved oxygen flux across the sediment-water interface in Lake Mývatn, Iceland. Three sampling stations were selected, each station reflecting a specific sedimentary environment, benthic communities, and water depth. During this study the phytoplankton density was low. Spatial and seasonal variations of bottom DO concentration and DO flux have been observed during this study. The oxygen consumption rate at all study sites had a mean of −89 (± 44) mmol m −2 d −1 while the oxygen production rate due to benthic algae had a mean of 131 (± 103) mmol m −2 d −1. There was a strong correlation (r = 0.91) between oxygen consumption rate and temperature. This was presumably because of the temperature influence on rates of microbial and macrobenthic processes. The mean benthic primary production rate at all study sites was 1216 (± 957) mg C m −2 d −1 between June 2000 and February 2001. Annual gross benthic primary production was estimated from the gross mean daily benthic DO production (P) and Redfield's C:O 2 ratio of 106:138 to be 420 g C m −2 y −1 at station HO, 250 g C m −2 y −1 at B2 and 340 g C m −2 y −1 at station 95. Thus, the mean gross benthic primary production was estimated as 1151 mg C m −2 d −1 at station HO, 685 mg C m −2 d −1 at station B2, and 932 mg C m −2 d −1 at station 95.

scenarios shows that a well-oxygenated eutrophic system favors the conditions for Hg species bioaccumulation with a potential adverse effect on the ecosystem. The research is relevant to the UN Minimata convention, EU policies on water, environmental quality standards and Mercury in particular.

for the Thesis Presented In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy (in Civil Engineering) August, 2007 Research concerning the fate and biogeochemical cycling of mercury (Hg) within coastal ecosystems has suggested that microbially-mediated diagenetic processes control Hg mobilization and that ligands with strong affinity for Hg control Hg Finally and most significantly of all-thank you, Elliot, for absolutely everything. When 1 woke up that day and announced that it was time to go back to graduate school you rearranged many things to make it possible. I want you to know that I saw all that you did and I appreciate it beyond measure. As Joe Strummer said: without your love I won't make it through. in TABLE OF CONTENTS ACKNOWLEDGEMENTS ii LIST OF TABLES viii LIST OF FIGURES ix PREFACE x GLOSSARY OF TERMS xiv CHAPTER 1. Mercury dynamics in sulfide-rich sediments: geochemical influence on mercury mobilization within the Penobscot River Estuary,

Research concerning the fate and biogeochemical cycling of mercury (Hg) within coastal ecosystems has suggested that microbiaHy-mediated diagenetic processes control Hg mobilization and that ligands with strong affinity for Hg, such as dissolved inorganic sulfide (S(-II)) and dissolved organic matter (DOM), control Hg partitioning between the dissolved and particulate phases. We have studied total Hg sediment-bound Hg and complexation of dissolved Hg. In Zone 2, FeS(S) precipitation occurs concomitant with Hg consumption. Net transfer within Zone 3 is consistent with the potential for ligand-mediated Hg efflux across the SWI. S(-II)-mediated Hg mobilization at depth in Penobscot estuary sediments suggests a broadening of the depth interval over which biogeochemical Hg cycling must be examined. Our results also show that, while estuary sediments act as a net sink for particulate Hg inputs, they may also function for a considerable time interval as a source of dissolved Hg.

research described in this article has been funded wholly by the U.S. Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.