Sediment-Water Interface

Measurements of dissolved 02, NzO, NH4+, NOz-, and NOs-are used to study the biogeochemistry of nitrogen in the Potomac River near Washington, D.C. The nutrient budget of the Potomac is dominated by sewage inputs from treatment plants at Blue Plains and Alexandria. Measurements taken in July and September 1977 indicate large supersaturated concentrations of NzO and exhibit significant short-period variations over distances >40 km. Concentrations of N20 and O2 appear to be inversely correlated and there are indications that N,O may be produced primarily near the sediment-water interface. Nitrous oxide is released to the air on a time scale of about 1 day and gas exchange may occur mainly in shallow embaymcnts in response to turbulence associated with tidal currents. Release of N20 to the atmosphere takes place at a rate of about 10' kg* d-' and may be attributed to prompt conversion to N,O of between 1 and 5% of the total nitrogen input. We report here results from an experimental study designed to quantify some aspects of the nitrogen cycle in the Potomac River near Washington, D.C. The Potomac drains a land area of about 3.8 x lo4 km2, from its source on the eastern slope of the Appalachian Mountains to the point where it enters Chesapeake Bay at Point Lookout downstream some 640 km. The river is tidal over the last 183 km of its course, with significant concentrations of chloride evident in waters below Indian Head, about 115 km from Chesapeake Bay. The drainage basin of the river serves a population of about 3.3 million people, with 2.8 million resident in the Washington metropolitan area. The Potomac is the main source of water for Washington and acts also as the receptacle for the city's waste. Flow rates in the river are variable, with an average value before diversion for urban use of 306 ms. s-l, a median value of 184 m3. s-r (Jaworski et al. 1972). Flow rates for the 1 Colonel and Mrs. J. W. Elkins provided laboratory facilities and hospitality during the course of these observations.

The distribution and partitioning of dissolved and particulate arsenic and phosphorus in the water column and sediments of the Saguenay Fjord in Quebec, Canada, are compared. In addition, selective and/or sequential extractions were carried out on the suspended particulate matter (SPM) and solid sediments to contrast their geochemical behaviors in this natural aquatic system. Results of our analyses show that both arsenic and soluble reactive phosphate are actively scavenged from the water column by settling particles. Upon their accumulation at the sediment-water interface some As and P may be released to porewaters following the degradation of organic matter to which they are associated. The porewater concentrations are, however, limited by their strong affinity for authigenic, amorphous iron oxyhydroxides which accumulate in the oxic sediments near the sediment-water interface. The geochemical behavior of arsenic and phosphorus diverge most strikingly upon the development of anoxic conditions in the sediments. Following their burial in the anoxic zone, amorphous iron oxyhydroxides are reduced and dissolved, releasing phosphate and arsenic to the porewaters. We observed, however, that porewater arsenic concentrations increase at shallower depths than phosphate in the sediments. The reduction of arsenate, As(V), to arsenite, As(III), and its desorption prior to the reductive dissolution of the carrier phase(s) may explain this observation. Driven by the strong concentration gradient established in the suboxic zone, phosphate diffuses up towards the oxic layer where it is readsorbed by authigenic iron oxyhydroxides. In the organic-rich and rapidly accumulating sediments at the head of the Fjord, porewater sulfate depletion and the resulting absence of a sulfide sink for Fe(II), may lead to the formation of vivianite in the fermentation zone, a potential sink for phosphate. Arsenite released to the porewaters in the suboxic and anoxic zones of the sediments diffuses either down, where it is adsorbed to or incorporated with authigenic iron sulfides, or up towards the oxic boundary. Arsenite appears to migrate well into the oxic zone where it may be oxidized by authigenic manganese oxides before being adsorbed by iron oxyhydroxides present at the same depth. Whereas, in the absence of authigenic carbonate fluorapatite precipitation, the ability of oxic sediments to retain mineralized phosphate is a function of their amorphous iron oxyhydroxide content, arsenic retention may depend on the availability of manganese oxides, the thickness of the oxic layer and, its co-precipitation with iron sulfides at depth.

Benthic primary production and nutrient dynamics were examined along a transect in the Bangrong mangrove forest in Thailand. Six stations were established extending from a high-intertidal site within the mangrove forest to low-intertidal flats and seagrass beds in front of the mangrove forest. Benthic processes (O2 and CO2 fluxes) and nutrient dynamics (mineralization, sediment-water fluxes, pore water and sediment pools)

We test the relationship of deep sea benthic foraminiferal assemblage composition to the surface ocean productivity gradient in the low latitude Atlantic Ocean using 81 surface sediment samples from a water depth range between 2800 and 3500 m. The samples are selected so that the surface ocean productivity gradient, controlling the flux of organic carbon to the seabed, will be the most important environmental variable. The first two principal components of the assemblage data account for 73% of data variance and are clearly linked to the productivity gradient across the Atlantic. These components show that under higher productivity the assemblages contain a higher abundance of Uvigerina perqrinu, Melonis barleeanum, Globobuliminu spp. and other taxa with probable infaunal microhabitats. Alubuminella weddellensis, a species linked to episodic phytoplankton debris falls, is also important in these assemblages. As productivity decreases there is a regular shift in assemblage composition so that low productivity assemblages are dominated by Globocassidulinu subglnbosa and several Cussidulinu species along with Epismminellu exiguu. We hypothesize that these taxa are epifaunal to very shallow infaunal since nearly all organic carbon oxidation occurs near the sediment-water interface in low productivity settings. Discriminant function analysis of the foraminiferal assemblages, with groups selected on the basis of surface ocean productivity, shows clear separation among five productivity levels we used. This analysis demonstrates that productivity variations have a strong influence on assemblage composition. Finally, we used two groups of samples from the Rio-Grande Rise representing water depths from 2007 to 2340 m and 2739 to 3454 m to test for effects produced by changing water depth. All these samples are from a low productivity region and represent nearly identical environmental conditions. Although the low productivity nature of all the Rio-Grande Rise samples is obvious, there are assemblage differences between our depth groups. We cannot account for the assemblage differences with changes in organic carbon flux, dissolution effects or other physical/chemical properties of the ocean. Thus there are as yet unidentified factors related to water depth which cause some assemblage variation in the low productivity setting we investigated.

The subsurface Middle Eocene Upper Dammam Subformation in Qatar displays a complex diagenetic history related to sea level variations. The studied rocks are made up of dolomitic limestones and pervasive dolomites. Two main types of dolomite are identified based on an integrated petrographic/isotopic/mineralogic study: dolomites type (I) and (II), which affect the lower (the Umm Bab member) and the upper (the Abaruq member) parts of the Upper Damrnam Subformation respectively. Dolomite type (I) is pervasive and composed of relatively coarse crystals that exhibit polymodal size distribution and different shapes of crystal boundaries (non-planar, planar-s and planar-e). Meanwhile, dolomite type (II) is selective, fine-textured and made up of unimodal equant euhedral rhombs. The marine 0 18 0 (+0.6 to +2.8%0) values of dolomite type (I) are consistent with formation during an early transgressive phase where the rising sea level possibly caused marine pore-water to push mixing and meteoric waters ahead of it. The active circulation in the marine pore-water zone led to pervasive dolomitization. The depleted 0 13C (-5.9 to-8.9%0 PDB) values of dolomite type (1)indicate that dolomitization possibly occurred within the sulphate reduction zone beneath the sediment/water interface when an oxygen-minimum zone impinged the bottom surface of the carbonate platform. The presence of mimetic and non-mimetic dolomite textures is related to kinetic factors including the rate of dissolution of the CaCO J precursor, reactant surface area and precipitation kinetics. Dolomite type (II) was formed during a subsequent highstand period during shelf progradation. The 0 180 (+0.4 to +2.6%0 PDB) and ol3C(-0.9 to-4.6%0 PDB) values, the fine crystalline fabric and the planar intercrystalline boundaries argue for its early diagenetic origin within the subtidal zone above the oxygen-minimum zone. During further shelf progradation the studied rocks have been emplaced within a mixing marine-meteoric zone. During this period the dolomitization process was interrupted by a silicification phase. The precipitation of quartz and the resultant silica fabrics are controlled by variations in the PC02 and the pH of the porewater. At the later stages of the highstand system tract, when sea level started to fall, a meteoric lens possibly developed at the shelf margin. The section was subjected to meteoric diagenesis. A late meteoric origin is suggested for calcite mosaics, that fill some secondary pores, based on their extremely depleted 0 18 0 values (-5.7 to-11.8%0), variable OBC values (-2.6 to-7.80/00 PDB), low-Mg composition and their equant form.

Two simple algorithms are suggested here to correct for the effect of diffusion and diagenetic sulfate reduction on radiocarbon age determination of marine porewater. The correction algorithms were developed from mass balances of sulfate, dissolved inorganic carbon (DIC), and 14C of the DIC (14CDIC) in vertical concentrations profiles in porewater starting from the sediment water interface. The algorithms were tested on data collected during our recent study of sediment porewaters extracted from the deep Eastern Mediterranean. The real ages of these porewaters varied from present (top of the core) to approximately 30 ka BP (bottom of the core) covering most of the dynamic range of the 14C method (approximately 5 half lives). These ages were markedly older than the ages calculated from 14CDIC analyses by the regular age equation. It is clearly demonstrated that in this case the correction of the apparent age for diffusion across the sediment/water interface is overwhelmingly larger t...

... We also Figure 2 Lithostratigraphic section of basal Emu Bay Shale where exposed on the shore platform at the mouth of Big Gully. ... Analyses for a suite of 44 trace and rare earth elements were conducted on an Agilent 7500cx solution ICP-MS at Adelaide Microscopy. ...

Previous research recognized most of the Northern Adriatic coastal lagoon environments as contaminated by mercury (Hg) from multiple anthropogenic sources. Among them, the Pialassa Baiona (P.B.) Lagoon, located near the city of Ravenna (Italy), received between 100 and 200 tons of Hg, generated by an acetaldehyde factory in the period 1957-1977. Further east, the Grado Lagoon has been mainly affected by a long-term Hg input from the Idrija mine (western Slovenia) through the Isonzo River since the 16th century. Hg cycling at the sediment-water interface (SWI) of the two lagoons was investigated and compared by means of an in situ benthic chamber, estimating diffusive Hg and Methyl-Hg fluxes in the summer season. Major chemical features in porewaters (Fe, Mn, H 2 S, dissolved inorganic (DIC) and organic carbon (DOC), nutrients) and in the solid phase (C org , N and S) were also explored to understand the general biogeochemical conditions of the system in response to benthic respiration. The daily integrated flux for the methylated Hg form was extremely low in P.B. Lagoon, accounting for only 7% of the corresponding flux calculated for the Grado Lagoon. Despite a higher sedimentary Hg content in the P.B. Lagoon (14.4-79.0 mg g À 1) compared to the Grado Lagoon (10.7-12.5 mg g À 1), the in situ fluxes of Hg in the two experimental sites appeared similar. A selective sequential extraction procedure was applied to the solid phase, showing that the stable crystalline mineral phase cinnabar (HgS) is the predominant Hg fraction (about 50%) in the Grado Lagoon surface sediments. Conversely, Hg mobilization and sequestration in the P.B. Lagoon is related to the extremely anoxic redox conditions of the system where the intense sulfate reduction, by the release of sulfur and the formation of sulfides, limits the metal recycling at the SWI and its availability for methylation processes. Thus, the environmental conditions at the SWI in the P.B. Lagoon seem to represent a natural ''barrier'' for the potential risk of Hg transfer to the aquatic trophic chain.

Limrtol Oceanogì . 43(1), 1998, 53-64 © 1998, by the American Society of Limnology and Oceanography, Ine Burial efficiency of phosphorus and the geochemistry of iron in continental margin sediments Pierre Anschutz,1 Shaojun Zhong,2 and Bj0rn Sundby ...

A new laboratory device was used to study microbial processes within a-1-mm layer of aggregated seston material during different seasons. The conditions below and above this sediment-water interface were simulated, and the concentrations of plant nutrients and oxygen were controlled. Nitrification occurred in the layer only at oxygen concentrations > 1.5-2 mg liter-I. Heterotrophic processes followed zero-order kinetics with respect to oxygen; thus, anoxic conditions were reached within the seston layer at depths of 800-1,400 pm. Oxygen consumption rates in the heterotrophic layer differed as much as 1 O-fold, and values were highest at the oxic-suboxic interface in the layer due to production of dissolved organic C by fermentation. Nitrification accounted for 100% of the total 0, consumption in the layer during the first 20 h. Thereafter, an increase in the oxygen flux into the layer by factors as high as 2-9 was due to initiation of heterotrophic processes. About 120 h after settling, the aggregated seston material became refractory to aerobic microbial degradation. The observed short-term succession of microbial autotrophic and heterotrophic 0, consumption in aggregated seston material might control environmental conditions both at the sediment-water interface and in floes in the open water.

Study of the U(IV) fraction in Mishash phosphorites of the Negev reveals an apparently paradoxical trend. The U(IV) fraction is very low (less than 10%) in the pristine, organic-rich laminated phosphorites while this proportion considerably increases (up to 60%) in the recycled (mechanically reworked) granular phosphorites which deposited under more oxidizing conditions. This U(IV) trend seems to differ from that reported in other phosphate sequences elsewhere, and is also quite unexpected in view of the sediment patterns and the specific formational conditions of the two phosphorite facies. In addition, a lower %U(IV) in the analyzed granular phosphorites is coupled with a lower rare earth elements (REE)/P 2 O 5 and Y/P 2 O 5 ratio, a higher Cd and Zn concentration, less negative Ce (Ce/Ce*) and Eu (Eu/Eu*) anomalies, a lower HREE (Dy-Er) enrichment, and a lower F/P 2 O 5 and higher CO 2 /F ratio in francolites. This U(IV) trend in the two Negev phosphorite facies may be explained by varying time-spans of apatite growth, changes in sediment burial rates, and principally, different residence times of the phosphate fraction in the suboxic apatite-forming diagenetic zone near the sediment-water interface. The associated varying concentrations of REEs, Y, Cd and Zn in phosphorites and of F and CO 2 in francolites in the two granular phosphorite facies are apparently collectively regulated by the same sedimentary dynamics during phosphate deposition. It is presumed that the U(IV) trend observed in the Negev phosphorites could be detected in other phosphate sequences as well if pristine phosphorites are carefully identified and selectively analyzed.

Nitrogen dynamics and microbial food web structure were characterized in subtropical, eutrophic, large (2,338 km 2), shallow (1.9 m mean depth), and polymictic Lake Taihu (China) in Sept-Oct 2002 during a cyanobacterial bloom. Population growth and industrialization are factors in trophic status deterioration in Lake Taihu. Sites for investigation were selected along a transect from the Liangxihe River discharge into Meiliang Bay to the main lake. Water column nitrogen and microbial food web measurements were combined with sediment-water interface incubations to characterize and identify important processes related to system nitrogen dynamics. Results indicate a gradient from strong phosphorus limitation at the river discharge to nitrogen limitation or co-limitation in the main lake. Denitrification in Meiliang Bay may drive main lake nitrogen limitation by removing excess nitrogen before physical transport to the main lake. Five times higher nutrient mineralization rates in the water column versus sediments indicate that sediment nutrient transformations were not as important as water column processes for fueling primary production. However, sediments provide a site for denitrification, which, along with nitrogen fixation and other processes, can determine available nutrient ratios. Dissimilatory nitrate reduction to ammonium (DNRA) was important, relative to denitrification, only at the river discharge site, and nitrogen fixation was observed only in the main lake. Reflecting nitrogen cycling patterns, microbial food web structure shifted from autotrophic (phytoplankton dominated) at the river discharge to heterotrophic (bacteria dominated) in and near the main lake.

A marine mesocosm system (boxcosm system) was developed for ecological and/or ecotoxicological studies of sediment community function and structure. The system consists of continuous flow-through incubations of intact sediment samples, each with a surface area of 0.25 m 2. The experimental setup enables repeated non-destructive measurements of benthic fluxes, such as of nutrients, oxygen and dissolved inorganic carbon, over the sediment-water interface. The benthic fluxes reflect the function of the sediment community, integrating over the chemical, biological and physical activities in the sediment. The suitability of the boxcosm system for controlled, highly ecologically relevant studies of intact sediment communities was evaluated in two experiments of six weeks and five months duration respectively, where the functional and structural development over time was compared to the development of the sampling site. The function of the sediment was measured as nutrient and oxygen fluxes, and the structural component consisted of microbial functional diversity and meio-and macrofauna composition. Differences between the boxcosm and the sampling site were detected especially in nitrate fluxes and meiofauna diversity and abundance, but all differences fell within seasonal and inter-annual variability at the sampling site. The cause of the differences could be referred to differences in oxygen availability, supply of organic matter particles, and recruitment of larvae. These factors can however be compensated for within the present setup. The study shows that the boxcosms are suitable tools for ecologically relevant studies generating comparable conditions to the natural environment.

Sediment-water dissolved inorganic nitrogen fluxes were measured monthly between November 1997 and January 1999 at two sites on the continental shelf in the Gulf of Lions. Exchanges of ammonium, nitrate and nitrite were measured using whole core incubations performed in the laboratory at in situ temperature and in the dark. Stations were located on the shelf and near the shelf break. Nitrate release represented the major part of total dissolved inorganic nitrogen fluxes (52-98%). At station Sofi, nitrate flux doubled from winter-early spring to August (4 to 10 µmol m-2 h-1). At station R, a large increase (2 to 14 µmol m-2 h-1) was recorded between December 1997 and January 1998, after which flux tended to decrease. Ammonium flux was generally directed out of the sediment, and was lower at station Sofi than at station R. Nitrite flux was always less than 1 µmol m-2 h-1. A large part of the temporal variability observed at station R could be attributed to the input of organic matter associated with the discharge of the Rhône River. On the contrary, the influence of the Rhône River is limited at station Sofi, and the spring phytoplankton bloom deposition was probably the major cause of temporal variability. Comparisons of total fluxes with diffusive fluxes suggested a high nitrification rate in the upper centimetre of the sediment. Annual dissolved inorganic nitrogen release from sediments, equivalent to 20-34% of the Rhône River inputs, may contribute 4-8% of the requirements for primary production in the Gulf of Lions. © 2001 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS Résumé − Variabilité temporelle des flux d'azote inorganique dissous à l'interface eau-sédiment et bilan annuel sur un plateau continental (Méditerranée nord occidentale). Afin de décrire la variabilité temporelle des flux d'azote inorganique dissous sur le plateau continental du golfe du Lion, un suivi mensuel (novembre 1997-janvier 1999) a été effectué à deux stations. Les flux de NH 4 + , NO 3-, NO 2 ont été mesurés par incubation de carotte. Le flux de nitrate hors

Mass, carbon, and nitrogen #uxes and carbon and nitrogen compositions were determined for particulate samples from plankton net tows, shallow #oating sediment traps, intermediate and deep moored sediment traps, and sediment cores collected along 1403W in the central equatorial Paci"c Ocean during the US JGOFS EqPac program. Mass, particulate organic carbon (POC), and particulate inorganic carbon (PIC)

Arsenic-containing minerals mobilized during mining activities and deposited to Lake Coeur d'Alene (CDA), Idaho sediments represent a potential source of soluble As to the overlying water. Our objective was to delineate the processes controlling porewater As concentrations within Lake CDA sediments. Sediment and porewater As concentrations were determined, and solid-phase As associations were probed using X-ray absorption near-edge structure (XANES) spectroscopy. Although maximum As in the sediment porewaters varied from 8.4 to 16.2 µM, As sorption on iron oxyhydroxides at the oxic sediment-water interface prevented flux to overlying water. Floods deposit sediment containing variable amounts of arsenopyrite (FeAsS), with major floods depositing large amounts of sediment that bury and preserve reduced minerals. Periods of lower deposition increase sediment residence times in the oxic zone, promoting oxidation of reduced minerals, SO 4 2efflux, and formation of oxide precipitates. Depositional events bury oxides containing sorbed As, transitioning them into anoxic environments where they undergo dissolution, releasing As to the porewater. High Fe:S ratios limit the formation of arsenic sulfides in the anoxic zone. As a result of As sequestration at the sediment-water interface and its release upon burial, decreased concentrations of porewater As will not occur unless As-bearing erosional inputs are eliminated.

Approximately half of the sedimentation flux of particulate phosphorus in the Laurentian Trough in the Gulf of St. Lawrence is mobilized within the sediment and returned to the water column. In the oxidizing surface sediment, a major portion of the sedimentation flux of organic phosphorus is mineralized, and the released phosphate is partitioned between the pore water and surface adsorption sites. Surface-adsorbed phosphate is released to the pore water as needed to replace dissolved phosphate that escapes to the overlying water. Most of the phosphate is released deeper in the sediment column from iron oxides undergoing reduction. The nonmobilized phosphorus, which is buried with the accumulating sediment, appears to consist mostly of stable minerals such as apatite. The concentration of dissolved phosphate in sediment pore waters increases sharp!y across the sediment-water interface from 2 pmol PO, liter-l in the bottom water to 6+3 pmol PO, liter-' in the top centimeter, remains almost constant at this value down to 5-l 5-cm depth, and then increases rapidly with further depth. In the region of constant concentration, phosphate is buffered by adsorption-desorption equilibria with the sediment. The production rate of phosphate, the buffering capacity of the sediment, and the thickness of the diffusive boundary layer at the sediment-water interface control the shape of the pore-water profile.

We examined the effects of seasonal salinity changes on sediment ammonium (NH 4 +) adsorption and exchange across the sediment-water interface in the Parker River Estuary, by means of seasonal field sampling, laboratory adsorption experiments, and modeling. The fraction of dissolved NH 4 + relative to adsorbed NH 4 + in oligohaline sediments rose significantly with increased pore water salinity over the season. Laboratory experiments demonstrated that small (∼3) increases in salinity from freshwater conditions had the greatest effect on NH 4 + adsorption by reducing the exchangeable pool from 69% to 14% of the total NH 4 + in the upper estuary sediments that experience large (0-20) seasonal salinity shifts. NH 4 + dynamics did not appear to be significantly affected by salinity in sediments of the lower estuary where salinities under 10 were not measured. We further assessed the importance of salinity-mediated desorption by constructing a simple mechanistic numerical model for pore water chloride and NH 4 + diffusion for sediments of the upper estuary. The model predicted pore water salinity and NH 4 + profiles that fit measured profiles very well and described a seasonal pattern of NH 4 + flux from the sediment that was significantly affected by salinity. The model demonstrated that changes in salinity on several timescales (tidally, seasonally, and annually) can significantly alter the magnitude and timing of NH 4 + release from the sediments. Salinity-mediated desorption and fluxes of NH 4 + from sediments in the upper estuary can be of similar magnitude to rates of organic nitrogen mineralization and may therefore be important in supporting estuarine productivity when watershed inputs of N are low.

Organic carbon #uxes through the sediment/water interface in the high-latitude North Atlantic were calculated from oxygen micropro"les. A wire-operated in situ oxygen bottom pro"ler was deployed, and oxygen pro"les were also measured onboard (ex situ). Di!usive oxygen #uxes, obtained by "tting exponential functions to the oxygen pro"les, were translated into organic carbon #uxes and organic carbon degradation rates. The mean C input to the abyssal plain sediments of the Norwegian and Greenland Seas was found to be 1.9 mg C m\ d\. Typical values at the seasonally ice-covered East Greenland continental margin are between 1.3 and 10.9 mg C m\ d\ (mean 3.7 mg C m\ d\), whereas #uxes on the East Greenland shelf are considerably higher, 9.1}22.5 mg C m\ d\. On the Norwegian continental slope C #uxes of 3.3}13.9 mg C m\ d\ (mean 6.5 mg C m\ d\) were found. Fluxes are considerably higher here compared to stations on the East Greenland slope at similar water depths. By repeated occupation of three sites o! southern Norway in 1997 the temporal variability of di!usive O #uxes was found to be quite low. The seasonal signal of primary and export production from the upper water column appears to be strongly damped at the sea#oor. Degradation rates of 0.004}1.1 mg C cm\ a\ at the sediment surface were calculated from the oxygen pro"les. First-order degradation constants, obtained from C degradation rates and sediment organic carbon content, are in the range 0.03}0.6 a\. Thus, the corresponding mean lifetime of organic carbon lies between 1.7 and 33.2 years, which also suggests that seasonal variations in C #ux are small. The data presented here characterize the Norwegian and Greenland Seas as oligotrophic and relatively low organic carbon deep-sea environments.

We present microelectrode profiles of oxygen, pH, and porosity measured at in situ depth across the sediment-water interface from stations above and below the calcite saturation horizon in the equatorial Atlantic. Resistivity electrode data indicate that the diffusion coefhcient for solutes is attenuated by nearly a factor of two within the top centimeter of the sediment lattice. Oxygen and porosity data are used to estimate that the organic carbon respiration rate of the sediment is IO-15 rmol cm-* yr-' in these locations. Comparison of the pH data with numerical model results reveals that sedimentary calcite dissolves in response to respiration in the top few centimeters of sediment even above the saturation horizon. We calculate that the effective in situ dissolution rate constant for calcite required to explain these data is the neighborhood of lo-100% d-'. Although more data are needed to confirm this result, it appears to be at least an order of magnitude less than the rate observed in laboratory stirred-reactor experiments.

The groundwater contribution into Green Lake and Black Lake (Vescovo Lakes Group), two cover collapse sinkholes in Pontina Plain (Central Italy), was estimated using water chemistry and a 222 Rn budget. These data can constrain the interactions between sinkholes and deep seated fluid circulation, with a special focus on the possibility of the bedrock karst aquifer feeding the lake. The Rn budget accounted for all quantifiable surface and subsurface input and output fluxes including the flux across the sediment-water interface. The total value of groundwater discharge into Green Lake and Black Lake ($540 ± 160 L s À1) obtained from the Rn budget is lower than, but comparable with historical data on the springs group discharge estimated in the same period of the year (800 ± 90 L s À1). Besides being an indirect test for the reliability of the Rn-budget ''tool'', it confirms that both Green and Black Lake are effectively springs and not simply ''water filled'' sinkholes. New data on the water chemistry and the groundwater fluxes into the sinkhole area of Vescovo Lakes allows the assessment of the mechanism responsible for sinkhole formation in Pontina Plain and suggests the necessity of monitoring the changes of physical and chemical parameters of groundwater below the plain in order to mitigate the associated risk.

The benthic macroinvertebrate community in a severely eutrophicated South Swedish lake, Lake Ringsjön, was found to re-establish after the lake was subjected to a substantial reduction of cyprinids (mainly bream and roach). Being totally dominated by chironomids and oligochaetes during the 1980s, the lake became repopulated by groups like Amphipoda, Ephemeroptera, Coleoptera and Mollusca after the fish reduction in 1988-1992. These groups appeared in numbers similar to that found in 1969, before the lake entered its worst stage of eutrophication. The re-establishment of the invertebrates mainly occurred in the littoral parts of the lake. The profundal community was still dominated by chironomids and oligochaetes after the fish reduction. The increase in numbers of benthic macroinvertebrates in the littoral zone of the lake was likely an effect of lowered predation pressure from fish. The potential role of an increased macroinvertebrate abundance due to fish removal and its effects on recirculation of nutrients over the sediment-water interface is discussed. It was observed that average total nitrogen retention in Lake Ringsjön before and after the reduction was 59±5% and 82±15%, respectively.

A halocline developed in the GotlandDeep, Baltic Sea, at c. 8.0 14C ky BP, as theresult of a transition from fresh to brackish water.The sediment-water interface changed from oxic topredominantly anoxic, depositional conditions wereperiodically euxinic and pyrite formation wasextensive. This environmental change led topyritization of the upper part of earlier depositedsediments. This study discusses how the distributionof trace elements (As,

This study investigated the spatial and seasonal variations of MeHg concentrations and burdens of different sediments and soils of the Tapajós river floodplain, one of the major clear-water tributaries of the Amazon. The small floodplain of the Tapajós is typical of Amazonian floodplain ecosystems. The studied lakes are bordered by inundated forest (igapó), while floating macrophyte mats (Paspalum sp.) develop at the margin of lakes during the flooded season. During the flood, we observed very low MeHg concentrations in the open water lake sediments (<0.5 ng g −1 d.w or <0.5 µg m −2 cm −1 d.w.) as compared to the semi-aquatic sediments of the macrophyte zone (0.2-1.4 ng g −1 d.w or 1-3 µg m −2 cm −1 d.w.) and the igapó semi-terrestrial soils (0.2-3 ng g −1 d.w or 2-12 µg m −2 cm −1 d.w.). The litter horizon from the igapó soils showed the highest value of MeHg (4-8 ng g −1 d.w.) representing 0.2-2 µg m −2 cm −1 d.w. at the sediment/water interface during the aquatic phase. The inundation had no effect on the concentrations and burdens of MeHg in the sediment of the central part of the lake. The inundation had a clear effect on the methylation of Hg at the surface of semi-aquatic shoreline sediments (macrophyte zone) and semi-terrestrial forest soils, where MeHg concentrations and burdens appeared to be 3 times greater following inundation. In all cores, total Hg concentrations follow those of Fe and Al oxy-hydroxides, whereas the MeHg concentrations are linked to organic matter quality and quantity. It is suggested that organic matter and inundation control MeHg production and accumulation at the benthic interface. These results confirm previous observations, in the same study area, of net 203 Hg methylation potentials. The fresh and labile organic matter in the litter of the igapó forest appears as the most important factor leading to significant enrichment of MeHg in these particular terrestrial/aquatic sedimentary environments.

Major-and minor-element determinations were carried out on a high-resolution sample set obtained from a sediment drill core at Wunstorf (N. Germany). This study interval includes the black shale-bearing Hesseltal Formation associated with the Oceanic Anoxic Event 2 (OAE 2), also referred to as Cenomanian eTuronian Boundary Event (CTBE). Seven black shale packages, each containing several black shale layers, were defined by elevated TOC values, with black shale packages 1e4 deposited during OAE 2. Packages 5e7 extend above the level of the positive carbon-isotope excursion defining OAE 2, indicating that conditions favouring organic carbon burial must have prevailed longer in the Wunstorf Basin than elsewhere. Geochemical analyses revealed no significant differences between black shale packages deposited during and after OAE 2. Enrichment patterns of sulphur, iron and redox-sensitive and sulphide-forming trace metals point to suboxic to anoxic conditions existing at the sedimentewater interface during black shale deposition, whereas sulphidic conditions prevailed deeper in the sediment. Variations in element/Al ratios follow cyclic patterns which are interpreted to represent climaticallyinduced changes in sediment supply. Reduced vertical mixing led to water-column stratification and caused black shale deposition.

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 em- phasis was placed on nutrient cycling at the sediment-water interface and on the self-purification capac- ity 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 ex- periments of &#39;SN-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 dis- tance to the main branches. The nutrient loading is highest in the inflow channels, and decreases to- wards the outflow of the lakes. In June, the uptake of NO3, TDP and Si(OH)...

Hypoxia has been observed in Hood Canal, Puget Sound, WA, USA since the 1970s. Four long sediment cores were collected in 2005 and age-dated to resolve natural and post-urbanization signatures of hypoxia and organic matter (OM) sources in two contrasting basins of Puget Sound: Main Basin and Hood Canal. Paleoecological indicators used for sediment reconstructions included pollen, stable carbon and nitrogen isotopes (d 13 C and d 15 N), biomarkers of terrestrial OM (TOM), biogenic silica (BSi), and redoxsensitive metals (RSM). The sedimentary reconstructions illustrated a gradient in RSM enrichment factors as Hood Canal [ Main Basin, southern [ northern cores, and pre-1900s [ 1900-2005. The urbanization of Puget Sound watersheds during the 1900s was reflected as shifts in all the paleoecological signatures. Pollen distributions shifted from predominantly old growth conifer to successional alder, dominant OM signatures recorded a decrease in the proportion of marine OM (MOM) concomitant with an increase in the proportion of TOM, and the weight % of BSi decreased. However, these shifts were not coincidental with an overall increase in the enrichment of RSM or d 15 N signatures indicative of cultural eutrophication. The increased percentage of TOM was independently verified by both the elemental ratios and lignin yields. In addition, isotopic signatures, BSi, and RSMs all suggest that OM shifts may be due to a reduction in primary productivity rather than an increase in OM regeneration in the water column or at the sediment/water interface. Therefore, the reconstructions suggested the Hood Canal has been under a more Electronic supplementary material The online version of this article (

Two piston cores (PCs), containing two full glacial cycles, were retrieved at Murray Ridge. One core (PC463) contains sediment deposited at 920 mbsl within, the other (PC464) has sediment deposited at 1470 mbsl below the current oxygen minimum zone, OMZ (O 2 : ≤2 μM). Variations in the organic carbon (OC) content correlate well between the two cores; high OC contents correspond with periods of high insolation and are accompanied by a stable and intensive OMZ. The lithogenic fractions are similar and have a rather constant composition. The V, Mn, Co, Cu, Se, Zn and Sb contents in these fractions are in fair agreement with the values in standard shales (SS). The Ni content of the lithogenic fraction is higher than in SS due to its high content in the regional eolian input, and the Mo, Re and U contents are higher as a result of syngenetic and(or) diagenetic overprints. In both cores, Mn and Co have a negative, the other elements a positive correlation with OC. Four processes, contributing to sedimentary enrichment in trace elements, can be envisaged: 1) delivery, as minor or trace components, in hydrogenous and biogenic particles, 2) scavenging in the water column, 3) (partial) retention in the sediment during decomposition of host phases, 4) diffusion into the sediment across the sediment-water interface and subsequent immobilization in response to reduction or precipitation as sulfides. Vanadium, Mo, Re, and U were found to be clearly enriched, and Ni, Cu, Zn and Se to be possibly enriched as a result of these processes. Enrichment or depletion in trace elements in PC463 relative to PC464 was tested as a proxy for bottom-water oxygen (BWO) conditions during periods of maximum accumulation rates of organic matter. Only Mo, Re and Re/Mo appear to be unambiguous proxies for differences in depositional conditions between sites 463 and 464. Using these proxies, it was found that the OMZ expanded to at least the depth of site 464 from 155 to 135 ka.

In the Bigadiç volcano-sedimentary basin, Palaeozoic-Mesozoic basement rocks are overlain unconformably by the Miocene rock units. The straligraphic sequence of Miocene units, from the bottom to the top in ascending order is, basement volcanic unit, basement limestone unit, lower tuff unit, lower borate bearing unit, upper tuff unit, upper borate bearing unit, and basalt unit. Miocene units are over- lain

This paper discusses the effects of organic carbon oxidation on the dissolution and precipitation of calcium carbonate (aragonite and calcite) in fine-grained continental shelf and slope sediments, using data obtained during the Canadian Joint Global Ocean Flux Study program in the Gulf of St. Lawrence and on the Scotia shelf. The oxygen-penetration depth in these sediments is on the order of 10-15 Para obtener copia del documento contacta con el autor (silverb@ipn.mx) o con el personal de la biblioteca

Precise age control is a prerequisite for reliable paleoenvironmental reconstruction. Almost all Holocene chronologies of lacustrine sequences are based on radiocarbon dating. Most reliable ages are obtained from fragile terrestrial vegetation remnants (e.g., leaves) as these neither yield a reservoir or hard water effect nor are reworked as they would be destroyed during this process. However, in many records no

The effect of phytodetritus derived from Phaeocystis sp. bloom on benthic mineralization processes has been determined at four intertidal stations along the French coast of the eastern English Channel. Sites were chosen to offer a diversity of sediment types, from permeable sandy beach to estuarine mudflats. Sediment Oxygen Demand (SOD) as well as total fluxes of Dissolved Inorganic Nitrogen (DIN) at the sediment–water interface were determined by using whole core incubation technique and diffusive fluxes were predicted from interstitial water concentrations. In the absence of phytodetritus deposits, a marked gradient of granulometric characteristics and organic matter contents were observed, and resulted in more intensive mineralization processes in muddy sediments. Highly significant correlations (P &lt; 0.05) were evidenced between SOD and porosity, bacterial biomass, Organic Carbon and Organic Nitrogen, evidencing the direct link between sediment texture, organic matter accumula...

In order to quantify the spatial and seasonal variations of sediment oxygen consumption and nutrient fluxes, we performed a spatial survey in the south west lagoon of New Caledonia during the two major seasons (dry and wet) based on a network of 11 sampling stations. Stations were selected along two barrier reef to land transects representing most types of sediments encountered in the lagoon. Fluxes were measured using ex-situ sediment incubations and compared to sediment characteristics. Sediment oxygen consumption (SOC) varied between 500 and 2000 lmol m À2 h À1 , depending on season and stations. Nutrient effluxes from sediment were highly variable with highest fluxes measured in muddy sediments near the coast. Inter-sample variability was as high as seasonal differences so that no seasonally driven temperature effect could be observed on benthic nutrient fluxes in our temperature range. Nutrient fluxes, generally directed from the sediment to the water column, varied between À5.0 and 70.0 lmol m À2 h À1 for ammonia and between À2.5 and +12.5 lmol m À2 h À1 for PO 4 and NO 2+3. SOC and nutrient fluxes were compared to pelagic primary production rates in order to highlight the tight coupling existing between the benthic and pelagic compartments in this shallow tropical lagoon. Under specific occasions of low pelagic productivity, oxygen sediment consumption and related carbon and nutrient fluxes could balance nearly all net primary production in the lagoon. These biogeochemical estimates point to the functional importance of sediment biogeochemistry in the lagoon of New Caledonia.

The effect of phytodetritus derived from Phaeocystis sp. bloom on benthic mineralization processes has been determined at four intertidal stations along the French coast of the eastern English Channel. Sites were chosen to offer a diversity of sediment types, from permeable sandy beach to estuarine mudflats. Sediment Oxygen Demand (SOD) as well as total fluxes of Dissolved Inorganic Nitrogen (DIN) at the sediment-water interface were determined by using whole core incubation technique and diffusive fluxes were predicted from interstitial water concentrations. In the absence of phytodetritus deposits, a marked gradient of granulometric characteristics and organic matter contents were observed, and resulted in more intensive mineralization processes in muddy sediments. Highly significant correlations (P \ 0.05) were evidenced between SOD and porosity, bacterial biomass, Organic Carbon and Organic Nitrogen, evidencing the direct link between sediment texture, organic matter accumulation and microbial activity. The spring bloom led to a massive input of organic matter in surficial sediments and mineralization rates significantly increased while higher DIN release towards the water column was observed. A modification of the mineralization pathways was evidenced but clearly depended on the sediment type. With a global view, benthic mineralization processes in the intertidal zone provided significant a part of DIN inputs in the coastal zone while water column was depleted in nutrients.

The ichnogenus Psammichnites Torell 1870 includes a wide variety of predominantly horizontal, sinuous to looped, backfilled traces, characterized by a distinctive median dorsal structure. Though commonly preserved in full relief on upper bedding surfaces, some ichnospecies of Psammichnites may be preserved in negative hyporelief. Psammichnites records the feeding activities of a subsurface animal using a siphon-like device. Several ichnogenera reflect this general behavioral pattern, including Plagiogmus Roedel 1929 and the Carboniferous ichnogenera Olivellites Fenton and Fenton 1937a and Aulichnites Fenton and Fenton 1937b. Based on analysis of specimens from the United States, Spain, and the United Kingdom, three Carboniferous ichnospecies of Psammichnites are reviewed in this paper: P. plummeri (Fenton and Fenton, 1937a), P. grumula (Romano and Meléndez 1979), and P. implexus (Rindsberg 1994). Psammichnites plummeri is the most common Carboniferous ichnospecies and is characterized by a relatively straight, continuous dorsal ridge/groove, fine transverse ridges, larger size range, and non-looping geometric pattern. It represents a grazing trace of deposit feeders. Psammichnites grumula differs from the other ichnospecies of Psammichnites by having median dorsal holes or protruding mounds. The presence of mounds or holes in P. grumula suggests a siphon that was regularly connected to the sediment-water interface. This ichnospecies is interpreted as produced by a deposit feeder using the siphon for respiration or as a device for a chemosymbiotic strategy. Psammichnites implexus is characterized by its consistently smaller size range, subtle backfill structure, and tendency to scribble. Although displaying similarities with Dictyodora scotica, P. implexus is a very shallow-tier, grazing trace. Changes in behavioral pattern, preservational style, and bedform morphology suggest a complex interplay of ecological and taphonomic controls in Carboniferous tidal-flat Psammichnites. A first distributional pattern consists of guided meandering specimens preserved in ripple troughs, probably reflecting food-searching of buried organic matter concentrated in troughs. A second is recorded by concentration of Psammichnites on ripple crests and slopes. In some cases, the course is almost straight to slightly sinuous and closely follows topographic highs, suggesting a direct control of bedform morphology on trace pattern. Occurrences of Carboniferous Psammichnites most likely represent an opportunistic strategy in marginal-marine settings. Analysis of Carboniferous Psammichnites indicates the presence of a siphon-like device in the producer and reestablishes the possibility of a molluscan tracemaker.

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.

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 basic composition of the lake's water was controlled mainly by the mixing of two distinct water types (Jordan River and peat water), and by evaporation. About 3/4 of the lake water originated from the Jordan Inlet, a quarter through the Z Canal Inlet (peat drainage) and a minor amount from groundwater seepage. Lake Agmon is unique among freshwater wetlands owing to its high SO 2− 4 content, which is ca. 1/3 that of sea water. This characteristic is ascribed to the dissolution of secondary gypsum, formed in the peat soils since the drainage of the historic Hula Marsh. Leaching gypsum from the shallow sediments during the first few months after flooding was followed by a later stage of constant diffusion and advection of SO 2− 4 from gypsum dissolution in deeper sediments. Gypsum dissolution in lake sediments contributed ca. half of the SO 4 2− and Ca 2+ inputs to the lake. The concomitant increase of Ca 2+ combined with alkalinity release due to organic matter decomposition in the sediments led to the precipitation of CaCO 3. This precipitation was enhanced by photosynthesis, particularly during summers, and consumed about a tenth of the Ca 2+ and third of the alkalinity outputs from the lake. Iron-hydroxide was the main agent for microbial oxidation of organic matter, surpassing by far the role of sulfate, nitrate and manganese as electron acceptors. The produced Fe 2+ was transported upward by diffusion and advection and oxidized to ferric iron at the sediment-water interface. There is evidence, however, that some sulfate was reduced, but most of the produced sulfide reacted with ferrous iron and accumulated in the sediments as FeS minerals. Therefore, despite high sulfate concentrations, the high iron availability restricted release of toxic sulfides into the water and thereby served to maintain reasonable water quality.

An analysis of sedimentological, sediment trap and hydrological data was performed to investigate the transport and fate of particulate matter in the Gulf of Lions. The sedimentological properties outline the major sedimentary units of the shelf (Rhone prodelta, mid-shelf mud belt, outer shelf) and slope. The geometry of these sedimentary units and the southwestwards increase of particulate fluxes on the slope highlight the influence of the general cyclonic circulation on the dispersion of land-derived particulate matter. Considering the known input and output terms, budgets of particulate matter and organic carbon in the Gulf of Lions are proposed. Inputs were river supply, atmospheric deposition and primary production; outputs were sediment burial and advective export on the slope. Degradation of particulate organic carbon in the water column and at the sediment-water interface was also estimated. Mass and POC budgets were balanced within uncertainties. Nevertheless, these results illustrate the difficulty in establishing budgets based on elements estimated at different time scales, for which the system is not necessarily in a steady state.

A 4 m long sediment core from the Gotland Deep, Baltic Sea, was investigated by means of biostratigraphical and chemical parameters and i4C datings. This multidisciplinary approach allows us to evaluate the regional changes in salinity and redox conditions during the Holocene. According to the diatom assemblages and 14C datings, the sediments were deposited in the brackish and freshwater phases of the Yoldia Sea (10,300-9600 r4C yr B.P.), the freshwater Ancylus Lake (9600-8000 14C yr B.P.) and in the brackish water Litorina Sea (8000-3000 14C yr B.P.). Of several palaeosalinity indicators evaluated, the diatoms are the least ambiguous. Nevertheless the B content and, under some premises, the C/S ratio may be used for palaeosalinity information. Iron sulphides in the lowermost freshwater sediments formed below the prevailing, oxic sediment-water interface. Pyrite in the uppermost freshwater sediments was to a large extent formed in Litorina time, by diffusion of XH,S from overlying brackish water sediments. The Ancylus Lake/Litorina Sea transition can be traced by the marked changes in both the siliceous microfossil assemblages and in the chemical properties of the sediments. An early Litorina phase is characterised by increased primary production, progressively increasing salinity and the development of euxinic conditions. In the Litorina Sea laminated clay gyttjas (def. as a clay containing 6-30% organic matter) containing pyrite and laminae of Ca-rhodochrosite, (Mn,Ca)CO,, accumulated. Both these minerals formed in close connection with sediment deposition. The rhodochrosite laminae formed during occasional inflow of oxygen rich, marine water through the Danish Straits. Rhodochrosite is by far the most common mineral in the laminae and formed on average with a periodicity of two to three years.

Diurnal variation of dissolved oxygen (DO), organic and inorganic carbon (DOC, DIC), nitrogen (DON, DIN), and phosphorus (DOP, DIP) flux across the sediment-water interface was assessed in fish farm impacted and pristine seagrass (Posidonia oceanica) meadows in the Aegean Sea (Greece). DIC consumption decreased by 52% and DO production decreased by 60% in the light, suggesting reduced photosynthetic performance of the plant community under the fish cages probably due to organic matter loading. In light there was 4 and 15 times higher release of dissolved inorganic and organic matter, respectively, compared to dark incubations under the cages, indicating that fish farming impact is more intense during daytime. DO was taken up, while DIC was released in the dark in both stations, representing a direct measure of mineralization. Dissolved inorganic matter flux (as the sum of DIN and DIP fluxes) was positively related to DIC flux, rendering mineralization as the main driver of nutrient flux under the cages. On average, the impacted meadow released DIN and DIP both in light and dark, while efflux of dissolved organic matter (as the sum of DOC, DON, and DOP fluxes) increased by 132% in the light and by 21% in the dark, implying that the degrading seagrass meadow is a source of dissolved matter to the surrounding water. Shoot density and leaf production were negatively correlated with both diel DIN and DIP fluxes, showing that meadow regression is accompanied by DIN and DIP release from the sediment. Hence, nutrient efflux can adequately illustrate meadow deterioration and, therefore, can be used as indicator of P. oceanica community health.

We present the distribution of Cd and Zn in sediment cores taken under Spartina alterniflora Loisel. marsh and compare with adjacent mud flats without plant cover. Vertical distribution of Cd and Zn in the mud flat and S. alterniflora cores were different. From 0 to 5 cm of depth Zn and Cd concentrations did not change with depth in the two cores but were higher in the mud flat. From 5 to 15 cm, corresponding to maximum root biomass under S. alterniflora, Zn and Cd concentrations remained constant in the mud flat cores, but increased by a factor of nearly 4 under S. alterniflora. Below this depth concentrations dropped in both cores. Manganese distribution in S. alterniflora cores suggests that oxides are precipitated at the depth of maximum root biomass. More oxic environment under S. alterniflora (average Eh = +5 mV) with low sulfide concentration (0.58 mg l ,1) relative to the mud flat cores (average Eh =-204 mV; sulfide 47.1 mg l ,1), may favor Fe and Mn oxi-hydroxides precipitation, causing an accumulation of Cd and Zn at the root zone. In the mud flat cores, Zn and Cd highest concentrations were at the sediment water interface, where oxic waters favor their precipitation with Fe and Mn oxi-hydroxides. The results discourages the indiscriminate use of salt marsh sediments in environmental monitoring of trace metals, since the possibility of post-deposition mobilization of trace metals due to root metabolism, will make correlation with trace metal loading rates impossible.

The major bioturbation features in an oligotrophic canyon, macrobenthic assemblage and biogenic structures have been studied simultaneously at a station of 1000 m depth in the channel of Toulon Canyon. Two different techniques of quantifying biogenic structures have been employed: X-rays of the sediment column and photographs of the sea floor. In this deep environment, a macrobenthic community dominated by 64.4% of subsurface deposit feeders provides slow bioturbation with intensity proportional to low macrobenthos density (13.1 + 5.6 individuals dm 2). Subsurface feeding appears to be the best feeding strategy as there arc numerous turbidite deposits observed in this canyon. Burrow records in the sediment column duc to macrobenthos are very scarce. Sediment mixing processes do not exceed 3 cm as 85% of the macrobenthos is encountered in the top 3 cm. From photographs of the sediment surface, the density of biogenic features induced by megabenthos is estimated to be 3.3 +_ 1.1 structures m-2. Organisms responsible for the more frequently encountered biogenic structures, groups of six, nine or 12 holes, are identified as the Thalassinidean shrimps Calocaris macandreae. In thc Toulon Canyon, the relatively low biological mixing associated with low sedimentation rates provide good maintenance of biogenic structures at the sediment surface. The density of biogenic structures at the sediment-water interface is inversely proportional to macrobenthos abundance which is in agreement with Kitchell's hypothesis. This canyon represents an inverse example to the Atlantic Ocean where macrobenthos is more abundant and biogenic structure density is smaller.

Palaeoenvironmental interpretations from analyses of lake sediment archives are dependent on our understanding of the modern lake environment and the link between neo-and palaeolimnology. Diatom assemblages represent different lake habitats and vary in species composition and abundance on seasonal and annual timescales. These factors, along with mixing at the mud-water interface, alter the information recorded in the sedimentary archive and the resulting interpre

Approximately half of the sedimentation flux of particulate phosphorus in the Laurentian Trough in the Gulf of St. Lawrence is mobilized within the sediment and returned to the water column. In the oxidizing surface sediment, a major portion of the sedimentation flux of organic phosphorus is mineralized, and the released phosphate is partitioned between the pore water and surface adsorption sites. Surface-adsorbed phosphate is released to the pore water as needed to replace dissolved phosphate that escapes to the overlying water. Most of the phosphate is released deeper in the sediment column from iron oxides undergoing reduction. The nonmobilized phosphorus, which is buried with the accumulating sediment, appears to consist mostly of stable minerals such as apatite.

We present inorganic geochemical analyses of pore waters and sediments of two Late Quaternary sediment cores from the western Arctic Ocean (southern Mendeleev Ridge, RV Polarstern Expedition ARK-XXIII/3), focussing on the composition and origin of distinct, brown-colored, Mn-rich sediment layers. Carbonate enrichments occur in association with these layers as peaks in Ca/Al, Mg/Al, Sr/Al and Sr/Mg, suggesting enhanced input of both ice-rafted and biogenic carbonate. For the first time, we show that the Mn-rich layers layers are also consistently enriched in the scavenged trace metals Co, Cu, Mo and Ni. Distinct bioturbation patterns, specifically well-defined brown burrows into the underlying sediments, suggest these metal enrichments formed close to the sediment-water interface. The geochemical signature of these metal-and carbonate-rich layers most probably documents formation under warmer climate conditions with an intensified continental hydrological cycle and only seasonal sea ice cover. Both rivers and sea ice delivered trace metals to the Arctic Ocean, while enhanced seasonal productivity exported reactive organic matter to the sea floor. The coeval deposition of organic matter, Mn (oxyhydr)oxides and trace metals triggered intense diagenetic Mn cycling at the sediment-water interface. These processes resulted in the formation of Mn and trace metal enrichments, and the degradation of labile organic matter. With the onset of cooler conditions, reduced riverine runoff and/or a solid sea ice cover terminated the input of riverine trace metal and fresh organic matter, resulting in deposition of grayish-yellowish, metal-poor sediments. Oxygen depletion of Arctic bottom waters under these cooler conditions is not supported by our data, and did not cause the sedimentary Mn distribution. While the original composition and texture of the brown layers resulted from specific climatic conditions and corresponding diagenetic processes, pore water data show that diagenetic Mn redistribution is still affecting the organic-poor deeper sediments. Given persistent steady state conditions, purely authigenic Mn-rich brown layers may form, while others may be partly or completely dissolved. The degree of diagenetic Mn redistribution largely depends on the depositional environment, the Mn and organic matter availability, and apparently affected the Co/Mo ratios of Mn-rich layers. Thus, brown Arctic layers are not necessarily synchronous features, and should not be correlated across the Arctic Ocean without additional age control.