A molecular marker-based assessment of sedimentary organic matter sources and distributions in Florida Bay

Seven surface sediment samples covering the general geographical area of Florida Bay were examined through the measurement of ¹³C isotopic abundance and lipid classes to assess the distributions and sources of organic matter (OM) in this estuarine environment. The bulk δ¹³C_org value shifted from a more isotopically depleted (−19.9‰) to a more isotopically enriched (−13.5‰) signal along the NE to SW transect. Two geochemical proxies (Paq and C₂₅/C₂₇ n-alkan–2-ones) indicative of seagrass-derived OM significantly increased from near-shore to offshore areas, while taraxerol, a biomarker for mangroves, substantially decreased from 7200 to 284 ng/g along that transect. A clear spatial variation of OM sources was observed in Florida Bay. Generally, the sites in the NE contained mixed OM sources of terrestrial (mangrove) and seagrass-derived OM, where the terrestrial component accounted for over 60% of the OM. In contrast, the sites in central and SW Florida Bay were strongly dominated by seagrass-derived OM. Other lipid fractions such as fatty acids, n-alcohols and sterols revealed an important contribution of algae and bacteria especially in the central and SW section of the Bay. Relatively abundant C₂₅ HBIs suggest important marine diatom inputs, while the presence of C₂₀ HBIs particularly in central Florida Bay possibly reflects the contribution of cyanobacterial mats. The molecular proxies developed in this study to assess OM sources in Florida Bay are promising tools for the characterization and seasonal variability assessment of OM in this and other similar subtropical and tropical estuaries and for paleoenvironmental studies. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Seasonality of sulfate reduction and pore water solutes in a marine fish farm sediment: the importance of temperature and sedimentary organic matter

Sulfate reduction and pore water solutes related to sulfur cycling and anaerobic processes (short chain fatty acids (SCFA), SO₄ ^2–, TCO₂, NH₄ ⁺, dissolved sulfides (H₂S) and CH₄) were examined during one year at a marine fish farm. Mineralization of fish farm waste products was rapid in this non-bioturbated, organic rich sediment. Stimulation of sulfate reduction rates (SRR) occurred primarily in the surface layers where the organic matter was deposited. Acetate was the most important (<99%) of the measured SCFA attaining high concentrations during summer months (up to 4.7 mM). The acetate profiles exhibited distinct seasonal cycles, where periods with high concentrations in the pore waters were found coincident with a high pool of particulate organic matter in the surface sediments and a low activity of the sulfate reducing bacteria (early spring and late summer). Periods with low acetate pools occurred when sulfate reduction rates were high in early summer and in winter were pools of particulate organic matter were decreasing. Methane production was observed concurrent with sulfate reduction in the microbial active surface layers in late summer. Subsurface peaks of SO₄ ^2–, TCO₂, NH₄ ⁺ and H₂S were evident in July and August due to rapid mineralization in these surface layers. With decreasing autumn water temperatures mineralization rates declined and subsurface peaks of these solutes disappeared. A strong relationship was found between pore water TCO₂, and NH₄ ⁺. Ratios between TCO₂, and NH₄ ⁺ were low compared to a control site, attaining minimum values in mid-summer. This indicated rapid nitrogen mineralization of nitrogen rich labile substrates in the fish farm sediment during the entire season.     

Establishment of anaerobic,reducing conditions in lake sediment after deposition of acidic,aerobic sediment by a major storm

Hurricane Danny resulted in the rapid deposition of 10cm of oxidized, acidic sediment in the Contrary Creek arm of Lake Anna, Virginia. Several biological and geochemical parameters were monitored with time to ascertain how long it took the newly-deposited lake sediments to attain the anaerobic, circumneutral, actively sulfate-reducing state normally observed in this portion of the lake. The sediment platinum-electrode potential dropped from 350 mV to 100 mV within the first week after the storm. The pH of the pore water increased from 4.5 to 5.8 within three weeks, and titratable alkalinity was detected within two weeks and three weeks at 3 cm and 1 cm depths, respectively. Accumulation of reduced products of sulfate reduction (acid volatile sulfide) began by three to four weeks after the storm event. Both methanogens and sulfate reducers were present in high and approximately equal numbers in the freshly deposited material. The rapid neutralization of the acidity in the fresh sediment prior to the onset of sulfate reduction suggests that reactions other than sulfate reduction caused the initial increase in pH and alkalinity in this system.     

Sulfur and iron cycling in a coastal sediment: Radiotracer studies and seasonal dynamics

The seasonal variation in sulfate reduction ana the dynamics or sulfur ana iron geochemistry were studied throughout a year in sediment of Aarhus Bay, Denmark. A radiotracer method for measuring sulfate reduction rates was applied with incubation times down to 15 min and a depth resolution down to 2 mm in the oxidized surface layer of the sediment. The radiotracer data were analyzed by a mathematical model which showed that, due to partial, rapid reoxidation of radioactive sulfide during incubation, the actual reduction rates in this layer were probably underestimated 5-fold. In the deeper, sulfidic zone, measured rates appeared to be correct. Sulfate reduction followed the seasonal variation in temperature with maximum activity at 1–2 cm depth in late summer. In spite of its rapid production, free H₂S was detectable in the porewater only below the depth of free Fe²⁺ at 6–7 cm throughout the year. Following the massive sedimentation from a spring phytoplankton bloom, anaerobic degradation of phytoplankton detritus was strongly stimulated over several weeks. A transient reversed redox zonation developed with a thin, black zone on top of the brown, oxidized sediment layer due to intensive sulfate and iron reduction. Mineralization through sulfate reduction was equivalent to two thirds of the annual net sedimentation of organic matter.Author for correspondence     

Effects of added organic matter on iron and manganese redox systems in sediment

Addition of five types of organic matter to Lake Washington sediments resulted in release of high concentrations of iron, organic carbon, and manganese into the interstitial water, and caused an increase in observed sediment oxygen consumption rates. The depressed electrode potentials (E_h < —150 mV) that should accompany such reduction processes did not occur, indicating that E_h was being poised by redox systems present in the sediment. Iron redox systems [Fe(OH)₃‐Fe²⁺, Fe₃(OH)₈‐Fe²⁺, and Fe(OH)₃‐Fe₃(OH)₈] were shown to be poising the E_h of control sediments throughout 13 weeks of incubation and dominating the potential of several of the organically amended sediments following the first three weeks of incubation. Depression of calculated iron system E_o values relative to that of the control sediment early in the incubation appeared to be due to the decreased pH and non‐equilibrium conditions in the organic matter‐amended sediment during the first weeks of incubation. Manganese redox systems exerted no discernable impact on the E_h of the sediment.     

The effect of sulfate and nitrate on methane formation in a freshwater sediment

A freshwater sediment from a ditch of a peat grassland near Zegveld (Province of Utrecht, The Netherlands) was investigated for its potential methanogenic and syntrophic activity and the influence of sulfate and nitrate on these potential activities. Methanogenesis started after a 10 days lagphase. After 35–40 days aceticlastic methanogens were sufficiently enriched to cause a net decrease of acetate. In the presence of sulfate methane formation was only slightly affected. The addition of nitrate led to an outcompetion of aceticlastic methanogens by nitrate reducers. When inorganic electron acceptors were absent, substrates like propionate and butyrate were converted by syntrophic methanogenic consortia. Addition of inorganic electron acceptors resulted in an outcompetition of the syntrophic propionate and butyrate degrading consortia by the sulfate and nitrate reducers.     

The role of sediment organic matter composition in larval habitat selection by the polychaete Capitella sp. I

The role of sediment organic matter composition in larval habitat selection by the polychaete Capitella sp. I was investigated using a still water multiple-choice experiment. Specifically, the following question was addressed: what are the roles of sediment labile organic compounds in larval habitat selection? A replicated Latin-square design was used to test larval preferences among sediments collected from five sites (TLC—Tung Lung Chau; VHE—Victoria Harbor east; VH—Victoria Harbor; VHW—Victoria Harbor west; PC—Peng Chau) in the vicinity of Victoria Harbor, Hong Kong. Although mean grain size was not different among sites, water content and the total organic matter were higher in VH Mud but were similar among other sites. Carbohydrate, lipid, and biopolymeric fractions followed the same trend. According to protein content, sites were ranked as: VH > VHW > TLC = VHE > PC. PC Mud had the highest carbohydrate/protein ratio, due to low concentration of protein. According to larval preference, sites were ranked as: TLC = VH = VHW = VHE > PC. Our study shows a precise relationship between sediment biochemical properties and larval habitat selection. Larvae tend to choose sediments with low carbohydrate/protein ratio. Results suggest that types of sediment organic matter may play more important role in the larval habitat selection process of Capitella sp. I than concentration of organic matter alone.     

Effects of temperature and oxygenavailability on greenhouse gas and nutrient dynamics in sediment of a eutrophic mid-boreal lake

The effects of oxygen conditions and temperature on dynamics of greenhousegases (CH₄, CO₂, N₂O) and nutrients(NH₄ ⁺, NO₂ ^–+NO₃ ^–, tot-P) were studied in sediment of hyper-eutrophic LakeKevätön, Finland. Undisturbed sediment cores were incubated at 6, 11,16, and 23 °C in a laboratory microcosm using a continuouswater flowtechnique with an oxic or anoxic water flow. The production of CO₂increased with increasing temperature in both oxic (Q₁₀ 3.2 ±0.6) and anoxic (Q₁₀ 2.3 ± 0.4) flows. The release ofCH₄ increased with temperature in anoxic conditions (Q₁₀2.3 ± 0.2), but was negligible with the oxic flow at all temperatures.The release of NH₄ ⁺ increased with temperature with the oxic and anoxic flows(Q₁₀ 2.4 ± 0.1). There was a net production of NO₂ ^–, NO₃ ^– and N₂O with the oxic flow at temperatures below16 °C. The release of phosphorus was greater from the anoxicsediments and increased with temperature with both the anoxic (Q₁₀2.9 ± 0.5) and oxic (Q₁₀ 1.9 ± 0.1) flows. It isprobable that the temperature of boreal lakes and the associated oxygendeficiency will increase as the climate becomes warmer. Our experiments showedthat this change would increase the global warming potential of greenhousegasesreleased from sediments of eutrophic lakes predominately attributable to theincrease in the CH₄ production. Furthermore, warming would alsoaccelerate the eutrophication of lakes by increasing release of phosphorus andmineral nitrogen from sediments, which further enhance CH₄productionin sediments.     

Bioavailability of organic matter in a freshwater estuarine sediment: long<Emphasis Type="Bold">-</Emphasis>term degradation experiments with and without nitrate supply

Organic carbon degradation experiments were carried out using flow-through reactors with sediments collected from an intertidal freshwater marsh of an eutrophic estuary (The Scheldt, Belgium). Concentrations of nitrate, nitrite, dissolved inorganic carbon (DIC), dissolved organic carbon, methane, dissolved cations (Ca²⁺, Mg²⁺, Na⁺ and K⁺), total dissolved Fe, phosphate and alkalinity were measured in the outflow solutions from reactors that were supplied with or without the terminal electron acceptor nitrate. Organic carbon mineralization rates were computed from the release rates of DIC after correcting for the contribution of carbonate mineral dissolution. The experiments ran for several months until nitrate reducing activity could no longer be detected. In the reactors supplied with nitrate, 10–13% of the bulk sedimentary organic carbon (SOC) was mineralized by the end of the experiments. In reactors receiving no nitrate, only 3–9% of the initial SOC was mineralized. Organic matter utilization by nitrate reducers could be described as the simultaneous degradation of two carbon pools with different maximum oxidation rates and half-saturation constants. Even when nitrate was supplied in non-limiting concentrations about half of the carbon mineralization in the reactors was due to fermentative processes, rather than being coupled to nitrate respiration. Fermentation may thus be responsible for a large fraction of the DIC efflux from organic-rich, nearshore sediments.     

The neutralization of acidic coal mine lakes by additions of natural organic matter: a mesocosm test

Cylindrical polyethylene enclosures 3 m in length and 1 m in diameter reaching from the surface to the bottom were constructed in an acid (pH=3.1) lake on a coal surface mine in southern Illinois. Wheat straw was added to the enclosures to test the effects of dissimilatory sulfate reduction on water chemistry. Added straw increased sulfide concentrations, raised pH to 6.5, reduced O₂ and increased acid neutralizing capacity of the enclosed water columns when compared with a control enclosure and with the open lake. Generation of acid neutralizing capacity exceeded the standing stock of sulfide indicating that sulfide was removed either by precipitation of FeS or outgassing of H₂S. The pH and acid neutralizing capacity within the enclosures eventually returned to the level of the surrounding lake because of water exchange around the enclosure walls. Our results show that additions of organic matter to acid surface mine lakes result in the generation of acid neutralizing capacity.     

Spectroscopic characterization and molecular weight distribution of dissolved organic matter in sediment porewaters from Lake Erhai, Southwest China

Dissolved organic matter (DOM) in sediment porewaters from Lake Erhai, Southwest China was investigated using dissolved organic carbon (DOC) concentration, UV absorbance, fluorescence and molecular weight distribution. DOC exhibited a high concentration at the sediment–water interface with a rapid decrease to the oxic–anoxic interface at approximately 7 cm, and then increased with depth. Similar trends were also found for the UV absorption coefficients at 254 and 280 nm in the porewaters. DNA in the sediment was also measured, which confirmed the high abundance of aerobic bacteria in the upper layer of the sediment. Both humic-like (peaks A and C) and protein-like (peaks B and D) fluorescence were observed in the porewater DOM, and their fluorescence intensities exhibited a similar porewater profile as DOC concentration. A strong correlation was found between the peak fluorescence intensity ratio r(A, C) and r(D, B). Both the fluorescence index and UV absorption coefficient at 254 nm suggested a dramatic increase in aromaticity of porewater DOM across the oxic–anoxic interface. Porewater DOM exhibited a multimodal distribution of molecular weight with a relatively low polydispersity. The results of this study offer significant insight into the nature and properties of DOM in freshwater ecosystems.     

Assessing the use of sediment organic,carbonate and biogenic silica content as indicators of environmental conditions in Arctic lakes

The objective of this paper is to examine three sediment parameters used in paleolimnological studies, sediment organic, carbonate and biogenic silica (BSi) content, and to quantify the relationship among these parameters and modern environmental conditions for a series of Canadian Arctic lakes. Sediment samples from 63 lakes were analysed for organic, carbonate and BSi content. The environmental and physical parameters of these lakes were recorded. Bedrock composition exerted a very strong influence on sediment organic, BSi and carbonate content, and differences in lake production accounted for much of the remaining variance. The strongest relationships existed among sediment carbonate content, BSi content and water pH. A statistical model developed for predicting water pH provides a new tool for reconstructing past environmental conditions for Arctic aquatic ecosystems. A positive relationship between mean July air temperature and sediment organic and BSi content was detected, but was confounded by effects of bedrock on these parameters. However, the relationship between mean July air temperature and sediment organic and BSi could be even more significant in the context of constant bedrock composition, as is the case in paleolimnological studies.     

The Origin and Age of Biogeochemical Trends in Deep Fracture Water of the Witwatersrand Basin,South Africa

Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0–2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH ₄ and hydrocarbon, occasionally N ₂ , originally formed at ～ 250–300°C and during cooling isotopically exchanged O and H with minerals and accrued H ₂ , ⁴ He and other radiogenic gases. The paleo-meteoric water ranges in age from ～ 10 Ka to > 1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO ₂ and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past ～100 Myr, a process which inoculates previously sterile environments at depths > 2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fracture water and that it is 10 ⁷ times that normally required for cell maintenance in lab cultures. Flux calculations also indicate that the potential bioavailable chemical energy increases with salinity, but because the fluence of bioavailable C, N and P also increase with salinity, the environment remains energy-limited. The ⁴ He concentrations and theoretical calculations indicate that the H ₂ that is sustaining the subsurface microbial communities (e.g. H ₂ -utilizing SRB and methanogens) is produced by water radiolysis at a rate of ～1 nM yr ^?1 . Microbial CH ₄ mixes with abiogenic CH ₄ to produce the observed isotopic signatures and indicates that the rate of methanogenesis diminishes with depth from ～ 100 at < 1 kmbls, to < 0.01 nM yr ^?1 at > 3 kmbls. Microbial Fe(III) reduction is limited due to the elevated pH. The δ¹³C of dissolved inorganic carbon is consistent with heterotrophy rather than autotrophy dominating the deeper, more saline environments. One potential source of the organic carbon may be microfilms present on the mineral surfaces.     

The effect of organic matter on sedimentary phosphorus release in an Australian reservoir

Australian reservoirs, compared to much of the world, are subjected to extreme arid and semi-arid climatic conditions where dam volumes can range from near-empty to full, often with rapid filling events. P-release, after re-flooding of desiccated sediments, can be important to water quality, and can be further influenced by dried macrophyte, exposed as water recedes and incorporated into sediments. P-release from Lake Rowlands (New South Wales, Australia) sediments was studied under different aerobic and sterile conditions with five carbon source amendments to the sediment (the macrophyte Isoetes sp. in different stages of senescence and acetate). Sedimentary P-release involved a complex array of factors modified by aerobic, biotic and abiotic processes, organic matter breakdown, iron content of sediments and turbulence. Under aerobic conditions, P-release from sterile non-amended sediments and sterile macrophyte-amended sediments was greater than from non-sterile sediments. Under anaerobic conditions, P-release was maximal from non-sterile macrophyte-amended sediments, probably via pathways involving fermentative Fe³⁺-reducing bacteria where electrons are transferred from organic matter to amorphous Fe(OOH) leading to Fe²⁺ and consequent release of P. Macrophyte addition (whether fresh or dried) enhanced P-release under anaerobic compared with aerobic conditions. P-release from acetate-amended sediments appeared to involve acetate aerobes. The re-flooding of sediments, therefore, has the potential to create conditions that are conducive to aerobic sedimentary P-release and should be taken into account in management strategies adopted for reservoirs where levels are likely to fluctuate.     

Dimethylsulfide and methane thiol in sediment porewater of a Danish estuary

Seasonal variation of dimethylsulfide (DMS) and methane thiol (MSH) concentrations in sediment porewater was determined in a Danish estuary. Dimethylsulfide (DMDS) was never found. Detectable DMS levels of up to 0.1 M were found only in the summer and only within the upper 5 cm of the sediment. The DMS accumulation was probably associated with decomposing fragments of macro-algae in the surface layer. Significant MSH accumulation of up to 1 M was found only in the deep, CH₄-rich sediment below the SO₄ ^2- zone. With depth, a detectable MSH level could thus be observed below the 1 mM SO₄ ^2--isopleth which also marked the SO₄ ^2--CH₄ transition. The transition zone was located deeper in the sediment in winter (20–25 cm depth) than in summer (5–10 cm depth). The absence of MSH in the SO₄ ^2- zone could be due to rapid utilization of the compound by SO₄ ^2--reducing bacteria. A possible involvement of MSH in anaerobic CH₄ oxidation at the transition zone is discussed; CH₄ and sulfide (HS^- form, pH 7) are proposed to form MSH and H₂ which in turn may be metabolized by, e.g. SO₄ ^2--reducing bacteria.     

The effects of bioturbation on particle redistribution in Mediterranean coastal sediment. Preliminary results

In order to quantify bioturbation processes in a coastal Mediterranean ecosystem, experiments were performed to determine sediment mixing rates resulting from macrobenthos activity. Particle flux was measured in situ for 22 days using luminophores, which are colored sediment particles with sizes ranging from 10 to 200 µm.In sediment depths from 0–5 cm, particle mixing was intensive due to high macrobenthos abundance. A small quantity of luminophores was transported down to a depth of 14 ± 2 cm, where the macrofauna was represented principally by Polychetes. In a control experimental structure — without benthic fauna — no transfer of luminophores into the sediment was recorded.Sediment particle mixing measured in the ecosystem studied is intensive, and is the result of high macrobenthos activity. Different mixing modes occur with scales and rates depending on the organisms present. The luminophore profile resulting from bioturbation processes is explained by an intensive bioadvection sediment mixing added to a biodiffusive mixing with an order of magnitude of 10^–6 cm² s^–1. Tracer accumulations between 1 and 2 ± 1 cm and between 4 and 5 ± 1 cm are attributed to bioadvection activity of two or more distinct populations. Studies over a larger time scale have been undertaken to monitor developments in the observed subsurface maxima.     

Nutrient pulsing as a regulator of phytoplankton abundance and community composition in Galveston Bay, Texas

Galveston Bay, Texas, is a large shallow estuary with a watershed that includes 60% of the major industrial facilities of Texas. However, the system exhibits low to moderate (2-20 μg l⁻¹) microalgal biomass with sporadic phytoplankton blooms. Both nitrogen (N) and phosphate (P) limitation of phytoplankton growth have been proposed for the estuary. However, shifts between N and P limitation of algae growth may occur due to annual fluctuations in nutrient concentrations. The primary goal of this work was to determine the primary limiting nutrient for phytoplankton in Galveston Bay. Nutrient addition bioassays were used to assess short-term (1-2 days) phytoplankton responses (both biomass and community composition) to potentially limiting nutrients. The experimental bioassays were conducted over an annual cycle using natural water collected from the center to lower part of the estuary. Total phytoplankton biomass increased in the nitrate (10 μM) additions in 11 of the 13 bioassays, but no significant increases were detected in the phosphate (3 μM)-only additions. Bioassay results suggest that the phytoplankton community was usually not phosphate limited. All major groups increased in biomass following nitrate additions but diatoms increased in biomass at a faster rate than other groups, shifting the community composition toward higher relative abundance of diatoms. The results of this study suggest that pulsed N input events preferentially favor increases in diatom biomass in this estuary. The broader implications of this study are that N pulsing events, primarily due to river discharge, play an important role in structuring the phytoplankton community in the Galveston Bay estuary.     

The effects of sediment slurrying on microbial processes,and the role of amino acids as substrates for sulfate reduction in anoxic marine sediments

In sediment slurry experiments with anoxic marine sediments collected in Cape Lookout Bight, NC, and a site in mid-Chesapeake Bay, the rates of sulfate reduction and ammonium production decrease with increasing dilution of sediment with oxygen-free sea-water. The effect of sediment dilution on the rates of these processes can be described by a simple mathematical relationship, and when these rates are corrected for sediment dilution they yield values which agree well with direct measurements of these processes.In sediment slurry studies of amino acid utilization in Cape Lookout Bight sediments, the fermentative decarboxylation of glutamic acid (to -aminobutyric acid) or aspartic acid (to alanine or -alanine) did not occur when either of these amino acids were added to Cape Lookout Bight slurries. The addition of glutamic acid did however lead to a small (1) transient build-up of -aminoglutaric acid. Measured rates of glutamic acid uptake in these slurries also decreased with increasing sediment dilution.Molybdate inhibition experiments demonstrated that dissolved free amino acids represent 1–3% of the carbon sources/electron donors used for sulfate reduction in Cape Lookout Bight sediments. The direct oxidation of amino acids by sulfate reducing bacteria also accounts for 13–20% of the total ammonium produced. Glutamic acid, alanine, -aminoglutaric acid, aspartic acid and asparagine are the major amino acids oxidized by sulfate reducing bacteria in Cape Lookout Bight sediments.     

Observations on the diagenetic behavior of arsenic in a deep coastal sediment

Vertical profiles of total dissolved arsenic, manganese and iron, pH, Eh and rates of sulfate reduction were determined in a freshly-collected box core from a 335m depth station in the Laurentian Trough. The relationships observed between the profiles were further examined in the laboratory by measuring these same parameters with time in surficial sediment slurries as the Eh decreased in response to biological activity or chemical alteration.Both field and laboratory observations have shown that arsenic is released predominantly as As(III) into reducing sediment porewaters. This occurs after the dissolution of manganese oxides and at the same time as the dissolution of iron oxyhydroxides and the onset of sulfate reduction. Laboratory experiments indicated that sulfate reduction and the production of sulfide ions are not solely responsible for the release of arsenic to the porewaters, although this process is necessary to create and maintain a highly reducing environment conducive to rapid iron dissolution.The diagenesis of arsenic in Laurentain Trough sediments involves the simultaneous release of arsenic and iron at a subsurface depth, followed by its removal from porewaters by precipitation and adsorption reactions after migration by diffusion along concentration gradients. A qualitative model is presented to describe the behavior of arsenic in coastal marine sediments.Present address: Department of Geological Sciences, McGill University, 3450 UniversityStreet, Montreal, Quebec H3A 2A7, Canada