Reactivity Of Organic Matter Research Articles

Characterisation of insoluble charcoal in Weipa bauxite

The role of charcoal like components (also referred to as char) in soil organic matter reactivity has become increasingly evident. Recently we have demonstrated the role of such material in bauxite. Sodium hydroxide is used at elevated temperatures to separate aluminium hydroxide from ferric oxide in bauxite in the Bayer process and charcoal like material may interfere with the precipitation of aluminium hydroxide. In this paper we study the solubility, structure and composition of charcoal in the feed stockpile of bauxite ore by solubility, laser Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and pyrolysis gas chromatography mass spectrometry (py-GC/MS). The charcoal was treated under simulated Bayer process conditions of 245 °C in 5 M sodium hydroxide. The alkaline digestion test showed that a large fraction of the charcoal was insoluble (∼90%). Surprisingly, the spectroscopic characterisation revealed not only typical polycyclic aromatic networks expected for charcoals (aromaticity f a = 0.64), but also showed an aliphatic character and that the sample contained alkyl chains ranging from nC 10 to nC 23 carbon chain lengths. The role of this material in bauxite refineries is discussed.

Allophane, aluminum, and organic matter accumulation across a bioclimatic sequence of volcanic ash soils of Argentina

An investigation was conducted to study Al fractions, organic matter accumulation, and their effects on physicochemical properties in soils derived from volcanic ash at the Andinopatagonian region in SW Neuquén, Argentina. Five sites were selected in a climatic biosequence, ranging from Nothofagus forest with a udic soil moisture regime at the footslope of the Andes mountain range to grass-shrub steppe with a xeric soil moisture regime in the hills and plains. The morphological characteristics showed young pedons, containing andic, mollic, and cambic horizons. A change from low bulk density from <0.82 Mg m −3 in Andisols under a udic regime to medium density values of ∼1.1 Mg m −3 in Xerands and Xerolls was observed. This was related to a lower rate of weathering probably coinciding with a lower content of allophane and higher clay crystallization and to a decrease in total soil organic carbon (C org). The rate of soil weathering was estimated from acid oxalate extractable Al, which together with C org significantly explained the bulk density variation ( R 2=0.7091, p<0.00002). Soil acidity varied from moderately and strongly acidic in Andisols with a udic soil moisture regime to slightly acid and almost neutral in Xerands and Xerolls. The higher base concentration and low soil acidity in Xerolls were attributed to less leaching, due to lower mean annual precipitation. However, pH NaF and C org were the variables which significantly explained the pH water variation ( R 2=0.7806, p<0.0000). This showed that soil acidity was closely related to Al activity and organic matter content, rather than to base content. The lower C/N ratios, in the surface horizons of the Xerands and Xerolls versus the Andisols with a udic soil moisture regime, suggested that the rate of mineralization was faster under warmer conditions with xeric moisture regime than with udic moisture regime. There was also a remarkable difference in the humic C/fulvic C ratio in A-horizons, which may reflect the input of vegetation residues indicating a sensible contribution of the cover of grass-shrub steppe to the formation of humic substances. The acid oxalate extractable Si and Al values reflected a low rate of weathering, which decreased when mean annual precipitation decreased and a xeric soil moisture developed being also higher under Nothofagus forest than under grass-shrub steppe, which reflects the possible effect of the mean annual precipitation gradient (2000–700 mm) and vegetation types on soil weathering. The differences in type and quantity of organic matter input, with a higher organic matter content and lower humic C/fulvic C ratio under Nothofagus, indicate a more reactive organic matter which may be somewhat responsible for the weathering degree. No significant relationship between acid extractable Al and C org was found suggesting that organically bound Al is not dominant. The detection of higher allophane estimations in soils with the highest C org levels, may indicate that the anti-allophanic hypothesis probably does not apply to these soils.

Carbonate dissolution in the guts of benthic deposit feeders: A numerical model

We present a numerical model to quantify calcite dissolution in the guts of deposit feeding invertebrates. Deposit feeder guts were modeled as constantly stirred reactors (CSTRs) following terminology from digestion theory. Saturation state and dissolution of calcium carbonate were calculated from changes in total dissolved carbon dioxide and alkalinity resulting from sediment passage through the digestive tract, while accounting for dissolution of calcite and respiration of organic carbon. Typical dissolution rates for a gut volume of 1 ml ranged between 0.5–4 mg calcite d −1. Sensitivity analysis revealed gut pH, sediment organic matter (OM) content and OM reactivity to be the critical parameters determining calcite dissolution rate. Carbonate dissolution rate was inversely related to gut pH. However, calcite dissolution was found to be possible even at alkaline gut pH due to respiration by intestinal microbes. The kinetics of calcite dissolution had only marginal influence on daily calcite dissolution rates: Varying the calcite dissolution rate constant κ by six orders of magnitude affected calcite dissolution rates by less than a factor of 10. Calcite dissolution rates were calculated for 4 different hydrographic regimes that differed in their content of sedimentary calcite and OM and furthermore in their OM reactivity. Highest dissolution rates were calculated for the shallow water setting, where relatively high OM content facilitated high microbial respiration rates depressing gut pH. However, dissolution rates for the deep sea setting were only slightly lower, due to greatly elevated ingestion rates resulting from low OM content. As a consequence of much higher faunal abundances, shallow-water benthos is likely to contribute the vast majority of gut-mediated carbonate dissolution. Nevertheless, the fraction of sedimentary calcite that dissolves during one gut passage is probably too small to be observable by simple gravimetric analysis. This may explain the notable scarcity of evidence for gut-mediated carbonate dissolution in the literature to date. Assuming depth-dependent calcite dissolution rates and deposit feeder abundances, we estimate gut-mediated carbonate dissolution to contribute approximately 5% of the annual global sedimentary carbonate dissolution rate, which corresponds to an average calcite dissolution rate of approximately 0.5 mg m −2 d −1 for the entire ocean floor.

Comparative geochemistry of cadmium, rhenium, uranium, and molybdenum in continental margin sediments

The concentrations of authigenic phases of Cd, Re, U, and Mo increase with depth in four 45-cm-long sediment box cores collected along the axis of the Laurentian Trough, Gulf of St. Lawrence. Average authigenic accumulation rates, estimated from element inventories, are similar to rates in other continental margin environments. Strong regional variations in sediment accumulation rate and sulfide concentration have little influence on the accumulation rates of Cd and Re. This suggests that slow precipitation kinetics controls the accumulation of Cd and Re in these sediments. The accumulation rate of authigenic U is more variable; it may be tied to the kinetics of microbially mediated U reduction and be controlled by the availability of reactive organic matter. Authigenic Mo is distinguished by a sharp subsurface concentration minimum, above which Mo cycles with manganese. Mo released to pore water upon reduction of Mn oxides diffuses downward and enriches the subsurface sediment. Mo accumulates most rapidly in the sediment with the highest sulfide content. Slow conversion of molybdate to thiomolybdate may explain the much slower Mo accumulation rate in the less sulfidic sediments. A component of authigenic Mo accumulates with pyrite in an approximately constant Mo:Fe ratio. The accumulation rate of pyrite and associated Mo is insensitive to AVS abundance. Pyrite formation may be limited by the reactivity of iron oxide minerals.

Reactivity of organic matter in aquifer sediments: geological and geochemical controls

Reduction rates in aquifers are commonly carbon limited, but little is known about the molecular composition and degradability of sedimentary organic matter (SOM) in aquifer sediments. The composition, source and degradation status of SOM in aquifer sediments of fluvio-glacial (Pleistocene) and shallow marine (Pliocene) origin, were determined using flash pyrolysis-gas chromatography/mass spectrometry. Incubation experiments (106 d) were used to assess the reactivity of SOM towards molecular oxygen. A dominance of lignin-derived components and long chain odd-over-even predominant alkanes indicate that terrestrial higher land plants were the main source of SOM even in the shallow marine sediments, while bacterial lipid-derived hopanoids and iso- and anteiso-C15 and C17 fatty acids indicate a minor contribution of microbial biomass. No compositional difference was observed between SOM present in the fine (<63 μm) and coarse fraction (63–2000 μm). A significant part of SOM was not present as low-molecular-weight compounds but was macromolecularly bound. For the fluvio-glacial sediments, a relatively higher abundance of resistant macromolecular compounds was in agreement with stronger signs of aerobic lignin, alkane and hopanoid oxidation. The more degraded status of SOM in the fluvio-glacial sediments was consistent with their significantly lower SOM mineralization (2–6%) during incubation, as compared with the shallow marine sediments (9–14%). The reactivity towards oxygen of SOM was controlled by the extent of past aerobic oxidation. Not the age of SOM, but the extent of oxygen exposure during syn- and postdepositional conditions seems most important in affecting the degradation status of SOM in aquifer sediments and thus their ability to reduce oxidants.

Reactivity of natural organic matter with aqueous chlorine and bromine

While both aqueous bromine (HOBr/OBr −) and chlorine (HOCl/OCl −) react with natural organic matter (NOM) during water treatment, limited direct parallel comparison of bromine versus chlorine has been conducted. Experiments with model compounds and natural waters indicated more efficient substitution reactions with bromine than chlorine. Kinetic experiments with NOM isolates with and without pre-ozonation were conducted to obtain second-order rate constants ( k) with bromine and chlorine. Two-stage reaction kinetics (rapid initial and slower consumption stages) were observed. Bromine reacted about 10 times faster than chlorine with NOM isolates during both stages. The rapid initial stage reactions were too fast to quantify k values, but qualitative estimates ranged between 500 and 5000 M −1 s −1. For the slower second stage k values for bromine were 15 to 167 M −1 s −1 over the pH range of 5–11, and lower for chlorine ( k = 0.7–5 M −1 s −1). Values of k correlated with initial SUVA values of NOM (UVA measured at 254 nm divided by DOC). Based upon UV/VIS and solid-state 13C-NMR spectroscopy, chlorine addition to a NOM isolate resulted in significant oxidation of aromatic and ketone groups while bromine had significantly less change in spectra. Overall, the improved knowledge that bromine reacts faster and substitutes more efficiently than chlorine will be useful in developing strategies to control disinfection by-product formation during water treatment.

Amino acid remineralization and organic matter lability in Chilean coastal sediments

The spatial distribution of proteinaceous amino acids was investigated in Chilean coastal surface sediments. Proteinaceous amino acids, measured as total hydrolyzed amino acids (THAA), were in the range 37–1335 μmol THAA-N gdw−1 and accounted for 6–70% of total nitrogen in surface sediments. Beggiatoa or Thioploca, motile, filamentous bacteria present in sediments with low oxygen bottom waters, may be a source of the non-protein amino acid, β-aminoglutaric acid. Mineralization of sedimentary THAA accounted for about one quarter of the ammonium production measured in these sampling sites. Reactivity of organic matter in these coastal sediments was assessed using a Degradation Index developed by Dauwe and Middelburg (1998; Limnology and Oceanography 43, 782–798) and Dauwe et al. (1999; Limnology and Oceanography 44, 1809–1814). Both production of ammonium and mineralization of amino acids follow a consistent trend with this index. Thus, we can relate the production of ammonium and mineralization of amino acids to the diagenetic state of the sediment.

Incorporating Reservoir Transfer into Treatment Optimization Decisions

The Massachusetts Water Resources Authority (MWRA) supplies unfiltered water from two large surface water reservoirs to the metropolitan Boston area, as well as to three smaller communities in central Massachusetts [the Chicopee Valley Aqueduct (CVA) communities]. Quabbin Reservoir is larger than Wachusett Reservoir, and has traditionally been used to supplement the Wachusett during the summer period. Quabbin water is also of better quality, with lower reactive natural organic matter (NOM). The MWRA began to add chlorine at Wachusett in 1997, and a new facility for adding chlorine at Quabbin for the CVA was also started up in 2000 to meet primary disinfection regulations to meet pathogen inactivation. The reaction of chlorine with NOM produces undesirable disinfection by-products (DBPs). The absorption of ultraviolet light at a wavelength of 254 nm was identified in chlorine decay studies to be the most important raw water quality parameter for predicting chlorine decay and DBP formation. This technical note summarizes the chlorine decay model for Wachusett and Quabbin water. The model is extended to ozonation of Wachusett water for the future Walnut Hill treatment plant. The models allowed the development of a trigger using UV-254 to time the Quabbin transfer to optimize treatment results. It is believed that the model for disinfectant decay and the use of UV-254 as a trigger for water treatment decisions are generalized and applicable to other water utilities.

Sorption of organic matter from four phytoplankton species to montmorillonite, chlorite and kaolinite in seawater

The sorption of organic matter from four species of phytoplankton to clean mineral surfaces was studied using sequential adsorption experiments. The soluble intracellular components from four phytoplankton species, Phaeocystis globosa, Gymnodinium sanguineum, Scrippsiella trochoidea, and Ditylum brightwellii, were repeatedly exposed to three minerals; montmorillonite, kaolinite, and chlorite. The surface-reactive fraction of organic matter in phytoplankton dissolved organic carbon (DOC) solutions ranged from 47% to 85% of the total. Adsorption partition coefficients ( K d) ranged from 53 to 175 l kg −1. These partition coefficients are significantly higher than the partition coefficients commonly observed for sedimentary porewater organic matter (≤30 l kg −1), and suggest that phytoplankton exudates contain a component of organic matter with considerable surface reactivity. S. trochoidea and D. brightwellii consistently had the highest K d values and the greatest reactive organic matter component relative to P. globosa and G. sanguineum. On average, the highest K d values were associated with chlorite (average K d of 119 l kg −1) and montmorillonite (average of 105 l kg −1) rather than kaolinite (average K d of 89 l kg −1). Organic matter from all four phytoplankton species interacted most with montmorillonite (average=82% reactive), followed by chlorite (72% reactive) and kaolinite (63% reactive). Significant variation in both the extent of surface-reactive material and the partition coefficients of the reactive material using this matrix of source organisms and mineral surfaces suggest that both the mineral and organic matter sources are influential in the sorption of organic matter to sediment surfaces.

Post-Cambrian closure of the deep-water slope-basin taphonomic window

Exceptional faunas (Konservat-Lagerstatten that preserve traces of volatile nonmineralized tissues) are statistically overabundant in the Cambrian Period; almost all examples preserved in continental-slope and shelf-basin environments are of this age. The hypothesis that an increase in the amount and complexity of bioturbation was an important agent in the elimination of this deep-water slope-basin taphonomic window is supported. Post-Cambrian ichnofaunal assemblages contain a higher proportion of pascichnia and agrichnia, ethologies produced by a mobile infauna. They also illustrate the lateral partitioning of organisms into different environmental niches; both opportunistic and equilibrium infaunas occur in low-oxygen environments in which the preservation of nonbiomineralized tissues was favored. Direct consumption of carcasses by bioturbating organisms was less important than changes to sediment properties as a result of bioturbation, notably enhanced microbial degradation of reactive organic matter, increased permeability, and the disruption of geochemical gradients necessary for mineral authigenesis.

Early diagenesis of marine organic-matter and magnetic properties of sedimentary rocks: the role of iron limitation and organic-matter source organisms

Abstract The magnetic parameters of sedimentary rocks can record accurately tectonic and climatic influences upon sedimentary processes, when they are not altered during diagenesis. This paper is focused on the possible alteration of the primary magnetic-susceptibility signal during early diagenesis of marine organic matter (OM). In the late Kimmeridgian-Tithonian Argiles de Châtillon Formation (Fm.) of the Boulonnais area (northern France – lateral time equivalent of the distal organic-rich sediments of the Kimmeridge Clay Fm. of Dorset), the organic content is dominated by amorphous OM (AOM), either brown or orange, originating from selective preservation or sulphurisation, respectively. Total sulphur content correlates well with orange + brown AOM abundance, and organic S content correlates well with orange AOM abundance. The magnetic signal of these claystones and paper shales is dominantly carried by clay minerals. The magnetic susceptibility vs. brown-AOM abundance relationship shows a clear correlation. Furthermore, if the part of the magnetic-susceptibility signal linked to the lithoclastic fraction of the sediments is removed from the total signal, a positive correlation is also drawn between the brown-AOM abundance and the ‘excess’ magnetic susceptibility. The cause for this resides in the iron-sulphide abundance. The presence of both types of AOM implies that intense sulphate reduction took place and that the HS−/H2S released reacted with either organic molecules (orange AOM dominating the palynofacies) or reactive iron (brown AOM dominating). When reactive iron was abundant enough and when sulphate reduction-induced sulphide ions could react with it, iron sulphides could form and this sediment component (present now as pyrite) influences the magnetic-susceptibility signal. The type of dominating source organisms (phytoplankton) may condition the reactivity of OM towards sulphide ions. This may influence iron-sulphide formation, and in turn the sediment magnetic signal.

Alkali-extractable polysaccharides in marine sediments: Abundance, molecular size distribution, and monosaccharide composition

—Carbohydrates from coastal sediments were characterized using differential extraction and ultrafiltration techniques. Carbohydrates adsorbed on sediment particles were extracted by various combinations of heating, ultrasonic radiation, and alkali treatment, and the extraction efficiencies of these treatments were compared. Heating (121°C, 60 min) in the presence of alkali (Na 2CO 3, 20 mM) followed by ultrasonication (20 min on ice) yielded maximal recovery. By repeating this extraction three times, 50% (at maximum) of total carbohydrate in the sediment could be extracted to solution. Carbon-based ratios of carbohydrate to total organic carbon in the extracts and residues were 19 to 28% and 10 to 13%, respectively. The major fraction (68–84%) of extracted carbohydrate was found in macromolecular fractions (>10 kDa). The extracted carbohydrates could be precipitated with 80% (v:v) ethanol with a recovery of 74 to 80%. The monosaccharide composition of the ethanol precipitable polysaccharides was then analyzed by capillary gas chromatography and compared with carbohydrates in the insoluble residue fraction. The molecular composition of residual polysaccharides was somewhat variable, presumably depending on whether the sediment was affected by terrestrially derived organic matter, whereas the composition of the extracted polysaccharides was strikingly similar irrespective of depositional environment. Implications of these data for mobility, reactivity, and origins of sediment organic matter are discussed.

The reactivity of natural organic matter to disinfection by-products formation and its relation to specific ultraviolet absorbance

Five natural waters with a broad range of DOC concentrations were fractionated using various coal- and wood-based granular activated carbons (GAC) and alum coagulation. Adsorption and alum coagulation fractionated NOM solutions by preferentially removing components having high specific ultraviolet absorbance (SUVA). UV absorbing fractions of NOM were found to be the major contributors to DBP formation. SUVA appears to be an accurate predictor of reactivity with chlorine in terms of DBP yield; however, it was also found that low-SUVA components of NOM have higher bromine incorporation. SUVA has promise as a parameter for on-line monitoring and control of DBP formation in practical applications; however, the effects of bromide concentration may also need to be considered. Understanding how reactivity is correlated to SUVA may allow utilities to optimize the degree of treatment required to comply with DBP regulations. The reactive components that require removal, and the degree of treatment necessary to accomplish this removal, may be directly obtained from the relationship between SUVA removal and the degree of treatment (e.g., alum dose).

Energy-filtering transmission electron microscopy (EFTEM) and electron energy-loss spectroscopy (EELS) investigation of clay–organic matter aggregates in aquatic sediments

High resolution (<10 nm) transmission electron microscopy (TEM), energy-filtering TEM (EFTEM), and electron energy loss spectroscopy (EELS) have been used for the direct microstructural imaging and analysis of clay-organic matter aggregates in fine-grained aquatic sediments from Jourdan River Estuary, MS, USA. EFTEM and EELS allow rapid, high-resolution spatial mapping and analysis of light elements such as carbon. The study area sediments are comprised of discrete organic matter masses and aggregates of clay plates. Clay aggregates often include organic matter. The comparison of clay aggregate images obtained by the TEM bright-field technique and the EFTEM carbon mapping technique shows that carbon within clay domains has spatial features that are in the same size scale as the features of individual clay plates within the aggregates (<∼20 nm). These intimate spatial associations suggest that organic matter in clay aggregates is either closely attached to the surfaces of individual clay plates or structurally incorporated into clay crystals. Organic matter within clay aggregates does not appear to exist as discrete or massive masses that fill the pore spaces within the clay aggregates. These intimate associations, whether they are due to chemical interaction or physical sequestration, should affect the reactivity of organic matter during early diagenesis. The EELS spectra of clay aggregates show that organic matter always coexists with calcium, suggesting that Ca-containing smectite, rather than Ca-poor clay minerals such as kaolinite or illite, is preferentially associated with organic matter in the study area sediments. Further research is required to determine whether this is due to the sources and depositional history or physico-chemical properties of different clay minerals.

Rates of C, N, P and Si recycling and denitrification at the US Mid-Atlantic continental slope depocenter

Benthic fluxes of O 2, titration alkalinity (TA), total inorganic carbon (TIC), Ca 2+, NO 3 −, NH 4 +, PO 4 3−, and Si(OH) 4 were measured by in situ benthic flux chamber incubations at 13 locations on the North Carolina continental slope. The majority of measurements were made at water depths of approximately 700–850 m, in the previously identified upper slope depocenter. This region is characterized by extremely high organic matter deposition rates and near saturation bottom water oxygen concentrations. Measured benthic fluxes of TA are reasonably correlated with O 2 benthic fluxes. Because bottom waters are supersaturated with respect to calcite and aragonite at these shallow water depths, these results demonstrate the importance of metabolically driven dissolution in this region. Subtraction of the calcium carbonate dissolution contributions from the TIC benthic fluxes suggests rates of organic matter remineralization ranging from 0.97 to 3.9 mol C m −2 yr −1 at the depocenter sites, a factor of 3–10 greater than estimated for the adjacent continental rise and upper slope areas. Because biological primary production in the overlying waters does not follow this pattern, these extremely high values are most likely supported by lateral inputs of highly reactive organic matter. Mass balance calculations indicate that despite the oxygenated bottom water conditions, 68% of the organic nitrogen released during organic matter remineralization processes is ultimately denitrified. The release of PO 4 3− from the depocenter sediments is equivalent to or larger than that predicted from the remineralization of Redfield organic matter. This implies either that PO 4 3− is preferentially released in this setting and that the accumulating sediments must be depleted in PO 4 3− relative to organic carbon or that another, non-organic, phase is contributing PO 4 3− to the system. The molar ratio of the Si benthic flux and organic carbon remineralization rate ranges from 0.30 to 0.86. This is significantly greater than the ratio reported for most pelagic diatoms. Possible reasons for this high ratio include the deposition of benthic diatoms that may have a larger Si : C ratio than pelagic diatoms, the near-bottom lateral input of partially reworked organic matter that may have an elevated Si : C ratio relative to fresh diatoms, preferential loss of carbon in sinking particulates or the release of Si from non-opaline materials.

Early diagenetic pyrite morphology in a mudstone-dominated succession: the Lower Jurassic Cleveland Ironstone Formation, eastern England

Abstract Diagenetic pyrite in the mudstones and ironstones of the Lower Jurassic Cleveland Ironstone Formation of eastern England exhibits two distinct morphologies: framboidal pyrite, commonly associated with organic matter, and euhedral pyrite, associated with detrital clay pellets. These two morphologies are mutually exclusive in occurrence. Framboidal pyrite is present in clay-rich mudstones, ooidal ironstones, apatite-rich units and some silt-rich mudstones. Euhedral pyrite is present in silt-rich and sand-rich mudstones. δ34S isotopic analysis of six samples of pyrite suggests that both types of pyrite morphology precipitated during early diagenesis from porewaters with open access to overlying sea-water, although both probably acted as sites for continued pyrite precipitation during burial. It is proposed that framboidal pyrite precipitated from iron-dominated porewaters at sites of sulfide supply (i.e. in the region of organic matter as a result of bacterial sulfate reduction) where, locally, sulfide production rates were high enough for porewaters to reach supersaturation with respect to FeS. Euhedral pyrite also precipitated from iron-dominated porewaters, but sulfide production rates from organic matter was such that FeS saturation was not reached at the sites of sulfide production. Instead, euhedral pyrite was precipitated directly from porewater when FeS2 saturation was reached. The control over pyrite morphology was probably the amount and reactivity of the organic matter within the deposited sediments. The sand-rich mudstones contained less reactive organic matter due to clastic dilution and deposition in shallower environments with O2-rich bottom waters. The ironstones and apatite-rich units were deposited under very low sedimentation rates, and as a result organic matter contents were very low and iron reduction dominated early diagenesis, which inhibited sulfate-reduction. The presence of minor framboidal pyrite within these units, however, suggests that sulfide reduction took place in micro-environments during early diagenesis.

Organic matter diagenesis at the oxic/anoxic interface in coastal marine sediments, with emphasis on the role of burrowing animals

The present paper reviews the current knowledge on diagenetic carbon transformations at the oxic/anoxic interface in coastal marine sediments. Oxygen microelectrodes have revealed that most coastal sediments are covered only by a thin oxic surface layer. The penetration depth of oxygen into sediments is controlled by the balance between downward transport and consumption processes. Consumption of oxygen is directly or indirectly caused by respiration of benthic organisms. Aerobic organisms have the enzymatic capacity for complete oxidation of organic carbon. Anaerobic decay occurs stepwise, involving several types of bacteria. Large organic molecules are first fermented into small moieties. These are then oxidized completely by anaerobic respirers using a sequence of electron acceptors: Mn4+, NO3-, Fe3+, SO42- and CO2. The quantitative role of each electron acceptor depends on the sediment type and water depth. Since most of the sediment oxygen uptake is due to reoxidation of reduced metabolites, aerobic respiration is of limited importance. It has been suggested that sediments contain three major organic fractions: (1) fresh material that is oxidized regardless of oxygen conditions; (2) oxygen sensitive material that is only degraded in the presence of oxygen; and (3) totally refractory organic matter. Processes occurring at the oxic/anoxic boundaries are controlled by a number of factors. The most important are: (1) temperature, (2) organic supply, (3) light, (4) water currents, and (5) bioturbation. The role of bioturbation is important because the infauna creates a three-dimensional mosaic of oxic/anoxic interfaces in sediments. The volume of oxic burrow walls may be several times the volume of oxic surface sediment. The infauna increases the capacity, but not the overall organic matter decay in sediments, thus decreasing the pool of reactive organic matter. The increase in decay capacity is partly caused by injection of oxygen into the sediment, and thereby enhancing the decay of old, oxygen sensitive organic matter several fold. Finally, some future research directions to improve our understanding of diagenetic processes at the oxic/anoxic interface are suggested.

A comparison of benthic nutrient fluxes from deep-sea sediments off Namibia and Argentina

Benthic fluxes of nitrate, phosphate, silicate, and oxygen, estimated C org input to the sea floor, and C org content of surface sediments at various stations in the Argentine Basin and the Cape Basin (South Atlantic) were used to characterize the benthic mineralization pattern in these areas. Diffusive benthic fluxes across the sediment–water interface reveal very intense subsurface biodegradation of organic matter and dissolution of biogenic silica. Across the Namibian continental margin diffusive benthic fluxes generally decrease with increasing water depth, but a considerable site-to-site variation exists along the 1300 m isobath. Mineralization patterns indicate that the input flux of organic matter seems to be significantly controlled by “vertical” transport through the water column. A nearly reverse pattern can be observed in the western Argentine Basin. Intense nutrient release is observed even at locations on the deeper slope. For this whole area a dependence of nutrient release and water depth is not observed. An exception is the Rio de la Plata region, where stations between 1800 and 3500 m water depth reveal the highest nitrate and phosphate fluxes, decreasing towards the shelf and into the deeper basin. Silicate fluxes off the Rio de la Plata increase continuously with water depth. Below 4000 m water depth this is probably due to the input of Antarctic diatoms by the Antarctic Bottom Water. Mineralization processes on the continental margin of the Argentine Basin do not correlate with the estimated organic matter input from surface waters. In accordance with previous sedimentological studies, we conclude that intense sediment mass flows from the shelf are the main source for the input of reactive organic matter to the deeper parts of the continental margin.

Linking diagenetic alteration of amino acids and bulk organic matter reactivity

Examination of amino acids in particulate samples from a variety of marine environments (fresh phytoplankton to deep‐sea sediments) revealed systematic compositional changes upon progressive degradation. These consistent trends have been used to derive a quantitative degradation index (DI) that is directly related to the reactivity of the organic material, as indicated by its lability to enzymatic decay and its first‐order degradation rate constant. This direct link between molecular composition and degradation rate allows us to quantify the quality of organic matter based solely on its chemical composition.

Influence of sediment‐organic matter quality on growth and polychlorobiphenyl bioavailability in Echinodermata (Amphiura filiformis)

AbstractSediment total organic carbon (TOC) content is considered to be a primary food source for benthic invertebrates and a major factor influencing the partitioning and bioavailability of sediment‐associated organic contaminants. Most studies report that both toxicity and uptake of sediment‐associated contaminants by benthic organisms are inversely proportional to sediment TOC content. The aim of this study was to determine the importance of the TOC quality for the bioavailability of sediment‐associated organic contaminants and the growth of benthic macrofauna. The common infaunal brittle star Amphiura filiformis was exposed to a base sediment covered by a 14C‐polychlorobiphenyl(3,3′4,4′‐14C‐tetrachloiphenyl (TCB)) contaminated top layer (0−2 cm), enriched to the same TOC content with 31 g TOC/m2 of different quality and origin. The following carbon sources, ranging from labile to refractory, were used: (1) green macroalga (Ulva lactuca), (2) brown macroalga (Ascophyllum nodosum), (3) eelgrass (Zostera marina), (4) phytoplankton (Ceratium spp.), and (5) lignins of terrestrial origin. Characterization of the organic matter quality was accomplished by measuring the content of amino acids, lipids, C, N, and polyphenolic compounds. The reactivity of the sedimentary organic matter was assessed by means of respiration and dissolved inorganic nitrogen flux measurements. The experiment was carried out in 1‐L glass jars, each containing four brittle stars and the contaminated and enriched sediment. The jars were circulated in a flow‐through mode with filtered seawater. Somatic growth (regeneration of a precut arm) and bioaccumulation of 14C‐TCB were measured at 10 sampling occasions during 48 d of exposure. Growth rates, TCB uptake rates, and steady‐state concentrations differed significantly between treatments and were correlated to the qualities of the organic substrates. The greatest TCB accumulation and growth were observed in treatments with the most labile organic substrates and the lowest accumulation and growth were seen with the most refractory substrates. Results from this experiment suggest that the bioaccumulation of organic contaminants in benthic infauna is dependent on the nutritional quality of the sedimentary organic matter.