Sedimentation Of Aggregates Research Articles

Volumetric Concentration Maximum of Cohesive Sediment in Waters: A Numerical Study

Cohesive sediment has different characteristics compared to non-cohesive sediment. The density and size of a cohesive sediment aggregate (a so-called, floc) continuously changes through the flocculation process. The variation of floc size and density can cause a change of volumetric concentration under the condition of constant mass concentration. This study investigates how the volumetric concentration is affected by different conditions such as flow velocity, water depth, and sediment suspension. A previously verified, one-dimensional vertical numerical model is utilized here. The flocculation process is also considered by floc in the growth type flocculation model. Idealized conditions are assumed in this study for the numerical experiments. The simulation results show that the volumetric concentration profile of cohesive sediment is different from the Rouse profile. The volumetric concentration decreases near the bed showing the elevated maximum in the cases of both current and oscillatory flow. The density and size of floc show the minimum and the maximum values near the elevation of volumetric concentration maximum, respectively. This study also shows that the flow velocity and the critical shear stress have significant effects on the elevated maximum of volumetric concentration. As mechanisms of the elevated maximum, the strong turbulence intensity and increased mass concentration are considered because they cause the enhanced flocculation process. This study uses numerical experiments. To the best of our knowledge, no laboratory or field experiments on the elevated maximum have been carried out until now. It is of great necessity to conduct well-controlled laboratory experiments in the near future.

Hair imprints in Pleistocene cave sediments and the use of X-ray micro-computed tomography for their reconstruction

Detailed studies of cave sediments in the Divje babe I cave, in western Slovenia, have revealed both fossil hairs and the imprints of such hairs present within phosphate aggregates in the clastic sediments. These aggregates consist of fine rock and bone fragments, as well as hairs cemented by phosphate precipitated from pore water. The hairs and their imprints were discovered by means of scanning electron microscopy and are likely to have belonged to cave bears, which were the most frequent inhabitants of this cave. From X-ray micro-computed tomography showing the spatial distribution of the hair imprints, and their number, it appears that the hairs did not belong to one particular animal which died at the location where the samples were collected, rather the hairs probably accumulated on the floor of the cave not only from cadavers, but also because hairs were shed during the hibernation of bears and their visits to the cave.

THE ROLE OF TEP IN ESTUARINE HYDROSEDIMENTARY FUNCTIONING: IMPACT ON SETTLING VELOCITY OF AGGREGATES

Transparent exopolymer particles (TEP) are widely recognized to promote sediment aggregation in eutrophic environments. Flocculation in presence of TEP of various suspended sediment concentrations of material sampled on the bank of the Cam River was quantified at the laboratory for turbulence level consistent with slack water and mid ebb conditions measured in the Cam River estuary during dry season of 2009. Stickiness and concentrations of TEP were let to naturally fluctuate by incubation (aging in the dark) for up to nine days. We found that the impact of turbulence on overall buoyancy of TEP-governed aggregation was always opposite between slack water and mid ebb conditions for any duration of incubation; always negative for slack water conditions but for 126 hours of incubation and significantly negative for mid ebb conditions but for 126 hours of incubation. Suspended sediment concentration (SSC) consistently limited aggregates buoyancy, negative or positive. We propose a conceptual model that relates measured and inferred parameters to observed hydrosedimentary processes.

Clogging-up of a stormwater infiltration basin: a laboratory approach using image analysis

The main function of stormwater infiltration basins is to favour stormwater drainage. However, the gradual clogging-up process caused by sediment accumulation inside these basins raises questions about their hydrodynamic functioning. Therefore, the objective of this work is to study the evolution of sediment pore distribution and its relationships with water retention and infiltration characteristics. Chevire basin (Nantes, France) ageing was simulated in the laboratory, using PVC columns (10 cm diameter, 32 cm height). Seven columns were first filled in with 20 cm of thick sandy Loire river alluvia, i.e. the same material as in the basin over which sediment accumulates. The objective of the experiment was to simulate 36 months of basin ageing within 9 weeks in the laboratory. Every day, the columns were submitted to 2 cycles composed of 4 h of rain separated by 8 h without rain. During the experiment, (1) the water flow at the bottom of the column, (2) the sediment layer thickness, (3) sediment water retention and hydraulic conductivity at saturation (Ks), and (4) pore space distribution by image analysis were measured. After 36 months of experimental simulation, 3 cm of sediment had accumulated (i.e. 1 cm year−1); this rate was representative of in situ observations. This progressive accumulation generated the formation of a water layer above the sediment, revealing early clogging-up by the sediment. Using HYDRUS 1D inverse resolution, a decreased Ks values from 25 to 6 × 10−6 m s−1 was observed after 6 and 36 months, respectively. The mean equivalent pore radius decreased 1.6-fold, from 606 to 380 μm after 6 and 36 months, respectively. These observations were confirmed by an image analysis study, whereby internal organisational changes were clearly evidenced in the sediment. Sediment particles, at first well individualised, progressively bound to one another, leaving hardly any voids. While the clogging-up process in stormwater infiltration basins has often been studied, very little has been done about sediment hydrodynamic properties. Pore space characterisation by image analysis is a major scientific progress and showed that the presence of high levels of organic matter did not favour sediment aggregation. On the contrary, sediment gradually constituted a barrier to water flow, leading to clogging-up.

Flood pulses control soil nitrogen cycling in a dynamic river floodplain

Flood pulses are major drivers of river–floodplain processes. We investigated their effects on soil nitrogen (N) transformations along a lateral gradient from the river to a mature alluvial forest in the Thur River floodplain (NE Switzerland). Selected N pools (ammonium and nitrate) and N transformations (mineralization, nitrification and denitrification) in the topsoils were repeatedly quantified over a period of six months. During this period, the floodplain was affected by two flood events of different magnitude.Our results showed a distinct difference in size and temporal changes of N pools and transformation rates among functional process zones (FPZs) differing in flooding disturbance. A strong temporary increase in N mineralization in a FPZ with young soil on the frequently flooded gravel bars was the most prominent flood-related effect. This was most likely related to the fast-flowing water during inundation that deposited fresh dispersed sediments with a coarse sandy texture containing highly bioavailable organic N, and/or stimulated the turnover of existing N by destroying sediment aggregates already present before the flood. Increased N mineralization appeared to stimulate coupled nitrification–denitrification in this FPZ during the drying phase. In the more stable FPZs, N mineralization was not strongly enhanced by flooding, whereas nitrification and denitrification were also increased during the drying phase, however to a lesser degree than in the gravel bar soils. Based on our results we propose that floodplain zones characterized by short intensive floods with fast over-flowing water are “hot spots” and the drying phases after the floods are “hot moments” of N transformations.

Benthic control upon the morphology of transported fine sediments in a low‐gradient stream

AbstractThe structure of fluvial sediments in streams has environmental implications to contaminant fate, nutrient budgeting and the carbon flux associated with fine particulate organic matter (FPOM). However, the influence of sediment structure is lacking in environmental predictive models. To this end, the present study links field‐based results of sediment aggregate structure to seasonal biological functions in the surface fine‐grained laminae (SFGL) of a low‐gradient stream. Fluvial sediment collection, microscopy and image analysis are used to show that aggregates collected over a 20 month time period support the concept that aggregate structure can vary seasonally in low‐gradient streams where temporarily stored sediment is prominent. Results show that the structure of the transported aggregates is more irregular in the summer with the structure being elongated about the long axes. In the winter, the aggregate structure is compacted and more spherical. Statistical analysis and results suggest that heterotrophic and autotrophic biological activity within the SFGL exhibits seasonal control upon the morphology of transported sediments. Implications of this research are highlighted through calculations of the reactive surface area of the transported suspended sediment load. The surface area of transported sediment is estimated to be 40% greater in the summer as compared to the winter time period, which implies that (i) the affinity of sediments to sorb contaminants is higher in summer months and (ii) the downstream reactivity of FPOM in large rivers, lakes and estuaries is not just a function of microbial drivers but also the seasonally dependent structure of transported FPOM derived from low‐order streams. Copyright © 2013 John Wiley & Sons, Ltd.

Pressure-assisted cyclic washing of heavy-metal-contaminated sediments

Remediation of heavy-metal-contaminated sediment is often hampered by the availability of heavy metals to the added chemical agents because the heavy metals are often shielded by the sediment matrix. Effective heavy-metal extraction technique becomes an important factor in enhancing the treatment efficiency. A novel extraction/washing technique utilizing chelating agent and elevated pressure in consecutive cycles of compression and decompression has been developed for heavy-metal-contaminated sediment washing in the presence of chelating agent. In this study, the optimal operational conditions of pressure-assisted cyclic washing of Cu-contaminated sediments (initial Cu concentration = 23.177 mg/kg) were determined in a laboratory-scale system. The control factors included applied pressure level, washing time, applied chelant [ethylenediamine-tertraacetic (EDTA)] concentration (0.01–0.5 M), pressure times, and application of consecutive batches washing. Results from the bench-scale study showed that up to 70 % of Cu can be removed from the sediments when 10 atm of pressure was applied for washing. The efficiency dropped to 55 % when the pressure dropped to 6 atm. Under the same operational conditions, the optimal cyclic washing time was 60 min. Results from the particle size analyses indicate that the mean particle size dropped from 100 to 50 μm after the pressure-assisted cyclic washing. Thus, cyclic pressure caused the fracture of sediment aggregates resulting in the exposure of Cu to chelating agents. With the assistance of pressure cyclic system, the total washing time and the amount of added chemical agent used can be significantly reduced.

Geoarchaeological Investigations of Midden‐Formation Processes in the Early to Late Ceramic Neolithic Levels at Çatalhöyük, Turkey ca. 8550–8370 cal BP

Anthropogenic midden deposits are remarkably well preserved at the Neolithic settlement of Çatalhöyük and provide significant archaeological information on the types and nature of activities occurring at the site. To decipher their complex stratigraphy and to investigate formation processes, a combination of geoarchaeological techniques was used. Deposits were investigated from the early ceramic to late Neolithic levels, targeting continuous sequences to examine high resolution and broader scale changes in deposition. Thin‐section micromorphology combined with targeted phytolith and geochemical analyses indicates they are composed of a diverse range of ashes and other charred and siliceous plant materials, with inputs of decayed plants and organic matter, fecal waste, and sedimentary aggregates, each with diverse depositional pathways. Activities identified include in situ burning, with a range of different fuel types that may be associated with different activities. The complexity and heterogeneity of the midden deposits, and thus the necessity of employing an integrated microstratigraphic approach is demonstrated, as a prerequisite for cultural and palaeoenvironmental reconstructions.

Brazilian Geological Sandstone Characterization and its Utilization as Aggregate in Structural Concrete

The sandstone extraction is an essential activity to the production of specific products. It is employed in the civil construction sector. However, there is still a deficiency in physical and geological characterization. This research develops two experiments in the sandstone matrix located in São Carlos region (City of São Paulo State – Brazil). The first experiment is related to the geological aspect of the specific sandstone. It identified the geographic location and investigated the porosity and absorption rates of the Brazilian sandstone rock, besides verifying its compressive strength. The purpose was to identify and evaluate possibilities of sandstone use in the structural concrete composition. The second experiment tested the concrete strength with this aggregate. Some quantities of crushed sandstone were used as fine aggregate in the manufactured concrete. The experiments presented sandstone properties and sandstone aggregate possibilities in concrete composition. The present study is a result of an undergraduate project, aiming to contribute with new information on the Brazilian sandstone characterization and concrete strength application with the sedimentary aggregate. This work showed some possibilities to use this type of yellow and red sandstone as aggregate.

Biotic and abiotic controls on sediment aggregation and consolidation: Implications for geochemical fluxes and coastal restoration

This study examined the influence of particle size and organic matter on aggregation and compaction of 3 hydraulically dredged sediments from coastal Louisiana (clay, silt loam, sandy loam) saturated under a range of salinity regimes (1 and 5 PSU, 5 and 10 PSU, and 15 and 25 PSU) for 4 time periods (1, 8, 16, and 26 weeks). Particle sizes were determined using a laser diffraction particle size analyzer, which allowed us to develop high-resolution results indicating changes in aggregate size across a spectrum of experimental conditions. The sediments with greater organic matter content exhibited approximately 60% aggregation, as indicated by fewer aggregates in the clay size fraction, and subsequently more aggregates in the sand size fraction, when organic matter remained in the sediment. Additionally, the sandy sediment compacted more than the organic sediments in the first 16 weeks. These findings suggest that sediments with greater clay and organic matter content behave as larger particles and may undergo particle rearrangement and compaction over longer time scales than sandy sediments with low organic matter. For coastal wetland restoration, models should include the effect of organic matter on particle aggregation to understand sediment dynamics over geologic time.

Using disposable solid-phase microextraction (SPME) to determine the freely dissolved concentration of polybrominated diphenyl ethers (PBDEs) in sediments

Polybrominated diphenyl ethers (PBDEs) are brominated flame retardants (BFRs). The ubiquity and persistence of PBDEs in sediment have raised concerns over their environmental fate and ecological risks. Due to strong affinity for sediment organic matter, environmental fate and bioavailability of PBDEs closely depend on their phase distribution. In this study, disposable polydimethylsiloxane (PDMS) fiber was used to derive the freely dissolved concentration (Cfree) of PBDEs in sediment porewater as a measurement of bioavailability. The PDMS-to-water partition coefficient (log KPDMS) was 5.46–5.83 for BDE 47, 99, and 153. In sediments, PBDEs were predominantly sorbed to the sediment phase, with Cfree accounting for <0.012% of the total chemical mass. The Cfree of PBDEs decreased as their bromination or sediment organic carbon content increased. The strong association with dissolved organic matter (DOM) implies a potential for facilitated offsite transport and dispersion in the environment that depends closely on the stability of sediment aggregates.

Seasonal variability of cohesive sediment aggregation in the Bach Dang–Cam Estuary, Haiphong (Vietnam)

In the Bach Dang-Cam Estuary, northern Vietnam, mechanisms governing cohesive sediment aggregation were investigated in situ in 2008-2009. As part of the Red River delta, this estuary exhibits a marked contrast in hydrological conditions between the monsoon and dry seasons. The impact on flocculation processes was assessed by means of surveys of water discharge, suspended particulate matter concentration and floc size distributions (FSDs) conducted during a tidal cycle at three selected sites along the estuary. A method was developed for calculating the relative volume concentration for the modes of various size classes from FSDs provided by the LISST 100X (Sequoia Scientific Inc.). It was found that all FSDs comprised four modes identified as particles/flocculi, fine and coarse microflocs, and macroflocs. Under the influence of the instantaneous turbulent kinetic energy, their proportions varied but without significant modification of their median diameters. In particular, when the turbulence level corresponded to a Kolmogorov microscale of less than ∼235 μm, a major breakup of flocs resulted in the formation of particles/flocculi and fine microflocs. Fluctuations in turbulence level were governed by seasonal variations in freshwater discharge and by the tidal cycle. During the wet season, strong freshwater input induced a high turbulent energy level that tended to generate sediment transfer from the coarser size classes (macroflocs, coarse microflocs) to finer ones (particles/flocculi and fine microflocs), and to promote a transport of sediment seawards. During the dry season, the influence of tides predominated. The turbulent energy level was then only episodically sufficiently high to generate transfer of sediment between floc size classes. At low turbulent energy, modifications in the proportions of floc size classes were due to differential settling. Tidal pumping produced a net upstream transport of sediment. Associated with the settling of sediment trapped in a near-bed layer at low turbulent energy, this causes the silting up of the waterways leading to the harbour of Haiphong.

Laser diffraction analysis of the disintegration of aeolian sedimentary aggregates in water

Abstract A new method for the analysis of aggregate disintegration in water was tested using aeolian sediment samples. The response of aeolian sedimentary aggregates to wetting and mechanical energy has important implications for the immobilization of aeolian sediments by rainfall and their susceptibility to post-depositional erosion by water. Repeated laser diffraction analysis of a single sample circulating in deionized water provides a detailed view of the changes in particle size distribution as aggregates disintegrate. This methodology also allows experiments on the effects of water chemistry, exchangeable cation chemistry, and prewetting. Results presented here suggest that aggregates of widely varying stabilities can occur in a wide range of particle size classes; that is, aggregate stability may to some extent be independent of the hierarchy of aggregate sizes emphasized in earlier research. The methods described here are broadly applicable to a variety of soils and sediments, but experiments are needed to relate the results more specifically to aggregate behavior under field conditions.

Fate and transport of oil in sediment laden marine waters

Spilled oil when present in sediment-enriched waters, is known to interact with sediments. Oil droplets form aggregates with sediments due to collision and dissolved oil partitions (absorption and adsorption) into sediments due to capillarity and surfactant ions. This may change the fate and transport of both oil and sediments. Interaction with sediments acts as a natural oil removal process. This has been recognized as a crucial component in developing oil spill countermeasures. However, no comprehensive models are currently available to describe the process. This paper presents a novel numerical model developed to simulate oil-sediment interaction and transport in near-shore waters. The present model simulates, oil and sediment transport, oil-sediment aggregate (OSA) formation, oil partitioning into sediments, and sediment aggregation. The critical information provided by the model such as the fraction of oil removed due to sediment interaction would be useful in developing oil spill countermeasures. The model results are compared with the limited data from laboratory experiments and show a good agreement. Scenario simulations showed that up to 65% of released oil may be removed from the water column as OSAs. They further showed that when oil droplets and sediment particles are smaller (less than 0.1 mm), the amount of OSAs formed is higher. According to the scenario simulations, the amount of oil partitioned into sediments is 4–5 orders of magnitude smaller than the amount of OSAs formed.

Effect of microstructure and curing conditions upon the performance of a mortar added with Treated Sediment Aggregates (TSA)

This experimental study investigates the performance of mortars added with water-saturated treated sediment aggregates (TSA), used as sand substitution (0, 33, 66 and 100% sand volume). As compared to industrial practice, two extreme curing conditions are used, either water immersion (most favorable to cement hydration), or air curing (highly deficient curing condition). Independently of curing conditions, porosity is shown to increase hugely with TSA amount, while apparent density decreases linearly with porosity increase. Nevertheless, up to 33% substitution, elastic Young's modulus, uniaxial compressive strength and apparent gas permeability are all improved as compared to reference 0%-substitution mortar. Maturity is attained from 28 to 60 curing days. The positive internal curing effect of TSA is more significant under air curing. The negative effect of brittle lightweight TSA is significant above 33% substitution. Micro-structural changes have also been observed by SEM and EDS analysis, along with thermo-gravimetry analysis (TGA).

Impact of wildfire on interdune ecology and sediments: An example from the Simpson Desert, Australia

The stability of many sand dunes and their interdunes is dependent on vegetation and surface crust cover. When this cover is removed, the sand can be activated and fine sediments deflated making the dunefields into sources of dust. This paper reports the impact of devegetation by wildfire on an interdune in the Simpson Desert, Australia. The fire occurred in 2001 and six years after the event pronounced differences between a pair of burnt and unburnt sites was clearly discernible. The variables examined included vegetation assemblage, cyanobacteria abundance and sediment aggregation, particle-size distribution and colour; but whether they apply to all such situations is uncertain. Rate of recovery has been slow and the differences are likely to have been sustained by a combination of negative feedback processes and climate.

Sediment bioavailable organic matter, deposition rates and mixing intensity in the Setúbal–Lisbon canyon and adjacent slope (Western Iberian Margin)

The role of the Setúbal–Lisbon canyon in accumulation and transport of labile organic matter from the coastal sea and ocean surface water towards the deep sea was assessed by investigating the distribution of organic matter of different quality in sedimentary aggregates and surface sediments of the canyon and adjacent slopes. Total hydrolysable amino acids (THAA) and organic carbon (Corg) were measured from aggregates, and contents of Corg, chlorophyll a (chl a), phaeopigments (phaeo), chloroplastic pigment equivalents (CPE) from sediments. As indices of organic matter (OM) quality THAA:Corg, degradation index (DI), chl a:phaeo, chl a:Corg and C:N ratio were determined. Sediment profiles of chl a and the isotope 210 of lead ( 210Pb) were used as tracers in a transport model to estimate deposition rates and background levels of the tracers, and sediment mixing rates (Db). Whereas bulk Corg contents of canyon and slope sediments were practically similar at all depths, higher contents of THAA, chl a and CPE, as well as higher THAA:Corg, DI and chl a:Corg, in aggregates and sediments from the upper reaches of the canyon indicate that labile organic matter accumulates in the upper canyon. This is confirmed by higher chl a and 210Pb deposition and Db calculated from the model. Hence, the Setúbal–Lisbon canyon, specially the upper region, acts as a natural trap of organic matter that is transported to the region via lateral transport and vertical settling from primary productivity. Organic matter might be further transported in downward canyon direction via rebound processes. The chl a and 210Pb profiles reveal active sediment mixing by physical processes and/or animal reworking.

Size and settling velocities of cohesive flocs and suspended sediment aggregates in a trailing suction hopper dredge plume

A field experiment was conducted to quantify settling velocities, aggregate states, and flocculation within a hopper dredge plume. Particular interest was in determining the abundance of dense, bed aggregates suspended from the consolidated bed during dredging. A suspended sediment plume from the hopper dredge Essayons was sampled for a period of 90 min after dredging. Settling velocities and suspended particle sizes were quantified through sampling with the Particle Imaging Camera System (PICS) and automated image processing routines. The sediment plume was identified and a profiling instrumentation frame was positioned within the plume using Acoustic Doppler Current Profiler (ADCP) backscatter. Results indicated that suspended bed aggregates (defined by densities of 1200–1800 kg m −3) represented 0.2–0.5 of total suspended mass, and flocs (densities<1200 kg m −3) represented 0.5–0.8 of total suspended mass. The peak diameter of bed aggregates and flocs occurred near 90 and 200 μm, respectively, corresponding to peak settling velocities of about 1 mm s −1 in each case. Floc settling velocities increased with particle size d 1.1, while bed aggregate settling velocity increased like d 1.3.

Anaerobic methane oxidation by archaea/sulfate-reducing bacteria aggregates: 2. Isotopic constraints

Recent studies employing novel analytical tools provide detailed, microscopic portraits of archaea/sulfate-reducing bacteria aggregates in sediments from methane seep and vent sites. One of the most striking features of these aggregates is that lipid and cell carbon are highly depleted in ¹³C (δ¹³C &lt; −60‰). Biogenic methane, with δ¹³C values of −50 to −110 permil, is a logical candidate for carbon source of these aggregates. Accordingly, it is widely assumed that the archaea oxidize and assimilate methane, and that methane-derived carbon is transferred to the sulfate-reducing bacteria (SRB) symbionts as CO₂ or as a partially oxidized intermediate. However, methane is not the only possible source of ¹³C-depleted carbon in archaea/SRB aggregates. ΣCO₂ in sediments at seep and vent sites tends to be isotopically “light” due to decomposition of organic matter derived from chemoautotrophic organisms. In addition, CO₂ is depleted in ¹³C by ∼10 permil compared to ΣCO₂ owing to the equilibrium isotope effect. Assimilation of this “light” CO₂ by methanogenic archaea and autotrophic SRB, combined with enzymatic isotope effects, could also yield lipid and biomass that are highly depleted in ¹³C. We derive general equations based on isotope mass-balance and calibrated with laboratory and field data to predict the isotopic composition of archaeal cell carbon and lipids derived from autotrophic methanogenesis and anaerobic methane oxidation. The calculations show that observed δ¹³C values for archaeal biomass and lipids at methane seep and vent sites are readily accounted for by isotope fractionation during methane production from CO₂, and that biomass produced during anaerobic methane oxidation is only slightly depleted in ¹³C relative to methane unless the enzymatic isotope effect associated with the anabolic arm of the assimilation-dissimilation branch point is considerably larger than the isotope effect associated with the catabolic arm. We also apply an isotope diffusion-reaction model to demonstrate that micro-gradients in δ¹³C-CO₂ cannot be maintained within archaea/SRB aggregates. However, ¹³C-depleted carbon in SRB members of the aggregate is readily explained by autotrophic sulfate-reduction with bulk porewater CO₂ as carbon source. These results illustrate that ¹³C-depleted biomass and lipids observed in sediments from methane seep and vent sites may be derived from CO₂-reducing archaea and autotrophic sulfate-reducing bacteria. The inference of anaerobic methanotrophy based on ¹³C depletion in archaeal and sulfate-reducing bacterial cell carbon and/or lipids should be considered tentative unless corroborated by independent, concordant evidence of net methane consumption.

Anaerobic methane oxidation by archaea/sulfate-reducing bacteria aggregates: 1. Thermodynamic and physical constraints

Aggregates of archaea and sulfate-reducing bacteria (SRB) recently discovered in methane-seep sediments are widely assumed to engage in anaerobic methane oxidation (AMO), but the reaction mechanism remains poorly understood. We used a spherical diffusion-reaction model that incorporates thermodynamic controls, realistic aggregate morphology, and essential elements of cell structure to quantify maximum reaction rates and energy yields for competing mechanisms, to determine how cellular energy yields are affected by aggregate size and morphology, and to investigate the impact of organic-matter remineralization on archaea and SRB in the aggregate. The model provides the following insights: (a) syntrophic AMO is thermodynamically and physically possible for a variety of intermediate compounds (including H 2 , formate, and acetate); (b) the energy yield for syntrophic AMO is low but compatible with the maintenance needs of non- or slowly-growing cells; (c) archaea and SRB engaged in syntrophic AMO face a substantial energetic cost for aggregating; (d) direct contact between archaea and SRB provides only a modest energetic advantage compared to a loose association; and (e) sulfidogenic-methanogenic aggregates that take advantage of fermentation products released during organic-matter decay have a substantial energetic advantage over aggregates that rely exclusively on syntrophic AMO. Moreover, the model calls attention to a discrepancy between the observed sulfate-reduction rate at a well-characterized methane-seep site and the theoretical upper-limit rate of syntrophic AMO by a mechanism involving interspecies transfer of H 2 , formate, acetate, or other chemical intermediates. An analysis of possible errors, ambiguities, and artifacts in modeling and experimental techniques leads us to a surprising conclusion: that archaea/SRB aggregates in methane-seep sediments may be methanogenic rather than methanotrophic. In contrast, AMO in non-seep (diffusion-dominated) sediments is best explained by a consortium involving methanogenic archaea (that oxidize methane and release H 2 ) and hydrogenotrophic SRB.