Kinds Of Sediments Research Articles

The role of liquefaction in the evolution of shallow submarine canyon heads from a geotechnical perspective: A case study of the Garrucha Canyon (SE Mediterranean)

This work aims to establish the role of liquefaction in a shallow submarine environment defined by a canyon head reaching the coast. The study area is the Garrucha submarine canyon head, which is located in the western Mediterranean Sea.The potential of liquefaction is approached empirically by two methods in parallel, undrained cyclic direct simple shear (UCDSS) test and piezocone penetration test (CPTu) analyses. For both approaches, considering the regional earthquake records, a cyclic load linked to Mw ≤ 6.5 earthquake events or a maximum ground surface acceleration amax of 0.25 g is considered.The sediment samples analysed are nonplastic sands with low silt/clay contents and can be defined as liquefiable. Geotechnical analysis reveals a high probability of triggering liquefaction in this kind of sediment at depths greater than 3 m below the seafloor. CPTu records are used to assess and improve the liquefaction model for the study area by defining 3 different stratigraphic configurations or liquefiable conditions: uniformly liquefiable, interbedded liquifiable and nonliquefiable.This work highlights the importance of liquefaction—a process normally underestimated in submarine environments—in the downslope transport of sediment from the upper part of a canyon and, more generally, in canyon head evolution with different potential morphosedimentary consequences.

Experimental study on the impact of effective stress and cyclic loading on permeability of methane hydrate clayey sediments

Permeability evolution directly determine gas production and its efficiency of gas hydrates in the South China Sea. Due to existence of submarine overlying rocks and change of soil internal mechanics during the hydrate exploitation, permeability evolution is significantly affected. Therefore, in this paper, influence of effective stress on the porosity and permeability of sediment with different hydrate saturation was studied, and functional relationship between them was revealed. Effect of cyclic loading and unloading stress on the permeability recovery and damage degree was also discussed. The results show that when hydrate saturation increases, the relation of permeability and effective stress are power function rules, while porosity and effective stress are exponential function rules in clayey sediments. An empirical prediction equation for gas permeability of clay sediments with effective stress was presented, which takes into account hydrate saturation and porosity. In addition, during cyclic loading experiments, the results showed a linear increase in permeability during unloading. The permeability recovery rate and permeability damage rate are completely opposite with change of hydrate saturation. Permeability damage rate of kaolin is different from that of other two kinds of sediment. With the increase of cycle times, the proportion of plastic deformation of sediment decreases gradually.

Wave-induced liquefaction in a silt and seashell mixture

This paper presents the results of an experimental investigation of wave-induced liquefaction in silt and seashell mixtures. Three kinds of sediments were used in the experiments: silt ( d 50 = 0.070 mm), coarse seashell ( d 50 = 2.87 mm), and fine seashell ( d 50 = 1.46 mm). With these sediments, three kinds of tests were carried out: silt-alone tests (the reference tests), tests with silt and coarse-seashell mixture, and tests with silt and fine-seashell mixture. The experiments showed that the influence of seashell content on wave-induced liquefaction is very significant. The susceptibility of silt to liquefaction is decreased with increasing seashell content. This is up to a certain point beyond which the mixture of silt and seashell becomes liquefaction resistant. For the seashells used in the experiments, this limiting value was found to be approximately S C ≈ 30 % , where S C is the shell content by weight. It is argued that this behavior is linked with the elastic modulus of the mixture. The present findings showed that the liquefaction criterion based on the initial mean normal effective stress in pure silt/sand can be extended to silt and seashell mixtures. A chart is proposed for an initial screening check of liquefaction potential. Furthermore, it is recommended that, if there is a liquefaction potential, standard assessment methodology may be implemented to check for liquefaction, provided that the elastic modulus and other properties of the soil and seashell mixture are incorporated into this assessment. • Wave-induced liquefaction in a silt and seashell mixture is experimentally investigated in a wave flume. • Influence of seashell content of the mixture on wave-induced liquefaction is found to be very significant. • The susceptibility of silt to liquefaction is decreased with increasing seashell content. • A chart is proposed for an initial screening check of liquefaction potential. • Mixing of the native soil with seashell with a sufficiently large seashell content offers an option as a counter measure in the prevention of backfill soils against liquefaction.

Evaluation of caridean ecological distribution in the Ubatuba region, southeastern Brazilian coast using unsupervised machine learning technique

AbstractWe used the unsupervised machine learning technique to evaluate the environmental factors responsible for modulating the spatial and seasonal distribution of caridean shrimps from a southeastern region of the Brazilian coast. Samplings were collected from seven transects with an artisanal shrimp fishery boat with two double‐rig nets. Samplings occurred every month from October 2008 to September 2009. The most frequently captured species were Exhippolysmata oplophoroides, Leander paulensis, and Nematopalaemon schmitti. The highest abundance of shrimps occurred in autumn at the II, III, and V transects, which present a higher amount of coarse sediment and biodetritic fragments on the bottom. During autumn, the temperatures were the highest and salinity values were the lowest. Data evaluation indicated efficiency in the visualization of interactions of different shrimp species and environmental data. This kind of sediment may be allowing shrimps to burrow in shelters that prevent predation. The seasons with high temperatures and low salinities can offer better conditions for the establishment of the studied species, despite the fact that there is no hypothesis to prove it. Additionally, the higher abundance of such shrimps coincides with vegetal debris deposition, which could serve as food and provide protection for these shrimps. In this region, the vegetation matter deposited at the bottom of the bay comes from the Atlantic Forest. Overall, the preservation of the coastal forest strongly influences the abundance of this taxon, as it provides protection and food for these shrimps.

Trace and rare earth element evidence for the provenances of aeolian sands in the Mu Us Desert, NW China

Tracing the provenances of aeolian sands in the Mu Us Desert is crucial to understanding surficial processes in arid and semiarid regions of China. However, the material source of the aeolian sands in the Mu Us Desert is still under debate. The provenances of modern aeolian sands in different areas and with different particle sizes in the Mu Us Desert remain unclear, as does the link between the sources of different kinds of sediments. Here, the Mu Us Desert is divided into two subzones in the east and the west with hierarchical cluster analysis. According to the characteristics of trace and rare earth elements, we studied the provenances of aeolian sands in different areas and with different particle sizes, and we analyze the provenance linkages among aeolian sands, sandstones, fluvio-lacustrine sediments, loess sediments, and floodplain sediments in the desert. We report the following results: (1) The sources of coarse-grained aeolian sands in the eastern and western Mu Us Desert are different. Aeolian sands from the eastern part are mainly derived from local sandstones and eastern coarse-grained fluvio-lacustrine sediments, whereas those from the western part mainly come from the Qilian orogenic belt in the northeastern Tibetan Plateau. (2) The fine-grained aeolian sands of the whole desert show obvious homogeneity and have geochemical characteristics similar to those of fine-grained fluvio-lacustrine, loess, and floodplain sediments, suggesting that these fine-grained fractions are fully mixed after long-distance transport. (3) The transport and deposition of aeolian sands are affected by aeolian processes and fluvial activity.

Study of Mangrove Forest Existing Condition using Remote Sensing Image in The Karawang Coast of 2018

ABSTRACTThe northern part of Karawang is a coastal area with mostly mud-sand substrates. This substrate tends to be unstable, so that naturally, this kind of sediment is supported by coastal vegetation that forms coastal ecosystems, such as mangroves; therefore, the importance of mangroves in Karawang coast is definite. Unfotunately the data regarding the condition of mangroves in Karawang Regency is quite insufficient. This information, especially about its existence, is needed as a database for further research and as basis to support government policies on coastal area management. The aim of this research is to provide information about the existence of mangrove in Karawang Regency. The method is by using Normalized Different Vegetation Index (NDVI) calculations on Landsat 8 2018 satellite imagery of Karawang to get the data that reveal the information. We have discovered that the existing of mangroves in Karawang Regency in 2018 is 305,14 Ha. Border coast that is vegetated is only 33.75 km of 77 km long coastline of Karawang. Only less than 5% of the total mangrove protected area in Karawang Regency is detected as mangrove from the total 9.055 Ha of the area.

An overview of the taxonomic groups of non-pollen palynomorphs

Abstract Non-pollen palynomorphs (NPPs) are ‘extra’ microfossils often found in palynology slides. These include remains of organisms within the size range of pollen grains ( c. 10–250 µm), resistant to laboratory treatments used for the preparation of palynological samples. NPPs are a large and taxonomically heterogeneous group of remains of organisms living in diverse environments. Taxonomically, they belong to a wide variety of groups such as cyanobacteria, algae, vascular plants, invertebrates and fungi. The aim of this chapter is to provide a general overview of NPP groups observed in palynology slides. It includes more than 40 of the most common groups starting with acritarcha, cyanobacteria and algae, moving through transitional groups to animals and fungi and finishing with human-made objects such as textile fibres. Although far from complete, it provides an updated overview of taxonomical diversity of NPPs and their indicator values. Further works on NPP identifications are of great importance to improve our current knowledge. Since NPPs occur in all kinds of sediments, their analysis is a powerful tool for reconstructing environmental changes over time. Further detailed studies of specific NPP groups and their indicator values will open the way for new fields of study.

Mathematical modeling of sedimentation processes and surfacing of solids, droplets and bubbles in an isotropic turbulent flow

Relevance. The problems of separation, stratification and classification of disperse systems that constitute the base of sedimentation and phase displacement are determined by hydrodynamic structure and direction of the flow, physical interaction of forces of different nature of the systems, diffusion transport and deposition in turbulent flows, physical and chemical properties of the particles themselves and the carrier medium and many other factors. Appears the necessity of development of the model of solid spherical particles deposition from a volume with small values of number Re under the condition of absence of effects of interaction of particles and model of constrained deposition from the concentrated disperse flow. The consideration of expressions for determination of effective viscosity of the dispersed system taking into account the concentration of particles in the flow and models of particle deposition from isotropic turbulent flow taking into account the scale of turbulence and specific energy of dissipation. This, in turn, is associated with the determination of the coefficient of resistance for deformable particles (droplets and bubbles), which is used in the model of deposition velocity and surfacing of droplets and bubbles for different numbers of Weber and Morton. Purpose of work. The purpose of this study is an analytical review of all kinds of sedimentation (deposition), separation and stratification of dispersed systems and model representations of their description in different flow conditions. Methodology. To solve given problem, it is necessary to analyze all the effects associated with particle migration, deposition and separation. An essential role in deposition and migration is determined by the forces of resistance, which depend on the number of Reynolds, shape and size, as well as physical and chemical properties of the particles and the corresponding environment. Results. The study of particle deposition in an isotropic turbulent flow for different scales of turbulence in pipes and channels allowed to express the deposition rate through the main parameters of turbulence – specific energy dissipation, scale of turbulence and viscosity of the medium. The deposition and formation of a dense layer of particles on the inner surface of the pipes has a significant influence on all parameters of substance transfer (mass, heat and pulse) and on hydrodynamic stability of the flow. It has been found that the deposition of polydisperse particles is characterized by the size inconstancy or the function of their size distribution, which is related to the confinement of deposition, collision and interaction of particles among themselves. Conclusions. It is concluded that the nature of deposition of particles from the polydisperse turbulent flow is significantly different from their free deposition from the volume. As the result of dispersed particle deposition on the walls of pipes and canals, the following mechanisms and models are distinguished: free-inertial, which is based on the principle of free inertial ejection of particles to the wall; elevating-migration, which binds the deposition of particles with their elevating migration (Magnus effect); convection-inertial, which binds the deposition rate of particles with inertial effects; efficient-diffusion; turbulent-migration, where turbulent migration of particles to the wall is considered as the driving force of deposition. Particle deposition and formation of a dense layer of particles on the inner surface of the pipes has a significant impact on the hydrodynamic flow and heat-mass transfer. Particle deposition and formation of a dense layer of particles on the inner surface of the pipes has a significant impact on the hydrodynamics of the flow and heat and mass transfer. All proposed models are compared with available experimental data, which confirms their effectiveness and adequacy.

Burrowing-site selection by the soldier crab Mictyris guinotae Davie, Shih & Chan, 2010 (Decapoda: Brachyura: Mictyridae)

AbstractThe soldier crab Mictyris guinotae Davie, Shih & Chan, 2010 inhabiting sandy tidal flats in the Ryukyu Is., Japan, are deposit feeders in two regions: its upper habitat where it burrows and the shoreline. The crabs usually migrate between these two regions during the period of emergence at the daytime and night-time low tide. The aim of this study was to determine which kind of sediments are chosen by the crabs when burrowing. Experiments designed to investigate burrowing-site selection revealed that the crabs preferably burrowed in sediment that had been conditioned through feeding by conspecific crabs, but not in sediment that had been conditioned by conspecific crabs walking on its surface. Moreover, the crabs never showed any preference for burrowing in sediment that had been conditioned through feeding by the competitor fiddler crab Austruca perplexa (H. Milne Edwards, 1852); both soldier (prey) and fiddler (predator) crabs sympatrically inhabit the tidal flat, but segregate their habitats with a transition area. These results suggest that chemical cues in sediment conditioned through feeding by conspecific soldier crabs affect where crabs burrow.

Exploring computed tomography in ichnological analysis of cores from modern marine sediments

Ichnological analysis is considered a very useful tool in several disciplines of Earth Sciences, including palaeoenvironmental studies and hydrocarbon exploration. Sediment cores provide excellent records, despite difficulties encountered during study runs due to specific core features. Previous studies using 2D images have proven the benefits of high-resolution image treatment in improving the visibility of ichnological features, but with limitations. 3D computed tomography (CT) techniques were applied to palaeoichnological studies in lithified cores and other disciplines of palaeontology to solve these limitations, but not used for ichnological studies in unconsolidated sediments due to the low density contrast between host sediment and trace fossils. In this study, a CT processing technique, previously tested in coral research, is applied to facilitate the characterisation of the ichnological signature of cores from modern marine soft sediments. This technique allows for the first time the isolation of burrows within these kinds of sediments and the differentiation of intervals based on burrow orientation. Data obtained from the technique are complemented with the ichnological information from conventional core description, thus providing a more complete characterisation of the trace fossil assemblage with additional ichnological properties such as burrow orientation and branching. This will improve palaeoenvironmental interpretations related to changes in energy or oxygenation, and the analysis of reservoir quality given the impact of burrows on porosity and permeability. Therefore, adopting CT to complement visual core description in the ichnological analysis of soft modern marine cores is a very informative approach.

Characterization and engineering properties of AMU Morasko soft clay

AMU Morasko soft clay is the normally consolidated, postglacial clayey silty sand to sandy clay, deposited during the last glaciation (Weichselian) within the marginal zone of end moraine of Posnanian Phase. These soils are characterized by low plasticity, high porosity and are fully saturated, therefore, from a geotechnical point of view they can be defined as soft clays. The over 10 meter thick complex of clayey glacial sediments is covered with fine and medium sands with single grains of gravel of the so-called first sandur level from the youngest glaciation. This kind of sediments are very common in the area of north-western Poland and, due to high compressibility, become a problematic subsoil for the shallow foundations. The high porosity and saturation also causes significant problems in obtaining high quality samples for laboratory tests. This means that in situ tests are very often the only possible and economically justifiable test methods. Extended investigations carried out on these clays, which provide reliable correlations between in situ and laboratory test results, will be very useful. For the purpose of this research paper, a wide variety of in situ (CPTU, DMT, FVT) and laboratory (physical properties, direct shear, oedometer) tests have been carried out on the material found at the depth of up to 16 m, in the area of 8 ha, within the campus of Adam Mickiewicz University (AMU) in Poznan, Poland. Preliminary investigations (performed four years ago) showed high uniformity of the test site. The current laboratory tests were carried out on both high quality natural samples and reconstructed samples. The rate effect on the results of some of the tests was also investigated. The obtained results allowed to present a wide geological and engineering characterization of these interesting glacial deposits.

Substrate Flow by Different Biochemical Activities in the Urban Sewage Network

In order to investigate the consumption mechanism of organic contaminants in different biological metabolism pathways within an urban sewage network, a pilot-scale system using two kinds of sediments (urban sewage sediment and artificial sediment) was constructed. The pilot system was operated to study the migration and transformation characteristics of COD, methane, nitrate, and sulfate between sediment and sewage. Results showed that the variation of COD in sewage was 170.58 mg·L-1, with change of COD by deposition and bioreaction of 101.53 mg·L-1 and 69.05 mg·L-1, respectively. Due to biological metabolism, the generation of methane in sewers was of 7.39 mg·L-1; the decrease of nitrate and sulfate in sewage was 0.33 mg·L-1 and 21.35 mg·L-1, respectively. Based on our calculations, the consuming concentration of COD was 32.51 mg·L-1for methane generation, 8.04 mg·L-1 for denitrification, and 6.41 mg·L-1for sulfate degradation by sulfate reducing bacteria. The decrease in COD by deposition was responsible for 65.38% of total variation of COD in sewage. The decrease in COD by bioreaction was responsible for 34.62% of total variation in COD. Meanwhile, total variation values of COD for methane generation, denitrification, and sulfate degradation were responsible for 68.01% of COD variation by bioreaction. Deposition was therefore the main pathway for removal of organic contaminants from the sewer system; biochemical activities of methane generation, denitrification, and sulfate degradation also played important roles.

Influence of hydrostatic pressure on the sound speed of surficial seafloor sediments

ABSTRACTChanges in seawater depth could cause variations of hydrostatic pressure and temperature conditions in surficial seafloor sediments. As a result, the sound speed of the sediments would change. Studies seldom have focused on the influence of hydrostatic pressure on such. According to calculations of the general model of sound speed (GMSS), the sound speed of sediment increased linearly when the hydrostatic pressure increased, the same trend as that of the seawater. Pressure-controlled experiments in the laboratory verified that the influence of hydrostatic pressure on the sediment sound speed was highly related to the pore water, which was consistent with the theoretical calculations from the GMSS. Hydrostatic pressure was found to have the same effect on sound speed for the three kinds of sediments: gravel-silty sand, clayey silt, and silty clay. For different hydrostatic pressures up to 40 MPa, the fluctuation of sound speed ratio calculated with the theoretical model was less than 1.96%, while that measured in the controlled experiment was less than 2.63%. This means that if the sound speed ratio is used as a constant to correct for the difference at different seawater depths of no more than 4000 m, the estimable error is about 2.63%.

Application Research on Stabilizing Treatment of Dredged Sediment

In order to improve water quality, to ensure the capacity of normal flood discharge and river transport, it carried out a lot of dredging work across the country recently. For harmful sediment’s second pollution and recycling use, this study selected five kinds of sediment. And point A was determined as the research object by using the geo-accumulation index to evaluate the heavy metal. Then the sediment was mixed with lime, fly ash, kaolin clay, sodium silicate, sodium carbonate and polyvinyl alcohol as mud stabilizing materials. Experimental research shows that, copper loss of stabilized soil’s toxicity leaching agent reaches more than 95%, and the permeability coefficient was 10-5 cm/s orders of magnitude. In this paper, it ensures GW7 as the best choice of plants experiments, which are 5% fly ash, 1% lime, 3% kaolin clay, 3% sodium silicate, 3% sodium carbonate and 1% polyvinyl alcohol, through the ways of permeability, water retention, SEM and XRD. The scheme effectively stabilizes copper, and keeps the plants as better form than others. But it has different effect on different plants. Thus it should consider planting varieties optimization in actual applications.

Research into the biosorption process of heavy metal ions by the sediments from stations of biological iron removal

The article considers the possibility of using the sediments from stations of biological iron removal for the biosorption of heavy metal ions (IHMs) from both natural groundwaters and washing waters of enterprises with galvanic cycles. The processes of extracting Zn 2+ ions (up to 50 mg/dm 3 ), Cu 2+ (up to 16 mg/dm 3 ), Ni 2+ (up to 1.3 mg/dm 3 ), Cr 6+ (up to 2.0 mg/dm 3 ) on the bacterial structures of iron bacteria consortia and the optimal parameters of passing this process are determined (for washing waters pH=8.5–9.0). The influence of constant magnetic field with intensity of 20–60 mT, as well as the influence of different kinds of sediments from stations of biological iron removal on the efficiency of the process of removing IHM have been investigated. A significant percentage of the removal of IHM using calcined precipitate (98 % for Cu 2+ , 97 % for Zn 2+ , 85 % for Ni 2+ ) is due to the exoeffect that occurs at a wide temperature range of 200–400 oС. In this case, there is a smooth decrease in the mass of the sediment, indicating the combustion of the organic component and the crystallization of the amorphous phase. As a result, goethite (a-FeOOH) and lepidocrocyte (g-FeOOH) are converted into hematite (a-Fe 2 O 3 ) and magnetite (g-Fe 2 O 3 ), which have magnetic properties. When using as a reagent a fresh sediment, active involvement in the process of adsorption of heavy metal ions is carried out by biominerals, cells and polymer matrices of iron bacteria that have a crystalline structure and a large specific surface. At the same time, the effectiveness of removing IHMs was, respectively: for Cu 2+ 93 %; for Zn 2+ 92 %; for Ni 2+ 70 %. The efficiency of one-, two- and three-stage water purification schemes have been investigated. It was established that using the three-stage scheme with hydromechanical mixers and water alkalinization to pH 9.0, the efficiency of IHM removal was: for Cu 2+ up to 96 %; For Zn 2+ up to 99 %; for Ni 2+ up to 85 %. The X-ray spectral microanalysis of the sediment of the surface of the Gallioella bacterium case revealed the accumulation of IHM in the structure. The process of adsorption of IHM on the bacterial structures of consortia of iron bacteria due to their electrostatic bonding with both anionic surface of cell wall and with organic polymers isolated by cells of iron bacteria is substantiated.

Comment on "Outburst flood at 1920 BCE supports historicity of China's Great Flood and the Xia dynasty".

Wu et al (Reports, 5 August 2016, p. 579) reported an enormous flood in the upper Yellow River that destroyed the Lajia Ruins. However, published research shows that the Ruins were destroyed at 3950 years before the present (B.P.) by earthquakes accompanied with mudflows, whereas the landslide-dammed lake terminated about 5650 years B.P. Various kinds of sediments with different ages were taken as evidence to verify an outburst flood.

Hydrogeochemical characteristics of interstitial water and overlying water in the lacustrine environment

This research was conducted in the lacustrine environment of Lake Wigry, one of the largest and deepest lakes in Poland. The main objective was to determine the relationship between the chemical composition of interstitial water, overlying water and bottom sediment in the different sedimentation conditions. Therefore, seven research positions were selected, taking into account different zones of lake and lithological type of deposits. Overlying water samples taken from zones with carbonate type of sediment (lacustrine chalk or calcareous gyttja) were characterized by slightly alkaline pH, similar hydrogeochemical type (HCO3–Ca–Mg, HCO3–SO4–Ca–Mg), low values of mineralization and low metal concentrations. The situation is different with the water taken from the zone of organic: detritus gyttja or dy deposits. Overlying water collected from the Hanczanska Bay (estuary of Czarna Hancza River) had the highest mineralization 552 mg/L, HCO3–Ca type and low metal concentrations, while water collected from the dystrophic lake was SO4–Ca–Mg type, low mineralized and acidic type. Studies of interstitial solutions collected from different lithological type of sediments showed noticeable chemical composition differences, both between the carbonate and organic sediments in the same reservoir, as well as in the same kind of sediment but exposed to different external conditions. The highest mineralization (2032 mg/L) was generally found in the interstitial water obtained from the detritus gyttja localized in the shallow positions, which are more exposed to pollution. The highest trace metals and nutrients content in the sediment and interstitial water found in the Czarna Hancza River estuary were a result of the input from increasing industrial wastewater, domestic sewage discharge and runoff from agricultural soil. A greater resistance to adsorption of contaminants characterizes sediments composed of the lacustrine chalk; mineralization of their pore solutions was significantly lower. This fact is probably also affected by very low mineralization and chemical composition of overlying water and sediment.

Chloride Ions Penetration and their Role in Sediments Corrosion of Buried Steel Structures in Closed Basin

Buried pipelines and steel structures passes or existed through different kind of sediments and saline water. This paper aims at optimize the best conditions to protect the buried pipelines from corrosive environments. Two ponds are selected for this study because of the big difference in chloride contents. The ponds were subjected to chemical, mineralogical, electrochemical analysis and the moisture content is also considered in this study. The results proved that mild steel is probably used in pond 1 at moisture content of 150% but in case of pond 4 mild steel is totally not recommended in all moisture content.

Influence of the Treatment Method on the Water Yielding Capacity of Natural Water Sediments

Low water yielding capacity of natural water sediments complicates the treatment and recycling processes. It is caused by their peculiar structure and hydroxide composition distinct from other kinds of sediments. Elemental composition of the sediments of treatment facilities has been studied with the use of a scanning electron microscope, the grain-size composition has been determined using light scattering of water suspensions. In order to intensify the sludge dehydration of the water-supply network treatment facilities, the following methods has been investigated: the temperature treatment (freezing with subsequent thawing), the flocculant treatment, and the joint treatment with a flocculant and a filler (vermiculite). The properties and the structures of natural and conditioned sediments have been studied, which allows choosing the optimal treatment method. It has been shown that the use of the polyacrylamide-based flocculant together with vermiculite is a promising technique to increase the water yielding capacity of sediments along with freezing. Optimal amounts of the flocculant and vermiculite have been suggested.

COMPOSICIÓN Y ESTRUCTURA DEL ENSAMBLAJE DE CRUSTÁCEOS BRACHYURA DE LA PLATAFORMA CONTINENTAL DE LA GUAJIRA COLOMBIANA Y SU RELACIÓN CON LA PROFUNDIDAD Y LAS CARACTERÍSTICAS DEL SEDIMENTO

Brachyura crustaceans composition and structure assemblage along be continental shelf of La Guajira and its variation according to depth gradient and sediment characteristics were determined. 44 bottom drag samples were taken at 10 and 50 m depth using a semi globe demersal net, during March and April 2005. 83 species belonging to 21 families were identified; the most important families due to their abundance of specimens and amount of species were Goneplacidae, Portunidae, and Mithracidae; and dominant species were Glyptoplax smithii, with, 12.53 % of total abundance; Portunus spinicarpus, 7.42 % and Nemausa cornuta, 6.84 %. The non-metric multidimensional scaling analysis (NMDS) did not show any well defined sampling station clusters, but meaningful statistical differences were found in the assemblage structure according to depth gradient and the different kinds of sediments. The ANOSIM tests and the ordination analysis (NMDS) pointed out that there is a single assemblage that varies gradually with depth, and to a lesser degree by physical sediments characteristics. The ordination (NMDS) also reveals that the assemblage is more heterogeneous at 10 m than at 50 m depth, possibly as a result of the larger environmental variability close to the coast, in comparison with the environmental stability that characterizes deeper zones.