Clay Mineral Fraction Research Articles

Clay mineral evidence of nepheloid layer contributions to the Heinrich layers in the northwest Atlantic

The clay fraction of four cores drilled in the north Atlantic Ocean was studied at a very high resolution over the last 150 ka in order to record the mineralogical signature of Heinrich events. Factor analysis of clay mineralogy establishes that three independent factors represent the main variations: a `detrital factor' (illite + chlorite + kaolinite), a `smectite factor', and a `mixed-layer factor' (IVML: illite-vermiculite mixed-layered clay). The clay mineral fraction of core SU90-38 drilled in the northeastern Atlantic basin did not record any Heinrich event, whereas large changes in the clay mineral fraction occurred during Heinrich events H1, H2, H4, and H5 in the three cores from the northwestern Atlantic basin (cores SU90-08, SU90-11, and SU90-12). Heinrich layers are characterized by increases in the detrital factor in cores SU90-08 and SU90-11, and sharp increases in the mixed-layer factor in cores SU90-11 and SU90-12. The geographical setting of the cores, the pattern of surface, intermediate and deep water circulation, and the main sources of clay minerals allow recognition of two major mechanisms involved in the deposition of the Heinrich Layers: (1) an increased supply of detrital clay minerals by the icebergs; and (2) a specific input of illite-vermiculite mixed-layer clay minerals by a nepheloid layer.

Integrating Surface and Borehole Geophysics in Ground Water Studies—An Example Using Electromagnetic Soundings in South Florida

Time domain surface electromagnetic soundings, borehole induction logs, and other borehole logging techniques are used to construct a realistic model for the shallow subsurface hydraulic properties of unconsolidated sediments in south Florida. Induction logs are used to calibrate surface induction soundings in units of pore water salinity by correlating water sample specific electrical conductivity with the electrical conductivity of the formation over the sampled interval for a two‐layered aquifer model. Geophysical logs are also used to show that a constant conductivity layer model is appropriate for the south Florida study. Several physically independent log measurements are used to quantify the dependence of formation electrical conductivity on such parameters as salinity, permeability, and clay mineral fraction. The combined interpretation of electromagnetic soundings and induction logs was verified by logging three validation boreholes, confirming quantitative estimates of formation conductivity and...

Adsorption and desorption of bensulfuron-methyl on Italian paddy field soils

A study on bensulfuron-methyl adsorption and desortion processes has been carried out in three different Italian paddy field soils (silt-loam, sandy-loam and loamy-sandy). The kinetic study has shown that equilibrium was reached in the early stages (twenty minutes) of contact between soil and herbicide. The results of the experiments on adsorption fit Freundlich's equation. K oc values have shown that bensulfuron-methyl adsorption mainly regards the clay mineral fraction. The desorption of the pesticide from sandy-loam and loamy-sand soil has resulted to be about 50% when soil washing is carried out with a water volume 7.5 times greater than soil mass, while the amount released from silt-loam soil is 21%.

Permeability prediction in anisotropic shaly formations

Abstract We present a unified model for simulating the permeability and electrical conductivity of anisotropic shaly formations. The model is based on Willis’ formulae and the concept of a host medium, the selection of which is crucial in predicting these transport properties. Different rock components, including shales and mudrocks, are characterized by parameters typifying their pore geometry, namely the aspect ratio, size and orientation distribution of the pores. In this regard the model is an extension of the elastic model of Xu & White for predicting P- and S-wave velocities in siliciclastic rocks. The electrochemical effect of clay minerals on electrical conductivity is simulated by Waxman & Smits’ model. A novel feature of the permeability model is that its percolation factor is estimated by a nonlinear transformation of the percolation factor found from conductivity measurements. The model was tested on the laboratory measurements published by Waxman & Smits. Comparison of the results with those from the Waxman & Smits, Dual-Water, and Kozeny-Carman models, and with multilinear and non-linear regression techniques, demonstrated that the unified model predicted conductivity and permeability more accurately than any of these models from the same number or fewer parameters. The improved prediction was most noticeable in samples containing a significant clay mineral fraction. Apart from Waxman & Smits’ data, we have found no published dataset that is comprehensive enough to test physical predictions of both conductivity and permeability.

Forms of fallout 137Cs and 239+240Pu in successive horizons of a forest soil

In order to understand better the mobility of Pu and Cs in a spruce forest soil, the forms of these radionuclides were examined by sequential extraction analysis (modified Tessier procedure). In this way the fractions ‘readily exchangeable’ (fraction I), ‘bound to sesquioxides’ (fraction II), ‘bound to organic matter’ (fraction III), ‘persistently bound’ (fraction IV), and ‘residual’ (fraction V) of fallout 239 + 240Pu and 137Cs were assessed at 5 plots in the top organic horizons (Of1, Of2, Oh) and in the subsequent 4 layers of the mineral soil (0–2, 2–5, 5–10, and 10–20 cm). The results show that 137Cs in the organic layers is present throughout all fractions although mainly in the clay mineral fractions IV and V. Within these layers, a significant dependence on depth is observed for this radionuclide in fraction I (increase with depth) and infraction V (decrease with depth). 239 + 240Pu is associated in the organic horizons essentially with fraction III. In the mineral horizons, 137Cs is found mainly in fractions IV and V, 239 + 240Pu in fractions III and IV. A significant dependence on depth is present for 137Cs in fraction I (continuous increase with depth) and fraction V (decrease with depth). This indicates that radiocesium is presently more mobile in the deeper soil layers. 239 + 240Pu decreases with depth in fraction III, but increases simultaneously with depth in fraction IV. This could be due to the decreasing amount of organic matter present in the deeper layers of the mineral horizons and to changes in the organic matter structure (e.g. loss of functional groups). The results are also discussed in relation to the long-term vertical migration rates as determined recently for both radionuclides in the same soil, and with respect to possible reclamation procedures.

Carbon Tetrachloride Retention by Modern and Buried Soil A Horizons

Carbon tetrachloride (CCl4) was commonly used as a fumigant for stored corn (Zea mays L.) and other grains. This practice and other industrial uses have resulted in instances of soil and groundwater contamination. To better understand the potential for downward CCl4 movement, retention by a buried soil A horizon was compared with that of the modern A horizon in that same location and its loess C parent material. Batch equilibration was used to determine sorption coefficients (Kd) to compare retention among horizons. Mean Kd values were 0.83 (standard deviation [SD] = 0.18) for the modern A, 0.36 (SD = 0.10) for the loess C and 0.41 (SD = 0.07) for the buried A. Mean organic carbon (OC) contents were 14.9 g kg−1 (SD = 2.6) for the modern A, 1.3 g kg−1 (SD = 0.50) for the loess C, and 5.3 g kg−1 (SD = 0.60) for the buried A. Sorption coefficients were most strongly related to percent OC (R = 0.91 across all soils). Log OC coefficients (log Koc) were 1.74 (SD = 0.04) for the modern A, 2.43 (SD = 0.18) for the loess C, and 1.89 (SD = 0.10) for the buried A. The OC coefficient varied with OC content of the horizons. This variation may be attributed to qualitative chemical differences in the organic matter fractions. This may also be caused by the increased importance of the clay mineral fraction at low OC contents, leading to falsely high Koc values. The presence of a buried A horizon may help retard movement of CCl4 to groundwater.

A geotechnical survey of some British Coal Measures mudstones, with particular emphasis on durability

Samples of fresh mudstones were obtained from near Tow Law and Kirkheaton, and of weathered mudstones from near Wrexham. The mineral composition of these mudstones was similar, kaolinite being the principal clay mineral, the clay mineral fraction dominating. Quartz was the chief non-clay mineral present. Some basic geotechnical and index properties were determined. The fresh mudstones from near Tow Law had a low plasticity, as did the moderately and highly weathered mudstones from near Wrexham. However, the completely weathered mudstones from the latter area possessed intermediate plasticity. The strength of the fresh mudstones from near Kirkheaton was notably higher than that of the mudstones from near Tow Law. Both groups of fresh mudstones displayed significant anisotropy ratios when they underwent axial and diametral point load testing. This, no doubt, was due to the influence of lamination. As durability is one of the most important engineering properties of mudstone, it was assessed by a number of tests. One cycle slake durability testing did not appear to offer an acceptable indication of the durability of mudstones when compared with cyclic wetting and drying, cyclic freezing and thawing, and geodurability classification. Cyclic slake durability testing and comparison with unconfined compressive strength would seem to be a more reliable way of determining durability. Both fresh and weathered mudstones underwent little expansion when subjected to swelling tests. However, in the case of the weathered mudstones, the amount of swelling stress and strain could be related to basic geotechnical properties.

Clay mineralogy and pedogenesis of volcanic ash soils influenced by tropospheric eolian bust in Changbaishan, Sanjiaolongwan, and Wudalianchi, Northeast China

Clay mineralogical characteristics of volcanic ash soils in Changbaishan and Sanjiaolongwan, Jilin Province and Wudalianchi, Heilongjiang Province, Northeast China, were investigated. The clay mineral fractions of 8 Changbai-shan pedons, 1 Sanjiaolongwan pedon, and 4 Wudalianchi pedons were characterized by the predominance of 2:1 layer silicates, kaolinite, fine-grained quartz, and opaline silica in the surface (O, OA, A, and AE) horizons. However, the C horizons of some pedons in Changbaishan were characterized by the presence of allophane and ferrihydrite, which are weathering products from Holocene tephra deposits. The Bw and BC horizons exhibited intermediate mineralogical properties between the surface horizons and the C horizon. We assumed that the predominance of 2:1 layer silicates, kaolinite, and fine-grained quartz in the surface horizons of pedons was due to the addition of tropospheric eolian dust transported from arid and semiarid regions in China and Mongolia. Oxygen isotope ratios of quartz samples substantiated this interpretation. Opaline silica identified in the surface horizons could be formed from the soil solution containing Si released from tephra by weathering. The possibility that the pedogenic process of volcanic ash soils was associated strong with the influence of tropospheric eolian dust under the cool or cold environment of Northeast China was demonstrated on the basis of clay mineralogical properties, quartz contents, and oxygen isotope ratios of fine-grained quartz.

Mineralogy and geochemistry of surface sediments from the bengal fan, Indian ocean

The mineralogy and composition of 75 sediment samples from the Bengal Fan. Andaman Sea and the eastern flanks of the upper Nicobar Fan have been determined. Sediments from the Upper and Middle Bengal Fan are mainly terrigenous and derived from the Ganges Brahmaputra river system, whereas sediments from the Andaman Sea and the eastern flank of the upper Nicobar Fan vary from terrigenous to largely biogenic. For the terrigenous sediments, the continental influence is illustrated by the distribution of the major elements (excluding Ca) which decrease away from the source. The Ti/Al and Fe/Al ratios of the sediments are higher in samples from the cast coast of India than from the fans, indicating the influence of material from the Deccan basalts as well as from mafic rocks of the Indian Peninsula. In the clay mineral fraction of Upper Bengal Fan sediments, illite and chlorite are the dominant constituents, with minor amounts of smectite and traces of kaolinite. Illite is mainly a weathering product of lithic material from the Himalaya and is transported by the Ganges Brahmaputra river system. Smectite is the dominant constituent of the sediments adjacent to the Indian Peninsula.

Diferenciación textural y mineralpogica de los minerales de Terrazas y bajos de la Cuenca del Río Guadalete.

Various terrace leveis of the present day course of the Guadalete viva have been studicd with the aim of finding differences between the deposits. Mineralogical ~rnd xnorpboscopic features of the quartz gratis diflerences xx'ere found between the low ano high terrace leveis. Although Ihe mineralogical associations of both clay aud hcavv mineral fractions are simular iii the two leveis: (smectite, illite and kaolinite+cblorite) aud (garnet,zireon, epidote ant! andalusite) respectively. The lower leveis are constituted by sands with 60% of quartz aud 21% of calcite, while ID the highest leveis grave) dominates wliich is formed by pebbles of limestone, sandstone, chert aud ophitic materiais, from the Cordillera Hetica, being the sandy fraction most quartzitic (80% of quarf.z and 1 2% of caRite). Tbis is interpreted as a consecuence of the presence of different materlais along the river's eonrse aud also because the evolution of tIie hasin from a tuctonicly active pitase fo more stable one. The predominance of angular quartz grains witb mechanic superficial textures (conehoidal fractures) jo the lower terrace leveis ana subronuded gratis with suJ)erhcial textures of chemical origin (disolution aud deposits of silica> iii the highest leveis, sliow thc biggest evolution grade of fliese aud the beginning of an edafie phase.

Heavy‐metal pollution in surficial sediments from the Southern Baltic sea off Poland

Abstract The Gulf of Gdańsk is one of the pollution “hot spots”; of the Baltic Sea. In order to investigate heavy‐metal pollution there, 31 bulk (<2 mm), surficial sediments from the Gulf of Gdańsk and the southern Baltic Sea were analyzed for 20 elements. Cobalt, Ni, Cs, Rb, Fe, Mg, Li, and K covary with Al and display an enrichment factor EFAl M of about one, indicating that these elements occur in the sediments dominantly in the clay mineral fraction. Zinc, Cd, Pb, and Ag display an enrichment factor much greater than one and are thought to be dominantly anthropogenic in origin. Grain size plays a significant role in determining element concentrations within the sediments. Systematic trends in the distribution of the heavy metals away from the mouth of the Vistula River were therefore not observed in bulk sediments. However, R‐mode factor analysis discriminated between sediments taken from near the mouths of the Vistula and Reda Rivers and the other sediments and indicated that sediments from near the mouths of those rivers are the most polluted with heavy metals. Present evidence suggests that the heavy metals are introduced into marine sediments in part from atmospheric sources.

Clay Mineral Diagenesis and Thermal History of the Thonex Well, Western Swiss Molasse Basin

AbstractResults are presented of a diagenetic study from the 1300 m thick Oligocene Molasse deposits penetrated by the Thônex geothermal exploration well (Geneva, Switzerland). The x-ray diffraction (XRD) studies of fine-grained rocks indicate the following diagenetic changes: a decrease of illite/smectite (US) expandability from approximately 90% to 30% with depth, a decrease of the amount of US in the clay mineral fraction, and the appearance of corrensite at depths &gt;750 m. The transition from random US to ordered I/S occurs at the base of the Thônex well (1200 to 1300 m) and is associated with a coal rank of about 0.7% Rr (mean random vitrinite reflectance) corresponding to paleotemperatures of 110 to 115 °C Corrensite appears at a vitrinite reflectance value of 0.6% Rr and a corresponding paleotemperature of 100 °C. The amount of post-Molasse erosion is estimated to be approximately 2 km. Thermal history modeling of the Thônex well suggests maximum paleotemperatures of 80 to 115°C and an average paleogeothermal gradient of 27 °C/km during Late Miocene maximum burial conditions.

Effect of organic cosolvents on adsorption of organophosphorus pesticides by soils

A study was conducted to describe the adsorption and mobility of diazinon and acephate in soils from aqueous medium and mixtures of methanol-water and hexane-water. Pesticide adsorption from aqueous system was related to organic matter content ( viz. diazinon), and to content and composition of clay mineral fraction ( viz. acephate). In the methanol-water systems, the adsorption of diazinon and acephate by soils decreases. In the hexane-water mixtures the adsorption of diazinon decreases, whereas the adsorption of acephate increases.

Alterations in the Non-Clay-Mineral Fraction of Pelitic Rocks Across the Diagenetic to Low-Grade Metamorphic Transition, Ouachita Mountains, Oklahoma and Arkansas

ABSTRACT The transformation of smectite to illite has been cited by many authors as a source of silica during diagenesis of mudrocks. Illites themselves, however, undergo chemical changes as they recrystallize into micas during high-grade diagenesis/low-grade metamorphism. Average compositions of phyllosilicates from the literature suggest that an equivalent amount of silica is available from transformation of illite to muscovite as from illitization of smectites. The fate of silica released by this process has not been reported, but could be a major contributor to the silt-size quartz population. The quartz and feldspar fraction of pelites from the Stanley Shale (Mississippian) in the Ouachita Mountains of Oklahoma and Arkansas was separated using sodium bisulfate fusions. The mineralogy and the grainsize distribution of this fraction were determined using standard petrographic and X-ray diffraction (XRD) techniques. Bulk rock samples were analyzed using X-ray fluorescence (XRF) and instrumental neutron activation analysis (INAA) methods. The data obtained were related to illite crystallinity and vitrinite reflectance as reported by Guthrie et al. (1986) and Houseknecht and Matthews (1985). Both the percentage of quartz and the mean grain size of the quartz and feldspar fraction increase with greater illite crystallinity values. The growth in quartz is especially apparent in the finest size fractions. A corresponding decrease in the silica content of the clay-mineral fraction is also observed. Development of quartz polycrystallinity occurs across the same interval. Whole-rock chemical abundances show no statistical correlation with thermal maturity. Relative to titanium, both the major-element and the trace-element concentrations show little variation. Rare-earth element ratios do not correlate with thermal maturity and remain essentially constant. Our results are consistent with reported differences between quartz in schists and their shale precursors, and suggest that release of silica during diagenesis of phyllosilicates continues after the smectite-illite transformation. This silica precipitates as quartz within the pelite, consistent with the suggestion by Blatt (1987) that metapelites are the source of abundant silt-size quartz. The lack of whole-rock chemical variation with thermal maturity implies closed-system behavior across much of the pelite-to-metapelite transition.

Characteristics of sediments in an altitudinal sequence of lakes in the Venezuela andes: Climatic implications

Analyses of water and sediments from lake basins along an altitudinal transect in the Central Andes of Venezuela (elevation range 1100 to 3700 m) suggest that some geochemical and clay mineral characteristics follow distinct altitudinal patterns. The distributino of extractable sedimentary iron shows that organically-bound iron dominates in the sediments of high-elevation lake basins in the Tierra Helada climatic zone (cold and humid, alpine). In the Tierra Templada zone (warm and wet, temperate), amorphous iron is most abundant and the concentration of organic matter is at a maximum. In the lowest lake basin, located in the Tierra Caliente zone (hot and semi-dry, subtropical), amorphous and crystalline iron show similar abundances. Illite and chlorite dominate the clay mineral assemblage of the high elevation (> 3400 m) lake sediments. At lower elevations (around 2000 m), gibbsite and kaolinite become the primary constituents of the clay mineral fraction, but in the lowest lake basin, at 1100 m, illite reappears as the main clay mineral. The observed variations in lake sediments and water chemistry can only be partially explained by the bedrock composition and other physical properties of the lake basins. It is suggested that, to a great extent, these characteristics are controlled by weathering processes in the drainage basins.

Paleoclimatic and structural environment at the end of the Cretaceous along the western flank of the Congo basin, with application of underground microdiamonds around Brazzaville

In Brazzaville (R.P. Congo), the study of outcrops and well sections permitted an investigation of the sedimentary evolution of the Continental Paleoenvironments during the Cretaceous and Tertiary, along a vertical thickness of 70 m (so-called Stanley Pool Formations). The base of the deposits is made of reworked red Precambrian sandstone of Inkisi which is associated with an argillaceous phase with dominant montmorillonite.The late-Cretaceous involved a renewal of erosion which was manifested by the deposition of coarse sands of alluvial-flats where general metamorphic minerals are associated with relatively important evidence of microdiamonds. Climatic changes indicate a more ferralitic pedogenesis of the drainage basins as evident by an increase in kaolin content. Towards the top of the section and the beginning of the Tertiary, kaolin tends to dominate the clay mineral fraction. The metakimberlites of Gabon and the diamond concentrations of Bouenzien at the flank of the granitic massif of Chaillu are considered as possible origins of the microdiamonds.

Absence of Altonian Glaciation in Illinois

AbstractIllinois was free of glacier ice from about 130,000 to 25,000 yr B.P. Deposits of this time interval in northeastern Illinois, represented by core samples from test-hole ISGS S-30, are composed of leached, nonglacigenic, stratified, or pedoturbated silty sediments that typically are organic-rich and pedogenically modified. These deposits overlie the Sangamon Soil, which is identified by its stratigraphic position, soil morphology, and distinct alteration products in the clay-mineral fraction, including a variably swelling vermiculite-like phase and a randomly interstratified kaolinite/10 Å phase. The regional extent of these deposits suggests that if Altonian ice existed in the Lake Michigan basin, it did not extend westward beyond the cuesta of Silurian dolomite that rims the southwestern shore of Lake Michigan.

Compositional Variation in Clay Mineral Fraction of Fine and Coarse-Grained Units in Westwater Canyon Member (Morrison Formation, San Juan Basin, New Mexico): ABSTRACT

Smectites and mixed-layer illite/smectite clays from a suite of core samples drawn from the Westwater Canyon Member of the Morrison Formation (USGS borehole S6) have been examined using analytical electron microscopy (AEM) and x-ray diffraction. Samples from 784 and 808 m deep are fine and coarse-grained, respectively. An intermediate sample (806-m depth) is a sandy siltstone with conspicuous shale intraclasts. The fine and mixed grain-size samples are characterized by illitic clays (14% expandable, ordered; 21% expandable, random). The coarse-grained sample is, by contrast, highly expandable (96%). All AEM analyses were performed at {minus}150{degree}C to minimize electron beam damage and retard loss of volatile interlayer cations. Interlayer sites for clays in all of these samples are dominated by Na and K with minor Ca. Mixed-layer clays of the fine-grained and intraclast-bearing units are characterized by high K/Na in interlayer sites, and clays of the coarse-grained sample include Na-rich varieties. All of the clays are dioctahedral. The octahedral chemistry of each sample is dominated by Al, Mg, and divalent Fe. The proportion of Al to divalent cations in the octahedral layer ranges from three to four in all samples. Charge on the TOT layer of the clays spans the expected range frommore » illite to smectite. Some of this variation may indicate that detrital illites or mixed-layer materials were present in the fraction analyzed. These data may reflect compositional variation of the diagenetic fluids that percolated through fine and coarse-grained units of the Westwater Canyon member.« less

Cysts and Sediments: Gonyaulax Polyedra (Lingulodinium Machaerophorum) in Loch Creran

The distribution of dinoflagellate cysts in sediments is a subject of increasing interest. There have been several approaches, of which two are relevant to this investigation. The studies of palynologists who endeavour to relate the distribution of cyst assemblages to environmental factors (for example Reid, 1972; Wall et al. 1977; Harland, 1983) and the studies of phycologists, concerned with toxic dinoflagellate blooms, who concentrate on the distributions of cysts of bloom species (for example Anderson, Kulis et al. 1982; Thayer et al. 1983). Consequently an increasing amount of information on quantitative distribution of cysts has become available. In 1976 Dale reported the investigation of sediments from Trondheimsfjord, relating the abundance of cysts to the percentage abundance of finer sediment. Wall et al. (1977) provided a more comprehensive picture of this relationship showing that cyst density (numbers of cysts per gram dry weight of sediment) increased logarithmically with increased percentage of the silt plus clay mineral fraction to a level where this comprised 50–60% of the sediment, after this no further increase occurred.

Geochemical signature of provenance in sand-size material in soils and stream sediments near the Tobacco Root batholith, Montana, U.S.A.

Trace-element geochemistry of sandstones are being used to determine provenance. We have conducted preliminary and limited experiments to determine to what extent daughter sands retain the geochemical signature of parent rocks. Six sets of first-order stream sediments, soils from adjacent slopes, and a variety of parent rocks were collected from southwestern Montana, U.S.A. Sampling in a low-relief area ensured that climate and residence time of soils on slopes could be eliminated as variables. Sand-size fractions of stream sediments and soils, and the corresponding parent rocks (granodiorite, quartz monzonite, granite gneiss, biotite-tonalite gneiss and amphibolite) were analyzed for most major elements and selected trace elements. Petrologic modal analysis of the parent rocks and the 0.25–0.50-mm fraction of each sand was done to monitor major mineralogic control, if any, on chemical compositions of the samples. Our data show that the abundances of the Si and Al in sediments do not discriminate provenance. Abundances of Ca, Mg, Fe and Ti may broadly distinguish between sands derived from metamorphic and igneous source rocks, at least in the area studied. Differences in abundances of the Ba and Th, and the ratio of La/Lu between granitic, tonalitic and amphibolitic parent rocks are preserved in the daughter sediments that we studied. However, the size of the Eu anomaly in the REE patterns of different daughter sediments is not diagnostic of parent rocks. Abundances of Co and Sc distinguish between sediments derived from felsic and mafic rocks. A better provenance discrimination is obtained if the ratios La/Sc, Th/Sc, La/Co, Ba/Sc and Ba/Co are used. Petrologic modal data show that mineral contents and chemical compositions of parent rocks are compatible with each other. The chemical composition of the sands may be roughly correlated to the petrological modal data but the abundances of some minor and trace elements of sediments cannot be inferred from modal mineralogy. This is expected because these elements may concentrate in accessory minerals and/or may weather out into aqueous or clay mineral fractions; it is also compatible with conclusions of previous studies that some of these elements do not reside in sand-size fractions of siliciclastic sediments.