Grain Size Classes Research Articles

Pathways of clay mineral transport in the coastal zone of the Brazilian continental shelf from Ceará to the mouth of the Amazon River

The transport pathways of fine sediments (fraction <2 μm) along the Brazilian continental shelf from Ceara to the Amazon River mouth were studied by means of clay mineral analyses. On the continental shelf southeast of the Amazon mouth, fluctuations in clay mineral compositions reflect simple mixing between the suspended load of the North Brazil Current and sediment from several smaller rivers. Previously, clay mineral variations west of the Amazon mouth have been explained by variable settling velocities of different grain-size classes within the <2 μm fraction or by selective coagulation of individual clay mineral groups. By contrast, our experiments with river bank samples show that selective coagulation does not occur in Amazon River sediments. A more appropriate explanation for observed variations in clay mineral composition off the Amazon mouth seems to be, similarly to that for the shelf between Ceara and the Amazon mouth, a mixing of Amazon sediments with suspended material of the North Brazil Current. This interpretation is supported by data on clay mineral composition east and south of the Amazon mouth, showing more affinity to sediments of the North Brazil Current than to the suspended load of the Amazon River. Additionally, relatively low sedimentation rates and low concentrations of fine-grained sediments on the shelf suggest that high riverine input by the Amazon River does not overprint the sediments of the North Brazil Current in this region. The strong North Brazil Current shunts the Amazon suspended load in a north-westerly direction along the north-eastern coast of South America. Hence, stronger sedimentation of Amazon sediments would occur only west of the river mouth.

Recycling of construction and demolition waste materials: a chemical–mineralogical appraisal

Building activity is currently demanding remarkable amounts of inert materials (such as gravel and sand) that are usually provided by alluvial sediments. The EU directives and Italian Legislation are encouraging the re-use of construction and demolition waste provided by continuous urban redevelopment. The re-utilisation of building waste is a relatively new issue for Italy: unfortunately the employment of recycled inert materials is still limited to general bulk and drainage fills, while a more complete re-evaluation is generally hampered by the lack of suitable recycling plants. In this paper, chemical–mineralogical characterization of recycled inert materials was carried out after preliminary crushing and grain-size sorting. XRF and XRD analysis of the different grain-size classes allowed us to recognise particular granulometric classes that can be re-utilised as first-order material in the building activity. Specifically, the presented chemical–mineralogical appraisal indicates that the recycled grain-size fraction 0.6–0.125 mm could be directly re-employed in the preparation of new mortar and concrete, while finer fractions could be considered as components for industrial processing in the preparation of cements and bricks/tiles.

Control of modern dinoflagellate cyst distribution in the Irish and Celtic seas by seasonal stratification dynamics

Surface sediments from seven stations located in the seasonally stratified, frontal and mixed water regions in the Celtic and Irish seas have been analysed for their dinoflagellate cyst assemblages and dinosterol content. A total of 45 dinoflagellate cyst taxa have been identified and the assemblages related to surface and sediment conditions. Sediments from the mixed water region, at 30 m water depth, are characterised by a relatively low cyst concentration (∼2300 cysts/g dry weight) and high relative abundances of Lingulodinium machaerophorum accompanied by Spiniferites membranaceus, Brigantedinium spp. and Dubridinium caperatum. Assemblages from stratified and frontal water stations are dominated by Spiniferites ramosus associated with Operculodinium centrocarpum, Brigantedinium spp., cysts of Polykrikos schwartzii and Selenopemphix quanta. Ordination techniques performed on a restricted number of 35 taxa from the assemblages differentiated the stratified and frontal assemblages based on the abundance of the less abundant species Bitectatodinium tepikiense and Spiniferites elongatus. Among the environmental parameters (sea-surface temperature and salinity, stratification index, chlorophyll concentration and sediment grain-size classes), the seasonal stratification and sedimentological context, itself a function of tidal dynamics, explain most of the variance in the environmental conditions. These results indicate that dinoflagellate cyst analyses of shelf sediment records can be used to document the planktonic signal of seasonal stratification dynamics.

Lead in grain size fractions of road-deposited sediment

Road-deposited sediment (RDS) is an important environmental medium for assessing contaminant levels in urban systems. Their atmospheric resuspension has significant implications for human health, and storm water transport can directly impact aquatic biota. Data from 20 RDS samples from Palolo Valley, Oahu, Hawaii, were fractionated into six grain-size classes and analyzed for Pb using a weak HCl (0.5 M) digestion. Data indicate significant Pb contamination in all samples. Median labile Pb concentration ( n=120) was 170 mg/kg, with a range from 4 to 1750 mg/kg. The five sediment fractions <1000 μm had statistically similar Pb concentrations, but all were significantly greater than the coarsest fraction examined (1000–2000 μm). Silt plus clay (<63 μm) was the single most important mass component with 38% of the total sediment stored in this fraction. Mass of sediment <63 μm combined with a median labile Pb concentration of 222 mg/kg accounted for 51% of the total Pb load stored in road sediments. These findings are significant from an environmental management perspective, and these issues are discussed in light of street sweeper sediment grain size removal efficiencies.

The Influence of Allochthonous Leaf Detritus on the Occurrence of Crustacean Detritivores in the Soft-bottom Macrobenthos of the Po River Delta Area (northwestern Adriatic Sea)

Core samples were examined quarterly at two coastal sites (S1 and S2) and at an offshore station (S3) located in the Po River delta area (northwestern Adriatic Sea). Analyses focused on (i) occurrence of coarse detritus of allochthonous origin in the sedimentary matrix and (ii) the relative influence of macrodetritus enrichment and other environmental factors on the vagile macrofauna.Plant debris occurred in site S1 sediments only in summer and autumn; in contrast, fragments of the phanerogams Cymodocea nodosa and Zostera spp. were found in site S2 sediment throughout the sampling period. Sediments from the offshore site S3 were characterized by negligible plant material, even though in summer and autumn samples debris of continental origin was observed. Even though leaf detritus occurrence at site S2 was ∼5-fold higher compared to the other coastal site S1, it did not influence the total organic matter and its distribution among grain-size classes. Conversely, the specific organic content of dimensional fractions provided an effective assessment of detritus enrichment processes occurring at the two coastal sites.A group of brackish-originated crustaceans (i.e. the amphipods Gammarus insensibilis and G. aequicauda and the isopod Idotea baltica) was the main determinant of among-site multivariate differences in the vagile macrofauna; depositivorous ophiuroids accounted for the residual differences observed during the study period. The analysis of taxa abundance and individual body size indicated that in both site S1 and S3 macrodetritus advection to the benthic system corresponded with passive dispersal of brackish crustaceans, that provided a negligible contribution to the macrobenthic production. In contrast, in site S2 allochthonous inputs from marginal environments could have represented the key factor for the persistence of an authochthonous population of Gammarus insensibilis. The amphipod provided a considerable (19·4%) contribution to the total macrobenthic production of the site, dominated by filter-feeding bivalves and tubicolous deposit feeders. The potential effects of detritivorous crustaceans on soft-bottom macrobenthic assemblages of the northern Adriatic Sea are discussed.

Distribution and Geochemistry of Trace Elements in Central Red Sea Coastal Sediments

Fifty-one surficial sediment samples were collected from 21 stations situated in the nearshore area of the central Red Sea. Sediments were analysed for the major elements Ca, Mg, Al and Fe and the trace elements Mn, Cu, Zn, Ni, Cr, Co, Cd and Pb. The major grain-size classes gravel, sand and mud as well as organic carbon and carbonates were determined. Some samples were subjected to X-ray mineralogy. Sediments were generally coarse-grained but the fine fraction increased in the sheltered areas. The major mineralogical constituents of the sediments were carbonate minerals (calcite and aragonite) and detrital silicates (quartz and feldspars). Organic carbon was low (0.38%) in the northern part of the area but greatly increased in areas receiving direct sewage discharge (1-9.5%). Concentrations of trace elements were highly variable and appeared to be related to the grain size and the mineralogical composition. Generally, trace element concentrations were positively correlated with the mud and Al and Fe contents. In the polluted sites these regular associations were perturbed and trace elements were associated with organic matter. Element/Al ratios were used to group the sampling sites according to their degree of enrichment. The resulting order was different from that based on the absolute concentrations. The use of a combination of the element/Al ratio and the absolute concentration resulted in the distinction of highly and moderately contaminated and uncontaminated sites.

Grain-size distributions of sandy sediments—sieving versus settling

Geografisk Tidsskrift, Danish Journal of Geography 99: 9–17, 1999 Settling analysis are becoming common practice in many sediment laboratories. By analysing sieved sand fractions of varied sample weight in a settling tube, it was found that the optimal sample size for the settling analysis can be calculated as (Sl-units): Sample size = 55.4 dsiv (dtube)2. Grain-size distributions based on settling analysis, differ systematically from those of traditional sieve analysis. The differences between the two methods have been examined in sediments from four different sedimentary environments in the southwest of Denmark. Samples from rivers, the Wadden Sea, the North Sea coast and aeolian dunes gave a mean relationship between sieve diameter and settling diameter: dset, = 0.8169dsiv + 0.3555. This relationship is almost identical to that observed in sediments from the East Frisian Wadden Sea. Thus, the settling diameters are larger than the sieve diameters for sand finer than approximately 2 phi (0.250 mm) and smaller for coarser grain sizes. This trend results from a larger volume diameter in the fine grain-size classes combined with the well known feature that the hydraulic resistance of irregular grains increases as the settling velocity becomes larger. Comparing sieve-based textural parameters with their settling-based counterparts, the following trends were observed: 1) sieve-based grain sizes finer than 2 phi correspond to coarser settling diameters, while coarser sieve-based sand fractions correspond to finer settling diameters; 2) the sorting gets better; 3) generally, fine well-sorted sediments with a negative skewness become more negatively skewed, whereas coarser and less well-sorted sediments become more positively skewed, and 4) the kurtosis of the distributions increases for well sorted sediments. The pattern is very clear for the mean grain size and the sorting, but less obvious, for the skewness and the kurtosis.

The influence of grain size and grain size distribution on methods for estimating paleostresses from twinning in carbonates

This study examines the relative differential stresses around a minor thrust fault within the Mountain City window, Tennessee, U.S.A. The fault zone developed within dolomite rocks and deformation took place by twinning, fracturing, pressure solution and the development of fine-grained deformation zones. Grain-size reduction is observed from undeformed wall rock to the center of the fault zone, and occurred by dynamic recrystallization. Methods to determine paleostresses in naturally deformed rocks from twinning assume a single, coaxial, strain-inducing event. The recrystallization within the center of the fault zone removed the effects of earlier deformation, so that the twinning more closely reflects a single, coaxial deformation event late in the fault history. Two methods were used to estimate the relative differential stresses across the fault zone and the results show opposite trends towards the center of the fault zone. The different results may be partially explained by the influence of grain size, as only one of these methods considers the influence of grain size. In addition, the grainsize data from this fault zone demonstrate that the tendency for a grain-size class to be twinned depends on the grain size distribution. The grain size distribution may result in grain-to-grain stress concentrations that induce twinning. Thus, grain size distribution should also be considered to achieve more accurate estimates of paleostresses.

A one-dimensional model for the transport of a sediment mixture in non-equilibrium conditions

The non-stationary one-dimensional continuity equations of each granulometric class of sediments are written for the waterstream (suspended transport), the bottom layer (bedload transport) and the mixing layer below the bottom surface. Proper closure equations have been introduced to express the transversal sediment (luxes between these three layers, accounting for the different adaptation-length of each grainsize class. A number of examples are given, showing the importance of non-equilibrium for non-uniform particles in erosion-deposition processes at short time-scale.

Partitioning of Sediments Among Shoreface Transport Paths: Analysis of Sediment Dispersal Patterns Using Empirical Orthogonal Functions: ABSTRACT

Recent geomorphic evidence from the inner shelf and shoreface to the east of Long Island's barrier island system indicates that reworking of glacial outwash deposits at the inner shelf-shoreface transition, as sea level rises, may be supplying much of the sediment needed to maintain barrier islands to the west. A conceptual model describing sediment dispersal from outwash source areas was developed from this hypothesis. It was reasoned that outwash sediments ranging from silts to coarse gravels would be subject to differential transport paths across and along the shoreface upon reworking. Coarser grain sizes would move onshore toward the intertidal beach, whereas finer sediments would move offshore. Sand of intermediate grain size would be concentrated in the surf zone an move alongshore in wave-generated longshore currents. To test this model, 400 samples from the beach and shoreface of Long Island were analyzed for grain-size frequency distribution and each grain-size class was examined for frequency of occurrence in the cross-shore and alongshore directions. On a spatially averaged basis, grain-size classes displayed peak abundance in specific zones across the shoreface as predicted by the model, but alongshore trends could not be recognized among the data. Therefore, empirical orthogonal functions (EOF) were used to examine uncorrelated (orthogonal) modes of variability in the occurrence of each grain-size class in the alongshore direction. The first function, representing more than 60% of the variability among the data, showed that grain sizes subject to longshore transport in the surf zone i crease in frequency in the alongshore direction relative to coarser grain sizes. Results also show that peak concentrations of coarse sediments correspond to zones subject to frequent overwashing. It is concluded that EOF analysis of individual grain-size classes holds promise for extracting trends from noisy data sets. End_of_Article - Last_Page 318------------

Generation and dispersal of fine ash and dust by volcanic eruptions

The volcanic eruptions which generate the greatest quantities of fine ash and dust are those of ignimbrite-forming, plinian, vulcanian and phreatomagmatic types; these are also the eruptions which produce the widest dispersal of this material, attributed to the superior height attained by their eruptive columns. However, much of the fine ash and dust may be rapidly flushed out of the eruptive plume by water, particularly in phreatomagmatic eruptions. Recent studies made on the dispersal and grain-size of pyroclastic deposits produced by examples of plinian and phreatomagmatic types, have yielded estimates of the quantities of material generated in each grain-size class, besides the extent of their dispersal. Not all of the fine volcanic particles are produced by fragmentation at the eruptive vent; in ignimbrite eruptions, there is good evidence for their large-scale generation in and loss from the moving ash flows.

Distribution of Si, Mn, Fe, Ni, Cu, Co, Zn, Pb, Mg, and Ca in reain-size fractions of sediment samples from a manganese nodule field in the Central Pacific Ocean

Deep-sea sediment samples from a manganese nodule field of the Central Pacific were separated into eight grain-size fractions. From these fractions chemical analyses were made for the elements Si, Mm, Fe, Ni, Cu, Co, Zn, Pb, Mg, and Ca. Mn, Cu, Ni, and, less significantly, Co and Zn are enriched in the 10–20 μm fraction. Fe and Mg show a continuous increase with decreasing grain size, whereas SiO 2 is characterized by diminution. Pb does not show any correlation with the other elements. More than 50% of each of Mg, Fe, Cu, Zn, and Pb is bound in the < 2 μm fraction. The metals Mn-Ni-Cu-Co on the one hand, and Mg-Fe on the other, show good positive correlations. In the finer fractions clay minerals and colloidal particles are important carrier matters for metals as are micronodules and opaline silica skeletons in coarser grain-size classes. Assuming that all the Mn in samples is fixed to micronodules, our calculations (based on chemical analyses of micronodules) prove that the following proportions of metal are not fixed to micronodules: Ni: 25%, Cu: 63%, Co: 85%, Zn: 82%, Pb: 88%. About 30% of the non-nodular metal amounts are fixed to the > 4 μm fraction. The significance of these results and implications for the metal supply during the growth of macronodules are discussed.

Dispersal and Geochemistry of Surface Sediments in Halls Bay, North-Central Newfoundland: Application to Mineral Exploration

Surficial palimpsest sediments in Halls Bay, north-central Newfoundland, are mixtures of gravel, sand, and mud deposited from a number of sources in varying quantities from late Wisconsinan to the present time. Shallow water gravel originated as till and glacio-fluvial outwash. Gravel in deep water probably is ice-rafted. Sand and mud, which occurs with shallow water gravel and in deeper water, is a combination of fluvial material and material winnowed out of till and outwash by shallow water waves and currents during early marine transgression. There also may be a contribution of fine-grained sediment from the adjacent shelf.Gravel (coarser than [Formula: see text]), very fine sand (3 to [Formula: see text]) and coarse silt (4 to [Formula: see text]) modal grain-size classes predominate in the sediments. The very fine sand mode occurs on the west side of the inlet and the coarse silt mode occurs on the east side regardless of water depth, indicating net or active easterly dispersal of fine-grained sediment. This dispersal path may result from the presence in Halls Bay of a counterclockwise gyre of the Labrador Current that has developed since early transgression, which suggests the sediment surface is adjusting to the Halls Bay modern hydraulic regime.Sandy and muddy sediments are composed of quartz, feldspar, amphibole, illite, chlorite, montmorillonite, organic matter, CaCO3, and FeS. Major, minor, and trace element concentrations vary with grain-size, owing to the different proportions of these components in different size fractions. Calculation of an average chemical composition of sediments is biased because of this grain-size effect. The grain-size effect on chemistry of a suite of sediments can be accounted for by ratioing element concentrations to clay content.Plots of the ratio trace metal concentration/clay content vs. clay content for six trace metals indicate anomalous Cu concentrations occur in surface sediments along the east side of Halls Bay in the direction of fine-grained sediment dispersal. The anomalous Cu is derived from onshore mineralization in Lushs Bight Group volcanic rocks, which Occur along the west side of the inlet.The results provide an example of the applicability of marine sedimentologic/sedimentary geochemical investigations to mineral exploration. Local geochemical anomalies in sediments can be detected by routine analysis of total metal content of bulk samples provided the grain-size effect on chemistry is accounted for. The anomalous metal can be traced to its onshore source by evaluating sediment dispersal paths from textural variations.

Texture of Ice-Cored Deposits from Ten Alaskan Valley Glaciers

ABSTRACT Texture of twenty-six sediment samples from ice-cored deposits at the termini of ten Alaskan valley glaciers was determined. The glaciers are eroding five different types of bedrock. Results of pebble roundness and grain-size frequency distribution analyses reveal two groups of samples: (1) those with mean pebble roundness values in Pettijohn's (1957) subangular class and which do not exhibit distinct grain-size modes (amodal sediment), and (2) those with mean pebble roundness values in the subrounded to rounded class and which exhibit either distinct modal grain-size classes (modal sediment) or pronounced deficiencies in particles finer than coarse or medium sand. The characteristics of samples in the first group suggest these sediments are the product of primary sediment-forming processes associated with valley glaciers (glacial erosion of bedrock and/or frost action on valley walls). These processes produce particles of all sizes irrespective of bedrock type. The occurrence of modal proglacial sediment of valley glacier origin, reported in the literature, is therefore probably a function of the environment of transport and deposition and is independent of the nature of the bedrock type or initially eroded detritus. The characteristics of samples in the second group suggest they are primarily mixtures of former proglacial deposits. Prior to present deposition, proglacial deposits were probably incorporated and mixed in the glacier load during ice advance.