Spatial Distribution Of Grain Size Research Articles

Integrated modeling of the gradient structure evolution during surface mechanical grinding treatment: Formation mechanism and mechanical properties

Gradient nanostructured materials are regarded as a promising class of architectures with tunable mechanical properties, primarily dependent on the optimization of well-controlled fabrication parameters. In this paper, a microvariable-based constitutive model is incorporated into an integrated finite analysis technique. This approach correlates the fabrication parameters of surface mechanical grinding treatment (SMGT) with the corresponding measured mechanical properties of gradient structured (GS) materials, quantifying the relationship between process parameters, microstructure, and mechanical properties. Through theoretical prediction and experimental verification, it is observed that grain refinement, mechanical strength, and surface hardness are enhanced by increasing processing times and reducing path spacing. The yield stress of the fabricated GS material ranges from 126.8 MPa to 162.2 MPa, an increase of above 2.5 times compared to the original material, with a slight decrease in uniform elongation by a factor of 25.8 %, indicative of an excellent strength-ductility trade-off. The underlying mechanism for improved strength-ductility synergy is discussed, focusing on the importance of the tunable spatial distribution of grain size. This work sheds light on the potential application of gradient nanograined structures at an industrial scale and advances the fundamental understanding of strengthening mechanisms in gradient nanostructures.

Optimal grain size distribution in gradient nano-grained nickel

Gradient nano-grained (GNG) metals are a unique class of metallic materials by manipulating the gradient size distribution, but the optimal grain size distribution producing the best strength–ductility synergy is still unclear. Here, the mechanical behavior and deformation mechanism of GNG Ni with different spatial distribution of grain size varying in the inverse Hall-Petch regime are investigated by molecular dynamics method. The results show that the GNG structure has the optimal strength–ductility synergy when the distribution of grain size gradient satisfies a linear relationship (i.e. the gradient rate n = 1). The strengthening and toughening mechanisms of the optimal GNG structure are further illustrated. It is found that the stress gradient and strain gradient in the GNG Ni with gradient rate n = 1 are the largest among different gradient structures. The optimal strength-ductility synergy in GNG structures with linear distribution of grain size is attributed to the remarkable enhancement of dislocations and the diminution of GB activities. These results would provide an insight into the mechanical performance tuning of GNG structures experimentally.

The spatial distribution of sedimentary compounds and their environmental implications in surface sediments of Lake Khar Nuur (Mongolian Altai)

AbstractLake sediments are valuable natural archives to reconstruct paleoclimate and paleoenvironmental changes which consist of inorganic and organic sediment compounds of allochthonous origin from the catchment and of autochthonous production in the lake. However, for robust paleo‐reconstructions it is important to develop a better understanding about sedimentation processes, the origin of inorganic and organic sediment compounds and their distribution within the lake. In this context, modern process studies provide important insights, although environmental and anthropological changes can affect the spatial distribution of sediment compounds through time. Therefore, in this study the spatial distribution of grain size and geochemical proxies in 52 surface sediment samples from Lake Khar Nuur, a small high‐altitude lake in the Mongolian Altai with a small and anthropogenically used hydrological catchment, is investigated. The results show a distinct sediment focussing in the two deep basins of the lake, which therefore act as accumulation zones. In those accumulation zones, total organic carbon (TOC), total nitrogen (N) and their isotopic composition (δ13CTOC, δ15N) as well as n‐alkanes indicate that organic sediment compounds are a mixture of both allochthonous and autochthonous origin. While the recent catchment vegetation consists of grasses/herbs and the shrub Betula nana (L.) with distinct differences in their n‐alkane homologue patterns, those differences are not reflected in the sediment surface samples which rather indicates that grass‐derived n‐alkanes become preferentially incorporated in the lake. Extensive anthropogenic activity such as grazing and housing in the southern part of the catchment causes soil erosion which is well reflected by high TOC, N and sulphur (S) contents and 15N depleted δ15N values at the central southern shore, i.e. increased allochthonous sediment input by anthropogenically‐induced soil erosion. Overall, the surface sediments of Lake Khar Nuur origin from allochthonous and autochthonous sources and are focussed in the accumulation zones of the lake, while their distribution is both environmentally and anthropogenically driven.

The paleoenvironmental significance of spatial distributions of grain size in groundwater‐recharged lakes: A case study in the hinterland of the Badain Jaran Desert, northwest China

AbstractFor lakes in desert hinterlands that are not recharged by river runoff, sediment input solely comes from wind transport. While the processes of sediment transport and deposition in these lakes differ significantly from those with river discharge, the spatial distribution of sediment grain size in these groundwater‐recharged lakes remains largely unknown. Moreover, whether the grain size distribution in these lake sediments can be used as a proxy in the study of past climatic change and environmental evolution studies is unclear. In this study, five lakes with a range of surface areas that had no runoff recharge were selected from the hinterland of the Badain Jaran Desert of north‐western China, and a total of 108 samples of lake surface sediments were collected to examine the spatial distribution of grain size. Moreover, an end‐member‐modeling algorithm was used to calculate end members from all grain size measurements. Our results showed that both the median and mean grain sizes in the lake sediments decreased from the nearshore to the offshore, deep‐water zone. However, the lowest median and mean grain sizes were not found in the center of the lakes, in contrast to lakes recharged by surface runoff. The median grain size of sediment in the lake center was negatively correlated with lake level, and thus could help reveal lake evolution at low resolutions. Moreover, EM1 and EM2 were interpreted as wind transported sediment, and sediment perturbed by lake waves after wind transport, respectively. The modal grain size of EM1 varied slightly between lakes, while changes in the modal grain size of EM2 were related to lake area. Given the positive relationship found between EM2 content and lake level, changes in the EM2 content (%) can serve as a rough indicator of lake level fluctuations at low temporal resolutions. Copyright © 2017 John Wiley & Sons, Ltd.

Factors influencing the accumulation of Pb in sediments of deep and shallow dam reservoirs

Abstract The study determines the differences in Pb accumulation in sediments of dam reservoirs with respect to locations of the old river beds and their depth (deep Czorsztyn Reservoir and shallow Goczałkowice Reservoir in southern Poland). Parameters (grain size, organic matter content and reservoir depth) that may influence the spatial distribution of Pb concentrations in the sediments were analyzed. Based on the hierarchical cluster analysis, sediment samples from the reservoirs were ranked with respect to particle size distribution. We found differences in the spatial distribution of grain size and organic matter in the studied reservoirs, caused mostly by the topography of these reservoirs. The spatial distribution of Pb concentrations in the sediments of the Czorsztyn Reservoir (range 4.8-35.8 μg g-1) and the Goczałkowice Reservoir (range 11.3-59.4 μg g-1), regardless of their average depth and their type (dimictic, polymictic), depends on the distribution of silty clay and clay fractions, organic matter and reservoir depth. Therefore, Pb spatial distribution was more regular in the Czorsztyn Reservoir than in the Goczałkowice Reservoir. Locations of the old river beds had a significant impact on the Pb distribution in the sediments of both reservoirs.

Can high resolution 3D topographic surveys provide reliable grain size estimates in gravel bed rivers?

High resolution topographic surveys such as those provided by Structure-from-Motion (SfM) contain a wealth of information that is not always exploited in the generation of Digital Elevation Models (DEMs). In particular, several authors have related sub-metre scale topographic variability (or ‘surface roughness’) to sediment grain size by deriving empirical relationships between the two. In fluvial applications, such relationships permit rapid analysis of the spatial distribution of grain size over entire river reaches, providing improved data to drive three-dimensional hydraulic models, allowing rapid geomorphic monitoring of sub-reach river restoration projects, and enabling more robust characterisation of riverbed habitats. However, comparison of previously published roughness-grain-size relationships shows substantial variability between field sites. Using a combination of over 300 laboratory and field-based SfM surveys, we demonstrate the influence of inherent survey error, irregularity of natural gravels, particle shape, grain packing structure, sorting, and form roughness on roughness-grain-size relationships. Roughness analysis from SfM datasets can accurately predict the diameter of smooth hemispheres, though natural, irregular gravels result in a higher roughness value for a given diameter and different grain shapes yield different relationships. A suite of empirical relationships is presented as a decision tree which improves predictions of grain size. By accounting for differences in patch facies, large improvements in D50 prediction are possible. SfM is capable of providing accurate grain size estimates, although further refinement is needed for poorly sorted gravel patches, for which c-axis percentiles are better predicted than b-axis percentiles.

Spatial distribution of grain size and depositional process in tidal area along Waikelo Beach, Sumba

ABSTRACTA study of characteristics and distribution of sediment along Waikelo Beach, Sumba, Nusa Tenggara Timur (NTT), Indonesia has been conducted using grain size train analysis. Grain size train analysis aims to determine physical properties and textural parameters such as mean, sorting, skewness, and kurtosis. Samples were taken from several locations on the beach that represent high-tide, transition, and low-tide areas. The observation of two physical characteristics has concluded two points: (1) changing in textural pattern that shows physical differences of sediments is a result of interaction between deposition process and marine activities; (2) the abundance of coarse to medium sand demonstrates general process of sediment deposition along Waikelo Beach deposited at moderate to low energy. Linear discriminate function indicates aeolian, shallow marine, and fluvial environments. CM diagram (C = one percentile in micron, M = median in micron) describes the mechanism of sediment deposition on the beach in rolling and ground suspension. These all conditions are supported by the plotting results in the Stewart diagram that shows the influence of current either from waves or from river.

Mid-late Holocene climate variability in the Indian monsoon: Evidence from continental shelf sediments adjacent to Rushikulya river, eastern India

We present elemental and grain-size distributions obtained from the sediment core of the continental shelf adjacent to the Rushikulya river mouth, eastern India to quantify the paleoclimatic changes. The retrieved 1.60 m long well dated core spans the past ca. 6800 cal BP. The modern spatial distribution of grain size and geochemistry of the inner-mid shelf sediments has been carried out to understand the seafloor morphology and sedimentary processes. Based on the modern investigations, the proportion of particle size (clay vs sand) and variation in elemental values (TiO2 vs Al2O3) has been used to interpret the changes in terrigenous supply. The grain-size and elemental distribution data from the core sediments indicates a period of enhanced surface water runoff from 6800 to 3100 cal BP followed by a drier condition (3100 cal BP to present) suggesting weakening of monsoon. The weakening of the monsoonal strength is coeval with other records from the Indian sub-continent and suggests response of Indian monsoon to changing solar insolation during late Holocene.

Evaluation of environmental status and geochemical assessment of sediments, Manasbal Lake, Kashmir, India

The present study was conducted on the Manasbal Lake (34°14′N: 74°40′E) to assess the geochemical characteristics of the lake bottom sediments, its environmental implications and its response in the local catchment area. This study tracks the spatial distribution of grain size, geochemical analysis, C/N ratio, calcium carbonate (CaCO3) and organic matter (OM) of the lake bottom sediments. It is observed that the clay fraction (49.79%) is predominant in the lake bottom sediments, followed by silt (35.88%) and sand (14.33%) and its spatial distribution is controlled by water depth. Geochemistry and normalized diagrams for the major oxides and trace elements reveal enrichment of CaO, K2O, P2O5, S, Cl, Ni, Zn and Sr. Chemical index of alteration (CIA) reflects low to moderate weathering intensity and near compositional similarity with the bedrock exposed in the catchment area around the lake. Environmental indices (EF, Igeo) suggest that the sediments are enriched in Cu, Ni, Zn, Cr, Co, Pb followed by Mn content. Pollution load index (PLI) reveal that all the sampling sites reflect low to moderately polluted category except for few stations that are towards the southern and southeastern side of the lake. OM (16.85%), CaCO3 (14.04%) and C/N ratio (15.5) of the lake bottom sediments is attributed to high organic activity within the lake, shell fragments, contributions from the lake flora and fauna adhering to the clayey silty sediments. The C/N ratio of 15.5 suggests a mixed source of organic matter both terrestrially and in situ formation within the lake. Sulphur and chlorine are high amongst the trace elements suggesting anthropogenic detritus input into the lake and this is due to the chemical fertilizers from the agricultural runoff and organic load into the lake. Thus, the present study suggests that in order to preserve the pristine lake ecology and the environment; continued monitoring and restoration efforts need to be undertaken.

Economic minerals of the Kovdor baddeleyite-apatite-magnetite deposit, Russia: mineralogy, spatial distribution and ore processing optimization

The comprehensive petrographical, petrochemical and mineralogical study of the Kovdor magnetite-apatite-baddeleyite deposit in the phoscorite–carbonatite complex (Murmansk Region, Russia) revealed a spatial distribution of grain size and chemical composition of three economically extractable minerals — magnetite, apatite, and baddeleyite, showing that zonal distribution of mineral properties mimics both concentric and vertical zonation of the carbonatite-phoscorite pipe.The marginal zone of the pipe consists of (apatite)-forsterite phoscorite carrying fine grains of Ti–Mn–Si–rich magnetite with ilmenite exsolution lamellae, fine grains of Fe–Mg-rich apatite and finest grains of baddeleyite, enriched in Mg, Fe, Si and Mn. The intermediate zone accommodates carbonate-free magnetite-rich phoscorites that carry medium to coarse grains of Mg–Al-rich magnetite with exsolution inclusions of spinel, medium-grained pure apatite and baddeleyite. The axial zone hosts carbonate-rich phoscorites and phoscorite-related carbonatites bearing medium-grained Ti–V–Ca-rich magnetite with exsolution inclusions of geikielite–ilmenite, fine grains of Ba–Sr–Ln-rich apatite and comparatively large grains of baddeleyite, enriched in Hf, Ta, Nb and Sc. The collected data enable us to predict such important mineralogical characteristics of the multicomponent ore as chemical composition and grain size of economic and associated minerals, presence of contaminating inclusions, etc. We have identified potential areas of maximum concentration of such by-products as scandium, niobium and hafnium in baddeleyite and REEs in apatite.

Distribution Patterns of Suspended Sediment in Radial Sand Ridges Area, South Yellow Sea, China

Radial sand ridges area is located in south Yellow Sea, China, which is a valuable area for tidal flat reclamation and port construction. To rationally develop and protect this resource, the spatial distribution of tidal current velocity, grain size and suspended sediment concentration was studied based on measured data in August 2006. Result shows that the velocity in study area has a decreasing tendency with the Tiaozini water area being centered towards surrounding areas. The grain sizes near shore are smaller than ones offshore. At the same time ,the spatial distribution of grain size appears Λ pattern from north to south. The high value of mean sediment concentration appears in near shore area as well as the center of radial sand ridges.

From grain size to tectonics

Regional grain size trends in fluvial successions can reveal important information regarding the dynamics of sediment routing systems. Self‐similar solutions for down‐system grain size fining have recently been proposed to explore how key variables, such as the spatial distribution of deposition, sediment discharge, and sediment supply characteristics, control spatial distribution of grain size in fluvial successions over time scales of 104–106 years. We explore the sensitivity of these solutions to changes in key variables and assess their applicability to ancient fluvial successions. Several sensitivity analyses are presented to investigate the relative control of the key model variables on the spatial pattern of down‐system grain size fining in fluvial successions. Sensitivity analyses demonstrate that (1) an increase in the initial value of sediment discharge to a basin causes a decrease in the rate of grain size fining in fluvial successions, an effect that becomes nonlinear for large values of initial sediment discharge; (2) a short‐wavelength/high‐amplitude subsidence regime generates a greater rate of down‐system grain size fining and a long‐wavelength/lower‐amplitude subsidence regime generates a lesser rate of down‐system grain size fining in fluvial successions; and (3) an increase in the spread of grain sizes in the sediment supply generates a greater rate of down‐system grain size fining. We apply this modeling technique to grain size data sets collected from two time surfaces within conglomerates of the Upper Eocene Montsor Fan Succession of the Pobla Basin, Spanish Pyrenees. These data sets exhibit approximately self‐similar grain size distributions; further, the observed increase in down‐system grain size fining associated with smaller depositional system lengths provides support for the application of self‐similar solutions to fluvial successions. By applying these solutions to carefully collected grain size data from fluvial successions, we are able to relate explicitly the initial grain size supplied to the system, the spatial distribution of subsidence and the sediment discharge into the basin to the rate of grain size fining in fluvial successions. This method thus offers a powerful means of elucidating sediment routing system dynamics over time.

Experimental Modeling of the Spatial Distribution of Grain Size Developed in a Fill-and-Spill Mini-Basin Setting

ABSTRACT In many deep-water slope settings, turbidity currents are inferred to fill discrete basins that are linked streamwise (e.g., Gulf of Mexico ponded mini-basin settings). As an upstream basin is filled with sediment, progressively more overspill is directed into the next basin downstream. Turbidity currents are, however, vertically stratified in terms of grain concentration and grain size, especially during deposition. Thus the degree of confinement should potentially affect the degree of grain size fractionation between the two basins. Accordingly, two separate experimental programs were conducted to assess spatial trends in flow and deposit character developed within a pair of linked basins. Two sills, sinusoidal in section and arranged transverse to flow, were positioned between confining lateral walls. In the Series 1 experiments, individual flows were obstructed by an upstream sill, whose height was varied between flows, and a downstream sill of fixed height. Measurements of flow velocity, concentration, grain size, and the resultant deposit thickness were taken. In the Series 2 experiments, both sills were fixed in height whilst 18 repeat flows were run one after the other. Each flow overran the deposit of its predecessor/s. Sampling both along and through the resultant composite deposit allowed the mapping of systematic changes in grain size both horizontally and vertically. Both sets of experimental results show a strong relationship between the depth of the experimental flow, the height of the confining topography, and the degree of grain size partitioning between the two basins. Progressively greater proportions of coarser-grained material are bypassed downstream as the degree of confinement is reduced, whilst the mean grain size of that retained in the upstream basin also increases. At the natural scale, this may result in the production of systematic vertical trends in mean grain size, sorting (skewness), and sand-to-shale ratio in both the upstream and downstream basins.

Spatial pattern of grain size in the Late Pliocene ‘Red Clay’ deposits (North China) indicates transport by low-level northerly winds

The confirmation of an eolian origin of the Tertiary ‘Red Clay’ from North China would make it a valuable terrestrial paleoclimatic record. However, it is still controversial as to which wind primarily transported the dust forming the ‘Red Clay’ onto the Loess Plateau. Careful consideration of this question is a prerequisite for better understanding the history of atmospheric circulation in central and East Asia during the Miocene and Pliocene epochs. In this paper, the grain-size distributions of bulk samples and chemically isolated quartz grains of the Late Tertiary ‘Red Clay’ were investigated to explore the potential transportation mechanisms of the dust as well as the atmospheric circulation at the time of deposition. The samples were taken from six localities spanning across 550 km on the Loess Plateau. The results show that the mean grain size of both bulk and quartz samples decreases southward, as does the coarse fraction content (>42 μm). These data not only give new evidence for the eolian origin of the ‘Red Clay’, but also indicate that the ‘Red Clay’ was probably transported mainly by northerly low-level winds, or the winds at least had a strong northerly component. Therefore, the spatial distribution of grain size of the ‘Red Clay’ is comparable to that of the overlying Quaternary eolian loess, which demonstrates a remarkable northwest–southeast decrease in grain size, sedimentation rate, and thickness during the Last Glacial, coinciding with the dominant wind direction of the northwesterly East Asian winter monsoon. However, the grain size of the ‘Red Clay’ is generally finer than the corresponding loess. This indicates that the ‘Red Clay’ may have been transported by a weaker winter monsoon associated with stronger pedogenic modification, and/or the dust source area was not extensive compared to that during the Quaternary. Moreover, the high content of grains coarser than 19 μm, transported mainly by energetic low-level winds over short distances, suggests that the dusts were not transported by upper-level westerly.