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1.
A controlled reservoir release from Llyn Celyn to the Afon Tryweryn, Wales, U.K., has been used to study suspended load and turbidity variations. Turbidity was monitored continuously at two sites and 235 suspended solids samples were obtained at these and three additional sites during the passage of the release wave. The results are compared with data for a natural tributary flood event. The reservoir release data relate to sediment source depletion and reflects changing sources along the channel. Close to the dam, fine organic matter dominates the seston which scanning electron microscopy revealed to be predominantly allochthonous organic matter, with algal fragments and inorganic diatom frustules, derived from the periphyton of the channel bed. Coulter Counter analysis showed the seston to be relatively coarse with a median particle-size of 20 μm. Within 3 km of the dam, however, minerogenic particles dominate the sediment load of which more than 90 per cent is finer than 10 μm. This represents the flushing of channel-bed accumulations derived from tributary sources. The relationships between suspended sediment concentration and turbidity during the release are characterized by a marked, anticlockwise hysteresis. This contrasts with the clockwise hysteresis for the tributary flood event, but the different relationships cannot be explained by particle-size variations alone; seston composition also appears to be an important control.  相似文献   

2.
This paper describes a series of tests designed to evaluate the capacity of a personal computer (PC) based statistical curve‐fitting program called MIX to quantify composite populations within multi‐modal particle‐size distributions. Three natural soil samples were analysed by a Coulter Multisizer, and their particle‐size distributions analysed using MIX software to identify the modes, standard deviations and proportions of their composite populations. The particle‐size distributions of the three natural soil samples were then numerically combined in equal proportions using a spreadsheet program to create synthetic particle‐size distributions of known populations. MIX was then tested on the synthetic particle‐size distributions to see if the modes and proportions it identified were similar to those modes and proportions known to characterize the synthetic particle‐size distributions. The main outcome is that MIX can very accurately describe the modal particle size and proportions of the major composite populations within a particle‐size distribution. However MIX has difficulty in identifying small populations (those contributing <10 per cent of a total particle‐size distribution), particularly when they are located in the central sections of particle‐size distributions, overlain by larger populations, or when positioned in the fine tails of distributions. Despite these minor shortcomings, MIX is a valuable tool for the examination and interpretation of particle‐size data. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

3.
Quartz sand from southern Hungary was agitated for 128 hours in an abrasion chamber using a constant airstream. The fines produced by abrasion were collected from the exhaust air using an electrostatic precipitator. Analysis of the fines at set intervals by Coulter Counter show particle size distributions after 16 hours with > 50 per cent in the 20-60 μm range (the size range normally associated with loess deposits). Results demonstrate the feasibility of a local, abrasion derived contribution to Hungarian loess.  相似文献   

4.
Grain-size characteristics and the flux of sediment transported by wind from a cultivated paddock in a Quaternary relict dune field are described. Sediments were collected at seven heights between 0.7 and 2.0 m. The distribution of sediment mass with height is explained by a power function (of the order of −1), which is highly skewed towards the bed. The distribution of <90 μm sediment mass is explained by a log function of height and is less skewed towards the bed because these finer particles are influenced by the vertical velocity component of the wind. The particle-size distribution (PSD) of the eroded sediments is strongly influenced by the PSD of the parent material. Enrichment of the suspended sediment (PSA<90 μm) was in the order of 2.3 times. Sediment flux measurements show that 93 per cent of the erosion occurred in 3 per cent of the time.  相似文献   

5.
Geochemical analysis of fine grained (<20 μm) tephra found in ice cores is inherently difficult, due to the typically low number and small size of available particles. Ice core tephra samples require specialized sample preparation techniques to maximize the amount of information that can be gained from these logistically limited samples that may provide important chronology to an ice record, as well as linking glacial, marine and terrestrial sediments. We have developed a flexible workflow for preparation of tephra and cryptotephra samples to allow accurate and robust geochemical fingerprinting, which is fundamental to tephrochronology. The samples can be prepared so that secondary electron imagery can be obtained for morphological characterization of the samples to ensure that the sample is tephra-bearing and then the sample can be further prepared for quantitative electron microprobe analysis using wavelength dispersive techniques (EMP-WDS), scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), laser ablation inductively coupled mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS). Some samples may be too small for typical instrumentation conditions to be used (i.e. 20 μm beam on the EMP) to analyze for geochemistry and we present other techniques that can be employed to obtain accurate, although less precise, geochemistry. Methods include analyzing unpolished tephra shards less than 5 μm in diameter with a 1 μm beam on an SEM; using the “broad beam overlap” EMP method on irregular particles less than 20 μm in diameter, and analyzing microlitic shards as well as aphyric shards using EMP to increase the number of analyzed shards in low abundance tephra layers. The methods presented are flexible enough to be employed in other geological environments (terrestrial, marine and glacial) which will help maximize and integrate multiple environments into the overall tephra framework.  相似文献   

6.
We report normalized AF demagnetization curves of anhysteretic remanences (ARM's) produced by 1-, 10- and 40-Oe steady fields and of saturation isothermal remanence (IRMs) in a suite of dispersed, unannealed magnetite powders with median sizes of 2, 4, 6, 10 and 14 μm (pseudo-single-domain or PSD size range) and 100 μm (multidomain or MD size). Interpreted in the light of the domain structure test first proposed by Lowrie and Fuller [12], the relative stability trend of curves for the 2 μm sample is of single-domain (SD) type, the 1-Oe ARM being most resistant to demagnetization followed by the 10-Oe and 40-Oe ARM's and IRMs. For the 100-μm sample, the trend is exactly reversed and is of MD-type. In the 4–14 μm samples, hitherto undescribed transitional trends between SD-type and MD-type occur. At 6 μm, 1-Oe, 10-Oe and 40-Oe ARM's preserve an SD-type trend but for all AF's > 75 Oe, IRMs is more resistant than any of these remanences. At 10 μm, this trend is unmistakable, and only at 14 μm do the 1-Oe, 10-Oe and 40-Oe ARM curves merge. We conclude (1) that the Lowrie-Fuller test distinguishes between small MD grains enhanced by PSD remanence and large MD grains lacking PSD remanence, rather than between SD and MD structures per se, and (2) that in the PSD transition region from 6 to 14 μm in magnetite, IRMs changes over to MD-type relative stability around 6 μm, whereas 10-Oe and 40-Oe ARM's achieve an MD-type trend around 14 μm, in accord with the predicted field dependence of the PSD threshold size.Our theoretical interpretation assumes that the intrinsic (internal field) coercive force spectra of weak-field and strong-field remanences are identical but that the observed (external field) spectrum is shifted to lower fields as a result of the internal demagnetizing field — NJr of the remanence Jr. The effect is slight for weak-field Jr's but substantial for IRMs. Since all coercivities, high as well as low, are shifted, the result of the Lowrie-Fuller test is determined simply by the shape of the intrinsic coercivity spectrum or the corresponding AF demagnetization curve. Depending on the model of self-demagnetization used, either subexponential or sublinear AF decay curves of weak-field remanence will automatically lead to an MD-type trend, whereas by either model the decay curves that characterize SD and PSD remanences (decaying slowly initially and then more rapidly) will always produce and SD-type trend.  相似文献   

7.
The effects of emulsified crude oil and salinity (15, 30 per thousand ) on the steady state aggregate volume distributions and fractal dimensions were determined for a range of mean velocity gradients, (G(m) =5-50 s(-1)). Aggregation was performed in a 40-L cylindrical tank with a 4-blade paddle mixer. Three-dimensional fractal dimensions (D3) and volume distributions were determined using a procedure integrating data from an electrozone and an in situ light scattering instrument. Two-dimensional fractal dimensions (D2) and derived volume distributions were determined using a recently developed submersible flow cytometer equipped with a digital camera and image analysis software. For latex beads or emulsified crude oil systems, the above listed instruments yielded consistent size distributions and fractal dimensions (D2=1.92 +/- 0.16, D3=2.94 +/- 0.12). Mean volume aggregate diameters determined using the FlowCAM were consistently larger that those determined using the LISST-100 or Coulter Multisizer due to aggregate orientations during measurements. With increasing G(m) values, all colloidal aggregates showed increasing D3 values due to reduced aggregate length. Because of the compactness of all the aggregates (D3 >2), D2 values remained constant at 2. Neither salinity nor sediment type significantly affected D2 values calculated for sediment-crude oil aggregates. However, clay-oil aggregates showed higher D3 values than clay aggregates. This suggests that colloidal oil and mixing shear are the more dominant factors influencing aggregate morphology in nearshore waters. Overall, the data suggests that the analysis methods provide consistent size distribution results. However, because of the shear and salinity of coastal waters, resulting aggregates are too compact to estimate their D3 values using image analysis alone.  相似文献   

8.
In order to evaluate the influence of the measuring technique on the determination of (micro‐)aggregation in soil and sediment samples, results of grain size distributions of undispersed silty soil samples obtained by the sieve‐pipette method are compared with those obtained using a laser diffraction grain size analyser, the Coulter LS‐100. Reduced major axis relationships are calculated which may be used to convert Coulter LS‐100 results to those obtained by the sieve‐pipette method. The relationships obtained are very similar to the reduced major axis relationships established for dispersed silty soil samples. The results also show that the Coulter LS measurements have a systematic bias compared to the sieve‐pipette data. This implies that, if the percentage of (micro‐)aggregation is determined, the (interpretation of the) results will be strongly dependent on the measurement technique used. Using the calibration relationships that were established, nomographs can be developed to predict the level of sieve‐pipette (micro‐) aggregation from Coulter LS‐100 data. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

9.
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10.
Stable-boundary-layer regimes from the perspective of the low-level jet   总被引：2，自引：0，他引：2  
This paper reviews results from two field studies of the nocturnal stable atmospheric boundary layer (SBL) over the Great Plains of the United States. Data from a scanning remote-sensing system, a High-Resolution Doppler Lidar (HRDL), provided measurements of mean and turbulent wind components at high spatial and temporal resolution through the lowest 500–1000 m of the atmosphere. This data set has allowed the characteristics of the low-level jet (LLJ) maximum (speed, height, direction) to be documented through entire nights. LLJs form after sunset and produce strong shear in the layer below the LLJ maximum or nose, which is a source of turbulence and mixing in the SBL. Simultaneous HRDL measurements of turbulence quantities related to turbulence kinetic energy (TKE) has allowed the turbulence in the subjet layer to be related to LLJ properties. Turbulence structure was found to be a function of the bulk stability of the subjet layer. For the strong-LLJ (> 15 m s−1), weakly stable cases the strength of the turbulence is proportional to the strength of the LLJ. For these cases with nearly continuous turbulence in the subjet layer, low-level jet scaling, in which lengths are scaled by the LLJ height and velocity variables are scaled by the LLJ speed, was found to be appropriate. For the weak-wind (< 5 m s−1 in the lowest 200 m), very stable boundary layer (vSBL), the boundary layer was found to be very shallow (sometimes < 10 m deep), and turbulent fluxes between the earth’s surface and the atmosphere were found to be essentially shut down. For more intermediate wind speeds and stabilities, the SBL shows varying degrees of intermittency due to various mechanisms, including shearinstability and other gravity waves, density currents, and other mesoscale disturbances.  相似文献   

11.
Very fine samples from the mineralized zones of the Jacupiranga complex at the Cajati mine were selected for crystallographic identification of Ti-magnesioferrite (TMf) nanostructures embedded in titanomagnetite (TM) using high-resolution transmission electron microscopy (TEM). A magnetic concentrate obtained of pyroxenite samples (sites 4 to 7) was reduced and divided into fractions of distinct range sizes: 26±2 μm, 19±1 μm, 13±1 μm, 9±1 μm, 6±1 μm and 6-0.1 μm. The mineralized samples of carbonatite and pyroxenite were characterized by X-ray diffraction, transmitted and reflected light microscope, and scanning electron microscope with multielemental analysis. The finest magnetic concentrate sample (MC6) was analyzed under high-resolution transmitted electron microscopy (TEM) and high angle annular dark field and Raman spectroscopy. Magnetic properties were measured for the distinct granulometric fractions, showing drastic changes when grain sizes go beyond the frontier from micro to nanometer sizes. Frequency-dependent magnetic susceptibility percentage (÷fd%) report higher values (10.2%) for the finer fractions (6±1 μm and 6-0.1 μm) attributed to dominant fractions of superparamagnetic particles. Nanometer and < 6 μm grain size TMf in TM particles require a magnetic field up to 249 mT to reach saturation during the isothermal remanent magnetization experiment. Coercivity and remanent magnetization of these samples increase when the particle size decreases, probably due to parallel coupling effects. Magnetic susceptibility versus temperature experiments were conducted two times on the same (< 35 nm) sample, showing that the repetition during the second heating is probably due to the formation of new TMf nanoparticles and growth of those already present during the first heating process.  相似文献   

12.
Four new gravity field models from GOCE, two of them combined with GRACE, are compared here with EGM2008. The objectives are to look into the differences in consecutive ranges of the spherical harmonic expansion globally as well as in selected geographical regions and in the regions of the various data sources used for EGM2008. In general, GOCE is able to contribute to improved global gravity models in the spherical harmonic range between 120 and 200 (and above). The agreement between EGM2008 and the GOCE models is very good in well-surveyed regions such as North America, Europe and Australia, with geoid RMS-differences on the order of 4–6 cm. In other regions, where the surface gravity data available for the development of EGM2008 were poor, such as South America, Africa, South-East Asia or China the RMS-differences are on a level of 30 cm. Here GOCE leads to a significant improvement. These findings are confirmed by the analysis of the areas of the various EGM2008 data sources. In the regions of the so-called “fill-in” data of EGM2008 RMS-geoid height differences are high. In Antarctica GOCE also gives important improvements in terms of spatial resolution and accuracy. In general, the agreement between EGM2008 and the GOCE-models up to degree and order (d/o) 200 is good, with a global (excluding the polar gaps of GOCE orbits, throughout) geoid difference RMS of 11 cm, in the ocean areas 8 cm and 20 cm in the continental areas. GOCE models are better suited for ocean circulation studies because no prior ocean information enters into the data reduction process, as it is the case when deducing gravity anomalies from an altimetric mean sea surface. On the other hand, the good consistency between GOCE-models and EGM2008 in ocean areas very likely indicates that the influence of ocean circulation information on EGM2008 is rather small. The four tested GOCE models behave similarly except at the highest latitudes where GOCE lacks data due to its orbit inclination of 96.5° and some form of regularization which has to be applied.  相似文献   

13.
Seawater samples were collected in the lagoon of Nouméa (New Caledonia) along two transects from coastal bays to the oligotrophic barrier reef. Dissolved (<0.2 μm) and sub-micrometer (0.2–0.8 μm) concentrations of chromium, nickel and zinc were measured. Concentrations in the 0.2–0.8 μm size range were correlated with transparent exopolymeric particles (TEP) turnover rates, suggesting that a TEP pool rapidly cycling in the system is more reactive toward metals. The role of TEP reactivity in the transfer efficiency of metals from dissolved to particulate phases was estimated through variations of the metal partitioning coefficient K between sub-micrometer TEP and solution, as a function of the water mass residence time. The log K (6.0–8.0) increased from high to low residence time, suggesting that TEP reactivity increases when water masses renew rapidly. This suggests that hydrodynamics control TEP reactivity toward metals. The characteristics (adsorption vs. complexation) of TEP-metal association are discussed.  相似文献   

14.
The glacial cirques of a mountainous region usually have comparable size. Cirque widths between 400 m and 800 m are relatively common, whereas very large and very small cirques are infrequent. Although the presence of an upper limit is probably a result of the limited time available since the epoch of formation, the absence of very small cirques is more problematic. Simple statistical arguments suggest that this feature can be explained if the formation of cirques is very selective and localized in relatively small areas of the landscape. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

15.
A guide is provided to the minimum sample masses required to obtain reproducible measures of the particle-size distributions of coarse sediments. This is based on studies of the actual particle-size distributions of a range of clastic deposits. Procedures are given to enable representative bulk samples of tills, fluvial gravels and beach gravels to be taken.  相似文献   

16.
Fine suspended materials in the particle-size range1 < r < 6 μm (r, equivalent sphere radius) from the equatorial North Atlantic, between the ocean surface and greater depths (30–5100 m), can be consistently described by a size-distribution relationshipdN/dr = Ar?b, where the parametersA andb are independent ofr. The shape of the particle-size distribution is essentially constant and independent of depth, as reflected in the values of the logarithmic slope of the size distributionb = 4.0 ± 0.3. The particle-number and mass concentration, however, decrease strongly with depth, most of the decrease taking place in the surface layer 200 m deep. In the surface layer, mass concentration decreases exponentially with the half-concentration depthz1/2 ? 60m; in the deep layer (200–5000 m),z1/2 ? 1000m. The computed removal half-lives of the particulate material are0.8 ± 0.6yr in the surface layer, and60 ± 20yr in the deep layer, both half-lives being considerably shorter than Stokes settling residence times. The fast turnover or removal rate in the surface layer is compatible with the rates of zooplankton growth and carbon assimilation.  相似文献   

17.
Twenty-seven apatite samples recovered from Precambrian intrusive rocks of Rwanda and Burundi have been dated with the fission-track method. The resulting ages range from 75 to 423 m.y., which is much younger than the ages of emplacement or metamorphism of these rocks according to other radiometric methods. They even postdate the late Proterozoic (Pan-African) tectonic event which caused a partial isotopic rehomogenisation in the western part of the basement rocks around 650–700 m.y.From the regional geology and from the length ratios of spontaneous to induced tracks determined on 18 of the 27 samples it can be inferred that the fission-track ages are not mixed ages caused by some relatively recent thermal event, but that they essentially date the last cooling history of the studied massifs. This last cooling is interpreted to be mainly the result of a slow epirogenetic uplift (less than 100 m/107 y) which affected the area during the major part of the Phanerozoic. In this way the large age variations can be ascribed to differential cooling caused by regional differences in the rate of epirogenetic uplift, by small (less than 1.5°C/100 m) but persistent spatial variations in the geothermal gradient or by a combined effect of both phenomena.The spatial distribution of the ages indicates that differential cooling took place along SW-NE to W-E directions which is nearly perpendicular to the general orientation of the rift and none of the ages can be related directly to late Tertiary and Quaternary rift tectonics. Uplift movements associated with rifting were probably responsible for the relatively younger ages found on samples from the high rift shoulders and the rift walls, when compared to the ages of nearby samples from the central plateau. However, it must be concluded that these movements were not strong enough to bring rocks to the surface yielding ages that would date this major tectonic event itself.  相似文献   

18.
A comparison involving both field and laboratory trials was performed to evaluate the utility of two continuous-flow centrifuges and a tangential-flow filtration system for dewatering suspended sediments for subsequent trace element analysis. Although recovery efficiencies for the various devices differ, the analytical results from the separated suspended sediments indicate that any of the tested units can be used effectively and precisely for dewatering. Further, the three devices appear to concentrate and dewater suspended sediments in such a manner as to be equivalent to that which could be obtained by in-line filtration. Only the tangential-flow filtration system appears capable of providing both a dewatered sediment sample and a potentially usable effluent, which can be analysed for dissolved trace elements. The continuous-flow centrifuges can process whole water at an influent feed rate of 41 per minute; however, when suspended sediment concentrations are low (<30mg?1), when small volumes of whole water are to be processed (30 to 401), or when suspended sediment mean grain size is very fine (<10 μm), influent feed rates of 21 per minute may be more efficient. Tangential-flow filtration can be used to process samples at the rate of 11 per minute.  相似文献   

19.
Fresh groundwater reserves on small coral islands are under continual threat of salinization and contamination because of droughts, storm‐surge overwash events, over‐extraction, island community urbanization, and sea level rise. Whereas storm‐surge overwash events can cause sudden groundwater salinization, long‐term changes in rainfall patterns and sea level elevation have the potential of rendering these islands uninhabitable in the coming decades. This study demonstrates the use of a tested freshwater lens thickness simulator to estimate the groundwater resources of a set of atoll islands in the coming decades. The method uses ranges of projected rates of annual rainfall and sea level rise (SLR) to provide a range of probable lens thickness for each island. Projected rainfall is provided by General Circulation Models that accurately replicate the historical rainfall patterns in the geographic region of the islands. Methodology is applied to 68 atoll islands in the Federated States of Micronesia. These islands have widths that range between 150 and 1000 m, and experience annual rainfall rates of between 2.8 and 4.8 m. Results indicate that under average conditions of SLR, beach slope, and rainfall, almost half of the island will experience a 20% decrease in lens thickness by the year 2050. For worst‐case scenarios (high SLR, low rainfall), average decrease in lens thickness is 55%, with almost half of the islands experiencing a decrease of greater than 75% and half of the islands having a lens thickness less than 1.0 m. Small islands (widths less than 400 m) are particularly vulnerable because of shoreline recession. Groundwater on islands in the western region is less vulnerable to SLR because of a projected increase in rainfall during the coming decades. Results indicate the vulnerability of small islands to changing climatic conditions, and can be used for water resources management and community planning. Methodology can be applied to any group of islands as a first approximation of the effect of future climate conditions on groundwater resources. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

20.
The composition and amount of colloidal and suspended participate matter transported during a small flood event in Magela Creek in tropical northern Australia was investigated. The flood studied constituted approximately 3 % of the total annual flow, most (90%) of which occurred between mid-January and mid-February of the study year. Three fractions were separated from water samples using a sequential method involving a continuous flow centrifuge to separate suspended particulate matter (SPM; nominally > 1 μm) followed by hollow fibre filtration, first using a 0.1 μm filter to separate course colloidal matter (CCM; nominal size 1–0.1 μm) and then a 0–015 μm filter to separate fine colloidal matter (FCM; nominal size 0.1–0.015 μm). The SPM was predominantly inorganic (organic matter 21 %), whereas the colloidal fractions were dominantly organic matter (CCM 60%; FCM 83%). Analysis of individual particles using electron microprobe and automated image analysis indicated that the mineral fractions in both the SPM and CCM were dominated by iron-enriched aluminosilicates (including kaolinite) (72–82%) and quartz (9–10%), indicative of a highly weathered and extensively laterized catchment. Surprisingly there was very little difference in the composition of the SPM or CCM fractions during the flood event studied, which may indicate either that sediment availability was restricted following the major run-off events in January and February, or that all the sediment sources within the catchment are geochemically similar. Approximately the same amounts of particulate (20 tonne), colloidal (21 tonne) and dissolved material (17 tonne) were transported during the 25 hour period of the main flood peak; over 90% of the colloidal matter was 0.1–1.0 μm in size. These data suggest that previous estimates of the amounts of particulate (and colloidal) matter transported by Magela Creek, which were based on suspended solids measurements, may have underestimated the particulate matter load by as much as 50%. It is possible that the relatively high proportion of colloidal matter is unique to Magela Creek because coagulation and aggregation of colloidal matter to particulate matter is slow due to the very low concentations of calcium and magnesium in these waters. However, if the result is more widespread, there are important implications for the global estimates of fluvially transported particulate and dissolved materials as many of the previous studies may have underestimated the particulate load and overestimated the dissolved load.  相似文献   

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