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1.
Antimony in the environment: Lessons from geochemical mapping   总被引：2，自引：0，他引：2  
The distribution of Sb in a variety of sample materials, including soils, plants and surface water, was studied at different scales, from continental to local, combining published data sets with the aim of delineating the impact and relative importance of geogenic vs. anthropogenic Sb sources. Geochemical mapping demonstrates that variation is high at all scales – from the detailed scale with sample densities of many sites per km2 to the continental-scale with densities of 1 site per 5000 km2. Different processes govern the Sb distribution at different scales. A high sample density of several samples per km2 is needed to reliably detect mineralisation or contamination in soil samples. Median concentrations are so low for Sb in most sample materials (below 1 mg/kg in rocks and soils, below 0.1 mg/kg in plants, below 0.1 μg/L in surface water) that contamination is easier to detect than for many other elements. Distribution patterns on the sub-continental to continental-scale are, however, still dominated by natural variation. Given that the geochemical background is characterised by a high variation at all scales, it appears impossible to establish a reliable single value for “good soil quality” or a “natural background concentration” for Sb for any sizeable area, e.g., for Europe. For such a differentiation, geochemical maps at a variety of scales are needed.  相似文献   

2.
High top-/bottom-soil ratios, or high values of “enrichment factors” (EFs), are used as a proof for major anthropogenic impact on the geochemistry of the Earth surface. The idea behind calculating such ratios is that soils taken at depth or “average crust” can provide the geochemical background for the soils collected at the Earth surface. However, a soil profile is not a closed system, element exchange between the different layers, depending on and varying with the chemical properties of the different elements, and their turnover in the biosphere is the essence of soil formation. High top-/bottom-soil ratios, or EFs, may thus highlight the geochemical de-coupling of the lithosphere from the biosphere rather than contamination. This is demonstrated by using regional data from 258 soil O- and B-horizon samples collected from the Czech Republic (76,800 km2). Results show no relationship between the ratios and the magnitude of anthropogenic emissions. The visible relationship between element concentrations and sources in a map of the spatial distribution of the elements is lost when maps for the top-/bottom-soil ratio or EFs are constructed. The value of the data lies in the spatial elemental distribution, and not in ratios calculated based on misconceptions.  相似文献   

3.
文章以浙江省平湖市为典型三角洲平原区,开展了多种尺度、不同采样方法的对比试验。结果表明,田块内土壤元素分布较为均匀,小范围内元素的空间分异性较小,而田块之间土壤元素含量差异性明显增加;Hg,Cd等典型污染元素空间分异性较强,而人为污染扰动较弱的元素的空间分布较为均一;不同采样方法取得的区域性资料,其统计值接近,空间分布模式也相似,但有些元素仍有较明显差异。因此,中大比例尺地球化学调查时,应根据调查研究的目标任务,充分考虑地块分布、土壤类型和土地利用方式,选择合理的采样点。土壤样品可采取多坑点采集、多子样组合的方式,以保证样品的代表性,同时减少分析测试工作量。  相似文献   

4.
The distribution of chemical elements at and near the Earth's surface, the so-called critical zone, is complex and reflects the geochemistry and mineralogy of the original substrate modified by environmental factors that include physical, chemical and biological processes over time.Geochemical data typically is illustrated in the form of plan view maps or vertical cross-sections, where the composition of regolith, soil, bedrock or any other material is represented. These are primarily point observations that frequently are interpolated to produce rasters of element distributions. Here we propose the application of environmental or covariate regression modelling to predict and better understand the controls on major and trace element geochemistry within the regolith. Available environmental covariate datasets (raster or vector) representing factors influencing regolith or soil composition are intersected with the geochemical point data in a spatial statistical correlation model to develop a system of multiple linear correlations. The spatial resolution of the environmental covariates, which typically is much finer (e.g. ∼90 m pixel) than that of geochemical surveys (e.g. 1 sample per 10-10,000 km2), carries over to the predictions. Therefore the derived predictive models of element concentrations take the form of continuous geochemical landscape representations that are potentially much more informative than geostatistical interpolations.Environmental correlation is applied to the Sir Samuel 1:250,000 scale map sheet in Western Australia to produce distribution models of individual elements describing the geochemical composition of the regolith and exposed bedrock. As an example we model the distribution of two elements – chromium and sodium. We show that the environmental correlation approach generates high resolution predictive maps that are statistically more accurate and effective than ordinary kriging and inverse distance weighting interpolation methods. Furthermore, insights can be gained into the landscape processes controlling element concentration, distribution and mobility from analysis of the covariates used in the model. This modelling approach can be extended to groups of elements (indices), element ratios, isotopes or mineralogy over a range of scales and in a variety of environments.  相似文献   

5.
《Applied Geochemistry》2001,16(7-8):963-983
The <2 mm fraction of 605 samples of the C-horizon of podzols collected from an area of 188,000 km2 in the European Arctic was analysed for more than 40 chemical elements. The results were used to construct geochemical maps which showed clear regional distribution patterns, notwithstanding the very low sample density of 1 site per 300 km2. Some of these patterns fit established lithological boundaries. Others fit lineament structures in the area and underline the relative importance of certain tectonic directions some of which have not yet been delineated on existing geological maps. Some mark large-scale hydrothermal events and related alteration. Finally, some are connected with prominent, known ore deposits occurring in the area. However, several large deposits and even famous metallogenic provinces (Fe, Ni/Cu) are hardly, or not at all, reflected in the regional geochemical maps. In their present definitions geochemical provinces and metallogenic provinces are thus not necessarily related. Special geochemical features can occur at very different scales. The term geochemical province is so imprecise in terms of processes leading to regional-scale geochemical features that it should be avoided. Low-density geochemistry can be used to aid the interpretation of the geological evolution of large regions. It can also be used to find prospective areas within such regions.  相似文献   

6.
The continental area of Portugal is now entirely covered by a soil geochemical survey (1 site/135 km2), taking as the sampling media topsoils (upper mineral horizons, A) and organic horizons (humus, O). Standard methods for sampling, sample preparation and analysis were used in order to achieve high quality and consistent data. Each sample was analyzed for 32 chemical elements, pH, electrical conductivity and organic matter content.The main purpose of the survey was to obtain baseline levels for various chemical elements. The compilation of all data (nearly 45,000 individual data) in an organised way, led to the production of the first Soil Geochemical Atlas of Portugal. In this Atlas it is possible to find for each chemical element a set of information statistics (basic statistical parameters, boxplots, cumulative frequency curves, etc.), maps of spatial distribution, among other information of geochemical and environmental interest. This paper gives an overview of the Soil Atlas and examples of application. The data were used to calculate reference values for 9 elements of environmental importance and to obtain empirical formulae allowing the estimation of elements in the coarse fraction of soils (< 2.00 mm) from known concentration in a finer fraction (< 0.18 mm).  相似文献   

7.
The Forum of European Geological Surveys (FOREGS) includes representatives from 33 European countries and is responsible for co-ordinating Geological Survey activities in Europe. The FOREGS Geochemistry Task Group was established in 1994 to develop a strategy for the preparation of European geochemical maps following the recommendations of the International Geological Correlation Programme (IGCP) Project 259 ‘International Geochemical Mapping’ (now the International Union of Geological Sciences (IUGS) /International Association of Geochemistry and Cosmochemistry (IAGC) Working Group on Global Geochemical Baselines).The FOREGS geochemistry programme is aimed at preparing a standardised European geochemical baseline to IGCP-259 standards. The principal aims of this dataset will be for environmental purposes, as a baseline for the assessment of the extent and distribution of contaminated land in the context of variations in the natural geochemical background, but it will also have applications in resource assessment and for the development of policy for the sustainable management of metalliferous mineral and other resources.The first phase of the programme was the compilation of an inventory of geochemical data based on the results of a questionnaire completed by Geological Surveys and related organisations throughout the FOREGS community. The results show that the sample types which have been used most extensively are stream sediment (26% coverage), surface water (19% coverage) and soil (11% coverage). Stream sediments have been collected using a narrow range of mesh sizes (< 150–< 200 μm), but soil samples have been collected according to two different conventions: some surveys used a similar mesh size range to that used for stream sediments while others employed the < 1000 or < 2000 μm fractions traditionally used by soil surveys. Sample densities range from 1 sample per 0.5 km2 to 1 per 3500 km2. Various analytical methods have been used, but most of the available data have been calibrated using international reference materials, and data for the most important of the potentially harmful elements (PHEs) are available for most datasets. Systematic radiometric data are available for only a small proportion of Europe, a situation which compares very unfavourably with that in Australia, North America, the former Soviet Union and many developing countries.Recommendations are made for increasing the compatibility of geochemical methods between national geochemical surveys as a basis for the preparation of a series of European geochemical maps. The next stage of the FOREGS Geochemistry Task Group will be the collection of the Global Reference Network of samples against which to standardise national datasets according to the methods recommended in the final report of the IGCP 259 programme.  相似文献   

8.
Agricultural (Ap, Ap-horizon, 0–20 cm) and grazing land soil samples (Gr, 0–10 cm) were collected from a large part of Europe (33 countries, 5.6 million km2) at an average density of 1 sample site/2500 km2. The resulting more than 2 × 2000 soil samples were air dried, sieved to <2 mm and analysed for their Hg concentrations following an aqua regia extraction. Median concentrations for Hg are 0.030 mg/kg (range: <0.003–1.56 mg/kg) for the Ap samples and 0.035 mg/kg (range: <0.003–3.12 mg/kg) for the Gr samples. Only 5 Ap and 10 Gr samples returned Hg concentrations above 1 mg/kg. In the geochemical maps the continental-scale distribution of the element is clearly dominated by geology. Climate exerts an important influence. Mercury accumulates in those areas of northern Europe where a wet and cold climate favours the build-up of soil organic material. Typical anthropogenic sources like coal-fired power plants, waste incinerators, chlor-alkali plants, metal smelters and urban agglomerations are hardly visible at continental scales but can have a major impact at the local-scale.  相似文献   

9.
The Turpan–Hami basin, covering an area of approximately 50,000 km2 in NW China, contains concealed sandstone-type U deposits in a Jurassic sequence of sandstone, mudstone and coal beds. Sampling of soil profiles over the Shihongtan concealed U deposit in this basin shows that fine-grained soil collected from the clay-rich horizon contains U concentrations three times higher than similar soils at background areas. Selective leaching studies of these soils show that U is mainly associated with clay minerals, which comprise from 17.9% to 40% (average 30.4%) of the total mineral content. This may indicate that U is converted to uranyl ions [UO2]2+ under oxidizing conditions and is sorbed on clay minerals to accumulate in anomalous concentrations. Fine-grained soil (<120 mesh, <0.125 mm) from the clay-rich horizon, generally occurring at a depth of 0–40 cm, is shown to be an effective sampling medium for deep-penetrating geochemical surveys. A wide-spaced geochemical survey at a density of approximately 1 site per 100 km2 was carried out throughout the whole basin using this sampling medium. Samples were analyzed for 30 elements by ICP-MS following a 4-acid extraction. Three large-scale geochemical anomalies of U and Mo were delineated over the whole basin. One of the anomalies is consistent with the known U deposit at Shihongtan in the western part of the basin. A new potential target in the eastern part of the basin was selected for a follow-up survey at a density of 1 sample per 4 km2. A drilling exploration programme at the center of the geochemical anomaly delineated by this follow-up survey discovered a new U deposit.  相似文献   

10.
Guidelines for a low-density geochemical survey were described in 1990 by the Western European Geological Surveys. A low-density geochemical survey of Hungary was carried out in 1991–1995. The results are useful for future surveys and for the IGCP 360 project ‘Global Geochemical Baseline’. In regions with well-developed drainage systems in Hungary, 196 catchment basins of approx. 400 km2 were delineated and flood-plain deposits sampled at their outlets. The samples were taken from 0 to 10 cm and from 50 to 60 cm depths. Samples were analysed by ICP-AES and AAS techniques in two laboratories. A Geochemical Atlas of Hungary is in preparation that will show the distribution of 25 elements in the two sampled layers. Maps for the lower layer represent regional geochemical baseline values and a geochemical subdivision of the country (maps showing the distribution of element associations) was made on the basis of factor variables. Maps constructed from the data of the upper sampling level show us the present state of contamination of the surface. The results of this survey have contributed to the establishment of guidance values for soils prepared by the Hungarian Ministry of the Environment in 1995. Safe levels were established for As, Cd, Cr, Cu, Hg, Pb and Zn and regional environmental loads plotted. Differences between the median values of the two levels are generally small. However, the concentrations of certain elements like P, Pb and S are significantly greater in the upper layer reflecting contamination from agriculture. In certain regions, the rate of sedimentation was fairly fast such that the environmental effects of ore mining in Transylvania and southern Slovakia as well as those of heavy industry in northern Hungary can be observed in samples from the lower level. The main factor controlling the geochemical pattern in Hungary is the predominance of young (Pleistocene or Miocene) clastic sediments at the surface. Approx. 90% of the surface is covered by these young sediments. This kind of survey has the disadvantage of not providing enough contrast to differentiate geologically dissimilar areas but it has the advantage to provide regional surface background geochemical data and it helps to outline areas of possible surface contamination. Based on the results of this survey we conclude that it would be much better to sample smaller, but geologically homogeneous areas in mountainous terrain to obtain data characteristic of the geochemical background of lithologic units. This approach would mean a sampling density of a few tens of km2/sample for hilly areas, and a few hundred km2/sample for lowland areas.  相似文献   

11.
《Applied Geochemistry》2001,16(11-12):1323-1331
This paper reports the first results of a low-density geochemical survey covering the whole continental area of Portugal, taking both topsoils and active stream sediments as the sampling media. The data were obtained in a consistent way from 653 sites at a sampling density of 1 site/135 km2. The samples were analysed for 31 elements by ICP-AES and after quality control the following 19 were selected: Al, As, Ba, Ca, Co, Cr, Cu, Fe, K, La, Mg, Mn, Ni, P, Pb, Sr, Th, V and Zn. The baseline levels for these elements are presented. The first geochemical maps of Portugal were prepared and the geochemical patterns are generally well correlated to geological factors sometimes combined with anthropogenic influences. Some general trends can be observed between geochemical patterns and soils distribution.  相似文献   

12.
The Council for Geoscience (CGS, South Africa) has a statutory mandate to carry out regional geochemical mapping in South Africa that needs to be rapidly and accurately analysed. Both simultaneous X-ray fluorescence spectrometry (S-XRF) and a newly developed method using inductively coupled plasma-mass spectrometry (ICP-MS) were employed. Various trace elements that could not previously be analysed by S-XRF can now be analysed by ICP-MS for the regional geochemical mapping programme, e.g., Cd, Mo, Te and Li. Using both techniques, the CGS aims to report element distributions for some fifty elements. To ensure that element concentration levels correlate over map boundaries, quality control measures in the sampling, sample preparation and analyses were of critical importance. This paper aims to discuss the sample preparation and quality control measures as applied to the ∼5500 samples of the Giyani and Tzaneen 1:100000 scale map sheets sampled at a density of one soil sample per km2. ICP-MS batch- and instrumental drift-correction procedures will be discussed. As a final step, geochemical data were overlain over simplified geological maps using geographical information system software. These maps complement existing geological information of South Africa, help in the identification of exploration targets, test exploration models and initiate further geological research.  相似文献   

13.
This paper reviews some aspects of the use of “loam” (soil) concentrates in geochemical surveys in arid, deeply weathered environments.An orientation survey at a small Ni-Cu-Co prospect in Western Australia has shown that discrimination between mineralized and unmineralized samples could be achieved using Ni, Cu, Co, Cr, Zn, As, Sn, Sc, Ti, Yb and Y in the coarse fraction of heavy concentrates. However, at the same prospect the best contrast for Ni, Cu and Co in surface samples was provided by analysis of the same fraction following a cold ammonium citrate/hydroxylamine hydrochloride digestion.At a nearby, larger prospect, some 54 km2 in area, concentrates were separated, by jigging, from bulk soil samples, themselves composites of representative subsamples. Sampling at a density of 4 samples per km2 revealed 1–2 km2 size anomalies of Cu, Ni, Co, Cr, As and Au which could be related, variously, to known Ni-Cu and Au mineralization.In Botswana, analysis of concentrates, separated by tabling from samples collected at a density of 1 sample per 7.5 km2 over an area of 5400 km2, identified distinctive geochemical districts. Enhanced values of Au and of Cu-Ni in the concentrates were relatable to known mineralization and the results suggested that there were also Sn-W-Mo-Bi (granitoid) and Au-Pb-Zn-Bi-Sn (volcanogenic) associations which could lead to new prospecting targets. Anomalies of certain elements (for example, Cu in an ultramafic environment) may be more readily detected in surface material by “enrichment indexing” the concentrate data.  相似文献   

14.
Regional-scale variations in soil geochemistry were investigated with special reference to differences among soil groups and lithology in an area of 9,699 km2 in Medak district, Andhra Pradesh, India. The concentrations of 29 elements (major: Si, Al, Fe, Mn, Mg, Ca, Na, K, Ti, P and trace: As, Ba, Cd, Co, Cr, Cu, F, Mo, Ni, Pb, Rb, Se, Sr, Th, U, V, Y, Zn, Zr) in 878 soil samples collected (557-topsoil, 321-subsoil) at a sampling density of 1 site/17 km2 from 557 sites representative of all the soil types present in studied area were determined, and their elemental composition are discussed. The baseline levels of these elements in soils are determined over different lithological units for the identification of anomalous values relative to these. For the first time, geochemical maps for Medak district are prepared on 1:50,000 scale and the lithogeochemical database generated provides information on the lateral and vertical distribution of elements in soil. The spatial variations in the distribution of elements reflect underlying geologic characteristics. Box-plots reveal that the concentration of most of the elements in soils were not strongly dependent on the soil group but the soil-geochemistry abruptly changes with the change in the soil parent materials indicating that the distribution of elements is mostly influenced by the bedrock lithology and other natural processes acting on them. For instance, the concentrations of Co, Cu, Fe, Mn, Ti, V and Zn are high in soils developed on basaltic terrain while the soils developed on granitic and gneissic terrain exhibit high elemental concentrations of K, Pb, Rb, Si, Th and Y. Alfisols had relatively high contents of elements while entisols had lower concentrations of most of the elements. The database can be used in the chemical characterisation of different geological units as well as applications in various environmental and agricultural fields. The results indicate that regional geology is an important determinant of soil geochemical baselines for soil pollution assessment and further emphasizes the importance of determining background levels locally. The defined baselines can be used to establish background values for future soil surveys.  相似文献   

15.
Arsenic concentrations are reported for the <2 mm fraction of ca. 2200 soil samples each from agricultural (Ap horizon, 0–20 cm) and grazing land (Gr, 0–10 cm), covering western Europe at a sample density of 1 site/2500 km2. Median As concentrations in an aqua regia extraction determined by inductively coupled plasma emission mass spectrometer (ICP-MS) were 5.7 mg/kg for the Ap samples and 5.8 mg/kg for the Gr samples. The median for the total As concentration as determined by X-ray fluorescence spectrometry (XRF) was 7 mg/kg in both soil materials. Maps of the As distribution for both land-use types (Ap and Gr) show a very similar geographical distribution. The dominant feature in both maps is the southern margin of the former glacial cover seen in the form of a sharp boundary between northern and southern European As concentrations. In fact, the median As concentration in the agricultural soils of southern Europe was found to be more than 3-fold higher than in those of northern Europe (Ap: aqua regia: 2.5 vs. 8.0 mg/kg; total: 3 vs. 10 mg/kg). Most of the As anomalies on the maps can be directly linked to geology (ore occurrences, As-rich rock types). However, some features have an anthropogenic origin. The new data define the geochemical background of As in agricultural soils at the European scale.  相似文献   

16.
The results of a first systematic study of spatial distribution of different elements in surface soil over of the Kavadarci region, Republic of Macedonia, known for its nickel industrial activity are reported. The investigated region (360 km2) is covered by a sampling grid of 2 × 2 km2; whereas the sampling grid of 1 × 1 km2 was applied in the urban zone and around the ferronickel smelter plant (117 km2). In total 344 soil samples from 172 locations were collected. At each sampling point soil samples were collected at two depths, topsoil (0–5 cm) and bottom soil (20–30 cm). Inductively coupled plasma-mass spectrometry (ICP-MS) was applied for the determination of 36 elements (Ag, Al, As, Au, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, Hg, K, La, Mn, Na, Mg, Mo, Ni, P, Pb, S, Sb, Sc, Se, Sr, Th, Tl, Ti, U, V, W and Zn). Data analysis and construction of maps were performed using the Paradox (ver. 9), Statistica (ver. 6.1), AutoDesk Map (ver. 2008) and Surfer (ver. 8.09) software. Four geogenic and three anthropogenic geochemical associations were established. Within the research, natural and anthropogenic enrichment with heavy metals was determined. Principally, the natural enrichment is related especially to Ni. Pollution by As, Cd, Co, Cr, Cu, Hg, Mo, Pb and Zn is basically insignificant.  相似文献   

17.
Geochemical patterns from local to global   总被引：5，自引：0，他引：5  
The historical development of geochemical exploration is, in a sense, a process of progressive enlargement of areal coverage by exploration projects and progressive widening of sampling space. Along with this process, a hierarchy of geochemical patterns from small to large is gradually discovered and understood. In this paper, we try to summarize systematically the whole hierarchy of geochemical patterns from local, regional, provincial, megaprovincial to global, using examples obtained in China.Local anomalies (various types of dispersion halos, trains and fans) within areas not exceeding a few km2, can be delineated when sampling is done with very close intervals in limited areas. Regional anomalies within areas of tens to hundreds km2 and threshold values lower than local anomalies can be identified only when large areas of more than thousands of km2 are mapped with wider space sampling. Geochemical provinces with areas of thousands or tens of thousands of km2 can be discovered if even larger areas, of more than tens or hundreds of thousands of km2, are covered with very low density sampling. More than millions of km2 should be covered in order to discover geochemical megaprovinces somewhere in the world. Such megaprovinces are often associated with extraordinarily large mineral resources. If ultra-low density geochemical mapping can be carried out across national boundaries on a continental or global scale, we could find even broader geochemical patterns which will reflect the global tectonic features.The classification of geochemical patterns according to their sizes is necessary because it will be extremely useful in planning sampling layout in order to hit targets of certain size ranges. This in turn is arranged in different geochemical projects for achieving specific aims.  相似文献   

18.
Regional, national and global scale geochemical mapping projects have been carried out in China since the late 1970s, due to the development of cost‐effective, low detection limit analytical methods. These projects have provided a huge mass of high‐quality, informative and comparable data for mineral resource exploration and are now making contributions to environmental assessment. In this paper, four national‐scale geochemical mapping projects are described. (1) The Regional Geochemistry‐National Reconnaissance Project (RGNR project), which is China's largest national geochemical mapping project, has covered 6 million km2 of upland regions since 1978. Generally, stream sediment samples were collected at a density of 1/km2 and four samples were composited into one sample and analysed for thirty‐nine elements. (2) The deep‐penetrating geochemical mapping project (DEEPMAP Project) has been conducted since 1994 in covered terrains, including sedimentary basins, at a density of 1 sample per 100 km2 with thirty to seventy elements determined per sample. In the past 10 years, an area of approximately 800 000 km2 has been covered and this project has played an important role in finding sandstone‐type uranium deposits in basins. (3) The seventy‐six geochemical element mapping project (76 GEM project) has been carried out since 1999 and involved the collection of stream sediment samples from the RGNR project targets which were analysed for seventy‐six elements. Samples from each 1:50 000 map sheet were composited into one analytical sample (approximately one composite sample per 400 km2). Approximately 1 million km2 have been surveyed to date. (4) The multi‐purpose eco‐geochemical mapping project has been conducted since 1999 in Quaternary plain areas for environmental and agricultural applications. Surface soils (depths from 0–20 cm) were collected at a density of one sample per km2, and four samples were composited into one for analysis. Deep soils (from a depth of 150 to 200 cm) were collected at a density of one sample per 4 km2 and four samples were composited into one analytical sample. All the composite samples were analysed for fifty‐four elements.  相似文献   

19.
A geochemical survey of Belgium and Luxembourg was carried out as part of an international research project entitled ‘Regional geochemical mapping of Western Europe towards the year 2000'. The aim of this research was to map regional background geochemical patterns based on pristine or at least pre-industrial overbank samples and to deduce regional information on the degree of environmental pollution of floodplain and present-day river sediments. Over the entire study area (about 33,000 km2), 66 overbank sites have been sampled. Catchment areas range between 60 and 600 km2. At each site an overbank profile has been dug out in the immediate vicinity of the river and described in detail. A first composite sample was taken 5–25 cm below the surface. This sample is supposed to represent deposition over the last centuries. Human interferences in this interval are often inferable based on changed sedimentary characteristics and the presence of anthropogenic particles such as charcoal, slags and brick fragments. A second composite sample was taken at depth, usually >1.5 m below the surface over an interval of about 20 cm. In most sites, the profile characteristics allowed to assume pre-industrial or even pristine conditions for this lower overbank sample. In some profiles this was confirmed by 14C-dating and/or by the absence of anthropogenic particles. Finally, a present-day stream sediment was sampled on the site to infer the actual pollution status. After drying at 80°C, disaggregation and sieving, the <125 μm fractions of the three sediment samples were analysed by XRF for major elements and several trace elements. Lower overbank samples generally show a direct link with the geological substrate and allow to assess natural background concentrations. Results from the mapping exercise as well as from the statistical analyses display a clear contrast between the northern part of Belgium where Cenozoic unconsolidated sandy and silty formations dominate which are especially vulnerable for erosion, and the southern part of Belgium and Luxembourg where Paleozoic and Mesozoic sandstones, carbonates, marls and shales are the prevalent lithologies. Here the shales are the most intensively eroded lithologies. This is especially reflected in the element patterns of Al2O3, MgO, K2O, Ga, Ni, Rb, Sc and V which negatively correlate with SiO2. Despite the human related pollution, the geological contrast between north and south Belgium is still recognisable in the geochemical pattern of the upper overbank and present-day stream sediment samples for the above-mentioned elements. Furthermore there is a clear increase in heavy metal contents (Zn, Cd, Pb, Cu), As and in certain locations in Ba from the lower to the upper overbank sediment, as well as to the present-day stream sediment. The relative increase in element content allows to assess the degree of pollution and helps to define those drainage areas where more detailed research is needed.  相似文献   

20.
A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO3) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of “hydrothermal” elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr−1, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores’ surface mixed layers are presented which, typically, are an order of magnitude greater than “background” authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time—perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting.  相似文献   

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