Contrasting the geochemistry of oxic sediments across ecosystems: a synthesis

The geochemistry of oxic sediments was contrasted across a range of Canadian aquatic ecosystems; 7 freshwater lakes, (3 circumneutral and 4 acidic), 15 peatlands (5 mineral-rich, 5 moderately-poor and 5 mineral-poor), 9 wetlands (3 oligosaline, 3 mesosaline and 3 euryhaline), an estuary (deposited and suspended sediments) and an intertidal region. Sediments were characterized by a simultaneous extraction that separated sediments into easily reducible (ER) metal (oxyhydroxides of Mn and easily reducible amorphous oxyhydroxides of Fe) and reducible (R) metal (more crystalline forms of oxyhydroxides of Fe), organic matter, and, the concentrations and partitioning of Zn, Cu and Cd associated with these 3 sediment components. Ecosystems were distinct with respect to ER Fe and organic matter [canonical variate analysis (CVA)] with 53% of among system variation in geochemistry attributed to these 2 components. Sediments of peatlands and wetlands contained greater amounts of organic matter whereas sediments of lakes, intertidal and estuarine deposited and suspended sediments were characterized by greater amounts of ER Fe. A further 21% of among system variation could be ascribed to organically bound Fe that was greater in acidic lakes and mineral-rich peatlands as compared to other systems. Concentration and partitioning of Cd within sediments was regionally dependent with 41% of among system variation (CVA) attributed to differences in ER Cd and R Cd. Cadmium from peatlands and lakes located in Ontario was recovered from all 3 sediment components whereas sediment from wetlands, the estuary and the intertidal regions of British Columbia (BC) contained no organically bound Cd with amounts recovered occurring mostly as ER Cd. Lakes and peatlands located in Ontario contained 3–5-fold total Cd as compared to ecosystems located in BC. A further 21% of among ecosystem variation was attributed to Zn partitioning. Zinc in peatland and wetland sediments occurred as R Zn as compared to lake and estuarine deposited sediments where Zn was recovered both as ER and R Zn. Total Zn was also 3–5-fold greater in sediments of systems in Ontario as compared to those sampled in BC. Concentration and partitioning of Cu was similar across all systems with Cu recovered from the organic component of sediment. The geochemistry of surficial oxic sediments with respect to ER Mn, R Fe and organic matter and the geochemical associations among these sediment components is ecosystem and region dependent. For assessing impacts of metals on sediment dwelling biota the geochemical characteristics of the system under study should first be defined.     

Overbank sediments as appropriate geochemical sample media in regional stream sediment surveys for gold exploration in the savannah regions of northern Ghana

Conventional stream sediment sampling in which sediments are taken from the active channels during reconnaissance regional geochemical surveys in gold exploration has over the years failed to delineate prospective target zones in northern Ghana, where the relict is flat. Whereas the technique has been successful in the south western Ghana, which is characterised by moderate to high relief, generally the savannah north is associated with low relief, commonly with flat topographies and generally decoupled stream channels. Geochemical comparison of active stream and overbank sediments in this study demonstrate that active stream channels may contain contaminated materials of recent origin, but overbank sediments, except for the uppermost horizons, represent alluvial regolith of earlier depositional cycles over time. Based on gold value repeatability, composite samples taken from the overbank sediment layers were relatively less erratic and are considered to be an appropriate geochemical medium in delineating potential regional gold targets for follow up. The results show that overbank sediment sampling can be used as a cost-effective method to define broad anomalous zones; and the technique must be considered useful during reconnaissance geochemical surveys in the savannah regions.     

Assessment of trace metals contamination in stream sediments and soils in Abuja leather mining,southwestern Nigeria

This study is aimed at determining the level of environmental degradation as well as the concentration of trace elements in soil and stream sediments in order to evaluate the environmental impact of the mining operation. Twenty-five (25) soils and ten (10) stream sediment samples were collected from the study area. The physicochemical parameters were determined using appropriate instrumentation with the aid of a digital pH meter (Milwaukee meter) to measure the pH and electrical conductivity, total dissolved solids, moisture content and loss on ignition of the soil and stream sediment samples. The pH of the soil sample ranged from (6.10 to 7.19); Electrical conductivity ranged from (21.3 to 279.4 µS/cm), moisture content varied from (0.60% to 7.20%), and the LOI ranged from (2.03% to 18.62%). The results of the analysis showed that the concentrations of the trace elements in the soils and stream sediment samples were slightly higher than the background values. Plots of the trace elements in stream sediment samples show moderate, consistent decrease downstream except at points where there was mine water discharge into the main river. The pollution levels of heavy metals were examined in stream sediment and soil samples using different assessable indices, such as the enrichment factor, which showed significant-moderate enrichment for Cr, Th, Nb, Zn, Pb, Y and Zr and the geo-accumulation index, which showed practically moderate contamination with Cr, Ni and Sr based on regional background reference values. Geo-accumulation index and contamination index for soils and stream sediment revealed uncontaminated to moderate contamination. Likewise, elements with moderate contamination were Cr, Ni and Sr. The Pearson correlation showed that there were significant positive associations among selected metals in soil and stream sediment samples.     

Organic matter scavenging of copper, zinc, molybdenum, iron and manganese, estimated by a sodium hypochlorite extraction (pH 9.5)

The importance of trace metal scavenging by organic matter in geochemical samples was estimated using an alkaline sodium hypochlorite extraction to leach copper, zinc, molybdenum, iron and manganese from a variety of soils, and stream and lake sediments collected on the Nechako plateau, central British Columbia. The reagent oxidizes or dissolves most forms of organic matter, together with any sulphide minerals, to give strongly coloured extracts containing the associated trace elements at a pH where solution of other sample fractions is at a minimum. Metals precipitated due to alkaline conditions are redissolved by a succeeding distilled-water leach (pH 3.0 ± 0.3).A large fraction of the copper, zinc, molybdenum, and manganese held within the organic fraction of the A soil horizon is liberated whereas only minor amounts of copper, zinc, and manganese are released from inorganic soil (B and C) horizons. Molybdenum, however, is relatively soluble in all soils as the molybdate ion. Despite similar concentrations of organic matter in A horizon soils and stream sediments the latter release a lower proportion of their trace element content. Behaviour of the organic fraction of lake sediments varies from lake to lake and there is great variability in the association of copper, zinc, molybdenum and manganese with organic matter even within the same lake.The presence of organic matter in samples subjected to other partial extractions can be a deleterious factor if the organic fraction is not first removed by a hypochlorite extraction.     

Impact of historical metalworks on the concentrations of major and trace elements in sediments: a case study in Finland

The concentrations of major and trace elements were determined (aqua regia leach and ICP-AES analyses) in stream, lake and dredged sediments downstream of the historical Antskog iron- and copperworks, S.Finland. The levels of Ag, Cd, Cu, Pb and Zn are highly elevated in all studied sediment types: roughly half of the studied lake-sediment samples contain >5 ppm Ag, >15 ppm Cd, >0.1% Cu, >0.1% Pb and >0.3% Zn. In the dredged sediment material located onshore, the concentrations of Ag, Cu and Pb are comparable to those in the polluted lake-sediment samples, while in stream sediments elevated metal concentrations are found especially in samples characterised by high concentrations of organic material. The source of the elevated metal concentrations is the historical metalworks at Antskog, mainly the copperworks of the 19th century. Compared to the limit values for contaminated soils in Finland, the concentrations of Cu, Pb and Zn are on average elevated by factors >10 in the polluted horizons of lake sediments, >5 in the dredged sediment located onshore and >2 at the most heavily contaminated site in the stream. Since the surface waters in the area are used for agricultural purposes and for various leisure activities, it is necessary to make further detailed investigations into the extent of the metal pollution and to determine species, mobility and bioavailability of the metals.     

Phosphate regeneration from sediments of the Peru continental margin by dissolution of fish debris

Generally, oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes. Stoichiometric modelling of such reactions takes into consideration varying proportions of ‘decomposable’ organically bound P to account for the ratios among nutrients in depth-concentration profiles of near-surface sediments. New results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off Peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound P.Dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations. Phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10% of the nutrient pool in the waters of the Peru undercurrent. Partitioning of P among the two sources reveals further that fish debris phosphate is about four times more important than organically bound P in nutrient generation from sediments of the Peru continental margin. Not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area.     

Possible effects of iron oxide coating in the recovery of particulate gold from stream sediments

The influence of iron hydroxide/oxide coatings on the recovery of gold was studied using 13 stream sediment samples collected from an area of known gold mineralisation. Magnetic products collected from a Frantz isodynamic separator showed higher gold values than did non-magnetic fractions. An acid-treated magnetic product when run through the isodynamic separator yielded a significant amount of non-magnetic component which, when viewed by binocular microscope, showed the presence of gold grains in both bulk sample and in bromoform-separated heavy mineral concentrate. It is suggested that concentrates are treated with 10% HCl prior to magnetic separation where iron oxide coatings are conspicuous.     

Multi-element signatures of stream sediments and sources under moderate to low flow conditions

This study assesses a simple sediment source tracing method using major- (Al, Ca, Fe, K, Mg, Mn, Na, P, Si, Ti) and trace-element (Ba, Be, Ce, Co, Cr, Mo, Nd, Pb, Sr, Th, V, Y, Zn) signatures of stream suspended particulate matter (SPM), bed sediments and soils in a small agricultural catchment in NE Scotland. Whilst most erosion studies characterise the large amounts of material mobilised at the highest flows, this study aimed to assess properties of sediments during moderate to low flow periods. These occur more frequent than intense storms and are important in linking stream sediments, near-channel sources and aquatic ecosystem impacts. Data were transformed by multivariate statistical methods to compare elemental signatures of SPM (ranging from 3 to 53 mg L⁻¹ in the stream) and stream bed sediments with a limited number of near-channel source soils. Increased concentrations of Ce, Nd, Th and Y in subsoils contributed to the ability to discriminate between surface fieldslope and stream bank erosion sources. Stream bed sediments showed close matches with compositions of stream bank and arable surface soils, but signatures of SPM differed greatly from any of the sources. Large concentrations of Cr, Pb and Zn in SPM, particularly during summer (677, 177 and 661 mg kg⁻¹, respectively) exceeded water quality standards and were linked to an accumulation of trace elements associated with biological material. The potential for within-stream alteration of SPM in relation to erosion sources was confirmed by changes in the nature of the SPM organic matter observed by IR spectroscopy. Thus the potential is shown for multi-element signatures to give information on catchment sediment sources to aid land management decisions, given careful consideration of the effects of in-stream alteration of eroded material. However, this combined information may be beneficial to process understanding linking land use and stream ecosystems at critical ecological periods.     

Mercury, copper and zinc contamination in soils and fluvial sediments from an abandoned gold mining area in southern Minas Gerais State, Brazil

Mercury, zinc and copper contamination was evaluated in soils and fluvial sediments from an abandoned gold mining site at Descoberto Municipality (southern Minas Gerais State, Brazil). Metals bioavailability and potential mobility were studied through physical, chemical and mineralogical characterization, geoaccumulation indexes calculations, mercury speciation and determination of potentially bioavailable contents of zinc and copper. Values of pH were in the neutral range, while organic matter concentrations were very low. Mineralogical characterization, in the total samples, indicated the presence of quartz, kaolinite and gibbsite for all samples. Total mercury, zinc and cooper concentrations were higher than the limits recommended by Brazilian documents. Geoaccumulations indexes revealed that most of the sediment samples were low to moderately polluted by zinc and copper, while just one sample was very strongly polluted by mercury. Mercury speciation revealed the predominance of the elementary form for all samples, and low concentrations for exchangeable, strongly bound and residual fractions. Zinc and copper behavior was strongly controlled by iron and aluminum concentrations, while their bioavailable contents were very low in comparison with the total concentrations.     

Geochemical dispersion of gold related to copper-gold mineralization in northeastern Thailand

The nature of gold dispersion in soils and stream sediments associated with a copper-gold-mineralized system in northeastern Thailand has been investigated as a basis for identifying appropriate geochemical exploration techniques for the search for comparable deposits in similar environments.Soils were collected with varying relationships to mineralization as a basis for determining sample representativity, size distribution of gold, variation with soil horizon and possible pathfinder elements. Similarly, stream sediments were collected to estimate sample representativity, size distribution of gold, variation of gold with depth in the stream sediment profile and to compare the relative recoveries of gold in field-panned and laboratory-prepared heavy-mineral concentrates. Samples were analyzed for Au and potential indicator elements by a variety of methods but mostly by instrumental neutron activation analysis.Results indicate the consistent distribution of fine-grained gold in soils which allows Au analysis of relatively small samples from B-horizon soils to be used effectively and reliably to identify the surficial patterns of gold mineralization in the study area. Anomalous patterns of other indicator elements, Co, As, Cu, Sb, W, Pb, Zn, Ag, Fe and Mn, may contribute additional information regarding type of mineralization. This finding indicates the effectiveness of soil surveys in gold exploration, particularly in areas of deep weathering where fresh bedrock exposures are infrequent.Unlike soils, size distributions of gold in stream sediments, as a result of the local flow regime, vary both between sampling sites and at depths within a sampling site. Exploration requires Au analysis of the fine fraction (minus 63 μm) of active stream sediments to reduce the problem of sampling representativity. The presence of coarse-grained gold in the stream channel has drawn attention to the possible benefit of using the conventional field-panning method as a semiquantitative technique for providing immediate results. However, highly erratic distribution of pannable gold on a very local scale together with variable proportions of the total gold recovered in field-panned or heavy-mineral concentrates highlights a potentially serious drawback of the method. Combination of analysis of the minus 63 μm fraction and field panning appears warranted to cover the possible existence of gold of a wide size range in stream sediments.The overall results indicate the utility of geochemical exploration techniques in the search for gold mineralization. However, particular care is necessary in the design and implementation of geochemical techniques to ensure maximum reliability of exploration.     

Distribution and dispersion of lead and zinc in anomalous soils and stream sediments, Mount Hermon area, Israel

Geochemical field and laboratory tests of soils and stream sediments in an area of weak surficial lead-zinc mineralization resulted in the delineation of two anomalous zones. Lead-zinc values are exceptionally high, but downslope and downstream mobility is low.The metals occur in base-exchangeable positions (5%), with iron oxides (45–60%), in clay lattice positions (15–30%), as resistant minerals (2–5%), and in organic matter (up to 20%).Intense dissolution and redeposition of metals occurred probably close to their point of origin in residual soils.     

A Microscopic and Nanoscale Understanding of the Formation of Gold Geochemical Provinces

Gold geochemical provinces in China were delineated through stream sediment or catchment sediment sampling in this study. Each gold geochemical province delineated by a threshold value of 2.5 ng/g covers an area of thousands of kilometers. It is a dilemma that geochemists traditionally thought that gold could not migrate for a long distance in rivers or streams to form a large-scale geochemical anomaly due to its chemical inertness and high specific density. The quantitative spectroscopic analysis and observations under a scanning electron microscope(SEM) indicate the presence of submicroscopic gold particles(5 μm in diameter) in large quantities, and observations under a Transmission Electron Microscopy(TEM) further suggest the presence of nanoscale gold particles(several tens of nanometre in diameter) in ores, rocks, soils and stream sediments. Particularly, submicroscopic and nanoscale gold particles less than 5 μm were only found in samples having a low gold content(10ng/g). This result shows that geochemical provinces, delineated by a threshold value of 2.5 ng/g, are formed by long-distance transport of ultrafine gold in streams. The findings may provide direct microscopic evidence for gold migration to form geochemical provinces.     

Geochemical distribution of arsenic in waters,sediments and weathered gold mineralized rocks from Iron Quadrangle,Brazil

The Iron Quadrangle has been the scenery of the most important gold production in Brazil. It is estimated that during the three centuries of gold mining in the Iron Quadrangle, at least 390,000 t of arsenic was discharged into the drainage system. This study presents geochemical data for the three river basins in the region, with focus on surface water and stream sediment monitoring. Samples of primary and oxidized sulfide ores as well as of tailings and groundwater from the major gold mines were also studied. The highest As concentrations in water and stream sediments occur in the vicinity of mining areas. In surface water, up to 300 g As/l were found whereas the As contents in stream sediments were in the range of 20 to 4,000 mg/kg. The As³⁺/As⁵⁺ concentration ratios obtained for some water samples range from 1.10^у to 4.10^ф. The As mobility associated with ore-deposit weathering could be traced in some closed gold mines by observation of in-situ pyrite and arsenopyrite oxidation, precipitation of scorodite and gippsite, As adsorption onto goethite, and final liberation of As into underground and surface waters. This process is likely to produce large volumes of mine effluents containing total As and trivalent As up to 1,960 and 60 g/l, respectively. River sediments and tailings pile samples were submitted to a leaching procedure showing maximal arsenic release from 1 to 4% of the original total As in the samples. There are potential risks for As hazards in some areas induced by, for instance, the dispersion of old tailings by flooding, occupation of poisoned soils for settlements, and occasional consumption of contaminated surface and groundwater.     

Effect of indigenous bacteria on geochemical behavior of arsenic in aquifer sediments from the Hetao Basin, Inner Mongolia: Evidence from sediment incubations

Incubation studies were carried out using 5 freshly collected sediments from shallow aquifers of the Hetao Basin, Inner Mongolia. The aquifer sediments covering a range of redox conditions, as indicated by their deep grey to yellow color were mixed with degassed artificial As solution or degassed deionized water at a ratio of solid to water of about 1:10 (wt./wt.). Suspensions which were either amended with glucose or autoclaved, were incubated in parallel with unamended suspensions. Five microcosm cultures of unamended sediments gradually release the equivalent of 0.03–0.30 μg/g As to the dissolved phase. The addition of glucose as a potential electron donor results in a marked stimulation in the mobilization of As (0.71–3.81 μg/g) in the amended incubations for all sediments. The quantity of As released accounts for 60–70% of As bound to Fe/Mn oxides in the original sediments. The microbially mediated mobilization of As with the organic nutrient as an electron donor is strongly associated with the As bound to Fe/Mn oxides, as well as the exchangeable As. During the incubations amended with glucose, 2–4% of the sediment Fe is released. The results suggest that the introduction of labile dissolved organic C into the yellowish sediment aquifers with As-free groundwater would reduce a significant proportion of the Fe(III) oxyhydroxides mediated by anaerobic bacteria respiration and increase groundwater As concentrations.     

Organic sulphur fixation in freshwater lake sediments and the implication for US ratios

A comparative analysis of two sediment cores from perialpine lakes (L. Zürich and L. Geneva) helped to clarify pathways of sulphur fixation in freshwater sediments. Despite greater than three-fold differences in lake water sulphate concentration equal amounts of sulphur are fixed in the sediments. The maximum sedimentary sulphur concentrations attain 1% (dry weight) which is similar to the sulphur content of modem, near-shore marine sediments. This is also expressed in low C _org/S_tot ratios which range from 2.5–8.
An analysis of the sulphur pools showed that organic sulphur compounds are the main components of the sedimentary sulphur, accounting for ˜ 80% and ˜ 60% of the total sulphur in Lakes Zürich and Geneva, respectively. The largest single sulphur pool is the sulphate-esters, which comprise 40–60% of the total sedimentary sulphur. Substantial amounts of the organic sulphur compounds must be formed within the sediment by boundary-layer microbial communities at the oxic-anoxic transition zone. This possibly suggests rapid recycling of sulphur in suboxic zones. Because of low concentration of SO²₄-_- in the watermass, the zone of sulphate reduction is very thin. This coupled with the mobility of some sulphide oxidizing microbial colonies (e.g. Beggiutoa ) potentially results in an almost complete recycling of the sulphur pool near the sediment-water interface. Reduced sulphur caught in this cycling process is unavailable for sedimentary pyrite formation. The C_org/S_pyriteratios for these sediments are thus high. If organically bound sulphur is not completely transformed into pyrite during late diagenesis, the C_org/ S_pyrite ratio can serve as a sensitive indicator for the salinities of anaent depositional environments.     

Influence of HCl/HF treatment on organic matter in aquifer sediments: A Rock-Eval pyrolysis study

Rock–Eval pyrolysis is increasingly used for the routine characterization of natural organic matter in soils and sediments. In this work the bulk composition of sedimentary organic matter (SOM) in sandy aquifer sediments is studied, as well as purified samples (isolation of SOM) by HCl/HF treatment. This treatment is necessary to avoid detection limit problems for samples with low SOM contents, but the results presented here indicate that this treatment influences the organic geochemistry of the aquifer sediment samples. The FID and CO₂/CO pyrograms show a shift of 10–40 °C of the major peak to a lower temperature. Organic matter alteration or removal of components containing O-bearing groups may explain this. It is also suggested that destruction of the mineral matrix may lead to the reduced retention of the material. For the change of the CO₂/CO pyrograms of the RC fraction only organic matter alteration seems to be likely. Concentrated organic matter samples may also accelerate the release of exothermic energy and influence the pyrograms. Results indicate that the organic matter concentration in the sample influences the measured total organic matter (TOM) content and the T_max of the FID pyrogram, while the sample loading (absolute organic matter amount) up to 80 mg in the Rock–Eval apparatus does not. The FID pyrograms can be deconvoluted into four subpeaks, which allows comparison of samples at various depths. Rock–Eval pyrolysis may only be routinely applied to characterize SOM in aquifer sediments when such systematic and analytical phenomena are taken into account.     

Distribution of chemical elements in calc-alkaline igneous rocks,soils, sediments and tailings deposits in northern central Chile

This study follows the paths of 32 chemical elements in the arid to semi-arid realm of the western Andes, between 27° and 33° S, a region hosting important ore deposits and mining operations. The study encompasses igneous rocks, soils, river and stream sediments, and tailings deposits. The chemical elements have been grouped according to the Goldschmidt classification, and their concentrations in each compartment are confronted with their expected contents for different rock types based on geochemical affinities and the geologic and metallogenic setting. Also, the element behavior during rock weathering and fluvial transport is here interpreted in terms of the ionic potentials and solubility products. The results highlight the similarity between the chemical composition of the andesites and that of the average Continental Crust, except for the higher V and Mn contents of the former, and their depletion in Mg, Ni, and Cr. The geochemical behavior of the elements in the different compartments (rocks, soils, sediments and tailings) is highly consistent with the mobility expected from their ionic potentials, their sulfates and carbonates solubility products, and their affinities for Fe and Mn hydroxides. From an environmental perspective, the low solubility of Cu, Zn, and Pb due to climatic, chemical, and mineralogical factors reduces the pollution risks related to their high to extremely high contents in source materials (e.g., rocks, altered zones, tailings). Besides, the complex oxyanions of arsenic get bound by colloidal particles of Fe-hydroxides and oxyhydroxides (e.g., goethite), thus becoming incorporated to the fine sediment fraction in the stream sediments.     

Characterization of arsenic occurrence in the water and sediments of the Okavango Delta,NW Botswana

Detailed chemical analyses were performed on surface water, groundwater and sediment samples collected from the Okavango Delta between February and November 2003 in order to examine the distribution and geochemistry of naturally occurring As in the area. Surface water in the Okavango Delta, which is neutral to slightly acidic and has high dissolved organic C (DOC), was found to be slightly enriched in As when compared to a global value for stream water. Of the 20 new borehole analyses from this project, six were found to have values exceeding 10 μg/L, the current World Health Organization provisional guideline value for As. The results from field speciation indicate that As(III) is slightly more predominant than As(V). There is a positive correlation between As and pH and between As and DOC in the groundwater samples. For the sediment samples, there is a positive correlation between As and Co, As and Fe, As and loss on ignition (LOI) and between As and the percent fines in the sample. Reductive dissolution of oxides and hydroxides in the sediments with organic C as an electron acceptor is the likely mechanism for the release of As from the sediments into the groundwater.     

森林沼泽景观元素次生分散富集特征

得尔布干森林沼泽区的景观特征是冬季寒冷,夏季降雨集中,中低山沟谷长而平缓,植被发育,土壤A层和水系沉积物中富泥炭,土壤呈弱酸性、弱还原环境.铅锌矿床形成的异常从岩石、土壤C层、土壤A层到水系沉积物元素含量降低,呈逐步分散.水系沉积物中Pb、Zn元素含量随粒度变细而降低,Ag、有机碳含量增高.水系中泥炭沉积物可吸附活动态金属元素,在背景地段可形成次生富集和"假异常".有机质影响水系沉积物异常,为干扰因素,应消除有机质影响.水系沉积物中碎屑沉积物能较好反映矿化异常.确定合理采样介质、密度,采样到位,可提高中大比例尺地球化学勘查效果.     

Evolution of organic matter in lignite-containing sediments revealed by analytical pyrolysis (Py–GC–MS)

Newly vegetated sites provide opportunities to enlighten organic matter (OM) transformation mechanisms in soils and sediments at very early stages of development which, in turn, is relevant to better understand general ecosystem functioning. Mine acid soils and sediments in the Lusatian open cast lignite mining district (Germany) contains a high concentration of fossil carbon (lignite) in ad mixture with recent OM from the local vegetation, both contributing to the humified OM pool. In this study, analytical pyrolysis (Py–GC–MS) was used to monitor the different C sources (lignite or plant derived) in developing mine tailing soils and sediments and their degree of degradation in contrasting environments. Representative vegetation and the organic carbon (OC) rich soil/sediment fraction (humus fraction) were sampled at two depths (0–5 and 5–10 cm) in three plots along a transect covering an upland forest soil, a partially submerged sediment at the land–water interface and a constantly submerged sediment. The analysis of plant (lipds, isoprenoids, methoxyphenols and carbohydrates) and possible lignite (alkyl napththalenes, alkyl benzenes and PAHs) biomarkers released after pyrolysis supports previous findings in the area using other proxies. It was possible to discern OM sources in soil/sediment humus fractions, both from the substrate (lignite) as well as from the prevailing vegetation of the area. Environmental conditions in the submerged sediment seem to favour OM protection and the accumulation of decomposing plant material, whereas more intense OM degradation seems to prevail in the land–water interface areas characterized by fluctuating water level. In addition, a well resolved series of organic sulfur compounds (OSCS) found in the submerged sediments of rehabilitated acid lakes, indicates the possible occurrence of particular mechanisms of C preservation in this extreme anoxic S rich environment, i.e. via sulphur “quenching” with plant derived lipids during early diagenesis.