Al speciation in tropical podzols of the upper Amazon Basin: A solid-state Al MAS and MQMAS NMR study

In the upper Amazon Basin, aluminum previously accumulated in lateritic formations is massively remobilised in soils by podzolization and exported in waters. We have investigated the speciation of aluminum in the clay-size fractions of eight horizons of waterlogged podzols lying in a depression of a plateau. The horizons illustrate the main steps involved in the podzolization of laterites. They belong to eluviated topsoil A horizons and illuviated subsoil Bhs, Bh and 2BCs horizons of weakly and better-expressed podzols located at the margin and centre of the depression. For the first time, aluminum speciation is quantitatively assessed in soils by spectroscopic methods, namely FTIR, ²⁷Al magic angle spinning (MAS) and multiple-quantum magic angle spinning (MQMAS), nuclear magnetic resonance (NMR). The results thus obtained are compared to chemical extraction data.Solid-state ²⁷Al MAS NMR spectra enable to distinguish Al bound to organic compounds from that incorporated in secondary mineral phases detected by FTIR. MQMAS experiments additionally show that both chemical shifts and quadrupolar constants are distributed for Al nuclei linked with organic compounds. Similar amounts of chelated Al are obtained from NMR spectra and chemical extractions. The study enables to highlight three major steps in the fate of aluminum. (i) Aluminum is first released by mineral weathering, feeds complexing sites of organic matter and accumulates in subsurface Bhs horizons of weakly expressed podzols (acidocomplexolysis). (ii) Complexes of aluminum with organic matter (Al-OM) then migrate downwards in sandy horizons of better-expressed podzols and accumulate at depth in less permeable 2BCs horizons. (iii) The minor amounts of aluminum present in the 2BCs horizon of the downslope podzol show that aluminum is eventually exported towards the river network, either complexed with organic matter or as Al³⁺ ions after desorption from organic compounds, due to decreasing pH or biodegradation of organic ligands. The direct spectroscopic determination of Al-speciation during the formation of podzolic soils opens new perspectives to trace metal loads in the rivers of the upper Amazon Basin.     

Fe and Al oxides distribution in some ultisols and inceptisols of southeastern Nigeria in relation to soil total phosphorus

Ten highly weathered soils in southeastern Nigeria were sampled from their typical A and B horizons for analyses. The objectives were to determine the different forms of Fe and Al oxides in the soils and relating their occurrence to phosphate availability and retention in the soils. The soils are deep and often physically degraded but are well drained and coarse in the particle size distribution. They are mostly dominated by kaolinite in their mineralogy with very high values of SiO₂. The soils are acidic with low soil organic carbon (SOC) contents. The elements in the exchange complex are also low thus reflecting in the low CEC of the soil. Available phosphorus (P) in the soils are generally low while total P ranged from 157 to 982 mg kg⁻¹ with an overall average of 422 mg kg⁻¹. Total Fe in the soil is highest and their order represented as follows: Fe_t > Fe_d > Fe_ox ≥ Fe_p. The pyrophosphate extractable Fe was always higher in the top soil than in the subsoil and was attributed to the fact that these forms of Fe are associated with organic matter which is more abundant in topsoil than in subsoil. Like in Fe forms, the order of Al occurrence could generally be presented as; Al_t > Al_d > Al_ox > Al_p. More Fe and Al oxides in the soils are strongly crystalline while a small quantity is poorly crystalline Fe forms. The amorphous forms of both Fe and Al are very low in the soils when compared with the crystalline forms. The oxides that show very strong affinity to total P are Fe_d–Fe_ox, Fe_d, Al_d, Fe_t, Fe_ox and Al_ox/Al_d. To overcome this problem of P retention in the soil, we recommend constant liming of these soils to neutralize them, application of organic matter and of high dosage of phosphate fertilizer to the soils.     

Sulphur behaviour in forest soils near the largest SO2 emitter in northern Europe

The impact of 60 a of SO₂ emissions from a Ni–Cu plant in the Kola Peninsula (Russia) on soil S contents was assessed in podzols under pine forests. Sulphate desorption and the possible delay of acidification reversal was investigated, because the plant will be reconstructed in 2006 with an expected emission reduction of 90%. Sites were sampled along a pollution gradient in the prevailing wind direction from 1 to 66 km. The investigated podzols stored S mostly in the organic form. The concentrations of total and organic S in soil organic horizons tended to be higher near the smelter but were only weakly correlated with S deposition. No relationship between distance to smelter and S contents was found for the mineral horizons. Sulphate content and desorption behavior were highly variable due to natural variations of texture and extractable Al and Fe contents of the soils. The lack of a clear strong trend with distance from the smelter except in the organic layer indicated that long range transport and diffuse input of SO₄ played a major role rather than point source impact. It was concluded that biological turnover is most likely the regulating process in these soils and thus low to medium release of SO₄ is expected under decreasing deposition scenarios because organic S was the dominant fraction of total S in all soils.     

Tracing source and evolution of suspended particles in the Rio Negro Basin (Brazil) using chemical species of iron

Suspended solids found in porewaters obtained in waterlogged soil sequences that included representative laterite–podzol transitions, associated brooks and major rivers of the Rio Negro watershed (Brazil) were studied using electron paramagnetic resonance (EPR) and Fourier-transform infrared spectroscopies. The main goal was to ascertain sources and track the evolution of suspended matter using a ubiquitous chemical species, FeIII complexed to organic matter (FeOM). Three size fractions were separated by tangential-flow (ultra)filtration: particulate (> 0.2 μm), dense (P) and light (Ps), and colloidal (5 kD < Col. < 0.2 μm) fractions. Quantitative results were acquired for Col. and Ps fractions which are predominantly organic in nature.FeOM concentration (in ‰ dry weight) was determined to be relatively low in suspended solids found in black waters from podzol porewaters and brooks whereas in the main rivers it was several times higher. FeOM concentrations were also correlated with Fe(II)/Fe(III) ratios in solution; these ratios were high in podzol porewaters and low in the rivers. Considering that organic complexation of Fe(II) is minor when compared to that of Fe(III), two interpretations were proposed to account for the above observation. First, [FeOM] was assumed to be distributed along a mixing line, with the clear waters from laterites and the black waters from podzols being its end-members. Consequently, [FeOM] can be used to trace the source of suspended material. Second, dissolved Fe(II) from podzol areas was considered to be progressively oxidized as pore waters move towards the mainstream. According to this mechanism, iron is complexed by organic matter or precipitated as oxides, thus producing an evolution of colloidal matter. As a result of these mechanisms' action, both the high production of Fe(II) and organic matter at the waterlogged podzol–laterite transition areas are major factors affecting iron export in the Rio Negro watershed.     

Iron solid-phase differentiation along a redox gradient in basaltic soils

Iron compounds in soil are multifunctional, providing physical structure, ion sorption sites, catalytic reaction-centers, and a sink for respiratory electrons. Basaltic soils contain large quantities of iron that reside in different mineral and organic phases depending on their age and redox status. We investigated changes in soil iron concentration and its solid-phase speciation across a single-aged (400 ky) lava flow subjected to a gradient in precipitation (2200-4200 mm yr⁻¹) and hence redox history. With increasing rainfall and decreasing Eh, total Fe decreased from about 25% to <1% of the soil mass. Quantitative speciation of soil solid-phase iron was constrained by combining ⁵⁷Fe Mössbauer spectroscopy (MBS) at 295 and 4.2 K with powder X-ray diffraction, selective chemical extractions, and magnetic susceptibility measurements. This approach allowed us to partition iron into (1) nanoparticulate and microcrystalline Fe^III-(oxy)hydroxides, (2) microcrystalline and bulk Fe^III-oxides, (3) organic/silicate bound Fe^III, and (4) ferrous iron. The Fe^III-(oxy)hydroxide fraction dominated solid-phase Fe, exhibiting a crystallinity continuum based on magnetic ordering temperature. The continuum extended from well-ordered microcrystalline goethite through nanocrystalline Fe^III-(oxy)hydroxides to a nano Fe^III-(oxy)hydroxide phase of extremely low crystallinity. Magnetic susceptibility was correlated (R² = 0.77) with Fe^III-oxide concentration, consistent with a contribution of maghemite to the otherwise hematite dominated Fe-oxide fraction. The Fe^III-(oxy)hydroxide fraction of total Fe decreased with increasing rainfall and was replaced by corresponding increase in the organic/silicate Fe^III fraction. The crystallinity of the Fe^III-(oxy)hydroxides also decreased with increasing rainfall and leaching, with the most disordered members of the crystallinity continuum, the nano Fe^III-(oxy)hydroxides, gaining proportional abundance in the wetter sites. This finding runs counter to the conventional kinetic expectation of preferential removal of the most disordered minerals in a reductive dissolution-dominated environment. We suggest the persistence of highly disordered Fe phases reflects the dynamic redox conditions of these upland soils in which periods of anoxia are marked by high water-throughput and Fe²⁺(aq) removal, while periodic Fe oxidation events occur in the presence of high concentrations of organic matter. Our ⁵⁷Fe Mössbauer study shows basalt-derived nano-scale Fe^III phases are more disordered than current synthetic analogs and have nano-structural characteristics that are linked to their formation environment.     

Revealing forms of iron in river-borne material from major tropical rivers of the Amazon Basin (Brazil)

The present study deals with the direct determination of colloidal forms of iron in river-borne solids from main rivers of the Amazon Basin. The contribution of different forms of colloidal iron have been assessed using ultrafiltration associated with various techniques including electron paramagnetic resonance spectroscopy (EPR), high resolution transmission electron microscopy (HRTEM), and micro proton-induced X ray emission analysis (μPIXE). EPR shows the presence of Fe³⁺ bound to organic matter (Fe³⁺-OM) and colloidal iron oxides. Quantitative estimate of Fe³⁺-OM content in colloidal matter ranges from 0.1 to 1.6 weight % of dried solids and decreases as the pH of the river increases in the range 4 to 6.8. The modeling of the field data with the Equilibrium Calculation of Speciation and Transport (ECOSAT) code demonstrates that this trend is indicative of a geochemical control resulting from the solubility equilibrium of Fe oxyhydroxide phase and Fe binding to organic matter. Combining EPR and μPIXE data quantitatively confirms the presence of colloidal iron phase (min. 35 to 65% of iron content), assuming no divalent Fe is present. In the Rio Negro, HRTEM specifies the nature of colloidal iron phase mainly as ferrihydrite particles of circa 20 to 50 Å associated with organic matter. The geochemical forms of colloidal iron differentiate the pedoclimatic regions drained by the different rivers, corresponding to different major weathering/erosion processes. Modeling allows the calculation of the speciation of iron as mineral, organic and dissolved phases in the studied rivers.     

Reactions of aqueous iron-DFOB (desferrioxamine B) complexes with flavin mononucleotide in the absence of strong iron(II) chelators

The mechanisms controlling microbial uptake of Fe^III-siderophore complexes and subsequent release of the metal for cellular use have been extensively studied in recent years. Reduction of the Fe^III center is believed to be necessary to labilize the coordinated Fe and facilitate exchange with cellular ligands. Previous studies report reduction of Fe^III-DFOB by various reducing agents in solutions containing Fe^II-chelating colorimetric agents for monitoring reaction progress, but the importance of these findings is unclear because the colorimetric agents themselves stabilize and enhance the reactions being monitored. This study examines the reduction of Fe^III complexes with DFOB (desferrioxamine B), a trihydroxamate siderophore, by the fully reduced hydroquinone form of flavin mononucleotide (FMN_HQ) in the absence of strong Fe^II-chelating agents, and Fe redox cycling in solutions containing DFOB and oxidized and reduced FMN species. Experimental results demonstrate that the rate and extent of Fe^III-DFOB reduction is strongly dependent on pH and FMN_HQ concentration. At pH ? 5, incomplete Fe^III reduction is observed due to two processes that re-oxidize Fe^II, namely, the autodecomposition of Fe^II-DFOB complexes (Fe^II oxidation is coupled with reduction of a protonated hydroxamate moiety) and reaction of Fe^II-DFOB complexes with the fully oxidized flavin mononucleotide product (FMN_OX). Chemical speciation-dependent kinetic models for the forward reduction process and both reverse Fe^II oxidation processes are developed, and coupling kinetic models for all three Fe redox processes leads to successful predictions of steady-state Fe^II concentrations observed over a range of pH conditions in the presence of excess FMN_HQ and FMN_OX. The observed redox reactions are also in agreement with thermodynamic constraints imposed by the combination of Fe^III/Fe^II and FMN_OX/FMN_HQ redox couples. Quantitative comparison between kinetic trends and changing Fe speciation reveals that FMN species react predominantly with diprotonated Fe^III-DFOB and Fe^II-DFOB complexes, where protonation of one hydroxamate group opens up two Fe coordination positions. This finding suggests that ternary complex formation (FMN-Fe-DFOB) facilitates inner-sphere electron transfer reactions between the flavin and Fe center.     

Geochemistry of radium in soils of the Eastern United States

The abundance and chemical/mineralogical form of ²²⁶Ra, ²³⁸U and ²³²Th were determined on samples of soil and associated vegetation at 12 sites in the eastern United States. Progressive, selective chemical extraction plus size fractionation determined the abundance and radiometric equilibrium condition of these nuclides in 6 operationally defined soil fractions: exchangeable cations, organic matter, “free” Fe-oxides, sand, silt, and clay.In soils, profile-averaged ²²⁶Ra/²³⁸U activity ratios (AR) are within 10% of unity for most sites, implying little fractionation of U and Ra when the entire soil profile is considered. However, ²²⁶Ra greatly exceeds ²³⁸U activity in most surface soil (AR up to 1.8, av 1.22), in vegetation (AR up to 65, av. 2.8), in the exchangeable+organic fraction (AR up to 30, av. 13), in some soil Fe oxides (AR up to 3.5, av. 0.83) and in the C horizons of deeply weathered soils (AR up to 1.5).A major factor in Ra behavior is uptake by vegetation, which concentrates Ra>U and moves Ra from deeper soil to surface soil. Vegetation is capable of creating the observed Ra excess in typical surface soil horizons (AR up to 1.8, av. 1.22) in about 1000 a. Of the total Ra in an average A horizon, 42% occurs as exchangeable ions and in organic matter, but only 6–8% of the parent U and Th occur in these soil forms. In contrast, U is slightly enriched relative to Ra in Fe-oxides of A horizons, implying rapid chemical partition of vegetation-cycled U and Ra.In deeper horizons, transfer by vegetation and/or direct chemical partitioning of Ra into organic and exchangeable forms provides a source for unsupported ²²⁶Ra in Ra-rich organic matter, and leaves all soil minerals Ra-poor (AR=0.73). Organic matter evidently has a strong affinity for Ra.The phenomena discussed above are relevant to evaluation of indoor Rn hazard, and behavior of Ra at sites affected by radioactive waste disposal, phosphate tailings, Ra-rich brine, and uraniferous fertilizer.     

Abundance and fractionation of Al,Fe and trace metals following tidal inundation of a tropical acid sulfate soil

Tidal inundation was restored to a severely degraded tropical acid sulfate soil landscape and subsequent changes in the abundance and fractionation of Al, Fe and selected trace metals were investigated. After 5 a of regular tidal inundation there were large decreases in water-soluble and exchangeable Al fractions within former sulfuric horizons. This was strongly associated with decreased soil acidity and increases in pH, suggesting pH-dependent immobilisation of Al via precipitation as poorly soluble phases. The water-soluble fractions of Fe, Zn, Ni and Mn also decreased. However, there was substantial enrichment (2–5×) of the reactive Fe fraction (Fe_R; 1 M HCl extractable) near the soil surface, plus a closely corresponding enrichment of 1 M HCl extractable Cr, Zn, Ni and Mn. Surficial accumulations of Fe(III) minerals in the inter-tidal zone were poorly crystalline (up to 38% Fe_R) and comprised mainly of schwertmannite (Fe₈O₈(OH)₆SO₄) with minor quantities of goethite (α-FeOOH) and lepidocrocite (γ-FeOOH). These Fe (III) mineral accumulations provide an effective substrate for the adsorption/co-precipitation and accumulation of trace metals. Arsenic displayed contrary behaviour to trace metals with peak concentrations (∼60 μg g⁻¹) near the redox minima. Changes in the abundance and fractionation of the various metals can be primarily explained by the shift in the geochemical regime from oxic–acidic to reducing-circumneutral conditions, combined with the enrichment of reactive Fe near the soil surface. Whilst increasing sequestration of trace metals via sulfidisation is likely to occur over the long-term, the current abundance of reactive Fe near the sediment–water interface favours a dynamic environment with respect to metals in the tidally inundated areas.     

Enthalpy of formation of sulfate green rusts

As a contribution to the systematic study of iron oxide thermodynamics, this work reports enthalpies of formation of green rust, a double layered (Fe^II, Fe^III) hydroxide with the ideal stoichiometry , with sulfate as the anion in the interlayer. Samples were characterized by X-ray powder diffraction, thermogravimetric analysis, infrared spectroscopy, and Mössbauer spectroscopy. Full chemical analysis was performed. Contents of Fe^II, Fe^III, water, and sulfate were obtained. We report standard enthalpies of formation for green rust with different Fe^II/Fe^III ratios. Enthalpies of formation from single cation compounds, namely, Fe(OH)₂, Fe(OH)₃, FeSO₄ and H₂O show reasonable agreement with Gibbs free energies of formation from single cation compounds recalculated from the reported literature values. These values show that green rust has little stabilization over a mechanical mixture of these single cation compounds and there is no thermodynamic preference for any particular Fe^II/Fe^III ratio.     

Mössbauer and ELNES spectroscopy of (Mg,Fe)(Si,Al)O3 perovskite: a highly oxidised component of the lower mantle

(Mg,Fe)(Si,Al)O₃ perovskite samples with varying Fe and Al concentration were synthesised at high pressure and temperature at varying conditions of oxygen fugacity using a multianvil press, and were characterised using ex?situ X-ray diffraction, electron microprobe, Mössbauer spectroscopy and analytical transmission electron microscopy. The Fe³⁺/ΣFe ratio was determined from Mössbauer spectra recorded at 293 and 80?K, and shows a nearly linear dependence of Fe³⁺/ΣFe with Al composition of (Mg,Fe)(Si,Al)O₃ perovskite. The Fe³⁺/ΣFe values were obtained for selected samples of (Mg,Fe)(Si,Al)O₃ perovskite using electron energy-loss near-edge structure (ELNES) spectroscopy, and are in excellent agreement with Mössbauer data, demonstrating that Fe³⁺/ΣFe can be determined with a spatial resolution on the order of nm. Oxygen concentrations were determined by combining bulk chemical data with Fe³⁺/ΣFe data determined by Mössbauer spectroscopy, and show a significant concentration of oxygen vacancies in (Mg,Fe)(Si,Al)O₃ perovskite.     

Fractionation of U,Th, Ra and Pb from boreal forest soils by sequential extractions

To determine the mobility of natural radionuclides in boreal forest soil, a five-step sequential extraction procedure was carried out on soil samples taken from various depths down to 3 m on Olkiluoto Island, Finland, where there are plans to construct a spent nuclear fuel disposal repository in the bedrock. The extracted fractions studied were exchangeable, acid-soluble, reducible, oxidizable and tightly bound. It was found that the extractability of most of the radionuclides studied was dependent on the sample grain size and depth. All the elements were concentrated in the smallest grain size samples (<0.063 mm). The extraction behaviour of Th, however, did not vary with sample depth, and only about 10% of the Th was extracted by the time of the final extraction step. Stable Pb and ²¹⁰Pb, as well as Ba and Ra concentrations were strongly correlated in the extractions. Radium and Ba were leached more readily than the other elements; approximately 17% of the total Ra was found in the first fraction extracted, representing exchangeable ions. Uranium was more mobile in the topsoil horizons than in the lower horizons. In the topsoil samples, an average of 51% of the extractable U was leached in the second extraction step, representing the elements soluble in weak acids, whereas only 13% of the U in the subsoil samples was extracted in this step. This is probably due to changes in soil redox conditions lower down the soil profile. The extraction behaviour of Pb and Fe also suggests the presence of more reducing conditions in the deeper soil horizons, because the percentage of extractable Pb and Fe in the oxidizable fraction increased with sample depth.     

Iron monosulfide formation and oxidation in drain-bottom sediments of an acid sulfate soil environment

Iron monosulfide formation and oxidation processes were studied in the extensively drained acid sulfate soil environment of the Tweed River floodplain in eastern Australia. Porewater profiles of pH, Eh, SO₄²⁻, Fe²⁺, Fe³⁺, Cl⁻, HCO₃⁻, and metals (Cd, Co, Cr, Cu, Ni, Pb and Zn) were obtained using in situ dialysis membrane samplers (`peepers'). Concentrations of acid volatile S (AVS), pyrite, total S, reactive Fe, total and organic C, simultaneously extracted metals (SEMs) and total elemental composition by X-ray fluorescence, were determined on sediment samples. The oxidation of pyrite in the surrounding landscape provides a source of acidity, Fe, Al, SO₄ and metals, which are exported into the drainage system where they accumulate in the sediments and porewaters. Negative porewater concentration gradients of SO₄²⁻ and Fe²⁺, and large AVS concentrations in the sediments, indicate Fe monosulfides form rapidly under reducing conditions and consume acidity and metals. Oxidation of the sediments during previous drought episodes has resulted in the conversion of monosulfides and pyrite to oxidised Fe minerals and the release of acidity, SO₄²⁻, Fe³⁺, and metals to the surface waters. These formation and oxidation cycles show that Fe monosulfides play an important role in controlling water quality in the drainage system.     

Using atmospheric fallout to date organic horizon layers and quantify metal dynamics during decomposition

High concentrations of metals in organic matter can inhibit decomposition and limit nutrient availability in ecosystems, but the long-term fate of metals bound to forest litter is poorly understood. Controlled experiments indicate that during the first few years of litter decay, Al, Fe, Pb, and other metals that form stable complexes with organic matter are naturally enriched by several hundred percent as carbon is oxidized. The transformation of fresh litter to humus takes decades, however, such that current datasets describing the accumulation and release of metals in decomposing organic matter are timescale limited. Here we use atmospheric ²¹⁰Pb to quantify the fate of metals in canopy-derived litter during burial and decay in coniferous forests in New England and Norway where decomposition rates are slow and physical soil mixing is minimal. We measure ²¹⁰Pb inventories in the O horizon and mineral soil and calculate a 60-630 year timescale for the production of mobile organo-metallic colloids from the decomposition of fresh forest detritus. This production rate is slowest at our highest elevation (∼1000 m) and highest latitude sites (>63°N) where decomposition rates are expected to be low.We calculate soil layer ages by assuming a constant supply of atmospheric ²¹⁰Pb and find that they are consistent with the distribution of geochemical tracers from weapons fallout, air pollution, and a direct ²⁰⁷Pb application at one site. By quantifying a gradient of organic matter ages with depth in the O horizon, we describe the accumulation and loss of metals in the soil profile as organic matter transforms from fresh litter to humus. While decomposition experiments predict that Al and Fe concentrations increase during the initial few years of decay, we show here that these metals continue to accumulate in humus for decades, and that enrichment occurs at a rate higher than can be explained by quantitative retention during decomposition alone. Acid extractable Al and Fe concentrations are higher in the humus layer of the O horizon than in the mineral soil immediately beneath this layer: it is therefore unlikely that physical soil mixing introduces significant Al and Fe to humus. This continuous enrichment of Al and Fe over time may best be explained by the recent suggestion that metals are mined from deeper horizons and brought into the O horizon via mycorrhizal plants. In sharp contrast to Al and Fe, we find that Mn concentrations in decomposing litter layers decrease exponentially with age, presumably because of leaching or rapid uptake, which may explain the low levels of acid extractable Mn in the mineral soil. This study quantifies how metals are enriched and lost in decomposing organic matter over a longer timescale than previous studies have been able to characterize. We also put new limits on the rate at which metals in litter become mobile organo-metallic complexes that can migrate to deeper soil horizons or surface waters.     

Iron isotope composition of the suspended matter along depth and lateral profiles in the Amazon River and its tributaries

Samples of suspended matter were collected at different locations, seasons, depths and lateral profiles in the Amazon River and three of its main tributaries, the Madeira, the Solimões and the Negro rivers. Their iron isotope compositions were studied in order to understand the iron cycle and investigate the level of isotopic homogeneity at the river cross-section scale. Samples from four depth profiles and three lateral profiles analyzed show suspended matter δ⁵⁷Fe values (relative to IRMM-14) between −0.501 ± 0.075‰ and 0.196 ± 0.083‰ (2SE). Samples from the Negro River, a blackwater river, yield the negative values. Samples from other stations (whitewater rivers, the Madeira, the Solimões and the Amazon) show positive values, which are indistinguishable from the average composition of the continental crust (δ⁵⁷Fe_IRMM-14 ∼ 0.1‰). Individual analyses of the depth and lateral profiles show no significant variation in iron isotope signatures, indicating that, in contrast to certain chemical or other isotopic tracers, one individual subsurface sample is representative of river deeper waters. This also suggests that, instead of providing detailed information on the riverine iron cycling, iron isotopes of particulate matter in rivers will rather yield a general picture of the iron sources.     

Heavy metal migration at a landfill site,Sarnia, Ontario,Canada—I. Thermodynamic assessment and chemical interpretations

The transition metals Fe, Cu, Zn and Pb have diffused only 10–20 cm into the clay barrier at the Confederation Road landfill compared to 130 cm for porewater chloride. Other major dissolved species, including the alkali and alkaline earth metals, have also diffused out of waste landfill and into clay subsoils more rapidly than the metals. Redox potentials, E_h, indicate strongly reducing conditions (E_h= − 130 mV) in the clayey soil at the subsoil/waste interface and increase to +50 mV at a depth of 45 cm below the interface. pH values are close to 8 within the subsoil but the slightly lower values (7.8–8) near the interface may result from production of organic acids during degradation of the wastes. Thermodynamic analysis of subsoil pore waters indicates that Fe, Cu, Zn and Pb exist primarily as metal-hydroxy complexes of the forms [MeOH]⁺ and [Me(OH)₂]⁰, although a complex of [PbCl]⁺ may be significant, but not predominant. The analysis also demonstrates that the dissolved transition metal concentrations of the subsoil pore waters are controlled at carbonate mineral saturation levels, whereas Fe concentrations in leachate solutions associated with the wastes are controlled at FeS₂ saturation levels.Thermodynamic calculations and E_h-pH diagrams suggest that Fe(OH)₂, Zn(OH)₂ and Pb(OH)₂ are not stable phases in the solids of the subsoil. This means that observed “hydroxide” phases reported in the selective dissolution analysis by Yanful and Quigley (1986) have to be re-evaluated.     

Crystal chemistry of ferric iron in (Mg,Fe)(Si,Al)O<Subscript>3</Subscript> majorite with implications for the transition zone

Fifteen samples of (Mg,Fe)SiO₃ majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O₃ majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe³⁺ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe²⁺–Fe³⁺ charge transfer, where band intensity increases with increasing Fe³⁺ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe³⁺ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO₃ majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe³⁺ concentration of (Mg,Fe)(Si,Al)O₃ majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O₃ perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.     

Dissolution and transport of manganese by organic acids and their role in sedimentary Mn ore formation

Dissolution experiments of Mn and Fe under natural conditions from fresh basalt, weathered basalt and Mn laterite by different organic acids show that Mn is highly enriched over Fe in solutions from the weathered rocks but that more Fe than Mn is dissolved from the fresh basalt. The enrichment of Mn is caused by sparingly soluble Fe-oxides and hydroxides and more soluble Mn-oxides. In addition from the weathered rocks the Mn concentration dissolved by the organic acids is up to 1000 times higher than in inorganic solutions. Mn enrichment is caused by acid attack, organic reduction of Mn⁴+ to Mn²+ and complexing by the organic acids. The complexed Mn is not attacked as easily by oxidation as free Mn ions. Higher concentrations of manganese in the organic dissolved stage can therefore be transported by rivers over greater distances. Organic complexed Mn, derived from lateritic weathered rocks may therefore contribute to the formation of low iron marine sedimentary Mn deposits.     

Effects of grazing and livestock exclusion on soil physical and chemical properties in desertified sandy grassland,Inner Mongolia,northern China

Heavy grazing is recognized as one of the main causes of vegetation and soil degradation and desertification in the semiarid Horqin sandy grassland of northern China. Soil physical and chemical properties were examined under continuous grazing and exclusion of livestock for 8 years in a representative desertified sandy grassland. Exclosure increased the mean soil organic C, total N, fine sand and silt + clay contents, inorganic C (CaCO₃), electrical conductivity, and mineral contents (including Al₂O₃, K₂O, Na₂O, Fe₂O₃, CaO, MgO, TiO₂, MnO), microelements (Fe, Mn, Zn, B, Cu, Mo), and heavy metals (Pb, Cr, Ni, As, Hg, Cd, Se), and decreased the coarse sand content, bulk density, and SiO₂ in the top 100 cm of the soil. Livestock exclusion also improved available N, P, K, Fe, Mn, and Cu, exchangeable K⁺, and the cation exchange capacity, but decreased pH, exchangeable Na⁺, and available S, Zn, and Mo in the top 20 cm of the soil. The greatest change in soil properties was observed in the topsoil. The results confirm that the desertified grassland is recovering after removal of the livestock disturbance, but that recovery is a slow process.     

Acidity status and mobility of Al in podzols near SO2 emission sources on the Kola Peninsula,NW Russia

Soil acidity status and Al mobility in podzols was examined on a broad scale near the large emission sources of SO₂ and heavy metals on the Kola Peninsula (the Severonikel and Pechenganikel smelter complexes) in NW Russia and in neighbouring parts of Norway and Finland. Acidification of the upper podzol horizons and depletion of mobile base cations were only evident at sites where ecosystems are severely destroyed, in the immediate vicinity of the Severonikel smelter complex. The high content of base cations in the parent material (till) near the emission sources may mask the acidification effect of pollution. Both strong anthropogenic (SO₂) emissions and natural acidification (in situ weathering of black schist) accelerate weathering and mobilize Al. However, drainage conditions seem to be the most important factor determining the content of mobile Al in the podzols.