Acid Weathering Research Articles

Mineralogy and evolution of the surface of Mars: A review

We review the mineralogy of the surface of Mars, using data from various sources, including in situ characterisations performed by landers, remote observations from orbit, and studies of the SNC meteorites. We also discuss the possible alteration processes and the factor controlling them, and try to relate the mineralogical observations to the chemical evolution of the surface materials on Mars in order to identify the dominant process(es). Then we try to describe a possible chemical and mineralogical evolution of the surface materials, resulting from weathering driven by the abundance and activity of water. Even if weathering is the dominant process responsible for the surface evolution, all observations suggest that it is strongly affected locally in time and space by various other processes including hydrothermalism, volcanism, evaporites, meteoritic impacts and aeolian erosion. Nevertheless, the observed phases on the surface of Mars globally depend on the evolution of the weathering conditions. This hypothesis, if confirmed, could give a new view of the evolution of the martian surface, roughly in three steps. The first would correspond to clay-type weathering process in the Noachian, under a probable thick H 2O/CO 2-rich atmosphere. Then, during the Hesperian when water became scarcer and its activity sporadic, linked to volcanic activity, sulfate-type acidic weathering process would have been predominant. The third period would be like today, a very slow weathering by strongly oxidising agents (H 2O 2, O 2) in cold and dry conditions, through solid-gas or solid-films of water resulting frost-thaw and/or acid fog. This would favour poorly crystalline phases, mainly iron (oxy) hydroxides. But in this scenario many questions remain about the transition between these processes, and about the factors affecting the evolution of the weathering process.

Rapid natural acid weathering, physical erosion, and debris-flow hazards in scar areas developed on hydrothermally-altered rocks along the Red River Valley near Questa, New Mexico, USA

In southern Rocky Mountains, catchments characterized by acidic, metalliferous waters that are relatively unaffected by human activity usually occur within areas that have active or historical mining activity. The US Geological Survey has utilized these mineralized but unmined catchments to constrain geochemical processes that control the surfaceand ground-water chemistry associated with near surface acid weathering as well as to estimate premining conditions. Study areas include the upper Animas River watershed, Lake City, Mt. Emmons, and Montezuma in Colorado and Questa in New Mexico. Although host-rock lithologies range from Precambrian gneisses to Cretaceous sedimentary units to Tertiary volcanic complexes, mineralization is Tertiary in age and associated with intermediate to felsic composition, porphyritic plutons. Pyrite is ubiquitous. Variability of metal concentrations in water is caused by two main factors: mineralogy and hydrology. Parameters that potentially affect water chemistry include: host-rock lithology, intensity of hydrothermal alteration, sulfide mineralogy and chemistry, gangue mineralogy, length of flow path, precipitation, evaporation, and redox conditions. Springs and headwater streams have pH values as low as 2.5, sulfate up to 3700 mg/L and high dissolved metal concentrations (for example: A1 up to 170 rag/L; Fe up to 250 mg/L; Cu up to 3.5 mg/L and Zn up to 14 mg/L). With the exception of evaporative waters, the lowest pH values and highest Fe and A1 concentrations occur in water draining the most intense hydrothermally altered areas consisting of the mineral assemblage quartz-sericite-pyrite. Stream beds tend to be coated with iron floc, and some reaches are underlain by ferricrete. When iron-rich ground water interacts with oxygenated waters in the stream or hyporheic zone, ferrous iron is oxidized to ferric iron, which is less soluble, leading to the precipitation of iron oxyhydroxides. Ground-water wells have been drilled and sampled in two unmined, alpine catchments to characterize constituent concentrations, to identify hydrogeochemical processes controlling constituent concentrations, to determine rock hydraulic properties, and to delineate flow paths. By using an integrated approach to investigating surface and ground waters in these acidic catchments a more complete understanding of the hydrogeologic framework is gained.

In situ and experimental evidence for acidic weathering of rocks and soils on Mars

Experimental data for alteration of synthetic Martian basalts at pH = 0–1 indicate that chemical fractionations at low pH are vastly different from those observed during terrestrial weathering. Rock surface analyses from Gusev crater are well described by the relationships apparent from low‐pH experimental alteration data. A model for rock surface alteration is developed, which indicates that a leached alteration zone is present on rock surfaces at Gusev. This zone is not chemically fractionated to a large degree from the underlying rock interior, indicating that the rock surface alteration process has occurred at low water to rock ratio. The geochemistry of natural rock surfaces analyzed by APXS is consistent with a mixture between adhering soil/dust and the leached alteration zone. The chemistry of rock surfaces analyzed after brushing with the RAT is largely representative of the leached alteration zone. The chemistry of rock surfaces analyzed after grinding with the RAT is largely representative of the interior of the rock, relatively unaffected by the alteration process occurring at the rock surface. Elemental measurements from the Spirit, Opportunity, Pathfinder, and Viking 1 landing sites indicate that soil chemistry from widely separated locations is consistent with the low‐pH, low water to rock ratio alteration relationships developed for Gusev rocks. Soils are affected principally by mobility of Fe and Mg, consistent with alteration of olivine‐bearing basalt and subsequent precipitation of Fe‐ and Mg‐bearing secondary minerals as the primary control on soil geochemistry.

Cinética de liberação de potássio e magnésio pelos minerais da fração argila de solos do Triângulo Mineiro

Estudos sobre cinética de liberação de K e Mg permitem uma melhor compreensão da dinâmica desses nutrientes no solo e podem fornecer subsídios para adequação das recomendações de adubação e aumentar a produção das plantas. A seleção do local de amostragem foi definida com base nas diferenças entre os materiais de origem (Grupo Bauru, Araxá, São Bento e Coberturas Detritico-Lateríticas Terciárias), estádio de desenvolvimento dos solos e representação espacial das litologias. Os teores de K total da fração argila foram determinados após digestão das amostras com HF, HNO3 e H2SO4 concentrados. Para avaliar o potencial dos minerais da fração argila de liberar K e Mg para as plantas, a partir de formas inicialmente não-trocáveis e estruturais, foram utilizadas nove extrações seqüenciais dos nutrientes com solução de ácido cítrico 0,1 mol L-1, com os seguintes tempos de contato com a amostra: 2, 12, 24, 48, 96, 144, 192, 288 e 576 h. O tempo total acumulado de extração foi de 1.382 h. A liberação da reserva de K e Mg foi definida pelo estádio de intemperismo e material de origem dos solos. A fração argila dos Argissolos desenvolvidos de arenito da Formação Uberaba e de micaxisto/granito (Grupo Araxá), de maneira geral, apresentou os maiores teores totais de K e Mg e teores acumulados até 1.382 h de extração com ácido cítrico 0,1 mol L-1. Contudo, mesmo nos Latossolos (baixos teores totais), verificou-se liberação relativamente alta de K e Mg (teores acumulados até 1.382 h), quando comparada aos teores trocáveis da TFSA, evidenciando o potencial das plantas na utilização de formas não-trocáveis e estruturais desses nutrientes ao longo dos cultivos. A descrição da cinética de liberação de K e Mg pela equação parabólica de difusão mostrou que o processo ocorreu, a diferentes velocidades, em duas fases, tendo sido, na maioria das amostras, a taxa de liberação na primeira fase maior para o K e menor para o Mg. Este comportamento diferenciado indica o predomínio de K na forma não-trocável em sítios de média/alta energia de adsorção e de Mg na estrutura de minerais ferromagnesianos, predominantemente a biotita, retardando a liberação do nutriente pela dependência de reações de intemperismo promovidas pelo ácido cítrico.

The Role of Silicon in Suppressing Rice Diseases.

Some of these soil orders such as Ultisols and Oxisol account for 34% of the area of major soil orders in the tropics. These orders occupy great amounts of land (approximately 1,666 million hectares) in Africa and South and Central America (Fig. 2). Histosols and sandy Entisols also contain low levels of Si (57). As a result of Si leaching, the soluble Si content of tropical soils, such as Ultisols and Oxisols, is generally less than in most temperate soils (16). This might be one of the unidentified causes of lower rice productivity of many Fig. 1. A simplified acid weathering sequence in soils. Source: Friesen et al. (17). Reprinted from Advances in Agronomy, Vol. 58, Savant, N. K., Snyder, G. H., and Datnoff, L. E., Silicon management and sustainable rice production, p. 157, 1997, with permission from Elsevier.

Geochemistry and mineralogy of the 2.45 Ga Pronto paleosols, Ontario, Canada

The Ontario, Canada Pronto paleosols, which formed around 2.45 Ga on Archean granite, were studied through field observation, mineralogical study by microscope and microprobe, and chemical analyses of major and 45 trace elements, including rare earth elements (REEs). Since the first full description of its mineralogy and chemistry of major elements by Gay and Grandstaff [Gay, A.L., Grandstaff, D.E., 1980. Chemistry and mineralogy of Precambrian paleosols at Elliot Lake, Ontario, Canada. Precambrian Res. 12, 349–373], the Pronto paleosols are among the most cited paleosols of the Archean–early Proterozoic era, though several questions have been raised as to whether they are true in-place ancient soils. Results of this study show that the studied section meets the criteria for being in-place paleosols. Furthermore, the profile has no explicit traces of being strongly affected by hydrothermal alteration. The REE-containing accessory minerals were destroyed through weathering, and secondary phosphate minerals with REEs formed. All of these observations support the weathering origin of the Pronto paleosols. However, the profile should be identified as a saprolith, rather than paleosol, as even the uppermost samples exhibit the granitic texture of the parent rock. The samples in this study were collected mainly from two sites (Site I and Site II). Saprolith samples from Site I show the retention of ΣFe and increases of the Fe(III)/Fe(II) ratios and the chemical index of alteration (CIA) values toward the unconformity. As for the Site II samples, the ΣFe/Ti ratios show slight decreases with increasing CIA values in the upper part of section, whereas these samples also show the same trend of increasing Fe(III)/Fe(II) ratios as the samples from Site I. Microprobe analyses show that Fe from the parent rock distributes mainly in mafic minerals as hornblende, biotite, or chlorite, which were completely destroyed during the early stages of weathering. In the saprolith section, Fe is present in sericite aggregates in the form of obscure patches of iron oxide tens of micrometers in size. All these characteristics indicate that: (i) the Fe was not lost during soil formation at Site I and slight amount of Fe was lost at Site II; and (ii) ferrous Fe was quantitatively transformed to ferric Fe (i.e., oxidation) during soil formation. In the samples from both sites, the ratios of Al/Ti and Si/Ti decrease toward the top of the profile. Together with the destruction of minerals toward the unconformity, such as apatite, biotite and chlorite, the depletion of Al and Si suggests that the parental granite was weathered by an acidic weathering solution. During the Archean–early Proterozoic era, the high atmospheric partial pressure of CO 2 ( pCO 2) is the most probable candidate for producing the acidic weathering solution.

Modeling aqueous ferrous iron chemistry at low temperatures with application to Mars

Major uncertainties exist with respect to the aqueous geochemical evolution of the Martian surface. Considering the prevailing cryogenic climates and the abundance of salts and iron minerals on Mars, any attempt at comprehensive modeling of Martian aqueous chemistry should include iron chemistry and be valid at low temperatures and high solution concentrations. The objectives of this paper were to (1) estimate ferrous iron Pitzer-equation parameters and iron mineral solubility products at low temperatures (from < 0 °C to 25 °C), (2) incorporate these parameters and solubility products into the FREZCHEM model, and (3) use the model to simulate the surficial aqueous geochemical evolution of Mars.Ferrous iron Pitzer-equation parameters were derived in this work or taken from the literature. Six new iron minerals [FeCl2·4H2O, FeCl2·6H2O, FeSO4·H2O, FeSO4·7H2O, FeCO3, and Fe(OH)3] were added to the FREZCHEM model bringing the total solid phases to 56. Agreement between model predictions and experimental data are fair to excellent for the ferrous systems: Fe-Cl, Fe-SO4, Fe-HCO3, H-Fe-Cl, and H-Fe-SO4.We quantified a conceptual model for the aqueous geochemical evolution of the Martian surface. The five stages of the conceptual model are: (1) carbonic acid weathering of primary ferromagnesian minerals to form an initial magnesium-iron-bicarbonate-rich solution; (2) evaporation and precipitation of carbonates, including siderite (FeCO3), with evolution of the brine to a concentrated NaCl solution; (3) ferrous/ferric iron oxidation; (4) either evaporation or freezing of the brine to dryness; and (5) surface acidification.What began as a dilute Mg-Fe-HCO3 dominated leachate representing ferromagnesian weathering evolved into an Earth-like seawater composition dominated by NaCl, and finally into a hypersaline Mg-Na-SO4-Cl brine. Weathering appears to have taken place initially under conditions that allowed solution of ferrous iron [low O2(g)], but later caused oxidation of iron [high O2(g)]. Surface acidification and/or sediment burial can account for the minor amounts of Martian surface carbonates. This model rests on a large number of assumptions and is therefore speculative. Nevertheless, the model is consistent with current understanding concerning surficial salts and minerals based on Martian meteorites, Mars lander data, and remotely-sensed spectral analyses.

Factors Affecting the Formation, Nature, and Properties of Iron Precipitation Products at the Soil–Root Interface

A number of iron oxides (hematite, goethite, lepidocrocite, maghemite, and magnetite) or short‐range ordered precipitates (ferrihydrite) may be found in soil environments, but in the rhizosphere the presence of organic ligands released by plants (exudates) or microorganisms promote the formation of ferrihydrite. Iron ions are liberated into soil solution by acidic weathering of minerals and then precipitated either locally or after translocation in soil environments. Humic and fulvic acids as well as organic substances produced by plants and microorganisms are involved in the weathering of primary minerals. Organic compounds play a very important role in the hydrolytic reactions of iron and on the formation, nature, surface properties, reactivity, and transformation of Fe oxides. Organic substances present in the rhizosphere interact with Fe promoting the formation of ferrihydrite and organo‐mineral complexes. The solubility of Fe precipitation products is usually low. However, the formation of soluble complexes of Fe(II) or Fe(III) with organic ligands, usually present in the rhizosphere increases the solubility of Fe‐oxides. Mobilization of Fe from Fe oxides by siderophores is of great importance in natural systems. They can form stable Fe(III) complexes (pK up to 32) and thus mobilize Fe from Fe(III) compounds. These higher Fe concentrations are important for the supply of Fe to plant roots which excrete organic acids at the soil–root interface. Iron oxides adsorb a wide variety of organic and inorganic anions and cations, which include natural organics, nutrients, and xenobiotics. There is competition between anions and cations for the surfaces of Fe‐oxides. Root exudates suppress phosphate or sulfate adsorption on Fe‐oxides. This is a mechanism by which plant roots mobilize adsorbed phosphate and improve their phosphate supply. Anions adsorption on iron oxides modify their dispersion/flocculation behavior and thus their mobility in the soil system. That can increase or decrease the possibility of contact between Fe‐oxides and organics or organisms able to dissolve them.

Aspects of weathering and solute acquisition processes controlling chemistry of sub‐Alpine proglacial streams of Garhwal Himalaya, India

AbstractAn analytical study of major cations and anions of the proglacial streams of Garhwal Himalaya has been carried out to assess the weathering and geochemical processes in high altitude river basins. Calcium and magnesium are the major cations, and bicarbonate and sulphate are the most dominant anions in these waters. A high correlation among HCO3, Ca and Mg, a relatively high contribution of (Ca+Mg) to the total cations (TZ+) and high (Ca+Mg&amp;#38;sol;Na+K) ratio indicate carbonate weathering could be the primary source of the dissolved ions. Carbonic acid weathering is the major proton‐producing reaction in the Alaknanda River, while in the Bhagirathi River it is the coupled reaction which controls the solute acquisition processes. To know the geochemical factors controlling the chemical nature of water, R‐mode factor analysis on major ion data from Ganga headwater streams has been performed. Factor 1 in the Alaknanda River is explicitly a bicarbonate factor showing strong loading of EC, Ca, Mg, HCO3 and TDS. In the Bhagirathi River Factor 1 explains the sulphide dissolution and silicate weathering and Factor 2 explains carbonate weathering. Wide downstream variations are observed in the total dissolved solids (TDS) and total suspended matter (TSM) in the headwater streams of the Ganga. Quartz and feldspar are the common detrital minerals, and kaolinite and illite the common clay minerals in the suspended sediment. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Understanding soil change: soil sustainability over millennia, centuries, and decades

Preface Acknowledgements Foreword William A. Reiners and Pedro A. Sanchez Part I. Soil and Sustainability: 1. Concerns about soil in the modern world 2. Managing soils for productivity and environmental quality 3. Biogeochemical sciences in support of soil management 4. The science of estimating soil change 5. Soil change over millennia, centuries and decades 6. The Calhoun forest: a window to understanding soil change Part II. Soil Change over Time Scales of Millennia: Long-Term Pedogenesis 7. Soil development from the Devonian to Mendocino and Hawaii 8. Genesis of advanced weathering-stage soils at the Calhoun ecosystem 9. The Calhoun soil profile 10. The forest's biogeochemical attack on soil minerals Part III. Soil Change over Time Scales of Centuries: Conversion of Primary Forests to Agricultural Fields: 11. Agricultural beginnings: Native American cultivation 12. Soil biogeochemistry in cotton fields of the Old South 13. Agricultural legacies in old-field soils Part IV. Soil Change over Time Scales of Decades: Conversion of Agricultural Fields to Secondary Forests: 14. The birth of a new forest 15. Accumulation and rapid turnover of soil carbon in a re-establishing forest 16. Satisfying a forest's four-decade nitrogen demand 17. Soil re-acidification and circulation of nutrient cations 18. Changes in soil-phosphorus fractions in a re-establishing forest Part V. Soil Change and the Future: 19. The case for long-term soil-ecosystem experiments Epilogue Recommended readings Appendix I. Carbonic acid weathering reactions Appendix II. Simulation of bomb-produced 14C in the forest floor at the Calhoun Experimental Forest, SC Appendix III. Sources of variation in the Calhoun Experimental Forest's main analysis of variance (ANOVA) Appendix IV. Total elemental concentrations for soils from the Calhoun Experimental Forest, SC References Index.

Effects of atmospheric deposition, soil pH and acidification on heavy metal contents in soils and vegetation of semi-natural ecosystems at Rothamsted Experimental Station, UK

The effects of acidification on the soil chemistry and plant availability of the metals Pb, Cd, Zn, Cu, Mn and Ni in new and archived soil and plant samples taken from the >100-year-old experiments on natural woodland regeneration (Geescroft and Broadbalk Wildernesses) and a hay meadow (Park Grass) at Rothamsted Experimental Station are examined. We measured a significant input of metals from atmospheric deposition, enhanced under woodland by 33% (Ni) to 259% (Zn); Pb deposition was greatly influenced by vehicle emissions and the introduction of Pb in petrol. The build up of metals by long-term deposition was influenced by acidification, mobilization and leaching, but leaching, generally, only occurred in soils at pH<4. Mn and Cd were most sensitive to soil acidity with effective mobilization occurring at pH 6.0–5.5 (0.01 M CaCl2), followed by Zn, Ni and Cu at pH 5.5–5.0. Pb was not mobilized until pH<4.5. Acidification to pH 4 mobilized 60–90% of total soil Cd but this was adsorbed onto ion exchange surfaces and/or complexed with soil organic matter. This buffering effect of ion exchange surfaces and organic matter in soils down to pH 4 was generally reflected by all the metals investigated. For grassland the maximum accumulation of metals in herbage generally corresponded to a soil pH of 4.0. For woodland the concentration of Pb, Mn and Cd in oak saplings (Quercus robur) was 3-, 4- and 6-fold larger at pH 4 than at pH 7. Mature Oak trees contained 10 times more Mn, 4 times more Ni and 3 times more Cd in their leaves at pH 4 than at pH 7. At pH values <4.0 on grassland the metal content in herbage declined. Only for Mn and Zn did this reflect a decline in the plant available soil content attributed to long-term acid weathering and leaching. The chief cause was a long-term decline in plant species richness and the increased dominance of two acid-tolerant, metal-excluder species

Vermiculitization of Phlogopite in Metagabbro, Central Turkey

AbstractDioctahedral vermiculite occurs in an isolated metagabbro klippe (Kurançalı Metagabbro) that belongs to the Central Anatolian Ophiolites from central Turkey. Both the metagabbro and the structurally underlying high-grade metamorphic rocks are intruded by granitic rocks. The Kurançalı Metagabbro is characterized by its well-developed compositional layering, and the presence of vermiculitized phlogopiterich layers. Petrographic and mineralogie studies show that the primary mineral phases in the host rock are diopside, tschermakitic hornblende, Fe-rich phlogopite, and plagioclase. Secondary minerals are hornblende, actinolitic hornblende, Fe-rich phlogopite, and vermiculite. A two-phase history of alteration involving acidic weathering and alkaline metasomatism is suggested for the dioctahedral vermiculite and secondary Fe-rich phlogopite, respectively. The alteration of phlogopite to dioctahedral vermiculite proceeded both along cleavage planes and at crystal edges. The vermiculite is colorless to pale yellow with weak pleochroism and shows optical continuity with the parent mineral. Vermiculite flakes, analyzed semi-quantitatively by scanning electron microscope-energy dispersive analysis (SEM-EDS) and electron microprobe (EMP), are characterized by partially expanded interlayers, K depletion, and Mg and/or A1 enrichment. X-ray diffraction (XRD) and differential thermal analysis-thermal gravimetric (DTA-TG) analyses indicate that phlogopite is not a pure phase, although it is the dominant one. The XRD patterns show the presence of both dioctahedral vermiculite having dehydrated interlayers and hydroxy-Al interlayers, and interstratified phlogopite-vermiculite. The transformation of phlogopite to vermiculite is thought to represent an initial stage of weathering in an acidic environment.

Atmospheric oxidation of the pyritic waste rock in Maardu, Estonia, 2: an assessment of aluminosilicate buffering potential

The assessment of the aluminosilicate buffering potential during acid weathering of the Estonian alum shale is provided. It is found that the stoichiometric interaction between dissolved pyrite oxidation products and illite of the shale best describe the buffering process and are consistent with earlier field studies. The scheme includes incongruent dissolution of illite with smectite and K-jarosite precipitating. This complex mechanism involves buffering of 8% of the acidity by K+ and temporary precipitation of 25% of the acidity as K-jarosite. Dissolution proceeds at a low pH (1.5–3) until all pyrite in the shale particle is oxidised. Hence, if the total amount of illite present is larger than needed for stoichiometric interactions, only part of it is involved in a buffering process, neutralising a certain percentage of acidity. The next stage in shale weathering is the incongruent dissolution of K-jarosite with the release of the precipitated acidity and the formation of ferric oxyhydroxide.

Low-Temperature Acid Weathering in Newhaven, Sussex, United Kingdom, and Its Application to Theoretical Modeling in Radioactive Waste-Disposal Sites

AbstractTertiary weathered sediments located immediately to the west of the harbor at Newhaven, Sussex, UK, were investigated by examination of major and trace elements by scanning electron microscope (SEM), microprobe, and inductively coupled plasma mass spectrometer (ICP-MS), and the mineralogy was studied by optical petrography, X-ray diffraction (XRD), transmission electron microscope (TEM), selective leaching, and thermodynamic modeling. Studied outcrops experienced acid leaching by sulfuric acid percolating downward through Tertiary sediments overlying Cretaceous chalk. The progressive neutralization of the percolating acid fluids resulted in “sequentially” layered neoformation of minerals: jarosite, iron oxides, aluminous minerals (sulfates, oxyhydroxides), gypsum, and Fe-Mn oxides. Substantial agreement was found between field observations and mineral assemblages obtained by modeling with the program CHILLER. These results suggest that the initial assumptions on the weathering process and mechanisms are correct. The relevance and implications of this study in the modeling of future denudation and weathering processes of radioactive waste-disposal sites (both deeply buried sites for high-leveI waste and surface sites for low-level waste) are discussed. Neoformed phases, such as jarosite, aluminous minerals, and silico-aluminous gels may play a significant role in the efficient trapping of mobilized pollutant radionuclides.

Palaeoclimatic interpretation of clay minerals in marine deposits: an outlook from the continental origin

As distribution of clay minerals in modern oceans seems to be controlled by contemporary climates, marine clays in ancient deposits have been widely used to reconstruct palaeoclimates. In this review, we examine this question mainly from the continental origin of the clay minerals and show the difficulties of retrospective analysis of the influence of climate on common marine deposits. We examine successively how soils develop and behave, are reworked and palaeoweatherings are preserved, focusing the review on kaolinitic soils indicative of tropical wet climates. Several stages intervene between the time of clay mineral development in soils and their final deposition in the basin: (1) The persistence over geological times of huge amounts of kaolinitic palaeoweathering materials in the landscapes may seriously alter the palaeoclimatic signal of kaolinite in the sedimentary record. Although the kaolinite deposited in recent sediments of the world oceans is consistent with wet climatic zones, this is mainly because the major kaolinitic and bauxitic palaeosurfaces coincide with the tropical areas where kaolinite is still developing at present. (2) Possible development of deep kaolinitic profiles at high latitudes and cold conditions, under CO 2-enriched palaeoatmospheres, or even by unusual acidic weathering linked with dry climates is misleading palaeoenvironments for palaeoclimatic reconstruction. (3) Erosion and transport processes always cloud the signal to some extent, because frequently, erosion products from the bedrock and different soil horizons will be mixed and major drainage systems may include soils of different climatic zones. (4) The arrival of the soil clay assemblages in the basin inevitably lags against their formation on the continent and may occur several million years after formation of the soils on the continent and climate may have considerably changed at that time. It is pointed out that sedimentologists and palaeoclimatologists should consider the complexity of the relations between soil and sedimentary clay minerals. Soil formation rates are slow and therefore the resolution of the palaeoclimatic record in marine clay may not be closer than 1 or 2 Ma. At present, the palaeoclimatic interpretations of marine clay assemblages are yielding, at best, nothing more than rather broad palaeoclimatic information. This approach cannot achieve the same degree of resolution as other techniques such as isotope or microfossil studies. Clay mineral assemblages may provide integrated records of overall climatic impacts, whereas other techniques are more likely to reveal local or temporary climates.

Characterization of artificially produced copper and bronze patina by XPS

Miniaturized photoacoustic and photothermal sensors have been developed for non-destructive in situ control of the state of patinated copper and bronze monuments. For calibration purposes, various systematic surface and microanalytical studies have been carried out, including x-ray photoelectron spectroscopy (XPS) studies of artificially patinated copper and bronze substrates. In order to prevent patina sample damage by x-ray-induced reduction of cupric sulphates, it has been found necessary to perform XPS measurements at liquid nitrogen temperature. In this way reliable qualitative and semi-quantitative analyses of copper and bronze patina could be accomplished. The XPS analyses have been successful in detecting partial chemical transformations of patina films upon artificial weathering. The acid weathering procedure applied, which was thought to imitate a very aggressive industrial atmosphere with rapidly changing relative humidity, produced less basic cupric sulphates such as chalcanthite both in pure antlerite and brochantite films and in mixed patina films.

Fixation of radiocaesium in an acid brown forest soil

SummaryThe OAh and Ah horizons of acid brown and podzolic forest soils are reported to fix more radiocaesium than the mineral B horizons beneath them. We determined the respective influence of organic matter and clay minerals on the magnitude of Cs+ retention in a strongly acid brown forest soil in Belgium. The soil contained mica throughout the profile. Vermiculite was identified in the OAh and Ah horizons, and hydroxy interlayered vermiculite (HIV) in the Bw horizon. The OAh and Ah clay fraction retained much more Cs+ than the Bw horizon. The extraction of Al interlayers by Na‐citrate resulted in a marked increase in Cs+ fixation in the Bw clays as well as the collapse of the vermiculitic layers after K+ saturation. Organic matter had a strong but indirect effect on Cs+ fixation. In the Bw horizon, acid weathering of layer silicates releases free Al and produces HIV minerals in which Al polymers block the access of radiocaesium onto Cs+‐specific sites. In OAh and Ah horizons, free Al is complexed by organic acids. Consequently, the interlayer specific sites remain accessible for Cs+ fixation.

Geochemistry, origin and recharge mechanisms of groundwaters from the Garoua Sandstone aquifer, northen Cameroon

This paper presents results of the geochemical and isotopic studies on groundwater samples from the Garoua Sandstone Basin, in northern Cameroon. Chemical and environmental isotope data are presented and discussed in terms of the origin of dissolved species and of groundwater. All of the investigated groundwaters are categorized into two chemical types: low mineralized and acidic waters of CaNaHCO 3 type, and relatively high mineralized waters of NaHCO 3 type, with relatively high pH, between 7.40 and 8.22. Interpretation of chemical data, based upon both thermodynamic calculations and stability diagrams, suggests that the chemical evolution of groundwater is primarily controlled by water-rock interactions, involving (1) acidic weathering of aluminosilicates, (2) dissolution of secondary carbonate minerals, and (3) cation exchange of Na + for Ca 2+. However, the original composition of groundwater may have been modified by further secondary processes such as mixing of chemically different water masses, and anthropogenic NO − 3 pollution. Tritium contents in the considered groundwaters are very heterogeneous, suggesting differences in the water turnover time or in the recharge conditions, or both. On the basis of the geographical distribution of 3H contents, a hydrogeological relation between river and the groundwater systems has been proposed for the study site. Interpretation of both 18O and 2H, as well as 3H, suggests that the recharge of the investigated groundwaters may result from three major mechanisms: (1) direct infiltration of local precipitation; (2) lateral inflow of river waters; (3) probably upward leakage of old groundwaters.

Effect of Permafrost and Palaeo-Environmental History on Soil Formation in the lower Kolyma Lowland, Siberia

Abstract Danish Journal of Geography 96: 40–50, 1996. The Geography of soils has been studied in the tundra of the sandy Kolyma Lowland between latitudes 69 ° and 70 ° N. Generally, soils show characteristics indicative of variable intensity of previous fossil Podzol formation in the present day cryoturbated active layer. Hence, the transformation, transfer and segregation of chemical substances in the soil profile can be interpreted as evidence of palaeo-climate changes. Fossil soil features indicate horizon differentiating processes active in a former warmer and wetter climate. The chemical composition reflects a general sequence of pedogenetic translocation processes. Early stages of soil formation primarily involved translocation of weathering products from a carbonic acid weathering system. Inorganic Al-Fe silicate and phosphate materials were translocated into B horizons as the A horizon became more acid. At a later stage the increasing activity of simple organic acids in the A horizons caused a mig...

Rates of oxidative weathering on the surface of Mars

The presence of sulfates and ferric‐bearing assemblages in the Martian regolith implies that oxidative weathering of iron‐ and sulfur‐bearing minerals has occurred through time on the surface of Mars. A model of acid weathering has been proposed for the iron‐rich basalts on Mars. In this model, aqueous oxidation of iron sulfides released SO42− and H+ ions that initiated the dissolution of basaltic ferromagnesian silicates and released Fe2+ ions. The Fe2+ ions eventually underwent ferrolysis reactions and produced insoluble hydrous ferric oxide phases. Measurements of the time‐dependence of acid weathering reactions have shown that pyrrhotite (Fe7S8) is rapidly converted to pyrite (FeS2) plus dissolved ferrous iron, the rate of pyrite formation decreasing with rising pH and lower temperatures. The rate of oxidation of aqueous Fe2+ ions derived from weathering of iron sulfides by dissolved atmospheric oxygen is known to be pH‐dependent; rates are fast above pH 4.5 at ambient temperatures on the Earth's surface and much slower in acidic groundwater. On Mars, oxidation rates of dissolved Fe2+ ions in equatorial melt‐waters in contact with the atmosphere (PO2 = 10−5 atm) are estimated to lie in the range 0.3–3.0 ppb Fe yr−1 over the pH range 2 to 6. These oxidation rates are more than 105 times slower than corresponding reactions near the freezing point of water on Earth. Oxidation of Fe2+ ions is estimated to be extremely slow in brine eutectic solutions that might be present on Mars and to be negligible in the frozen regolith. Such slow oxidation rates of aqueous Fe2+ ions in acidic groundwater (now permafrost) during the evolution of the Martian regolith would aid the stability, solubility and transport of iron in solution and other dissolved species over the surface of Mars.