Saprolite Formation Research Articles

Kaolinite dating from Acrisol and Ferralsol: A new key to understanding the landscape evolution in NW Amazonia (Brazil)

Ferralsols and Acrisols are major types of soils of the Amazon basin observed on various landform units comprising plateau surfaces, incised hills at their edges and slopes. The present study focuses on an Acrisol developed on plateaus surfaces from northwest Amazonia and a Ferralsol from the convex hills of the incised plateau edges. Local geomorphologic models of weathering covers suggest that Ferralsols are younger than plateau Acrisols but the absolute chronology of their formation is still lacking. This type of information is however critical to understand the evolution of northwest Amazon Basin landscapes and to identify its link with major climatic and geomorphologic events. In this article, kaolinite-rich samples from soils and saprolites belonging to a transect in the São Gabriel da Cachoeira region (Amazon state, Brasil) are investigated. Based on their crystal-chemical characters, several types of kaolinites are identified. Using a previously developed methodology based on electron paramagnetic resonance (EPR) spectroscopy, crystallization ages are proposed for these different kaolinites. Saprolite kaolinites are dated from 6 to 3.6 Ma in the Acrisol profile and display significantly more recent ages (<1 Ma) in the Ferralsol saprolite. Kaolinite from solum (soils horizons above the C horizon) display ages ranging from 2.5 to 1 Ma for both the Acrisol and Ferralsol. Three distinct weathering stages are thus unraveled by kaolinite dating. The Acrisol saprolite displays the older weathering stage preserved in the investigated soil sequence. It is followed by a single weathering stage leading to the formation of both soils. These two stages can be correlated to the formation of two paleosurfaces recognized at the scale of the South America subcontinent as the Miocene Vehlas and the Quaternary Paraguaçu surfaces, indicating that the Curicuriari profiles evolved during periods favorable to tropical weathering surfaces development in Amazonia. The last weathering stage corresponds to the saprolite formation in the Ferralsol profile, which is still developing under the present Amazonian climate. This still active, late weathering stage is tentatively related to the more significant drainage and relief dissection occurring on the plateau edges.

Geochemistry of canga (ferricrete) and evolution of the weathering profile developed on itabirite and iron ore in the Quadrilátero Ferrífero, Minas Gerais, Brazil

Mineralogical and whole rock geochemical analyses for 60 elements on 31 samples of hard ferruginous crust (canga) provide insights into the evolution of the lateritic profile developed on itabirite. Canga can form in two environments: in situ canga that typically caps itabirite and transported canga that covers country rock. Both have similar mineralogical and chemical compositions. Detrital haematite and rare quartz inherited from the itabirite and iron ore comprise the matrix of canga, cemented by goethite, minor gibbsite, and rare manganese oxides and secondary phosphates. Fe2O3 represents more than 91% of its chemical composition and the concentrations of trace elements are low, generally less than 50 ppm. A comparison of the chemical weathering of dolomitic itabirite against the quartz itabirite shows that, although weathering processes are less effective in the former, the geochemical trends of major and trace elements are similar. Negative Ce anomalies (Ce/Ce* = 0.8) and U/Th ratios lower than 1.5 suggest that saprolite formation occurred under slightly anoxic and mildly acidic conditions, allowing rare earth elements (REEs) to remain in the saprolite and also the formation of secondary Al phosphates, instead of Fe phosphates. These conditions became more aggressive during the canga formation process, resulting in further removal of trace elements from the system. The canga formation (pedogenesis) and the chemical weathering of the itabirite (saprolite formation) are independent, but interrelated processes that have been occurring since the Palaeocene.

Evidence for climatic and hillslope‐aspect controls on vadose zone hydrology and implications for saprolite weathering

AbstractThrough the delivery of water in snowmelt, climate should govern the rate and extent of saprolite formation in snow‐dominated mountain watersheds, yet the mechanisms by which water flows deeply into regolith are largely unexplored. In this study we link rainfall, snow depth, and water content data from both soil and shallow saprolite to document vadose zone dynamics in two montane catchments over 2 years. Measurements of snow pack thickness and soil moisture reveal strong contrasts between north‐ and south‐facing slopes in both the timing of meltwater delivery and the duration of significant soil wetting in the shallow vadose zone. Despite similar magnitudes of snowmelt recharge, north‐facing slopes have higher sustained soil moisture compared to south‐facing slopes. To help interpret these observations, we use a 2D numerical model of vadose zone dynamics to calculate the expected space–time moisture patterns on an idealized hillslope under two wetting scenarios: a single sustained recharge pulse versus a set of short pulses. The model predicts that the duration of the recharge event exerts a stronger control on the depth and residence time of water in the upper unsaturated zone than the magnitude of the recharge event. Model calculations also imply that water should move more slowly through the subsurface and downward water flux should be substantially reduced when water is applied in several pulses rather than in one sustained event. The results suggest that thicker soil and more deeply weathered rock on north‐facing slopes may reflect greater water supply to the deep subsurface. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Mapping bedrock lithologies through in situ regolith using retained element ratios: a case study from the Agnew-Lawlers area, Western Australia

Large, high-quality multi-element geochemical datasets are becoming widely available in the exploration industry, and afford excellent opportunities to investigate geochemical processes. A dataset of over 2500 analyses of unweathered and variably weathered mafic and ultramafic rocks for over 50 elements has been collected by Gold Fields Ltd. in the auriferous Agnew-Lawlers area of the eastern Yilgarn Craton of Western Australia. This dataset is used to investigate changes in element abundances and inter-element ratios through varying degrees and styles of weathering in an area of thick regolith characterised by deep in situ weathering. Systematic interrogation of the data, using lithostratigraphic controls derived from regional mapping and geophysics, reveals that a suite of elements, including Ti, Al, Zr, Th, La, Sc and Nb, and to a lesser extent Cr and Ni, behave as essentially immobile components during saprolite formation. In some cases diagnostic element ratios persist into siliceous duricrust. Ratios of these elements are used as reliable discriminants of bedrock type, and delineate features such as cryptic layering within fractionated sills and subtle geochemical variants in a sequence of tholeiitic and komatiitic basalts. Mapping on the basis of discriminant element ratios greatly extends previous trace-element ratio-based schemes for rock type discrimination. The potential to determine several of these elements with adequate precision and accuracy using portable XRF technology opens a potentially useful technique for rapid geochemical bedrock mapping in residual terrains.

Slow advance of the weathering front during deep, supply-limited saprolite formation in the tropical Highlands of Sri Lanka

Silicate weathering – initiated by major mineralogical transformations at the base of ten meters of clay-rich saprolite – generates the exceptionally low weathering flux found in streams draining the crystalline rocks of the mountainous and humid tropical Highlands of Sri Lanka. This conclusion is reached from a thorough investigation of the mineralogical, chemical, and Sr isotope compositions of samples within a regolith profile extending >10m from surface soil through the weathering front in charnockite bedrock (a high-grade metamorphic rock), corestones formed at the weathering front, as well as from the chemical composition of the dissolved loads in nearby streams. Weatherable minerals and soluble elements are fully depleted at the top of the profile, showing that the system is supply-limited, such that weathering fluxes are controlled directly by the supply of fresh minerals. We determine the weathering rates using two independent means: (1) in situ-produced cosmogenic nuclides in surface soil and creek sediments in the close vicinity of the regolith combined with immobile element mass balance across the regolith and (2) river dissolved loads. Silicate weathering rates determined from both approaches range from 16 to 36tkm−2y−1, corresponding to a weathering front advance rate of 6–14mmky−1. These rates agree across the 101 to 104ytime scales over which our rate metrics integrate, suggesting that the weathering system operates at steady state. Within error these rates are furthermore compatible with those obtained by modeling the advance rate of the weathering front from chemical gradients and mineral dissolution rates. The silicate weathering flux out of the weathering profile, measured on small creeks, amounts to 84% of the profile’s export flux; the remaining 16% is contributed by non-silicate, atmospheric-derived input. The silicate weathering flux, as measured by dissolved loads in large catchments, amounts to ca. 50% of the total dissolved flux; the remainder being contributed by dust, rain, and weathering of local marble bands. Spheroidal weathering is the key processes of converting the fresh bedrock into saprolite at the weathering front. The mineralogical composition of weathering rinds shows that the sequence of mineral decomposition is: pyroxene; plagioclase; biotite; K-feldspar. Observable biotite alteration does not appear to initiate spheroidal weathering within corestones; therefore, we infer that other processes than biotite oxidation, like pyroxene oxidation, clay formation from pyroxene and plagioclase decomposition, the development of secondary porosity by plagioclase dissolution, or even microbiologic processes at depth enable the coupling between slow advance of the weathering front and slow erosion at the surface. The comparison to tectonically more active tropical landscapes lets us conclude that the combination of hard rock with tightly interlocked mineral grains and slow erosion in the absence of tectonically-induced landscape rejuvenation lead to these exceptionally low weathering rates.

Meteoric 10Be concentrations from saprolite and till in northern Sweden: Implications for glacial erosion and age

We examine 10Be concentration in two pit profiles in the Parkajoki area at ∼67°N on the northern Fennoscandian shield in northern Sweden. Due to repeated cover by cold-based, non-erosive ice sheets, the area retains many relict non-glacial features, including tors and saprolites. In the examined pit profiles, gruss-type saprolite developed from weathering of intermediate igneous rocks is overlain unconformably by Weichselian till.Our results show that 10Be concentrations found in the till greatly exceed the levels of 10Be that can have accumulated since deglaciation at ∼11 ka and are comparable to those reported from Pliocene and Early Pleistocene tills in North America. Old tills with grussified boulders at depth were excavated in the Parkajoki area and correlations with neighbouring parts of Finland indicate a Middle Pleistocene or older age. Evidence from pit excavations and geochemistry shows that the underlying saprolites have been truncated by glacial erosion and that previously weathered material has been incorporated into the till sequence. Hence, 10Be inventories in the tills are dominated by material recycled from Middle Pleistocene or older soils, near-surface sediments and saprolite, and cannot be used to date the periods of till deposition. The retention of relict 10Be in the tills nonetheless confirms minimal glacial erosion.Concentrations of meteoric 10Be in the saprolites are lower than any reported saprolite concentrations measured in other settings. Uncertainty in the pre-glaciation 10Be concentrations in the saprolites makes age determinations difficult. One possibility is that that the saprolite had higher 10Be concentrations in the past but that saprolite formation ended after glaciation and burial by till and that the 10Be has substantially decayed. Modelling of the meteoric 10Be depth profiles in this case suggests that the saprolites in the Parkajoki area were formed at a minimum of 2 Ma. Erosion of the saprolite allows an older age of up to ∼5 Ma, with up to 250 cm of material removed and incorporated into later tills. A second possibility is that concentrations of meteoric 10Be in the saprolite were originally lower, with formation of the saprolite in a period or periods of ice- and permafrost-free conditions before 0.8 Ma.

A reactive-transport model for weathering rind formation on basalt

Saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution. Here we investigate the chemical and physical transformations that occur during weathering by studying small-scale “saprolites” in the form of weathering rinds, which form on rock in soil or saprolite and grow in thickness without physical disturbance with time. We compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in Costa Rica to diffusion-reaction simulations of rind formation using the fully coupled reactive transport model CrunchFlow. The four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include (1) the mineralogy of weathering products, (2) weathering rind thickness, (3) the coincidence of plagioclase and augite reaction fronts, and (4) the thickness of the zones of mineral reaction, i.e. reaction fronts. Four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations. To fit the model to all four criteria, however, it was necessary to (1) treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9%, and (2) treat mineral surface area as a fitting factor. This latter approach was presumably necessary because the mineral-water surface area of the connected (accessible) porosity in the Costa Rica samples is much less than the total porosity ( Navarre-Sitchler et al., 2009). The model-fit surface area, here termed reacting surface area, was much smaller than the BET-measured surface area determined for powdered basaltic material. In the parent basalt, reacting surface area and diffusivity are low due to low pore connectivity, and early weathering is therefore transport controlled. However, as pore connectivity increases as a result of weathering, the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics. The transition point between transport and kinetic control appears to be related to a critical porosity (9%) at which pore connectivity is high enough to allow rapid transport. Based on these simulations, we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface, and that this porosity increases because of mineral dissolution following a rate that is largely surface-reaction controlled.

Exploring links between vadose zone hydrology and chemical weathering in the Boulder Creek critical zone observatory

Understanding the relationship between subsurface flow paths on hillslopes and chemical weathering of bedrock is fundamental to understanding the timing and mechanisms that weather bedrock to saprolite. The link between chemical weathering of bedrock and contact time with reactive water along flow paths motivates this study. Water drives the chemical alteration of rock into saprolite, yet connected porosity generally declines with depth into the weathered profile. Saprolite formation, therefore, reflects coupled weathering and permeability development over time. This study uses numerical modeling and soil-moisture monitoring to explore the hydrology of the unsaturated zone and the influence of fracture density, hillslope gradient, and permeability contrasts within the saprolite development horizon on saprolite development.

Vadose zone controls on weathering intensity and depth: Observations from grussic saprolites

Abstract An investigation of vadose zone weathering processes has been undertaken on grussic saprolites developed on Californian granitoids. Preliminary results indicate strong climatic control, through infiltration, on the depth and intensity of weathering. At sites with higher infiltration, the vadose zone is comprehensively altered to grussic saprolite and saprock. Conversely, lower infiltration sites display only thin grussic saprolites, strongly influenced by rock texture. Both vadose zone and weathering depth appear to be governed by local base level, and vadose zone hydrology exerts a fundamental control on the effective operation and relative dominance of the key weathering reactions. In zones of matrix permeability, oxidation of biotite comprehensively disaggregates the rock but results in little mass loss and clay mineral formation. Conversely, the higher transient flow rates that characterize zones of fracture permeability result in plagioclase hydrolysis, significant mass losses and accompanying clay mineral formation. A variable hydrological regime may also contribute to high partial pressures of O 2 in vadose zone pore waters and pore spaces, thereby enhancing the oxidative environment and further predisposing grussic saprolite formation.

Phosphorus and iron cycling in deep saprolite, Luquillo Mountains, Puerto Rico

Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut. Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ 56Fe = 0 to − 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ 56Fe = − 0.26 to − 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10 − 14 mol m − 2 s − 1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite.

Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing

In the mountainous Rio Icacos watershed in northeastern Puerto Rico, quartz diorite bedrock weathers spheroidally, producing a 0.2–2 m thick zone of partially weathered rock layers (∼2.5 cm thickness each) called rindlets, which form concentric layers around corestones. Spheroidal fracturing has been modeled to occur when a weathering reaction with a positive Δ V of reaction builds up elastic strain energy. The rates of spheroidal fracturing and saprolite formation are therefore controlled by the rate of the weathering reaction. Chemical, petrographic, and spectroscopic evidence demonstrates that biotite oxidation is the most likely fracture-inducing reaction. This reaction occurs with an expansion in d (0 0 1) from 10.0 to 10.5 Å, forming “altered biotite”. Progressive biotite oxidation across the rindlet zone was inferred from thin sections and gradients in K and Fe(II). Using the gradient in Fe(II) and constraints based on cosmogenic age dates, we calculated a biotite oxidation reaction rate of 8.2 × 10 −14 mol biotite m −2 s −1. Biotite oxidation was documented within the bedrock corestone by synchrotron X-ray microprobe fluorescence imaging and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2 μm resolution revealed that oxidized zones within individual biotite crystals are the first evidence of alteration of the otherwise unaltered corestone. Fluids entering along fractures lead to the dissolution of plagioclase within the rindlet zone. Within 7 cm surrounding the rindlet–saprolite interface, hornblende dissolves to completion at a rate of 6.3 × 10 −13 mol hornblende m −2 s −1: the fastest reported rate of hornblende weathering in the field. This rate is consistent with laboratory-derived hornblende dissolution rates. By revealing the coupling of these mineral weathering reactions to fracturing and porosity formation we are able to describe the process by which the quartz diorite bedrock disaggregates and forms saprolite. In the corestone, biotite oxidation induces spheroidal fracturing, facilitating the influx of fluids that react with other minerals, dissolving plagioclase and chlorite, creating additional porosity, and eventually dissolving hornblende and precipitating secondary minerals. The thickness of the resultant saprolite is maintained at steady state by a positive feedback between the denudation rate and the weathering advance rate driven by the concentration of pore water O 2 at the bedrock–saprolite interface.

Geomorphic inferences from regolith thickness, chemical denudation and CRN erosion rates near the glacial limit, Boulder Creek catchment and vicinity, Colorado

Granitic regolith, developed in the Boulder Creek catchment and adjacent areas, records a history of deep weathering, some of which may predate Quaternary time. Field and well-log measurements of weathering, chemical denudation and rates of erosion derived from 10Be cosmogenic radionuclide (CRN) data help to quantify rates of landscape change in the post-orogenic Rocky Mountains. The density of oxidized, fractured bedrock ranges from 2.7 to about 2.2 g cm − 3 , saprolite and grus have densities between ∼2.0 and 1.8 g cm − 3 , and 30 soil samples averaged 1.6 ± 0.2 g cm − 3 . Highly weathered regolith in ∼540 wells averages ∼3.3 m thick, mean depth to bedrock in 1661 wells is ∼7 m, and the weathered thickness exceeds 10 m in relatively large local areas east of the late Pleistocene glacial limit. Thickness of regolith shows no simple relationship to rock type or structure, local slope, or distance from channels. Catchments in the vicinity of the Boulder Creek have an average CRN erosion rate of 2.2 ± 0.7 cm kyr − 1 for the past 10,000 to 40,000 yr. Annual losses of cations and SiO 2 vary from about 2 to 5 g m − 2 over a runoff range of 10 to nearly 160 cm. Using measured rates in simple box models shows that if a substantial fraction of void space is created by volume expansion in the weathering rock materials, ∼7 m of weathered rock materials could form in as little as 230 kyr. If density loss results mainly from chemical denudation and some volume expansion, however, the same weathering profile would take > 1340 kyr to form. Rates of erosion measured by CRN could be balanced by the rate of soil formation from saprolite if the annual solute loss from soil is 2.0 g m − 2 and 70% of the density decrease from saprolite to grus and soil results from strain. Saprolite, however, forms from oxidized bedrock at a far slower rate and rates of saprolite formation cannot balance soil and grus losses to erosion. The zone of thick weathered regolith is likely an eroding relict landscape. The undulating surface marked by relatively low relief and tors is not literally a topographic surface of Eocene, Oligocene or Miocene age unless it was covered with deposits that were removed in Pliocene or Quaternary time.

The coupling of biological iron cycling and mineral weathering during saprolite formation, Luquillo Mountains, Puerto Rico

ABSTRACTCorestones of quartz diorite bedrock in the Rio Icacos watershed in Puerto Rico weather spheroidally to form concentric sets of partially weathered rock layers (referred to here as rindlets) that slowly transform to saprolite. The rindlet zone (0.2–2 m thick) is overlain by saprolite (2–8 m) topped by soil (0.5–1 m). With the objective of understanding interactions between weathering, substrate availability, and resident micro‐organisms, we made geochemical and microbiological measurements as a function of depth in 5 m of regolith (soil + saprolite). We employed direct microscopic counting of total cell densities; enumeration of culturable aerobic heterotrophs; extraction of microbial DNA for yield calculations; and biochemical tests for iron‐oxidizing bacteria. Total cell densities, which ranged from 2.5 × 106 to 1.6 × 1010 g−1 regolith, were higher than 108 g−1 at three depths: in the upper 1 m, at 2.1 m, and between 3.7 and 4.9 m, just above the rindlet zone. High proportions of inactive or unculturable cells were indicated throughout the profile by very low percentages of culturable heterotrophs (0.0004% to 0.02% of total cell densities). The observed increases in total and culturable cells and DNA yields at lower depths were not correlated with organic carbon or total iron but were correlated with moisture and HCl‐extractable iron. Biochemical tests for aerobic iron‐oxidizers were also positive at 0.15–0.6 m, at 2.1–2.4 m, and at 4.9 m depths. To interpret microbial populations within the context of weathering reactions, we developed a model for estimating growth rates of lithoautotrophs and heterotrophs based on measured substrate fluxes. The calculations and observations are consistent with a model wherein electron donor flux driving bacterial growth at the saprolite–bedrock interface is dominated by Fe(II) and where autotrophic iron‐oxidizing bacteria support the heterotrophic population and contribute to bedrock disaggregation and saprolite formation.

Saprolite remnants as indicators of pre‐glacial landform genesis in southeast sweden

Twenty‐six sites with remnants of gravelly saprolites (grus) have been located in southeast Sweden. Joint block hills (castle kopjes) and steep rock walls with weathered joints as well as rounded boulders are documented to have an origin in deep weathering and subsequent stripping of saprolites. The saprolite remnants and landforms result from the fragmentation of the re‐exposed sub‐Cambrian peneplain along fracture systems. Only shallow saprolites occur on the elevated intact parts of the sub‐Cambrian peneplain, while saprolites up to 20 m thick are encountered in areas where the sub‐Cambrian peneplain is fractured and dissected. Neogene uplift with reactivation of the weathering system is thought to be the main cause of saprolite formation. Deep weathering is thus judged to have been the major agent of landform formation in the study area, while glacial and glaciofluvial erosion has contributed mainly by stripping saprolites, detaching corestones, and plucking joint blocks along weathered joints.

Elemental translocation and loss from three highly weathered soil–bedrock profiles in the southeastern United States

Chemical weathering of soils and rocks during pedogenesis is significant because it provides many essential elements for life and because it is coupled with the rise and fall of atmospheric CO 2. We used geochemical mass-balance equations to quantify the net result of pedogenic weathering, i.e. elemental loss and gain, in three residual soil–bedrock profiles on the Piedmont of North Carolina. Soils are located on interfluves and derived directly from the bedrock below: a Kanhapludult (Tarrus series) from phyllite, a Kanhapludult (Cecil series) from granitic gneiss, and a Hapludalf (Enon series) from diabase. Bulk density ratios of soils and bedrock as well as elemental concentrations referenced to Zr, Ti, Y, and V were used to estimate strain and open-system mass-transport functions through the soil profiles. Estimated strains of the three soils indicated substantial volumetric changes during C horizon and saprolite formation. Overall, desilication was the most predominant pedogenic process removing chemical elements from the three soils. Losses of Si were about 50% of total elemental molar losses in the 8.5-m deep Tarrus profile, 75% of total losses in the 3.8-m deep Cecil profile, and 39% of total losses in the 4-m deep Enon profile. Base cations were also lost in great amounts following desilication. Losses of base cations accounted for about 50% of the total elemental losses in the Tarrus, 20% of the total losses in the Cecil, and 37% of the total losses in the Enon profiles. The specific base cations lost in greatest amounts differed among the three soils and depended on bedrock mineralogy. Sodium and Mg accounted for 24% and 16% of total elemental loss from Tarrus profiles, Na and K accounted for 14% and 4% of total elemental loss from Cecil profiles, and Ca and Mg accounted for 19% and 12% of total elemental loss from Enon profiles. The vertical pattern of loss of base cations was not always gradual from surface soil horizons to saprolite to bedrock. For example, almost 100% of Ca in the bedrock had been lost from throughout the upper 4.5-m deep Cecil and 8-m deep Tarrus profiles. Aluminum and iron were lost from A and E horizons but were accumulated in B and C horizons due to translocation as well as secondary clay and sesquioxide formation at depth. Physical and chemical data from all three soils and geologic substrata indicate that the entire regolith profile (A through C horizons or solum plus saprolite) is formed by pedogenic processes of elemental inputs, transformations, translocations, and removals.

Chemistry, mineralogy and micropedology of highland soils on crystalline rocks of Serra da Mantiqueira, southeastern Brazil

A typical altitudinal sequence of four soil profiles from one of the uppermost areas of the Mantiqueira Highlands (Minas Gerais State, southeastern Brazil) was studied regarding physical, chemical, mineralogical and micromorphological attributes. The soils were classified as a Terric Haplosaprist, a Humic Xanthic Hapludox, a Typic Haplorthod and a Humic Psammentic Dystrudept. All soils are gibbsitic, highly acidic and distrophic, with high aluminium saturation. The high organic carbon content and coarse texture favour the occurrence of podzolization. Intense hydrolysis and mineral dissolution occur in the studied pedoenvironments, with the presence of adsorbed Al polymers and organic matter bound Al forms. Podzolization can only be confirmed with the combined analysis of soil humic fractions, Al forms and micromorphological attributes. The conspicuous presence of gibbsite is attributed to past conditions of deep weathering and saprolite formation. Closed nutrient cycling is essential for environmental sustainability due to the very low nutrient reserve of soils and deep pre-weathered saprolites. The highland subtropical environment is an important site of natural carbon immobilization in the tropics in which histic/humic horizons are the most important loci of carbon inputs.

Mineralogical and (sub)microscopic aspects of iron oxides in polygenetic Oxisols from Minas Gerais, Brazil

Iron oxides are known to be useful paleo-environmental indicators, and their properties may give information about past and present circumstances of weathering and soil formation. Iron oxides from polygenetic Oxisols developed on saprolites of metamorphic rocks (soil-saprolite sequences) and on sediments (soil-sediment sequences) from Minas Gerais, Brazil, were studied by X-ray diffraction, Mössbauer spectroscopy, micromorphology, and electron microscopy to establish the connection between their properties and circumstances of formation. Hematite and goethite are the only iron oxides found in the clay and silt fractions. Micromorphology and mineralogy indicated the presence of three genetically different forms of hematite and four forms of goethite. XRD and Mössbauer spectroscopy indicate that these forms have differences in mean coherence length (MCL) and Al substitution. In soil-saprolite sequences, hematite is mainly formed during saprolite formation, while goethite formed mainly during soil formation. Microscopic hematite formed in the saprolite has low Al substitution and uniform MCL. In response to soil formation, it diminishes to submicroscopic size and goethite is formed towards toposoils. Goethite formed in yellow surface soils reflects recent soil formation and has high Al substitution. Hematites and goethites related to gley phenomena have low Al substitution and large MCLs. Different coexisting forms of either goethite or hematite point to varying pedo-environments, either in time, corroborating the polygenetic nature of the studied soils, or at micron-scale.

Crystal chemistry of trace elements in natural and synthetic goethite

The crystal chemistry of Cr (0.73%), Mn (0.88%), Co (0.166%), Ni (0.898%), Cu (0.263%), and Zn (0.026%) in natural goethite (αFeOOH) from an earthy saprolite formation in West-Africa was investigated by fluorescence-yield extended X-ray absorption fine structure (EXAFS) spectroscopy. Synthetic goethites and phyllomanganates were used as model compounds for structural determinations. The measurement of EXAFS spectra at energies higher than the Fe K-edge (Co, Ni, Cu, and Zn) is notoriously difficult because the fluorescence signal from trace elements is typically two orders of magnitude less intense than the Fe fluorescence from the matrix. This difficulty was circumvented by measuring total intensity (signal + background) with high precision on the ID26 undulator EXAFS spectrometer of the European Synchrotron Radiation Facility (ESRF) with a fast, highly linear, and low-noise diode detector. Cr, Cu, and Zn have the same local structure in natural and synthetic goethites. With the exception of the Cu polyhedron, which is distorted by the Jahn-Teller effect, Me-O and Me-Fe distances are similar to Fe-O and Fe-Fe distances in goethite. No significant steric effect was observed around Cu and Zn in spite of the ∼14% increase in ionic radii compared to Fe 3+. The compliance of the Fe site to these substitutional impurities probably is accomplished by displacement of nearest O and Fe shells (relaxation effect) and modification of interpolyhedral angles (compliance effect) owing to the corner-sharing topology of the goethite structure. X-ray absorption near-edge structure analysis reveals an average valence of ∼3.7 to 3.8 for Mn, and EXAFS indicates that manganese is present as a phyllomanganate impurity having a hexagonal layer symmetry like asbolane, lithiophorite, and hexagonal birnessite. Cobalt is trivalent and located in the phyllomanganate layer, and also possibly in the interlayer, substituting for Mn. Selective uptake of cobalt by the Mn oxide impurity results from the oxidation of soluble Co 2+ to insoluble Co 3+ by Mn 3+/Mn 4+, this mineral-surface redox reaction accounting for the well-known geochemical affinity of Co for Mn at the earth’s surface. No more than ∼20% of the amounts of Mn and Co in natural goethite substitute for Fe in the structure, if present at all. About 75% of total Ni is substituted for Fe in natural goethite and ∼25% is associated with the phyllomanganate impurity as a Ni(OH) 2–MnO 2 mixed-layer phase (Ni-asbolane). The Ni site in synthetic goethite is strongly relaxed with a contraction of the goethite structure in the plane of edge-sharing double chains ( bc plane), and an expansion in the direction of corner linkages (approximately the ab plane). This anisotropic relaxation of the Ni site locally reduces the distortion of the goethite structure, and could be due to a clustering of Ni atoms.

Geoelectrical investigation of old/abandoned, covered landfill sites in urban areas: model development with a genetic diagnosis approach

Geoelectrical methods have an important, albeit difficult role to play in landfill investigations. In the present economic conditions, with the environmentally sensitive regime, adequate desk-study and model development are essential ingredients for a successful site investigation of landfills. This paper attempts to develop a genetic investigative model for old/abandoned landfill sites where the records of operations are not available. The main elements of the model are the site boundaries, age and nature of anthropogenic deposits, depth and dip of the layers of refuse and sealing materials, the integrity and shape of the capping zones or separating walls and basal floor slopes, the position of concealed access roads in the site, the water table (or perched water bodies within the refuse) and the presence of leachate. The attendant geotechnical, hydrogeological, and bio-geochemical constraints at such sites are also incorporated in the model for consistency of practical solutions to landfill problems. The nature of anthropogenic deposits and the spatial–temporal characteristics of leachates are reviewed in a geoelectrical context. The analogy between waste degradation and leaching, and the well-known weathering processes of supergene mineral enrichment and saprolite formation in crystalline rocks is explored, and used to develop a conceptual resistivity-vs.-depth model for landfill sites. The main tenet of the model is that vertical conductivity profiles will attain maximum values in the zone of mineral enrichment near the water table and tail-off away from it. This conceptual resistivity model is shown to be consistent with non-invasive observations in landfill sites in different geographical environments. Power–law relationships are found to exist between some geoelectrically important hydrochemical parameters (fluid conductivity, chloride content and total dissolved solids) in leachates and leachate-contaminated groundwater from some landfill sites. Since some chemical parameters of fill are known to vary consistently with time, a plausible hydrochemical and age-deductive scheme for saturated fill is proposed for geoelectrical models of landfills without significant amounts of metal. Practical suggestions are made for a consistent approach in geoelectrical investigation and diagnosis of old landfill sites. A few field examples are used to illustrate the diagnosis approach.

Characteristics of Regolith developed on basalt in Pahang, Malaysia

The characteristics and degree of weathering in a deep regolith developed on basalt were investigated. A representative 15m deep regolith of soil‐saprolite‐rock sequence, located in Pahang state, Malaysia, was selected. The intensity of weathering in this deep regolith was assessed by various weathering indices as well as changes in the physicochemical properties and clay mineralogy of the regolith. The values of all weathering indices and other assessments gave strong evidences of intense weathering during saprolite formation. The low CËC and base cation values even in the saprolite layers were accounted By the extreme depletion of major elements such as potassium (K), sodium (Na), calcium (Ca), and magnesium (Mg), and significant enrichment of aluminum (Al), iron (Fe), titanium (Ti), copper (Cu), and niobium (Nb) occurred during saprolitization. The extreme weathering pattern of basalt differed from those described in granite and metamorphic rocks found in Peninsular Malaysia.