Pedogenic Minerals Research Articles

Linking speleothem and soil magnetism in the Pau d'Alho cave (central South America)

AbstractMineral magnetism of Pau d'Alho cave sediments, soils outside the cave, and in the stalagmite #6 (ALHO6) in Midwest Brazil is presented. This high growth‐rate speleothem (~168 mm/ka) encompasses the past 1355 years. Oxygen and carbon isotope data from the same stalagmite allow for a direct comparison of the magnetic signal with changes in paleoprecipitation and soil dynamics at the surface. Magnetic experiments include isothermal remanent magnetization, anhysteretic remanent magnetization, hysteresis loops, first‐order reversal curves, and low‐temperature superconducting quantum interference device magnetometry. The main magnetic remanence carriers in ALHO6 are magnetite and goethite, with a nearly constant relative proportion. Remanent coercivities of magnetite in all our samples are within 14–17 mT for an average grain‐size of ~1–2 µm, in the range of pedogenic magnetite, thus suggesting the detrital grains deposited in the stalagmite were produced in the soil above the cave. Magnetic remanence variations follow δ13C and δ18O data, suggesting a climatic control on the input of magnetic minerals into the Pau d'Alho cave system. The concentration of magnetic minerals in the stalagmite is governed by soil erosion above the cave, which by its turn is controlled by soil erosion and vegetation cover. Dry periods are associated with less stable soils and result in higher mineral fluxes carried into karst systems. Conversely, wetter periods are associated with soils topped by denser vegetation that retains micrometer‐scale pedogenic minerals and thus reduces detrital fluxes into the cave.

Elevation-dependent changes in n-alkane δD and soil GDGTs across the South Central Andes

Surface uplift of large plateaus may significantly influence regional climate and more specifically precipitation patterns and temperature, sometimes complicating paleoaltimetry interpretations. Thus, understanding the topographic evolution of tectonically active mountain belts benefits from continued development of reliable proxies to reduce uncertainties in paleoaltimetry reconstructions. Lipid biomarker-based proxies provide a novel approach to stable isotope paleoaltimetry and complement authigenic or pedogenic mineral proxy materials, in particular outside semi-arid climate zones where soil carbonates are not abundant but (soil) organic matter has a high preservation potential. Here we present δD values of soil-derived n-alkanes and mean annual air temperature (MAT) estimates based on branched glycerol dialkyl glycerol tetraether (brGDGT) distributions to assess their potential for paleoelevation reconstructions in the southern central Andes. We analyzed soil samples across two environmental and hydrological gradients that include a hillslope (26–28°S) and a valley (22–24°S) transect on the windward flanks of Central Andean Eastern Cordillera in NW Argentina. Our results show that present-day n-alkane δD values and brGDGT-based MAT estimates are both linearly related with elevation and in good agreement with present-day climate conditions. Soil n-alkanes show a δD lapse rate (Δ(δD)) of −1.64‰/100 m (R2=0.91, p<0.01) at the hillslope transect, within the range of δD lapse rates from precipitation and surface waters in other tropical regions in the Andes like the Eastern Cordillera in Colombia and Bolivia and the Equatorial and Peruvian Andes. BrGDGT-derived soil temperatures are similar to monitored winter temperatures in the region and show a lapse rate of ΔT=−0.51°C/100 m (R2=0.91, p<0.01), comparable with lapse rates from in situ soil temperature measurements, satellite-derived land-surface temperatures at this transect, and weather stations from the Eastern Cordillera at similar latitude. As a result of an increasing leeward sampling position along the valley transect lapse rates are biased towards lower values and display higher scatter (Δ(δD)=−0.95‰/100 m, R2=0.76, p<0.01 and ΔT=−0.19°C/100 m, R2=0.48, p<0.05). Despite this higher complexity, they are in line with lapse rates from stream-water samples and in situ soil temperature measurements along the same transect. Our results demonstrate that both soil n-alkane δD values and MAT reconstructions based on brGDGTs distributions from the hillslope transect (Δ(δD)=−1.64‰/100 m, R2=0.91, p<0.01 and ΔT=−0.51°C/100 m, R2=0.91, p<0.01) track the direct effects of orography on precipitation and temperature and hence the combined effects of local and regional hydrology as well as elevation.

Retention of sterically and electrosterically stabilized silver nanoparticles by soil minerals

Engineered silver nanoparticles (Ag ENP) enter the environment and lead to concerns about their environmental effects. Clay‐sized particles are assumed to govern the environmental mobility and bioavailability of ENP, although little is known about their interaction with pedogenic minerals. The interactions of sterically (AgNM–300k, OECD standard) and electrosterically (AgCN30) stabilized Ag ENP with iron and clay minerals were investigated at a range of pH values from 4 to 7. The Ag ENP were separated from mineral suspensions by filtration at &lt;7 and &lt;0.45 µm and by ultracentrifugation at &lt;0.003 µm. The colloidal stable Ag ENP were analysed by dynamic light scattering (DLS). The retention of AgNM–300k (pH adjusted to 6, Ag content ≈ 8 mg l−1) was greater than 85% for all minerals except silica and showed the tendency to increase in the following order: ferrihydrite, smectite, illite, kaolinite, goethite and allophane. The retention of AgCN30 (Ag content ≈ 8 mg l−1) was less than for AgNM–300k, except for silica at pH 6, and showed the tendency to increase in the following order: goethite, smectite, allophane, kaolinite, illite and silica. The zeta potential in the fraction &lt;0.45 µm was not sufficient to explain the difference in stability of colloidal Ag ENP in the Ag ENP–mineral suspensions. However, the retention increased for Ag ENP with decreasing pH, most probably because of homo‐ and hetero‐aggregation, but it was not necessarily related to dissolution of the Ag ENP. In general, iron and clay minerals showed clearly the potential to retain Ag ENP under relevant environmental conditions even though the retention was incomplete.HighlightsInteraction of different Ag ENP with soil minerals was tested in short‐term batch experiments. Ag ENP showed hetero‐aggregation with minerals, but the retention was incomplete. Hetero‐aggregation between minerals and Ag ENP was favoured by acidic conditions (pH ≤ 5). Iron and clay minerals showed the potential to retain Ag ENP in the environment.

Climatic control of sediment transport from the Himalayas to the proximal NE Bengal Fan during the last glacial-interglacial cycle

Clay mineralogy, siliciclastic grain-size, major elements, 87Sr/86Sr, and eNd analyses of deep-sea sediments cored in the north-eastern Bay of Bengal are used to reconstruct evolution of detrital sources and sediment transport to the proximal part of the Bengal deep-sea fan during the last climatic cycle. eNd values (−13.3 to −9.7) and 87Sr/86Sr ratios (0.721–0.733) indicate a mixture of sediments originating from the Ganges-Brahmaputra rivers and the Indo-Burman ranges. Interglacial Marine Isotopic Stages (MIS) 5 and 1 are associated with a higher contribution of sediments from the Ganges-Brahmaputra river system than is the case for glacial MIS 6, 4, 3, and 2. Siliciclasitic grain-size combined with Si/Al and Si/Fe ratios indicate coarser glacial sediments with numerous turbidite layers. Glacial turbidite layers display similar clay mineralogical compositions to hemipelagic sediments. Only few of turbidite layers (MIS 6, 4, and 2) are slightly unradiogenic (eNd −13.3), suggesting a higher contribution of Ganges-Brahmaputra river sediments. Independently of changes in the sedimentary sources, the smectite/(illite + chlorite) ratio of cores located on the NE Bengal Fan indicates higher inputs of primary minerals (illite and chlorite) from the highlands of the river basins (relief) during glacial MIS 6, 4, 3, and 2 and an increased contribution of pedogenic minerals (smectite and kaolinite) during interglacial MIS 5 and 1. Maximum smectite/(illite + chlorite) ratios during the warm sub-stages of MIS 5 suggest an intensification of summer monsoon rainfall associated with higher rates of physical erosion of the Indo-Gangetic flood-plain and/or dominant summer hydrological conditions transporting a higher proportion of sediments deriving from the Ganges-Brahmaputra rivers to the NE Bengal Fan. In addition, a higher production of smectite in soils of the Indo-Gangetic flood-plain during periods of intensification of monsoon rainfall cannot be excluded.

Ubiquitous occurrence of basaltic-derived paleosols in the Late Archean Fortescue Group, Western Australia

The 2.76Ga old Mount Roe Basalt paleosols (MR#1 and MR#2), recognized near the base of the 2.76–2.69Ga Fortescue Group in Western Australia, represent some of the oldest definite examples of Archean paleoweathering profiles. The loss of Fe and the absence of pedogenic carbonates in these reference paleosols have been considered as strong evidence for low oxygen and moderate carbon dioxide in the Late Archean atmosphere, respectively. However, the robustness of such interpretations suffers both from the scarcity of paleosol exposures and the superposition of post-weathering alteration over primary soil profiles. Here we report new exposures of the MR#1 paleosol as well as a number of new basalt-derived paleosol occurrences distributed in the Mount Roe Basalt and the 2.73Ga old Kylena Formations of the Fortescue Group. We show that all these paleosols, including MR#1 and MR#2, were strongly affected by post-weathering reductive alteration. Nevertheless, we discovered early lithologic units, locally preserved within the hydrothermally altered paleosols, which feature distinct chemical and mineralogic compositions. These include: (i) 13C-depleted carbonaceous-rich, diaspore–pyrophyllite boudins likely inherited from the hydrolysis and bauxitization of the parent basalt and (ii) green hard core material, mostly composed of Fe–sericite associated with authigenic sphene and containing early relics of sulfate-bearing iron-rich smectite or berthierine. We argue that smectite relics may constitute a part of the primary pedogenic mineral assemblage, while berthierine and sphene formed during diagenesis through the circulation of reductive fluids. Because the depletion of iron observed in the Fortescue paleosols is not solely due to pedogenesis, but also to post-weathering alteration, particular care has to be taken when bulk chemical profiles of iron in such paleosols are used as atmospheric paleobarometer. Recognition of the regional-scale, syn-depositional alteration of the Fortescue subaerial basalts over the north Pilbara suggests the onset of intense continental weathering associated with the uplift and emergence of the Pilbara craton during continental break-up and rifting. Such a regional onset of continental weathering may have provided large amounts of nutrients to nearby marine and/or lacustrine systems, favoring the development of microbial life in shallow waters.

Relevant magnetic and soil parameters as potential indicators of soil conservation status of Mediterranean agroecosystems

SUMMARY The main sources of magnetic minerals in soils unaffected by anthropogenic pollution are iron oxides and hydroxides derived from parent materials through soil formation processes. Soil magnetic minerals can be used as indicators of environmental factors including soil forming processes, degree of pedogenesis, weathering processes and biological activities. In this study measurements of magnetic susceptibility are used to detect the presence and the concentration of soil magnetic minerals in topsoil and bulk samples in a small cultivated field, which forms a hydrological unit that can be considered to be representative of the rainfed agroecosystems of Mediterranean mountain environments. Additionalmagneticstudiessuchasisothermalremanentmagnetization(IRM),anhysteretic remanent magnetization (ARM) and thermomagnetic measurements are used to identify and characterize the magnetic mineralogy of soil minerals. The objectives were to analyse the spatial variability of the magnetic parameters to assess whether topographic factors, soil redistribution processes, and soil properties such as soil texture, organic matter and carbonate contents analysed in this study, are related to the spatial distribution pattern of magnetic properties. The medians of mass specific magnetic susceptibility at low frequency (χlf )w ere 36.0 and 31.1 × 10 −8 m 3 kg −1 in bulk and topsoil samples respectively. High correlation coefficients were found between the χlf in topsoil and bulk core samples (r = 0.951, p < 0.01). Inaddition,volumetricmagneticsusceptibilitywasmeasuredinsituinthefield(κis)andvalues varied from 13.3 to 64.0 × 10 −5 SI. High correlation coefficients were found between χlf in topsoil measured in the laboratory and volumetric magnetic susceptibility field measurements (r = 0.894, p < 0.01). The results obtained from magnetic studies such as IRM, ARM and thermomagnetic measurements show the presence of magnetite, which is the predominant magnetic carrier, and hematite. The predominance of superparamagnetic minerals in upper soil layers suggests enrichment in pedogenic minerals. The finer soil particles, the organic matter content and the magnetic susceptibility values are statistically correlated and their spatial variability is related to similar physical processes. Runoff redistributes soil components including magnetic minerals and exports fine particles out the field. This research contributed tofurtherknowledgeontheapplicationofsoilmagneticpropertiestoderiveusefulinformation on soil processes in Mediterranean cultivated soils.

Sulfur accumulation in Melanudands during development by upbuilding pedogenesis since 14–15 cal. ka

To understand the contribution of upbuilding pedogenesis to the accumulation of sulfur (S) in Melanudands, we investigated the vertical distribution of S species using K-edge X-ray absorption near-edge structure (XANES) spectroscopy of S species and vertical variation of δ34S values of total S in two Japanese Melanudands developed above tephra Nt-S, dated at 14–15cal.ka. These soils have been developed by upbuilding pedogenesis, in which episodic accumulations of volcanic ash and eolian dust transported from the Asian continent are added incrementally to the soil surface while topdown pedogenesis occurs concomitantly forming soil horizons. The oxidation states of S compounds determined by XANES analysis varied widely in the soil profiles. The predominant S species was highly oxidized S in ester sulfates and inorganic sulfates (+6 oxidation state). Proportions of S with reduced and intermediate oxidation states increased episodically to as much as 66%. Ester sulfate-S was found in the soil horizons with estimated ages less than 10calka. A close correlation was found between ester sulfate-S concentrations and Al-associated organic S concentrations (p<0.05); indeed, the concentrations themselves were roughly similar. The δ34S values of total S increased with depth in the two soil profiles. These results suggest that episodic deposits of volcanic ash and eolian dust trapped detritus and humus beneath them during the buildup of the soil surface. The organic S compounds in this material might be decayed as topdown pedogenic processes in the soils. Production of ester sulfate-S may be related to the warm and humid climate of the Holocene, which may have accelerated microbial activity responsible for transformation of reduced to highly oxidized S. Ester sulfate-S might have become Al-associated forms and accumulated in the soils due to their high pedogenic mineral contents.

Testing the applicability of vacuum-encapsulated 40Ar/39Ar geochronology to pedogenic palygorskite and sepiolite

The difficulty of isolating intact, mineralogically pure pedogenic crystals from cemented soil is one of the most significant obstacles to quantifying rates of soil formation, geomorphic processes, and climate change in arid regions. We evaluate the applicability of vacuum encapsulated 40Ar/39Ar geochronology to pedogenic palygorskite and sepiolite extracted from the 4 to 5 Ma, extant Mormon Mesa petrocalcic soil-geomorphic surface of southern Nevada, and from the 780 ka to 2 Ma Jornada Experimental Range La Mesa soil-geomorphic surface near Las Cruces, New Mexico. Selective dissolution of cements using NaOAc and Tiron, accompanied by particle size fractionation, was used to isolate the pedogenic Mg-phyllosilicates. Scanning electron microscopy, inductively-coupled plasma spectrometry, X-ray diffraction, gas chromatograph mass spectrometry, and Ar isotope analysis were used to determine whether extraction impacted palygorskite/sepiolite suitability for 40Ar/39Ar geochronology. We found no adverse morphological or mineralogical effects, but meaningful ages could not be obtained due to small amounts of old, detrital phyllosilicates in the samples. While the potential of pedogenic palygorskite and/or sepiolite for geochronology now seems limited, results from this study may prove relevant for samples from other, non-pedogenic surface environments. It is hoped that this work will encourage further research towards successful 40Ar/39Ar geochronology of pedogenic phyllosilicates, as well as inform future geochemical or isotopic studies of individual pedogenic mineral species.

Oxygen and hydrogen isotope compositions of paleosol phyllosilicates: Differential burial histories and determination of Middle–Late Pennsylvanian low-latitude terrestrial paleotemperatures

The clay mineralogy, chemistry, and stable hydrogen and oxygen-isotope compositions were measured from 20 phyllosilicate samples representing 11 Pennsylvanian-age paleosol profiles taken from three cores in the Illinois basin in order to assess their utility as proxies of low-latitude terrestrial paleotemperatures. The majority of the samples are mineralogical mixtures of illite–smectite (I/S), kaolinite, and rarely discrete illite. Samples from a shallowly-buried locality in the northern part of the basin are dominantly composed of smectite-rich I/S, with variable amounts of kaolinite, and no discrete illite. Samples from deeply-buried, interior parts of the basin are composed of illite-rich I/S, variable amounts of kaolinite, and discrete illite.These phyllosilicate mixtures have δ18OV-SMOW and δDV-SMOW values that range from 17.2‰ to 23.0‰ and −56‰ to −27‰, respectively. Assuming that the phyllosilicates preserve a record of isotopic equilibrium with Pennsylvanian meteoric waters, these oxygen and hydrogen isotope values correspond to crystallization temperatures ranging from 22±3°C to 55±3°C. The clay mineralogy, phyllosilicate δ18O and δD values and calculated crystallization temperatures of 44°C to 55°C from deeply buried localities in the interior of the basin are not consistent with a pedogenic origin. Instead, these trends are considered to be the result of diagenetic recrystallization of pedogenic minerals in response to greater depths of burial (by ~1.5 to 3km) in the southerly, basin-center localities, as well as an interval of middle Permian elevated heat flow associated with magmatic intrusions in the southern part of the basin.Phyllosilicate mineralogy, δ18O and δD values, and calculated phyllosilicate crystallization temperatures from a shallowly buried, northern locality in the Illinois basin are consistent with a pedogenic origin, and reveal a long-term warming trend from an average temperature of 23±3°C in the lower Desmoinesian to an average temperature of 32±3°C in the Missourian. This temperature change is coincident with a significant change in the composition of wetland vegetation in Euramerica, which has been attributed to a shift in low-latitude Pennsylvanian climate towards warmer and drier conditions in the Late Pennsylvanian. This study reveals the presence of a dynamic Late Paleozoic paleoequatorial icehouse climate characterized by significant low-latitude temperature variability unprecedented on modern Earth.

Three‐Dimensional Investigation of Petrocalcic Materials: Insight into Pedogenic Processes and Future Applications

Computed tomography (CT) imaging was applied to Mormon Mesa petrocalcic and related materials to observe the location, arrangement, and interconnectedness of void spaces and to visualize density differences that may define pedogenic mineral relationships. Density variations visualized by these scans reveal features that may result from long developed pedogenic processes within these unique soil materials. Voids are present as cracks (shown to interconnect three‐dimensionally), elongated spaces filled with unconsolidated materials, and oblate‐spheroidal shapes that appear to be associated with detrital carbonate clasts. Density variations viewed through CT imaging may show the products of mineral neoformation, mineral packing differences during crystal growth, and/or older, denser petrocalcic materials eroded and incorporated via subsequent recementation (pisoliths). Computed tomography scanning can be used to understand unique forms and processes that are vital to our understanding of dissolution/precipitation and other pedogenic processes at work in these mature petrocalcic soils. We encourage the application of CT imaging to similar soil materials.

Soil sulfur content and its spatial distribution in a small catchment covered by volcanic soil in the montane zone of central Japan

High concentrations of organic C and Al/Fe oxides, and low bulk density are responsible for the accumulation of large amounts of S compounds in volcanic soils. The purpose of our study was to clarify the spatial distribution of S compounds and determine the factors governing that distribution within the Katsura Headwater Catchment, a small drainage basin covered by volcanic soil in a mountainous area of the North Kanto district, central Japan. Geostatistical analysis revealed a strong spatial dependence of total S concentration throughout the catchment area. Total S concentrations were higher on upper slopes than on lower slopes, and higher in deeper soils (20–50cm depth) than in surface soils (0–20cm depth). Our analyses also revealed similar spatial variations of the characteristic pedogenic volcanic minerals properties (i.e., concentrations of oxalate extractable Al, Fe, and Si), total C concentrations, and bulk density. The strong relationships of total S with the pedogenic mineral properties indicate that the accumulation of S in the soils is governed predominantly by the abundance of those pedogenic minerals, and that the influence of volcanic ash as a parent material is reflected more strongly in residual soils than in colluvial soils. Because the total S concentrations of the residual and colluvial soils differ, so too do the S pools in the uppermost 50cm of these soils: residual soils retain larger amounts of S (2030±590kgSha–1 on average) than colluvial soils (1400±590kgSha–1 on average). Our results indicate that volcanic ash that remains in situ on the upper slopes of the small Katsura Headwater Catchment contributes strongly to the accumulation and spatial variation of S. Total C inputs from plant communities might also influence S accumulation, particularly in surface soils.

Multi-proxy evidences of Mid-Pleistocene dry climates observed on calcretes in Central Turkey

Palaeosols are very important in reconstructing palaeoclimate studies, since they are accepted as useful climate markers and as potential providers of proxy data. The study of the palaeosols of the Plio-Pleistocene rock sequences around Ankara revealed evidence for a reconstruction of the Quaternary palaeoclimate of the region. The study area is located in Bala, south-east of Ankara, which contains Middle Pleistocene red palaeosols with powdery to nodular calcrete developments, alternating with channel deposits. During this warm and arid period in the Pleistocene, the limited water available in the soil led to the accumulation of low magnesian carbonates, forming calcretes. The clay fractions of the samples were X-rayed using air-drying, ethylene-glycol solvation and heating treatments. The clay mineral contents were determined as smectite, kaolinite, illite and chlorite. Palygorskite was also identified. In the relative abundances of clay minerals, smectite, the most abundant clay mineral, is depleted towards the upper part of the section while the amount of palygorskite increases. Since palygorskite is the only pedogenic mineral formed during the Pleistocene, its presence can be accepted as evidence of the dominant arid climatic conditions. The isotopic composition of carbonates in the Bala section exhibits a slightly wider range in \delta^{13}C composition from -5.98%o to -9.22%o and a narrower range in \delta^{18}O composition from -7.19%o to -8.66%o. The carbon isotope values clearly imply that arid to semiarid flora C4 is dominant, with C3:C4 mixed vegetation. This study suggests that the Middle Pleistocene is the time of the Mid-Brunhes Event when the dominantly warm climatic temperatures led to the development of calcretes in Bala, Ankara, as with all Quaternary Mediterranean-type calcretes.

Late Pleistocene climate history of the Baranja loess plateau – evidence from the Zmajevac loess-paleosol section (northeastern Croatia)

The Zmajevac loess-palaeosol succession (LPS) of the northeastern Baranja loess plateau is exposed along the southern slope of Bansko Brdo, on the western bank of the Danube River. The investigated 17.5-m-thick section shows 4 palaeosol, 1 loess-like and 6 loess horizons. Their integrative palaeoenvironmental analysis combines quantified dana from the mollusc record, magnetic susceptibility, grain-size, calcimetry and mineral abundances to reconstruct the pattern of regional palaeoclimate evolution. This result combined with infrared optically stimulated luminescence age est CLEAL imates by GALOVIC et al. (2009) enabled correlation of the depositional units to Middle to the Late Pleistocene Marine Isotope Stages (MIS) 6 to 2. Magnetic susceptibility measurements show strong peaks in the palaeosol horizons pointing to increased concentrations of pedogenic ferrimagnetic minerals. Sedimentological and mineralogical parameters are in good agreement with other Pannonian Basin LPS. Terrestrial gastropod palaeoecology based on 1705 specimens of 13 species counted from loess and loess-like horizons documents cyclic transitions between cryophilous to cold resistant and mesophilous to thermophilous assemblage types. Whereas Helicopsis striata, Arianta arbustorum and Chondrula tridens are common throughout the succession, the typical loess representatives Pupilla sp., Vallonia tenuilabris and Columella columella are abundant only in certain horizons. Nevertheless, species tolerating open and dry habitats are abundant throughout the succession. The faunal spectra for the samples prove the dominance of transitional palaeoecological assemblage types, whereas uniformly defined types are rare. One of these, the Columella columella assemblage from the base of the section proved to be indicative of the Penultimate Glacial Maximum.

K-Ar Age Constraints on the Origin Of Micaceous Minerals in Savannah River Site Soils, South Carolina, USA

AbstractK-Ar measurements were used in this study of upland Savannah River Site soils to distinguish between sorbed K and the K remaining in remnants of primary minerals. Study of sorbed K contributes to understanding further the interaction of alkali metals (Cs in particular) with the soils. Primary mineral K and the associated radiogenic Ar were studied to characterize soil mica with respect to its provenance and its relationship to hydroxy-interlayered vermiculite. K-Ar age values of Na-saturated clay fractions from five samples of these soils range in age from 270 to 370 Ma. After a moderate acid treatment (6% HNO3 v/v, ~1 mol dm-3, 3 h, 80°C) of the clay fractions, K-Ar age values (270-325 Ma) were little changed on the whole, but they were more closely grouped near 300 Ma. Earlier work had shown that most of the K in these soils is found in material resistant to moderate acid extraction. The K-Ar age values show that this acid-resistant material is much older than any pedogenic minerals could be, even much older than the sedimentary parent rocks from which the soils were derived. These observations support earlier inferences by others that the K in these well leached soils is largely in remnants of primary muscovite from the parent sediments. Age values near 300 Ma suggest that the muscovite is largely from proximal Piedmont terranes of the Appalachian orogen, where the K-Ar relationship in most micas was set by Alleghanian tectonic processes late in the Paleozoic Era. The structural location of the K within mica, shown by the retention of the associated radiogenic Ar, is in contrast to the sorption-dominated behavior of the Cs and most of the Rb in these soils during pedogenesis. Stronger acid treatment (~6 mol dm-3 HNO3, 3 h, 100°C) extracted substantial fractions of both the K and the radiogenic Ar from bulk-soil portions, indicating destruction of some of the primary mica. K-Ar age values for the sand-rich bulk soils were not useful for this study because the sand contains excess radiogenic Ar, probably in sand-sized vein quartz.

Effects of CBD Treatment on Magnetic Minerals of Natural Samples

AbstractLoess‐paleosol sequences are good archives for research on paleoclimate over the last 2.5 million years and even earlier. Magnetic susceptibility of loess‐paleosol sequences has been used extensively as a proxy for paleoclimate interpretations. Magnetic minerals in loess/paleosols are of lithogenic (allochthonous) origin or pedogenic (authigenic) origin. It is of great significance for loess magnetism to separate magnetic signals of them and investigate its magnetite or maghemite that mainly contributes to magnetic susceptibility enhancement in paleosol. Though the combination of CBD (Citrate‐Bicarbonate‐Dithionite) procedure with loess magnetism sheds light on these questions, the CBD method is still debated and its mechanism unclear. To better understand the effects of CBD method on magnetic minerals, CBD treatment were made to loess/paleosol samples from Western Sichuan plateau, Tianshan mountains and Kurtak, Siberia and non‐aeolian samples from Subtropics in this work. Magnetic parameters, including low‐field magnetic susceptibility, frequency dependent magnetic susceptibility, anhysteretic remanent magnetization, saturation isothermal remanent magnetization and thermomagnetic curves (J‐T curves), were measured before and after the CBD treatment. Changes in magnetic mineralogy, magnetic concentration and magnetic domain were analyzed. The results suggest that the CBD method is not apparently selective in dissolving various magnetic minerals, and the dissolving quantity is controlled by grain size distribution if the temperature and duration are given. The CBD method can efficiently remove fine grains which have higher ration of surface to volume, and dissolve a rim from the outside of the coarse grains resulting in finer grain size. There are multiple possibilities of changes in magnetic susceptibility after CBD treatment. The CBD method can be used to obtain magnetic signal of pedogenic original. It is not advisable to use the method to separate lithogenic and pedogenic magnetic minerals in loess from arid or over‐wet areas.

Clay-Mineral Formation in Soils Developed in the Weathering Zone of Pyrite-Bearing Schists: A Case Study from The Abandoned Pyrite Mine in Wieściszowice, Lower Silesia, SW Poland

AbstractIntense mineral transformations that produce acid soils from weathering zones of pyritebearing rocks, including alterations of layer silicates, are of critical importance to agricultural and environmental interests in various regions of the world. To date, the transformations of layer silicates in these soils have not been studied in detail. The aim of the present investigation was to examine the weathering pathways controlling processes of clay-mineral formation in acidic soils developed near the abandoned pyrite mine in Wieściszowice (Lower Silesia, SW Poland). A sequence of soils, from weakly developed technogenic soils (located on the mine dumps) to well developed natural soils, was selected. Bulk soil material and separated clay fractions were analyzed using X-ray diffractometry, Fouriertransform infrared spectroscopy, and scanning electron microscopy-energy dispersive spectrometry. The profiles analyzed were developed on pyrite-bearing schists containing trioctahedral Mg,Fe-chlorite and dioctahedral micas (muscovite and paragonite). Because of pyrite weathering, all the soils studied were strongly acidic (pH 2.8–4.4). Inherited chlorite and micas (K- and Na-mica) predominated in the clay fractions of soils developed on the mine dumps, whereas clays from natural soils were rich in pedogenic minerals (i.e. smectite, vermiculite, and mixed-layer minerals containing hydrated interlayers). The formation of both vermiculite and smectite at the expense of chlorite was observed in the soils studied. The transformation probably led to smectite formation via intermediate stages of mixed-layer minerals (i.e. chlorite-vermiculite, chlorite-smectite, and/or vermiculite-smectite). The process of chlorite dissolution took place simultaneously with the transformation. Micas were mainly transformed to smectite and mixedlayer mica-smectite. Neoformation of kaolinite occurring in A horizons of the soils investigated was also documented. Metal-hydroxy interlayers in Bw horizons of well developed soils were found. The process of interlayer development appeared to be pH dependent and took place at pH ⩾4.2. The processes of claymineral formation in soils developed in the weathering zone of a pyrite-bearing schist are similar to those occurring in podzols (Spodosols).

Oxygen and hydrogen isotopes in pedogenic minerals — Implications for paleoclimate evolution in Amazonia during the Cenozoic

Stable isotopes in gibbsite and kaolinite were analyzed to reconstruct the Amazonia palaeoclimate during the Cenozoic and to investigate the environmental conditions of the weathering process and the origin of the Oxisol that cover the lateritic/bauxite crust. At least two types of kaolinite were identified in the Oxisol; near the surface, the kaolinite is less fractionated and was developed at a higher temperature (35 °C). This is probably due to the effect of vegetation and/or evapotranspiration or is a consequence of surface microenvironmental conditions or allochthonous inputs. The other kaolinite samples ranged in formation temperatures from 21 °C to 26 °C and developed under conditions similar to the Pleistocene and the modern climate of Amazonia. At least two types of gibbsite were found. One type developed in the modern Amazonia climate, in a way similar to most kaolinite samples, whereas the samples of the other type are more fractionated. This is because they may have been formed in low recycling rainwater and/or under a low amount of precipitation. These two conditions indicate a seasonal climate for bauxite development that can be linked to the low exhumation of the Andes cordillera and its geographical position relative to the Trombetas region during the period ranging from the Late Oligocene to the Early Miocene. At the end of the Cenozoic, the climate was less seasonal and wetter and the temperature was lower In addition, there was the probable inland position of the Trombetas region, which allowed Oxisol development. The gibbsite developed earlier than the kaolinite of the Oxisol, but there is a partial inheritance, or temporal equivalence, among the bauxite horizon and the Oxisol, as well as multiple generations of gibbsite and kaolinite, which is to be expected for materials developing over long periods in hard leaching conditions.

Soil development of three Chernozem-like profiles from North Bulgaria revealed by magnetic studies

A detailed magnetic study of three soil profiles of Calcic Chernozem, Luvic Phaeozem and Orthic Luvisol, described in the Soil Atlas of Bulgaria, was carried out in order to reveal the potential of magnetic methods for characterization of soil forming processes and properties of pedogenic iron oxides. Thermomagnetic analysis of susceptibility and SEM/EDX observations revealed that the main pedogenic mineral in all three soils is fine grained superparamagnetic to single domain maghemite. Thermal stability of this pedogenic phase up to temperatures of 500 °C is revealed by the partial thermomagnetic analyses of susceptibility. Depth variations of magnetic parameters and ratios point to the presence of stable single-domain to pseudo-single domain grains in humic horizons, the quantity of which decreases sharply towards lower soil horizons. These magnetic grain size changes are best expressed in Orthic Luvisol and reflect intensified processes of translocation of substances downward.

A magnetic investigation of a loess/paleosol sequences record in Ili area

The pedogenic ferrimagnetic minerals have been considered to be the cause of magnetic susceptibility enhancement in loess deposits distributing in the Chinese Loess Plateau and Central Europe, while “wind intensity” mechanism is proposed to be responsible for the magnetic susceptibility enhancement of loess in Alaska and Siberia. However, the magnetic enhancement mechanism is still open for loess in Ili valley, Xinjiang Uygur Autonomous Region, China. To understand this, we conducted a rock magnetic investigation on Axike section that is located in Ili valley. Results show that transitional stage from magnetic (χlf) trough to magnetic (χlf) peak corresponds to soil units. The PSD and MD-grained magnetite dominate the magnetic properties of AXK sequences, and the main factor affecting magnetic properties is the concentration of ferrimagnetic fraction. For the “pedogenesis enhancement” and “wind intensity” model, it seems hard to explain the magnetic susceptibility enhancement in this area. For the concentration of fine-grained magnetite in magnetic mineral shows positive relationship with the intensity of pedogenesis, the magnetic parameters of loess deposit in Ili valley can be used to recover paleoclimatic variations.

Oxygen isotope ratios in Holocene carbonates across a climatic gradient, eastern Washington State, USA: Evidence for seasonal effects on pedogenic mineral isotopic composition

Oxygen isotope ratios of pedogenic carbonates were analyzed along a climatic gradient in the Palouse region of eastern Washington State to evaluate changes in their isotopic composition with changing climate. A modern analog approach was used to assess the relationship between the istotope ratios in the pedogenic carbonates with those in soil water (measured in June when we anticipated carbonate formation might occur because of low soil water content), and meteoric water from two National Atmospheric Deposition Program (NADP) sites adjacent to our sequence. Mean annual meteoric water δ18O values (wrt VSMOW) from the two NADP sites varied slightly with elevation, —13.9‰ at the Starkey site (elevation 1253 m) and —13.0‰ at the Palouse site, (elevation 766 m) but showed distinct seasonal trends with weighted means ranging from —11.9‰ during autumn to —15.0‰ during the winter. Carbonate δ18O (wrt VPDB) varied significantly with MAP, ranging from —14.1‰ at the most arid site to —11.4‰ at the wettest site but displayed a trend opposite to that of soil water δ18O (—4 to —12‰ VSMOW for the 70 cm depth). Neither measured soil water δ18O values nor mean meteoric water δ18O values generated realistic temperatures of formation for the observed trend in carbonate δ18O values. When mean annual temperatures across the gradient were used for temperature of formation, calculated soil water δ18O in equilibrium with carbonate δ 18O was similar to winter precipitation δ18O at the most arid sites but became increasingly similar to mean annual meteoric δ 18O as MAP increased. We suggest that the unexpected trend in carbonate δ18O is largely due to the diminishing influence of winter precipitation/snowmelt on soil moisture as MAP increases across the climatic gradient and that seasonal effects of meteoric water may need to be considered in pedogenic mineral isotopic equilibrium in temperate regions.