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Stable isotope compositions of pedogenic clay minerals can be used as a paleoclimatic proxy. The underlying fundamental assumption is that pedogenic clays are formed in isotope equilibrium with the environment, which is well-documented for hot and humid climates. However, observational and experimental data on oxygen and hydrogen isotope composition of pedogenic clay minerals developed in colder climates remain scarce. The present study aims to fill this gap by providing data on the stable isotope composition of the fine clay fraction (

Rare earth elements (REEs) have gained attention as tracers of pedogenic processes over the last few decades. Clay-sized fractions (CSFs, < 2 μm) may play a crucial role in hosting REEs. To better understand the pedochemical signals of REEs in clay-sized phases, such as iron oxides and phyllosilicates, we analyzed REE speciation in CSFs of carbonate rocks (limestone), clastic rocks (sandstone and shale), and their derived saprolites. Our results quantified the REE content (41.2–144.6 mg kg−1) and speciation in CSFs of parent materials, revealing that REEs were primarily hosted in amorphous iron oxides (Feox1), followed by crystalline iron oxides (Feox2) and phyllosilicates. Compared to parent materials, saprolites derived from carbonate rocks exhibited more than a two-fold enrichment of REEs in major clay-sized phases, confirming the role of CSFs as REE sinks during carbonate rock weathering. Furthermore, the initial REE patterns of CSFs of carbonate rocks underwent alteration throughout the weathering process, likely due to water–mineral interactions. Our findings suggest that REEs in CSFs record the weathering fingerprint for soils derived from carbonate rocks, while they are indicative of provenance for soils originating from clastic rocks.

Phytoliths are an important component in the cycling of silicon (Si) in rice cultivation, yet little is known about their medium to long-term stability. While it is commonly accepted that phytolith solubility in soil decreases with time, the mechanisms that cause this decrease remain unclear. Most studies on phytolith aging to date have been conducted under laboratory conditions and field studies are rare. Here, we present a comprehensive field study of phytolith aging in paddy and non-paddy fields in the Philippines and Vietnam. Phytoliths extracted from rice straw were placed in mesh bags and exposed to ambient soil conditions for up to 550 days. Following exposure, total mass loss and changes in physical (particle size, specific surface area, ζ potential) and chemical (surface chemical composition by acid digestion and X-ray photoelectron spectroscopy) properties as well as dissolution kinetics (56 days in 0.1 M CaCl2 at pH 6.5) were determined. Phytolith dissolution in the field was rapid (up to 30 % mass loss) and three times faster in paddy than in non-paddy soils. Differences in phytolith properties were unexpectedly little between paddy and non-paddy soils. Laboratory and field-derived dissolution rates decreased with exposure time but were one order of magnitude lower in the field. While laboratory dissolution rates were negatively related to surface Al contents this was not observed for field-exposed phytoliths. Our findings suggest that under natural field conditions, phytolith dissolution is overwhelmingly governed by water regime, while the influence of protective surface coatings is only minor. We conclude that reliable estimates of phytolith dissolution can be best obtained in field experiments.

• δ18O and δ2H values of fine clay fractions from four alpine Podzols were measured. • Stable isotope compositions of pedogenic phyllosilicates were modeled. • Isotope compositions reflect weathering conditions. • Detailed mineralogical investigation is needed to interpret isotope data correctly. Stable isotope compositions of pedogenic clay minerals can be used as a paleoclimatic proxy. The underlying fundamental assumption is that pedogenic clays are formed in isotope equilibrium with the environment, which is well-documented for hot and humid climates. However, observational and experimental data on oxygen and hydrogen isotope composition of pedogenic clay minerals developed in colder climates remain scarce. The present study aims to fill this gap by providing data on the stable isotope composition of the fine clay fraction (<0.2 μm) from four Podzols developed in the alpine climate of the Tatra Mountains, Poland. Investigated material was dominated by common products of granite weathering in alpine conditions (mean annual temperature: 2.6 °C, mean annual precipitation: 1100 mm): mixed-layer minerals of illite–smectite type and kaolinite. Oxygen and hydrogen isotope compositions of the fine clay fractions were measured, elemental budgets of the studied samples were calculated, and the pedogenic end-member isotope composition was estimated with two methods: (i) using linear regressions of samples' isotope compositions on the fraction of O or H contained in the pedogenic pool and (ii) isotope modeling assuming isotope equilibrium between pedogenic component and soil water for a range of temperatures. The obtained results revealed that the isotope composition of fine clay fraction mostly plots between the composition of bulk parent granitoids and assumed weathering end-member in the δ2H-δ18O space, with isotope compositions of pedogenic phyllosilicates reconstructed from mass-balance calculations broadly reflecting alpine climate conditions. Oxygen isotope composition was correlated with the cation exchange capacity (CEC) of the investigated samples, which reflects the weathering nature of expandable phyllosilicates in the soil. Weathering in the alpine climate leads to the formation of material containing less pedogenic components that record the environmental conditions in their oxygen and hydrogen isotope composition than weathering in tropical environments. [ABSTRACT FROM AUTHOR]

• REEs in clay-sized fractions (CSFs) of sedimentary rocks were quantified. • Iron oxides and phyllosilicates were main REE hosts in CSFs of parent materials. • CSFs were REE sinks during carbonate rock weathering, with 2–3.8-fold enrichment. • REE patterns in CSFs recorded weathering for carbonate rock derived soils. • REEs in CSFs indicated provenance for clastic rocks derived soils. Rare earth elements (REEs) have gained attention as tracers of pedogenic processes over the last few decades. Clay-sized fractions (CSFs, < 2 μm) may play a crucial role in hosting REEs. To better understand the pedochemical signals of REEs in clay-sized phases, such as iron oxides and phyllosilicates, we analyzed REE speciation in CSFs of carbonate rocks (limestone), clastic rocks (sandstone and shale), and their derived saprolites. Our results quantified the REE content (41.2–144.6 mg kg−1) and speciation in CSFs of parent materials, revealing that REEs were primarily hosted in amorphous iron oxides (Fe ox1), followed by crystalline iron oxides (Fe ox2) and phyllosilicates. Compared to parent materials, saprolites derived from carbonate rocks exhibited more than a two-fold enrichment of REEs in major clay-sized phases, confirming the role of CSFs as REE sinks during carbonate rock weathering. Furthermore, the initial REE patterns of CSFs of carbonate rocks underwent alteration throughout the weathering process, likely due to water–mineral interactions. Our findings suggest that REEs in CSFs record the weathering fingerprint for soils derived from carbonate rocks, while they are indicative of provenance for soils originating from clastic rocks. [ABSTRACT FROM AUTHOR]

• Phytoliths dissolved three times faster in paddy soils than in non-paddy soils. • Phytolith morphology and surface chemistry were not affected by paddy management. • Laboratory dissolution rate decreased with time in soil and with surface Al. • Field dissolution was unrelated to changes in surface chemistry. • Field dissolution is overwhelmingly governed by water regime. Phytoliths are an important component in the cycling of silicon (Si) in rice cultivation, yet little is known about their medium to long-term stability. While it is commonly accepted that phytolith solubility in soil decreases with time, the mechanisms that cause this decrease remain unclear. Most studies on phytolith aging to date have been conducted under laboratory conditions and field studies are rare. Here, we present a comprehensive field study of phytolith aging in paddy and non-paddy fields in the Philippines and Vietnam. Phytoliths extracted from rice straw were placed in mesh bags and exposed to ambient soil conditions for up to 550 days. Following exposure, total mass loss and changes in physical (particle size, specific surface area, ζ potential) and chemical (surface chemical composition by acid digestion and X-ray photoelectron spectroscopy) properties as well as dissolution kinetics (56 days in 0.1 M CaCl 2 at pH 6.5) were determined. Phytolith dissolution in the field was rapid (up to 30 % mass loss) and three times faster in paddy than in non-paddy soils. Differences in phytolith properties were unexpectedly little between paddy and non-paddy soils. Laboratory and field-derived dissolution rates decreased with exposure time but were one order of magnitude lower in the field. While laboratory dissolution rates were negatively related to surface Al contents this was not observed for field-exposed phytoliths. Our findings suggest that under natural field conditions, phytolith dissolution is overwhelmingly governed by water regime, while the influence of protective surface coatings is only minor. We conclude that reliable estimates of phytolith dissolution can be best obtained in field experiments. [ABSTRACT FROM AUTHOR]

[Display omitted] • 5 RDs neutralise 500 up to 10 000 mmol H+/kg, variation in mineralogy and target pH. • Automatic titration (pH stat) estimates pH-dependent dissolution rates and slopes. • Novel soil-RD suspension estimates fast/slow pools of acid neutralising capacity. • Low and high doses (12 Mg/ha, 340 Mg/ha) for validation in 2 year soil-RD mesocosm. • Mechanistic model using pH stat and soil-RD suspension predicts mesocosm soil pH. The ongoing acidification of soil poses a significant threat to the proper functioning of various ecosystems worldwide. Silicate rock dusts (RD) are increasingly amended to acid soils to restore their pH, but the acid neutralising capacity (ANC) and dissolution rate of these products are highly variable and lack proper assessment protocols. It is expected that pH-dependent RD ANCs and dissolution rates dictate the pH increase in soils depending on the initial pH and pH buffer power of the soil. This study addressed these questions by comparing and validating three accelerated weathering tests for their capacity to predict the gradual liming effects in a two-year outdoor mesocosm. Five commercial RDs (two basalts, phonolite, foidite, and trachy-andesite) were tested in four acidified forest soils varying in initial pH, in texture and associated pH buffer power. First, RD dissolution was measured in aqueous batch renewal systems during one year at various starting pH (3.5, 4.5, and 5.5) and constant temperatures (20 °C, 37 °C, and 65 °C). These showed that the ANCs of RDs exhibit a fast fraction (half-life < 1 day) followed by a slower fraction. Second, titration tests of RDs at fixed solution pH, i.e., pH stat , performed between pH 3.5 and 4.5 revealed surface normalised dissolution rates that decreased factors 10–100 per unit pH increase, the slope depending on the RD's mineralogy. Indeed, the rate ranking of observed surface area normalised dissolution rates confirmed that of literature-based values, weighed by the XRD-derived mineralogical fractions. Third, lime-calibrated agitated soil-RD suspension tests were conducted for two months, illustrating the dependency of RD ANC on soil context, and yielding ANCs that were factor 2–5 times larger than in the batch renewal test because of pH buffering effects of soil particulates. Finally, an outdoor soil mesocosm on the four acid soils amended with RDs at 12 Mg/ha and 340 Mg/ha was established and soil pH was monitored for 2 years. The observed pH trends corresponded well for both the low dose (R2 a = 0.67) and the high dose (R2 a = 0.93) with model predictions made using the pH- and temperature-dependent RD dissolution rates (pH stat test) and the soil pH buffer power and RD slow ANC-fraction (soil-RD suspension test). This model predicts half-lives of RD dissolution ranging between <1 month to >100 years depending on the starting pH of the soil, its pH buffer power, the RD mineralogy (XRD based) and its specific surface area. This study shows that the dissolution and ANC of RD can be most pragmatically predicted with a series of lime-calibrated soil-RD suspension tests of maximally two months. [ABSTRACT FROM AUTHOR]

The disintegration of rocks into soil plays an important role in geomorphological processes, such as the evolution of hillslopes and river valleys. Nevertheless, the factors and processes controlling the physical weathering of rock particles are poorly understood. In this study, the surface and subsurface distribution of rock fragments abundance and size is measured in ten soil profiles along three hillslopes transects. For the surface horizons, we observed a significant linear relation between the ninth decile of the diameter, d 90 , and hillslope gradient and for both surface and subsurface horizons, a significant logarithmic relation between d 90 and rock fragment abundance. For the first time, the performance of various fragmentation models is compared against field data to evaluate if the surface rock distribution can be explained from fragmentation of the subsurface material only. In six profiles these fragmentation models adequately reproduced the observed particle size distribution. In the other four profiles, all located on eroding hillslopes, armouring dominates over fragmentation and the surface rock distribution is coarser compared to the subsurface. Generally, the profiles have a good fit to almost all models with the exception of two. However, we can differentiate two zones in terms of fragmentation mechanism according to the modelling results. The profiles in the transect located along a gently sloping hillslope, and the two other profiles, along a steep river valley. In the models, we consider physical weathering as the single factor of particle size evolution, except or two models where conceptual chemical weathering of the fine fraction is considered, although this chemical weathering was not specifically designed to explore soil water chemistry or mineralogy in detail. The observed discrepancies between field data and the results of this study, especially in the fine fraction, reveal the possible relevance of chemical or biological weathering, redistribution processes and the lower accuracy of the number of fine fraction classes established and measured than in the coarse fraction for the field data. • Different rock fragmentation models are tested with field data from three transects in a Mediterranean environment. • In six soil profiles, surface rock distribution can be explained from the fragmentation of the subsurface material • In the remaining four soil profiles, all located in gentle or steep slopes, armouring effect due to erosion is detected [ABSTRACT FROM AUTHOR]

Abstract The fate and transport of cesium (Cs+) is strongly related to the presence of clay minerals in soils, and their weathering due to natural cycles of wetting and drying. In this study, the adsorption characteristics of Cs+ under repeated wetting and drying conditions were investigated using the 1:1 lattice clay mineral, kaolinite, the non-expanding 2:1 lattice clay mineral, illite, and the expanding 2:1 lattice clay minerals, hydrobiotite and montmorillonite. Unlike the cation exchange capacity of clay minerals (montmorillonite > hydrobiotite > illite > kaolinite), the amount of Cs+ adsorbed followed the order of hydrobiotite > montmorillonite > illite > kaolinite. The expanding 2:1 lattice clay minerals (hydrobiotite and montmorillonite) displayed a fixed d-spacing when Cs+ was adsorbed and dried, after which they had a structure similar to that of illite. The desorption efficiency of Cs+ in illite and montmorillonite also was lowered after drying at 80 °C. Desorption efficiencies of Cs+ in clay minerals were in the order of kaolinite > illite > montmorillonite > hydrobiotite. Therefore, there are differences on desorption efficiencies of Cs according to clay minerals due to each clay mineral structure and characteristics. Highlights • Cs was fixed in the interlayer of montmorillonite by drying. • Wetting and drying cycles changed structure of montmorillonite into illite-like structure. • Desorption of Cs from expanding minerals hardly occurred. [ABSTRACT FROM AUTHOR]

Abstract The aim of the present study was to investigate the process of glauconite weathering in an alkaline soil under temperate climatic conditions. Calcaric Cambisol developed on Miocene sand in eastern Poland, and the sand containing parent glauconite, were sampled and analyzed using optical microscopy, X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIRS), Mössbauer spectroscopy, and scanning electron microscopy (SEM) with energy dispersive spectroscopy. The bulk soil material contained quartz, calcite, feldspars, glauconite, smectite, and traces of kaolinite. Both physical disintegration and chemical dissolution of the parent glauconite were evidenced by morphology of the pellets observed by SEM. Textural pedofeatures of the soil observed under the optical microscope indicated that no intense translocation of the finest material occurred within the profile. The green pellets separated from the sample representing the parent material contained almost pure glauconite with only traces of glauconite-smectite mixed-layered minerals. The pellets separated from middle and upper soil horizons contained mixed-layered smectite-rich glauconite-smectite, kaolinite, and minor amounts of goethite, as well as glauconite. Separated clay fractions contained glauconite, smectite-rich glauconite-smectite mixed-layered minerals, kaolinite, and traces of goethite. Observed increases, upward in the profile, in the proportions of glauconite-smectite mixed-layered minerals and kaolinite in the pellets and in the clay fraction suggest that both smectite and kaolinite are products of the glauconite weathering. The aluminous character of the smectitic phases, indicated by the position of the 060 XRD reflection and the FTIRS analyses, which show a gradual decrease in the content of Fe3+ in fine clays upward in the profile, suggest that leaching of iron from the glauconite took place during the weathering. The iron most likely precipitated in the form of goethite and XRD-amorphous iron compounds. Since Fe2+ constituted only 4% of the total iron present in the parent glauconite and accounted for only 0.05 of total layer charge, the oxidation of iron should not be considered the main mechanism of the glauconite smectitization. Highlights • Disintegration and dissolution of glauconite pellets occurred in the studied soil. • The clay material did not undergo an intense translocation within the soil profile. • Smectite, kaolinite and Fe-hydroxides are the products of glauconite weathering. • Formation of smectite was likely due to solid-state transformation of glauconite. • Formation of kaolinite occurred most likely by precipitation from soil solutions. [ABSTRACT FROM AUTHOR]

Abstract We present the results of a study of chromium (Cr) content, Cr speciation and its stable isotope composition in a terra rossa weathering profile, which includes ferromanganese concretions (FMCs) and gibbsite spots, developed on dolomite in the Yunnan–Guizhou Plateau in south central China. Sequential extraction results show that the Cr in all samples is present mainly within the residual fraction and oxidizable fraction, and that the Cr content of the exchangeable fraction and reducible fraction is very low. The significantly high Cr content (mean of 21.0 ppm) of the oxidizable fraction is a potential environmental hazard. The Cr in the terra rossa and gibbsite spots exhibits absolute accumulation throughout the profile; correspondingly, the FMCs are significantly enriched in most trace elements, except for Cr. Re -oxidation of Cr(III) to soluble Cr(VI) by MnO 2 is likely responsible for the relative depletion of Cr in the FMCs. In the bulk dolomite, the δ53Cr values of the carbonate component and detrital component are 0.447‰ and −0.437‰, respectively. The δ53Cr signal in both fractions is heterogeneous, while both δ53Cr signals are significantly different from that of bulk silicate Earth (BSE; −0.124 ± 0.101‰). Our results provide a proof of concept that the allogenic Cr-isotopic composition of terrigenous minerals in carbonate rocks, can provide new insights into paleo-terrestrial oxidative weathering. Overall, the terra rossa is characterized by the enrichment of heavy Cr isotopes compared to the detrital component of dolomite. Hence, the isotopic signature of Cr does not support the idea that the detrital component in dolomite is the sole parent material of the terra rossa. The absolute accumulation of Cr and heavy Cr isotope enrichment in terra rossa indicate that the extraneous input of dissolved Cr(VI) with a heavy isotopic signature occurred during dolomite weathering and terra rossa formation. Highlights • Cr exhibits absolute accumulation within a terra rossa profile. • The high Cr contnet of the oxidizable fraction is a potential environmental hazard. • Extraneous sources introduce isotopically heavy Cr(VI) into the weathering system. • δ53Cr of detrital component reflects paleo-terrestrial oxidative weathering. • The detrital component in dolomite is not the sole parent material for terra rossa. [ABSTRACT FROM AUTHOR]

Abstract Recent findings suggest that soil organic carbon mineralization and stabilization depend to a substantial degree on the soil geochemistry and the degree of weathering. We hypothesized that this dependence can be translated to decay rate modifiers in a model context, and used data from the Merced chronosequence (CA, U.S.A., 100 yr–3 Myr), representing a weathering sequence, to test, on a 1000-year time scale for model spin-up, a simple soil organic carbon (SOC) model based on the RothC26.3 model concepts. Model performance was tested for four levels of information: (1) known decay rates for each model SOC pool at individual chronosequence locations, obtained by calibrating the model to measured SOC-fractions and measured site-specific C-inputs; (2) average decay rates for each SOC-pool, corrected per location with rate modifiers based on geochemical proxies and measured site-specific C-inputs; (3) uncorrected average decay rates per SOC-pool and measured site-specific C-inputs; (4) uncorrected average decay rates per SOC-pool and averaged C-inputs. A lumped root mean square error (RMSE) statistic was calculated for each information level. We found that using local measurements of fresh C-input led to a decrease in RMSE of near 15% relative to information level (4). Applying geochemical rate modifiers led to a further reduction of 20%. Thus, we conclude that there is a benefit of including geochemical rate modifiers in this SOC-model. We repeated this analysis for a five-pool and a four-pool SOC model that either included or excluded an inert organic matter pool. In terms of the lumped RMSE both models performed similarly, but by comparing measured and simulated percentage Modern Carbon (pMC) for bulk SOC we concluded that measured pMC was best approximated using a four-pool SOC model (without an Inert Organic Matter pool). Furthermore, it is likely that a five-pool model including a very slowly decaying pool would further improve model performance. Highlights • Decay rates for RothC-pools correlate to degree of weathering and soil development. • Decay-rate modifiers based on weathering indices improved simulated SOC storage. • A SOC-model without Inert Organic Matter is appropriate for millennial timescales. • Local, site specific estimations of C-input generally increased simulation quality. [ABSTRACT FROM AUTHOR]

Abstract Mineral evolution in weathering zones is usually a complex process. Mixed-layered minerals (MLMs) can form as a result of progressive weathering advancement, promoting important geochemical changes in soils. In this contribution, four soil profiles located in Paraná State (southern Brazil) and formed from different fine-grained Permian sedimentary rocks (one siltstone, two Al-rich shales and one organic black shale) were characterized on mineralogical and geochemical bases. General physico-chemical/morphological characterization, XRD profile modeling of clay fractions, geochemical mass balance and weathering index calculations were performed to relate clay minerals transformation with the geochemical transformations in soils. MLMs dominate clay mineral suite, in progressive compositional ranges. The three soils developed from Al-rich shales (P1, P2 and P4) are dominated by several R0 K-I (kaolinite-illite) phases, and exhibit a trend of progressive kaolinization towards surface soil horizons. The soil developed from Mg-rich organic black shale (P3), however, has R0 I-S (illite-smectite) as the dominant phase, and a progressive dominance of smectite in I-S phase prevails towards surface. These results suggest the prevalence of topotactic transformations (solid-state reactions) during the transition of one clay into another as weathering advances. In the three soils where kaolinization of K-I phases prevails, geochemical mass balance calculations indicate massive losses of SiO 2 , Al 2 O 3 and alkalis, in addition to high values for weathering indices. Conversely, SiO 2 and MgO accumulate, and values for weathering indices are lower in the P3 soil profile. The agreement between the pathways of clay mineral transformations and the mass balance calculations and weathering indices suggests that transformations involving MLMs drive geochemical transformation in soils. The nature of clay suites, in turn, is highly dependent on the chemical compositions of fresh rocks, which determine how transformations involving clays proceed. Highlights • Mixed-layered minerals (MLM) prevail in soils as weathering advances. • Diagenetic illite transforms into R0 kaolinite-illite or illite-smectite. • Kaolinite/smectite contents in MLM increase towards highly weathered horizons. • Chemical composition of rocks defines the pathways of mineral transformations. • Mass balance calculations/weathering indices support the models of clays evolution. [ABSTRACT FROM AUTHOR]

Pedogenic processes imprint their signature on soils over the course of thousands to millions of years in most soil systems. Variation in soil forming processes – such as parent material weathering, organic material additions, hydrologic processes, and atmospheric additions – account for the distribution and sourcing of cations in ecosystems, and hence exert a strong influence on ecosystem productivity. Soil nutrient dynamics of cations also provide an indication of the dominant soil forming processes at work in a particular system. To gain insight into the generation and distribution of the soil cation pool in the Fraser Experimental Forest (FEF), we combined geochemical mass balance techniques and isotopic analyses of soil geochemical data to pedons across eight soil catenas in complex mountain terrain typical of the central Rocky Mountains. We found that mass gains in the FEF soils are primarily attributable to pedogenic additions of Ca to the soil mantle via atmospheric dust, and specifically that soil catenas on the summit landscapes were most enriched in Ca. Our data also show that atmospheric deposition contributions (calculated using Sr isotope ratios) to soils is as high as 82% (±3% SD), and that this isotopic signature in A-horizons and subsurface soil horizons diverges along a soil catena , due to both vertical and lateral hydrologic redistribution processes. Our results suggest that long term soil development and associated chemical signatures at the FEF are principally driven by the coupling of landscape scale cation supply processes, snow distribution, and snowmelt dynamics. Soil development models describing pedogenesis across catenas in montane ecosystems must pay special attention to atmospheric inputs and their redistribution. Any changes to these dynamics will affect productivity and soil/water chemistry in such ecosystems as investigated here. [ABSTRACT FROM AUTHOR]

A simplified soil model is presented for evolution of the mineral soil profile. The model provides a compromise between highly empirical models and highly mechanistic/geochemical models, and represents some of the main features of observed profiles, with features that can be identified with ‘A’, ‘B’ and ‘C’ horizons. The model is responsive to a range of global environments, which can be represented through climate and parent material parameters. In many cases there is an almost single-valued relationship between surface weathering and soil depth, allowing further simplification of the model, and allowing it to be included within a parsimonious landform evolution models. A key parameter and assumption of the model is the degree of weathering below which no further solution occurs, which limits the maximum extent of soil development. This is speculatively linked to CO 2 turnover rates and the degree of aridity. [ABSTRACT FROM AUTHOR]

• The chemical weathering intensity in Maritime Antarctica is modeled. • A geophysical survey and lithological characteristics as input data. • Proximal geophysical sensors and terrain attributes are combined for modeling. • Use of leave-one-out-cross-validation method in modeling. • Periglacial processes control the distribution of geophysical variables. The chemical weathering intensity in Antarctica is underestimated. As the chemical weathering intensity increases, hydrological, geochemical and geophysical changes occur in the different environmental spheres and at their interfaces through reactions and energy flows. Thus, once chemical weathering rates are understood and estimated, they can be used to predict and assess changes and trends in different environmental spheres. Few studies on the chemical weathering intensity have been performed in Antarctica. We used radiometric and magnetic properties associated with terrain attributes and the chemical degree of alteration of the igneous rock to model the chemical weathering intensity in Maritime Antarctica by using machine learning. Then, we related the chemical weathering intensity and geophysical variables with periglacial processes. To do this, gamma-spectrometric and magnetic readings were carried out using proximal-field sensors at 91 points located on different lithologies in a representative area of Maritime Antarctica. A qualitative analysis of chemical alteration for the different lithologies was carried out based on field observations and rock properties, and the levels of the chemical weathering degree were established. The geophysical data associated with terrain attributes were used as input data in the modeling of the weathering intensity. Then, the levels of the rock weathering degree were used as the "y" variable in the models. The results indicated that the C5.0 algorithm had the best performance in predicting the weathering intensity, and the most important variables were eTh, 40K, 40K/eTh, 40K/eU, the magnetic susceptibility and terrain attributes. The contents of radionuclides and ferrimagnetic minerals in different lithologies, concomitantly with the intensity at which chemical weathering occurs, determine the contents of these elements. However, the stability and distribution of these elements in a cold periglacial environment are controlled by periglacial processes. The chemical weathering intensity prediction model using gamma-spectrometric and magnetic data matched the in situ estimate of the chemical degree of alteration of the rock. The pyritized andesites showed the highest intensities of weathering, followed by tuffites, diorites, andesitic basalts and basaltic andesites, and the lowest weathering intensity was shown by undifferentiated marine sediments. This work highlighted the suitability of using machine learning techniques and proximal-field sensor data to study the chemical weathering process on different rocks in these important and inhospitable areas of the cryosphere system. [ABSTRACT FROM AUTHOR]

The biogeochemical weathering of the mineral apatite links the lithosphere to the biosphere by releasing the essential nutrient phosphorus (P) into the soil ecosystem. In Taylor Valley, Antarctica, faster rates of apatite weathering may be responsible for the higher concentrations of bioavailable soil P that exist in the Fryxell Basin as compared to the Bonney Basin. In this study, we use scanning electron microscopy to quantify the morphology and surface etching of individual apatite grains to determine whether the degree of apatite weathering differs between the Fryxell and Bonney Basins as well as saturated and dry soil sediments. We show that apatite grains from the Fryxell Basin are rounder, have fewer intact crystal faces, and are more chemically etched than grains from the Bonney Basin. In the Bonney Basin, apatite grains from dry soils show few signs of chemical dissolution, suggesting that soil moisture is a stronger control on the rate of apatite weathering in the Bonney Basin than in the Fryxell Basin. In addition, etch-pit morphologies in the Bonney Basin are more clearly controlled by the hexagonal crystal structure of apatite, while in the Fryxell Basin, etch pits demonstrate a wide range of morphologies without clear crystallographic control. Higher rates of apatite weathering in the Fryxell Basin may be due to the legacy of the physical abrasion of apatite grains during transport by a warm-based ice sheet, as well as the higher levels of precipitation and soil moisture closer to the coast. Our grain-scale approach provides a new perspective on P cycling in the McMurdo Dry Valleys and has implications for apatite weathering and P dynamics in the early stages of soil development. [ABSTRACT FROM AUTHOR]

Assessments of soils by reflectance spectroscopy have potential to facilitate and optimize soil survey, classification, and mapping of large areas. Visible and near-infrared (vis–NIR) spectra have been moderately used in pedological studies regarding characterization of soil samples and profiles, weathering and pedogenetic alterations along toposequences and landscapes, and soil classification. Therefore, there still is a lack of information about vis–NIR spectral pedology. For soil samples, our aims were to characterize the effects of physical, chemical, and mineralogical properties on vis–NIR soil spectral behavior, and to evaluate its potentiality in clustering 1259 soil samples according to weathering levels. For soil profiles, our aims were to evaluate the influence of pedogenesis in spectral behavior of some typical Brazilian soil classes, and to discriminate them by integrating proximal sensing and distance metrics. Continuum removed spectral data were transformed by Principal Component Analysis (PCA), and Fuzzy K-means algorithm and taxonomic distance were applied to cluster soil samples and profiles, respectively. Differences in reflectance intensity and absorption features caused by weathering intensification enabled to distinguish soil samples regarding similarity of particle size distribution, mineralogy, and some chemical properties. Soil formation processes, in particular, lessivage, dessilication, and ferralization similarly affected the spectral behavior soil profiles considering changes in depth caused by re-distribution of soil properties by horizons. A coherent discrimination of soil profiles was possible by combining spectral data and pedological distance metrics where 30% of the soil classes could be individually clustered as follow: Ferralsol, Nitisol, Acrisol, Lixisol, Arenosol, Gleysol, Cambisol, and Leptsol. vis–NIR spectral data enabled a coherent grouping of Ferralsol profiles with different contents of clay and iron oxides. vis–NIR reflectance spectroscopy is presented here as a useful and reliable tool for direct applications in pedological assessments, in particular, soil survey and classification. [ABSTRACT FROM AUTHOR]

Mineral weathering transforms rock into soils that supply nutrients to ecosystems, store terrestrial carbon, and provide habitat for organisms. As a result, the mineralogy and geochemistry of soils from contrasting environments are well-studied. The primary objective of this research was to examine how climate, topography, and dust interactively control the mineral and geochemical composition of granitic soils that span an environmental gradient in southern Arizona. Two field sites were selected within the Catalina Critical Zone Observatory that exhibit significant range in precipitation (25 to 85 cm yr − 1 ), temperature (24 to 10 °C), and vegetation composition (desert scrub → mixed conifer). Within each site, two catena end member pairs were selected to represent variation in local topography which included divergent, water-shedding summits and convergent, water-gathering footslopes. Soils and parent rock were studied using x-ray diffraction and x-ray fluorescence. Dust samples were collected from ridgetop dust traps at the desert scrub site and examined using x-ray fluorescence. The desert scrub soils showed enrichment in biotite, total feldspar, and Fe + Mg whereas the mixed conifer soils were depleted in feldspars and enriched in Fe + Mg. Depletions of Na, Si, and K + Ca occurred in both the desert and mixed conifer ecosystems, with the convergent soils in the conifer sites exhibiting the greatest degree of elemental loss. We examined dust in the regolith after identifying mineral and elemental enrichments in both ecosystems. Dust fraction estimates ranged from 2 to 21% in desert soils and 9 to 19% for the mixed conifer soils. Our results confirm the interactive role of bioclimate, topography, and dust in driving the geochemical evolution of soils and cycling of nutrients in desert and conifer ecosystems. [ABSTRACT FROM AUTHOR]

Global biomass production is fundamentally affected by the hydrological cycling of elements at the Earth's surface. Continental weathering processes are the major source for most bio-essential elements in marine environments and therefore affect primary productivity. In addition, critical zone biomass depends on energy and chemical exchange reactions in weathering profiles. The latter reservoirs are in turn influenced by different climatic conditions that control weathering and pore water parameters like pH and Eh, which regulate mineral break down rates and dictate the mobility and mass flux of elements. Two Deccan Traps basalt weathering profiles of contrasting age and alteration intensity provide a natural laboratory for investigating the effects of rock alteration on Si and Cr and their isotopic systematics. The Quaternary Chhindwara profile has progressed to a moderate degree of alteration (saprolite), while the Paleogene Bidar example displays an extremely altered laterite. The Chhindwara saprolite profile shows a near uniform Cr and Si concentration and isotopic composition, whereas the Bidar laterite profile is characterised by an intense loss of Si, a large enrichment of Cr within the most altered uppermost levels, and a wide range of Cr stable isotope ratios (− 0.85 to 0.36‰ δ 53/52 Cr). A co-variation between Si and Cr isotopes, as well as their co-variation with iron content, provides empirical evidence that iron redistribution within the profile has a large effect on Cr mobility and Si isotopic fractionation. Therefore, it is concluded that iron oxides exert a primary control over the isotopic composition of both Cr and Si in pore waters of laterites. Since laterite formation is promoted by tropical climates, the results of this study provide new evidence to suggest that the hydrological Cr and Si fluxes originating from continental weathering have changed in accordance with large-scale, deep time climate variation and continental plate configuration. An increased flux of Si and greater magnitude of Cr mobility and isotopic fractionation are possibly amplified under CO 2 -rich, greenhouse episodes and/or when large landmasses were tectonically arranged at near equatorial latitudes. [ABSTRACT FROM AUTHOR]

Quantitative soil mineralogy has been identified as a key factor influencing PROFILE weathering estimates, and is often calculated with normative methods, such as the “Analysis to Mineralogy” (‘A2M’) model. In Sweden and other countries, there is a large request for accurate base cation weathering estimates in order to establish how sustainable harvest of biomass should be performed in the future. However, there is a lack of knowledge around the accuracy of the arithmetic mean output of A2M estimates, the most common A2M solution used in weathering studies. To our knowledge, a thorough investigation of how A2M input data affect the arithmetic mean output (center of gravity of the A2M solution space) is missing. In this study, the indirect geochemical normative method (A2M) was compared with a direct x-ray powder diffraction method (XRPD) to quantify soil mineralogy at two sites and 8 soil profiles, at a 10 cm depth interval. We explored the hypothesis that normative calculations performed with A2M produce an output in closer agreement with the mineralogy obtained from XRPD, if site specific mineralogical input data are used rather than regional data. Site-specific mineralogical input data consisted of mineral stoichiometry data measured by electron microprobe analysis (EMPA) and mineral identity determined by XRPD, whereas regional mineral input data were based on previously reported data on mineral stoichiometry and mineral identity, derived from three geological regions in Sweden. The results from this comparison showed that the site-specific approach yielded relatively low average biases and root mean square errors (RMSE) for most minerals, with the exception of quartz (Average bias of − 4.8 wt%, RMSE of 5.3 wt%) at the Asa site. The regional approach yielded deviating results for K-feldspar and dioctahedral mica, with high average biases and RMSE for dioctahedral mica (Asa: 7.8 wt%, 9.0 wt%; Flakaliden: 12.8 wt%, 15.5 wt%) and for K-feldspar (Asa: − 5.2 wt%, 6.1 wt%; Flakaliden: − 5.6 wt%, 6.7 wt%). The results from this study were supported by a close agreement between measured geochemistry and normalized geochemistry derived from a back calculation of the XRPD mineralogy (i.e. mineral budgeting). In conclusion, our findings suggest that A2M results in combination with site-specific mineralogical input data are improved independent of study site and soil profile. However, for future weathering studies it might be beneficial to find constraints of how to select a solution from the entire A2M solution space which is in better agreement with the XRPD mineralogy. [ABSTRACT FROM AUTHOR]

• Weathering paths and soil depth serve as a first order control of soil organic carbon. • Soil deeper than 100 cm, can store significant portions of the total SOC stock. • SOC stock and poorly crystalline mineral concentrations peak in soil <30 cm at 30 kyr. Characterizing the distribution and dynamics of organic carbon in soil is critical for quantifying changes in the global carbon cycle. In particular, weathering controls on near-surface and deep (>1 m) soil organic carbon (SOC) dynamics have been proposed but limited data prevents us from predicting SOC over topographically complex landscapes and quantifying how changes in climate and perturbations, such as wildfire or land management, influence SOC stocks. To advance our understanding of how weathering alters soil geochemistry and influences SOC storage, we synthesize previous data with a new analysis of the Siuslaw River soil chronosequence from terraces in the Oregon Coast Range, a region that harbors the richest SOC inventories in the continental US. We analyze how the relationships between soil geochemistry, physical properties, and SOC storage vary with weathering status and pathways across soils that span 0.041 to 990 kyr and vary in depth from 1 m to >10 m. To distinguish the key properties and processes influencing SOC storage at different depths, we break our analysis into three depth intervals: 0–30, 30–100, and >100 cm. Our results suggest that the processes that control SOC stocks vary systematically with time and depth owing to weathering impacts on soil properties and pedogenic development. At 30 kyr we observe a peak in SOC stock in the top 100 cm coincident with a peak in oxalate extractable Al and Fe concentrations, representing secondary poorly crystalline minerals, which is consistent with previous studies. We also observe a decline in shallow SOC stock for >30 kyr soils as poorly crystalline minerals are replaced by more stable crystalline forms and soils become clay dominated. At 120 kyr, SOC below 100 cm starts to contribute significantly to the total SOC profile inventory and by 990 kyr, this fraction composes >40% of the total SOC stock. Taken together, our results indicate that total SOC stock increases with soil age as the increased intensity of bedrock weathering deepens the critical zone, creating accommodation space for deep SOC storage. These findings reveal the intimate link between poorly crystalline minerals and SOC and suggest that systematic analysis of soil development in the critical zone provides a first-order constraint on SOC stocks. [ABSTRACT FROM AUTHOR]

• Forest P nutrition reflects both root system and soil development. • As roots grow, forests can exploit deeper, mineral-bound sources of P. • Land use governs ecosystem nutrient cycling by altering root depth and P stocks. Nearly 50 y ago, Walker and Syers hypothesized that sources of most terrestrial nutrients shift in dominance from mineral- to organic matter-derived over millennia as soils weather. We investigated how overlaying this soil development framework with vegetation dynamics that can feed back to soil development on relatively short timescales offers insight into ecosystem functioning. To test the hypothesis that forest nutrient economies mediate the nutritional importance of organic matter as mineral weathering proceeds, we paired litterfall decay experiments with soil mineralogical data from diverse forests across the Critical Zone (CZ) Observatory Network, USA. Our findings suggest that dominant sources of tree P may shift from organic matter-bound stocks to minerals as roots expand during the transition from mid to late stages of forest growth and encounter deeper soils that have experienced a lesser degree of weathering. Thus, plants may develop nutritional strategies that do not necessarily rely most heavily on the dominant P form present in an ecosystem, typically driven by stage of soil development, but rather on root proliferation over time, which governs the ability of plants to mine soil volumes at a diversity of depths. Ecosystem P nutrition therefore depends strongly on the interaction between dominant P form and root system growth, particularly as it reflects past land use for both plants and soils. We use these findings to produce a novel framework of vegetative nutrient economics that highlights how root system growth and land use change can influence nutrient transformations and bioavailability, and soil development, across Earth's critical zones. [ABSTRACT FROM AUTHOR]

Grave fills from seven human burials from a late-Viking age - early medieval cemetery at Hofstaðir, Mývatnssveit (Iceland) were examined by soil micromorphology and organic chemical analysis. Detailed analysis of the weathering of the mineral constituents of the grave fills demonstrates a relationship between the extent of weathering of volcanic silicates within the fills and the presence of buried human remains. Gas chromatography-mass spectrometry (GC–MS) of extracts from the fills and controls of two graves revealed organic signatures dominated by plant-derived organic matter, with no evidence of degradation products of the body tissues. Transformation of n -alkanes into n -alkan-2-ones provides evidence for microbial activity within the fills. GC–MS analysis of the organic extract from under one of the skulls and pyrolysis gas chromatography of wood fragments found in that grave provide compelling evidence for the former presence of a conifer wood coffin. The use of this non-native wood in the burial provides evidence for either the import of foreign lumber or the utilisation of driftwood, most likely originating from Russia/Siberia. [ABSTRACT FROM AUTHOR]

Many areas in the world are characterized by shallow soils underlain by weathered bedrock, but root-rock interactions and their implications for regolith weathering are poorly understood. To test the role of tree roots in weathering bedrock, we excavated four pits along a catena in a shale-dominated catchment at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in central Pennsylvania. We measured a variety of biological, physical, and chemical properties including: 1) root density, distribution, and respiration, 2) soil gas, and 3) elemental compositions, mineralogy, and morphology of soil, rock, and rock fracture fill at ridge top, mid-slope, toe-slope, and valley floor sites. As expected, root density declined rapidly with depth; nevertheless, fine roots were present in rock fractures even in the deepest, least weathered shale sampled (~ 180 cm below the land surface). Root densities in shale fractures were comparable between the ridge top and mid-slope pits. However, they were significantly lower in the toe-slope, despite increasing rock fracture densities, which is likely due to a shallower water table depth at the downslope site. Average root respiration (per mass of dry root tissue) in rock fractures was comparable to rates in the soil. Thus, the total flux of CO 2 from root respiration tracked root densities, decreasing with depth. Potential microbial respiration, estimated as the laboratory C mineralization potential, was about an order of magnitude lower than measured root respiration in both the soil and shale fractures. Roots were only observed in large aperture (> 50 μm) shale fractures that were filled with particulate material. The fill in these fractures was mineralogically and geochemically similar to the lowest soil horizons with respect to clay composition, element mobility, extractable dissolved organic C (DOC), inorganic N-species, and potentially mineralizable C and N, while total C and total N values for the fracture fill were similar to the shale bedrock. In the bulk soil, depletion profiles (Al, Fe, K, Mg, and Si) relative to unweathered shale reflected characteristic weathering of illite and vermiculized chlorite to kaolinite and are similar between soils and fracture fill. Such similarities indicate that the fracture coatings are likely the result of pedogenic processes that occur at depth in the fractures rather than translocation of soil particles downward into the fractures. Overall, our data suggest that roots and fill in shale fractures down to ~ 180 cm are qualitatively similar to those in surface soil horizons. Thus, the deepest manifestation of the chemical depletion profiles observed in the pits consists of the rock fracture fill, and this fill is present at low concentrations with similarly low concentrations of fine roots. [ABSTRACT FROM AUTHOR]

Patchy alpine grassland with soil excavated by plateau pika ( Ochotona curzoniae ) and with bald patches is common on the Qinghai-Tibetan Plateau (QTP) where desertification has developed rapidly over the last few decades. This may have significant effects on alpine grassland restoration, by changing soil properties. In this study, the contribution of plateau pika disturbance and erosion on patchy alpine grassland soil were examined by classifying the surface of the patchy grassland into 4 types—vegetation patch, new pika pile, old pika pile and bald patch—and comparing the gravel content of the top layer of soil, soil moisture, soil hardness, soil organic carbon (SOC), and soil total nitrogen (TN) among them in the four study areas with different climatic conditions, altitudes, and grassland types on the QTP. We also analyzed the relationship between the amount of soil surface gravel and the Green Fractional Vegetation Cover (GFVC) using aerial photos. The results showed that (1) gravel content was significantly greater in bald patches and pika piles than in vegetation patches (p < 0.05); (2) soil moisture, hardness, SOC and TN were the highest in vegetation patches, and significantly lower in pika piles than in vegetation patches (p < 0.05); (3) GFVC was negatively and linearly correlated with the amount of soil surface gravel, with the amount of soil surface gravel in non-vegetation patches significantly greater than in vegetation patches (p < 0.001). Our results suggested that pika burrowing activity may increase the gravel content of the top layer of soil and decrease soil moisture, hardness, SOC and TN, which can increase soil erosion and hinder vegetation restoration. [ABSTRACT FROM AUTHOR]

A catenary sequence of soils formed on reworked loess under semiarid climatic conditions was studied. Four soil profiles located on the piedmont of Sierra Chica de Córdoba, central Argentina, were described, classified and geochemically analyzed. All soils, developed on summit, shoulder, backslope, and toeslope positions, were classified as Mollisols. Decarbonatation-carbonatation, melanization, and argilluviation are the main pedogenic processes recognized in these soils, which appear to control the differentiation of genetic horizons along the catena . Different geochemical approaches indicate that there are not substantial variations in the chemical composition of the studied soils along the catena , with the exception of the soil in the toeslope position, which exhibits slightly differences. In general, all profiles show weak depletions of mobile elements (Ca, Na, Mg, Sr, U) in the upper continental crust UCC-normalized diagrams which are attributed to a slight chemical alteration. Other elements, such as Fe, Cr, Co and Ni, also exhibit depletions compared to UCC, which can be explained by the alteration of ferromagnesian silicates, but can also be an inherited feature from the parent material. The significant enrichment in As compared to UCC, evident in all profiles along the catena , is also a typical feature of the pampean plains' loess of Argentina. So, the chemical differences in the profile located on the toeslope are mainly attributed to the supply of materials from local sources, i.e., crystalline basement and sedimentary rocks, due to its position in the catena . Statistical correlations and multivariate cluster analyses reinforce the assumption that the geochemistry of the studied soils is inherited from the parent material. In addition, chemical indices (CIA, ICV), elemental ratios (Ba/Sr, Rb/Sr) and the A-CN-K ternary diagram indicate an incipient degree of chemical alteration for these soils, compatible with the weathering regime prevailing in the region. Thus, the differentiation of genetic horizons along the catena is the result of weak weathering and pedogenic processes, which have not been strong enough to mask the chemical imprint of the parent material. [ABSTRACT FROM AUTHOR]

Ferricrete – sediment cemented by iron oxides and hydroxides – is common in subaerial weathering environments around the world. Formed under alternating oxidising and reducing conditions, ferricretes record pedogenetic processes of translocation and concentration of iron and trace elements in the soil. Given their stability and high preservation potential, ferricretes can provide insights to ancient soil forming and weathering processes. In this study the key processes controlling ferricrete geochemistry in a Neogene strandplain are identified and interpreted in the context of a coastal acid sulphate weathering system. Textural and geochemical variations in sediment indurated by hematite and goethite represent a record of in situ induration, erosion, and reworking. This development took place within an environment of fluctuating pH and Eh and subaerial wetting and drying cycles. We distinguished depositional and post-depositional processes based on the results of whole rock geochemistry, hyperspectral mineralogy, and major and trace element maps of petrographic thin sections. The ferricretes have three morphological types: flat-lying indurations, concentric pisoliths, and rounded nodules with fragmented internal textures. Successive laminae of Fe-oxides and hydroxides in all morphological types of ferricrete have variable Fe, Al, and Si abundances, reflecting cyclic precipitation and groundwater chemistry changes. Episodic wetting and drying of near coastal sediments was superimposed on a long-term trend of marine regression and local tectonic uplift (from ~ 7 Ma to present). This resulted in the diachronous exposure of relatively reduced shoreline sediments and concomitant acid production due to ferrolysis and the oxidation of biogenic sulphide. Local landform variations contributed to a wide variety of pedogenic processes and subsequent ferricrete formation. Acid sulphate weathering recorded by these indurated sediments is similar to conditions that are observed at present in the shallow water estuary of the Lower Lakes, near the mouth of the Murray River. [ABSTRACT FROM AUTHOR]

The surface chemistry of pyrogenic organic matter (PyOM) is altered by a variety of abiotic and biotic oxidative and sorption/desorption processes in the environment. Exposure of PyOM to high energy light prior to addition to soil or sediment, or while entrained in the atmosphere, may induce significant surface photooxidation, i.e., photochemical weathering, altering its environmental reactivity. We report on a 30-day soil incubation experiment testing the effects of the photochemical weathering of a 13 C-enriched ponderosa pine PyOM, produced by pyrolysis at 450 °C, on PyOM and soil organic carbon (SOC) mineralization. PyOM C mineralization was measured for both the photochemically weathered (i.e. UV treated PyOM or PyOMUV) and PyOM not exposed to high-energy light (i.e. PyOMW serving as a dark control). PyOMW exhibited a 3.7 times faster C mineralization rate across the 30-d study, which was driven by a large early mineralization of accessible/labile C during the first 6 d. In contrast, PyOMUV had faster C mineralization rates in the later part of the experiment (days 11–30). Overall, PyOMUV had a 13% lower net C mineralization than the untreated PyOMW where the MRT of accessible PyOMUV-C and PyOMW-C was calculated at 25.7 ± 6.8 d and 1.7 ± 0.2 d, respectively. Both forms of PyOM promoted a similar net reduction in native SOC mineralization (i.e., negative priming) of approximately 50% relative to the unamended, control soil. Addition of either PyOM form resulted in an equivalent minor decrease in the concentration of extractable soil lignin phenols and substituted fatty acids chemistry with respect to the unamended soil. At 30 d, soil phenol oxidase and peroxidase enzyme activities were higher with additions of either form of PyOM compared with the unamended control soils with PyOMUV exhibiting lower activities than PyOMW. Our results indicate that PyOM photochemical weathering can impart important changes to short-term PyOM reactivity and soil microbial activity, which could have important implications for soil systems by ultimately lowering turnover rates for both NSC and PyOM-C. [ABSTRACT FROM AUTHOR]

Airborne gamma-ray imagery (AGRI) provides coarse-resolution (approximately 400 × 400 m pixel) spatial information on gamma-ray emitting elements, i.e. potassium (K), thorium (Th) and uranium (U), in the upper half meter of the soil. These radioelements are a potential information source for soil mapping since their abundance is related to soil geochemistry, specifically the chemical composition of parent materials and their weathering products resulting from geomorphic and pedogenic processes. The aim of this study was to evaluate the potential of AGRI for improving digital soil survey process in two phases: (1) a preliminary phase, where hypotheses of soil-landscape relationships are developed, and (2) a phase where soil map unit boundaries are placed or revised. The study also evaluated the match between AGRI and existing lithologic, geopedologic, and soil series maps. The study was conducted in a well-characterized complex soil landscape: the 560 km 2 upper Pa Sak watershed in Petchaboon province, Thailand. The relationship between gamma-ray data and geological units was examined with box-and-whisker plots and confirmed by rock and soil samples. Rock and soil sample classifications were compared with the gamma-ray image and to typical radioelement responses found in the literature. To interpret AGRI data in terms of regolith and soil genesis, we compared AGRI to two existing soil maps: geopedologic and soil series maps. First, the geopedologic map was split into four maps according to the geopedologic hierarchy: landscape, lithology, relief, and landform; at the latter (lowest) level, soil units are also associated. Secondly, soil series and geopedologic soil units were used to examine the distribution of radioelement response to selected soil characteristics: parent material, texture, mineralogy, and thickness. The interpretations from best correlation in both soil maps were described in terms of the radioelement changes during pedogenic and geomorphic processes, based on a review of literature and supported by soil samples. In the hypotheses-generating stage, AGRI provided useful information in three forms (single signal, ratio, and ternary images enhanced with a hill shaded DEM) by relating these to lithology, material transport, and internal pedogenetic processes. AGRI correlated well with the classes of the geopedologic map (1:50,000) at the two higher levels (landscape and lithology) but to a lesser degree at the two lower levels (relief and landform in geopedologic approach). In the mapping stage, AGRI showed deficiencies in the soil series map (1:50,000) made by conventional aerial photo analysis and limited field surveys, especially in inaccessible areas but also in low-relief terraces and flood plains, which provided a basis for future field sampling to correct these deficiencies. AGRI suggested new boundaries, differentiating topsoil properties and the presence of plinthite, despite its coarse resolution. Clustering of gamma ray and elevation data (DEM) was carried out using fuzzy logic to generate various classification layers. Final map was produced using total weighted inverse distance of all the classes in a 3 by 3 window. Class label was assigned to the one with the largest total inverse distance over the entire set of fuzzy classification bands. The result shows relatively higher classification accuracy for soil parent material differentiation (overall accuracy of 72%) as compared to the classification for soil types (67%). [ABSTRACT FROM AUTHOR]

The properties of and processes in calcareous soils are important in the context of dryland soil management and moderation of atmospheric CO 2 by carbonate weathering. This study was conducted to determine the major and trace elemental composition in soils of the Kurdistan region of Iraq, and their geochemical features. Soil samples were taken from seven pedons, five of them from forest soils and two others from non-cultivated soils. Selected physical properties, total concentrations of elements and mineralogical properties were determined on samples from each horizon. Weathering processes in the soils are characterised by loss of Ca, Sr and several trace elements, and accumulation of less mobile elements such as Zr, Ti, and Al. The progress of carbonate weathering was best expressed using Ba/Sr and Rb/Sr ratios. Principal components analysis confirmed that distinctive geochemical differences for two profiles (significantly higher Mg, Co, Cr, Mn, and Ni) were likely to represent control by parent material, a conclusion reinforced by both mineralogical differences and the location of profiles relative to surficial geology. The other main geochemical processes differentiating the pedons were chemical weathering of carbonates and accumulation of residual elements. Calculation of absolute mass flux ( τ ) values showed that, for some profiles, concentration of mobile elements such as Na and K by evapotranspiration was potentially important, despite evidence for clay eluviation in most profiles. Absolute loss of several elements was also correlated with increased weathering as measured by Ba/Sr ratios. None of the forest soils showed enrichment-depletion ( τ ) profiles for Fe or Al consistent with podzolisation. This study highlights useful geochemical approaches to understanding chemical weathering and pedogenesis in calcareous soil profiles. [ABSTRACT FROM AUTHOR]

The skeleton water-extractable fines (SWEF) is a soil fraction smaller than 2 mm located at the interface between rock fragments (RF) and fine earth (FE). It is made of illuvial and/or RF weathering products comprising neoformed clay-size particles and organic substances. Contrary to FE and even to soil RF, SWEF represents a poorly documented soil fraction despite its unique ion exchange properties. To fill this knowledge gap, we compared mineralogy, effective cation exchange capacity (ECEC), and exchangeable cation composition of SWEF with those of FE and RF from three acidic soils derived from either mica schist (Mottarone, Italy), granite (Aubure, France) or glacial till of varied lithology (Gårdsjön, Sweden), and subjected to distinct climatic conditions. Further, we provided a general hypothesis on the formation of SWEF. The SWEF had a different acidity level, a significantly higher accumulation of organic C, a higher Al p and/or Fe p content (in two over three soils), was enriched in neoformed minerals, and had a larger concentration of exchangeable cations than both FE and RF. A key role in the development of the SWEF properties was attributed to mineralogy, weathering and pedogenesis. The interactions between mineralogical composition, pH, and organic C content not only determined the extent of ECEC in SWEF but also the source of exchangeable cations originating either from organic complexes (Mottarone soil) or from the dissolution of the mineral phases (Aubure and Gårdsjön soils). However, the abundance of SWEF appeared to be independent from the actual abundance of RF. Its formation was rather controlled by the combination of mineralogy of the parent material, weathering intensity, pathway of mineral dissolution (congruent and/or incongruent dissolution), and to the duration of pedogenesis. [ABSTRACT FROM AUTHOR]

• Definitions of regolith and soil examined. • Case made for regolith and soil being the same object of study. • Discussion of whole-regolith pedology. • The pedosphere/regosphere as an integral component of the Earth System. • Soil in the Critical Zone. Definitions of soil and regolith abound, the traditional view seeing soil as the top part of the regolith. Leighton (1958) claimed that soil and regolith are the same thing. Definitions of soil arising from research on soil and regolith in extraterrestrial, Precambrian, and some human-made settings tends to reinforce this view. Over the last two decades, the work of some researchers, including whole-regolith pedologists, has reinforced the case for Leighton's claim, suggesting that soil and regolith are indeed the same and form pedo-weathering profiles. This view partly reflects a growing realization that the substratum plays an important role in some soil-forming processes, and should be included in soil classification schemes. The regolith-or-soil debate also applies when looking at soil in the contexts of the Earth System and the Critical Zone, both of which stress the connections between unweathered rock, weathered rock, fluid flows, and biological activity at local, regional, and global scales. It is unlikely that the debate will end: realistically, research on the whole-regolith pedological studies, including those that tackle wider links with the Earth System, will accompany pedological studies that focus on the solum and soil-forming processes within it, although they will assuredly inform each other; and, even though a case is made for regolith and soil being the same thing, it is will doubtless spark disagreement. [ABSTRACT FROM AUTHOR]

I- and S-type granites comprise large areas and play a key role in global soil weathering patterns. I-type granites are originated from melting of igneous source rocks, whereas S-type granites result from melting of sedimentary sources. This gives rise to differences in mineralogical and geochemical characteristics of the rocks. The objective of this study was to address the effect of petrology and mineralogy of I- and S-type granites on weathering, pedogenesis and total organic carbon stocks across a climosequence. Significant differences in morphological, physical and chemical properties of soils derived from I- and S-type granites were observed and directly linked to petrological, mineralogical and geochemical signatures of the underlying granites, e.g., presence of more mafic minerals in the I-type granite, and therefore a higher clay and iron content in their derived soils. In addition, this higher iron content favored the hematite pathway and, thus, the red soil development. Despite the influence of the climate on weathering patterns, i.e., higher soil development in humid areas, parent material seems to play a decisive role in determining soils characteristics in the studied area. The type of parent material also governed the rates of carbon accumulation in soils. I-type granites originated soils with higher TOC stocks than soil profiles developed from S-type granites. This carbon accumulation is mainly due to soil chemical and physical characteristics driven by the type of granite, notably soil fertility and clay content. [ABSTRACT FROM AUTHOR]

Schematic representation of the development of the pore network during weathering. Macrocracks are shown in red, microcracks in blue, and the microporosity of the minerals is shown in grey (D µ = density of microcracks; DM = density of macrocracks; ΦMp = microporosity; ΦM = porosity of macrocracks; Φ µ = porosity of microcracks). [Display omitted] • Pores densities and apertures were determined by 14C-PMMA autoradiography and XRCT. • Cracks densities and cracks apertures both increase during weathering. • Total porosity mainly increases by increase of the aperture of pre-existing cracks. • Initial porosity of un-weathered rock controls development of pores during weathering. The development of porosity during rock weathering is a key process controlling nutrients release, water holding capacity available for plants and water flow. Here we used X-ray Computed Tomography (XRCT) and 14C PolyMethylMethAcrylate (PMMA) autoradiography to show how cracks are created and enlarged during initial weathering stages (saprock and saprolite) of granodiorite in southern Brazil (Viamão - RS). The physical evolution is characterized by imaging the pore network, using 14C-PMMA and XRCT methods. Combined with bulk porosity measurements, they highlight the increase in porosity with the degree of weathering (un-weathered rock Φ = 1.66 %, saprolite Φ = 11.7 %). This increase is related to the joint increase of the density of the cracks (un-weathered rock D = 0.28 mm−1, saprolite D = 0.94 mm−1) and of the average opening of the microcracks (un-weathered rock w = 2.4 µm, saprolite w = 3.9 µm) and macrocracks (un-weathered rock w = 176 µm, saprolite w = 400 µm). However, these average crack openings do not account for the variability of the openings that govern the flows, characterized here by specific distribution ranging from the submicrometre to the centimetre scale. The results highlight that the pore network of the un-weathered rock plays a key role in the initial stages or incipient weathering. The density and aperture and cracks increase following the subcritical cracking concept and new pores are formed by chemo-mechanical processes. The presence/absence of initial fractures in the regolith is certainly a key parameter controlling the weathering of different rock types (mafic vs felsic). [ABSTRACT FROM AUTHOR]

Geochemical weathering indices are widely used in evaluating the stage of soil development. However, their compatibility with Soil Taxonomic Classes has not been studied yet. This study was conducted in a hilly region to illustrate the relations between soil development indices and topographic attributes and Soil Taxonomic Classes. A grid sampling method was carried out with 100 m intervals. Physico-chemical analyses and total elemental analyses were performed on 184 and 56 soil samples respectively. In addition, eight topographic attributes and 12 common soil development indices were determined. Results showed that among the topographic attributes, slope gradient and topographic wetness index (TWI) had the highest correlation with Soil Taxonomic Classes and development indices. Morphological index (r = 0.60) and Fe o /Fe d ratio (r = 0.52) increased with progress in soil development from Entisols to Alfisols and Mollisols. Despite the significant correlations between most of the geochemical weathering indices and Soil Taxonomic Classes (Vogt index had the highest correlation coefficient (r = − 0.74)), these indices represented lower weathering intensity in more developed soils. These relationships were due to the predominance of more developed soils in low gradient slopes, where the weathering products of upper slopes accumulate and their parent materials are carbonatic. [ABSTRACT FROM AUTHOR]

The vertical distribution of soil exchangeable cations (Ca 2 + , Mg 2 + , K + , and Na + ) results from the integration of multiple processes: weathering of primary minerals, atmospheric input, leaching, and biological cycling. While weathering and atmospheric input affect the location of cation inputs to the soil system, leaching and biological cycling translocate cations in opposing directions within the profile. Little research has been conducted on deep soil relative to surface soil, and thus attempts to verify hypotheses about the drivers of exchangeable cation vertical distributions have not been possible on a broad scale. This study excavated soils down to 2.5 or 3 m at 22 sites across the coastal Pacific Northwest ranging from northern Washington to southern Oregon. Samples were analyzed for soil carbon (C), nitrogen (N), and exchangeable cations. PERMANOVA was used to evaluate the effect of soil chemical (C, N, and pH), environmental (climate, parent material, elevation), physical (texture) and spatial (horizon, depth) gradients on the distribution of exchangeable cation contents and stocks. The majority of exchangeable cation stocks are located in deeper soil horizons. On average, 66% of Ca 2 + , 76% of Mg 2 + , 57% of K + , and 63% of Na + stocks were below 1.0 m. Master soil horizon was the most significant predictor of the distribution of cations within the soil profile with substantial separation between A horizon samples and B and C horizon samples. There was a significant interaction between horizon and soil C, with higher soil C concentrations and stocks corresponding with lower Ca 2 + and Mg 2 + contents and stocks in horizons of the same type. Between sites, climate and cumulative carbon stocks were the dominant controls over cation distribution, with high C stocks and wet, cool climate leading to low exchangeable Ca 2 + and Mg 2 + . These results suggest that leaching with dissolved organic matter could be a driver of exchangeable Ca 2 + and Mg 2 + distributions in the Pacific Northwest. On the other hand, K + and Na + were largely uncorrelated with these environmental gradients. Biological uptake is a more important control over the distribution of exchangeable K + , while atmospheric deposition drives the relatively uniform distribution of Na + both between sites and within profiles. [ABSTRACT FROM AUTHOR]

Enhanced weathering associated with the use of phosphate fertilisers has been identified in some of the major farming areas of New Zealand and an evaluation of its effects on soil properties and the implications for soil management are needed. We assessed changes in the character of volcanic glass shards in topsoils (0 − 10 cm depth) of two tephra-derived Andisols of differing ages (Udivitrands, Hapludands), and with and without long-term fertilisation, to test if fertilising accelerates the weathering of soil constituents. Using visual assessment based on scanning electron microscopy and electron microprobe analyses of glass shards from samples from paired sites, we showed that the average sizes of the shards and the sharpness of the glass-shard edges (angularity) diminished with the age of the soil, and that these decreases were more marked in the soils that had been fertilised. Silica polymorphs were observed only in the older soil (Hapludand) that had been fertilised. We concluded that the addition of phosphate-containing fertiliser enhanced the dissolution of volcanic glass, consistent with phosphoric acid and F − -induced dealumination and desilication, and thus soil weathering has been accelerated. The Al and Si may subsequently coprecipitate as secondary minerals, such as allophane. However, silica polymorphs may form where Al activity is low. The occurrence of neogenic silica in topsoils (despite high annual rainfall that should result in desilication of the topsoil and reprecipitation of silica deeper in the soil) is explained by the seasonality of fertiliser application and the spring–summer climate, where the soil remains moist for periods sufficiently long enough for the dissolution of volcanic glass (and possibly of other aluminosilicates), the formation of Al–humus complexes, and the subsequent precipitation of silica in dry periods. Implications of the accelerated weathering for soil and land management are discussed. [ABSTRACT FROM AUTHOR]

A new approach to estimate weathering indices (WIs) in soil was developed using proximally-sensed spectral reflectance over visible to shortwave-infrared (vis–NIR) and mid-infrared (MIR) region of electromagnetic spectrum. Partial-least-squares regression (PLSR) analysis was used to develop spectral algorithms for estimating twelve different WIs commonly used in geochemical literature. For each index, two different models were calibrated: 1) based on all the features in spectra and 2) based on the features obtained by variable importance projection, which we denote by vis–NIR vip , MIR vip , and (vis–NIR + MIR) vip . The residual prediction deviation (RPD) was used for checking the robustness of spectral models. Results showed that the MIR reflectance data provided superior estimation capability for all WIs compared with the vis–NIR reflectance data with the best possible prediction obtained for index of laterization (IOL; RPD = 5.86) in the MIR and Mg Index (MgI; RPD = 2.43) in the vis–NIR approach. The highest RPD values of 3.12, 4.13, 3.78, 6.13, and 5.08 were obtained for chemical index of alteration (CIA), MgI, mafic index of alteration (MIA(O)), IOL, and weathering index of Parker (WIP), respectively, when the PLSR model was based on (vis–NIR + MIR) vip . Best predictions were obtained when vis–NIR and MIR were combined and important spectral features were selected through variable importance projection (VIP) approach. Both vis–NIR and MIR technologies are available in the form of portable devices and are amenable for remote sensing mode of operation. This study demonstrates for the first time that the WI values in soil may be estimated in a rapid and non-destructive way in situ. [ABSTRACT FROM AUTHOR]

The regolith is defined as weathered in situ and transported material overlying unweathered bedrock. As fresh bedrock exposures form a small proportion of the landscape, regolith is common in Australia and varies in depth from less than a metre in upland settings to many hundreds of metres (e.g. the Cenozoic Basins). Because of crucial fluxes in gases, water, nutrients, and dissolved salts and variation in permeability and connectivity, knowledge of the nature and depth of regolith is important to many land-based industries, including agriculture and forestry. The minerals industry sees regolith as a barrier to discovery of mineralised rock at depth or as a host of minerals in economic concentrations. There are clearly a range of important characteristics of regolith; depth (and therefore the quantity of regolith) is fundamental for resource inventory and biophysical modelling applications. A method is described that maps the depth of regolith (with estimates of mapping uncertainty) to the moderately weathered/saprock boundary for the whole of Australia. Our approach draws on an extensive legacy of publically available drillhole data (of variable attribute consistency), and applies an environmental correlation-style of digital mapping prediction that utilises relevant spatial covariates, e.g. terrain analysis, climate and gamma radiometric datasets. From the original database of > 350,000 records we filtered and harmonised a useable dataset of 128,000 records. Key predictive datasets used in the depth model included weathering intensity, lithology age, distance to out crop, elevation and relief. Shallowest cover corresponded to high relief erosional landscapes and deepest corresponded to Cenozoic basin sediments. The final map with a ground resolution of approximately 90 m (3 arcsec) was generated using 100 bootstrap model iterations. The reliability of the model was assessed using measures of r-square (0.38), Lin's concordance (0.51), mean error (− 5.73 m) and root mean squared error (24.56 m) calculated on a withheld test dataset. The regolith depth map is consistent with known areas of deep in-situ weathering and accumulation of recent sediment associated with the distribution of Cenozoic Basins and provides a testable estimate in areas with little drilling or survey history. The predictive modelling approach provides a framework to further build and improve regolith depth prediction across varying spatial scales where sufficient quality drillhole data and environmental predictors exist and set priorities for new data acquisition. [ABSTRACT FROM AUTHOR]

The analysis of γ-radiometric data from proximal soil sensing measurements can be used to predict key soil properties for environmental and agricultural issues. These predictions require prior calibrations that use a large sampling of field data that could be significantly reduced if regional predictions can be established. However, such predictions may be challenging for Mediterranean regions because of the highly contrasted soils and specific geopedological characteristics (sedimentary rocks, low weathering, etc.). Our study focuses on the regional prediction of a key soil property (clay content), within highly contrasting geopedological groups (GPG) from the Languedoc region. The aims of the research were (i) to understand the relationship between γ-radiometric data and the clay content within the different complex GPGs and (ii) to predict the clay content from γ radiometry with a regional calibration based on the GPG as an auxiliary variable . A proximal soil survey using the γ radiometer, called “The Mole”, was conducted in 8 areas of the Languedoc region. The plots were selected over different GPGs that are representative of the soil diversity and materials according to their weathering intensity and igneous components. In total, 242 sites were sampled based on a random grid cell for soil laboratory measurements. The soil texture and stoniness were analysed, and the particle size fractions were separated for mineral identification via X-ray diffraction. The activity of 232 Th [1 to 71 bq/kg] and 40 K [40 to 771 bq/kg] exhibits the largest range between the different GPGs, which spans the range of values measured in previous European studies. The relationship between the clay content and 232 Th is significant at a regional level [R 2 = 0.60; RMSE = 42 g/kg]. The residuals are significantly distributed according to the different GPGs. A new linear model based on the 232 Th activity and the GPGs improves the prediction of the clay content [R 2 = 0.72; RMSE = 35 g/kg]. Our study showed that ground γ radiometry can be used to predict the clay content within a complex Mediterranean sedimentary context. The 232 Th radionuclide is the best predictor. The disturbance of the relationship between the 232 Th and the clay content corresponds to the fine earth mineralogy type and the proportion of igneous pebbles. [ABSTRACT FROM AUTHOR]

Forty-three soils (130 horizons), sampled by the geo-pedological mission organized by the State University of Gent (Belgium) in 1962 on Isla Santa Cruz (Galápagos Islands), were analysed in order to determine their degree of chemical evolution. Several weathering indices (Weathering Index of Parker – WIP –, Chemical Index of Alteration – CIA –, Chemical Index of Weathering – CIW –, Plagioclase Index of Alteration – PIA – and Silica–Titania Index – STI –) and multivariate statistical analysis (principal components analysis), based on chemical composition, were used. With the only exception of the STI, the indices were highly correlated (r > 0.85). The highest WIP and STI values (20.9 ± 8.2 and 70.2 ± 2.2 respectively) were found for soils developed on basalt flows near the coast. Slightly lower values (WIP 16.8 + 5.1 and STI 61 ± 3.4) were shown by brown soils developed from basaltic flows at elevations between 140 and 225 m a.s.l. While the lowest values (WIP 9 ± 5 and STI 47 ± 6.8), representing the more weathered materials, were found for soils located at the highest elevations (> 400 m a.s.l.) and mostly developed on pyroclastic materials (tuff and tephra). As the chemical composition of the geological material (basalt and tephra) is highly homogeneous, the degree of weathering is likely to depend on climatic conditions controlled by altitude and orientation. On the windward slopes of the island a gradient of increasing weathering is observed from the arid conditions predominant at the coast to elevations of 400–500 m a.s.l., where much more humid conditions prevail. Principal component analysis on elemental composition also supported the interpretation that the degree of weathering (first component) and soil horizonation (second component) are both related to climatic conditions. Both, the variation of the chemical indices and the principal components of the geochemical composition are related to the bioclimatic zones: soils with the lowest degree of weathering are located in the arid coastal zone; slightly higher intensity was found for soils located in the transition and Scalesia zones; while the most weathered soils appear in the brown zone. Compared to other volcanic soils studied in the literature, soils from Isla Santa Cruz are in the upper range of chemical weathering intensity, only comparable to soils from Azores Islands and Canary Islands (Tenerife and La Gomera) developed on basalts, under oceanic conditions. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau has one of the largest distributions of alpine meadow in the world. Due to the cold climate and dominance of physical weathering conditions, coarse or massive debris prevails. Thus, pedogenesis is controlled by both the temperature and parent material. In this study, the alpine soils (Gelic Cambisol) at altitudes of 4768–4980 m in the Amdo zone of the northern Tibetan Plateau were investigated. We found that the parent materials throughout the alpine soil profiles were rich in clayey and silty minerals. Here, we focused on the origin of the soil forming materials of alpine meadows. We examined the grain size distribution, mineral compositions, trace element concentrations, and Sr–Nd isotopic compositions of the alpine soils and their potential source materials in various grain size fractions. Based on a comparison of many fingerprints, our results indicated that a red clay-rich bed at the bottom of the alpine soil profile is a residual paleo-weathering crust. Nevertheless, the parent materials in the alpine soils did not have a clear inherited relationship with the weathering products of the in situ bedrock. Correspondingly, we confirmed that the parent materials in the alpine soils have an aeolian origin. The alluvial sediments, and widespread beach and lacustrine delta sediments in the Amdo zone are the main source of alpine soil materials. However, it was not clear whether sediments are the main source of aeolian dust. Aeolian dust, with a short transport distance, is one of the main components (< 20 μm and 20–300 μm) of alpine soils. Correspondingly, we confirmed that the minor coarse components (> 300 μm) of alpine soils had a local origin, i.e., from adjacent bedrock outcrops. In contrast, long-range transport of dust had a minor role in the formation of alpine soils. In the region investigated, the aeolian dust began accumulating in the middle Holocene. On the Tibetan Plateau, one positive effect of aeolian processes is that the aeolian dust accretion is a source of parent materials and nutrients for alpine soils. [ABSTRACT FROM AUTHOR]