What controls selenium release during shale weathering?

This study demonstrates that only a combination of a chromous chloride reduction with dual sequential extraction schemes can clearly separate the proportions of Se present in the sulphide versus the organic pools in shales. The data reveals that even small amounts of pyrite outcompete the organic matter for the available Se and pyrite oxidation will control the release of selenium during shale weathering.     

Sulfuric acid as a weathering agent of carbonate weathering constrained by δ^13C： Examples from Southwest China

Rock weathering by carbonic acid is one of the important atmosphere CO2 sequestration. Actually, it depends on whether carbonic acid or other acids as weathering agents, which is important to understand the model of global carbon cycle. For example, sulfuric acid derived from oxidation of pyrite takes part in the rock weathering, which might counteract one part of CO2 drawdown by silicate weathering. In this study, chemicals and carbon isotopic composition of waters were determined in the Beipan River, Guizhou. The δ^13C values of dissolved inorganic carbon in the province, Southwest China. The values of the samples range from -13.1‰ to -2.4‰, which show a good negative correlation with the equivalent ratio of [HCO3]/（[Ca^2＋＋Mg^2＋]） and a good positive correlation with the equivalent ratio of [SO4^2-]/（[Ca^2＋＋Mg^2＋]） and [SO4^2-]/[HCO3^-]. The relationships suggest that sulfuric acid took part in carbonate weathering. Acid rain is thus a significant source of sulfuric acid to the karstic rivers of Guizhou Province.     

Effect of Aspergillus niger on weathering of phosphorite rock

Weathering mechanism of phosphorite rock by Aspergillus niger has been studied. Ten strains of phosphate-solubilizing fungi were isolated from rhizosphere soils by using dilute plate method in solid inorganic phosphorus culture. According to the size of soluble phosphate circle in the medium, a strain of stronger phosphate-solubilizing fungus was acquired, which was identified as, Aspergillus niger using identification methods of morphology and ITS sequences comparison in Genbank. In liquid culture, incubation in static state, the weathering of apatite rock with the fungus by contrast method： putting apatite powder （200 mesh） in liquid culture medium of flask, inoculation fungus after autoclave （setting sterile control） for studying direct weathering process; putting dialysis bag with apatite powder （200 mesh） in liquid culture medium of flask, inoculation fungus after autoclave （setting sterile control） for studying indirect weathering process.     

Sulfuric acid as a weathering agent of carbonate weathering constrained by δ~(13)C: Examples from Southwest China

Rock weathering by carbonic acid is one of the important atmosphere CO2 sequestration. Actually, it depends on whether carbonic acid or other acids as weathering agents, which is important to understand the model of global carbon cycle. For example, sulfu…     

Behaviour of REE during weathering of granite in a tropical climate

Weathering is essentially a leaching process and thus, one way of determining the extent of weathering is to monitor the mobility of different elements. Rate of loss of elements in a weathering profile can be determined by comparing the concentrations of …     

Carbonate weathering rates in the Jura Mountains, France-The influence of vegetation

In order to better understand what controls carbonate weathering rates, we report results from the Jura Mountains （East France）, an area exclusively composed of carbonate rocks. This region presents an altitude gradient increasing from 250 m to 1300 m. Over the basin, this gradient generates climatic contrasts of 5℃ and a runoff three times higher in altitude. This place offers a good opportunity for understanding the controlling factors of carbonate weathering. Contrary to thermodynamic calculations that predict the highest concentrations at low temperature, we observe that carbonate dissolution is two times higher at low elevation than in the mountains. This observation can only be explained by a variation in soil pCO2. In order to better constrain the observed variation in cation contents （principally Ca and Mg） of rivers, we used the ASPECTS ecological model （Rasse et al., 2001）. Based on both hourly climatic data, vegetation type and soil type, ASPECTS may reconstruct pCO2 in soil.     

Sodic metasomatism in a dacite weathering profile in Pinxiang, Guangxi, China

The Pinxiang weathering profile is well developed on Early Triassic dacite lavas of the Baisi Formation. At the top of the profile is developed a red clay zone which is characterized mineralogically by kaolinite, iron oxide minerals, quartz, and a small amount of illite, montmorillonite and vermiculite. In going downwards the red clay zone gives way to a saprolite zone in which plagioclase pseudomorphs have been well preserved although replaced by kaolinite. Beneath the saprolite zone is the saprock zone characterized by less weathering for dacite. At the bottom of the weathering profile is the parent material, dacite, which is composed mainly of plagioclase, quartz, K-feldspar and biotite which have been largely altered into chlorite owing to submarine extrusion of dacite lavas. Some layers in the weathering profile show obvious sodium enrichment and potassium depletion relative to others. In the Al2O3-(CaO* Na2O)-K2O triangular diagram, the weathering trends of these layers in the middle stage are remarkably deviated from normal ones. Both mineralogy and micro-morphology of these layers indicate such deviation resulted from sodic metasomatism of orthoclase.     

Geochemistry of large river suspended sediments: silicate weathering or recycling tracer?

This study focuses on the major and trace element composition of suspended sediments transported by the world’s largest rivers. Its main purpose is to answer the following question: is the degree of weathering of modern river-borne particles consistent with the estimated river dissolved loads derived from silicate weathering?In agreement with the well known mobility of elements during weathering of continental rocks, we confirm that river sediments are systematically depleted in Na, K, Ba with respect to the Upper Continental Crust. For each of these mobile elements, a systematics of weathering indexes of river-borne solids is attempted. A global consistency is found between all these indexes. Important variations in weathering intensities exist. A clear dependence of weathering intensities with climate is observed for the rivers draining mostly lowlands. However, no global correlation exists between weathering intensities and climatic or relief parameters because the trend observed for lowlands is obscured by rivers draining orogenic zones. An inverse correlation between weathering intensities and suspended sediment concentrations is observed showing that the regions having the highest rates of physical denudation produce the least weathered sediments. Finally, chemical and physical weathering are compared through the use of a simple steady state model. We show that the weathering intensities of large river suspended sediments can only be reconciled with the (silicate-derived) dissolved load of rivers, by admitting that most of the continental rocks submitted to weathering in large river basins have already suffered previous weathering cycles. A simple graphical method is proposed for calculating the proportion of sedimentary recycling in large river basins. Finally, even if orogenic zones produce weakly weathered sediments, we emphasize the fact that silicate chemical weathering rates (and hence CO₂ consumption rates by silicate weathering) are greatly enhanced in mountains simply because the sediment yields in orogenic drainage basins are higher. Hence, the parameters that control chemical weathering rates would be those that control physical denudation rates.     

Sub‐basaltic weathering,damsites, palaeomagnetism,and the age of lateritization

Investigations described from three Victorian damsites indicate that weathering of Ordovician bedrock is increased where it is overlain by basalt, even though the basalt is little weathered. Sub‐basaltic weathering may account for the fact that weathered rock beneath a Jurassic lava flow gives a weathering remagnetization age of Mid‐Cainozoic, consistent with palaeomagnetically determined ages of lateritization from elsewhere in Australia.     

Carbonate weathering rates in the Jura Mountains, France—The influence of vegetation

In order to better understand what controls carbonate weathering rates, we report results from the Jura Mountains (East France), an area exclusively composed of carbonate rocks. This region presents an altitude gradient increasing from 250 m to 1300 m. Ov…     

Variations of Sr–Nd–Hf isotopic systematics in basalt during intensive weathering

The Sr–Nd–Hf isotopic compositions of both saprolites and parent rocks of a profile of intensively weathered Neogene basalt in Hainan, South China are reported in this paper to investigate changes of isotopic systematics with high masses. The results indicate that all these isotopic systematics show significant changes in saprolites compared to those in corresponding parent rocks. The ⁸⁷Sr/⁸⁶Sr system was more seriously affected by weathering processes than other isotope systems, with ε_Sr drifts 30 to 70 away from those of the parent rocks. In the upper profile (> 2.2 m), the Sr isotopes of the saprolites show an upward increasing trend with ε_Sr changing from ~ 50 at 2.2 m to ~ 70 at 0.5 m, accompanying a upward increasing of Sr concentrations, from ~ 10 μg/g to ~ 25 μg/g. As nearly all the Sr of the parent rock has been removed during intensive weathering in this profile, the upward increasing of Sr concentrations in the upper profile suggests import of extraneous Sr. Rainwater in this region, which enriches in Sr (up to 139 μg/L) from seawater, may be the important extraneous source. Thus, the Sr isotopes of the saprolites in the upper profile may be mainly influenced by import of extraneous materials, and the Sr isotopic characteristics may not be retained. In contrast, the ε_Nd and ε_Hf of the saprolites drift only 0–2.6 and 0–3.7 away from the parent rocks, respectively. The negative drifts of the ε_Nd and ε_Hf are coupled with Nd and Hf losses in the saprolites; i.e., larger proportions of Nd and Hf loss correspond to lower ε_Nd and ε_Hf. Compared with the relative high Nd and Hf concentrations of the saprolites, the contributions of extraneous Nd and Hf both from wet and dry deposits of aeolian input are negligible. Thus, the ε_Nd and ε_Hf changes in the profile are mainly resulted from consecutive removal of the Nd and Hf. Calculation indicates that the ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios in saprolites are all significantly lower than their initial values in the parent rock. Simply removing part of the Nd and Hf by incongruent decomposing some of the minerals may not account for this. Fractionation should be happen, which ¹⁴³Nd and ¹⁷⁶Hf may be preferentially removed from the profile relative to ¹⁴⁴Nd and ¹⁷⁷Hf during intensive chemical weathering, resulting in lower ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios in saprolites relative to the parent rock, even though details for this process is not known. A positive correlation is observed between the ε_Nd and ε_Hf of the saprolites. Interestingly, the saprolites with a net loss of Nd and Hf in the upper profile show good positive correlation, and the regression line parallels the terrestrial array. By contrast, saprolites with a net gain of Nd and Hf in the lower profile generally show higher ε_Hf values at a given ε_Nd value, and the regression line between these ε_Nd and ε_Hf appears to parallel the seawater array. This supports the hypothesis that the contribution of continental Hf from chemical weathering release is the key to the obliquity of the seawater array away from the terrestrial array of the global ε_Nd and ε_Hf correlation. Our results also indicate that caution is needed when using ε_Sr, ε_Nd, and ε_Hf to trace provenances for sediments and soils.     

Chemical weathering rates in the Alaknanda–Bhagirathi river basins in Himalayas,India

The Alaknanda and Bhagirathi rivers flow through the Higher and Lesser Himalayas and confluence at Devprayag, which represents the origin of the Ganga (or Ganges) river. In the present study, a vast number of temporal and spatial samples of the river waters were collected and analyzed for major cations and anions. In addition, more recent and time series water flow data have been obtained and based on these inputs, a more refined dissolved flux rates have been estimated. The Alaknanda and Bhagirathi rivers show significant variations in chemical compositions during different seasons. Carbonate rock weathering is responsible for more than 70% of the chemical compositions in the river waters. The chemical weathering rates show seasonal variations and are much higher during non-monsoon season. The dissolved flux of Alaknanda river is much higher (1.80 × 10⁶ tons yr^?1) as compared to the Bhagirathi river (0.34 × 10⁶ tons yr^?1). The chemical weathering rates in the basin vary between 85 and 155 tons km^?2 yr^?1, which is significantly higher compared to the global average of ～24 tons km^?2 yr^?1.     

Transformation of organic carbon and its potential impact on lead weathering in firing range soils

Lead （Pb） is normally considered as a trace element in soils and sediments for geochemical study. However, the concentration of Pb in firing range soils is generally so high that it should be considered as a major element during the evaluation of the soil geochemical properties. Soil organic matter （SOM） has been reported as one of the major factors to expedite the corrosion of metallic lead （Pb） in acidic and organic-rich soils. The main impacts of SOM on the fate and transport of Pb in firing range soils lie in the following two aspects; （1） the complexation of organic matter with Pb, which has received lots of attention, and; （2） changes in soil redox potential due to the transformation of SOM and its subsequent impact on Pb speciation, which has rarely been investigated. Soils from 6 different firing ranges are selected for this study. These samples have been stored under a closed condition for more than 3 years. The soil moisture contents were well-retained, as all the samples were kept in closed plastic buckets. The analytical data showed that the summation of the soil total organic carbon content （TOC） and inorganic carbon contents （TIC） were consistent with soil total carbon contents （TC） measured in previous years, although the TOC and TIC contents have changed respectively after years of storage. In general, it is observed that the soil TOC decreased against an increase of TIC. The mass balance on such a transformation suggested a major conversion of organic carbon （Corg） to inorganic carbon （CO3^2-） in the stored soils.     

Degree of weathering in the identification of laterite materials for engineering purposes — a review

Laterite materials include a large succession of reddish, tropically weathered, decomposition products starting with fresh rock and ending with sesquioxide-rich pedogenic rock (cuirasse). Identification of the various grades of such materials for engineering purposes is a problem which repeatedly faces the engineer engaged in the design and construction of structures on or with laterite materials.The author has attempted to assemble available information useful for the identification and evaluation of all grades of laterite materials for engineering purposes. In the first part he summarises information on the significant genetic characteristics which appear to underlie the deviation of engineering behaviour of laterite materials from the expected properties based upon conventional soil mechanics as developed for temperate-zone soils.The second part summarises available information on the identification of significant geotechnical properties of various grades and genetic groups of laterite materials for engineering purposes.The engineering behaviour of laterite materials forms the substance of the third part of the paper.A literature study has revealed that the geotechnical characteristics and engineering behaviour of laterite materials depend mainly on the genesis and degree of weathering (i.e., decomposition, laterisation, dessication and/or hardening). Other significant genetic characteristics include morphological characteristics as well as the type and content of secondary minerals.Progress in the field of identification and evaluation of laterite materials for construction appears to depend on the simultaneous consideration of all the major factors which affect the behaviour of rocks and their derived soils (i.e., rock type, weathering condition, degree of weathering, type of derived materials as well as their chemical and mineralogical composition).Such an approach is desirable to increase the accuracy of predictions and assessments of the engineering behaviour of most laterite materials.     

Micro area transportation of residues： A style forming the red weathering crusts of carbonate rocks

The red weathering crusts of carbonate rocks are usually characteristic of thick-bedded intensely weathered profiles and strongly undulate basement rock surface （i.e., alternative distribution of solution grooves and stone teeth）. In this work a typical red weathering crust of carbonate rock, whose parent rock is of homogeneity in composition, is selected in karst terrace, middle Guizhou Province, China. Via field geology, geochemistry, mineralogy, grain size analyses, and in comparison with two neighbouring limestone soils as well, the authors have discussed forming process of the red weathering crusts of carbonate rocks, and demonstrated that micro area transportation of acid-insoluble residues of carbonate rocks is a style of forming the red weathering crusts of carbonate rocks. The weathering front is a main place of geochemical reaction, and at this limited-thick interface, with quick dissolution of carbonates, acid-insoluble residues begin to obviously decompose.     

Physical weathering of building stones induced by freeze–thaw action: a laboratory long-term study

Damages to natural building stones induced by the action of frost are considered to be of great importance. Commonly, the frost resistance of building stones is checked by standardised freeze–thaw tests before using. Corresponding tests normally involve 30–50 freeze–thaw action cycles. In order to verify the significance of such measurements, we performed long-term tests on four selected rocks over 1,400 freeze–thaw action cycles. Additionally, numerous petrophysical parameters were analysed to compare the behaviour of rocks in the weathering tests according to the current explanatory models of stress formation by growing ice crystals in the pore space. The long-term tests yield more information about the real frost sensibility of the rocks. A clear deterioration cannot be determined in most cases until 50 weathering cycles have been completed. In the freeze–thaw tests, the samples are also stressed by changing temperature and moisture, indicating that different decay mechanisms can interfere with each other. Thus, thermohygric and moisture expansion are important damage processes.     

Early weathering of palladium gold under lateritic conditions,Maquiné Mine,Minas Gerais,Brazil

Since the 80's, studies have shown that Au is mobile in supergene lateritic surficial conditions. They are based either on petrological, thermodynamic studies, or experimental works. In contrast, few studies have been done on the mobility of the Pt group elements (PGE). Moreover, at the present time, no study has addressed the differential mobility of Au, Ag and Pd from natural alloys in the supergene environment. The aim of this study is to understand the supergene behavior, in lateritic conditions, of Au–Ag–Pd alloys of the Au ore locally called Jacutinga at the Maquiné Mine, Iron Quadrangle, Minas Gerais state, Brazil.The field work shows that the host rock is a “Lake Superior type” banded iron formation (BIF) and that the Au mineralization originates from sulfide-barren hydrothermal processes. Primary Ag–Pd-bearing Au has developed as xenomorphous particles between hematite and quartz grains. The petrological study indicates that the most weathered primary Au particles with rounded shapes and pitted surfaces were found, under the duricrust, within the upper friable saprolite. This layer, however is not the most weathered part of the lateritic mantle, but it is where the quartz dissolution resulting porosity is the most developed. The distribution of Au contents in the weathered rocks are controlled by the initial hydrothermal primary pattern. No physical dispersion has been found. Most of the particles are residual and very weakly weathered. This characterizes early stages of Au particle weathering in agreement with the relatively low weathering gradient of the host itabiritic formations that leads essentially to the development of isostructural saprolite lateritic mantle. Limited dissolution of primary Au particles issued from the friable saprolite induces Pd–Ag depleted rims compared to primary Au particle Pd–Ag contents.In addition, limited very short distance in situ dissolution/reprecipitation processes have been found at depth within the primary mineralization, as illustrated by tiny supergene, almost pure, Au particles. The supergene mobility order Pd>Ag>Au as reflecting early weathering stages of Au–Ag–Pd alloys under lateritic conditions is proposed.