Chemical Weathering Processes Research Articles

Spatial characterization of chemical weathering in a proglacial river system, southcentral Alaska

Major constituent concentrations (Na+, K+, Ca2+, Mg2+, HCO3−, SO42−, Cl−, NO3−) were determined for a 121 km reach of the proglacial Matanuska River, associated tributaries, and five additional glacial rivers on the Kenai Peninsula of southcentral Alaska during peak flow conditions in July 2019. Mass balance and mixing models were utilized to gain insight to chemical weathering processes and carbon feedback implications along the sampling transect. Solute compositions of the Matanuska River varied spatially, shifting from carbonate dominance near the terminus of the Matanuska Glacier to a carbonate-silicate weathering signature in the lower reaches of the river. Geochemical modeling suggests that carbonate dissolution closest to the Matanuska Glacier terminus is driven by sulfuric acid produced from the oxidation of sulfide minerals in the subglacial and proglacial system that acts as a source of CO2. This contrasts with the lower reaches of the Matanuska River, its tributaries, and other Kenai glacial rivers, where mineral dissolution is dominated by carbonic acid and weathering acts as a sink of atmospheric CO2. The spatial variations observed in stream and river compositions are predominantly attributed to the local lithological variations across the catchment, with subglacial weathering signatures observed in the meltwater solute compositions closest to the Matanuska Glacier terminus. This study highlights the importance of spatial sampling in proglacial systems, as proglacial water chemical signatures and carbon feedback implications can shift significantly away from the glacial terminus.

High-temporal-resolution of lithium isotopes in Yangtze River headwater: Hydrological control on weathering in high-relief catchments

How climate change regulates silicate weathering in tectonically active areas remains clear. To evaluate the roles of temperature and hydrology in continental-scale silicate weathering in high-relief catchments, we applied a high temporal resolution of lithium isotopes in the Yalong River, which drains the high-relief borders of the eastern Tibetan Plateau. The dissolved δ7Li values range from +12.2‰ to +13.7‰ in the non-monsoon season and are higher and significantly vary from +13.5‰ to +19.4‰ in the monsoon season. The negative correlation between dissolved δ7Li and the Li/Na ratio is attributed to the formation of various proportions of δ7Li-low secondary minerals during weathering. From non-monsoon to monsoon season, the weathering intensity decreases with increasing secondary minerals formation and the weathering transforms from a supply limited to a kinetically limited weathering regime, indicated by a negative correlation between dissolved δ7Li value and SWR/D ratio (SWR = silicate weathering rate and D = total denudation rate). No correlations between temperature and dissolved δ7Li values were observed, and SWR suggested that temperature is not the direct control factor of silicate weathering in high-relief areas. The dissolved δ7Li values display positive correlations with discharge, physical erosion rates (PERs), and SWR. This positive correlations was attributed to an increase in the PER which caused the formation of more secondary minerals with increasing discharge. These results indicate the rapid temporal variability of riverine Li isotopes and chemical weathering process in response to changes in hydrology rather than temperature. Combined with the compiled PER, SWR, and Li isotopes at various altitudes, we further suggest that weathering in high-altitude catchments is more sensitive to hydrological changes than weathering in low-altitude catchments. These results highlight the key role of the hydrologic cycle (runoff and discharge) and the geomorphic regime in controlling global silicate weathering.

Chemical weathering processes impacted by pyrite oxidation in the upper Indus River basin, Western Himalaya

Chemical weathering processes impacted by pyrite oxidation in the upper Indus River basin, Western Himalaya

Application of fuzzy C-means clustering and fuzzy EDAS to assess groundwater irrigation suitability and prioritization for agricultural development in a complex hydrogeological basin.

The current research is focused on detecting a river basin suitable for agriculture and priority for management using a new clustering tool of groundwater quality with fuzzy logic technique in R and Geographical Information System. A new fuzzy clustering-soft computing technique has been executed to determine the different hydrochemical zones considering 13 essential parameters such as electrical conductivity, hardness, chloride, sodium adsorption ratio, residual sodium carbonate, soluble sodium percent, magnesium hazard, permeability index, potential salinity, residual sodium bicarbonate, Kelly's ratio, synthetic harmful coefficient, and exchangeable sodium percentage. The derived fuzzy C-mean clustering (FCM) outperformed other available hard computing techniques like hierarchical clustering, K-means clustering, and agglomerative clustering. It divided the sampling sites into 2 clustering groups (FCM I and FCM II) which has been validated using fuzzy silhouette index (0.85), the partition coefficient (0.76), the partial entropy (0.68), and the modified partition coefficient (0.52). The hydrogeochemical analysis confirmed that the rock-water interaction, chemical weathering, and ion exchange process are predominant in the aquifer system of the study area. According to the correlation plots, the studied groundwater samples largely evolved from [Formula: see text], mixed [Formula: see text] types, and [Formula: see text] types. The spatial distribution map and the hydrochemical analysis also gives a clear depiction of the fluoride (> 1.0mg/l) and high iron (> 0.3mg/l) contamination in groundwater quality, making it unsuitable for both drinking and irrigation. A fuzzy EDAS priority map has been prepared based on all the irrigation suitability parameters which concludes that the groundwater at the upstream and downstream section of the basin requires the most attention. Based on the highest priority for management, five zones have been delineated: very high (5.98%), high (22.31%), medium (16.39%), low (32.30%), and very low (23.02). The findings of this study will be beneficial to planners and policymakers as they can develop schemes to solve similar problems across the country.

Intensity of Chemical Weathering in Late Precambrian: New Data on the Riphean Stratotype (Southern Ural)

The values of Ni/Co, La/Sc, and Th/Cr (indicators of the composition of rock complexes eroded in paleowatersheds) and the values of CIA, Rb/Al, K/Al and K/Rb (indicators of the intensity of chemical weathering on land), typical for fine-grained clastic rocks of the Riphean of the Southern Urals were compared to the data given in the publication “Long-term evolution of terrestrial weathering and its link to Earth’s oxygenation” by Bayon G., Bindeman I.N., Trinquier A., Retallack G.J., and Bekker A. (Earth Planet. Sci. Lett., 2022, v. 584. 117490). It was shown that the average Rb/Al ratio (0.0014 ± 0.0006, n = 66) typical of the Riphean clayey rocks of the Southern Urals, together with other data, suggests that for more than 1.10 Ga (from ~1.75 to ~0.60 Ga) the intensity of chemical weathering in paleowatersheds was relatively low. This is in good agreement with the data obtained by Bayon et al. The material presented in the article allows us to consider that the composite “world” curves of K/Al, Rb/Al, and K/Rb by Bayon et al. and the same curves for clayey rocks of the Riphean of the Southern Urals are sufficiently comparable. Thus, data on the lithogeochemical characteristics of fine-grained clastic/clayey rocks of the Riphean stratotype significantly expand our understanding of the nature of chemical weathering processes on land in the interval 1.75–0.60 Ga ago.

Silica-based consolidants: Enhancement of chemical-physical properties of Vicenza stone in heritage buildings

Natural stones interact with the environment and undergo several chemical and physical weathering processes that cause the progressive loss of cohesion and degradation/disintegration of the material in the longer term. So, in order to retain the integrity of the historical patrimony and its undeniable impact on society, innovative materials and methodologies have been used in stone conservation field in recent years, also using a more environmentally-friendly approach. In this study, the interaction of three inorganic silica-based consolidant compounds with a valuable artistic calcareous rock, Vicenza stone, is discussed. This specific lithotype results to be still scarcely investigated, despite its high historical relevance and intrinsic features, especially porosity, which make it extensively susceptible to degradation. In particular, a colloidal suspension of silica made by particles with an average diameter of 10–20 nm, and two alkaline silicate solutions (lithium silicate and sodium silicate) are evaluated. An extensive characterization of both treated and untreated Vicenza stone samples was achieved by means of chemical–physical and morphological investigations (water absorption, open porosity, capillary absorption, ultrasonic pulse velocity and static contact angle measurements, Hg intrusion porosimetry, color evaluation, FTIR spectroscopy and SEM analyses). FTIR analyses revealed that all the consolidants exhibited a moderate penetration within the substrate, anyway a significant surface interaction with the stone, especially in the case of alkaline silicates, is proved by the results of SEM analyses, capillary absorption, static contact angle and color change. The improvement of the surface properties after treatments represents an essential and preliminary assumption to enhance the durability performance of Vicenza stone.

Secondary minerals of Fe, Pb, Cu in sulfide-containing tailings: sequence of formation, electrochemical reactions and physico-chemical model (Talmovskie Sands, Salaire, Russia)

The results of studies of the composition and form of secondary minerals Fe, Pb, Cu, formed under contrasting physico-chemical conditions of the accumulated tailings from the enrichment of barite-polymetallic ores of the Salair ore field, are presented. The complex mineral composition of ores (pyrite, chalcopyrite, sphalerite, galena, fahlore) and long-term processes of chemical weathering of the substance contributed to the formation of monomineral and zonal secondary rims and fillings of the intergranular space, which were identified using modern research methods. Plumbojarosite, anglesite, cerussite, and iron hydroxides predominate among them; pyromorphite, ginsdalite, and covellite occur less frequently. The method of thermodynamic modeling was used to solve the inverse problem - the restoration of the composition of solutions that led to a change in the associations of secondary minerals. The nature of the ongoing processes can be characterized not only by chemical interactions, but also by electrochemical reactions in the systems under consideration, where various mineral components act as galvanic pairs, which, in combination with the physico-chemical parameters of the medium (pH, Eh, ionic composition of solutions ), leads to stepwise or incomplete oxidation of the original minerals with subsequent selective deposition of secondary compounds.

Mineral surface area in deep weathering profiles reveals the interrelationship of iron oxidation and silicate weathering

Abstract. Mineral specific surface area (SSA) increases as primary minerals weather and restructure into secondary phyllosilicate, oxide, and oxyhydroxide minerals. SSA is a measurable property that captures cumulative effects of many physical and chemical weathering processes in a single measurement and has meaningful implications for many soil processes, including water-holding capacity and nutrient availability. Here we report our measurements of SSA and mineralogy of two 21 m deep SSA profiles at two landscape positions, in which the emergence of a very small mass percent (<0.1 %) of secondary oxide generated 36 %–81 % of the total SSA in both drill cores. The SSA transition occurred near 3 m at both locations and did not coincide with the boundary of soil to weathered rock. The 3 m boundary in each weathering profile coincides with the depth extent of secondary iron oxide minerals and secondary phyllosilicates. Although elemental depletions in both profiles extend to 7 and 10 m depth, the mineralogical changes did not result in SSA increase until 3 m depth. The emergence of secondary oxide minerals at 3 m suggests that this boundary may be the depth extent of oxidation weathering reactions. Our results suggest that oxidation weathering reactions may be the primary limitation in the coevolution of both secondary silicate and secondary oxide minerals. We value element depletion profiles to understand weathering, but our finding of nested weathering fronts driven by different chemical processes (e.g., oxidation to 3 m and acid dissolution to 10 m) warrants the recognition that element depletion profiles are not able to identify the full set of processes that occur in weathering profiles.

Behaviour of Sr, Ca, and Mg isotopes under variable hydrological conditions in high-relief large river systems

To assess how chemical weathering processes in large high-relief river systems respond to climatic variability, we studied seasonal changes in radiogenic strontium (87Sr/86Sr) and stable calcium (δ44/40Ca) and magnesium (δ26Mg) isotopes in the Jinsha and Yalong rivers, which drain the southeastern Tibetan Plateau. During the low-runoff season, with discharge (Q) < 2000 m3/s, the river waters reflect the Sr, Ca, and Mg isotope signatures of recharge meltwaters, with additional isotope fractionation signals for Ca and Mg related to secondary mineral precipitation, which might imply that meltwater flushes soil solutions from the soil. During medium-runoff intervals (2000 m3/s < Q < 4000 m3/s), the Sr, Ca, and Mg isotope signatures in the Jinsha river waters are similar to those of the headwaters, which are influenced by evaporite dissolution, while the Yalong is affected by greater carbonate weathering relative to silicate weathering. In both rivers, bedrock dissolution governs the chemical composition of the river waters. During the high-runoff season (Q > 4000 m3/s), storms generate rapid overland flow, which transfers large volumes of soil into the rivers, such that soil weathering plays an important role in regulating riverine chemical compositions. At these times, the riverine Ca and Sr isotope evolution is influenced by secondary mineral dissolution and sediment–water cation exchange. Overall, this study highlights the potential of combining multiple isotope systems (Sr, Ca, Mg) to trace the dynamics of water–rock interaction under variable hydrological conditions.

High precision measurements of lithium isotopic composition at sub-nanogram by MC-ICP-MS with membrane desolvation.

The geochemistry of Li and Li isotopes is a promising tracer of chemical weathering processes for both modern and ancient times. Therefore, accurate and precise determination of the isotopic composition of Li is required for a large variety of complex geological samples with different Li concentrations and matrix/Li ratios. Especially, geochemical studies of precious geological samples with ultra-low lithium content and high matrix. In this study, the accuracy and the precision corresponding to Li isotopic measurements of low-level samples using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) with membrane desolvation introduction system was evaluated. The method of MC-ICP-MS with membrane desolvation and a high-sensitivity X-skimmer cone, together with a simple one-step column separation enabled the high-precision isotopic analysis of Li quantities as small as 2 ng. The long-term instrumental external reproducibility of δ7Li values for the L-SVEC and SPEX-Li were 0.0 ± 0.1‰ (n = 20) and 12.1 ± 0.4‰ (n = 20), respectively. Based on the measurements on a series of international reference materials over the last two years. The measured δ7Li values for the standards with a variety of matrices, including BHVO-2, AGV-2 and seawater (NASS-6). The δ7Li values of BHVO-2 (4.58 ± 0.35‰), AGV-2 (6.85 ± 0.40‰) and NASS-6 (30.88 ± 0.20‰) are in agreement with the published data within the uncertainty. We also present analytical results for South China Sea surface seawater water, meteorite, limestones and rain water. These results demonstrate the validity of the method for obtaining highly precise and accurate outcomes.

Mineral Weathering and Element Migration in Granite Weathering Pits (Gnammas): A Case Study in Eastern China

This paper investigates weathering pits at three granite sites located on mountain tops, in a former river bed, and on the coastline of an island, respectively, from north to south in eastern China and aims to characterize weathering pit formation in the above settings in terms of mineral weathering and elemental transport. In these settings, the main elements, and mineral groups in the debris in the weathering pits and the fragments of the rock surface directly adjacent to the pits were analyzed. The chemical index of alteration (CIA), the quartz/feldspar (Q/F) ratio and the Na/K (Na2O/K2O) ratio were applied to identify the chemical origin of the weathering pits and assess the difference in the chemical weathering processes of the weathering pits in the different settings; the mass transfer coefficient was used to measure the characteristics of element migration during weathering pit formation at the three sites. The result of CIA, Q/F, and Na/K analysis shows that debris in a weathering pit suffered from higher chemical weathering intensity than nearby rock surfaces, indicating that the weathering pits of the study sites originated from chemical weathering. However, the differences in the CIA values of weathering pits in different areas are only the result of different chemical weathering durations and cannot be used to identify the climate types of the areas. The calculation of element mass transfer indicates that only Na and K are continuously leached during the formation process of weathering pits regardless of whether in valleys, mountains or on the coast. Other elements may or may not be the external source for the formation of weathering pits resulting in different natural tendencies for element mass transfer in weathering pits. Seawater can also be a factor contributing to the different patterns of element migration in weathering pits in coastal and inland areas. In addition, the environment of river valleys is more conducive to weathering pit formation than mountain tops.

Spatiotemporal variation of Li isotopes in the Yarlung Tsangpo River basin (upper reaches of the Brahmaputra River): Source and process

The increase in marine Li isotope composition (δ7Li, ∼9‰) since the Cenozoic is possibly related to continental chemical weathering associated with tectonic uplift (e.g., the Tibetan Plateau [TP]). However, the reasons for the spatiotemporal variations of δ7Li in the rivers flowing through the TP are still under debate, hindering the understanding of the changes in seawater δ7Li. Herein, various geological samples, i.e. river waters, river suspended particulate materials, river sediments, hot springs and silicate rocks, from the Yarlung Tsangpo River basin, the largest river system in the TP, have been analyzed to understand the source and isotopic fractionation of dissolved Li in rivers flowing through high-altitude and tectonic regions. The Yarlung Tsangpo River has an unusually high dissolved Li concentration ([Li]dis) (0.5–313 μg/L, mean 58.4 μg/L, n=93) and low δ7Lidis values (+1.0‰–+14.7‰, mean +6.4‰, n=92) compared with most river waters. These results can be explained by the input of geothermal water with extremely high [Li]dis (5.5–34.4 mg/L, mean 15.7 mg/L, n=9) and low δ7Lidis (−1.7‰–+3.1‰, mean +1.0‰). Silicate weathering is probably responsible for elevated δ7Lidis in river water compared to geothermal water, but the binary mixture model results of silicate weathering and geothermal water do not support this speculation. Ongoing Li isotope fractionation between dissolved loads and secondary minerals has been suggested to be the main reason for the increased δ7Lidis in river water. Field study and adsorption experiment results support the view of continuous Li isotope fractionation in rivers. Physical erosion and chemical weathering processes that provide fresh secondary minerals to rivers as well as dissolved Li from geothermal water transported in rivers over long residence times promote Li isotope fractionation. Hence, with the emergence of a continental geothermal system, the tectonic activity may directly or indirectly induce the simultaneous increase in dissolved Li flux and δ7Lidis in rivers that eventually flow into the ocean. This can partially explain the increase in seawater δ7Li in the Late Cenozoic.

The effect of morphoclimatic processes on archaeological sites in the Warka urban area

The research dealt with analyzing the impact of morphoclimatic processes on the urban area of Al -Warkaa city, which represents the most important archaeological cities in southern Iraq region. The study aims to analyze the most important characteristics of natural geography and its role in the activity or weakness of morphoclimatic processes affecting archaeological sites, and the studies showed that the morphoclimatic processes on the Al-Warkaa historical region are most effective on the historical sites, which represented by chemical, biological and mechanical weathering processes and their different effects, Mechanical weathering is considered one of the most active types of weathering in the region, in addition to wind and rain processes erosion and its prominent impact on archaeological monuments. Through field work and the use of mathematical equations, the activity of morphoclimatic processes was measured. Represented by the processes of rain and wind erosion and the rates of annual climatic susceptibility to wind erosion, the evidence of erosion in the Samawah and Nasiriyah climatic stations was very high. It became clear that the processes of rain erosion have a clear and significant impact on the archaeological sites represented by collision and plate erosion and the erosion of water bodies and grooves, and the sedimentary morphoclimatic processes represented by rain and wind precipitation have a clear and significant effect represented by the burial of many archaeological sites in the historical region, and various environmental disturbances contribute, such as fluctuation and change Climatic and geological disturbances to a very large extent in the instability of those morpho-climatic and its effects on the study area.

Petrography and Lithofacies of the Siwalik Group in the Core of Hazara-Kashmir Syntaxis: Implications for Middle Stage Himalayan Orogeny and Paleoclimatic Conditions

The present field and petrographic investigations of the Tortonian to Gelasian Siwalik Group in the core of the Hazara-Kashmir Syntaxis have been carried out to comprehend the middle stage Himalayan orogeny that resulted from the collision of Indian and Asian plates. The Chinji, Nagri, Dhok Pathan, and Soan Formations of the Siwalik Group were deposited by river meandering flood plains, braided rivers, and alluvial fan systems, respectively. The Siwalik Group is classified into seven major facies and many minor facies based on sedimentological properties. According to the petrographic analysis, the Siwalik Group sandstone is classified as litharenite and feldspathic litharenite petrofacies. The sandstone of the Siwalik Group is texturally mature, but compositionally it is immature. The data shown on the tectonic discrimination diagrams point to a recycled orogen provenance field for the Siwalik sandstone. In addition to quartz and feldspar, the sandstone includes clasts of volcanic, metamorphic, and sedimentary rock types. The igneous and metamorphic rock clasts were derived from the Lesser and Higher Himalayas. The sedimentary lithic fragments, on the other hand, are derived from both the earlier molasse and pre-molasse rocks. The presence of lithic fragments of the earlier molasse sandstone in the Siwalik sandstone indicates that the Siwalik Group sandstones were deposited during the Middle Stage of the Himalayan orogeny. The paleoclimatic conditions were semi-arid to semi-humid during the Siwalik Group’s deposition. The presence of clay minerals in the shale reveals the intense chemical weathering processes that occurred during their deposition on the flood plains of the river meandering system.

Influence of Weathering on Pore Size Distribution of Soft Rocks

Soft rocks can weather and lose their structure within a short time due to drying out and rewetting. Thus they are very sensitive to weathering. Since these rocks are often found in the shallow subsurface, they are of great practical relevance in the foundation of constructions. The rock properties change during the weathering process. Particularly relevant is the softening of the material and the decrease of its mechanical properties, which are determined typically using mechanical laboratory or field tests. The objective of this study is to examine in more detail how the microstructure of rocks changes over the course of weathering, which results in a decrease in mechanical properties. Sulfate rocks of the Grabfeld-Formation in Stuttgart, Germany were investigated. Using XRD analyses it was revealed that initially a chemical weathering processes occurs whereby the rocks leach out, followed by a physical weathering process. Investigations with a mercury porosimeter showed that during chemical weathering the intraaggregate pore content greatly increases, whereas the proportion of interaggregate pores only slightly increases.

Chemical weathering processes generating the raw material used in porcelain stoneware industry in the São Paulo State, Brazil

The Paraná Sedimentary Basin (PSB) is an intracratonic basin located in the South America and some litostratigraphic units can be exploitable for different segments of non-metallic's industry. Brazil is one of the main producers in ceramic tiles market, specifically in the Santa Gertrudes Ceramic Pole (SGCP), São Paulo State. The Tatuí Formation is an Eopermian post-glacial marine unit from the PSB composed essentially of massive or laminated siltstones and pelites, interbedded with limestones and sandstones. These lithotypes are exploited and used as raw material to enhance the strength to porcelain stoneware industry in the São Paulo State, a product with greater added value in relation to the common “dry route” ceramic tiles produced in the SGCP. In order to understand the influence of the chemical weathering processes to generate the raw material used in the porcelain stoneware, we studied the mineralogy and geochemical characteristics of the weathered samples at two mines located in the city of Tatuí and compared the results with fresh rock collected nearby. The fresh rocks are composed of quartz, albite, microcline and muscovite. The weathered samples from the top of the weathering profiles are composed of kaolinite, illite, quartz, iron hydroxides and rare microcline, while the bottom horizons are richer in montmorillonite and feldspar. From the bottom to top layers, geochemical results indicate progressively enrichment of Al2O3, Fe2O3 and TiO2 and depletion of the other major oxides, trace elements (except for Ni, Th and U) and REE. The Chemical Index of Alteration (CIA) and Chemical Index of Weathering (CIW) values increased from the bottom to the top in the isalteritic saprolite. The weathering trends indicated the partial hydrolysis as the main chemical weathering process at both study areas. The chemical weathering reactions generated an added value ceramic product higher than the traditional clay materials exploited at SGCP due to the presence of montmorillonite and the low content of Fe2O3.

Luminescence Sensitivity of Quartz From Rocks Under in situ Chemical Weathering Conditions

In provenance investigation of Quaternary sediments, quartz luminescence sensitivity (QLS) has been employed as a technique. The weathering and influence on the earliest signs of luminescence sensitivity of sedimentary quartz require more consideration. In this study, the QLS properties of eight samples of Permian biotite monzogranite from a borehole in the southern tropics of China are investigated. The degree of chemical weathering does not correspond with the QLS values of samples that have undergone varying degrees ofin situweathering. We hypothesize that the variation of the irradiation field during thein situchemical weathering process may affect the QLS, but not considerably within 200 ka. In addition, we discovered that quartz from the same source undergoes varying degrees ofin situchemical weathering, but has a constant response to irradiation-bleaching cycles (IBCs). The presence of variances in the thermal activation curves (TACs) of the same rock body indicates that further research is required to apply TAC as a provenance indicator.

The Effect of Size Distribution on the Geochemistry and Mineralogy of Tropical River Sediments and Its Implications regarding Chemical Weathering and Fractionation of Alkali Elements

Abstract The mineralogical and geochemical compositions of the sediments deposited by rivers have been used extensively to evaluate past chemical weathering and the physical erosion history of drainage basins at different time scales. However, the role of sediment sorting in the mineralogical and geochemical compositions of river sediment needs to be better constrained because it could significantly modify proxies used to evaluate weathering intensity in the past. In this study, major and trace element concentrations and mineralogical compositions were determined on seven different grain-size fractions of riverbed samples taken from the Pahang and Kelantan Rivers in the Malay Peninsula. The main aims are to assess chemical weathering and the fractionation of alkali elements during sediment transportation under tropical climate conditions. Fine sediments (from &amp;lt;2 μm to 4-8 μm) were characterized by higher concentrations of Al2O3, P2O5, V, Sc, and Cs than coarse sediments (from 8-16 μm to 32-63 μm). The presence of heavy minerals, such as zircon and monazite, was found to have critical effects on the concentration of several elements (e.g., Zr and Th), leading to extremely high elemental concentrations in the coarse-sediment fractions. Variations in elemental concentrations with particle sizes are mainly attributed to changes in the mineralogical composition during mineral sorting and weathering. In both rivers, the intensity of chemical weathering increased with decreasing grain sizes. However, the relationship between grain size and chemical weathering trends in the Pahang and Kelantan River basins was different in each case due to differences in plagioclase and K-feldspar concentrations, especially in the coarse-sediment fractions (16-32 μm and 32-63 μm). Kaolinite percentage, illite chemistry index, and illite crystallinity were directly proportional to the chemical index of alteration (CIA), suggesting that these mineralogical parameters are suitable proxies for determining chemical weathering intensity in sediments that have undergone significant changes due to mineral sorting during transportation. The comparison of Rb/K and Cs/K ratios and the CIA suggested that K and Rb mainly derive from primary minerals during the process of chemical weathering, whereas Cs is mainly present in fine weathered particles due to the reabsorption of Cs on clay minerals. Overall, our results highlight the relationship of grain size and mineral assemblages to different states of chemical weathering (and alkali fractionation) in tropical river basin sediments. These relationships need to be considered when using mineralogical and geochemical compositions to reconstruct weathering history in a “source to sink” approach.

Chemical weathering intensity and geochemical characteristics of Cretaceous terrigenous clastic rock‐purple soil profiles in the Pushi area, Xichang

The Pushi area of South‐west China features a large area of purple soil that formed by the weathering of Cretaceous terrigenous clastic rocks. In this study, four vertical profiles of the purple soil (P1, P2, P3, and P4) derived from different pedogenic parent rocks were investigated. Specifically, the chemical weathering intensity and element migration, enrichment, and differentiation characteristics were analysed and the influencing factors of chemical weathering were explored. According to the discrimination map of major and trace elements and content characteristics of rare earth elements, the soil profiles are controlled by complex chemical weathering processes. Various geochemical parameters and indicators exhibited good correlation, and the chemical weathering intensity of the profiles followed the order P1 (mudstone) &gt; P3 (silty mudstone) &gt; P4 (argillaceous siltstone) &gt; P2 (sandstone). The overall chemical weathering intensity of argillaceous rocks was slightly higher than that of sandstone, and the four profiles generally reflected medium to strong chemical weathering. The migration, enrichment, and differentiation characteristics of geochemical elements were found to be mainly affected by different chemical weathering intensities and stages, followed by leaching deposition. The chemical weathering characteristics varied for the four profiles with different pedogenic parent rocks, which could be mainly attributed to internal factors such as the lithology of the pedogenic parent rock and external topographic factors such as altitude.

Acidic condition accelerates cation release from purple rock in Southwestern China

In spite of the fact that rock weathering performs an essential task in the evolution of the Earth’s surface, the quantitative assessment between pH and rates of chemical weathering remain unclear. This study aims to characterize the chemical weathering rate of purple rocks and then develops a model to calculate the release rates of cations (K+, Na+, Ca2+ and Mg2+) under various pH conditions. Two types of purple rock were sampled from the Shaximiao Group (J2s) and Penglaizhen Group (J3p), and a series of laboratory experiments were performed by soaking the purple rocks in solutions with pHs from 2.5 to 7.0, over 24 treatment cycles. The results showed that the release rates of cations apparently increased as the pH decreased. The release of Ca2+ was the dominant process of chemical weathering in J3p under various pH treatments, while K+ and Na+ were remarkably high in J2s (with the exception of the pH 2.5 treatment). Quantitative analysis revealed that the rate of cation release was significantly related to the H+ concentration (p < 0.001) and the air temperature (p < 0.001). The relationship between cation release and acidity was found to be an exponential function. Our results suggested that solution acidity serves as an important driving force for cation release rates from purple rocks and that environmental acidification would enhance rock weathering.