Effects Of Chemical Weathering Research Articles

Erosion modulates the effect of chemical weathering on atmospheric pCO2, Qinghai-Tibet Plateau

Chemical weathering plays an important role in modulating Earth's climate. However, chemical weathering is usually influenced by multiple factors simultaneously (e.g., climate, topography, lithology), and it is difficult to understand how these variables modulate the effect of chemical weathering on atmospheric pCO2. In this study, we collected and analyzed water chemistry and multiple isotopes (δ34SSO4, δ18OSO4 and δ18OH2O) in the Bailong River catchment to decipher how these variables influence chemical weathering and its effect on atmospheric pCO2. We used the Monte Carlo inversion model to partition the sources of major cations and SO2–4, and the results reveal that carbonate weathering is the dominant source contributing cations in river. Sulfide oxidation is the most important source for SO2–4 and a supply-limited process that is highly dependent on slope. And slope is main factor modulating the effect of chemical weathering on atmospheric pCO2. Steep slope increases sulfide oxidation rate by elevating erosion, resulting in chemical weathering acting as a CO2 source. In contrast, gentle slope could facilitate a thick regolith that contributes long-term silicate weathering and limits sulfide oxidation, resulting in chemical weathering acting as a CO2 sink. And it has been found that the influence of temperature on chemical weathering becomes progressively pronounced with increasing altitude. This study investigated sulfide weathering in eroding Qinghai-Tibet Plateau, highlighting the geomorphologically controlled CO2 effects of chemical weathering.

Weathering resistance of Linz–Donawitz (LD) slag as ballast material using freeze-thaw and sulfate soundness

As nonrenewable natural aggregates with acceptable geotechnical properties become scarce, costly, and entail negative environmental impacts, the study of alternatives remains a first-order challenge for sustainable railway design. This paper focuses on the physical and chemical weathering effects of the industrial byproduct Linz–Donawitz (LD) slag as a ballast material. For this purpose, 75 freeze–thaw (F-T) and 40 sulfate soundness (SS) cycles were carried out on the byproduct. We present a series of laboratory experiments involving particle characteristics, durability and strength for different F-T and SS cycles. To benchmark the performance of LD slag, we also performed our experiments on two natural aggregates: gneiss and basalt. Our main findings reveal that: (i) the shape of LD slag ballast and its particle size distribution are unnafected by the F-T and SS cycles, (ii) the basalt exhibits higher magnitudes of fouling after SS cycles, (iii) losses in Los Angeles abrasion and shock resistance were much more pronounced in SS tests for all ballast materials, (iv) LD slag is more resistant and less susceptible to the degrading effects of freezing and thawing, (v) point load tests indicate that the loss of resistance of basalt is small compared to that of gneiss, (vi) the byproduct showed a decrease in strength of 87% after 40 SS cycles, suggesting that chemical weathering exerts a dominant control on the performance of LD slag. The findings are relevant to elucidate the physical and chemical weathering effects of LD slag and to promote its sustainable use.

Effects of chemical weathering on the exceptional preservation of mineralized insects from the Crato Formation, Cretaceous of Brazil: implications for late diagenesis of fine-grained Lagerstätten deposits

AbstractMany studies have improved our understanding of the mode of preservation at the Crato fossil Lagerstätte. The high degree of preservation of the Crato mineralized insects is thought to be a consequence of the diffusion of ions through carcasses and envelopment by bacteria that, in turn, created microenvironmental conditions that led to mineralization, mainly pyritization. Pyritized insects have been oxidized by in situ weathering to more stable oxide/hydroxy minerals during Quaternary time. This transformation is essential to maintain the palaeontological information acquired during microbially induced pyritization in an oxidizing atmosphere. However, intense weathering can diminish or obscure the morphological fidelity, and little attention has been paid to the post-diagenetic processes experienced by these fossils. Here, we aim to determine the degree of alteration undergone by Crato pyritized insects using the following combination of analytical tools: scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared and Raman spectroscopy. Our results show that well-preserved insects are preferentially replaced by haematite and poorly preserved fossils are replaced by goethite. In addition, we recorded three types of post-diagenetic alteration: insects with iron-oxide overgrowths; insects associated with black coatings, sometimes with the formation of dendrites; and insects preserved as an impression, where only the outline of the body remains. All of these alterations have the potential to distort or tarnish palaeontological information. Here, we measured the effects of such telodiagenetic alterations at macro and micro scales. Therefore, this taphonomic approach has wide applicability wherever fine-grained deposits bearing mineralized insects are found.

Riverine molybdenum isotopic fractionation in small mountainous rivers of Taiwan: The effect of chemical weathering and lithology

The molybdenum (Mo) concentration and its isotopic composition from 25 major river catchments throughout Taiwan have been measured to better understand the geochemical behavior of Mo and the mechanism that controls Mo isotopic fractionation via weathering and erosion processes, in a tectonically active high-stand island. In addition, the Mo isotopic composition and its abundance in bedrock and riverine bedload sediments were also studied for source identification. The concentration of Mo in river water varies between 1.94 and 45.09 nM, and the δ98/95Mo ranges from −0.53‰ to +1.35‰, with an average of +0.70 ± 0.31‰ (1SD, n = 42) for the wet season. In the dry season, Mo concentration varies between 2.15 and 58.27 nM, and the δ98/95Mo ranges from −0.48‰ to +1.09‰, with an average of +0.74 ± 0.28‰ (1SD, n = 43). The seasonal variability of δ98/95Mo composition is very small in Taiwan river catchment.The dissolved riverine δ98/95Mo are heavier than those of the bedrock (∼ −0.72 to +1.03‰). These rivers flow through quite different lithologies and are thus difficult to identify a common cause responsible for the observed Mo isotopic fractionations. However, δ98/95Mo of riverine bedload sediments from three rivers show a negative trend with trace elements, e.g., Nb, Mn, and Ti, that are enriched in fine-grain residual Fe-Ti oxides, such as titanite, rutile, ilmenite, and niobite, important hosts for Mo. Consequently, the riverine bedload sediments may be the sink for light δ98/95Mo, which drives the riverine dissolved load towards a heavier δ98/95Mo isotopic signature through adsorption processes. In general, the rivers of Taiwan discharge a significant flux and a heavier mean δ98/95Mo (+0.75‰) to the oceans than that of the mean global rivers (∼ +0.55‰). While the results will help better constrain the global Mo cycle, short-term seasonal variations, e.g., precipitations, tend to have an insignificant effect on the Mo cycle in the oceans.

Mineralogy and geochemistry of modern Red River sediments (North Vietnam): Provenance and weathering implications

Abstract This study illustrates the clay mineralogy and sedimentary geochemistry of the Red River and its major tributaries and distributaries in northern Vietnam and shows how these methods can be used to unravel grain size, provenance, hydraulic-sorting, and chemical weathering effects. All sand samples are SiO2-rich and consequently depleted in most chemical elements (but Sn and Pb) relative to the upper continental crust (UCC). The order of element mobility indicated by αAlE values, which estimate the degree of depletion in mobile element E relative to the UCC standard, is Ca ≥ Na > Sr > Mg > Ba ≥ K ≥ Rb. In mud fractions, SiO2 decreases, and other elements consequently increase. The grain size-dependent intrasample chemical variability of fluvial sediments reflects the grain size distribution of detrital minerals, which is strictly controlled in turn by the settling-equivalence principle. The 87Sr/86Sr ratio in Red River sands varies widely between 0.716 and 0.748, and εNd ranges from −8.5 to −13.8. The negative εNd values and high 87Sr/86Sr ratios point at a significant contribution from Precambrian crystalline basement, directly or through recycling of Triassic siliciclastic strata. Clay-mineral assemblages, dominated by illite and smectite with subordinate kaolinite and minor chlorite, suggest largely physical erosion in the upper catchment and stronger weathering in the monsoon-drenched lower catchment. Extremely intense weathering is demonstrated by a Quaternary soil sample from the Red River valley in northernmost Vietnam, which is a pure quartzose sand yielding a tourmaline-dominated heavy-mineral suite and a kaolinite-dominated clay-mineral assemblage. In the humid landscapes of northern Vietnam, no detrital mineral, excepting quartz, muscovite, tourmaline, prismatic sillimanite, anatase, and zircon, can resist even shallow early pedogenesis.

Evolution of earth pressure coefficient of sand undergoing varying rate of dissolution

In fluid-saturated granular materials, the physicochemical interaction between pore-fluids and grain minerals alters packing conditions, which in turn leads to stress change, deformation and, in extreme cases, even collapse. Chemical weathering, either naturally occurring or induced by human activities, is among such processes. This paper presents an experimental study illustrating the major effects of chemical weathering on the deformation and stress state of granular materials, emphasising particulate systems entirely made by highly soluble carbonate grains. Laboratory experiments are conducted by subjecting granular assemblies to acidic environments under oedometric conditions. The reaction rate is controlled by regulating various testing parameters, such as acid concentration and pore fluid flow rate. Experiments revealed that the lateral earth pressure steadily reduces in some cases, while others exhibit non-monotonic evolution. From a macroscopic standpoint, the rate of the chemical reaction was critical to determine the emergence of either of these trends. Such findings are relevant for any particulate system in which the stress conditions are controlled by multi-physical processes proceeding at different rates, such as waste products within bioreactors, gouge materials within faults and natural deposits subjected to the injection/extraction of reactive fluids.

The thermodynamics of rare earth element liberation, mobilization and supergene enrichment during groundwater-regolith interaction

Rare earth elements (REEs) mobilize, fractionate, and are re-distributed during supergene processes, and thus provide a powerful tool with which to quantitatively reconstruct the effects of chemical weathering. Moreover, under certain conditions, the REEs can concentrate to levels in the regolith sufficient to form giant regolith-hosted REE deposits, that are now responsible for much of the World’s production of heavy REEs (HREEs). Understanding the supergene behavior of the REEs is an important first step towards applying the REEs as a geochemical tool and meeting the growing global demand of the REEs.Thermodynamic calculations predict that dissolution of the main REE minerals in REE-rich protoliths, namely synchysite-(Y), gadolinite-(Y), hingganite-(Y), yttrialite-(Y), allanite-(Ce), eudialyte, chevkinite-(Ce), britholite, euxenite and loparite-(Ce), should occur spontaneously during weathering. It therefore follows that a high abundance of these minerals in the protolith implies high mobility of the REEs during weathering and consequently a high potential for the discovery of economic REE resources. Dissolution of apatite is promoted by metamictization or structural distortion and could be also important at low pH. In contrast, some LREE-fluorocarbonate minerals, notably bastnäsite-(Ce) and parisite-(Ce), and monazite-(Ce) are likely thermodynamically stable in acidic environments. Thus, they would be preserved in the regolith. Zircon, titanite, aeschynite, fergusonite, and xenotime-(Y) are resistant to acidic dissolution, consistent with their common occurrence as residual minerals.In the cases of the world-class regolith-hosted REE deposits in South China, the groundwater is mildly acidic to circumneutral and carbonate-rich. The REEs are consequently transported dominantly as hydrated cations and carbonate complexes, depending on the pH. The general inheritance of the REE pattern of the regolith groundwater in the clay-sorbed fraction in the regolith indicates that the REEs in regolith are scavenged from the regolith groundwater. Elemental anomalies of specific REEs in the clay-sorbed fraction are very likely caused by an anomalously high REECO3+ fraction in the corresponding regolith groundwater, suggesting a preferential uptake of the REECO3+ complexes by the clay minerals, feasibly by halloysite through intercalation as interlayer complexes. This reaction is expected to be particularly important for the sorption and enrichment of the HREEs in the regolith. Depending on the pH and carbonate concentration of the water, surface complexation on clay minerals or interlayer intercalation particularly in halloysite control the pattern of REE enrichment. Mixing of the regolith groundwater with the alkaline and carbonate-rich aquifer groundwater increases the pH and carbonate concentration and, in turn, affects the ability of the mixed water to transport the HREEs. Interplay of aqueous complexation with the regolith mineralogy significantly affects the REE fractionation and re-distribution during groundwater-regolith interaction.

Instability analysis of jointed rock slope subject to rainfall using DEM strength reduction technique

Uncertain climatic factors like rainfall can induce the complex hydro-chemo mechanical processes within the rock mass, and thus substantially increase the failure probability of rock slope. This paper presents a numerical investigation into the rainfall-induced instability mechanism of jointed rock slope with two joint inclinations of dipping 45° and 60° and the corresponded safety factor using the two-dimensional Distinct Element Method (DEM), where the non-uniform strength reduction approach is employed. For this purpose, some crucial aspects were mainly discussed, i.e. the quantifications of water softening and chemical weathering effects in the micro bond contact model, the necessary fitted functional relationships used in the safety factor, and determinations on the intensities of water softening and chemical weathering. The results show that the proposed DEM strength reduction provides a feasible approach to analyze the rainfall-triggered instability of jointed rock slope. The significant rise of bond breakage and the accompanied large displacement of sliding block indicate the achieved critical infiltration depth of rainfall and the consequent destabilization of jointed rock slope. The jointed rock slope with steeper dipping joints has lower safety factor, and thus is more prone to instability under the same rainfall effect.

Effects of chemical weathering and CO2 outgassing on δ13CDIC signals in a karst watershed

As the important part of carbon cycle research, riverine dissolved inorganic carbon (DIC) has attracted continuous attentions for its close relationship with global climate change. In order to investigate the dominant factors of dissolved inorganic carbon species and its stable carbon isotopic signals in large watershed under base flow condition, 81 river water samples were collected in the Xijiang River during the dry season in 2015. The elemental ratios in river water revealed the hydro-chemistry in Xijiang River were mainly controlled by the carbonate weathering. The characteristics of the most samples were the high DIC concentrations and narrow range of δ13CDIC, which can be interpreted as the result of chemical weathering under open system based on the simulated calculation of soil respiration and mineral dissolution. The relatively lower DIC concentrations and δ13CDIC values have been observed in tributaries draining silicate terrains, this result may be controlled both by the mineral dissolution and CO2 outgassing. Most samples had the over-saturated pCO2 levels compared to the atmosphere, the respiratory quotient (RQ) demonstrated that high pCO2 values were not from the biologic respiration, instead, the groundwater influx with the high soil pCO2 increased pCO2 level in river waters. The CO2 concentration gradient between river water and atmosphere caused a violent CO2 outgassing accompanied isotope fractionation. The outgassing significantly dropped the pCO2 in river water and caused an enrichment of C13 in DIC. Based on the theoretical calculation and previous observation, the negative correlation between the δ13CDIC and pCO2 in river water was caused by the CO2 outgassing. Additionally, the isotope exchange between the DIC and atmosphere CO2 can also partially increase the riverine CO2. This study examined the impacts of soil respiration, minerals dissolution and CO2 outgassing on DIC and δ13CDIC, and found that the riverine δ13CDIC is probably not reflect the mixing of source signals such as soil CO2 and carbonate, instead, it is mainly controlled by the fractionation in minerals dissolution and CO2 outgassing.

Reassessing the dissolved molybdenum isotopic composition of ocean inputs: The effect of chemical weathering and groundwater

Reassessing the dissolved molybdenum isotopic composition of ocean inputs: The effect of chemical weathering and groundwater

Luminescence dating of weathered sediments from the Paleolithic site of Fengshuzui in northern Hunan province, China

Abstract South China is an important region for the investigation of the origin of modern humans and the dispersal of Homo sapiens in Asia. However, the accurate dating of Paleolithic sites, especially for open-air sites, in the region has been difficult due to the lack of datable materials like bones that were destroyed due to intense chemical weathering. In this study, we applied optically stimulated luminescence (OSL) dating techniques to the Paleolithic site of Fengshuzui in south China, in which the deposits are intensely weathered. We first investigated the degree of chemical weathering of the deposits and the effect of weathering on dose rate, and then measured the equivalent dose of quartz extracts from five sediment samples from a ∼5 m thick archaeological section. The contents of mobile and immobile elements determined by different analysis methods show that the studied sediments are homogenous in chemical and mineral compositions. Enrichment or depletion of major and trace elements including U and Th were not detected. The sediments consist of ∼60% quartz, ∼30% illite and ∼6% feldspars, ∼3% kaolinite and ∼2% hematite. The calculated chemical weathering indices show that the sediments have been intensely weathered, and the weathering degree of the five studied samples at different depths are similar. Radioactive disequilibrium in the uranium decay series caused by weathering was not detected. All these indicate that the effect of chemical weathering on dose rate is negligible for our samples. The quartz extracts displayed excellent luminescence characteristics, and the five samples, from bottom to top, were dated to 123 ± 7, 98 ± 5, 98 ± 4, 81 ± 3, 68 ± 2 ka, respectively. The ages of the cultural layers range from 100 to 70 ka, mainly corresponding to MIS 5. This also implies that the OSL techniques can be employed to date open-air sites in south China, but the effect of weathering on dose rate should be investigated on a case by case basis.

Lower Palar River Sediments, Southern Peninsular, India: Geochemistry, Source-Area Weathering, Provenance and Tectonic Setting

Abstract This present study describes the geochemistry of fluvial sediments of the Palar river (lower reaches), Southern Peninsular India, with an aim to evaluate their provenance, weathering and tectonic setting. The bulk sediment chemistry is influenced by grain size. The river sediments are enriched with SiO2 and depleted in Al2O3, K2O, CaO, Na2O, MgO, P2O5, MnO, Fe2O3 as compared with UCC values. Geochemical classification indicate that the sediments are mainly arkose, wacke and shale in composition. Discriminant diagrams together with immobile element ratio plots reveal that, the Palar river sediments are mostly derived from rocks formed in an active continental margin. Additionally, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate felsic rock sources. The chemical indices of alteration suggest that Palar river sediments are chemically immature and have experienced low chemical weathering effects. This is further supported by the Th/U Rb/Sr ratio and A-CN-K ternary diagram, with most of the sample data points falling close to the plagioclase-smectite line. The bivariate plot of Th/Sc versus Zr/Sc suggest a moderate recycled origin of the sediments.

Using surface regolith geochemistry to map the major crustal blocks of the Australian continent

Multi-element near-surface geochemistry from the National Geochemical Survey of Australia has been evaluated in the context of mapping the exposed to deeply buried major crustal blocks of the Australian continent. The major crustal blocks, interpreted from geophysical and geological data, reflect distinct tectonic domains comprised of early Archean to recent Cenozoic igneous, metamorphic and sedimentary rock assemblages. The geochemical data have been treated as compositional data to uniquely describe and characterize the geochemistry of the regolith overlying the major crustal blocks across Australia according to the following workflow: imputation of missing/censored data, log-ratio transformation, multivariate statistical analysis, multivariate geospatial (minimum/maximum autocorrelation factor) analysis, and classification. Using cross validation techniques, the uniqueness of each major crustal block has been quantified. The ability to predict the membership of a surface regolith sample to one or more of the major crustal blocks is demonstrated. The predicted crustal block assignments define spatially coherent regions that coincide with the known crustal blocks. In some areas, inaccurate predictions are due to uncertainty in the initial crustal boundary definition or from surficial processes that mask the crustal block geochemical signature. In conclusion, the geochemical composition of the Australian surface regolith generally can be used to map the underlying crustal architecture, despite secondary modifications due to physical transport and chemical weathering effects. This methodology is however less effective where extensive and thick sedimentary basins such as the Eromanga and Eucla basins overlie crustal blocks.

High-resolution stratigraphy of the Lower Badenian deposits in selected cores of the Carpathian Foredeep

Drill cores from IK-1 Ivaň and LOM-1 Lomnice u Tisnova (Lower Badenian) was evaluated by several various proxy methods (X-ray fluorescence spektrometry XRF, gammaspectrometry, grain size analysis, RTG diffraction and chemical analysis) in order to obtain data about paleoclimatic and paleogeographic conditions of sedimentation as well as to veryficate applications of these methods on monitored Neogenous sediments. Both wells are situated in the southern part of the Karpathian Foredeep in the Czech Republic. IK-1 is situated in the central area of the basin while LOM-1 lies on the western border of it. According to the biostratigraphic data, both of the observed wells belong to the NN5 zone (Tomanova Petrova & Svabenicka 2007). Facial and litologic studies (Nehyba et al. 2007) point to similar sedimentary conditions (shallow-sea sediments of inner and outer shelf) and similar source of clastic material settling in the basin. Results from the X-ray fluorescence spectrometry were used as proxy date for grain size, weathering, oxide-reductive ratios and changes of high of sea-level. For the study, chemical elements were used, especially those which has been confirmed as reliable for given methods, specifically Al, Si, S, K, Ti, Mn, Fe, Rb and Zr. On the other hand, elements showing higher deviations during measuring were excluded, for example Cr, V, T or Ni. According to the results of grain analysis, the sediments can be classified as clay silts and, in only a fraction of cases, silt clays. Medium size of Mz grain (Folk & Ward 1957) was counted and was used to confirm grain proxies. Chosen ratios Si/Al, Ti/Al and Zr/Al (Ver Straeten et al. 2011) were used as grain proxies. Mutual correlations were insignificant, only in case of LOM-1 higher correlation between Mz and Ti/Al was proved. Therefore, the grain proxies cannot be considered reliable. Proxies for weathering show the effects of chemical weathering, which is also proved by high values of the CIA index. For surface-level changes, we used proxy Fe/Al (Clarkson et al. 2014). In IK‑1, the results were compared with ratio of plankton and bentos (Nehyba et al. 2007) and stronger similarity of trends was established. To sum it up, proxy data of studied sediments can be considered partially reliable.

Relationships between chemical weathering indices and physical and mechanical properties of decomposed granite

Chemical weathering is one of the dominant mechanisms for weathering of granite in subtropical and tropical regions resulting in modifications of its chemical, physical and mechanical properties. Thus, the changes in chemical properties may provide insight about the changes in physical and mechanical properties of weathered granite. In this paper three quantitative chemical weathering indices are correlated to the dry density of decomposed granite. A linear relationship is found between each chemical weathering index and dry density for granitic saprolites. Mobiles index (Imob) gives the highest coefficient of correlation among the three chemical weathering indices. Further investigations were conducted to establish the relationships between Imob and other physical and mechanical properties of granitic saprolites. The test results showed that fines content increases with increasing Imob, but peak strength increases with decreasing Imob. The peak states of granitic saprolites are consistent with Rowe's stress–dilatancy relationship. Dilatancy increases with decreasing Imob that contributes to a higher peak strength. Based on some test results presented in this study, it seems that Imob could be a good indicator to quantify the effects of chemical weathering on physical and mechanical properties of granitic saprolites.

Mackenzie Inuit Lithic Raw Material Procurement in the Lower Mackenzie Valley: The Importance of Social Factors

Oral and written historical records indicate that the Mackenzie Inuit traveled up the Mackenzie River from the Arctic Coast to procure lithic raw material in the interior from a quarry at the mouth of the Thunder River, which is known locally by the Gwich’in of the lower Mackenzie Valley as Vihtr’ii Tshik. We evaluate this proposition using non-destructive polarized energy dispersive X-ray fluorescence to compare the geochemical signatures of the lithic raw material from Vihtr’ii Tshik (MiTi-1) and flakes and tools from the Mackenzie Inuit village of Kuukpak (NiTs-1), which is located more than 400 km downriver of the quarry source. The concentrations of nine selected elements—three major elements expressed as oxides (SiO2, Fe2O3T, and K2O) and six trace elements expressed as metals (Rb, Sr, Y, Zr, Ba, and Ce)—are compared using descriptive statistics, spider diagrams, and principal components analysis. The geochemical effects of chemical weathering on the surfaces of artifacts are evaluated by measuring element concentrations before and after removal of the weathering rind from select artifacts. The results of our analyses demonstrate that the lithic raw material available at Vihtr’ii Tshik is best characterized as chert, and that 86% of the flakes and tools from Kuukpak analyzed in this study are chemically similar to the raw material from Vihtr’ii Tshik. Historical records and archaeological data indicate that the people of Kuukpak traversed a complex social landscape to obtain stone from Vihtr’ii Tshik through direct procurement.

Quantifying chemical weathering intensity and trace element release from two contrasting basalt profiles, Deccan Traps, India

Weathering profiles developed on basalt substrate contain information relevant to climate, atmospheric composition and evolution, nutrient release into the hydrosphere, and understanding Martian regolith. In this study, the chemical compositions of two profiles developed on Deccan Trap basalt are examined. One is sub-Recent and has only progressed to a moderate degree of alteration (Chhindwara profile), whereas the other is ancient (Paleocene) and the degree of alteration is extreme (Bidar laterite). In an attempt to better quantify the chemical changes during incipient to intermediate weathering of mafic substrates, a new index is proposed: the mafic index of alteration (MIA). Similar to the chemical index of alteration (CIA), the MIA quantifies the net loss of the mobile major elements (Ca, Mg, Na, K±Fe) relative to the immobile major elements (Al±Fe). The redox-dependent weathering behaviour of Fe is factored into two separate arrangements of the MIA that apply to oxidative [MIA(O)] or reduced [MIA(R)] weathering. The MIA can be visualised in a variety of ternary diagrams in the Al–Fe–Mg–Ca–Na–K system. To chemically quantify the stages of advanced to extreme weathering, at which the MIA and CIA are ineffective, the SiO2 to (Al2O3+Fe2O3) mass ratio, based on the established Si–Al–Fe (SAF) ‘laterite’ ternary diagram, is used; we propose that this ratio be referred to as the ‘index of lateritisation’ (IOL).Major element chemical variations, as expressed by weathering indices, are used to relate the extent of weathering with the behaviour of trace elements (alkali, alkaline earth, rare earth, and Nb) in the profiles. During the early stages of basalt weathering, the mobile trace elements (Sr, Be, Li) are anti-correlated with the chemical weathering indices and thus released during these stages. By contrast, the monovalent elements (K, Rb, Cs, Tl), excluding Na and Li, appear to be associated with the pedogenetic clay minerals. Of these elements, those with the most similar ionic radii are closely related in their weathering behaviour. Fractionation of the REE (Sm/Nd, Eu/Eu*, Ce/Ce*) is evident during weathering of the basalt. The loss of Eu is linked with that of Sr, Ca, and Na and thus associated with plagioclase dissolution during the stages of incipient to intermediate weathering. The fractionation of Sm/Nd suggests that basaltic weathering products may not always preserve their parent rock ratio and, consequently, their Nd isotope composition over time.Finally, weathering in the sub-Recent profile is shown to have progressed across two lava flows, whose morphology initially controlled the extent of weathering. Certain compositional variations in the original flows (e.g., immobile element ratios) are preserved through the effects of chemical weathering and have the potential to influence mass balance calculations across the entire profile.

Si cycling in a forest biogeosystem – the importance of transient state biogenic Si pools

Abstract. The relevance of biological Si cycling for dissolved silica (DSi) export from terrestrial biogeosystems is still in debate. Even in systems showing a high content of weatherable minerals, like Cambisols on volcanic tuff, biogenic Si (BSi) might contribute > 50% to DSi (Gerard et al., 2008). However, the number of biogeosystem studies is rather limited for generalized conclusions. To cover one end of controlling factors on DSi, i.e., weatherable minerals content, we studied a forested site with absolute quartz dominance (> 95%). Here we hypothesise minimal effects of chemical weathering of silicates on DSi. During a four year observation period (05/2007–04/2011), we quantified (i) internal and external Si fluxes of a temperate-humid biogeosystem (beech, 120 yr) by BIOME-BGC (version ZALF), (ii) related Si budgets, and (iii) Si pools in soil and beech, chemically as well as by SEM-EDX. For the first time two compartments of biogenic Si in soils were analysed, i.e., phytogenic and zoogenic Si pool (testate amoebae). We quantified an average Si plant uptake of 35 kg Si ha−1 yr−1 – most of which is recycled to the soil by litterfall – and calculated an annual biosilicification from idiosomic testate amoebae of 17 kg Si ha−1. The comparatively high DSi concentrations (6 mg L−1) and DSi exports (12 kg Si ha−1 yr−1) could not be explained by chemical weathering of feldspars or quartz dissolution. Instead, dissolution of a relictic, phytogenic Si pool seems to be the main process for the DSi observed. We identified canopy closure accompanied by a disappearance of grasses as well as the selective extraction of pine trees 30 yr ago as the most probable control for the phenomena observed. From our results we concluded the biogeosystem to be in a transient state in terms of Si cycling.

Major element geochemistry of purple soils/rocks in the red Sichuan Basin, China: implications of their diagenesis and pedogenesis

Major element compositions and chemical weathering features of the purple soils and their corresponding rocks in the Red Sichuan Basin, China were studied in this paper to infer the diagenetic features of the purple rocks and the pedogenetic features of the purple soils. The results showed that all of the rock and soil samples, except those from Yibin, have strongly similar major element compositions and the distribution patterns of their major element compositions are similar to those of the upper continental crust. Chemical index of alteration, chemical index of weathering and Na2O/K2O indicate that the purple soils and rocks have similar chemical weathering intensities, whereas the Al2O3–CaO* + Na2O–K2O (A–CN–K) relationship, suggests that chemical weathering was strong during diagenesis of the purple rocks, but weak during pedogenesis of the purple soils. Variations in major element compositions of the purplish rock samples, except that from Yibin, do not primarily reflect differences in compositions of the sediment source, sediment recycling and potassium metasomatism, and therefore the effects of chemical weathering on the major element compositions were mainly dependent on climate condition and the residence time of material exposed to chemical weathering. Chemical weathering of the purple rocks reached only moderate degrees under the general influence of warm and humid climate during Jurassic and Cretaceous. Warmer and more humid climate conditions partly resulted in stronger chemical weathering of rocks in the southern region of the basin than in the central and western region, whereas shorter residence time of material in upslope position partly resulted in weaker chemical weathering of rocks in the eastern region than in the central and western region. The same climate and stable tectonic setting led to comparable chemical weathering intensities of rocks in the Ziliujing and Jiaguan Formations. Gradually colder and dryer climate from Early Jurassic to Middle Jurassic then to Late Jurassic and shorter residence time in tectonically active setting of Middle Jurassic and late Late Jurassic resulted in the following order of chemical weathering intensity of the purple Jurassic rocks: Ziliujing Formation ≫ Shaximiao Formation ≈ Suining Formation > Penglaizhen Formation.

Evaluating effects of chemical weathering and surface contamination on the in situ provenance analysis of building stones in the Cuzco region of Peru with portable XRF

As the only type of instrument capable of determining artifact geochemistry in a wide variety of settings, portable X-Ray Fluorescence (PXRF) may be frequently utilized as probably the best option for collecting data when removal of objects in part or in whole is unfeasible. However, using PXRF in circumstances where sample selection and preparation diverge from standards developed for lab-based instrumentation requires assessing the impact of new and potentially limiting factors to establish the validity of the method in these new contexts. Here we examine the effects of surface contamination and chemical weathering on the ability to assess provenance of igneous building stones used in the Cuzco region of Peru. Surface contamination was assessed through comparing low-impact cleaning methods on diorite and andesite blocks, and weathering was examined by comparing weathered vs. fresh surfaces of samples from two andesite quarries. In general, the lightest elements were most impacted by contamination and weathering while measurements of many of the heavier elements commonly used in provenance studies were relatively unaffected. Concentrations of Pb and Zn were problematic in the urban context of Cuzco because of pollution. Ability to determine provenance of building stones within those limitations (i.e., the validity of this application of PXRF) was demonstrated in a preliminary study by comparing measurements from uncleaned, exposed building stones in the region to fresh quarry samples.