Chemical Index Of Alteration Values Research Articles

Geochemistry of siltstones in the North Qilian Zhongbao–Mayinggou formations (Late Ordovician–Early Silurian): implications for paleoclimate, paleoweathering, and tectonic setting

The Late Ordovician–Early Silurian period witnessed the Phanerozoic mass extinction, glacial events, and volcanic events. Paleoweathering indexes chemical index of alteration (CIA), chemical index of weathering (CIW), and plagioclase index of alteration (PIA) indicated that the source area weathering changed from weak to moderate to intense. CIA values in the upper Zhongbao formation ranged from 66.71% to 73.97%, indicating a drier and colder climate. Upward, the CIA values on the bottom of the Mayinggou formation returned to the high value quickly (from 73.86% to 81.31%), suggesting that the ice age ended, the climate became warmer and wetter, and the sea level rose. The Al2O3–(CaO*+Na2O)–K2O triangular plots, Hf-La/Th, and SiO2–Al2O3/TiO2 bivariate plots inferred that the source of the siltstones in the two formations is mostly from the felsic igneous rocks. The samples from the Zhongbao–Mayinggou formations have chondrite-normalized rare earth element (REE) patterns similar to that of the North Qilian volcanic arc rocks. Geochemical discrimination plots displayed that the sediments of the Zhongbao–Mayinggou formations came from the active continental margin setting.

Fractionation Mechanism and Flux Estimation of Strontium Isotopes During Basalt Weathering

AbstractThe fluxes of metal cations and isotopes released by weathering of silicate rocks are crucial and a prerequisite for constraining geochemical fluxes to rivers and oceans. This study presents mineral and elemental compositions along with 87Sr/86Sr and δ88Sr data from a basaltic weathering regolith on Hainan Island, South China to elucidate Sr isotope fractionation and weathering fluxes. The 87Sr/86Sr ratios vary from 0.703936 to 0.706338 as a result of differential weathering of the minerals. δ88Sr values in the weathering regolith range from −0.29 to 0.37‰, with the majority of the weathering regolith having lower δ88Sr values than the parent rock. Sr is leached into the soil solution during plagioclase decomposition, while 86Sr is preferentially adsorbed on the surface of secondary minerals. As weathering progresses, smectite decomposes and kaolinite desorbs under weakly acidic conditions, releasing the previously adsorbed 86Sr into the soil solution. The differential weathering of kaolinite and smectite controls the δ88Sr values of the weathering regolith, with pH being an important determinant of isotope fractionation. Furthermore, Sr elemental fluxes (SrFlux) and Sr isotopic fluxes (δ88SrFlux) of this weathering regolith were calculated using a mass balance model, yielding mean values of 0.20 (mg cm−3 Myr−1) and 0.052 (‰ (mg cm−3 Myr−1)), respectively. The δ88SrFlux exhibits a nonlinear positive correlation with the Chemical Index of Alteration (CIA), indicating that enhanced weathering leads to significant stable Sr isotope fractionation at CIA values below 95%. Our research promotes the understanding of Sr recycling and the fractionation behavior of stable Sr isotopes during consecutive weathering.

Geochemistry and petrology of Early Permian lacustrine shales in the Lodève Basin, Southern France: Depositional history, organic matter accumulation and thermal maturity

The lacustrine shales in the Lodève Basin, southern France, serve as excellent archives of paleo-lake development as well as climatic evolution during the Early Permian. In this study, an extensive dataset is presented encompassing organic petrographic data, major and trace element quantification, bulk and molecular organic geochemical proxies, as well as compound-specific stable carbon isotope data derived from the analysis of 36 black shale outcrop samples originating from the Usclas-St. Privat Formation (USPF), Tuilières-Loiras Formation (TLF), and Viala Formation (VF). All sample are thermally oil-mature, as evident from a vitrinite reflectance (VRr) of around 0.8%. The lower section of the USPF, characterized by notably higher total sulfur (TS) concentrations, displays total organic carbon (TOC) and TS contents spanning from 2.69 to 7.83 (avg. 4.28) wt% and 0.42–1.55 (avg. 1.09) wt%, respectively. In contrast, the upper section of the USPF (average TOC of 2.59 wt%), TLF (average TOC of 2.66 wt%), and VF (average TOC of 3.17 wt%) exhibit considerably lower TS contents of 0.26 wt%, 0.22 wt%, and 0.17 wt%, respectively. The lower section of the USPF, characterized by the lowest pristane/phytane (Pr/Ph) ratio and the highest TS/TOC ratios and chemical index of alteration (CIA) values, was deposited in oxygen-depleted and saline lacustrine environments. These conditions prevailed under more humid climatic conditions and were probably related to marine incursions. In contrast, the upper section of the USPF, the TLF, and the VF display elevated Pr/Ph ratios but reduced TS/TOC and CIA values, signifying deposition within oxic-dysoxic and freshwater-brackish bottom water conditions with a significant change towards arid conditions. All samples are characterized by low vitrinite and inertinite contents together with consistently similar average values of Al2O3 and TiO2. The biomarker analysis suggests that the organic matter (OM) in most samples mainly originates from planktonic/algal biomass with additional microbial OM. Only the lower section of the USPF displays a slightly enhanced contribution of terrestrial OM input and also more detrital elements. It is concluded that the structural evolution of the basin from narrow deep towards wider and shallower settings as well as the postulated marine transgressive events during early stages played a crucial role in shaping the deposition environments of the two distinct sets of lacustrine shales, thereby influencing the OM accumulation mechanisms. In contrast, the substantial climatic aridification seems to have a relatively minor impact on the sources of OM and the conditions prevailing within the water column.

Carbon‐cycle perturbations and intensified continental chemical weathering linked to volcanism during the Jenkyns Event in the Ordos Basin

The Jenkyns Event (~183 Ma) represents an exceptional warming period in the Jurassic, which is characterized by intense environmental and climate perturbation. The eruption of the Karoo‐Ferrar Large Igneous Province (K‐F LIP) triggered pronounced carbon isotope negative excursion (N‐CIE) in exogenic carbon reservoirs, global widespread organic carbon burial and extensive ocean anoxia. However, the synchronic coupling relationship between the carbon release, volcanic activity, and intense weathering induced by the K‐F LIP during this period remains poorly understood. In this study, detailed sedimentologic and geochemical studies were conducted on the terrestrial stratigraphy of the Lower Jurassic Fuxian Formation in the Ordos Basin. Three spore‐pollen assemblages have been identified, with the Cyathidites‐Deltoidospora‐Chasmatosporites assemblage suggesting a Pliensbachian age, the Classopollis‐Cycadopites‐Chasmatosporites assemblage indicating an early to middle Toarcian age, and the Cyathidites‐Cycadopites‐Psophosphaera assemblage indicating a late Toarcian age. The lower part of the Classopollis‐Cycadopites‐Chasmatosporites assemblage shows a negative CIE with a magnitude of 2.7‰, accompanied by organic carbon enrichment. Mercury concentrations and Hg/total organic carbon ratios also show peaks during the initial stage of the carbon isotope negative shift, implying a synchrony between volcanic activities and carbon perturbation. Furthermore, the Chemical Index of Alteration value has reached its maximum and the kaolinite content shows a sharp increase at the same position as the carbon isotope negative shift initiation, indicating a simultaneous enhancement of continental weathering during volcanic activity. The changing patterns of the continental weathering intensity encompassing the Jenkyns Event also indicate that continental weathering may have functioned as a significant approach to counteract the greenhouse effect caused by substantial volcanic CO2 emissions.

The effect of weathering on cohesion in granitic fault rocks: A case study from the Yeongdeok Fault, South Korea

Weathering can alter fault-rock cohesion, which is an important criterion in the classification of fault rocks, and can also alter fault-rock mechanical and hydraulic properties. To investigate the effects of weathering on fault rocks, we studied relatively fresh and weathered fault rocks collected from a drill core and outcrop, respectively, of a granitic fault zone (the Yeongdeok Fault, a northern branch of the Yangsan Fault) on the southeastern Korean Peninsula. At the studied outcrop, the Yeongdeok Fault has juxtaposed Triassic granite against Cretaceous sedimentary rocks. Granitic fault rocks in the outcrop are incohesive, whereas the drill core includes both cohesive and incohesive granitic fault rocks. A detailed correlation of the fault rocks from the drill core with those from the outcrop reveals that some incohesive fault rocks from the outcrop are weathered cataclasite. Cataclasites from the drill core are composed of feldspar, plagioclase, and quartz fragments embedded in a matrix of quartz, plagioclase, muscovite, and chlorite. Locally, foliation defined by the preferred orientation of muscovite is developed. The weathered cataclasite comprises quartz, plagioclase, and feldspar fragments, with a fine-grained matrix composed mostly of clay minerals. The measured Chemical Index of Alteration (CIA) values for cataclasite and weathered cataclasite are 47–49 and 60–67, respectively. Measurements of hydraulic properties show that weathered cataclasite has higher porosity and permeability compared with unweathered cataclasite. Physical and chemical weathering are interpreted to have increased the fracture density of the weathered cataclasite and enhanced the dissolution of vein minerals, respectively. These processes have not only contributed to the higher permeability and porosity of the weathered (cf. unweathered) cataclasite but have also resulted in a loss of cohesion. When cohesion is lost due to weathering, distinguishing between cataclasite and fault breccia becomes challenging. Our study demonstrates that the cohesion-based classification of fault rocks becomes limited for fault rocks with CIA value more than 60.

Four volcanically driven climatic perturbations led to enhanced continental weathering during the Late Triassic Carnian Pluvial Episode

The arid climate of the Late Triassic was interrupted by a particularly humid episode known as the Carnian Pluvial Episode (CPE; ca. 234–232 million years ago). The CPE is often linked to eruptions in the Wrangellia Large Igneous Province (LIP), and is assumed to have led to global warming, enhanced weathering, water deoxygenation, and biotic changes. However, direct evidence for a temporal link between volcanic activity and chemical weathering has not yet been established due to the lack of comprehensive records across the CPE. In this study, geochemical and mineralogical analyses are applied to a lacustrine stratigraphic succession of the Jiyuan Basin (North China) that captures the CPE in high resolution. We identify four distinct pulses of enhanced continental chemical weathering characterized by elevated Chemical Index of Alteration values and kaolinite contents. These peaks in continental weathering coincide with Hg/TOC enrichments and negative organic carbon isotope excursions that mark four short (∼400 kyr) but intense pulses of Wrangellia LIP volcanism. In combination with signs of increased humidity, our findings provide direct and independent evidence that Wrangellia LIP eruptions significantly altered CPE chemical weathering rates in response to global warming and wetting. The lake experienced eutrophication and water deoxygenation after each volcanic pulse but the swift recovery of carbon isotopes suggests that the system rapidly returned to conditions prior to the volcanic perturbation. Organic carbon burial facilitated by widespread dysoxic and anoxic waters, and CO2 consumption via enhanced weathering likely played crucial roles in the rapid climatic recovery after each volcanic pulse.

Influence of micro-scale factors in weathering and elements mobility: Evidence from a comparative study of granite and basalt weathering profiles across India

In an attempt to constrain the micro-scale factors during weathering, large number of samples from four weathering profiles developed on granite and basalt rocks under different climatic conditions from western (Gujarat, GJ) and central (Bundelkhand, BK) India are reported in this study and compared these profiles with profiles from north eastern (Arunachal Pradesh, Lesser Himalaya, AP) India. In granitic profiles, the chemical index of alteration (CIA) values ranges from 50 to 85 whereas 42–90 CIA values are observed for basalt profiles. This highlights the significance of climate conditions (i.e., precipitation) over lithology in controlling weathering advancement. The results of the mass-transfer coefficient (τ) of major oxides significantly vary among the three granite and basalt profiles. The trace elements distribution in weathering profiles seems to be strongly controlled by the degree of weathering and formation of secondary minerals. Regardless of the nature of parent rocks, HFSEs (high field strength elements) tend to be enriched in the upper portion of all the weathering profiles; whereas REEs (rare earth elements) display variable behavior in each profile. The principal component analysis (PCA) reveals that three components are accounted for 89.14% of the variance. PC1 is attributed to weathering intensity and formation of secondary minerals, PC2 can be related to the lithology of parent material, and PC3 might be a location-specific factors. The present study shows that the geochemical characteristics of weathering profiles cannot be explained by the influence of a single weathering agent rather, there seems to be a significant role of local (or micro-scale) factors that controls the weathering and soil formation.

An integrated petrographical and geochemical study of the Tredian Formation in the Salt and Trans‐Indus Surghar ranges, North‐West Pakistan: Implications for palaeoclimate

AbstractThe Middle Triassic Tredian Formation is well exposed in the western region of the Salt Range. It is mostly composed of sandstone with minor alternations of shales and dolomite. This is the first integrated attempt using petrographical and geochemical features to reveal palaeoclimate during deposition of the Tredian Formation. This work sheds light on the petrographical and geochemical properties of the Tredian sandstone in order to date various layers and evaluate the palaeoclimate. The formation was sampled at two distinct stratigraphic layers for petrographic and geochemical analysis of major and trace elements. The sandstone of the Tredian Formation is sub‐feldspathic to feldspathic arenite with sub‐angular to rounded grains that are moderately to extensively sorted. The relative proportions of the quartz, feldspars and lithoclasts in the examined sandstone samples shows that the Tredian sediments originated from the interior of the craton during a transitional continental regime. Chemical index of alteration values of 59, chemical index of weathering values of 67, and plagioclase index of alteration values of 74.5 indicate a low to moderate degree of weathering in the Tredian sediment source region. Based on the silica content, SiO2/Al2O3 ratios (2.7–6.1; mean 4.1), and chemical maturity index, it is deduced that the Tredian Formation was deposited in chemically immature to sub‐mature and dry to semi‐arid conditions. Geochemical proxies indicate the acidic source of sediment and deposition on the passive margin of the Indian Plate. The trace element characteristics of the sediments, especially the Rb/Sr, Cu/Zn, Ni/Co, V/Cr and Sr/Ba ratios, indicate that the sediments originated from the first weathering cycle and support the notion that they were deposited in an oxidising continental environment. A semi‐arid to arid palaeoclimate predominated through the Middle Triassic at the north‐western passive continental margin of the Indian Plate in the south‐eastern Neo‐Tethys.

Investigating weathering signatures in terrestrial muds: Can climatic signatures be separated from provenance?

Siliciclastic muds (clay- and silt-sized sediment) concentrate physical and chemical weathering products. However, both rock composition and climate can affect the mineralogy and geochemistry of these sediments. We quantitatively assessed the influence of provenance and climate on muds collected from end-member climates to test, which, if any, of these potential weathering signatures are indicative of climate in fine-grained, fluvial sediments. Granulometry, mineralogy, and geochemistry of the studied muds indicated that provenance and mineral sorting hinder interpretation of (paleo)climate signals. These issues also affect chemical index of alteration (CIA) values, as well as mafic-felsic-weathering (MFW), Al2O3−(CaO* + Na2O)−K2O (A-CN-K), and Al2O3−(CaO* + Na2O + K2O)−(FeOT + MgO) (A-CNK-FM) ternary plots, decreasing their utility as paleoclimate proxies. CaO content is heavily weighted within the calculations, resulting in even felsic-sourced sediment commonly plotting as mafic owing to the relative enrichment in CaO from preferential sorting of Ca-rich minerals into the mud-sized fraction during transport. These results cast doubt on the indiscriminate use of CIA values and ternary plots for interpreting chemical weathering and paleoclimate within muds, particularly from glacial systems. Most notably, the positive correlations between CIA and climatic parameters (mean annual temperature and mean annual precipitation) diminished when sediments that had formed in nonglacial settings were filtered out from the data sets. This implies that CIA may only be applicable when used in nonglacial systems in which the composition of the primary source material is well constrained—such as soil/paleosol profiles. Within this end-member climate data set, CIA was only useful in discriminating hot-humid climates.

GEMAS: Chemical weathering of silicate parent materials revealed by agricultural soil of Europe

The chemical composition of soil represents to a large extent the primary mineralogy and geochemistry of the source bedrock, the effects of pre- and post-depositional weathering and element mobility, either by leaching or mineral sorting with the addition of secondary products such as clays. Agricultural soil in Europe (0–20 cm, 33 countries, 5.6 million km2) was sampled during the continental-scale soil mapping project (GEochemical Mapping of Agricultural and grazing land Soil - GEMAS). Total element concentrations were determined by wavelength dispersive X-ray fluorescence spectrometry. Bulk content of major elements (SiO2, TiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, P2O5) in soil has been used to calculate weathering indices such as chemical index of alteration (CIA), reductive and oxidative mafic indices of alteration (MIA), the change in mass balance (τ calculation relative to immobile Nb) for soil derived from silicate bedrocks (granite, gneiss and schist) at the European continental-scale, which are useful tools for evaluating chemical weathering trends.The greatest variation of CIA values is exhibited by soil derived from granite, followed by soil derived from schist. The CIA results show clearly that Ca and Na are removed from agricultural soil (0–20 cm) during soil development. Geographical distribution of weathering indices shows that weak chemical weathering of agricultural soil samples, sourced from gneissic and granitic bedrock, occurs in the Fennoscandian Shield; intermediate chemical weathering in granite, schist and gneiss occurring in the Iberian Peninsula and France, and intense chemical weathering in schist sourced agricultural soil occurring in areas with intense rainfall and moderate temperature (Wales, England, Ireland and western Iberia). In addition, values of CIA and MIA, higher than those for standard rock compositions, suggest more active enrichment/depletion processes during soil formation and subsequent soil weathering/maturation.The range of the elemental mass transfer coefficient τ is wide for agricultural soil derived from gneiss and granite, and narrow for agricultural soil sourced from schist parent rocks. The weathering behaviour of chemical elements, assessed by using the τ mass transport model, suggests an order of susceptibility of K ≈ Na > Al > Fe > Ca > Mg > Ti.Weathering indices and gain-loss mass transfer coefficients, tested on agricultural soil samples, provide an insight to the weathering processes affecting the silicate parent rocks and their impact on soil development at the European scale. This can have further implications for evaluation of soil nutritional conditions and mitigating of soil erosion and deprivation.

Integrated organic, inorganic geochemical and palynofacies analyses to characterize the paleoenvironment of the Tortonian (Late Miocene) Wakar Formation, offshore Nile Delta, Egypt

The subsurface Wakar Formation (Fm) has been identified in all wells of the offshore concessions in the eastern Mediterranean in Egypt. This formation is important because it represents the end member of the Late Miocene (Tortonian) siliciclastic sedimentary rock of a vertical thickness between 500 and 900 m in these fields and is a promising formation for gas production in Egypt. There are no comprehensive studies on the geochemistry and palynofacies of this Wakar Fm in literature. Therefore, sixteen collected ditch samples of the entire thickness of the Wakar Fm from a PFM-SE -1 well were investigated. The main objective is to reveal the history of depositional events that prevailed in North Africa at this time. TOC content (average = 0.81 wt%) and generative potential of the Wakar Fm are of poor and fair quantity. The organic matter (OM) belongs to types IIIC and III of coastal and delta settings as revealed by the palynofacies and geochemical data. The Tmax of the samples does not exceed 435 °C (419–432 °C) of the immature stage, which corresponds to the carbon preference index (CPI >1) (1.15–1.4). The thermal alteration index range between 2 and 2+, indicating immature to marginally mature OM. GC of the two extracted samples shows a predominance of short-chain and mid-chain n-alkanes derived mainly from algae and seagrass; the Pr/Ph ratio (3.24–3.02; suboxic) and low terrigenous/aquatic ratio (TAR; 0.23–0.47) indicate that the OM bearing sediments were accumulated in a transitional setting of the suboxic aqueous medium. The dominance of terrestrial OM and low amorphous organic matter also suggests deposition in a marginal basin under dysoxic-anoxic to suboxic-anoxic conditions. Pr/n-C17 versus Ph/n-C18 diagrams confirm that preservation occurred in weakly oxidized environments from a source of kerogen type III gathered from a swamp and transitional marine zones. The inorganic geochemical analyses (XRD and XRF) and the calculated paleoenvironmental indices (i.e; log (Fe2O3/K2O) versus log (SiO2/Al2O3)) indicate that the main composition of the Wakar Fm is a combination of iron shale and sandstone facies. The Fe-sandstone facies occupies the middle part of the formation with a thickness of 170 m, which is the pay zone in the present concession. The parent protolith of Wakar Fm is a combination of intermediate and mafic igneous rocks. The prevailing paleoclimate is arid and semi-arid conditions synchronize the base, and top of the Wakar Fm, which is responsible for weak to moderate chemical weathering as revealed from chemical index of alteration values (CIA average = 67.99). Discriminant functions exhibit a depositional setting belonging to an active continental margin and oceanic island arc between marine and terrestrial zones under oxic-dysoxic environments.

Geochemistry of Weathering Cover and the Main Influencing Factors in Karst Area of Guilin, Southwest China

The chemical weathering of carbonate rocks is the main form of interaction between earth spheres in the karst critical zone. The karst weathering cover, which is composed by residua from carbonate rocks weathering, contains important information about the climate environment and material cycle. We present the chemical composition of weathering covers in karst area of Guilin, Guangxi province, analyze their weathering process and strength, and compare them with the other weathering covers in other karst area in China, including Yunnan, Guizhou, Hunan, and Qinghai Tibet Plateau. The results showed: (1) the chemical composition of Guilin weathering covers were similar to that of carbonate weathering covers in other areas of China, and had the common characteristics of uniform distribution of chemical composition in the profile and obvious enrichment of Fe, Al and trace elements. During the formation of carbonate weathering cover and the residua, the rapid dissolution of Ca and Mg had an important impact on the migration and enrichment of other elements. (2) The chemical index of alteration (CIA) of carbonate weathering covers in Guilin and other karst areas of China was much larger than that in the upper crust (UCC) (60.13), which showed strong chemical weathering characteristics of the humid and hot climate. (3) The weathering process of carbonate rock was different from that of silicate and loess. In the early stage of carbonate rock weathering, soluble components (calcite and dolomite) had been rapidly dissolved and leached. All the carbonate weathering residua was measured to be in the stage of K2O weathering and Al2O3 increasing. Therefore, the weathering degree of carbonate weathering cover was mainly determined by the leaching of K2O and the increase of Al2O3. As a result, there was no correlation between CIA and Na/K (molar ratio), but was a significant negative correlation between CIA and K/Al (molar ratio) in the carbonate weathering cover. (4) The CIA values of weathering residua and carbonate weathering cover in southern China were negatively correlated with latitude, reflecting the influence of climate factors. From the influence of lithology, the chemical compositions of parent rock can affect the CIA of weathering cover, and the content of insoluble matter in carbonate rock was negatively correlated with CIA. From the influence of topography, the CIA value of weathering residua decreased from the high to the low position and from the shallow to the deep part of the profile.

Mica in a sedimentary feature as evidence of humid conditions for a classic desert in Paraná Basin, southern Brazil

In Paraná Basin, oriented mica appears into horizontal layered fine sediments feature defined as geodes with sediments. Mica is identified as biotite in a volcano-sedimentary context of Botucatu and Serra Geral formations. Throughout the accomplishment of multiple analyses such as petrography, rock chemical evaluation, Rare Earth Elements (REE) values and also the Chemical Index of Alteration (CIA) some contributions for local paleoenvironment and paleoweathering for the classic Botucatu paleodesert is proposed. Since biotite is not regular in sediments of a desert environment, the mica presence suggests that it was transported by aqueous input and deposited within the provided spaces. The arrangement of these sediments as planar cross-stratification also confirms that this process is water dependent, otherwise the preservation of the layers' horizontality would not be preserved. The REE results of negative Ce (Cerium) anomaly in some features, geochemically implies the contribution of water column during deposition. Some geodes are intensely silicified, and in these, there is a relative preservation of Ce attributed to an intense siliceous cementation, avoiding the oxidation of Ce. Despite some differences in Ce values, these features hold similar REE patterns and CIA values indicating similar provenance for the geodes’ sediments and they were probably deposited under similar chemical conditions. The Ce difference between the slightly silicified and intensely silicified geodes shows local conditions for the values, influenced by the presence of water presence and silicification.

Sediment provenance signal of the discontinuous retroarc topography in the northern Andes during the Early Cretaceous

AbstractA large dataset of detrital zircon U–Pb ages (N = 5940) of Aptian‐Albian strata of the Colombian‐Ecuadorian retroarc region suggests that these rocks were sourced from Proterozoic cratonic and Permian–Triassic to Cretaceous rocks of the Andean proto‐Cordilleras. The nonconformity between Aptian‐Albian strata and Proterozoic rocks of the Garzón Massif indicates the existence of positive relief in this region. Topographic highs could have caused local basin compartmentalization and the prevalence of a localized provenance of coeval strata. Areas of positive relief were seemingly exposed to intense chemical weathering as suggested by the high Chemical Index of Alteration values, and low Rb/Sr and SiO2/Al2O3 ratios of analysed strata. Our results highlight the value of provenance analysis to study the ancient topography of retroarc systems and open the avenue for further research on the role of extensional tectonics in the topographic and basin evolution in the Andes.

Spatiotemporal variability of the late Paleozoic glacial-to-postglacial transition across eastern Paraná Basin, Brazil

The late Paleozoic glacial-to-postglacial turnover evolved complexly across Gondwana. Successions bearing volcaniclastic material that can be radiometrically dated provide crucial information about the timing of those climate events. The southernmost part of the Paraná Basin, for instance, has a high-precision geochronological framework. The eastern sector of this basin (Paraná State and north of Santa Catarina State), however, lacks radiometric ages, but conversely, has a more complete stratigraphic record, and paleontological information still poorly explored for the purpose of biostratigraphic correlation. This work examines the glacial-to-postglacial interval in the Paraná State, represented by the upper Itararé Group (Taciba Formation; glacial) and lower–middle Rio Bonito Formation (postglacial). Sedimentological, paleontological and geochemical data from outcrops, cores and well logs were used to decipher the timing, paleoclimatic and paleogeographic scenarios of this transition. The examined succession comprises four stacked units (U1 to U4, from older to younger). Diamictite-dominated units (U1 and U3), here interpreted as consecutive glaciation–deglaciation events, are separated by non-glacial, continental to shallow marine deposits, commonly bearing fossil plants and coal seams (U2). An important transgression followed the first deglaciation, which is equivalent to the “Eurydesma transgression”, based on the presence of marine invertebrates of the homonymous fauna in the Passinho Shale. U2 holds elements of two different floras, i.e., Phyllotheca–Gangamopteris (P–G), predating the “Eurydesma transgression”, and Glossopteris–Brasilodendron (G–B), above the transgression. Therefore, U2 is interpreted as an interglacial interval, once it records a climate improving before the last glacial episode of U3, which is further supported by relatively high values of the Chemical Index of Alteration (CIA). Deposits of U3, associated with a decrease in the value of the CIA, are unconformably overlain by U4. The occurrence of coal-bearing postglacial facies (U4) associated with the G–B Flora, coincides with an increase in the CIA values. Sediment transport was toward the SW in all units and in the same direction it is observed an overall thinning of U1 and U3 and thickening of U2. The interglacial P–G Flora of the study area correlates with postglacial southernmost floras, based on U–Pb CA-TIMS Asselian ages of tonsteins. Correlation of the Eurydesma fauna-containing Passinho Shale with equivalent successions with high-precision age control in southern Africa, allowed us to position both deglaciations of the Taciba Formation (U1 and U3) in the Asselian. These findings suggest that U1 and U3 record two early Permian glacial episodes, with the younger one (U3) disappearing southward. Accordingly, our results indicate that the glacial-to-postglacial turnover was diachronous along the eastern belt of the Paraná Basin, being progressively older southward, considering that interglacial fossil plant assemblages in the eastern margin correspond in time to postglacial assemblages farther south.

Provenance of the conglomerate and siliciclastic rocks from the Gadag Greenstone Belt, Western Dharwar Craton, India: Implications for understanding Neoarchean basin margin sedimentation

Petrographic, whole‐rock geochemical and geochronological analysis of polymictic conglomerate, associated siltstone and greywacke in the Neoarchean Gadag Greenstone Belt in the Western Dharwar Craton, India, provides new insights into sedimentary provenance and tectonic setting. The polymictic conglomerate contains granitic, gneissic, felsic and mafic volcanic, quartzite and banded ferruginous chert clasts within a quartzo‐feldspathic matrix. Framework grains in the siltstone and greywacke are quartz, feldspar with rock fragments, embedded in a chlorite‐dominant matrix. The chemical index of alteration values (~70) and A‐CN‐K plot for the rocks suggest a low‐to‐moderate degree of weathering. Rare earth element (REE) patterns show moderate fractionation (LaN/YbN ~ 12.31) with a weak negative europium anomaly (0.82). Enrichment in transition elements (Ni, Cr, Co, Sc) and depletion in high‐field‐strength elements, as well as relatively low La/Co (0.62), moderate to high Th/Yb (3.06), La/Th (5.16), La/Yb (17.5), and La/Sc (~2.37) ratios, suggest a mixed felsic‐mafic provenance. U–Pb dating of detrital zircon from the conglomerate and greywacke revealed major age peaks at ca. (3.2, 2.9, 2.8, 2.6, 2.5 Ga) and a maximum age of deposition of 2,508 ± 26 Ma, 2,493 ± 20 Ma, respectively. The above data suggest low to moderately weathered proximal sources for both conglomerate and greywacke, indicating that these two rock types were not deposited simultaneously, but rather separately. The association of these conglomerates and greywacke with continental arc‐related volcanics indicates the closing of an arc at the time of sedimentation. Complex provenance consisting of recycled and magmatic arc during basin closure confirm the notion of modern style plate tectonics operating during Neoarchaean.

Climatic and weathering conditions in southern high latitudes during the Turonian-Santonian interval: New insights from IODP Site U1512 (Bight Basin, Southern Australia)

A detailed study, based on a coupled mineralogical and geochemical approach, was done on IODP Site U1512 sediments (Bight Basin, Southern Australia). The mineral assemblages of the clay size fraction (<2 μm), obtained by X-ray diffraction, have been compared to inorganic (major and trace elements) geochemical signals, obtained from both whole rock and clay size fraction, and Sr/Nd isotopes, obtained from clay size fraction to determine climatic and weathering conditions during the Turonian to Santonian interval (∼94–84 Ma, Late Cretaceous) in the southern high latitude band. This study reveals that the mineral assemblages of clay fraction consist of R0-type illite/smectite mixed-layers (considered as smectites), illites, and kaolinites associated with opal-CT and clinoptilolites. Electron microscopy observations have demonstrated a negligible impact of burial diagenesis on clay assemblages while authigenesis contributions of clay minerals are limited and associated with minor volcanic contributions. Clay assemblages would be thus dominantly primary in origin. Based on Sr and Nd isotopic values, clay minerals are interpreted as the resulting from the weathering of rocks and pedogenic blankets of Eastern Australian Highlands and from the remobilization of older sediments from nearby uplifted basins. The chemical compositions of both whole rock and clay size fraction show a relative stability during the studied interval with two exceptions: during the Middle/Late Turonian times for Si, P and Ba and at the Turonian-Coniacian boundary for Ti, K and Zr, associated respectively to an increase of biogenic fraction and to an enhanced input of terrigenous material. The clay assemblages, dominated by smectites, show weak variations along the borehole reflecting stable climatic conditions during the 10-Myr Turonian-Santonian interval. The dominance of smectites, coupled to the absence of chlorite and the low proportions of illite, which are both considered as primary clay minerals, indicates a dominant warm/temperate and sub-humid climate during the studied interval. However, changes in weathering conditions occurred during the Turonian-Santonian interval in southern high latitude. The Early Turonian, characterized by high Chemical Index of Alteration values (>75), is followed by an interval of lower chemical weathering with enhanced biogenic silica during the Middle/Late Turonian. The slight increase of kaolinites, recorded during Coniacian and coeval with increase of Ti/Al and Zr/Al ratios, probably reflects an increase in coarser terrigenous inputs associated with a sea-level fall.

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.

Late Permian ∼ 6 My cooling induced by basaltic weathering of the Emeishan large igneous province: Evidence from interbasaltic paleosols

Land surface temperature variations during the Emeishan large igneous province volcanism helps to reveal the effect on the climate in the late phase of the late Paleozoic Ice Age. Paleosols preserved between the Emeishan basalts record hiatuses in basalt eruptions and provide detailed information on the Earth's surface environment. Forty-seven paleosol profiles in eleven locations were identified, described, and assigned to four pedotypes: I, II, III, and IV. The majority of pedotypes I, II, and III had a dominant mineral assemblage of kaolinites, hematite, and anatase, high chemical index of alteration values of 90–99 and Al2O3 and Fe2O3 enrichments, and were classified as Oxisols or Ultisols (alternatively, laterite). Pedotype IV contained large amounts of primary minerals from basalts, yielded low chemical index of alteration values of 35–65, and were classified as Entisols or Inceptisols. The eastern Emeishan large igneous province was dominated by highly weathered pedotypes I and II, with small amounts of pedotype III, while the western Emeishan large igneous province was dominated by pedotype III and weakly weathered pedotype IV. We interpreted this spatial distribution pattern of pedotypes as a result of the paleotopographic difference formed by the subaerial mafic lava accumulation, with steep highlands in the western Emeishan large igneous province and lowlands in the eastern areas. Based on the silica-alumina‑iron ratio climofunction, a land surface temperature cooling trend of ∼2–4 °C was recognized in the upper part of the Emeishan basalt succession (i.e., the waning phase) in three sections and was constrained to ∼260 to ∼253 Ma by new high-precision zircon UPb dates. This land surface temperature cooling trend was temporally coupled with the sea surface temperature cooling trend and the alpine P4 glaciation in eastern Australia. We propose that the intense weathering of extensive Emeishan basalts in the tropical humid belt, directly evidenced by the numerous thick laterites in the upper part of the basalt succession across the broad regions of the Emeishan large igneous province, elevated atmospheric CO2 consumption flux, leading to global cooling and further triggering the P4 glaciation.

The expression of late Cenomanian–Coniacian episodes of accelerated global change in the sedimentary record of the Mexican Interior Basin

Climate and oceanographic changes during the Oceanic Anoxic Event 2 (OAE 2) and the Late Turonian–Coniacian Event (LTCE) are reported from a succession deposited in the central part of the Mexican Interior Basin, based on sedimentological, microfacies, mineralogical, and geochemical analysis. Typical δ13C positive excursions that characterize both the OAE 2 and the LTCE were identified. Organic-rich sediments during the initial stage of OAE 2 (before and around peak “A”, late Cenomanian) and the middle stage of LTCE (around the Hitchwood event) accumulated under increasingly warm and humid conditions, as evidenced by high chemical index of alteration values. Elevated concentrations of detrital proxies coupled with high phosphorus mass-accumulation rates suggest that this scenario led to a rise in detrital and nutrients fluxes that induced eutrophication and anoxic/dysoxic bottom waters. Eutrophic-anoxic/dysoxic marine conditions are further supported by the highest TOC values, coinciding with significant enrichments in redox- and productivity-sensitive trace elements. Moreover, these conditions can also be inferred from the abundant presence of radiolarians and filaments in the OAE 2 interval, and the occurrence of opportunistic foraminifera in the LTCE interval. Oxygen-depleted bottom waters are also indicated by Mo–U systematics. The onset of the Mexican Orogen tectonic uplift and increased upwelling strengthened the transference of nutrients, enhancing organic matter burial during the initial stage of OAE 2. In the mid-OAE 2 δ13C trough interval (between peaks “A” and “B”) equivalent to the Plenus Cold Event, bioturbated sediments with low TOC values accumulated during a short episode of cold climate conditions probably reflecting the southward flow of boreal water throughout the Mexican Interior Basin. The minimum δ34Spy value occurring within the OAE 2 interval in the Mexican Interior Basin is lower than elsewhere because of a local increase in sulfate concentrations.