Intense Chemical Weathering Research Articles

Late Miocene Uplift and Exhumation of the Lesser Himalaya Recorded by Clumped Isotope Compositions of Detrital Carbonate

AbstractThe Himalaya orogen evolved since the Eocene as the Tethyan‐, Greater‐, Lesser‐ and Sub‐Himalaya thrust sheets were uplifted and exhumed in sequence. Reconstructing the provenance of sediment in Himalayan River systems can inform on stages in the tectonic history of the orogen. Here, we analyze the oxygen, carbon and “clumped” isotope compositions of carbonate minerals from Himalayan bedrock, Ganga River sediments and Bengal Fan turbidite deposits. We demonstrate that river sediments consist of a mixture of Himalayan‐derived and authigenic calcite precipitated in the river system. The relative abundance and clumped isotope apparent temperatures of detrital calcite in turbidite deposits decreased between the Late Miocene and Pliocene, while chemical weathering intensity did not increase during this interval. Considered together, these results reflect the establishment of the Lesser Himalaya as an important carbonate sediment source for Himalayan rivers, driven by the uplift and exhumation of this thrust sheet.

Chemical weathering in the Mekong River Basin: Clay mineralogy and element geochemistry of lower-reach river sediments

Chemical weathering of parent rocks in river basins plays a significant role in controlling the global geochemical cycle and climate change, especially in the world’s largest river basins such as the Mekong River Basin in tropical regions. However, the chemical weathering process of the Mekong River Basin is still not well understood. In this study, clay mineralogy and major/trace-element geochemistry of fluvial sediments (clay, silt, and sand fractions) collected from the lower Mekong River Basin (Cambodia and Vietnam) were utilized to investigate the sediment provenance and chemical weathering process. The major-element compositions of the clay, silt, and sand fraction sediments from both the mainstream and tributaries consist of dominant SiO2, Al2O3, and Fe2O3, (84 wt.%, 89 wt.%, and 95 wt.%, respectively) and minor K2O, Na2O, MgO, CaO, TiO2, P2O5, and MnO. The clay mineral assemblages in mainstream sediments are high in illite (36 wt.%), moderate in kaolinite (28 wt.%) and chlorite (26 wt.%), and low in smectite (10 wt.%), whereas those in tributary sediments are high in smectite (37 wt.%), moderate in kaolinite (26 wt.%) and chlorite (22 wt.%), and low in illite (15 wt.%). The different clay mineral assemblages between mainstream and tributary sediments can be significantly related to the parent rock lithology and/or the weathering process in the source region. On the basis of clay mineralogy and elemental geochemistry, river sediments of the mainstream in the lower reach may be derived mainly from felsic rocks in the lower part of the middle reach of this basin, with secondary contributions from the upper and lower reaches as well as the upper part of the middle reach. Sediments in the Kampi and Speu tributaries highly originate from felsic rocks, whereas sediments in the Srepok, Ter, and Chhiong tributaries can be mostly weathering products of mafic rocks. The clay mineral proxies (smectite/(illite + chlorite) and kaolinite/(illite + chlorite)) combined with the elemental geochemistry (CIA, αAlE values, and weathering trends) of the clay fraction sediments indicate intensive chemical weathering in the lower and middle reaches. The high-relief topography and cold and dry climatic conditions in the upper reach result in high illite and chlorite contents in the soil and moderate chemical weathering. The chemical weathering intensity increases from the upper to middle reaches and further to the lower reach. Tectonics in the middle and upper reaches of the Mekong River Basin play the most important role in controlling weathering and erosion processes, whereas East Asian–Indian monsoon climate conditions with warm temperature and predominant rainfall throughout the year and lithology are the main forcing factors for the intensity of chemical weathering in the lower reach.

Evolution of pore structure and cement composition during weathering of conglomerate at Mogao Grottoes in alkaline and arid regions, NW China

Numerous conglomerate grottoes have been excavated and preserved in the arid and alkaline regions of NW China. After millennia of weathering, these conglomerate grottoes have suffered from various weathering issues (such as seepage and collapse) that are internally governed by the distribution of pore networks and the degree of cementation. However, studies on conglomerate weathering have mostly been reported in humid and tropical climates, where intensive chemical weathering occurs. Additionally, characterizations of pore evolution during conglomerate weathering have been restricted due to limitations in detecting pore size and pore type using a single pore characterization method. To fill this gap, this study jointly used mercury intrusion porosimetry (MIP) and computed tomography (CT) to characterize the pore evolution process during conglomerate weathering at Mogao Grottoes, a world heritage site in NW China. Mineralogical, hydrological, mechanical and microscopic characterization were also conducted to understand the changes in compositions of bulk samples and cements during weathering. Results revealed that the majority of pores in conglomerate are concentrated in macropores > 5 μm, followed by mesopores and micropores. During weathering, volume fraction of macropores significantly increases and that of micropores decreases, with median/average pore diameters experiencing an 8- to 10-fold increase. Permeability, porosity and fractal dimension exhibited strong positive linear correlations with median/average pore diameters. Throughout weathering, the abundances of calcite and clay minerals show notable decrease, particularly in cements, resulting in an enrichment of feldspars. Frequently occurring sand-carrying wind activities and salt weathering in arid regions are inferred to be responsible for the major cement loss and porosity generation through physical processes. Our combined MIP-CT method for pore network characterization is also applicable to other lithologies with a wide range of pore sizes. The proposed mechanism of pore evolution and cementation failure provides a scientific base for protecting stone heritages in arid zones.

The influence of weathering and pedogenesis on the geochemical record of Miocene marls and Plio-Quaternary sediments, Medvednica Mt., Croatia

The geochemical signature of weathering and pedogenesis in a temperate humid climate on two parent material types in the foothills of Medvednica Mt. was studied. Five soil profiles on Miocene marls and three sections of Plio-Quaternary (PlQ) proluvial sediments with overlying soil and weathered material were analyzed. The (clay) mineralogy of all profiles and sections had been determined in previous studies. The chemical composition of the samples was determined by inductively coupled plasma emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectroscopy (ICP-MS). Poorly ordered Fe and Mn oxides were determined in the PlQ sediment and overlying soil samples by atomic absorption spectroscopy (AAS), after oxalate dissolution in the dark. The concentrations and element ratios were used to determine element enrichment/mobility and intensity of chemical weathering, material provenance, and to compare the geochemical signatures with previously obtained clay mineralogy results. Trace elements in the Miocene marls, including rare earth elements (REE), indicate the continental origin of the marl siliciclastic component, while more scattered geochemical data of the PlQ sediments reflect their proluvial/torrential nature. The mass transfer coefficient (τ) for major elements and element ratios of the Miocene marl profiles indicate chemical weathering and pedogenesis of lower intensity. The geochemistry of these samples shows homogeneity within the profiles. In the geochemical signature of the PlQ sections, a chaotic proluvial deposition of the material is visible, as well as the heterogeneity with the overlying soil and weathered material. Overall, the geochemistry results largely support the clay mineralogy of the samples and demonstrate how a multiproxy approach can help test hypotheses about past environments and provide valuable additional information for complex paleoenvironmental studies.

Sedimentary factors controlling the organic matter enrichment in oil shale, Seyitömer Lacustrine Basin, Western Anatolia: New implications from organic and inorganic geochemistry

AbstractWestern Anatolia, an important segment of the Alp‐Himalayan Belt, hosts numerous Neogene lacustrine basins with potential oil shale reserves surpassing 1.6 billion tonnes. Among these basins, the Seyitömer Basin (Kütahya) stands out, containing oil shales that are intercalated with claystone, marl, limestone and coal layers. This study presents a comprehensive dataset of organic and inorganic chemistry to gain insight into the palaeoclimate and palaeoenvironmental factors that control the enrichment of organic matter. The studied samples exhibit high total organic carbon contents, averaging 12.85% (ranging from 2.22 to 36.21%), high hydrogen indices (486–812 mgHC/g rock) and low oxygen indices (33–70 mgCO2/g total organic carbon), indicating their substantial hydrocarbon‐source potential. These characteristics indicate predominantly ‘excellent’ to occasionally ‘very good’ source rock qualities and very high oil potential. Their pyrolysis, gas chromatography and gas chromatography–mass spectrometer parameters indicate an immature‐early mature characteristic for Seyitömer oil shale samples. They comprise dominantly Type‐I kerogen with minor Type‐II kerogen and lacustrine algal organic matter. Their sedimentological characteristics, along with various geochemical values, such as total organic carbon versus S, B versus Ga and dibenzothiophene/phenanthrene, reveal a moderately deep fresh/brackish to saline lacustrine environment. The Ga/Rb, K/Al and Sr/Cu ratios suggest dominantly humid and warm climate conditions, occasionally interrupted by periods of less humidity. The prevalence of warm and humid climate conditions leads to intense chemical weathering processes, supported by high Chemical Index of Alteration and low Rb/Sr ratios in the associated oil shale samples. Intense chemical weathering and high runoff resulted in dissolved nutrient enrichment, promoting ecological dynamics favourable to increased productivity. Their low Pr/Ph and Mo/total organic carbon ratios, high Ni/Co ratio and, relatively low MoEF/UEF ratio, along with well‐developed lamination of the oil shales, indicate the presence of anoxic conditions in the bottom water. These anoxic conditions would have facilitated the preservation potential of organic matter in the samples. Thus, the palaeoclimate conditions integrated with sedimentary factors have an important role in the ecological dynamic and physical–chemical environmental conditions, ultimately contributing to the organic matter enrichment of the studied samples. This work provides a case study to better understand the sedimentary factors controlling organic matter enrichment in the lacustrine basins of the Alp‐orogenic belt.

Lithium isotopes track changes in continental weathering regimes across the end-Permian mass extinction in Southwest China

It has been assumed there was a massive amount of volcanic CO2 injection into the Permian-Triassic atmosphere and ocean systems, leading to rapid climatic warming and expansion of marine anoxia. However, it remains intriguing how Earth recovered from such a CO2-driven hyperthermal condition. One potential mechanism involves the negative feedback between continental silicate weathering and atmospheric CO2, which could have helped maintain habitability across the end-Permian mass extinction (EPME) interval. This process can be examined using lithium isotopes (δ7Li), which reflect the balance of physical erosion and chemical weathering, and chemical weathering indices such as the Chemical Index of Alteration (CIA), which indicates the chemical alteration of parent materials during weathering. In this study, we analyze siliciclastic sedimentary rocks from the Lopingian to Lower Triassic depositional sequences in the HK-1 drill core at the Lengqinggou section, a terrestrial coastal depositional environment in Southwest China, to reconstruct changes in continental chemical weathering intensity across the EPME. We observed a significant ∼4 ‰ positive shift in δ7Li, accompanied by a marked decrease in Li content from 26 ppm to 6 ppm. Our corrected CIA data (CIAcorr) also exhibits a considerable decrease from 94 to 59 across the EPME. The new δ7Li and CIAcorr data from the terrestrial section indicate a decrease in overall chemical weathering intensity in Southwest China, alongside an increase in physical erosion rates, suggesting a shift from a transport-limited to a kinetically limited weathering regime across the EPME. These changes in the continental weathering regime appear to be linked to active volcanic activity near the South China Block, which led to massive deforestation and the collapse of soil systems. Dramatic reductions in chemical weathering intensity may result in inefficient atmospheric CO2 consumption through silicate weathering if other climatic and tectonic conditions remain constant, potentially contribute to maintaining high global surface temperatures and prolonged marine anoxia into the Early Triassic.

Geochemistry of shales of middle Permian Barren Measures Formation, West Bokaro Basin, India: implications on provenance, paleodepositional and paleoclimatic conditions

Geochemical analyses of fine-grained clastic sedimentary rocks, such as shales, are significant since they portray the nature of the control factors from source to sink in a sedimentary basin, and help characterizing the heterogeneity in potential source and/or reservoir rocks. Bulk rock geochemistry (including major element oxides, trace elements and rare earth elements) of 20 shale samples from the Permian Barren Measures Formation, West Bokaro Basin, Peninsular India, is presented to decipher the provenance, paleo-weathering, paleotectonic, paleo-depositional and paleoredox conditions of the basin within the Lower Gondwana paleogeographic framework. X-ray diffraction (XRD) peaks and scanning electron microscopy (SEM) reveals an abundance of quartz, muscovite and clay minerals, viz., illite, kaolinite, etc., with less abundant feldspar and glauconite. The concentrations of the major element oxides, the trace elements and rare earth elements (REEs) of the shale samples are compared with predefined standards such as PAAS and UCC. The CIA (chemical index of alteration) and the A–CN–K values, based on the ratios of the major element oxides, signify intense chemical weathering. The ratios of Sr/Cu versus Rb/Sr and K2O/Al2O3 versus Ga/Rb, and high ΣREE values indicate a temperate paleoclimate. Ratios of the major element oxides (e.g., K2O/Na2O versus SiO2) suggest a passive tectonic setting. The Eu/Eu∗ and (Gd/Yb)n values depict predominantly post-Archean granitic and gneissic source rocks. The ratios of trace elements, viz., Th/Sc, Th/Co, Th/Cr, La/Sc and Eu/Eu∗, and the distribution pattern of the REEs (higher LREE in comparison to HREE) attest to a felsic-intermediate source from a nearby provenance. The redox sensitive trace elements, such as V and Ni, indicate a dysoxic condition that prevailed during deposition. The ratios of Fe2O3 versus MgO and Log (MgO/Al2O3) versus Log (K2O/Al2O3), and the ternary plots of Fe2O3–MgO–SiO2/Al2O3 suggest a non-marine to deltaic transitional environment. Such evidence of marine influences during the sedimentation of the Permian Barren Measures Formation confirms the fluvio-marine transgressive paleogeography inferred from sedimentological and paleontological inputs from the Gondwana Basins in Peninsular India, and opens up a scope for regional correlations of paleogeographic changes during the Middle Permian (Guadalupian) across the Gondwanaland.

Reconstructing the past environmental conditions of southwestern India using estuarine sediment core

Geological archives can be examined via multiple proxies to uncover significant information about historical environmental changes. In comparison to single proxy approach, the use of multiple proxies can provide better resolution of the paleoenvironmental record. Thus, in the present study, to understand the paleoenvironmental conditions in the Kali coast in southwestern India, sedimentological, geochemical and isotopic (210Pb, 137Cs) proxies were used. The findings demonstrated that, in previous decades, the sedimentation rate varied from 0.5 to 1.0 cm/year under conditions with relatively higher hydrodynamic energy that were more common and fluctuating, allowing for larger sand particle deposition. However, in more recent years, finer particle deposition towards the surface has been observed under conditions with lower and more stable hydrodynamic energy, with a sedimentation rate of 1.87 cm/year. Additionally, the finer fractions displayed a strong correlation with the metal distribution, which was mostly governed by Fe-Mn oxides. Furthermore, it can be revealed that the environment was warm, humid, and marine-like between 1995 and 2000 based on chemical weathering intensity values and Rb/K ratios. A subtle shift to a freshwater habitat with relatively less warm, less humid climate occurred between 2000 and 2020. Therefore, similar research with longer depositional histories coupled with multiple proxies can help predict the future climatic shifts in decadal time scales.

Mineralogy and Major Element Geochemistry of the Oligocene Barail Group Sandstones from the Sylhet Trough, Bengal Basin: Provenance and Tectonic Implications

The origin of Oligocene sediments in the Bengal Basin and associated tectonic setting remain poorly understood. This study investigates the framework mineralogy and major element geochemistry of the Barail Group sandstones from the Sylhet Trough within the Bengal Basin to clarify the provenance and tectonic history of the Oligocene. Modal analysis (Q83F7L10) and geochemical data support a classification of sublitharenite to subarkose, some with Fe enrichment. The heavy mineral assemblage is dominated by opaque minerals, followed by ultrastable minerals with zircon > tourmaline > rutile. The sub-angular to sub-rounded sand grains with a compositionally moderate mature nature suggest that the sediments were deposited close to the source area. The mineralogical and geochemical provenance discrimination diagram suggests contributions from felsic igneous, sedimentary/metasedimentary, and low-grade metamorphic sources, with detritus derived from the Indian craton and proto-Himalaya region. Data suggest moderate to intense chemical weathering, indicative of low relief and a sub-humid to humid climate in the source area. The tectonic analyses indicate that the Bengal Basin transitioned from a predominantly passive margin to an active tectonic margin setting during the Oligocene.

Provenance and tectonic setting of sandstones of the Lomas Coloradas Formation, Cabullona Group, Sonora, México: Constraints on petrography and geochemistry

The sedimentary rocks of the Cabullona Group are well exposed in the Cabullona Basin of northeastern Sonora, México, which represent syntectonic sedimentation associated with Laramide-style deformation during the Campanian−early Maastrichtian. The sandstones of the Lomas Coloradas Formation deposited at the upper part of the Cabullona Group. Petrography and geochemical (major, trace, and rare earth elements) studies were carried out on the sandstones to interpret the palaeoweathering, provenance, and tectonic setting of the Lomas Coloradas Formation sandstones. Most of the sandstone modal analysis data plotted in the QFL and QmFLt ternary diagrams fall into the fields of dissected arcs and recycled orogens, respectively. Geochemically the sandstones are classified as litharenites while few samples plot into the arkosic field. The chondrite-normalized REE diagrams for sandstones have LREE-enriched, relatively flat HREE patterns with negative Eu anomalies. CIA values and the A‒CN‒K diagram indicate a low intensity of chemical weathering in the source area. The tectonic setting diagram suggests both arc and rift settings for the studied sandstones. The bivariate and ternary diagrams and elemental ratios suggest a felsic source for the Lomas Coloradas Formation. The average REE patterns of the studied sandstones are comparable with REE patterns of the granite from the Caborca block, the rhyolite and granite of the Tarahumara Formation, and the Mesozoic felsic volcanic rocks. The modeled mixture of 40% Proterozoic granite, 30% Triassic arc, 10% Jurassic arc, and 20% Laramide arc represents the probable source rocks for the Lomas Coloradas Formation.

The Hydrochemistry Characteristics and Chemical Weathering Intensity of an Anthropogenically Involved Catchment, South China

The hydrochemical characteristics of watersheds are influenced by many factors, with chemical weathering and human activities exerting the most substantial influence. Performing a quantitative evaluation of the factors contributing to the chemical weathering of rocks is of significant scientific importance. This research zeroes in on the Qingtang River basin to elaborate on the hydrochemical characteristic, explore the origins of ions, and quantify the influence of anthropogenic discharges amidst cation interferences, thus improving the accuracy of chemical weathering rate estimations. The samples encompassed surface water, groundwater, and water from dripping in karst caves. The findings indicate that human-induced alterations significantly influence hydrogeochemical dynamics, although chemical weathering of rocks in their natural state is the controlling factor. The mean contributions of cations from atmospheric deposition, human inputs, carbonate weathering, and silicate weathering were 17.56%, 21.05%, 51.77%, and 9.54%, respectively. The chemical weathering rate for carbonate rocks was 62.4 t·km−2·a−1, which increased by 27.87% due to the influence of exogenous acids. The anthropogenic impact is predominantly evident in two aspects: (1) the alteration of hydrochemical processes within the watershed through direct input of ions, and (2) the acceleration of rock weathering rates in the watershed due to the exogenous acids.

Mineralogical Characterization and Geochemical Signatures of Supergene Kaolinitic Clay Deposits: Insight of Ropp Complex Kaolins, Northcentral Nigeria

This study presents the chemical and mineralogical composition of clay deposits and associated rock types within the Ropp Complex in order to assess the influence of parent lithology on the kaolinization, genesis, and utility of the deposit. Representative kaolin samples from E horizons of the weathering profiles and their bedrocks were collected from six sites in the Ropp Complex. Clay mineralogy was determined via the XRD technique, while a geochemical analysis was conducted using XRF, SEM coupled with EDS, and ICP-MS. The results showed that all kaolins dominantly contain kaolinite with a content of 77%–98% except for the AS1 kaolin with only minor kaolinite (20%) but mainly illite (65%). The notably lower crystallinity of kaolinite (HI value of 0.53–1.1) as well as its markedly small grain size is consistent with the formation of kaolinite from intensive chemical weathering of igneous rocks. The AS1 kaolin was probably formed from hydrothermal alteration in the burial stage due to the heating of groundwater by the late volcanism. Mobile trace elements (Sr, Ba, and Eu) exhibited a depletion trend, while immobile elements (Hf, Ta, Th) showed enrichment. The relatively more zirconium in kaolins implies the formation of low-temperature kaolinization. The notably high kaolinite content, accompanied by reasonable levels of Fe2O3 and TiO2, signifies a medium-grade quality. Furthermore, chondrite-normalized rare earth element (REE) patterns exhibit congruent trends in rocks and kaolin samples, indicating a relative enrichment in light rare earth elements (LREEs) alongside a discernible negative Eu anomaly. The abundant kaolinite and silicon–aluminum composition make the kaolins suitable for refractories, pharmaceutics, cosmetics, and supplementary cementitious material (SCM).

Enhanced erosion and silicate weathering of the West African craton during the late Cretaceous cooling evidenced by mineralogical and Hf[sbnd]Nd isotope proxies

The evolution of oceanic temperatures between the Turonian and the K/T boundary indicates a long-term cooling coincident with a decrease of atmospheric CO2 levels, yet the cause of climate cooling at that time still remains debated. In this study, we evaluated the possible implication of enhanced silicate weathering as a sink for atmospheric CO2 by applying paired NdHf isotope measurements to detrital clay records from the West African margin. The use of this novel proxy for chemical weathering intensity (ΔɛHf(t)clay) was complemented by additional mineralogical and major-trace element analyses in order to investigate the variability of mechanical erosion patterns and further explore potential linkages between tectonics, weathering and climate during the late Cretaceous.Our ΔɛHf(t)clay data suggest more intense silicate weathering on the West African Craton during the Santonian to the middle Campanian period, coincident with enhanced physical erosional inputs as inferred from higher Quartz/Clays and Feldspar/Clays ratios. This observation suggests that the shift towards intensified chemical weathering at that time was driven by enhanced mechanical erosion, possibly related to a moderate tectonic event on the West African craton. Evidence for increasing kaolinite contents and higher ΔɛHf(t)clay values during the Maastrichtian point towards more hydrolysing conditions, inducing either destabilization of older Mesozoic lateritic material or favouring the development of kaolinite-rich soils.Overall, this study was compared with several new data of chemical weathering evolution along the south Atlantic margins, adding new insights on tectonic-weathering-climate interactions during the late Cretaceous, suggesting a possible link between silicate weathering feedbacks and global cooling at that time.

Redox changes in the Iapetus Ocean during the Late Ordovician extinction crises

The cause of the Late Ordovician mass extinction (LOME) is widely debated, with glaciation, volcanism and oceanic redox fluctuations being proposed as possible drivers. Here, we apply a multi-proxy approach to deep-water Iapetus Ocean samples from Dob's Linn, Scotland, to determine oceanic redox conditions and changes in chemical weathering intensity. We document major redox fluctuations between anoxic ferruginous and oxic conditions during the first, end-Katian extinction pulse, whereas the end-Hirnantian extinction phase witnessed more persistent anoxia. These two episodes were separated by oxic conditions and a major, short-lived decline in chemical weathering during the Hirnantian glaciation, suggesting that although global cooling may have placed stress on certain biota, it was unlikely to be the cause of the extinction crisis. Late Hirnantian anoxia persisted into the Silurian, with widespread euxinia resulting in global drawdown of redox-sensitive trace metals. Recent studies have identified a mid-Katian biotic crisis and recovery prior to the LOME, although the precise stratigraphic position is not yet defined. The mid-Katian record at Dob's Linn shows a major redox change, with dysoxic to anoxic ferruginous deep ocean waters giving way to well-oxygenated conditions at this time. However, links between the mid-Katian biotic crisis and these redox changes remain unclear.

Climatic and Environmental Impacts on the Sedimentation of the SW Taiwan Margin Since the Last Deglaciation: Geochemical and Mineralogical Investigations

AbstractMany scientific studies have been conducted to constrain present and past source‐to‐sink processes and their controlling factors. The role of typhoon and monsoon rainfall on chemical weathering and soils erosion in east Asia is still not well established. Clay minerals and major elements, combined with Nd and Sr isotopic compositions were analyzed on sediments from Core MD18‐3569 located on the Taiwan margin in the northeastern South China Sea. The aim was to reconstruct the weathering history of small river basins of southwest Taiwan and to establish the impact of East Asian summer monsoon and typhoon rainfall on source to sink processes since the last glacial period. 87Sr/86Sr and ɛNd values and predominant illite‐chlorite indicate that the rivers of southwest Taiwan is the sole source of sediments to Core MD18‐3569 since last deglaciation. Variations in clay mineral assemblages and major elements allowed us to evaluate the intensity of past weathering in the rivers of southwest Taiwan. We demonstrated that long‐term changes of chemical weathering intensity record from Taiwan are driven by changes in the summer monsoon rainfall. The deglaciation is characterized by a progressive increase in the intensity of chemical weathering, which peaked at the beginning of the Holocene. The chemical weathering degree of sediments from Taiwan decreases during the Holocene concurrently with a weakening of the summer monsoon, increase in typhoon activity, and changes in the vegetation cover. All these processes induce soil erosion, regressive pedogenesis, due to shorter residence time of minerals in the soils of southwest Taiwan.

Paleoclimate Controls on West African Dust Inferred from Rb/Sr and Si/Al of Sediments in an Eastern Equatorial Atlantic Marine Core

Increased dust emissions from dryland areas and their effects on human health, ecosystem viability, and environmental change are a global concern in the face of the growing climate crisis. Dust plume emissions from the West African landmass, Sahara, and Sahel areas comprise a major fraction of the global aerosol budget. Dust plume intensity is closely related to regional winds (e.g., Harmattan, Sahara Air Layer), the Intertropical Convergence Zone, monsoonal seasonality, marine currents, and physiography. To study terrigenous material emitted from the continent over the last ~260 kyr (late Quaternary), we used X-ray fluorescence spectroscopy (XRF) to analyze a ~755 cm long marine sediment core from the eastern equatorial Atlantic Ocean, resulting in nearly 1400 discrete measurements. Spectral analysis results suggest that concentrations of elements (Rb, Sr, Si, Al) preserved in the sediments are correlated to different types of orbital climate forcing. Chemical weathering intensity indicated by the Rb/Sr ratio was sensitive to seasonal insolation variations controlled by precession cycles (23–18 kyr), which presumably reflects the relationship between monsoonal rainfall and sensible heating of the continent. Spectral analysis of silicate mineral grain size (Si/Al) showed significant 40 kyr cycles that were paced by obliquity. Based on these data, we infer that winter tradewind activity accelerated in response to the intertropical insolation gradient induced by high obliquity. High Rb/Sr ratios during the last glacial maximum and penultimate glacial maximum may have been due to a predominance of mechanical weathering over chemical weathering under dry/cool climates or the dissolution of Sr-bearing carbonates by corrosive glacial bottom waters.

Geochemical Split Among the Suspended and Mud Sediments in the Nethravati River: Insights to Compositional Similarity of Peninsular Gneiss and the Deccan Basalt Derived Sediments, and Its Implications on Tracing the Provenance in the Indian Ocean

AbstractMajor, trace, and rare earth element (REE) geochemistry of sediments from a tropical mountain river is investigated to understand the behavior of chemical elements during weathering and transportation in a Peninsular Gneissic terrain. The results from this study are compared with the Deccan Basalt‐derived River sediments and the eastern Arabian Sea sediments with an intent to highlight the challenges associated with the provenance determination of sediments along the continental margin of India. The geochemistry of suspended particulate matter (SPM) and mud sediments (<63 μm) of the Nethravati River indicate that sediments are derived from a relatively homogenous lithology, characterized by intermediate rocks of tonalitic (low‐Al TTGs) composition. The tectonic origin of the source rocks discriminated using sediment geochemistry suggests an ocean island arc origin. The sediments experience intense chemical weathering in the source region. The elemental composition and their inter‐element relationships suggest differential chemical weathering of mineral phases fractionate mafic components and their secondary mineral products in SPM, and mixed sources dominated by felsic components and their secondary mineral products in mud sediments. Intense chemical weathering induces significant geochemical splits among the suspended and mud sediments. The transport of mafic‐biased sediments from Peninsular India to Oceans, and the geochemical similarity with Deccan Basalt‐derived sediments makes it challenging to track the Peninsular Gneiss‐derived sediment provenance along the continental margin of India using conventional elemental geochemistry. The inferences from this study have important implications for determining the sediment provenance along the continental margin of India.

Clay minerals control silicon isotope variations of fine-grained river sediments: Implication for the trade-off between physical erosion and chemical weathering

Stable silicon (Si) isotopes in fluvial sediments can provide insights into understanding silicate weathering processes on the Earth's surface. However, a lack of comprehensive studies has hindered full understanding of the factors influencing Si isotope fractionation during continental weathering. In this study, through the analysis of Si isotopes in fine-grained sediments from 13 rivers surrounding the South China Sea, significant variation of Si isotopes in bulk detrital sediments (<63 μm) was observed, with δ30Si values ranging from −0.17‰ to −1.09‰. At basin scale, the δ30Si values are influenced by multiple controlling factors such as climatic conditions, lithology, and tectonic settings, which have a close relationship with the content of clay minerals. The characteristics of weathering types and intensities are ultimately reflected in the weathering products, specifically clay minerals. Compiling data across multiple grain sizes from major rivers globally, robust correlations based on clay mineral classification between δ30Si and Al/Si ratio have been observed, which are unaffected by regional and grain-size variations. As the dominant clay mineral group transitions from illite/chlorite to smectite and kaolinite, the degree of Si isotope fractionation increases progressively. This sequence indicates a shift from stronger physical erosion to more intensive chemical weathering, suggesting a transition in the weathering regime from weathering-limited to transport-limited. This study reveals the intrinsic link between Si isotopic compositions and clay mineral assemblages, providing implications for similar stable isotope research and offering a potential indicator for understanding continental weathering processes and their contributions to the global carbon cycle.

Effect of Depositional Environment and Climate on Organic Matter Enrichment in Sediments of the Upper Miocene—Pliocene Kampungbaru Formation, Lower Kutai Basin, Indonesia

The Upper Miocene–Pliocene Kampungbaru Formation crops out in the easternmost part of the Lower Kutai Basin, Indonesia. The sedimentological analysis of seven outcrops was carried out, and a total of twenty-five samples from these outcrops was analyzed for bulk geochemistry, organic petrography, and bulk and clay mineralogy to assess the effect of the climate and depositional environment on organic matter enrichment. The Kampungbaru Formation consists of interbedded sandstone, siltstone, claystone, and thick coal beds, which were classified into eleven lithofacies. Subsequently, seven facies associations were identified, namely the fluvial-dominated distributary channel, sheet-like sandstone, tide-influenced distributary channel, mouth bar, crevasse splay, delta plain, and delta front. The coal facies generally have a high amount of total organic carbon (TOC, 5.1–16.9; avg. 10.11 wt.%), and non-coal layers range from 0.03 to 4.22 wt.% (avg. 1.54 wt.%). The dominant maceral is vitrinite, while liptinite occurs only rarely in the samples. Organic matter is inferred to have originated from terrestrial plants growing in mangrove swamps. Identified clay minerals include varying proportions of kaolinite, illite, chlorite, and mixed layer illite/smectite (I/S). Kaolinite, which commonly constitutes up to 30% of the clay volume, indicates intensive chemical weathering during a warm and humid climate. In accordance with the Köppen climate classification, the paleoclimate during the deposition of the Kampungbaru Formation is classified as type Af, which is a tropical rainforest. Tropical climate was favorable for the growth of higher plants and deposition of organic matter under anoxic conditions and led to higher amounts of TOC in the Kampungbaru Formation.

Factors responsible for Witwatersrand gold mineralisation

Abstract The exceptional gold mineralisation in quartz pebble conglomerates of the Witwatersrand Basin is attributed to a combination of factors. These factors are linked to the co-evolution of the atmosphere, hydrosphere, lithosphere and biosphere, at a very specific time in Archaean geological history and the evolution of the Kaapvaal Craton. Following craton stabilisation and its subaerial emergence, intense chemical weathering and erosion of large volumes of granitoid-greenstone basement released detrital and dissolved gold. Shallow-marine reworking in a long-lived and slowly subsiding basin subjected to episodic compressional deformation and relative sea-level oscillations led to sedimentary concentration of detrital gold. The interaction between acidic, anoxic, and sulfurous surface runoff and more oxidizing marine waters in a near-coastal oxygen oasis supported microbially mediated diagenetic pyrite formation and incorporation of dissolved gold in the pyrite crystal lattice. Erosion and reworking of diagenetic pyrite gave rise to detrital pyrite that characterise most reefs. Abundance of detrital uraninite in conglomerates, derived from erosion of Mesoarchaean granites, and episodes of hydrocarbon migration through sedimentary strata during deep burial set the scene for further enhancement of gold grades in the reefs. Granular and seam pyro-bitumen formed by radiation-induced polymerisation of hydrocarbons around detrital uraninite. Gold dissolved in migrating hydrothermal fluids was then reduced and precipitated upon interaction with the reef pyro-bitumen during hydrothermal placer modification.