Chemical Index Of Alteration Research Articles

Geochemical Analysis of Paleocene-Eocene Sediments from Subathu Formation, Dehradun District, Uttarakhand: Implication for Provenance, Tectonic Setting and Paleoclimate

The Palaeocene-Eocene Subathu Formation in the Nilkanth region near Rishikesh, Uttarakhand provides insights into the provenance, depositional environment, and tectonic setting prior to final India-Asia continent collision. Geochemical analysis of major oxides, trace elements, and rare earth elements from shale-dominated sedimentary rocks reveals a complex provenance. Discriminant function plots indicate the derivation of sediments from quartzose sedimentary sources, with minor input from mafic igneous sources. Trace element patterns suggest contributions from felsic igneous rocks, recycled sediments, and subduction-related components. The Chemical Index of Alteration (87.2*Avg.) indicates moderate weathering in the source area under semi-arid paleoclimatic conditions. Tectonic discrimination diagrams indicate that the deposition of sediments from Subathu Formation was occurred predominantly in an active continental margin setting. Rare earth element patterns exhibit varying degrees of fractionation, implying diverse sources ranging from mafic to felsic compositions. This multi-proxy geochemical study sheds light on the evolving paleoenvironmental conditions and sediment routing systems in the western Himalayan foreland basin leading up to the terminal India-Asia collision. Keywords: Geochemistry, Subathu Formation, Provenance, Tectonic Setting, Paleoclimate, Depositional Environment, Himalaya

The reasons for the inheritance of the composition and properties of red-colored soils of the modern weathering crust within the western part of the Sichuan Basin of China

In this work, we conducted an experimental evaluation and analysis of the characteristics and makeup of red-colored rocks from four representative Cretaceous and Jurassic sedimentary sequences in the western Sichuan Basin of China, as well as their weathering products. To determine the direction of weathering processes and the cause-and-effect link between the characteristics and composition of the parent rocks and the excess chemical components in the weathered soils, the chemical index of alteration was calculated. The findings demonstrated that, with regard to features like color, density of solid component, organic carbon content, granulometric composition, pH, mineral composition, and content of main chemical elements, all samples of weathered (dispersed) soils are quite comparable to the parent rocks. All samples of weathered soils and parent rocks exhibit similar chemical weathering characteristics and an index, according to the results of the CIAcorr calculation and the examination of the A–CN–K triangle diagrams. Furthermore, all of them have attained a moderate level of chemical weathering, with samples from Jurassic rocks (J3p, J3s, and J2s) having far lower weathering intensities than those from Cretaceous rocks (K1c). The red-colored rocks are characterized by weak chemical weathering and rapid development of physical weathering. This weathering characteristic can be attributed to the parent rock’s uniform texture and mostly clayey composition, which break down readily in environments with high humidity and plenty of heat. The low level of chemical weathering is due to the fact that the parent rock itself has undergone a significant change in chemical composition during sedimentation and subsequent diagenesis, and has acquired a high degree of resistance to subsequent transformation under modern conditions. Therefore, the characteristics and makeup of purple soil are mostly determined by the parent rock.

A geochemical and palynological study of lake sediments at Mboandong, Cameroon: chemical weathering and vegetation linked to Holocene palaeoclimate

Geochemical and palynological analyses of core samples from Mboandong, a crater lake in western Cameroon, provide a record of response to climate and vegetation change in West Africa over the last 7000 years. Three palaeoclimatic phases were identified: a humid phase (6800-5700 cal. BP), characterized by a humid and warm palaeoclimate; a drying phase (5700-2000 cal. BP) with a less humid palaeoclimate; finally, a seasonal phase (2000 cal. BP to present), characterized by a seasonally varied, mainly humid and warm palaeoclimate. Pollen components show swamp and humid rainforest taxa being gradually replaced by semi-deciduous, regrowth and non-tree taxa such as grasses (Poaceae). There is a suggestion of increasing aridity over the last 200 years. The sediments are primarily weathered deposits mixed with volcanic eruptive material, rather than detritus derived from the UCC (upper continental crust). Chemical Index of Alteration (CIA) values are mostly high (mean 90.65%) and indicate a variable but mostly humid palaeoclimate. CIA values are most likely influenced by climate, as well as by runoff and physical erosion. Bands of volcanic ash are associated with frequent freshwater sponge spicules and diatoms, probably due to increased availability of silica. Ca and Na depletion is probably linked with soil leaching, while K depletion and Mg enrichment were probably caused by chemical alteration. Humid intervals may have higher organic carbon content, whereas increased amounts of iron occur in potentially drier intervals. A greatly increased presence of oil palm pollen (Elaeis guineensis) is interpreted as evidence of cultivation after 1700 cal. BP.

Characteristics and Paleoenvironment of the Niutitang Shale Reservoir in the Zhenba Area

The lack of in-depth analysis on the reservoir characteristics and the paleoenvironmental conditions of the Niutitang Formation in the study area has led to an unclear understanding of its geological background. In this study, core samples from well SZY1 were selected, and X-ray diffraction (XRD), scanning electron microscopy (SEM), and quantitative elemental analysis were employed to systematically investigate the reservoir properties and paleoenvironment of the shales. The results indicate that the Niutitang Formation shales form a low-porosity, low-permeability reservoir. By utilizing indicators such as the chemical index of alteration (CIA) and elemental ratios, the study delves into the paleoclimate and paleoproductivity of the region. The (La/Yb)n ratio is approximately 1, indicating a rapid deposition rate that is beneficial for the accumulation and preservation of organic matter. The chondrite-normalized and North American Shale Composite (NASC)-normalized rare earth element (REE) distribution patterns of the shales show consistent trends with minimal variation, reflecting the presence of mixed sources for the sediments in the study area. Analysis reveals that the Niutitang Formation shales are enriched in light rare-earth elements (LREEs) with a negative europium anomaly, and the primary source rocks are sedimentary and granitic, located far from areas of seafloor hydrothermal activity. The NiEF and CuEF values suggest high paleoproductivity, and the shales were deposited in an anoxic-reducing environment. The depositional environments of the Marcellus and Utica shales in the United States, the Wufeng-Longmaxi black shales in the Changning area of the Sichuan Basin, and the shales in the study area are similar, characterized by anoxic reducing conditions and well-developed fractures. The thermal evolution degree of the study area is relatively moderate, currently in the peak gas generation stage, with the reservoir quality rated as medium to high, indicating good potential for hydrocarbon accumulation and promising exploration prospects.

Molecular-level insight into the role of soil-derived dissolved organic matter composition in regulating photochemical reactivity

Soil-derived dissolved organic matter (DOM) links soil and water carbon pools and is an important source of photochemically produced reactive intermediates (PPRIs) in aquatic environments. Despite its importance, the variations in photochemical reactivity of soil-derived DOM molecules in producing PPRIs across broad geographical regions, and the factors driving these variations, remain unclear. Herein, we resolved the apparent quantum yields (Φ(PPRIs)) of hydroxyl radicals (•OH), singlet oxygen (1O2), and excited triplet-state DOM (3DOM*) for irradiated DOM from 22 representative soil reference materials in China, and linked them to soil pH, mineral weathering degree, and DOM characteristics. Generally, the average Φ(PPRIs) values of the soil-derived DOM followed the order of Φ(3DOM*) (1.67× 10−2) > Φ(1O2) (1.47× 10−2) > Φ(•OH) (7.31× 10−5). The DOM from less weathered soils showed higher Φ(•OH) and Φ(3DOM*) and comparable Φ(1O2) than that from more weathered soils. The differences were mainly regulated by the abundance of humic-, lignin-, tannin-, and aromatic-like compounds, as indicated by the correlation and random forest model analyses. Partial least squares and multiple linear regression analyses identified DOM molecular weight, nominal oxidation state of carbon, and soil chemical index of alteration as effective predictors of •OH yields. Soil chemical index of alteration emerged as a prioritized predictor of 3DOM* yields, while the electron-donating capacity and humic-like compound content of the soil-derived DOM were effective predictors of 1O2 yields. This study advances our understanding of how mineral weathering processes regulate the photochemical reactivity of soil-derived DOM in the aquatic environment across wide geographical regions.

Triple oxygen and hydrogen isotopes of glacial diamictites record crustal maturation and changing surface conditions through geologic time

The triple oxygen and hydrogen stable isotopic compositions of 24 glacial diamictite composites with depositional ages from 2.9 to 0.3 Ga are used to reconstruct the evolution of the continental crust and surface environments. The δ18O of the diamictite composites increases from the Archean to the Phanerozoic by ∼4 ‰ (3–5 ‰ in Mesoarchean to 13.5 ‰ in Neoproterozoic), a trend that is comparable to those seen in shales and granites, although the diamictites plot on the lower δ18O end of the range seen in shales. The Δ’17O0.528 decreases from −0.04 to −0.15 ‰, overlapping with shales and granites. Nine zircon separates extracted from individual diamictites also show a ∼3 ‰ increase in δ18O from 3.75 to 4.5 ‰ to 7 ‰ between 2.9 and 0.6 Ga. The δ18O of diamictites and zircon separates negatively correlates with ɛNd(i) of the diamictites, suggesting that the longer the rocks reside in the upper continental crust (the lower the ɛNd), the more they are subjected to repeated weathering cycles, thus acquiring a higher δ18O and lower Δ’17O0.528 during each cycle. On a triple O isotope plot (δ17O vs δ18O), diamictites define an array with a slope of 0.523, analogous to granites and shales.The δD values of nearly all diamictites overlap with the typical mantle and crustal ranges of −58 ± 12 ‰ and − 69 ± 24 ‰, respectively, but are on the lower end of coeval shale values. Water, contained primarily in sheet silicates, ranges from 2.85 wt% (pre-2.4 Ga) and 2.35 wt% (post-2.4 Ga), showing no temporal change, likely reflecting post-depositional reset. The youngest diamictites (0.3 Ga Dwyka Group) exhibit low δD values of −105 to −111 ‰, lower than the typical crustal values, potentially reflecting the near-polar position of South Africa at the time of their deposition, which is corroborated by the reconstructed δ18O value of −26 ‰ based on triple oxygen isotopic systematics.The chemical index of alteration (CIA) allows inverting measured δ18O and Δ’17O values of diamictite composites into a pure weathering product (wp) end member by subtracting values of coeval unweathered crust based on tzircon δ18O. This reveals that the δ18Owp also increases with time, suggesting that the increase in δ18O (decrease in Δ’17O) of the maturing crust is insufficient to fully explain the trends. Therefore, at least part of the increasing δ18Owp likely reflects increasing δ18O of the hydrosphere. Reconstructed surface temperatures around the time of diamictite deposition using δ18Owp and Δ’17Owp display a general decrease since the Archean. Given the diagenetic and post-diagenetic history experienced by the diamictites, the temporal and global trends should be interpreted with caution. Nonetheless, reconstructed δ18Owater shows an increase from −21 ‰ at 2.9 Ga to −11 ‰ at 0.6 Ga. Thus, diamictites, like shales, broadly monitor the evolution of surface conditions on Earth from warmer to colder and from lower to higher δ18Owater values of the hydrosphere.

Intensified continental weathering and reductive surface runoff during the Triassic–Jurassic transition

Abstract The end-Triassic mass extinction (ETE) is characterized by profound declines in biodiversity in both marine and terrestrial ecosystems, widely accepted to have been triggered by the Central Atlantic Magmatic Province (CAMP). In response to the CAMP, global continental weathering intensity and terrestrial redox conditions at the extinction interval may have substantially changed, but these are not fully constrained yet. Here we report high-resolution Fe (δ56Fe) and Zn (δ66Zn) isotope data on sedimentary successions of terrestrial open systems across high-latitude and low- to middle-latitude continental Triassic–Jurassic transition (TJT) sections. Both sections recorded a positive δ56Fe excursion and an increase in chemical index of alteration along with fluctuating δ66Zn values during the ETE–TJT period, indicating enhanced continental weathering. The notable increase of δ56Fe with lower total Fe2O3 contents suggests a reductive continental weathering condition considering that Fe isotope fractionation during silicate weathering requires loss of fluid-mobile, reduced Fe(II). Notably, the positive δ56Fe excursion was concurrent with and proportional to the increasing concentration of polycyclic aromatic hydrocarbons from wildfires in corresponding coal layers, implying that the surface runoff during intensified weathering may have been reduced by biochar from CAMP-triggered wildfires. The reductive terrestrial runoff might have influenced the surface ocean across the TJT to some extent.

Characterization of the Major Elements and Paleoenvironmental Significance of the Shiyang Profile in the Weinan Basin, China

The enrichment and migration patterns of different chemical elements record paleoclimatic information in loess formations. The chemical elemental measurements of 245 samples from the Shiyang profile in the Weinan Region were compared with the geochemical characteristics of typical wind-formed profiles, and the paleoclimatic evolution was discussed. The results showed the following: (1) the standardized curves of the cumulative concentrations of SiO2, Al2O3, and CaO along with the Upper Continental Crust (UCC) in the Shiyang profile exhibited significant similarities with typical wind-formed profiles. This strongly suggests that the Shiyang profile has a wind-formed origin. (2) The mean value of the chemical index of alteration (CIA) of the Shiyang profile is 62.06, indicating that the Shiyang profile has been in the stage of primary chemical weathering. (3) The ratios of K2O/Al2O3, TiO2/Al2O3, and Fe2O3/Al2O3 in the Shiyang profile are comparable to those found in typical wind-formed profiles, suggesting a common source area and supporting the premise that the Shiyang profile is of wind-induced origin.(4) The regional climate has undergone a series of transitions: from a dry and cool phase in the early Holocene to warm and humid yet unstable conditions in the middle Holocene, and returning to dry and cool during the late Holocene.

A Regional Paleoclimate Record of the Tropical Aeolian Sands during the Last Deglaciation in Hainan, China

The KLD segment of the Kenweiyuan section in Wenchang, Hainan, China is a set of aeolian sand deposits of the Last Deglaciation. The chemical element and heavy mineral analysis performed in this study reveals the chemical index of alteration (CIA) in the segment to be as high as 93–95, with all the heavy minerals identified as stable and extremely stable making up 38–45% of the total. Furthermore, the zircon, tourmaline, and rutile content (ZTR index) of the segment is determined to range between 48–71. The (Al2O3 + TOFE)/SiO2 ratios display obvious fluctuations from old to new strata in the segment, with the low values corresponding to Heinrich event (H1), Dansgaard-Oeschager (D-O), and Younger Dryas (YD) and the high values corresponding to Bølling and Allerød. Our study suggests that these fluctuations are attributed to the alternation of the East Asian winter and summer monsoons. Hainan Island is also impacted by the surface ocean climate of the South China Sea, and characteristics of the KLD segment may be connected to the climate changes in the North Atlantic related to the winter monsoon season or westerlies. Furthermore, the segment presents a clear response to millennium-scale climate changes during the Last Deglaciation on Hainan Island. Based on the high CIA values in the KLD segment, and particularly due to the observed stable detrital minerals, the ratios can be linked to the overall tropical climate, indicating a relatively warm tropical climate environment in the Last Deglaciation in Hainan. The high CIA values also reveal the cause of aeolian sand formation under the tropical environmental conditions in the low latitude region of China in the Late Quaternary.

Generation and geochemical characteristics of acid rock drainage (ARD) in Barton Peninsula, King George Island (KGI), maritime, Antarctica

This study deals with the generation, geochemical characteristics, and environmental impacts of acid rock drainage (ARD), a global environmental problem, on the Barton Peninsula. To elucidate the governing processes and to assess the environmental hazards of ARD, we present chemical data from lakes, ponds, and creeks with a wide range of pH values. We also provide mineralogical and geochemical compositions of sediments and bedrocks. Compared to weak-acidic and neutral waters, waters that display typical characteristics of ARD with low pH (3.7 to 4.2), high sulfate (46 to 92 mg/L), and Fe (0.8 to 16.5 mg/L) occur in the northern tip of the peninsula. Acidic waters with the highest cation (e.g., K, Na, Si, and Ca) and anion (e.g., SO4) compositions indicate ARD-enhanced rock weathering in the peninsula. Consistently, quantifying of chemical weathering degree yields the highest chemical index of alteration (CIA) and the mafic index of alteration (MIA) with the lowest ICV values for sediments from the acidic waters. Enrichment factors (EFs) calculated for As, Co, Cd, Cu, Pb, Zn, and Ni indicate severe to minor enrichment for As and Pb metals, respectively in the acidic water-associated sediments. Heavy metal concentrations of acidic waters also display the highest values for the peninsula, with Fe, Cu, and Cd metals exceeding the chronic aquatic toxicity limit (CAT). Therefore, geochemical records of acidic waters and sediments, especially lakes, may help in tracing the long-term environmental impacts of ARD, while sediments obtained from the weak acidic and near-neutral waters, together with water chemistry data, may provide a better representative composition of the bedrocks with neutralizing potential. The data presented here may contribute to predicting the source/s, and extent of future ARD generation in the peninsula, which is likely to be enhanced by increased chemical weathering due to climate warming.

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.

Detrital clay mineral input reconstructed based on weathering records and its influence on organic matter enrichment: A case study of the Paleogene Shahejie Formation in the Dongpu Sag, Bohai Bay Basin

As major components of shales, clay minerals are important for investigating unconventional petroleum geology. However, there are few quantitative methods for investigating possible inputs of clay minerals during sedimentary processes. In this study, shales from the Shahejie Formation in the Dongpu Sag, Bohai Bay Basin, were chosen for investigating clay mineral inputs. The shales are abundant in a variety of clay minerals. The shales, uniformly derived from felsic igneous rocks, underwent mild K metasomatism. Thus, the detrital clay mineral input was primarily influenced by weathering intensity. Accordingly, representative samples subjected to varying weathering intensities were selected to identify detrital and diagenetic clay minerals. Kaolinite and chlorite are of detrital origin. Illite and illite/smectite mixed layers (I/S) have both detrital and diagenetic origins. The precursor minerals of diagenetic minerals can be identified based on their morphological and elemental characteristics. After quantifying the area proportions of various clay minerals, the detrital clay mineral compositions of selected samples were calculated. Further analysis indicated that the detrital illite content and the detrital chlorite content are closely related, while the detrital smectite content correlates well with the chemical index of alteration (CIA). Based on the preceding analysis, the fitting formulas were established to quantitatively characterize the input of detrital clay minerals. The shales from the north sag had a high input of detrital smectite, whereas the shales from the south sag had a high input of detrital illite. Consequently, the shales from the north sag contain more organic matter than those from the south sag.

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.

Recognizing microbial manganese reduction in lacustrine carbonate and its linkage to terrestrial biogeochemical processes

Carbonate authigenesis, the in situ precipitation of carbonate minerals within the sediment porewaters, is a key pathway for carbonate deposition and plays a crucial role in global biogeochemical cycles. Heterotrophic microorganisms are essential in regulating authigenic carbonate formation, primarily through the consumption of organic matter. Although bacterial manganese reduction is known to influence the formation of rhodochrosite and dolomite, its role in limestone deposition is unclear. Here, we present a systematic investigation of mixed calcareous siliciclastic rocks from a Paleogene freshwater lake to identify the formation of authigenic carbonate, decode the role of microbial Mn reduction, and understand the microbial response to ancient lacustrine environmental changes. The positive correlation between carbonate fraction in bulk samples (Carb%) and Mn content in carbonate minerals (Mncarb) suggests that carbonate precipitation is stimulated by Mn2+ enrichment. The dissimilarity between Mncarb and Fecarb, along with the synergic variations of Mncarb and diagenetic indicators, support an authigenic rather than a hydrogenetic origin for the carbonates. Using a one-dimensional diffusion–advection-reaction model, we quantify the impact of Mn reduction on promoting carbonate precipitation. Furthermore, correlations between Pcarb and other values–positive with the chemical index alteration (CIA), negative with Mncarb, and none with TOC–suggest that nitrogen availability, regulated by continental weathering, is likely the primary factor limiting both the primary productivity and the bacterial reduction intensity at the study site. Overall, this study uncovers the role of microbial Mn reduction in stimulating authigenic carbonate precipitation, and reveals the modulation mechanism of Mn-reducing microorganisms in an ancient lake. These findings shed new light on the authigenic limestone formation mechanisms and provide a new perspective on interpreting the authigenic impacts on carbonate chemistry.

Cenozoic tectono-sedimentary evolution of the external rif chain (Morocco) derived from mineralogical and geochemical analysis of mudrocks

A model of the Cenozoic tectono-sedimentary evolution of the External Rif Chain (Morocco) is provided by means of the study of the mineralogical and geochemical composition of mudrocks. To date there was a lack of homogeneous data and of a complete and extensive study of the whole External Rif Zone (ERZ). Therefore, this work shows the study of the whole ERZ where the most representative stratigraphic sections have been selected. This work provides important information about the geodynamic evolution and the variations in source-area provenance related to the growing of the Rif orogenic belt. Although there is still much work to be done, this study aims to improve the knowledge of the Cenozoic tectono-sedimentary evolution of the entire western ERZ with a homogeneous method, with a focus on the paleogeographic and paleotectonic evolution, the paleoweathering and the source areas deduced from mineralogical and geochemical data of the Cenozoic mudrocks. The bulk mineralogy is mainly characterized by the presence of calcite, quartz and dolomite plus ankerite. Feldspars have few percentages. The clay minerals are principally represented by mixed-layer illite/smectite (I/S). Illite and kaolinite are in little amount. Femic minerals, mixed-layer chlorite/smectite (C/S) and chlorite are the most abundant. The I/S features suggest a different thermal condition for the three domains. The chemical composition indicates that the mudrocks can be described as mixtures of carbonates with aluminosilicate components. The Al/Ti, Th/Cr, Th/Sc, La/Th and La/Sc ratios, the Cr/V vs. Y/Ni plot, the V-Ni-Th∗10 and La-Th-Sc ternary diagrams indicate a predominantly felsic source with a minor mafic input more evident in the Paleocene-Eocene samples of the External Intrarif and Mesorif. The External Rif Zones changed in the Cenozoic from a passive margin to a complex foreland system with the incoming of the Alpine tectonic phases. In general, the felsic contribution should be linked to the foreland area consisting in the Middle Atlas and Mesetas massifs made of a crystalline domain. This margin probably presented an intermediate narrow oceanic branch in the External Intrarif-Mesorif boundary that surprisingly should start to close during Paleogene times providing the mafic contribution. This Paleogene tectonic activity in these domains is corroborated by the thermal maturity indicating late diagenesis. The chemical weathering indices, such as the CIA (Chemical index of Alteration) and its modifications, show medium-high values and thus suggest generally moderate paleoweathering conditions in agreement with the predominant amount of I/S.

Geochemistry of fine-grained sedimentary rocks: Implications for source area, paleoweathering, paleoclimate, and provenance of the Santa Maria Formation (Triassic, Paraná Basin, Brazil)

The integration of ICP-OES, XRD, chemical analysis, and scanning electron microscopy was used to investigate the source area, weathering, paleoclimate conditions, and provenance of the siltstones of the Santa Maria Formation (Middle to Upper Triassic, Southern Brazil). This unit is regionally subdivided, from base to top, into the Passo das Tropas Member (Late Anisian–Middle Ladinian) and the Alemoa Member (Middle Ladinian–Carnian). The Chemical Index of Alteration (CIA) was calculated for both members to highlight the weathering processes of primary minerals. The CIA results indicate moderate to extreme weathering of the source rocks for the siltstones. The extreme chemical alteration observed in the Passo das Tropas siltstones (CIA = 76.97–90.83) and in the mid-section of the Alemoa Member (CIA = 87.96–89.63) suggests weathering under more humid conditions, related to an increase in paleorainfall. Paleoclimate was interpreted as semi-arid to arid, with alternating wet and dry seasons. These results align with the geochemical behavior of uranium and thorium. Samples with higher CIA values show higher Th/U ratios, indicating a shift toward a more humid climate. X-ray diffraction analysis of the fine fraction (FF < 2 μm) of the samples identified mixed-layered illite-smectite (I/S) clay minerals with minor amounts of quartz. Regarding provenance, Th/U ratios coupled with Th vs. Sc and La-Th-Sc plots, suggest an upper crustal protolith for the southern Santa Maria siltstones, consisting of uplifted rocks from the crystalline basement of the Sul-Rio-Grandense shield and sedimentary rocks of the Camaquã Basin. The Th/Sc, Sc/Th, Co/Th, and La/Sc ratios, along with the contents of Al₂O₃, SiO₂, TiO₂, Co, V, Ni, and Sc, indicate a more felsic composition for the Passo das Tropas Member. Meanwhile, the source rocks of the Alemoa Member sediments demonstrate a more mafic composition in the mid-section, transitioning to a felsic composition in the uppermost succession.

Weathering and pedogenesis of the Holocene aeolian loess-paleosol sections in the Guanzhong Basin on the Chinese Loess Plateau

The aeolian loess-paleosol sections on the Chinese Loess Plateau (CLP) represent invaluable archives for elucidating the dynamics of the East Asian Monsoon (EAM) across Quaternary glacial-interglacial cycles. In this study, a Holocene aeolian loess-paleosol section in the Guanzhong Basin on the CLP was meticulously investigated to assess the intensity of weathering and pedogenesis, and to discern its paleoclimatic implications. Through comprehensive analysis of chemical element composition and micromorphology, it was revealed that the majority of stratigraphic units within the Holocene aeolian loess-paleosol section exhibit indications of weak weathering and pedogenesis, while the mid-Holocene paleosol approaches to moderate weathering and pedogenesis. Hierarchically, the intensity of weathering and pedogenesis among the stratigraphic units can be ranked as follows: paleosol > topsoil > recent loess > transitional loess. Furthermore, the chemical index of alteration (CIA) values of the mid-Holocene paleosol exhibit a gradient decline from southeast to northwest, aligning with present-day temperature and precipitation distributions under the predominant influence of the East Asian summer Monsoon (EASM). Thus, it can be inferred that the robust impact of the EASM on weathering and pedogenesis of paleosol was pronounced during the mid-Holocene Climatic Optimum. These results are of significance in exploring the Holocene weathering and pedogenesis and paleoenvironmental evolution on the CLP.

Constraining the Effect of Climate and Rock Porosity on Weathering Extent in the Volcanic Island of Santa Cruz (Galápagos, Ecuador)

AbstractVolcanic soils are among the most productive soils in the world as they can accumulate large amounts of organic carbon and nitrogen and have good water storage capacity. They are extensively used worldwide for agriculture, which makes it difficult to study the soil‐landscape dynamics under natural conditions. By working in the Galápagos Islands, a UNESCO World Heritage Site, we aimed to constrain soil development over millennial timescales using empirical data. Our monitoring sites on Santa Cruz Island cover a 10 km long NW‐SE transect with an 8‐fold increase in precipitation and associated vegetation changes. By controlling for age and chemical composition of the basaltic parent material, we investigated the influence of precipitation rates on soil weathering. At the landscape scale, soil weathering degree increased with increasing precipitation, as shown by the spatial patterns in soil depth, pH, mass loss coefficients, chemical index of alteration, chemical depletion fraction, and total reserve in bases. In addition to the climatic effect, rock porosity strongly enhanced basalt weathering. Porosity‐enhanced weathering is particularly important in the humid and perhumid precipitation regimes: soils developed on porous scoriae developed weathering mantles that are ∼10‐fold thicker and have 10‐fold higher mass losses due to weathering compared to soils developed on basalt lava flows. Our results demonstrated that variations in rock pore dimensions and distribution can lead to large variations in basalt weathering rates, particularly in humid and perhumid climates where deep leaching can be facilitated by rock porosity.

Provenance, Intensity of Weathering, and Economic Potential of Placer-bearing Sediments on the Shelf off Quilon, Southwestern Coast, India

ABSTRACTIn this study, the mineralogy and geochemistry of sediments of the continental shelf off Quilon, southwest coast, India is used as proxies to constrain their provenance, the intensity of weathering at source, and economic potential. The weathering index calculations were carried out on the sediments and the results showed that Chemical Alteration Index (CIA) (Average = 72.88), Chemical Index of Weathering (CIW) (Average = 92.72), and Plagioclase Alteration Index (PIA) (Average = 89.74) are high, indicating moderate to intense source area weathering. High source area weathering is also supported by high Th/U ratio (Average = 5.68) which is above the average crustal Th/U ratio of 3.8. The sediments are first-cycle sediments derived from the nearby source area, as evidenced by the identification of unaltered heavy minerals and rock-forming minerals, along with a high Average Composition Variability Index (ICV) (Average = 1.20). The high K2O/Na2O ratios (Average = 3.88), and LREE/HREE ratios (Average = 20), plotting of samples in the felsic field of various binary plots (SiO2 vs. Al2O3/TiO2, Zr vs. TiO2, Ce vs. La/Yb and La/Th vs. Hf binary plots) and normalized REE patterns with LREE enrichment and negative Eu anomaly indicate the sediments were primarily derived from Khondalites of the Trivandrum block and were transported by the Kallada and Ithikkara rivers. Average Σ REE, Th, and U concentrations are 222 ppm, 15.2 ppm, and 2.58 ppm respectively in the bulk sediments. The heavy mineral separates showed significantly increased concentrations of Σ REE (Average = 3009 ppm), Th (Average = 478 ppm), and U (Average = 30 ppm) due to the presence of monazite. Due to high Σ REE, Th, and U concentrations in the sediments due to monazite, present study recommend for further exploration of these sediments, which could contribute significantly to the resource base of the nation.