Low Chemical Index Of Alteration Research Articles

Tungsten stable isotope composition of the upper continental crust

Tungsten (W) stable isotope data for twenty-four composites made from the fine-grained matrix of glacial diamictites, deposited between the Mesoarchean and the Paleozoic, as well as two Archean tonalite-trondhjemite-granodiorites (TTG) are used to refine the W isotope compositions (expressed as δ186W deviations from the NIST 3163 W standard) of the upper continental crust (UCC). To do this we evaluate W isotope fractionation in the diamictites as the result of both magmatic and low-temperature processes. The δ186W values of the diamictites are heterogeneous (0.017 ± 0.011 ‰, 2SE to 0.182 ± 0.007 ‰, 2SE), encompassing the range of previously published values for igneous rocks. We find that δ186W correlates positively with the diamictite’s chemical index of alteration (CIA), a measure of the degree of chemical weathering in the provenance, suggesting that the continental regolith (i.e., surface material composed of weathered residues) is isotopically heavy for W. Since W in rivers and ocean water is also isotopically heavy, this suggests that equilibration with Fe-Mn-oxides controls the W isotope composition of surface waters and that such oxides are not important constituents of the regolith, whose W is likely accommodated in clays. Using diamictites with low CIA (<60, n = 9), we calculate an average δ186W of 0.046 ± 0.036 ‰ (2SD) for the UCC, from 2.3 to 0.3 Ga. This average is combined with the TTG data of this study, as well as data from the literature for upper crustal rocks to obtain a representative δ186W value of the UCC of 0.046 ± 0.046 ‰ (2SD). The W stable isotope composition of the UCC is lighter than that of mantle-derived melts (MORB and OIB, with an average δ186W = 0.082 ± 0.026 ‰, 2SD), and much lighter than the weighted average δ186W of intra-oceanic arcs (δ186W = 0.104 ± 0.052 ‰, 2SD), though the spread in the arc rocks is large. The significant difference in δ186W between average intra-oceanic island arc lavas and UCC is intriguing given that continental crust is thought to form primarily via arc magmatism. This difference suggests that not all modern intra-oceanic arcs are representative of those that formed the continental crust. Rather, only arcs that are enriched in incompatible trace elements, which also tend to have lighter W isotope compositions, may have been involved in making new continental crust. Alternatively, there may have been a secular change in the W isotope composition of arc magmas.

Texture and major element geochemistry of channel sediments in the Orsang and Hiren River Basins, Gujarat, India: Implications for provenance and weathering

Size, shape, degree of sorting, and composition of sediments in the river channels are controlled by climate, lithology, weathering, sorting, and medium of transportation. The present investigation is focused on the grain-size and geochemical analysis of the channel sediments of the Orsang and Hiren river basins. Major outcrops in the study area are Archaean granites, granitic gneisses, Upper Cretaceous to lower Eocene Deccan Volcanic Basalts (DVB), Quaternary sediments and minor proportion of Proterozoic low grade metamorphic rocks. The sediments are poorly to moderately sorted, very finely skewed, suggesting its derivation from heterogenous sources, while the kurtosis value indicates a high-energy depositional environment. The sediments are with gravelly sand texture and the mean grain size is varying from 581.9µm to 1284.2µm. The DVB provenance of the Hiren river basin and granitic provenance of the Orsang river basin is clearly reflected in the texture and geochemical composition of sediments. The TiO2 and Fe2O3 contents of sediments from the Hiren river basin are distinctly higher and are comparable to the basalts of the Saurashtra region of the Deccan Province. Sediments collected after Orsang and Hiren rivers confluence and from Narmada river show higher concentration of felsic sources, indicating that Orsang river's sediment supply significantly outweighs Hiren rivers. The arkosic-litharenite nature points towards less transportation and moderate chemical weathering for the Orsang river sediments. The low Chemical Index of Alteration (CIA) values (Avg. 48.45 and 56.99 for Orsang and Hiren rivers, respectively) and A-CN-K plot also suggest the supply of sediments from minimally weathered detritus under a semi-arid condition.

Prodigious shift in provenance across Permian-Triassic Boundary at Guryul Ravine Section, Kashmir, Tethys Himalaya, India: Evidences from Sr and Nd isotopes

The Permian-Triassic Boundary (PTB) section (Late Permian Zewan Formation and Early Triassic Khunamuh Formation) at Guryul Ravine (Kashmir, India) preserves potentially important information about the depositional conditions and provenance changes across the PTB. Several palaeobiological, sedimentological and geochemical studies have been conducted on this archetypal section to understand the causes of mass extinction at the PTB, however no integrated chemo-stratigraphic study focusing on provenance and geodynamics has been attempted. In the present study, samples from a 36 m thick interval across the PTB are analysed for major and trace element concentrations, Sr-, Nd- and Corg isotope geochemistry along with TOC (total organic carbon) contents to understand the contemporary environmental and provenance changes. The Late Permian Zewan Formation have relatively low chemical index of alteration (CIA) values (55–77) and low αAlCa (0.1–12.6), αAlNa (0.7–9.6), αAlMg (1.0–6.1) values, implying moderate weathering conditions; whereas the Early Triassic sediments of overlying Khunamuh Formation show higher CIA (69–79) and αAlCa (1.4–14.0), αAlNa (2.2–11.5), αAlMg (2.0–6.4) values indicating more intense chemical weathering conditions. The vertical profiles of δ13Corg, δ34Spyr and various redox-indicators suggest euxinic-anoxic condition during the latest Permian, and show a significant change in depositional conditions from anoxic to oxic across the PTB. The significant finding of the present study is the dramatic change of Sr- and Nd isotopic compositions across the PTB, indicating remarkable shift in provenance. The Late Permian samples show much less-radiogenic ɛNd(0) (avg. –15.27), radiogenic 87Sr/S6Sr(0) (avg. 0.72409) and older TDM ages (1.35–1.56 Ga) compared to the early Triassic samples (avg. εNd(0) = −9.49,87Sr/S6Sr(0) = 0.719282, TDM = 1.71–1.76 Ga). The εNd(0) and 87Sr/S6Sr(0) composition of Late Permian rocks overlap with the signatures of Tethyan Himalaya and Neoproterozoic-Cambrian Marwar Supergroup of NW India, indicating similar old cratonic sources. Higher εNd(0), younger TDM ages and lower 87Sr/86Sr(0) in Triassic samples suggest contribution from juvenile sources and Panjal volcanics, Cadimian arc of Cimmeria or African-Nubian shield could be the likely source terrains.

Petrography and geochemistry of Late Cretaceous clastic rocks in the northern Songliao Basin, north‐eastern China: Implications for provenance and weathering

Petrography and geochemical data of the Late Cretaceous clastic rocks in the northern Songliao Basin were used to infer the provenance and chemical weathering history. Petrographic data indicates that the composition of the sandstones is mainly quartz (34.48–50.43%), feldspars (14.16%–24.07%) and lithic fragments (33.91%–45.9%). Similar REE patterns and trace element ratios suggest that both the sandstones (LaCN/YbCN = 12.97; Eu/Eu* = 0.81; Th/Sc = 1.10) and mudstones (LaCN/YbCN = 10.02; Eu/Eu* = 0.71; Th/Sc = 1.10) were derived from a similar source area with felsic volcanic rocks as dominant contributors. The main provenance area is probably the eastern Zhangguangcai Range, the north‐eastern Lesser Xing'an Range and the south‐eastern Songliao Basin. A modelled mixture composed of 65% granodiorite in the Lesser Xing'an Range, 30% granite in Zhangguangcai Range and 5% epidiorite in the south‐eastern Songliao Basin, can be interpreted as the potential source compositions for the clastic sediments. This conclusion is reinforced by the major element of geochemistry composition and sandstone petrography. Relatively low Chemical Index of Alteration (CIA) values in mudstones (48.14–69.26) and sandstones (50.78–59.96) indicate mild chemical weathering conditions in the source area. The chemical weathering history suggests that a cold and arid climate prevailed during the Late Cretaceous.

Geochemistry and zircon geochronology of the Late Triassic volcanic–sedimentary successions in northern Tibet: Implications for the provenance and tectonic evolution of the Mesozoic Qiangtang Basin

The Late Triassic volcanic–sedimentary successions deposited in the Qiangtang Basin are linked with the initiation of the Mesozoic Qiangtang Basin. However, the nature of this basin is hotly debated. Here, we synthesize petrographic, mineralogical, geochemical, and detrital zircon U–Pb age data of these volcanic–sedimentary successions to constrain their chemical weathering history, provenance, and tectonic setting and to reconstruct the tectonic evolution of the Mesozoic Qiangtang Basin. The chemical weathering indices, especially the chemical index of alteration (CIA), are controlled by grain size and hydraulic sorting rather than sediment recycling. Low compositional and textural maturities, high index of compositional variation (ICV) values, and low CIA and plagioclase index of alteration (PIA) values in most samples suggest that the sediments from the Fanghu and north Woruoshan sections are predominantly from low-maturity sources that may have experienced a weak chemical weathering history. Based on provenance-sensitive geochemical indices (La/Th versus Hf, La/Sc versus Co/Th, and Sc/Th versus Cr/Th) and petrographic features, the detrital contributions to the Nadi Kangri Formation siliciclastic rocks are predominantly proximal felsic rocks. Detrital zircon age spectra suggest that the Nadi Kangri Formation siliciclastic rocks exhibit a provenance change from the Fanghu to the well QK-1 area, although all of the rocks were derived from the Qiangtang terrane itself. The Fanghu section was primarily sourced from synchronous magmatic rocks and pre-Triassic strata from the North Qiangtang and Central uplift belt, while well QK-1 was primarily sourced from recycled sediments from the South Qiangtang and Central uplift belt. Therefore, we propose that the Nadi Kangri Formation volcanic–sedimentary successions were deposited in a rift basin and that its evolution was likely related to the interaction of the rapid expansion of the Meso-Tethys Oceanic crust, collapse of the collisional orogen, and rollback of the Jinsha Paleo-Tethys subducting slab during the Late Triassic.

Petrological and geochemical characteristics of the Ordovician–Silurian black shale in eastern Sichuan and western Hubei, South China: Differential sedimentary responses to tectonism and glaciation

During the Ordovician–Silurian transition period, the Yangtze area experienced significant environmental changes, which were recorded by the widely distributed black shale of the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations. However, the petrological and geochemical analyses of the Upper Ordovician-Lower Silurian black shale in Well JY1 (eastern Sichuan Basin) and Well YY2 (western Hubei Province) drill cores show that there are some significant differences with respect to lithology, thickness, and geochemical characteristics despite the long-term similar variation trend in petrology and geochemistry. This may suggest some major environmental changes and their differential influence on black shale deposition in the Yangtze area. Based on the analyses of lithology, total organic carbon (TOC), MoEF, and UEF, four long-term sedimentary cycles (one for the Wufeng Formation and three for the Longmaxi Formation) were identified in Well JY1. In Well YY2, only three long-term sedimentary cycles can be recognized (one for the Wufeng Formation and two for the Longmaxi Formation). Different from the gradual increase of the geochemical proxies (such as TOC, MoEF, and NiEF) followed by an abrupt decrease in the Wufeng Formation, these proxies of the Silurian sedimentary cycles in both wells exhibit an abrupt increase at the base followed by a gradual decrease, showing that controlling mechanisms for the Lower Silurian black shale deposition in the study areas were different from those of the Upper Ordovician black shale. However, the black shale of each Silurian cycle in Well JY1 is much thicker than that in Well YY2. Compared with the thick silty mudstone developed in Well JY1 during the Rhuddanian–Aeronian transition, there is a stratigraphic gap in Well YY2 except for a thin calcareous dolostone formed in a strongly restricted environment. These differences indicate that tectonism and glacial eustasy should be the key factors for black shale development, but their influences on the deposition of the Lower Silurian black shale varied between eastern Sichuan Basin and western Hubei province. Since the early Rhuddanian stage, continuous tectonic subsidence in eastern Sichuan Basin promoted the development of thick black shale, while a persistent tectonic uplift in western Hubei Province led to the formation of thin black shale. Meanwhile, the study areas experienced a significant sea-level drop related to intensified glaciation during the Rhuddanian–Aeronian transition, which is recorded by the low chemical index of alteration (CIA) values in both wells. This resulted in the formation of a stratigraphic gap in western Hubei Province and the deposition of thick silty mudstone in eastern Sichuan Basin. Moreover, abrupt increases in CIA, TOC, MoEF, and NiEF observed in the lower part of the Aeronian black shale in the two wells suggest that eastern Sichuan Basin and western Hubei Province experienced a significant sea-level rise during the early Aeronian stage, which may be related to a global warming event. Subsequently, eastern Sichuan Basin and western Hubei Province both suffered a significant tectonic uplift related to the Kwangsian Movement leading to the termination of the Aeronian black shale formation. However, the thickness variations of the Aeronian black shale in the two study areas indicate that remarkable geomorphological differences persisted in these two areas until the middle Aeronian stage. Thus, this study not only reveals differences of the Upper Ordovician-Lower Silurian black shale in eastern Sichuan Basin and western Hubei Province, but also interprets the causes of these differences, which provide new data and perspectives for the understanding of global paleoclimate and sea-level changes during the Ordovician–Silurian transition.

Relationships between geochemical properties and microbial nutrient acquisition in tropical forest and cropland soils

Land-use conversion can profoundly modify geochemical and microbial properties that drive organic carbon (C) dynamics in tropical soils. However, it is unclear how microbes adjust nutrient acquisition strategies to changing geochemical properties across deeply weathered soils developed from geochemically contrasting parent material. Here we show that along a geochemical gradient in forest and cropland, a proxy variable, the chemical index of alteration (CIA), is sufficiently sensitive to reflect geochemical controls on microbial nutrient acquisition in tropical soils. We found that the CIA negatively correlates with rock-derived nutrient concentrations and pH, but positively with clay content, iron oxides, as well as total iron, aluminum, and manganese concentrations. Thus, using the CIA, which integrates effects of soil fertility and C stabilization by minerals, reduced the complexity of relating microbial nutrient acquisition to geochemical soil properties. Effects of the CIA on microbial C and phosphorus (P) acquisition were stronger in cropland than in forest soil. Microbial nutrient acquisition strategy shifted with increasing CIA from predominating C demand to P. Changes in soil properties at higher CIA (less rich in rock-derived nutrients) were favorable to fungi, which pursue a conservative nutrient allocation strategy to cope with acidic and nutrient-depleted soil conditions, reducing C loss through respiration. In low CIA soils (more rock-derived nutrients), bacteria-dominated communities increasingly invested in C acquisition at the expense of the community biomass, with subsequent greater C loss through respiration. We conclude that microbial communities adapt nutrient acquisition strategies to changing geochemical soil properties in a way that might affect C input versus C storage and release in tropical soils. Here, high soil fertility may favor plants to build up more C in biomass, thus increasing C input, whereas, it may also favor the establishment of microbial communities whose nutrient acquisition and allocation strategies limit long-term soil C storage.

Luminescence chronology of loess-palaeosol deposits in the Central Shandong Mountains region: Provenances and paleoclimate implications

The loess-palaeosol deposits of the Central Shandong Mountains (CSM) to the east of the Chinese Loess Plateau (CLP) potentially provide valuable archives for the reconstruction of East Asian monsoon patterns. However, compared to the abundant attention given to the loess layer, fewer studies have explored the palaeosols documenting the processes and characteristics of interglacial climate changes. The high-resolution chronologies and provenances of the palaeosol in the CSM area are still unclear. In this work, the luminescence ages and paleoclimate proxies in the Shaozhuang (SZ) and Focun (FC) sections were studied, by combining detrital zircons U–Pb ages from the loess-palaesosol in Jingbian, Licheng, Focun, Pianguan and Dongming Yellow River sediments. Quartz single-aliquot regenerative dose protocol (SAR) and K-feldspar post-infrared IRSL (pIRIR 290 ) dating results were obtained in the SZ (8.0 ± 1.1 ka −50.8 ± 2.7 ka) and FC (3.8 ± 0.3 ka—144.0 ± 7.8 ka) sections to develop the most detailed CSM region chronologies to date. The analyzed grain sizes and detrital zircon U–Pb ages suggest that the provenance of the CSM loess was dominated by local Yellow River sediments. The palaeosols observed in the field in these two sections were composed of both aggradation soils deposited in the interglacial period and non-aggradation soil formed by the weathering and leaching of the underlying loess. We found evidences for the presence of non-aggradation soils as indicated the relatively high 5–16 μm particle percentages, relatively low chemical index of alteration (CIA) values and the percentages of >63 μm particles compared to those of the overlying palaeosol layers. Nevertheless, the loess-palaeosol deposits in the CSM are still the product of the East Asian monsoon and global climatic variations, as the deposits have recorded the glacial-interglacial cycles.

Controls on organic matter accumulation in marine mudstones from the Lower Permian Zhanjin Formation of the Qiangtang Basin (Tibet), eastern Tethys

Organic-rich mudstones in the Lower Permian Zhanjin Formation may be primary source rocks in the Qiangtang Basin. However, the factors controlling organic matter accumulation remain unclear. Based on total organic carbon (TOC), geochemical, and mineral data, the paleoclimate, redox conditions, paleosalinity, hydrothermal activity, and primary productivity of the Zhanjin Formation mudstones (well QZ-5) were studied to discuss the organic matter accumulation mechanism. The Zhanjin Formation mudstones at 688-675 m and 672–622.8 m have higher TOC contents (avg. 1.52%) than those (avg. 0.94%) at 674-673 m and 662–659.5 m. Medium chemical index of alteration (CIA) values of samples at 688-675 m and 672-668 m indicate a warm and humid climate, whereas those at 674-673 m and 666–659.5 m indicate rapid cooling due to low CIA values. High Sr/Ba and S/TOC ratios indicate a high-salinity water column during the Zhanjin Formation mudstones deposition. Fluctuating UEF and MoEF values and low to medium Th/U, V/Cr, and Corg/P ratios suggest that these studied mudstones formed in an oxic to dysoxic environment. Relatively low Al/(Al + Fe + Mn) ratios, positive Eu anomalies, and Fe–Mn-(Cu + Co + Ni) × 10 and Co–Zn–Ni ternary diagrams indicate that these mudstones were influenced by atypical (weak) hydrothermal activity associated with relatively high femic terrigenous detrital influx. Relatively high P/Al ratios reveal that the level of primary productivity in the surficial water column was relatively high, but high Ni/Al and Cu/Al ratios may be influenced by paleoclimate and hydrothermal activity. Good correlations among the TOC concentrations and paleoclimate, detrital influx, and hydrothermal activity indicators combined with poor correlations between TOC and primary productivity, paleosalinity and redox proxies demonstrate that organic matter accumulation in the Zhanjin Formation mudstones was mainly controlled by detrital influx (adsorption of clay minerals) and hydrothermal activity, but not by primary productivity levels and preservation conditions such as paleosalinity and redox conditions.

Geochemical characteristics and the constraints on paleoenvironment, provenance, and tectonic setting of Precambrian Xifangshan Formation in the northwestern Tarim Basin, NW China

Combining geochemistry and petrology to clarify paleoenvironment, provenance and tectonic setting is the most reliable approach. Few studies have been conducted on the Xifangshan Formation, a set of turbidite deposits from the Precambrian in the northwestern Tarim Basin, NW China. The tectonic setting during this period is not clear. This study reports the petrological characteristics and geochemical data of the Xifangshan sandstones, aiming to constrain the paleoclimate, paleoredox conditions, paleoweathering, provenance, and finally establish the tectonic development pattern of this period. Xifangshan Formation is mainly composed of litharenite and feldspathic litharenite. Most of the trace elements (Rb, Sr, Pb, U, Y, Sc, V, Cr, Cu, Co, Ni, Zn) are depleted relative to the upper continental crust. And, the rare earth elements show a slight negative Eu anomaly. The sandstones were first-cycle, displaying immaturity of composition and structure. The major element composition and low chemical index of alteration indicate that the sandstones have experienced weak weathering during transportation. Multiple geochemical indicators show that the Xifangshan Formation was deposited in an oxic-suboxic marine environment with moderate salinity and semiarid climate. Based on the geochemical and petrographic characteristics of the rock fragments, the source of the Xifangshan Formation has a felsic provenance, and the parent rock is predominantly acidic magmatic rocks, with a small amount of intermediate magmatic rocks. Influenced by the subduction of the ancient oceanic plate, the blueschist-bearing Aksu Group and the continental margin magmatic arc were formed. This magmatic arc provided the source for the Xifangshan Formation sandstones.

Geochemical comparisons of weathering, provenance and tectonics in the fluvial sediments from Yarlung Zangbo to Brahmaputra River

Globally, large river sediments record crucial signatures of geology and climate change from source to sink. The Yarlung Zangbo River (YZR) is upstream of the Brahmaputra River (BJR). It originates in the active Himalaya-Tibetan Plateau (HTP) and finally enters the Indian Ocean. Therefore, YZR-BJR sediments are best for studying Earth surface processes and climate variations in Asia. However, few comparisons of their sediment chemistry have been conducted with respect to the weathering, climate, provenance and tectonic setting. Here, we examined 80 fluvial sediments measuring major and trace elements with chemical indicators. The YZR-BJR sediments are derived from a felsic source on an active continental margin with a continental island arc. Both the Lhasa terrane (LT) and Himalayan orogen are possibly the main sources of upstream (YZR) sediments that are chemically close to the upper crustal continent (UCC). In contrast, downstream BJR sediments, such as the post-Archean Australian Shale (PAAS), are predominantly sourced from the eastern Himalayan orogen, followed by LT. Due to the uplift of HTP and global cooling, the YZR sediments underwent low to moderate silicate weathering, while the downstream BJR sediments experienced weaker weathering processes with lower chemical index of alteration (CIA) values. This means dominant physical weathering and erosion in the source area and thus provides substantial sediments. Although the CIA values of finer overbank sediments are slightly higher than those in coarser riverbank sediments, they can still reveal weathering combined with other chemical indicators. The results suggest that there is no obvious contrast between YZR and BJR in the HTP due to river mixing and strong physical weathering.

Mineralogical and physico-chemical properties of the Cretaceous shales of the Calabar Flank, south-eastern, Nigeria: Implication for paleo-weathering conditions, provenance and industrial applications

The Physico-chemical and mineralogical properties of the Cretaceous shales of Calabar Flank southeastern Nigeria were examined for their Provenance, Paleo-weathering Conditions and Industrial Applications. Twenty samples of shale from Ekekpon and Nkporo Formations were analysed using X-Ray Fluorescence, X-ray diffraction (XRD), scanning electron microscopy (SEM) and geotechnical techniques. Results from the major oxides of elements revealed that the Nkporo Shale is enriched with SiO2 > Al2O3 > LOI > Fe2O3 > SO3 while the Ekepkon shale is also enriched in SiO2 > Al2O3 > LOI > Fe2O3 > CaO > K2O. The XRD revealed kaolinite as the major clay mineral, while other non-clay minerals are quartz, vallerite, mica, nacaphite, sodalite. The SEM shows that the Ekepkon Shale is enriched in calcium. The correlation matrix revealed SiO2 resides more in the quartz phase of the Nkporo Shales, while in the Ekekpon Shale the SiO2 is associated with the clay. The element ratios and discrimination diagrams revealed that the provenance of the shales is of intermediate to mafic igneous rocks and a tectonic setting of continental to oceanic arcs. The presence of kaolinite in the Nkporo Shale with the chemical index of alteration (CIA) values ranging from 83.58 to 96.93, indicated that the shale is derived from intensive chemical weathering in the source area. The low CIA values (37.90–76.96) in Ekenkpon Shale is attributed to CaO and calcium enrichment in the shale and the shale contains high quartz and kaolinite therefore the shale is suggested to have been derived from an intermediate to intense weathering activities in the source area. The geotechnical properties revealed that both shales are suitable for ceramic and refractory industries. The Ekenkpon Shale will be suitable for use as filling material and as raw material in cement industries. However, the shales do not meet the specifications for paper and rubber production due to the high contents of iron oxide, lime, magnesia, potash and soda.

Geochemistry and mineralogy of the sediments in the New Britain shelf-trench continuum, offshore Papua New Guinea: Insights into sediment provenance and burial in hadal trenches

Seven short cores collected along two transects from the shelf to trench bottom in the New Britain Trench (NBT) area, Papua New Guinea (PNG), were studied to illuminate the provenance and burial of sediments across the shelf-trench continuum. Here, we present geochemical data, including major, minor and rare earth elements, bulk and clay mineralogy, grain size, and Sr–Nd isotopes. The results indicate that sediments in the NBT area were mainly derived from intermediate volcanogenic materials of the surrounding islands with the contribution of volcanogenic materials up to 90%. Volcanogenic materials from Rabaul Volcano had a significant influence on the western NBT (Transect 1), whereas the eastern NBT (Transect 2) was more strongly affected by the Tabar-Lihir-Tanga-Feni (TLTF) volcanic chain. In addition, pollen and spores analyses showed a decreasing trend in their concentrations in the order of the landward site, the trench axis site, and the abyssal site on the Transect 1 and the dominance of Cyperaceae in the herb fraction at the trench axis site NBT01. This observation, accompanied by the significant differences in the major and minor elements as well as Sr–Nd isotopes between upper 4 cm sediments and those below at NBT01, suggests that the upper 4 cm sediments were probably sourced from river sediments. Together with low chemical index of alteration (CIA) values, we suggest that terrigenous riverine materials are rapidly transported and buried at the trench axis. Our findings highlight the important role of hadal trenches in trapping terrigenous materials and have implication for understanding the fate of terrigenous materials in the deep ocean.

Geochemical record of the sediments in the continental shelf of the northwestern South China Sea: Implications for the provenance and sedimentary evolution

The marine stratigraphic record in the northwestern South China Sea provides an excellent case for understanding the provenance and sedimentary evolution of the marginal sea. However, considerable uncertainty exists in unraveling the geochemical record since the Oligocene due, in part, to a lack of systematic measurements of the sediments. In this study, the major and trace elements of sedimentary rocks from the northern Yinggehai-Song Hong Basin were analyzed to ascertain the provenance and reconstruct the sedimentary evolution in the northwestern South China Sea. The chondrite-normalized rare earth element (REE) patterns show fractionation between light REEs (LREEs) and heavy REEs (HREEs) with significant negative Eu anomalies (Eu/Eu* = 0.59–1.10), combining with discriminant function analysis, implies that the sediments were predominantly eroded from granitic rocks, which are mostly felsic igneous materials. The relatively high index of chemical variability (ICV) values (1.30–3.54) and low chemical index of alteration (CIA) values (36.2–61.5) indicate that these sediments are immature and might have experienced weak to intermediate chemical weathering during transportation and deposition. The geochemical data also show that the Red River catchment was the primary source region for the northern Yinggehai-Song Hong Basin since the late Oligocene. The variations in the element contents and ratios are consistent with the multi-stage tectonic activities of the basin and the provenance shift caused by the recombination of the Red River system in different sedimentary periods.

Behavioral differences between weathering and pedogenesis in a subtropical humid granitic terrain: Implications for chemical weathering intensity evaluation

Continental weathering plays a crucial role in the evolution of the Earth’s surface by linking Earth's spheres, shaping landscapes and regulating chemical cycles and global climate. Regolith weathering studies in some cases ideally assume successive and progressive bottom-up physicochemical variation trends. To validate this assumption, we target a granitic regolith profile in a subtropical monsoon climate-dominated region (southeast China). We present mineralogical, petrographic and geochemical data from soil, saprolite and bedrock samples to characterize physical and chemical alterations. The petrographic and mineralogical results indicate that both plagioclase and K-feldspar are depleted and kaolinite is the major neoformed mineral phase in top-soil samples, revealing intensive chemical weathering on the regolith. However, these top-soil samples have much lower chemical index of alteration (CIA, ca. 64–67) values than the underlying oxidized-soil samples (CIA values ranging 65–80). Some chemically immobile elements, such as Al, Ti, Zr and Hf, are depleted in top-soil samples but are comparatively enriched in oxidized-soil samples. This is attributed to vertical leaching and translocation of fine-grained minerals (e.g., kaolinite, illite and zircon) through the soils. Furthermore, the analyzed top-soil samples are comparatively rich in elements K and P, which are most likely due to the evident biological fixation process. These physical and biological processes in the unconsolidated, porous and biotic top-soil layer can be responsible for the unexpected low CIA values. Our findings demonstrate the complex physical, chemical and biological alterations on soils and the differential behaviors between weathering and pedogenic processes, especially for those tropical–subtropical high rainfall regions. This case study also highlights the importance of petrographic and mineralogical proxies to chemical weathering intensity evaluation for regolith profiles and siliciclastic sediments.

Genesis of sediment‐hosted stratiform Cu–Co deposits of the Zambia Copperbelt: Geochemistry and S–Pb isotopes constraints from the Chambishi deposit

In this study, we present new field geology, whole‐rock geochemical and S–Pb isotope data for the Chambishi deposit in the Zambia Copperbelt (ZCB), in order to constrain the mineralization process and ore sulphur and metal source. The Lower Roan Subgroup clastic rocks are compositionally similar to the Post‐Archean Australian Shale and have low Chemical Index of Alteration, indicating a felsic and slightly weathered source, which we proposed to be the felsic Lufubu Metamorphic Complex (LMC). The Th, Ti, and Zr elemental features suggest that the Lower Roan Subgroup was deposited in a rift basin of passive margin. The 34S values of early‐stage diagenetic Cu‐Co sulphide (−6.2 to +6.6‰, Vienna Canyon Diablo Troilite [V‐CDT]) from the nodular and layered ores are lower than those of the late‐stage vein‐type hydrothermal Cu–Co sulphides and disseminated molybdenites (3.9–11.9‰, V‐CDT), indicating different sulphur precipitation mechanism in the two mineralization stages. Integrated with previous fluid inclusion studies, we suggest that the sulphur for the diagenetic sulphides was from bacterial reduction of sedimentary anhydrite and seawater sulphates, whereas that for the orogenic‐stage sulphides was from both diagenetic sulphide remobilization and from thermochemical sulphate reduction. The mean 238U/204Pb ratios of the diagenetic‐ and orogenic‐stage sulphides are 10.21 and 10.44, respectively, overlapping with those of the average Lufubu schist (10.38) and the Mufulira granodiorite (10.21), and suggest the LMC as the main ore metal source.

Factors Controlling the Lower Radioactivity and Its Relation with Higher Organic Matter Content for Middle Jurassic Oil Shale in Yuqia Depression, Northern Qaidam Basin, China: Evidence from Organic and Inorganic Geochemistry.

Organic-rich oil shale with unusual lower radioactivity (expressed by GR) was found in Member 7 of Dameigou Formation, middle Jurassic (J2d7) in Yuqia depression of northern Qaidam Basin, China. In order to systematically and contrastively investigate the factors controlling the lower GR and its relation with higher organic matter (OM) content (expressed by total organic carbon, abbr. TOC), organic and inorganic geochemical analyses were performed on samples consisting of oil shale and the underlying conformable contact dark shale from Well YQ-1Y. Our study shows that GR of J2d7 oil shale is mainly derived from uranium and thorium. Compared with dark shale, oil shale is characterized by higher OY and TOC, lower GR, and clay mineral content. During oil shale deposition, the paleoclimate was relatively arid, indicated by a decreased C value and siderite content as well as an increased carbonate content and Classopollis. Under such paleoclimate conditions, sedimentary water became more anoxic, suggested by higher V/(V + Ni), pyrite content and lower pristane/phytane (Pr/Ph). From oil shale to dark shale deposition, according to analyses of Al2O3/TiO2, TiO2 versus Zr, La/Sc versus Th/Co, La/Th versus Hf, and La–Th–Sc, the felsic igneous rock could always be deduced as the parent rock of provenance; however, the increasing arid paleoclimate resulted in weakened chemical weathering of provenance, inferred by relatively low chemical index of alteration, chemical index of weathering, and plagioclase index of alteration corresponding to the input degree of radioactive materials and other terrigenous detrital materials (TDMs), evidenced by Ti and Al contents and terrigenous (%). Meanwhile, the relatively high P/Ti and Ba/Al both indicated increased primary paleoproductivity. Together with the maximum flooding stage of oil shale deposition, the relatively low radioactivity tends to be associated with the inhibited input of clay minerals and radioactive materials, largely caused by increasing arid paleoclimate. The accompanying decreased TDM benefited primary paleoproductivity and anoxic conditions; their combined influence could induce sapropelic OM accumulation.

Biomarker evidence of algal-microbial community changes linked to redox and salinity variation, Upper Devonian Chattanooga Shale (Tennessee, USA)

AbstractLate Devonian marine systems were characterized by major environmental perturbations and associated biotic community changes linked to climate change and widespread oceanic anoxia. Here, we provide high-resolution lipid biomarker chemostratigraphic records from the Upper Devonian Chattanooga Shale (Tennessee, USA) to investigate algal-microbial community changes in the southern Illinois Basin that were related to contemporaneous shifts in marine redox (as proxied by trace metals, Fe-species, and Corg/P) and salinity conditions (as proxied by B/Ga, Sr/Ba, and S/total organic carbon). The Frasnian was characterized by dominantly bacterial lipids (high hopane/sterane), near-marine salinity, and a shift from oxic to increasingly reducing conditions in response to increasing organic carbon sinking fluxes. Aryl isoprenoids and aryl isoprenoid ratios reveal that the O2-H2S chemocline was unstable and intermittently shallow (i.e., within the photic zone). The Frasnian-Famennian boundary was marked by a shift in microalgal community composition toward green algal (e.g., prasinophyte) dominance (lower C27 and higher C28 and C29 steranes), a sharp reduction in watermass salinity, and a stable O2-H2S chemocline below the photic zone, conditions that persisted until nearly the end of the Famennian. We infer that changing watermass conditions, especially a sharp reduction in salinity to possibly low-brackish conditions (&amp;lt;10 psu), were the primary cause of concurrent changes in the microalgal community, reflecting tolerance of low-salinity conditions by green algae. Transient spikes in moretane/hopane (M/H) ratios may record enhanced terrestrial weathering at the Frasnian-Famennian and Devonian–Carboniferous boundaries, triggered by coeval glacio-eustatic falls and increased inputs of soil organic matter. High M/H and pristane/phytane, in combination with low chemical index of alteration and K/Al, record a decrease in chemical weathering intensity during the Famennian that may have been due to contemporaneous climatic cooling, and a concurrent reduction in silt content may reflect stabilization of land surfaces by vascular plants and resulting reduced sediment yields. This study demonstrates the effectiveness of combining organic and inorganic geochemical proxies (including novel paleosalinity indices) for determination of environmental controls on the composition and productivity of plankton communities in paleomarine systems.

Chemical weathering of crystalline rocks in contrasting climatic conditions using geochemical proxies: An overview

Weathering profiles of granitoid rocks from the Sila Massif (southern Italy) have been studied to characterize weathering processes that affect this area. Chemical weathering indices of the Sila Massif weathered granitoids were compared to those of different areas of the world characterized by different climates (Malaysia, Puerto Rico, Georgia, China, Nigeria, Colorado, Switzerland, and Sweden) to decipher their weathering characteristics. The geochemical indices show values of CIA (Chemical Index of Alteration) ranging from 55 to 97, CIW (Chemical Index of Weathering) ranging from 60 to ~100, PIA (Plagioclase Index of Alteration) ranging from 55 to ~100, WIP (Weathering Index of Parker) ranging from 7 to 64 and RR (Ruxton Ratio) ranging from 2.14 to 3.96. All these geochemical proxies closely resemble each other and show variations mainly related to the different climatic and tectonic settings. Malaysia and Puerto Rico granitoids showing high CIA values (respectively 97 and 95 on average) underwent very intensive chemical weathering alteration; Georgia, China and Nigeria showing medium-high CIA values (respectively 86, 90 and 92 on average) underwent moderate-intensive chemical weathering alteration; southern Italy and Colorado granitoids showing medium-low CIA values (68 on average) underwent moderate-weak chemical weathering alteration, whereas Switzerland and Sweden granitoids showing low CIA values (respectively 58 and 55 on average) underwent weak chemical weathering alteration. Those values are mainly related to the climatic conditions in term of temperature and precipitation of each area. CIA vs. climate histograms display a shift towards higher CIA values with increasing of warm-humidity (‘tropical’) conditions, whereas granitoids weathered under conditions inferred to be cold-arid-polar climate show lower CIA values.

Variation of chemical index of alteration (CIA) in the Ediacaran Doushantuo Formation and its environmental implications

The Ediacaran Period (635–541 Ma) witnessed rapid turnovers of marine organisms that eventually led to the early Cambrian radiation. The Ediacaran Doushantuo Formation (ca. 635–551 Ma) serves as one of the most important and intensively studied stratigraphic units for understanding the co-evolution of life and environment during this critical time interval. To better understand the early-middle Ediacaran climate change, we reconstruct a temporal variation curve of chemical index of alteration (CIA) from a drill core and an outcrop of the Doushantuo Formation, in combination with previously published CIA data from segments of time-equivalent strata. The results show three distinct decreases in CIA values that correlate well with the positive carbon isotope (δ13C) excursion at the low Doushantuo Formation (EP1) and the negative δ13C excursion at the middle (EN2) and upper (EN3) Doushantuo Formation. The decline of CIA in EP1 is consistent with occurrence of glendonites at the same stratigraphic interval that suggests a cooling event. The low CIA values in EN2 and EN3 also suggest relatively colder periods that are consistent with evidence from strontium isotope (87Sr/86Sr) and oxygen isotope (δ18O) data. With existing bio- and chemostratigraphic constraints, the cooling event associated with EN2 might be synchronous with the Gaskiers glaciation. The other two cold periods associated with EP1 and EN3, however, have not been documented from other successions globally, which needs to be tested in future studies.