Terrigenous Detritus Research Articles

Mineralogical and Geochemical Composition of Late Permian Coals from Dengfeng Coalfield, North China: Conversion of Clay Minerals in Coal during Coalification

Dengfeng Coalfield represents a significant coalfield in Henan Province, North China. It is therefore essential to gain an understanding of the mineralogy and geochemistry of the Dengfeng coal, both from a geochemical perspective and in terms of the wider environmental context. In this study, a total of 27 coal bench samples were collected from the No. II1 coal of the Dengfeng Coalfield. The mineral species and major elements were quantitatively analysed using the X-ray diffraction and X-ray fluorescence methods, respectively. The minerals in the Dengfeng coal are dominated by ammonian illite and kaolinite with average contents of 3.73% and 7.47%, respectively. These are followed by calcite (2.74% on average) and ankerite (0.49%). The mean value of the kaolinite Hinkley index, which is a quantitative measure of kaolinite crystallinity, is 1.26. This suggests that kaolinite formation is primarily driven by diagenetic recrystallisation. The ammonian illite exhibits an average d001 of 10.2995 Å, indicative of a prevalence of NH4+ interlayer cations, with K+ also present in notable quantities. The ratio of NH4⁺ to (NH4⁺ + K⁺) has an average value of 0.90, which is indicative of the predominance of NH4⁺. The mean value of the illite Kübler index, which is a quantitative measure of illite crystallinity, is 0.264. This suggests that the diagenetic conditions correspond to the rank of the Dengfeng coal. The kaolinite present in the Dengfeng coal is suggested to have been derived from terrigenous detritus and subsequently subjected to diagenetic recrystallisation, resulting in a relatively high Hinkley index. The ammonian illite in the Dengfeng coal was predominantly formed through the conversion of the precursor kaolinite, with the influence of seawater during peat accumulation favouring the conversion of kaolinite to ammonian illite.

Controls and Geological Significance of Macerals in Hybrid Shales: A Case Study on the Gaoyou Sag, Subei Basin, East China.

In the Gaoyou Sag located within the Subei Basin, the hybrid shales from the second member of the Funing Formation (E1f2) have been identified as a prolific source of shale oil production, despite their characteristically low organic matter content (TOC < 1.5%). This observation suggests that specific macerals within these hybrid shales demonstrate a pronounced hydrocarbon generation potential, thus unveiling a new frontier for shale oil exploration endeavors. In this study, 16 samples were rigorously extracted from both mudstone and hybrid shale strata within the E1f2. An exhaustive suite of organic and inorganic geochemical analyses was conducted on these specimens. The analyses elucidated several key findings: (1) The maceral composition within the hybrid shales is predominantly comprised of alginite, solid bitumen, and inertinite. Remarkably, the variability of alginite content within the hybrid shales is more pronounced than that observed in high-TOC mudstones. High-TOC mudstones are characterized by a preponderance of lamalginite and a paucity of telalginite, leading to a diminished aggregate hydrocarbon potential. (2) Biomarker ratios (e.g., sterane/hopane, C23 tricyclic terpane/αβ C30 hopane, C24 tetracyclic terpane/C26 tricyclic terpane, etc.) suggest a primary derivation from lower aquatic organisms, with a secondary contribution from terrigenous organic matter within the hybrid shales. (3) The accretion of macerals is governed by an intricate set of factors, including the input of terrigenous detritus, paleosalinity, and paleoproductivity. (4) Alginite is identified as the principal constituent responsible for hydrocarbon genesis in hybrid shales. The proliferation of alginite facilitates the concurrent enrichment of shale oil and organic matter within the hybrid shales of the Subei Basin, illustrating a cooperative mechanism underlying the accumulation of shale oil and organic matter. This indicates that even hybrid shales with scant organic content exhibit considerable potential for shale oil exploration.

Enrichment mechanism of organic matter and silicon in lower Cambrian shale of the Yangtze Platform

Early Cambrian time witnessed accumulation of high-quality organic-rich siliceous shale deposits that comprise an especially promising shale gas exploration target, especially in China. The present study of the Cambrian Niutitang Formation of the shelf area of the Upper Yangtze Platform utilizes mineralogical and elemental geochemical data to aide in the paleoenvironment reconstruction of these organic-rich deposits. We are especially interested in elucidating organic carbon and silicon enrichment mechanisms of the Niutitang Formation that will benefit shale gas exploration and development of these deposits. The Niutitang Formation can be subdivided into three intervals based on lithology, gamma ray (GR) signature, and TOC abundance. Enriched TOC (average = 6.7%) and nano-size authigenic microcrystalline quartz (Qn) contents of Interval I deposits likely reflect deposition in an environment that experienced persistent euxinic and elevated organic matter delivery associated with enhanced paleoproductivity driven by hydrothermal activity. Interval II shale is interpreted to have accumulated under euxinic-ferruginous conditions that experienced the effects of robust paleoproductivity and diminished terrigenous input related to persistently warm, humid climatic conditions. Enriched TOC (average = 4.9%) and moderate microcrystalline quartz content of these deposits were likely the result of redox conditions and elevated paleoproductivity. Interval III strata appears to have accumulated under oxygenated bottom conditions that experienced moderate paleoproductivity, brackish water and elevated terrigenous detritus input driven by the establishment of cold, dry climatic conditions. The combined results of these conditions are diminished TOC content (average = 0.7%) and enrichment of the sediment in terrestrial quartz. The elevated content of authigenic quartz appears to have diminished the reservoir capacity of Niutitang Formation shale. Interval II organic-rich siliceous shale deposits of Upper Yangtze Platform shelf areas, which have the comparable authigenic microcrystalline quartz content, reservoir performance, and greater organic matter abundance as the rift trough, appear to comprise the stratigraphic horizon with the greatest exploration and development potential. Results of the present study offer theoretical guidance for future shale gas exploration and development efforts in geologically similar regions.

Comprehensive Comparison of Lacustrine Fine-Grained Sedimentary Rock Reservoirs, Organic Matter, and Palaeoenvironment: A Case Study of the Jurassic Ziliujing Formation and Xintiangou Formation in the Sichuan Basin

Lacustrine sedimentary formations potentially contain hydrocarbons. The lacustrine sedimentary rocks of the Ziliujung and Xintiangou Formations have been investigated for their hydrocarbon potential using low-pressure nitrogen adsorption (LP-N2A), low-field nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), total organic carbon (TOC), rock-eval pyrolysis (Rock-Eval), gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), X-ray fluorescence spectrometry (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). The results show that the normalized difference of the pore parameters between the two formations is less than 10%, and the pores are mainly slit-like mesopores with high porosity. Macropores and micropores are often developed in the quartz skeleton, while mesopores often occur among organic matter, clay minerals, carbonate minerals, and pyrite particles. The organic matter abundance of the Ziliujing Formation is relatively high. Additionally, the organic matter types of the two formations are mainly type II and type III, and the sources of the organic matter are plankton and bacteria which have reached the mature gas production stage. The palaeoenvironmental differences between the depositional periods of the two formations lie within 10% of each other. The warm and humid climate promotes the development of quartz minerals to further enhance the proportion of both micropores and macropores, and the clay minerals, carbonate minerals, and pyrite carried in the terrigenous detritus are closely associated with the total organic carbon (TOC), which promotes the development of mesopores to enhance the porosity. The reservoir, organic matter, and palaeoenvironmental characteristics of fine-grained sedimentary rocks in the two formations are similar, and both of them have good potential for development. The above results provide a basic geological theoretical basis for unconventional oil and gas exploration in the northeastern margin of the Sichuan Basin.

Evolution of the sedimentary environment on the continental shelf of southeastern Hainan Island under the influences of sea level and climate during the Holocene

The South China Sea, which connects the Indian Ocean and the Pacific Ocean, is an important part of the Indo-Pacific Warm Pool. This study examined the provenance and evolution of the sedimentary environment on the southeastern Hainan Island continental shelf in the northwestern South China Sea through AMS 14C dating, grain size analysis, and clay mineral analysis of sediments from Core QZ4. The clay mineral analysis indicated that the terrigenous detritus originated from Hainan Island, while the grain size data identified three endmembers: EM1 and EM2, corresponding to the fine-grained material transported by the oceanic current system, and EM3, corresponding to the coarse-grained material input by rivers from proximal sources. The sedimentary environment in the study area underwent three stages: foreshore (11.2–9.2 ka BP), nearshore (9.2–6.2 ka BP), and shallow shelf (6.2 ka BP to present). From 11.2 to 6.2 ka BP, the sedimentary environment was mainly controlled by sea level changes, although extreme cold events (such as the 9.4 ka BP and the 8.2 ka BP event) had a significant effect on the southeastern continental shelf of Hainan Island. From 6.2 ka BP to the present, the sedimentary record has mainly been influenced by climate change, and an increase in riverine contribution and the enhancement of chemical weathering may have been caused by rising typhoon intensity or frequency and possibly by an increase in human activities. These findings will be valuable for conducting high-resolution research on typhoons by analyzing sediments on the eastern Hainan Island shelf and neighboring areas.

Sedimentary Environment and Organic Accumulation of the Ordovician–Silurian Black Shale in Weiyuan, Sichuan Basin, China

The sedimentary environment and organic matter enrichment relationship of the Upper Ordovician Wufeng Formation black shale, Guanyinqiao mudstone, and Lower Silurian Longmaxi Formation black shale in the Sichuan Basin of Weiyuan are analyzed using geochemical methods such as organic carbon, sulfur, major elements, and trace elements. The experimental results illustrate that the upper section of the Wufeng Formation and the lower section of the Longmaxi Formation are organic matter enrichment layers. The presence of P indicates a high productivity level in the Sichuan Basin from the Late Ordovician to the Early Silurian. In addition, indicators such as V/Cr, Ni/Co, and S/C suggest that the Wufeng Formation was deposited under anoxic reductive conditions, that the ice age Guanyinqiao segment was in an oxygen-rich to oxygen-poor environment, and that the Longmaxi Formation was in a sulfidic environment. Mo/TOC indicates that the Wufeng Formation shale was controlled by a restricted basin and that the Guanyinqiao segment and the Longmaxi Formation were in a medium-to-weak retention environment. The weak correlation of TOC with P/Al and Al indicates that the level of primary productivity and terrigenous detritus had a minor effect on the organic matter enrichment of the Wufeng and Longmaxi Formation black shale. Conversely, the positive correlation of TOC with V/Cr and Ni/Co illustrates that the anoxic reductive sedimentary environment is the main factor affecting the organic matter enrichment of the Wufeng and Longmaxi Formation black shale. Based on these studies, the development model of organic-rich shales of the Ordovician–Silurian in Weiyuan, Sichuan Basin is proposed. This paper may provide a reference for shale gas exploration in the Wufeng–Longmaxi Formation and a sedimentary response to the major geological events of Ordovician–Silurian.

Genesis of Fibrous Calcite in the Chang 7 Member of the Yanchang Formation, Ordos Basin

AbstractTo explore the genesis of the laminated calcite veins developed in the black shale of Chang 73 submember of the Ordos Basin, the petrology, microstructure and geochemistry of calcite veins are studied using thin sections, fluid inclusions, trace elements and isotopic geochemistry. The source, the time of formation, the mechanism of formation, and the dynamic background of the veins are discussed. The veins are mostly made up of calcite, mixed with lenticular or spindle‐shape solid wall rock inclusions. Three structures are identified in the calcite minerals: fibrous, rhombic cleavage, and wedge‐like structure. Trace elements and isotopes of carbon and oxygen confirm that the calcite veins were formed from a high density hydrothermal fluid. It is assumed that calcite veins formed prior to wall rock consolidation during the formation of the Qinling Orogenic Belt in the Middle and Late Triassic. The results show that the sedimentary sources of Chang 73 submember were influenced by hydrothermal materials besides terrigenous detritus. The fibrous calcite is different from the fibrous calcite reported in the morphology and mechanism of formation. In this way, the research complements and improves the morphology and the mechanism of formation of fibrous calcite.

Weathering Intensity Response to Climate Change on Decadal Scales: A Record of Rb/Sr Ratios from Chaonaqiu Lake Sediments, Western Chinese Loess Plateau

The Rb/Sr ratio of lake sediments has been widely adopted as an indicator of weathering intensity in studies of past climate change, but the geochemical significance of this ratio varies with timescale. Here, we present Rb/Sr data for the past 300 years for sediments collected from Chaonaqiu Lake in the Liupan Mountains of the western Chinese Loess Plateau as a decadal-scale record of weathering intensity. To validate the application of this weathering proxy, we correlated the record with those of other major elements, rock-forming minerals, and paleoclimatic proxies. We found that Rb/Sr ratios are influenced mainly by Sr activity within the lake catchment (where Sr is likely sourced from albite). In addition, higher (lower) Rb/Sr ratios of bulk sediments from Chaonaqiu Lake are correlated with lower (higher) fractions of terrigenous detritus (SiO2, Ti, K2O, Al2O3, and Na2O). These indicate that the Rb/Sr ratios of bulk sediments in Chaonaqiu Lake are closely linked to terrigenous detritus input on decadal scales and also correlate well with TOC (a precipitation indicator) and other high-resolution paleoclimate records (e.g., tree rings and drought/flood index) in neighboring regions, with higher (lower) Rb/Sr ratios corresponding to more (less) precipitation. Lake bulk sediment Rb/Sr ratios are dominated by the input of terrigenous detritus over decadal timescales. Our data show that physical and chemical weathering in the Chaonaqiu Lake watershed have opposing influences on Rb/Sr ratios of bulk sediment, competing to dominate these ratios of lake sediments over different timescales, with ratios reflecting the relative importance of the two types of weathering.

Benthic foraminiferal morphogroups at the end-Guadalupian extinction in eastern Sichuan Basin, China

Benthic foraminiferal morphogroups near the Permian Guadalupian-Lopingian (GLB) reveal their responses to palaeoenvironmental and palaeoecological changes in the eastern Sichuan Basin, China. According to the shell forms, coiling modes, and life positions linked to the life styles and feeding strategies, seven types of benthic foraminiferal morphogroups were recognized in Guadalupian and lower Wuchiapingian carbonate rocks from four lithological profiles. During the Guadalupian Epoch, highly diversified and large number of benthic foraminifera and their various feeding strategies indicated sufficient trophic resources for food. However, the foraminiferal assemblages dropped dramatically after the end-Guadalupian extinction event, reflecting remarkable changes in the nutrient sources and oxygen conditions in the ocean. In the early Wuchiapingian Epoch, nutrient input and plant debris decreased rapidly, while terrigenous detritus increased. As a result, the number of herbivores significantly declined, and suspension feeders and detritivores dominated. Changes in foraminiferal morphogroups in the eastern Sichuan Basin confirmed the existence of a end-Guadalupian extinction event. Surviving benthic foraminifera in Wuchiapiangian, such as Codonofusiella and Eotuberitina, prefer epiphytism, multiple feeding strategies, and low oxygen concentrations, which helped them sustain in a severe ecological environment. The dominant Wuchiapingian foraminifera has changed to other genera, which had to alter their living habits by enlarging the bottom contact area to resist waves and expanding semi-shells to increase feeding capacity. Benthic foraminiferal assemblages from adjacent regions (south China, Tibet, Iran, and Greece) were consistent with our investigation of the morphogroups and showed a similar response to the end-Guadalupian extinction event.

Depositional response to the Emeishan mantle plume: evidence from the eastern Sichuan Basin, South China

ABSTRACT The Emeishan large igneous province (ELIP) is among the prominent large igneous provinces in the world and has been associated with mantle plume activity. In this study we investigated the impact of the Emeishan mantle plume on generating the sedimentary rock suites of the eastern Sichuan Basin. We present petrology, detrital zircon U – Pb ages, trace elements, and Hf isotopic compositions of the Middle-Late Permian sedimentary sequences in the Huayingshan region to constrain the provenance and characterize the depositional response to the Emeishan mantle plume activity. The detrital zircon U – Pb ages of the Middle Permian Maokou Formation indicate that terrigenous detritus was mainly sourced from the Kangdian paleoland in the southwestern margin of South China. Detrital zircon grains from the bottom of the Late Permian Longtan Formation yield a lower intercept age of 257 ± 28 Ma, interpreted as the contemporaneous metamorphism related to the ELIP magma. The detrital zircon U – Pb ages of the middle part of the Longtan Formation are characterized by unimodal age spectra with a peak at 259 Ma. Provenance analysis indicates that the Longtan Formation was mainly sourced from the Emeishan volcanic rocks in the Huayingshan region. We propose that the Kangdian paleohigh may have been regionally uplifted as the result of the pre-eruption rise of the Emeishan mantle plume in the late Middle Permian. Moreover, this paleohigh was translated into a denuded zone and provided voluminous detritus into the eastern Sichuan Basin. In the early Late Permian, the ELIP magmas upwelled and outpoured from the Huayingshan fault, and flowed past the peripheral rocks to the low terrain in the Huayingshan region. The ELIP volcanic rocks in the Huayingshan region underwent intense post-eruption erosion, resulting in the deposition of voluminous volcanic detritus in the eastern Sichuan Basin.

Geochemical characteristics of shales from the first member of Qingshankou Formation in Gulong Sag, Songliao Basin, China: Implication for mechanism of organic matter enrichment

Commercial oil production from the first member of the Qinshankou Formation in the Songliao Basin, China has proven it can be a promising target for shale oil recovery. However, geochemical characteristics and the mechanism of organic matter (OM) enrichment in this layer remain unknown. Herein, a large number of samples collected from the first member were analyzed through a combination of organic and inorganic geochemical methods. Results show, bedding oil shale and laminated shale are prominent facies in this member. Moreover, this member mainly consists of type-I and –II kerogen at highly thermal mature stage, belonging to good to excellent source rock. Overall, this member has been deposited in the semi-humid to humid and warm paleoclimate, fresh to brackish water mass, and dysoxic to anoxic conditions. This member is enriched in P2O5, indicating the high primary productivity. Moreover, lack of relation between P2O5, with Al2O3 and Fe2O3 concentrations, suggests that the contribution of terrigenous detritus influx and the effect of Fe cycle for P enrichment in sediments can be disregarded. The terrigenous detritus influx, rapid sedimentation rate and marine transgression were found to be detrimental to the OM enrichment. Moreover, more serious oxygen deficiency in water mass along with slightly lower sedimentation rate, made the lower part of this member to yield better preservation conditions and weaker dilution effect, which resulted in the formation of OM with better quality and higher abundance. Collectively preservation conditions were found to be significant for the OM enrichment.

The Significance of Nanomineral Particles during the Growth Process of Polymetallic Nodules in the Western Pacific Ocean.

As a huge reservoir of economic metallic elements, oceanic polymetallic nodules have important strategic significance and are one of the main research objects in marine geology, especially their formation process and genetic mechanism. In this study, polymetallic nodules from the cobalt-rich crust exploration contract area in the Western Pacific Ocean were taken as the research object. Optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS) were used for observation and testing. The results indicate that many nanomineral particles, mainly composed of Fe and Mn, developed in polymetallic nodules from the western Pacific Ocean. The solid-liquid interface process of nanomineral particles plays an important role in the growth and evolution of nodules. We propose that the growth process of polymetallic nodules in the western Pacific Ocean can be divided into three stages. First, terrigenous detritus nucleates, and nanomineral particles composed of Fe, Mn, and other elements form, aggregate and attach to the core to form the initial shell. Second, a dense layer of the shell forms under stable conditions. In the third stage, the redox conditions of the nodules change, and the polymetallic nodules experience a variety of interface process modifications.

Spatial analysis, ecological risk assessment, control factors, and sources of heavy metal pollution in the shelf surface sediments of the southwest Bay of Bengal, India

Heavy metals such as Mn, Zn, Ni, Cu, Pb, and Co besides Hg were determined to assess the distribution, contamination status, and possible sources in the shelf surface sediment of the southwest Bay of Bengal. Physico-chemical parameters significantly influenced the metals. Higher concentrations of all the metals except Hg were recorded in the Andhra Pradesh and Chennai waters than in waters south of these in Tamil Nadu and Puducherry. Significant differences between areas were found only for Co, Cu, Mn, and Ni. Generally, the pollution indices, namely Contamination factor (CF = <1), Geo-accumulation index (Igeo= <0), Potential contamination index (Cp=<1), and Potential ecological risk factor (Eri=<40) and Risk factor index (RI =<95) revealed the unpolluted nature of marine sediments by Mn, Zn, Ni, Cu, Pb, Co. However, the Cuddalore-SIPCOT transect was moderately and considerably contaminated with Hg (CF = 1.538, Igeo=>0.04, Cp=<1.6, Eri=<80- >40, RI = 95> - <190–190> - <380). The source of Hg was traced to the discharges from the nearby industries. Principal component analysis (factor analysis) suggested the natural input derived from terrigenous detritus materials (lithogenic factor) to influence the distribution of most of the considered metals in the southwest Bay of Bengal. The results are likely to be useful for the coastal zone regulation authorities to develop sustainable management strategies.

Rare earth elements and neodymium and strontium isotopic constraints on provenance switch and post-depositional alteration of fossiliferous Ediacaran and lowermost Cambrian strata from Arctic Norway

The Digermulen Peninsula in northeastern Finnmark, Arctic Norway, comprises one of the most complete Ediacaran–Cambrian transitions worldwide with a nearly continuous record of micro- and macrofossils from the interval of the diversification of complex life. Here, we report on the provenance and post-depositional alteration of argillaceous mudstones from the Digermulen Peninsula using rare earth elements and Sm–Nd and Rb–Sr isotopic systematics to provide an environmental context and better understand this important transition in Earth’s history. The studied sections comprise a mid-Ediacaran glacial–interglacial cycle, including the Nyborg Formation (ca. 590 Ma) and Mortensnes Formation (related to the ca. 580 Ma-old Gaskiers glaciation), and the Stáhpogieddi Formation (ca. 560–537 Ma), which yields Ediacara-type fossils in the Indreelva Member and contains the Ediacaran–Cambrian boundary interval in the Manndrapselva Member and basal part of the informal Lower Breidvika member (ca. 537–530 Ma). Three sample groups, (1) Nyborg and Mortensnes formations, (2) the lowermost five samples from the Indreelva Member and (3) the remaining samples from the Indreelva as well as from the Manndrapselva and Lower Breidvika members, can be distinguished, belonging to distinct depositional units. All samples have negative εNd(T) values (−6.00 to − 21.04) indicating a dominant input of terrigenous detritus with an old continental crust affinity. Significant shifts in Sm–Nd isotope values are related to changes in the sediment source, i.e. Svecofennian province vs Karelian province vs Svecofennian province plus in addition likely some juvenile (late Neoproterozoic volcanic) material, and probably reflect palaeotectonic reorganisation along the Iapetus-facing margin of Baltica. The combined Rb–Sr isotopic data of all samples yield an errorchron age of about 430 Ma reflecting the resetting of the Rb–Sr whole-rock isotope systems of the mudstones during the Scandian tectono-metamorphic event in the Gaissa Nappe Complex of Finnmark. Preservation of palaeopascichnids coincides with the sedimentation regimes of sample groups 2 and 3 while other Ediacara-type fossils, e.g. Aspidella-type and frondose forms, are limited to the sample group 3. Our results are similar to those of earlier studies from the East European Platform in suggesting oxic seafloor conditions during the late Ediacaran.

The Sedimentary Geochemical Characteristics and Geological Significance of the Wufeng-Longmaxi Formation Accumulation of Organic Matter Black Shale on the Southeastern Sichuan Basin, China

At present, the understanding of the sedimentary paleoenvironment and organic matter enrichment law of black shale in the Wufeng-Longmaxi Formation is still insufficient. This time, we investigated the total organic carbon (TOC) content, X-ray diffraction mineralogical composition, and the major and trace element abundances of the newly recovered cores of three shale gas exploration wells in Changning, Qianjiang, and Xiushan areas which were, respectively, selected to discuss and compare the varying paleoenvironmental conditions and the factors that control organic matter accumulation, such as terrigenous input, redox conditions, primary productivity, and the degree of water retention in the basin. The results show that black siliceous shale lithofacies, gray-black shale lithofacies, gray-black silty shale lithofacies, gray argillaceous siltstone lithofacies, and gray shell marl lithofacies are mainly deposited in the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin. Organic-rich black shale is mainly concentrated in the lower part of the Wufeng Formation and Longmaxi Formation. The continuous thickness of organic carbon content greater than 2% in the Changning area is 32 m, the continuous thickness in the Qianjiang area is 22 m, and the continuous thickness in the Xiushan area is 11 m. The hydrocarbon generation potential gradually weakens from west to east. The identification of the geochemical behavior characteristics of major and trace elements shows that enrichment of the organic matter black shale section in the Changning-Qianjiang-Xiushan areas is mainly formation in the anoxic-moderate water mass restriction environment with high primary productivity and less terrigenous detritus input under the low hydrodynamic conditions. However, the correlations of terrigenous detrital proxies Ti/Al, redox proxies V/Cr, Ni/Co, and paleoproductivity proxies Babio with the TOC contents show that the Changning area is a favorable area for the formation of organic-rich black shale and the Qianjiang area is affected by the Wuling underwater uplift; the thickness of the black shale deposits becomes thinner; the Xiushan area is adjacent to the Xuefeng uplift, and the deposition conditions of the accumulation of organic matter black shale deteriorate, which is unfavorable for shale gas exploration.

Differential enrichment mechanisms of organic matter in the Chang 7 Member mudstone and shale in Ordos Basin, China: Constraints from organic geochemistry and element geochemistry

Source rocks in the seventh member of the Upper Triassic Yanchang Formation (Chang 7 Member) are characterised by lacustrine organic-rich mudstone and organic-rich shale in the Ordos Basin. There are obvious differences in organic and inorganic geochemical characteristics between mudstone and shale in the Chang 7 Member, indicating that they are significantly different in genetic mechanism. Presently, the difference of organic-matter enrichment mechanism between shale and mudstone in the Chang 7 Member is still obscure. In this study, we explored the organic-matter accumulation mechanisms in the Chang 7 Member mudstone and shale by investigating their parent-rock properties, palaeoproductivity, palaeoenvironment and dilution conditions using biomarkers and elemental geochemical data. According to our analysis results, the mudstone and shale parent rocks are composed of sedimentary rock and granite. The material source of the mudstone was mainly from the south of the Ordos Basin, whilst the source of the shale was mainly from the south and northeast of the basin. The organic-matter enrichment of the shale was affected by high palaeoproductivity and anoxic environment conducive to the preservation of organic matter. The palaeoproductivity during the deposition of the mudstone was moderate, and the dilution caused by the high sedimentation rate was the main reason for its low abundance of organic matter. Affected by frequent volcanic activities, a grater algae bloom occurred in the process of the shale deposition, which made the palaeoproductivity during shale deposition higher than that of the mudstone deposition. In addition, the degradation of a large quantity of algae resulted in a reducing environment during shale deposition. Therefore, the high palaeoproductivity and anoxic preservation conditions resulted in large amounts of aquatic organisms in the shale, resulting in high organic-matter abundance in the shale. The palaeoclimate during mudstone deposition was warmer and wetter than during shale deposition, which was more conducive to parent-rock weathering and the transportation and deposition of terrigenous detritus. Therefore, the organic matter of mudstone is a mixture of terrestrial higher plants and aquatic organisms. However, the high input rate of terrigenous detritus caused the dilution of organic matter in the mudstone. The mudstone was mainly deposited under weakly oxidising to weakly reducing conditions. The moderate palaeoproductivity, oxygen-containing preservation conditions and dilution caused by the high sedimentation rate made the aquatic and terrestrial organic matter in the mudstone easily oxidised, degraded and diluted. Therefore, the differences of primary productivity and preservation conditions make the organic-matter abundance of the mudstone lower than that of the shale.

Response and Potential Indication to Hypoxia in the Changjiang River Estuary and its Adjacent Waters: Insight From Redox-Sensitive Trace Elements in Sediment Core

The migration and enrichment of redox-sensitive trace elements (RSEs) from seawater to sediments are controlled not only by their geochemical properties but also by marine redox conditions. Therefore, RSEs, such as Mo, U, and V, are extensively used to indicate redox states in marine environments. To retrieve the historical redox state in the hypoxic zone near the Changjiang River Estuary and its adjacent waters, the distribution and enrichment degree of Mo, U, and V in a sediment core collected from the recurrently hypoxic region were investigated. The correlation analysis shows that the authigenic enrichment of Mo and U in sediment core 3,050–2 is primarily controlled by redox conditions, rather than sediment grain size, adsorption on organic matters, Fe–Mn (hydr)oxides, and terrigenous detritus input. Mo-U covariation is selected to analyze the evolution of redox conditions in the seasonally hypoxic zone. The MoEF/UEF values in this sediment core are almost between 0.1 and 0.3×Mo/Umodern seawater value and show a general decreasing trend with depth, indicating an overall progressive transition from oxic to suboxic conditions in this area since 2005. The close MoEF/UEF values and similar chronological Mo-U variations by comparing our results with previous studies in adjacent areas offer common evidence for the gradually intensified bottom water–oxygen stress, suggesting the possible application of MoEF/UEF in retrieving redox development in this coastal hypoxic zone.

Natural and anthropogenic effects on the elemental compositions of surficial sediments across Tajikistan, Central Asia

The elemental compositions of surficial sediments are conducive to understanding various environmental conditions and surface processes. Tajikistan is situated in Central Asia. The eastern region of Tajikistan locating on higher-altitude Pamir Plateau has a dry and cold climate, whereas the western part has a contrasting natural environment and most populations live in this region. To investigate the effects of natural and anthropogenic processes on the elemental compositions of surficial sediments across Tajikistan, this study examines the major, trace and rare earth elements (REEs), and grain size distributions of various types of surficial sediments (e.g., topsoil, fluvial, lake sediments, loess deposit, and bedrock sample). We find that most of the topsoil and fluvial sediments show spatially homogeneous elemental compositions in Tajikistan. The source bedrocks of these various sediments are primarily felsic. Although the sediments in the western region display slightly higher chemical weathering intensity, the sediments across Tajikistan have experienced the overall incipient chemical weathering intensity and weak sorting process. In comparison with topsoil and fluvial sediments, elemental compositions of lake sediments are complex due to the differential proportions of terrigenous detritus with physical and chemical weathering of the bedrock in the catchment. The surficial sediments in western Tajikistan show higher Cd concentrations. The good correlations between Cd, lithogenic element Ti and Chemical Index of Alteration (CIA) suggest that Cd is largely derived from the underlying bedrock and positively correlated with the slightly higher chemical weathering intensity in this region. In addition, anthropogenic input likely contributes to the unusual Cd contamination of the sediments in the western Tajikistan where industrialization has been well-developed.

Carbon, oxygen and strontium isotopic and elemental characteristics of the Cambrian Longwangmiao Formation in South China: Paleoenvironmental significance and implications for carbon isotope excursions

The biosphere experienced episodic perturbations during the Neoproterozoic–early Cambrian, accompanied by major fluctuations of the carbon cycle. However, compared to the Ediacaran–early Cambrian, geochemical records of the Cambrian Toyonian Stage (Longwangmiao), which contains the significant Redlichiid–Olenellid Extinction Carbon Isotope Excursion (ROECE), are scarce. To address this data gap, we conducted measurements of carbon, oxygen and strontium isotopes and element contents of a continuous series of carbonate samples from the Longwangmiao Formation of Well HS2, located in the central Sichuan Basin of South China. Our specific aims were to reconstruct the paleoenvironment and to determine the origin of the ROECE. Strict sample screening was used to ensure that the isotopes and elements were not affected by diagenesis. The 87Sr/86Sr values of Well HS2, and for sites elsewhere, show that sea level gradually decreased during the Cambrian Toyonian Stage. Additionally, the pronounced negative δ13C excursion in the late Toyonian is globally associated with the ROECE, which is widely thought to have resulted from extinctions caused by a transgression. However, a transgression is not shown in the 87Sr/86Sr profile of Well HS2, which we attribute to local karst formation and erosion caused by frequent regional regressions. The interrelationships among several paleoenvironmental proxies show that continental weathering was climatically controlled, which affected the supply of terrigenous detritus, and that the carbonate platform was semi–restricted. Furthermore, profiles of paleoenvironmental proxies indicate that the environment was warm and humid, favoring high biological productivity and diversity during Interval I (early Toyonian), and that it gradually became colder and drier during Interval II (middle Toyonian). We suggest that by the time of Interval III (late Toyonian), the adverse effects of a cold and arid environment were superimposed on the transgression, triggering the ROECE. Our results provide a reference and possible interpretation for other carbon isotope excursion events.

Significant enrichment of Rb and Cs in the Late Triassic coals from the Coc Sau surface mine, Cam Pha Coalfield, Quang Ninh Province, Vietnam

The current study investigates the mineralogical and geochemicalcharacteristics of the Late Triassic coals from the Coc Sau open-pit mine, Cam Pha Coalfield (Quang Ninh Province, Northeastern Vietnam) using Powder X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), inductively coupled plasma atomic-emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). Coals from Coc Sau open-pit mine are characterized by medium ash yield (avg. 29.7%), low sulfur content (avg. 0.17%), and low moisture content (avg. 1.8%). The volatile matter content (avg. 8.4%) indicates a semianthracite rank. The crystalline phases identified in Coc Sau coals and host rocks (roof, floor, parting) are primarily quartz, kaolinite and muscovite, with small proportions of ankerite, clinochlore, anatase, calcite, and tobelite. Illite, pyrite, and siderite were occasionally detected in individual samples. In addition, several minerals below the XRD detection limit, such as barite, apatite, zircon, xenotime, and florencite were observed by SEM. The low sulfur concentrations, low Sr/Ba ratio and relatively high ash yield of these coals indicate a continental lake basin sedimentary environment with a relatively strong supply of terrigenous detritus. Quartz, kaolinite, and muscovite were most likely derived from the terrigenous source region surrounding the basin, and the other minerals are mainly of authigenic origin. Relative to worldwide coals, the Coc Sau coals and host rocks are highly enriched in Rb and Cs, which can be a potential source for Rb and Cs recovery during coal utilization and combustion. Rubidium and Cs enriched in the coals are positively correlated with Al and K, indicating that they most likely occur in potassium-rich clay minerals (e.g., muscovite), and originate from weathered felsic terrigenous detritus.