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The Early Toarcian “Oceanic Anoxic Event” (T-OAE) is recorded by marked disruption to both the climate system and marine ecosystems. Here, we present intergraded high-resolution carbon-isotope data ( δ 13C), bulk geochemistry, mineral characterization from an open-ocean setting in the eastern Tethys. With these data, we (1) construct the high-resolution record of the T-OAE from an open-ocean setting in the eastern Tethys; (2) show that the T-OAE in the Sewa succession was marked by coarser-grained deposits associated with high-energy conditions within the otherwise low-energy claystone deposits that likely linked to a globally increased supply of clastic sediments into marginal and deeper marine basin; (3) propose that the lowCorg:Ptotal ratios, in combination with bioturbated structure and depletion or slight enrichment in redox-sensitive trace elements of V, Mo, and U suggest a long-term oxygenation event throughout the T-OAE interval at the Sewa succession, and hence, anoxia may not play a fundamental role during the Toarcian negative CIE in this setting; (4) exhibit that a warming and more humid climate began at the start of the T-OAE, and many episodic changes in sediment provenance throughout the T-OAE interval occurred at this location; and (5) suggest that accumulation of organic-matter sediments during the T-OAE is generally controlled by global climatic changes, but a regional environmental perturbation also might influence the preservation of organic matter.

The Kungurian Stage in the early Permian was a transitional glacial age between the late Paleozoic icehouse and the early Mesozoic super-greenhouse period This stage offers an excellent opportunity to study the co-evolution between global carbon cycles and environments. This study presents facies and carbon isotope variations in a new carbonate section in the Lower Yangtze region of South China in order to understand the linkage between carbon cycle fluctuation, sedimentary environment, and climate change. Based on the sedimentary facies analyses of the Chihsia Formation (Kungurian), seven facies types were identified and grouped into lower slope, upper slope, and platform facies associations. The facies analyses show that the Kungurian Stage experiences two transgressive-regressive cycles; paleoclimatic changes controlled the sedimentary records and sea level fluctuations. Early Kungurian carbonate rocks record the presence of the short-lived Kungurian carbon isotopic event (KCIE). The rapid negative carbon isotope of the KCIE was closely related to the huge CO2 emission. A warming climate could have slowed down oceanic ventilation rates and accelerated stratification of seawater. The resulting anoxic environment led to a sharp decline in biological species. In the middle Kungurian, the intensity of volcanic activity gradually weakened and the climate turned cold, which accelerated oceanic ventilation rates and led to increased oxygenation of deep-shelf water masses. The higher Δ13C values supported enhanced primary productivity and photosynthesis, which promote the prosperity of biological species. This study provides a new perspective for better understanding the links between marine carbon cycle fluctuations, climate change, and environments during the icehouse to greenhouse conversion period.

The Permian–Triassic extinction was the largest Phanerozoic mass extinction event, yet its ultimate causes remain unresolved. Ocean redox conditions, global warming, and carbon cycle perturbations owing to massive volcanism and their inter-relationships are keys to resolve the origins of this event. We here assess such relationships using a method to obtain an instantaneous stratigraphic response from high-resolution (millennial-scale) records of carbonate-δ13C and pyrite framboid size distributions across the Permian–Triassic boundary (PTB) at Meishan, China. We report two gradual long-term negative δ13C shifts before and after the PTB. Late Permian ocean redox conditions varied through three successive oxic–dysoxic, dysoxic–euxinic, and high-frequency euxinic stages. Oxic and high-frequency euxinic events occurred in the early Griesbachian and middle Griesbachian, respectively. The short-term euxinia events were not associated with the negative δ13C excursions. Siberian volcanism could have produced large light-carbon inputs, global warming, and low-oxygen conditions at intermediate ocean depths, but it may not directly drive high-frequency euxinia across the PTB. Long-term dysoxia and occasional euxinia well before the PTB stressed mobile macroorganisms and caused gradual extinction before the major extinction event. High-frequency latest Permian euxinia reduced overall biotic adaptability, and caused the initial rapid mass extinction. Increasing temperatures and other deleterious environment factors then reinforced the extinction process. Oxic conditions contributed to biotic recovery immediately after the mass extinction in the early Griesbachian. Further millennial-scale dysoxic/euxinic alternations in the middle Griesbachian would have prolonged Early Triassic biotic recovery.

The Middle Permian Guadalupian witnessed significant environmental changes in the Phanerozoic such as large-scale sea-level drop, supercontinent Pangaea assembly, and transition from Early Permian glaciation to Late Permian non-glacial intervals. Carbonisotope tracers can provide insights for these environmental changes. The δ13C studies of the entire Guadalupian epoch are rare, and most of them has focused on near the end of Guadalupian and carbon isotopes of inoragnics. Here, we present carbon isotopic compositions of organic matters in the Guadalupian from two sections (Chaohu and Xiaolao) in the Lower Yangtze area, South China. Our results show that δ13Corg profiles in the Guadalupian show a peak in the Roadian and a gradual negative shift from the Roadian to the middle Capitanian. These trends can be matched by δ13C changes of carbonate rocks or organic matter in South China and other places in the world, representing a global carbon cycle signal. The Roadian positive peak was probably due to high productivity which was caused by upwelling during cooling time. The gradual negative shift of δ13C was caused mainly by a decrease of organic matter burial on land and in the ocean, resulting from global sea-level drop and anoxia-caused benthos decline, respectively. The less important causes for the gradual δ13C negative shift are volcanic-gases releasing, decreased mountain belts, and the resultant reduced silicate weathering-consumption of CO2. The gradual negative shift of δ13C coincides with the gradual extinction in the Guadalupian. Therefore, global sea-level drop and marine reducing conditions may be the main causes of the gradual extinction in the Guadalupian.

The siliciclastic rocks in the Xishanyao (XSY) and Toutunhe (TTH) formations recorded the Middle Jurassic palaeogeographic evolution processes of the Bogda Mountains. To explore their provenance, t...

The greenhouse climate in the Cretaceous period promoted widespread organic-rich marine/lacustrine black shales or mudrocks. Organic matter (OM) accumulation is generally affected by preservation conditions and surface water primary productivity. The detail of organic-rich mudrock formation in lacustrine system still remains enigmatic. The widespread OM-rich mudrocks in the Cretaceous period as petroleum source rocks in many large terrestrial petroliferous basins in China offer an opportunity to explore the mechanism of lacustrine OM accumulation. Fe speciation, C S Fe system, pyrite morphology, pyrite-sulfur isotopic composition, elements, and biomarkers in the Bayingebi Formation from a drill core in the Bayingebi Basin, Inner Mongolia, NW China are used to constrain redox conditions, OM source and primary productivity in a lacustrine system. Our results show that the dark-gray mudrocks in the Bayingebi Formation were deposited in ferruginous and stratified water-column in the Bayingebi Basin. The biomarkers suggest a mixed organic matter source of algae, bacteria and high plant with a low algal contribution. The surface-water primary productivity during the deposition of the Bayingebi Formation was low and shows a weak correlation with total organic carbon (TOC) content. The OM accumulation in early Cretaceous lacustrine system was mainly controlled by redox conditions. Ferruginous anoxic conditions in lacustrine system may have high potential for hydrocarbon source rocks owing to the reduction of oxygen decomposition.

The Late Ordovician mass extinction (LOME) was a severe biocrisis during the Phanerozoic and in this extinction, trilobites, brachiopods, mollusks, graptolites, conodonts and other types suffered severe mortality, and 85% of marine invertebrate species were eliminated. The potential cause of this mass extinction remains unclear, although glaciation and marine anoxia have received the greatest attention. Herein, we present findings on pyrite morphology, pyritic sulfur isotopic composition and total organic carbon content (TOC) to analyze the evolution of redox conditions across the Ordovician-Silurian boundary in the Yangtze area of South China. Our results show that long-term euxinic conditions interrupted by episodic dysoxic/oxic events occurred from the upper Katian stage to the lower Rhuddanian stage. The two extinction pulses of LOME were coincident with euxinic conditions, suggesting that marine euxinia may have been the main cause of this mass extinction event.

The Guadalupian mass extinction took place during the major global environmental changes during Phanerozoic time. Large-scale sea-level fluctuations and a negative shift ofδ13C were associated with this crisis. However, the diagenetic or primary origin of the decreasedδ13C across the Guadalupian–Lopingian (G–L) boundary and the potential causes for this biotic crisis are still being intensely debated. Integrated analyses, including detailed petrographic examination, identification of foraminifer and fusulinid genera, and analysis of carbonateδ13Ccarband bulkδ13Corgacross the G–L boundary were therefore carried out at Tianfengping, Hubei Province, South China. Our results show that: (1) some foraminifer and most fusulinid genera disappear in the upper Maokou Formation (upper Guadalupian); (2) the negative shift ofδ13Ccarbin the uppermost Maokou Formation is of diagenetic origin, but the values ofδ13Ccarbin the remainder of the Maokou Formation and in the Wuchiaping Formation represent a primary signal of coeval seawater; and (3) the bulkδ13Corgperturbation across the G–L boundary at Tianfengping is mainly controlled by organic matter (OM) source, that is, terrestrial OM contribution. We suggest that theδ13Ccarbnegative shift in the lower Wuchiaping Formation (Wuchiapingian) compared to that in the lower–middle Maokou Formation (Capitanian) were probably caused by the re-oxidization of12C-rich OM during regression. Global regression resulted in the negative shift ofδ13Ccarbat the G–L boundary in South China and led to the loss of shallow-marine benthic habitat. Large-scale global regression is probably one of the main causes for this bio-crisis.

The formation of carbonate concretions is a cementation process which passively infills the pore spaces within sediments. They record the original environments of deposition and diagenetic conditions of the host rocks. Little is known about the precise mechanisms responsible for the precipitation of carbonate concretions. The most common host rocks are mudstones/shales, sandstones, and limestones. This study presents an example of large carbonate concretions from an unusual host rock, the black bedded cherts of the Gufeng Formation (Guadalupian) at Enshi on the northern Yangtze Platform, South China. Petrographic observations (X-ray diffraction, optical microscopy, scanning electron microscopy) and multiple geochemical analyses (pyrite- and carbonate-associated-sulfate (CAS)-sulfur isotopes, carbon isotopes) indicate that (a) the studied carbonate concretion are mainly composed of micritic calcite with subordinate dolomite; (b) the concretions may have been mainly formed in the bacterial sulfate reduction (BSR) zone during very early diagenesis near the sediment-water surface; (c) the paleo-bottom water overlying the sediments during formation of the concretions was mainly euxinic; and (d) the growth of the studied concretions proceeded via a pervasive model, where later cementation phase initiated in the lower part of the concretions and progressed upward.

The recovery of marine ecosystems following a mass extinction event involves an extended interval of increasing biotic diversity and ecosystem complexity. The pace of recovery may be controlled by intrinsic ecosystem or extrinsic environmental factors. Here, we present an analysis of changes in marine conditions following the end-Permian mass extinction with the objective of evaluating the role of environmental factors in the protracted (~ 5-Myr-long) recovery of marine ecosystems during the Early Triassic. Specifically, our study examines changes in weathering, productivity, and redox proxies in three sections in South China (Chaohu, Daxiakou, and Zuodeng) and one in northern India (Mud). Our results reveal: 1) recurrent environmental perturbations during the Early Triassic; 2) a general pattern of high terrestrial weathering rates and more intensely reducing marine redox conditions during the early Griesbachian, late Griesbachian, mid-Smithian, and (more weakly) the mid-Spathian; 3) increases in marine productivity during the aforementioned intervals except for the early Griesbachian; and 4) stronger and more temporally discrete intervals of environmental change in deepwater sections (Chaohu and Daxiakou) relative to shallow and intermediate sections (Zuodeng and Mud). Our analysis reveals a close relationship between episodes of marine environmental deterioration and a slowing or reversal of ecosystem recovery based on metrics of biodiversity, within-community (alpha) diversity, infaunal burrowing, and ecosystem tiering. We infer that the pattern and pace of marine ecosystem recovery was strongly modulated by recurrent environmental perturbations during the Early Triassic. These perturbations were associated with elevated weathering and productivity fluxes, implying that nutrient and energy flows were key influences on recovery. More regular secular variation in deepwater relative to shallow-water environmental conditions implies that perturbations originated at depth (i.e., within the oceanic thermocline) and influenced the ocean-surface layer irregularly. Finally, we compared patterns of environmental disturbance and ecosystem recovery following the other four “Big Five” Phanerozoic mass extinctions to evaluate whether commonalities exist. In general, the pace of ecosystem recovery depends on the degree of stability of the post-crisis marine environment.

Bedding-parallel fibrous veins occurring as lenticular to flattened intercalations were found in the organic-rich marlstone/calcareous shale of the upper Lower Permian Chihsia Formation in western Hubei Province, South China. They dominantly consist of fibrous calcite crystals with smooth and tight boundaries, forming fence-like inward, syntaxial growth clusters toward the vein center along which a median suture line generally occurs. Petrographic evidence indicates that these veins may form at relatively shallow burial depth, where fluid overpressures would have incrementally created the bed-parallel vein space, resulting in displacive growth of fibrous calcite. On the other hand, the C, O and S isotopic data across the vein reveal slightly depleted δ13Ccarb values (−3.32 ‰ to +0.19 ‰ VPDB) and moderately depleted δ18Ocarb values (−9.6 ‰ to −7.3 ‰ VPDB) with respect to those of coeval seawaters and slightly heavier δ34Spyrite values (−7.88 ‰ CDT) with respect to those of ambient rocks. Stable isotope evidence consistently suggests significant contribution of bacterial sulfate reduction (BSR) to the formation of the fibrous calcite cements in the vein. The BSR could have been intensive with the availabilities of residual sulfate and abundant organic matters in the Chihsia sediments during shallow burial, increasing the alkalinity of pore waters and further promoting carbonate precipitation. Thus, the bedding-parallel fibrous calcite vein in the upper Lower Permian Chihsia Formation is an important time-specific petrographic capsule, providing clues for understanding the diagenetic process in organic-rich sediments.

The latest Permian mass extinction (LPME) coincided with major changes in the composition of marine plankton communities, yet little is known about concurrent changes in primary productivity. Earlier studies have inferred both decreased and increased productivity in marine ecosystems immediately following the end-Permian crisis. Here, we assess secular and regional patterns of productivity variation during the crisis through an analysis of the burial fluxes of three elemental proxies: total organic carbon (TOC), phosphorus (P), and biogenic barium (Babio). Primary productivity rates appear to have increased from the pre-crisis Late Permian through the Early Triassic in many parts of the world, although the South China Craton is unusual in exhibiting a pronounced decline at that time. Most of the 14 Permian–Triassic study sections show concurrent increases in sediment bulk accumulation rates, suggesting two possible influences linked to subaerial weathering rate changes: (1) intensified chemical weathering, resulting in an increased riverine flux of nutrients that stimulated marine productivity, and (2) intensified physical weathering, leading to higher fluxes of particulate detrital sediment to continental shelves, thus enhancing the preservation of organic matter in marine sediments. An additional factor, especially in the South China region, may have been the intensified recycling of bacterioplankton-derived organic matter in the ocean-surface layer, reducing the export flux rather than primary productivity per se. The ecosystem stresses imposed by elevated fluxes of nutrients and particulate sediment, as well as by locally reduced export fluxes of organic matter, may have been important factors in the ~ 2- to 5-million-year-long delay in the recovery of Early Triassic marine ecosystems.

Barite concretions and bands are widely distributed in black shale–chert horizons in the Yurtus Formation of Lower Cambrian in Aksu area, northwestern Tarim Basin, NW China. They mainly consist of coarse-grained anhedral to euhedral barite crystals with minor dolomites and pyrites. Petrological features indicate these concretions grew from the porewater in unconsolidated sediments at shallow burial below sediment-water interface. The slight deviation of 87Sr/86Sr ratios (0.7083 to 0.7090) and significant elevated δ34S values (56.8–76.4‰ CDT) of barite samples with respect to those of the Early Cambrian seawater further support that barite deposits precipitated from the enclosed porewater in sediment column, which evolved from the penecontemporaneous seawater with weak interaction with the host fine-grained siliciclastic sediments and highly-depleted sulfate in response to prolonged strong bacterial sulfate reduction without necessary renewal. The abundant organic matters in the basal Yurtus Formation should have facilitated developing sulfate-depleted methanogenesis zone and sulfate–methane transition zone (SMTZ) slightly after deposition. Therefore, barite deposits in the Yurtus Formation most likely resulted from diagenetic barium cycling and persistently grew from the porewater in the static SMTZ with a low sedimentation rate in the Early Cambrian. In comparison with the distribution of sedimentary barites in geological records, we tentatively proposed that a transition in diagenetic barium cycling and associated mineralization may have occurred from the Precambrian to Cambrian periods; this scenario may be causally linked to the changes in marine ecology (the advent of mesozooplankton and associated faecal pellet) and geochemistry (the increase of seawater sulfate concentration). Thus, the occurrence of diagenetic barite deposits in the Yurtus Formation implies that diagenetic barium cycling and more effective scavenging of barium from CH4- and Ba-rich porewaters within sediments might have become an nonnegligible process in continental margin areas, at least, since the earliest Cambrian, which could have significantly impacted the marine barium cycling.

The Late Permian environmental change, connecting the Guadalupian–Lopingian (G–L) (Middle–Upper Permian) boundary mass extinction and the Permain–Triassic (P–Tr) boundary mass extinction, has attracted more and more attentions. A significant negative shift for carbon isotope had been found at the Wuchiapingian–Changhsingian (W–C) boundary in the Upper Permian recently. However, the cause(s) of this negative excursion is still unknown. To resolve this problem, we analyzed the bulk organic carbon isotope, total organic carbon (TOC) content, pyritic sulfur (Spy) content, major element concentrations, and molecular organic biomarkers in the Wujiaping and Dalong formations in the Upper Permian from the Zhaojiaba section in western Hubei province, South China. Our results show that (1) there was a significant negative excursion in organic carbon isotopes at the W–C boundary and again a negative excursion at the top of Changhsingian stage; (2) the significant negative excursion at the W–C boundary was probably a global signal and mainly caused by the low primary productivity; and (3) the negative carbon isotope excursion at the top of Changhsingian was probably caused by the Siberian Traps eruptions. A decline in oceanic primary productivity at the W–C boundary probably represents a disturbance of the marine food web, leading to a vulnerable ecosystem prior to the P–Tr boundary mass extinction.

The Penglaitan section, as the Global Stratotype Section for the Guadalupian–Lopingian boundary (GLB), displays continuous deposition with a complete succession of pelagic conodont zones across the GLB. However, there is no reliable radiometric age from the Penglaitan section itself to constrain the GLB. Here, we report SIMS zircon U-Pb ages from two bentonite layers (Bed 7c) in the Penglaitan Global Stratotype Section near the GLB. The sample PL-62-1 yields a weighted mean 238U/206Pb age of 257.1 ± 2.2 Ma, and the sample PL-62-2 yields a weighted mean 206Pb/238U age of 257.0 ± 4.2 Ma. Therefore, we consider 257.0–257.1 Ma as the age of deposition of Bed 7c (the end of the C. postbitteri postbitteri conodont Zone, ca. 86 cm above the GLB), and, considering the depositional rate of chert, we suggest 258.6 Ma as the age of the GLB. Copyright © 2015 John Wiley & Sons, Ltd.

The Lower Triassic of the Guiyang area of southern China contains abundant and well-preserved trace fossils, which are useful in the reconstruction of ecological conditions following the end-Permian mass extinction. A total of 16 ichnogenera and 22 ichnospecies were identified; their vertical distribution and evolution indicate that benthic ecosystems experienced two periods of episodic recovery after the P–T event. Episode 1 occurs in the Daye Formation during the Dienerian, when trace-makers first appear in the upper portion of Member D1. Member D2 exhibits the first abrupt increase in ichnodiversity, and trace-makers shift from simple to complex, and undergo a substantial increase in size. These trends indicate a strong capability to rework sediments and an improvement of feeding strategies of trace makers, and points to the development of an early, initial, benthic ecosystem. Despite the emergence of complex trace fossils in D2, such as Phycodes and Phycosiphon , most of the trace fossils are small, horizontal burrows that are not fully developed in three dimensions. By the end of Dienerian, the regional environment deteriorated, and the benthic ecosystem was destroyed, as evidenced by the disappearance of trace fossils from upper D3 to lower A1, which also corresponds to a marked negative shift of δ 13 C carb . As a result, this newly established benthic ecosystem was vulnerable. Episode 2 occurs from the late Smithian to the Spathian. Trace fossils reappear in the lower Anshun Formation. The trace fossils from this stage are larger, more complex, and are fully three-dimensional. Trace maker diversity increases from single polychaeta, followed by the appearance of crustaceans, as represented by Rhizocorallium in Member A3 of the Anshun Formation. During this period, the marine environment improved and benthic organisms gradually established a basis for a new benthic ecosystem. The marine benthic communities had a transformative effect on the ecological environment of the ocean bottom and played an essential role in paving the way for the recovery of other organisms. Two episodes of evolution of trace fossils reflect that initial restoration of benthic ecosystems was apparently reset at least once during the Early Triassic, recovery was not delayed because of persistent environmental stress but short episodes of deleterious conditions. Marine benthic ecosystems had a full and lasting recovery occurred at the global scale during the Spathian, when oceanographic conditions ameliorated and allowed the recovery of ecosystems worldwide.

A multiproxy study of a new Upper Permian–Lower Triassic section (Xiaojiaba) in Sichuan Province, China, documents large changes in marine productivity, redox conditions and detrital input prior to the latest Permian mass extinction. Marine productivity, as proxied by total organic carbon content (TOC), biogenic SiO 2 , and excess barium, displays a long-term decline through most of the Changhsingian stage (late late Permian), culminating in very low values around the Permian–Triassic boundary. Concurrently, redox proxies including pyrite framboid, δ 34 S py , Mo auth and U auth , and C org /P document a shift from suboxic to dysoxic/oxic conditions that was interrupted by several episodes of benthic euxinia, and detrital siliciclastic proxies (Al, Hf, Nb, and REEs) suggest an increased flux of weathered material from land areas. The long-term changes in productivity, redox conditions, and terrigenous detrital fluxes were probably caused by a regional sea-level fall across the South China Craton. On the other hand, the brief euxinic episodes occurring during the late Permian had oceanographic causes, probably related to the transient upward expansion of the chemocline at the top of the oceanic oxygen-minimum zone. These euxinic episodes may have been harbingers of the more widespread anoxia that developed concurrently with the latest Permian mass extinction and that may have played a major role in triggering the largest biotic crisis of the Phanerozoic.

The Late Paleozoic Ice Age across Carboniferous and Permian had a significant impact on the Kungurian (Upper Cisuralian series of Permian) Chihsia Formation in South China. This resulted in a unique interval with features such as the lack of reef in Chihsian limestone, widespread stinkstone and nodular/bedded chert. The Chihsia limestone (Kungurian stage) deposited during a time of cooling was resulted from oceanic upwelling. Here we present evidence for this upwelling using several geochemical analyses: bulk organic carbon isotope, biomarker molecular geochemical data, and authigenic silica of the stinkstone member in the lower Chihsia Formation of the Kuangurian stage from the Enshi Section in western Hubei Province, South China. The lower part of the stinkstone member shows a rapid organic carbon isotope excursion with a −3‰ shift triggered by the upwelling of 13C-depleted bottom water. The concurrent rapid increasing of authigenic silica content resulted from the enhanced supply of dissolved silica in the upwelling water mass. This upwelling at the Enshi Section also led to relative high TOC content, accounting for the widespread stinkstone in the lower Chihsia Formation during the Kungurian stage in Permian.

From the Middle to Late Permian, the Laibin area in Guangxi, South China, was situated on the slope of an isolated carbonate platform, on which continuous marine successions were deposited. Two global stratotype sections for the boundary between the Guadalupian (Middle Permian) and Lopingian (Late Permian) are located at Penglaitan and Tieqiao in the Laibin area, respectively, and thus are chosen for study. At the two locations, 14 facies are recognized in the Maokou and Heshan Formations, and they are further grouped into four facies associations (basin, lower slope, upper slope, and platform margin). Six main transgressive–regressive (TR) sequences are identified in strata from the Roadian (Middle Permian) to the Wuchiapingian (Late Permian). They are conformable marine sequences that were little influenced by regional uplift (Dongwu Movement) and so provide a good record of the sea-level changes in South China at this time. Based on the significant taxonomic selection and controversial marine faunal loss in the end-Guadalupian mass extinction, and the Middle-Late Permian sea-level changes recorded by the TR sequences in the Laibin area, it is suggested that this extinction event might have been triggered by the reduction and loss of shallow-marine habitat area caused by the end-Guadalupian regression. The global cooling and Emeishan volcanism also occurring at this time could have further enhanced this extinction event.

During the late Permian, the Yuanba area of the northeastern Sichuan Basin, China, was predominantly occupied by a carbonate platform in an extensional setting. On the basis of wireline log, core sample, and cutting fragment, two depositional systems are recognized in the Changxing Formation, including a homoclinal ramp and a rimmed carbonate platform system. During deposition, the platform was evolved from a homoclinal ramp through a poorly rimmed platform into a rimmed platform. High-energy deposits of shoals and reefs preferentially formed on the pre-existing palaeohighs and grew upward to develop an aggradational architecture. Two depositional (third-order) sequences, consisting of several parasequences, are recognized in the Changxing Formation, the development of which was mainly controlled by eustatic fluctuations, with minor influence of tectonism. Individual sequences were defined by conformable surfaces along the depositional dip section. The distribution of facies and parasequence composition are variable both laterally across the platform-basin profile and vertically through time due to a combination of relative sea-level changes, palaeogeographic architecture, and tectonism. The sedimentological and stratigraphic characterization will aid further future exploration and production of hydrocarbon from the Changxing Formation.

article i nfo The organic-rich Chihsia Formation (Middle Permian) on the Yangtze platform, western Hubei, South China, is considered to be the major hydrocarbon source rock in South China. The lower part of the Chihsia Formation, reported here, was deposited in an intrashelf basin and is characterised by black laminated marlstones/shales intercalated with dark-grey limestones/dolomites in which metre-scale shallowing (or cleaning)-upward cycles are ubiquitous. Each cycle is composed of basal marlstone/shale and upper lime- stone. The organic matter is concentrated exclusively in the shaley bases of these cycles, and shows system- atic, cyclic variations in abundance up through the succession. An integrated approach of pyrite morphology and multiple geochemical proxies was used to determine the mechanism of organic accumulation. In the organic-rich sediments, many redox proxies, including pyrite framboid size distribution, C-S-Fe relation- ships, iron speciation, and δ 34 S values of pyrite suggest deposition beneath the dysoxic bottom water, which evolved into a more oxic water body when the organic-poor limestones were deposited. However, the coincidence of increased abundance of total organic carbon (TOC) with increased Ba and Mo contents in- dicates that organic matter accumulation was controlled mainly by primary productivity. These data indicate that increased organic accumulation resulted mainly from higher organic carbon export with increased nutri- ent flux during superimposed sea-level rises of different orders. This in turn could have increased the oxygen consumption by respiration of microorganisms and organic matter decay, leading to the depletion of oxygen in the water columns. Furthermore, the rapid sedimentation rate further limited bioturbation and organic matter decomposition, facilitating the preservation of buried organic matter.

The Ediacaran–Cambrian transition, one of the most critical intervals in Earth's history, is marked by dramatic biological, oceanic and geochemical turnovers. Here high-resolution carbon and sulfur isotopic data respectively for organic carbon and pyrite, and iron speciation data are presented from the deep-water Liuchapo and Niutitang Formations on the Yangtze block, South China. The carbon isotopic data, together with biostratigraphic and radiometric dating, offer the compelling evidence for the placement of Ediacaran–Cambrian boundary within the Liuchapo Formation (chert succession), and for its correlation with shallow-water equivalents elsewhere. In this context, iron speciation and sulfur isotopic data further suggest a predominant anoxic and ferruginous deep ocean over the transitional time until the middle Early Cambrian (Atdabanian or Stage 3) when the deep ocean was rapidly oxygenated. Coincidently, during this interval, large-body metazoans (i.e., sponges) abruptly appeared in the deep ocean, which was temporally associated with the highly diversified large-body skeletonized animals (i.e., Chengjiang Biota) which colonized in shallow-water niches particularly in southwestern China. This scenario suggests a causal link between deep oceanic oxygenation and the explosive diversification of large-body skeletonized organisms in the Early Cambrian.

The end-Paleozoic biotic crisis is characterized by two-phase mass extinctions; the first strike, resulting in a large decline of sessile benthos in shallow marine environments, occurred at the end-Guadalupian time. In order to explore the mechanism of organisms’ demise, detailed analyses of depositional facies, fossil record, and carbonate carbon isotopic variations were carried out on a Maokou-Wujiaping boundary succession in northwestern Sichuan, SW China. Our data reveal a negative carbon isotopic excursion across the boundary; the gradual excursion with relatively low amplitude (2.15‰) favors a long-term influx of isotopically light 12C sourced by the Emeishan basalt trap, rather than by rapid releasing of gas hydrate. The temporal coincidence of the beginning of accelerated negative carbon isotopic excursion with onsets of sea-level fall and massive biotic demise suggests a cause-effect link between them. Intensive volcanic activity of the Emeishan trap and sea-level fall could have resulted in detrimental environmental stresses and habitat loss for organisms, particularly for those benthic dwellers, leading to their subsequent massive extinction.