Yangtze Platform In South China Research Articles

Geochemical Characteristics of Trace Elements and Mineralization Model of the Ediacaran-Early Cambrian Phosphorites, South China.

As a nonrenewable resource, phosphate rock is an important support for the development and survival of the national economy. The regional distribution and output of phosphate rock in China are extremely uneven, and the amount of high-quality ore resources is relatively poor, which seriously restricts the development and utilization of phosphate rock resources in China. This paper briefly summarizes the distribution characteristics of phosphate rock resources and summarizes the characteristics and research progress of Ediacaran-early Cambrian phosphorus mineralization types, geological characteristics, and deposit genesis of the Yangtze platform in South China. The Ediacaran-early Cambrian sedimentary phosphorite deposits in China are mainly distributed in Yunnan, Guizhou, Hubei, Sichuan and Hunan provinces of the Yangtze platform, in which the early Cambrian phosphate deposits are also rich in rare earth elements, associated with uranium, nickel, molybdenum, vanadium, and other beneficial metal elements. The increase of atmospheric oxygen content at the Ediacaran-Cambrian boundary may have promoted the extensive oxygenation of the late Neoproterozoic oceans, so the Ediacaran-early Cambrian oceans generally showed a reductive environment, and there may be dynamic chemical stratification of the oxidation zone-sulfide zone-iron zone. Up to the early Cambrian, the redox stratified structure of Precambrian seawater may still be inherited, showing that the surface water is an oxidizing environment, changing to a reduction environment, and even wedge-shaped sulfide water is developed at the bottom of the deep basin. The main phosphorus sources are deep phosphorus-rich seawater, continental weathering, and deep hydrothermal activity of Ediacaran-early Cambrian marine sedimentary phosphorite deposits in South China. The genetic mechanisms of phosphorite deposits in the Yangtze platform in South China are mainly biogenic, upwelling phosphorus-forming theory, mechanical mineralization, and syn-sedimentary hot water mixed genesis. In the future, it is still necessary to further explore the internal relationship between phosphorus deposits and major geological events, the in situ analysis of microstructure of phosphate rock ores, and the genetic mechanism of phosphate deposits and the reconstruction of paleo-marine environment.

A high-resolution record of the late Hirnantian to Aeronian marine redox change in South China and its relationship with the record of graptolite biodiversity

Originating in the late Hirnantian (latest Ordovician) and extending into the Rhuddanian (early Silurian), the global Rhuddanian Oceanic Anoxic Event (Rhuddanian-OAE), characterized by habitat-specific anoxic-euxinic seafloor conditions led to the deposition of widespread graptolitic black shales. However, the intricate relationship between habitat-specific seafloor redox conditions and graptolite diversity record of the Rhuddanian-OAE remains enigmatic. Here, we present a continuous sequence of seafloor redox records obtained from a core spanning from the upper Katian (Ordovician) to the middle Aeronian (Silurian) in the Changning region of SE Sichuan Province. This region represents the Inner Yangtze Sea within the upper Yangtze Platform of South China. Following the glaciation peak of the Hirnantian, our study unveils the evolution of bottom waters, transitioning from oxic to permanently euxinic, intermittently euxinic, anoxic, and suboxic–oxic transitional states. This evolution is supported by independent evidence, including trace metals (Mo, U, V) and Corg/TP ratios, lithology, and TOC content. Local factors and global seawater drove redox changes of bottom water in Changning. In contrast to the increasing oxygenation in Changning and the northern hemisphere's suboxic bottom waters in Yukon, Canada, the bottom water of the southern hemisphere in the early Aeronian maintained a euxinic environment, potentially influencing the graptolite paleogeography of the early Aeronian. The graptolite diversity record was likely influenced by both paleoecological changes in the preferred habitat of deeper-water species combined with taphonomic processes that influenced their preservation potential.

Geochemistry of Cryogenian Datangpo manganese deposits in the southeastern Yangtze Platform of South China: implications for the origin of metallogenesis and depositional environment

ABSTRACT Sedimentary manganese (Mn) deposits in the southeastern Yangtze Platform of China have been newly discovered with considerable potential for resource exploration. Such deposits formed due to dynamic changes in the global environment, including the break-up of the supercontinent Rodinia, Snowball Earth glaciation, and the Neoproterozoic Oxygenation Event. The Mn-bearing sequences are hosted in the basal Datangpo Formation and are interbedded with black carbonaceous shale in a series of graben sub-basins in the Nanhua Rift Basin. We investigated the origin of these Mn deposits by analysing the major and trace elements, total organic carbon, and stable carbon and oxygen isotopes. Positive Eu anomalies were observed, indicating that the Mn was derived from a hydrothermal system. Redox-related elements, including Ce, Mo, U, V, S, and P, indicated that the depositional environment of the Mn layers was oxidative and that Mn2+ was initially oxidized to Mn oxides. However, the host rock (i.e. black carbonaceous shale) was deposited under dysoxic – anoxic and even sulfidic water conditions. The alternating distribution of Mn ore and host rock suggested alternating redox conditions in the sub-basins. A negative correlation between the Mn content and δ 13Ccarb values (−10‰–−7‰) and a deficiency of sulphide minerals indicate that the diagenetic degradation of organic matter (δ 13Corg from − 34‰ to − 31.9‰) took place through Mn oxide reduction rather than microbial sulphate reduction and can contribute to a considerable source of bicarbonates. The approximately homogeneous δ 13Ccarb depletion and their apparent discrepancy with δ 13Corg indicate that the post-depositional genesis of the Mn carbonate ore occurred in a relatively open early diagenetic pore water system with a considerable contribution from seawater-derived carbon sources.

A unique early Triassic (Spathian) conodont community from the Nanzhang‐Yuan'an Fauna, Hubei Province, South China

The largely stagnant marine ecosystem following the Permian–Triassic mass extinction (PTME) was superseded by the appearance of marine reptiles. One of the most abundant Early Triassic (Spathian) marine reptile faunas occurs in the Nanzhang‐Yuan'an area of Hubei Province, South China, and provides remarkable information for understanding biotic recovery after the PTME. Situated on the north edge of the Middle Yangtze Platform in South China, the well‐exposed Zhangjiawan stratigraphic succession in Yuan'an County represents one of the most productive sections, yielding abundant marine reptiles. Intensive sampling of this section reveals a unique conodont community both before and during the emergence of the Nanzhang‐Yuan'an Fauna (NYF). Occurrences of Neostrachanognathus tahoensis below the NYF and deposition of ʻGreen Bean Rock (GBR)ʼ above the NYF restricted this fauna to the latest Early Triassic. Except for the coniform Neostrachanognathus, this conodont community consists of abundant Ellisoniidae characterized by ramiform P1 elements. Foraminifers and fish found together with the conodont elements form other low‐level parts of the local food web. Other temporally coincident sections nearby, for example, the Yanduhe section of western Hubei Province, yield only rare conodonts. Hence, the unique conodont community from the Zhangjiawan section suggests that the Nanzhang‐Yuan'an area might be a “refuge area” for the Early Triassic marine ecosystem, or at least for the marine reptiles and the conodont communities. Although the direct connection between conodonts and marine reptiles is still uncertain, low‐level consumers—conodont animals‐played important roles in the food webs of NYF. The collision of the South China and North China blocks removed this “refuge area” and witnessed the disappearance of endemic Hupeisuchians.

Morphological reconstruction of the Ediacaran macroalga Gesinella from South China

Macroalga Gesinella, a carbonaceous compression, is found in the upper part of Doushantuo Formation (Ediacaran) in the Yangtze Platform of South China. Morphologically, it is composed of a thallus (including an oval or oblong lamina and a short-rod-like stipe) and a holdfast (including a cone-shaped rhizome and many filamentous rhizoids). The cone-shaped rhizome connects with the stipe and the long rhizoids grew on the rhizome, meaning that the rhizome grew downwards and inserted into the sediment, while the rhizoids grew in the gaps between the sediment grains to anchor its body on the seafloor. On the lamina, branching and/or unbranching filaments are regularly distributed, so that the oval or oblong lamina can be interpreted as a sac-like body that has been compressed. The filaments on the lamina extend from the three-dimensionally preserved stipe, suggesting that the short-rod-like stipe may have been flexible to support the sac-like body in the water column. With tissue and organ differentiations, Gesinella can be regarded as a eukaryotic macroalga and a high-level metaphyte. Based on measurement and analysis of 108 specimens, the Ediacaran macroalga Gesinella from South China can be divided into three stages: juvenile stage (<3 mm maximum width) identified with difficulty, adult stage (3–10 mm maximum width) interpreted to have grown upwards for sunlight, and senescent stage (>10 mm maximum width) that grew sideways.

New Early Triassic marine δ13C record from the northeastern Yangtze Platform: Implications for contemporaneous temperature changes and volcanic eruptions

The Early Triassic records the largest inorganic carbon isotope excursions of the Phanerozoic. The causes of these enormous δ13C shifts remain unclear. Here, we present a new high-resolution marine carbonates δ13C record from the Yashan section in the northeastern Yangtze Platform of South China. The δ13C profile shows two significant positive excursions (with maximum δ13C values of ∼ +6‰ and ∼ +9‰, respectively) at the Induan–Olenekian boundary and Smithian–Spathian boundary, which is similar to what is recorded in the shallow platform in Nanpanjiang Basin and in the upper Yangtze Platform area. Negative δ13C excursions during the Griesbachian–early Dienerian, Smithian and middle Spathian, are coincident with warming intervals and accounted for by coeval peaks of volcanic activity (e.g., the release of CO2 from the Siberian Traps and arc volcanism). Enhanced marine primary productivity, as a final effect of massive carbon release, could account for the two positive δ13C excursions, with minor influence of local effects on the magnitude of the shifts. Although the age of volcanism after the Permian–Triassic mass extinction is still not constrained, more lines of evidence support a volcanism-temperature-weathering-productivity mechanism of δ13C perturbations during the Early Triassic.

Mercury isotope evidence for a non-volcanic origin of Hg spikes at the Ordovician-Silurian boundary, South China

Ordovician-Silurian transition (OST) sections of South China contain extremely high mercury (Hg) concentrations (>1000 ppb) of uncertain provenance. The main hypotheses concerning their origin are: (1) contemporaneous elevated seawater Hg concentrations (e.g., due to volcanogenic inputs) combined with normal Hg-uptake processes by marine sediments, and (2) normal seawater Hg concentrations combined with enhanced Hg uptake by marine sediments (e.g., due to strong adsorption by sulfides). Here, we investigate Hg isotopes, which are a promising tool to track the sources of Hg in sediments, in OST strata of the Jiaoye drillcore from the Yangtze Platform of South China. Black shales and pyritic beds exhibit mass-independent fractionations of odd Hg isotopes (odd-MIF, i.e., Δ199Hg) of +0.13±0.05‰ and +0.13±0.03‰, respectively. These values are similar to those of modern and ancient marine sediments, supporting a seawater source of Hg in the study units and providing evidence against a volcanic source. We infer that the extreme Hg enrichment of the OST beds was due to elevated rates of Hg uptake from seawater through adsorption to pyrite. Local environmental conditions (e.g., intense euxinia and microbial sulfate reduction) played a dominant role in Hg enrichment of OST strata in South China.

Orbital forcing of late Early Devonian storm events on a mixed carbonate-siliciclastic shelf, Longmenshan area, Sichuan Province, China

AbstractThe Yangmaba Formation of latest Early Devonian age in the Longmenshan area of Sichuan Province, China, is a shelfal facies that consists of four types of carbonate-siliciclastic deposits: clay-rich, siliciclastic sand-rich, carbonate-dominated, and hybrid mixed. Storm deposits vary in their thickness, composition, and abundance-per-meter within these mixed carbonate-siliciclastic deposits. Meter-scale statistics of the relative storm frequency (events-per-meter) and magnitude (bed thickness) were compiled in two coeval sections through each of the hybrid facies in the lower Yangmaba Formation, which spans approximately the entire conodont Polygnathus patulus Zone of the uppermost Emsian Stage. Low-pass filtering, spectral analysis, and Acycle software interpretation of these tempestite statistics yield 3.5 main oscillations in each section, with an average wavelength of ~23 m. These long-wavelength trends are semi-coincident with interpreted long-term variations in sea level, where shallower depths allowed a greater influence by storms. Superimposed on the long-wavelength cycles are medium-wavelength cycles of 5.5–6.3 m. The estimated ~1.5 m.y. time-span of this conodont zone of the lower Yangmaba Formation and the approximate 1:4 ratios of these wavelengths indicate that frequency and intensity of major storms and the recording of tempestites in the sedimentary record were modulated by ~100- and 95-k.y.-short-eccentricity orbital-climate oscillations superimposed on a main 405-k.y.-long-eccentricity cycle. These eccentricity climate cycles governed storm intensity and regional sea level on this margin of the tropical Yangtze Platform of South China.

Llandovery (Silurian) conodont biofacies on the Yangtze Platform of South China and their palaeoenvironmental implications

• Silurian (Telychian) conodont distribution on the Yangtze Platform of South China is quantitatively studied for the first time. • Water-depth controlled Dapsilodus-Decoriconus Biofacies and Apsidognathus-Galerodus Biofacies are recognised on the Yangtze Platform. • Distribution of these biofacies can be closely related to Early Silurian sea-level changes. Diverse Telychian (Llandovery Series, Silurian System) conodont faunas have been documented from the South China Palaeoplate. Based on data from six selected sections in the Yangtze Platform, the spatial distribution of the Telychian conodont associations is investigated in detail by use of multivariate statistical methods. Two conodont biofacies are recognised within the Pterospathodus eopennatus Biozone: Dapsilodus-Decoriconus Biofacies and Apsidognathus-Galerodus Biofacies. The former is interpreted as representing a relatively deeper-water environment. The Apsidognathus-Galerodus Biofacies is widely distributed on the Yangtze Platform and is indicative of a shallower depositional setting. Vertical changes of conodont biofacies in the Baizitian and Xuanhe sections permit the recognition of transgressive–regressive patterns which are closely similar to published sea-level curves for the Early Silurian.

Origin of K-bentonites in the Doushantuo cap dolostones from South China and its potential link with the sedimentary model of the Marinoan cap dolostones

A study of the K-bentonites in the Doushantuo cap dolostones on the Yangtze Platform in South China would facilitate understanding the origin of the Marinoan cap dolostones and testing the snowball Earth hypothesis. To date, sporadic work has been conducted on the whole-rock geochemistry of the K-bentonites from the platform and slope of the Yangtze Platform, to preliminarily understand the genesis of the K-bentonites. However, a study of the whole-rock geochemistry of the K-bentonites from the basin and on zircon geochemistry of the K-bentonites from the entire Yangtze Platform has not been carried out. Therefore, the nature of the primary magmas, source and tectonic setting of the source volcanoes, and the episodes of source volcanic eruptions of the K-bentonites are unknown, which hinders unraveling the possible link between the sedimentary model of the Marinoan cap dolostone and the K-bentonites. Here, we perform a systematic study of the whole-rock and zircon geochemistry of the K-bentonites. The major and trace element concentrations of the K-bentonites and trace element contents in their volcanic zircons suggest that the primary magmas of the K-bentonites from the Jiuqunao and Heyu sections are different in nature. The same geochemical results indicate that the source volcanoes of all the K-bentonites may occur in the extensional setting on the northern margin of the Yangtze Block. Hf isotopic compositions of the volcanic zircons in the K-bentonites imply that the source magmas of the K-bentonites are mainly mantle-derived, and may have mixed with the crustal materials from the northern margin of the Yangtze Block. Based on the difference in whole-rock and zircon geochemistry of the K-bentonites from Heyu and Jiuqunao, two episodes of the source volcanic eruptions of the K-bentonites could be identified. The K-bentonite at Heyu most probably represents the record of the old episode, while that at Jiuqunao represents the record of the young one. The here proposed dual episodic deposition of K-bentonites within the Doushantuo cap carbonates holds potential significance to constrain 1) the duration of the cap carbonate deposition and 2) their genesis in potential slushball or snowball Earth environments.

Geochemistry of lower Ordovician microbialites on the Yangtze Platform, South China: Implications for oceanic oxygenation at the onset of the GOBE

The causes of the Great Ordovician Biodiversification Event (GOBE) remain a topic of debate. Recent studies indicate that global and regional oceanic oxygenation was a plausible trigger of the GOBE. The Ce anomaly of rare earth and yttrium (REY) elements in shallow marine carbonates is thought to be a powerful indicator of marine redox conditions of local waters. In this study, we analyzed REY, major and trace element compositions, and C and O isotopes of the Lower Ordovician microbial-dominant reefs on the Yangtze Platform, South China. We evaluate which carbonate components within the reefs most reliably record the seawater REY composition. The elemental correlations show that the REY elements of oncoids, stromatoporoids, sponges and carbonate infillings were affected by clastic influence, diagenetic processes, and the vital effects of carbonate secreting organisms; whereas the microbialites REY were partially modified by synsedimentary terrigenous and little diagenetic modification. Such introduction of synsedimentary terrigenous REY increased ΣREY and flattened the REY patterns of the microbialites, but the redox signals (Ce anomalies) of input source of the water column might still preserved in microbialites formed in situ. Although the magnitude of Ce anomalies is small, variation might still reflect authentic redox history of primary local seawater. In the middle Tremadocian, microbialites have positive Ce anomalies which were coeval to positive δ13Ccarb shifts. The positive δ13Ccarb shifts were documented from other sections of central Yangtze Platform, North American and Argentina, which might indicate global anoxic event. In the late Tremadocian to early Floian, microbialites have more visible negative Ce anomalies, which might indicate regional oceanic oxygenation event. The negative Ce anomalies provide direct local-geochemical evidence of oceanic oxygenation in the late Tremadocian-early Floian on the Yangtze Platform of South China, which might facilitate the first acme of biodiversification of local faunas predating the main pulse of the GOBE on the other palaeoplates.

Trace element and C-S-Fe geochemistry of Early Cambrian black shales and associated polymetallic Ni-Mo sulfide and vanadium mineralization, South China: Implications for paleoceanic redox variation

Early Cambrian black shales are widely exposed along the southeastern margin of the Yangtze Platform in South China for a length of over 1600 km. The basal part of this sequence host a large spectrum of mineralization, including polymetallic Ni-Mo sulfide, vanadium, phosphorite, barite, as well as combustible shale (stone coal). In particular, the polymetallic Ni-Mo sulfide ore layer is stratigraphically equivalent but geographically separated from the V mineralization. In the Zhangjiajie region in Hunan province, the tenor of the polymetallic Ni-Mo sulfide mineralization in the northeastern segment decreases gradually and grades into vanadium mineralization in the southwestern segment. Based on the stratigraphic correlation, we compared the trace element composition and carbon–sulfur-iron system of the polymetallic Ni-Mo sulfide ore, vanadium ore, and their host black shales.The polymetallic Ni-Mo sulfide and V ores display similar PAAS-normalized REE distribution patterns with negative Ce and positive Y anomalies, similar to present-day seawater. The polymetallic sulfide ores show elevated seawater-like Y/Ho ratios (mean: 52.7 ± 6.8) and the V ores with an average Y/Ho ratio of 40.2 ± 3.8 reflect the influence of terrigenous material. Although both the polymetallic Ni-Mo sulfide ore and the V ore formed under anoxic conditions, the degree of oxygen deficiency was variable, i.e., euxinic condition for the polymetallic sulfide ore but euxinic and suboxic conditions for the vanadium ore. This model is supported by the more elevated redox-sensitive elements (e.g., U and Co) in the polymetallic Ni-Mo sulfide ores compared to the V ores. Excess sulfur of the polymetallic Ni-Mo sulfide ores with S/Fe ratios>1.15 suggests that they formed under iron-limited euxinic conditions. In contrast, the V ores formed under euxinic to suboxic conditions with S/Fe ratios close to or lower than 1.15. The host black shales display positive correlations for TOC and redox-sensitive elements, suggesting that they formed under suboxic conditions. We infer that the polymetallic Ni-Mo sulfide ores formed in a sediment-starved semi-restricted euxinic marine basin. In contrast, the V ores formed under both euxinic and intermittently suboxic to euxinic conditions at the slope of the basin with terrestrial input from enhanced continental weathering. Repeated transgressive–regressive episodes led to the formation of V ores interlayered with suboxic black shale with more subdued oxygen deficiency. The results of this study suggest that exploration for V mineralization should focus on the southeastward lateral stratigraphic equivalents of the polymetallic Ni-Mo sulfide mineralization.

Constraints on the Ediacaran-Cambrian boundary in deep-water realm in South China: Evidence from zircon CA-ID-TIMS U-Pb ages from the topmost Liuchapo Formation

The placement of the Ediacaran-Cambrian boundary in deep-water realm in South China and the high-precision temporal framework for the Ediacaran-Cambrian transition in this region have not yet been completely solved. Recently, we have found two K-bentonite beds in the top of the Liuchapo Formation at the Pingyin section, Guizhou Province. It provides an opportunity for constructing the temporal framework of the transitional strata on the Yangtze Platform in South China and for determining the Ediacaran-Cambrian boundary in this area. In this study, we conducted high-precision CA-ID-TIMS U-Pb dating on zircons from the two K-bentonites. The ages of the two K-bentonites were precisely constrained at 536.40±0.47/1.1/1.2 Ma (2σ, n=7, MSWD=0.92) and 541.48±0.46/1.1/1.2 Ma (2σ, n=8, MSWD=1.3). Combining the geochronological results with organic carbon isotope data of chert in the topmost Liuchapo Formation from the section, we suggest that the EdiacaranCambrian boundary should be consistent with a significant negative organic carbon isotope excursion between the two K-bentonites. The scheme of the Ediacaran-Cambrian boundary in this study is of great significance for global correlation, and further for unravelling the information of the terminal Ediacaran-early Cambrian ocean.

Suboxic-dominated conditions in the Ediacaran shallow ocean: Geochemistry of pelitic siltstones in the Doushantuo Formation, South China

The Ediacaran witnessed a significant oxygenation of the Earth's surface in line with the emergence and diversification of early animals, but the redox conditions of the shallow waters during this period are still uncertain. Here, we present geochemical profiles from neritic pelitic siltstones of the Doushantuo Formation at the village of Xiaoyang on the northwestern margin of the Yangtze Platform in South China. Most sediments show moderate to low FeHR/FeT ratios (≤0.38), low FePy/FeHR ratios (<0.7), and low enrichment factors (EF ≤ 1) for most redox-sensitive elements, suggesting predominately oxygenated shallow waters. However, we also find moderately enriched Re (EF ~10–25) combined with depleted U and Mo (EF ≤ 1), indicating suboxic conditions. Furthermore, a few samples show FeHR/FeT > 0.38, suggesting episodic anoxia in the shallow ocean. Overall, the Xiaoyang section represents a suboxic shallow ocean with occasional episodes of anoxia during the Doushantuo stage, implying that the oxygenation level in the early-mid Ediacaran ocean was still limited, even under some shallow conditions. It also supports the high redox heterogeneity in the early-mid Ediacaran ocean, which could have restricted the evolution and distribution of early metazoans.

The architecture of organic matter and its pores in highly mature gas shales of the lower Silurian Longmaxi Formation in the upper Yangtze platform, south China

The nanometer-scale architecture of organic matter (OM) and associated pores in highly mature gas shales from the lower Silurian Longmaxi Formation in the upper Yangtze platform of south China were investigated using field emission scanning electron microscopy (SEM), focused ion beam SEM, and low-pressure gas (N2 and CO2) adsorption bulk pore characterization. The Longmaxi shale comprises fine-grained siliciclastic rocks deposited in a marine shelf environment, which was dominated by quartz and clay minerals. Four porous OM types were found in the Longmaxi shale on the basis of the chemical composition and spatial occurrence of OM, including (1) isolated original OM particles, (2) OM–clay mineral complexes, (3) OM–heavy mineral complexes, and (4) secondary migrated bituminous OM. The pores in the particulate OM are not homogeneously distributed, and the processes leading to different pores depend on the specific OM type. The nature of OM-hosted pores is a result of several factors, such as primary porous kerogen, mechanical compaction, organic–inorganic interactions, gaseous and liquid hydrocarbon generation, retention, and expulsion. Pore volumes and specific surface areas of the Longmaxi shale derived from low-pressure N2 and CO2 adsorption experiments reveal positive linear relationships with total organic carbon contents, which indicates that the pore systems in the highly mature Longmaxi shale are dominated by OM-hosted pores. Additionally, the OM-hosted pores appear connected compared to pores in the mineral matrix. Therefore, the OM-hosted pore systems offer the preferential storage space and primary migration pathways for natural gas in the Longmaxi shale reservoir.

High primary productivity during the Ediacaran Period revealed by the covariation of paired C-isotopic records from South China

Remarkable carbonate carbon isotope (δ13Ccarb) excursions accompanied by invariant organic carbon isotope (δ13Corg) values have been observed in many late Neoproterozoic successions. This decoupled relationship between δ13Ccarb and δ13Corg in the Precambrian differs from most Phanerozoic records which have been substantially imprinted by aqueous photosynthesis, and it has been tentatively interpreted as resulting from exotic organic matter inputs or diagenesis. In this study, we present high-resolution δ13Corg records in combination with previously published δ13Ccarb data from a continuous drill core from the Yangtze platform of South China. The studied interval spans the Cryogenian Nantuo Formation, the Ediacaran Doushantuo and Dengying formations, and the base of the Cambrian Yanjiahe Formation. The δ13Ccarb profile of this drill core can be reliably correlated with contemporaneous sequences and it reveals multiple negative δ13Ccarb excursions associated with phosphorite deposits. Intriguingly, the δ13Corg profile exhibits a co-varying trend with the δ13Ccarb profile, and the coupled relationship between δ13Ccarb and δ13Corg is rare in geological records of the Precambrian. The δ13Corg values of the drill core vary from −22.13‰ to −34.12‰, and the overall isotopic difference (Δ13Ccarb-org) between δ13Ccarb and δ13Corg ranges between 19.61‰ and 34.58‰. Most of the δ13Corg and Δ13Ccarb-org data can be interpreted in terms of oxygenic photosynthesis, probably mainly by eukaryotic algae. However, it is possible that exotic organic matter, most likely DOC from the deep ocean, may also have contributed to the sedimentary organic matter composition of specific intervals studied herein. Moreover, high primary productivity in shallow water may have been sustained for a long period in phosphate-rich conditions, and the organic matter generated by photosynthesis may have been quantitatively sufficient to overwhelm the DOC buffering effect in the total δ13Corg signal throughout the Ediacaran Period. The enhanced burial of organic matter in sediments would gradually result in the accumulation of oxygen, both in the ocean and atmosphere, which would likely have paved the way for macroscopic animal evolution.

The interplay of dynamic topography and eustasy on continental flooding in the late Paleozoic

Global sea level change can be inferred from sequence stratigraphic and continental flooding data. These methods reconstruct sea level from peri-cratonic and cratonic basins that are assumed to be tectonically stable and sometimes called reference districts, and from spatio-temporal correlations across basins. However, it has been understood that long-wavelength (typically hundreds of km) and low-amplitude (<2 km) vertical displacements of the Earth's surface due to mantle flow, namely dynamic topography, can occur in the absence of crustal deformation. Dynamic topography can drive marine inundation or regional emergence of continents and must be taken into consideration for eustasy estimates. Our analysis indicates that the long-term trend in global-scale maximum flooding over the late Paleozoic generally correlates with global sea level curves. The first-order flooding history of North America correlates with some estimates of eustasy. The Paleozoic inundation of South America does not follow long-term sea level variations. The flooding lows during the Early Carboniferous and high during the Late Carboniferous are at odds with estimates of eustasy and can be explained by dynamic uplift and subsidence, respectively. Our dynamic topography models indicate that the Yangtze Platform of South China experienced significant dynamic subsidence during the transition from Permian to Triassic largely due to proto-Pacific subduction and its northward motion to collide with North China. The reference districts – Western New York, Oklahoma and Kansas, and West Texas in North America – were to some degree affected by dynamic uplift and subsidence associated with long-lived Panthalassa subduction zones, closure of the Rheic Ocean and large-scale upwelling above the African deep-mantle structure during late Paleozoic times. This indicates that some published global sea level curves may include non-eustatic signals such as dynamic uplift or subsidence. The interpretation of stratigraphic data gathered from these reference districts should be treated with caution to estimate global sea level variations.

Multi-proxy analysis of organic matter accumulation in the Upper Ordovician–Lower Silurian black shale on the Upper Yangtze Platform, south China

Abstract There is no doubt that organic matter plays an important role in shale gas accumulation and storage. Thus, successful evaluation and production strategies of organic-rich shale deposits require an understanding of the range of factors that contribute to the accumulation and preservation of organic matter in these deposits. This paper reports results of a multi-faceted study of two Ordovician–Silurian transition sections of the Upper Yangtze Platform of South China. Organic and inorganic geochemical data are used to reconstruct paleoenvironmental changes across this critical interval of Earth history and to identify those factors that were most important to organic matter accumulation. Deposition of organic-rich shale of the Katian lower Wufeng Formation was favored by rising sea level and diminished clastic sediment content. Black shale of the upper Wufeng preceded Hirnantian glaciation whereas the overlying lower Longmaxi Formation accumulated in association with a post-glacial rise of sea level as well as the peak of Caledonian tectonism. The carbonaceous deposits appear to reflect the combined effects of elevated primary productivity as suggested by Si/Al and Ti/Al ratios, especially in the upper Wufeng, and consequent depletion of water column oxygen (preservation). Consideration of the relationships of redox-sensitive trace element concentrations and total organic carbon content suggest that bottom water conditions fluctuated between anoxic and euxinic during this period, perhaps related to rapid oscillations of sea level. Moreover, the basin appears to have remained connected with the global ocean in spite of Caledonian uplift of bordering areas. Thus, black shale sedimentation on the Upper Yangtze Platform at the Ordovician-Silurian boundary reflects the interplay of climate change and tectonism and their influences on sea level, clastic sediment content, and paleo-productivity.

Paired δ13Ccarb and δ13Corg records of the Ordovician on the Yangtze platform, South China

During the Ordovician, huge biological revolutions and environmental changes happened in Earth’s history, including the Great Ordovician Biodiversification Event, global cooling and so on, but the cause of these events remains controversial. Herein, we conducted a paired carbon isotopic analysis of carbonate (δ13Ccarb) and organic matter (δ13Corg) through the Ordovician in the Qiliao section on the Yangtze platform of South China. Our results showed that the δ13Ccarb trend of the Qiliao section can be correlated with local and global curves. The δ13Corg trend seems is less clear than the δ13Ccarb trend for stratigraphic correlations, but some δ13Corg positive excursions in the Middle and Upper Ordovician may be used for correlation studies. These carbon isotopic records may have global significance rather than local significance, revealing several fluctuations to the global carbon cycle during the Ordovician. Several known δ13Ccarb and δ13Corg negative and positive excursions have been recognised in this study, including the early Floian Negative Inorganic Carbon (δ13Ccarb) Excursion (EFNICE), as well as the early Floian Positive Organic Carbon (δ13Ccarb) Excursion, the mid-Darriwilian Inorganic Carbon (δ13Ccarb and δ13Corg) Excursion (MDICE), and the early Katian Guttenberg Inorganic Carbon (δ13Ccarb and δ13Corg) Excursion (GICE). These positive excursions and a smooth decline trend of δ13Corg values during the early to mid-Floian may imply multiple episodes of enhanced organic carbon burial that began at the early Floian stage, probably resulting in further decline in atmospheric pCO2 and then global cooling.

Onset of the middle Telychian (Silurian) clastic marine red beds on the western Yangtze Platform, South China

Marine red beds (MRBs) occur in many locations in the world during the Telychian Stage but it is unclear how they were related with the highly dynamic Silurian Earth system. To address this question, we conducted a case study involving geochronology, palaeogeographic reconstruction, facies analysis and sea-level history of the middle Telychian MRBs on the western Yangtze Platform of South China. Their distribution pattern indicates that the clastic MRBs owe their origin to the nearby oldlands. Facies analysis indicates that rocks during the study interval generally fall into three depositional belts: shallow subtidal, deep subtidal and shaly basinal belts. A chronostratigraphic framework constrained by both biostratigraphic data and eustatic events shows that, during the early Monoclimacis crenulata interval, olive-gray strata dominated both shallower (shallow and deep subtidal belts) and deeper (shaly basinal belt) settings, while distribution of the MRBs was restricted to deeper waters (shaly basinal belt) on the open-sea side of the platform. This situation changed after the middle M. crenulata interval when the MRB deposition was dominant in both shallower and deeper settings. The diachronous onset pattern of the MRBs coincided with prominent transitions in climatic conditions (from cooler to warmer), eustatic states (from regression to transgression), benthic faunas (decline of normal marine fauna and development of euryhaline assemblages) and reduced primary productivity in the shelf settings, most likely indicative of a change between different oceanic modes. Comparative studies suggest that, the widespread Telychian MRBs in both shallow and deeper water settings owe their origin to a very low organic carbon burial rate, which could be results of higher oxygen content in the ocean water and lower primary productivity.