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The basal unit of the Early Cambrian black shale sequence of South China hosts sulfide-rich polymetallic units, non-sulfidic vanadium-rich black shales, sapropelic alginite (combustible shale), barite, and phosphorite. This rock spectrum occurs in a paleoceanographic similar, and stratigraphically correlated, transgressive upwelling setting on the passive continental margin of the Neoproterozoic Yangtze Platform. Several centimeter-thick polymetallic sulfidic units (3–13 wt% Mo+Ni, 100–600 ppm U) have relatively light Mo (δ98/95Mo = 1.1 ± 0.2‰) and relatively heavy U isotope composition (δ238/235U = 0.2 ± 0.1‰). Several meter-thick V-rich shales with multiple ore-grade layers (0.1–0.8 wt% V, < 100 ppm U, Mo and Ni ~ 100 ppm) have isotopically lighter Mo (δ98/95Mo = 0.3 ± 0.4‰) and heavier U composition (δ238/235U = 0.4 ± 0.2‰ and up to 0.7‰). The inverse Mo versus U isotope correlation suggests that both metals were enriched by removal from anoxic to strongly euxinic bottom water in restricted basins along the rifted continental margin. Metal replenishment probably occurred via the cycling of Fe–Mn-oxyhydroxide particles across the redox boundary, with sorption/desorption of Mo (and likely Ni) in a stratified water column. In contrast, V enrichment with much lower Mo, Ni, and U contents, but more fractionated Mo and U isotope composition, reflects non-sulfidic anoxic depositional conditions in a partly open system with higher bottom water renewal rates. While Mo isotope fractionation likely occurred in the water column, U isotope fractionation may dominantly have occurred at the water-sediment interface, perhaps in a benthic organic flocculent layer. These findings indicate that local hydrodynamic control and stratified water column redox conditions may explain the observed variation of metal enrichment (Mo–Ni versus V) in the black shales. Furthermore, the high δ98/95Mo values up to 2.6‰ of the black shales studied and the correlated U and Mo isotope data suggest that Early Cambrian seawater was at least episodically broadly similar to modern seawater. [ABSTRACT FROM AUTHOR]

The early Cambrian witnessed profound environmental changes and biological evolution in Earth' history. During this period, organic-rich shales were widely distributed over almost the entire Yangtze Block. However, the dominant factor that drove the significant accumulation of organic matter (OM) remains controversial and is still debated. Here, we analyzed TOC, organic carbon isotopes, iron speciation, major and trace elements for the lower Cambrian Niutitang Formation in the upper slope Meiziwan section, to investigate the dominant factor controlling OM accumulation. High contents of TOC and Baxs reveal an OM-enriched feature of the Niutitang Formation, and the coupled relationship between them suggest a strong production control on OM accumulation at Meiziwan. Meanwhile, negative relationships between TOC and chemical index of alteration (CIA) values as well as Al contents suggest that influence of chemical weathering and terrestrial input on OM accumulation were limited. Fairly low CoEF × MnEF values provide strong evidence that the deposition of organic-rich shales was under the control of oceanic upwelling event. The upwelling event would bring nutrient-rich deep waters into surface water, stimulating phytoplankton bloom and primary productivity in surface water and facilitating OM enrichment. Meanwhile, enhanced accumulation of OM would have promoted subsequent bacterial sulfate reduction, leading to the occurrence of occasional euxinia (evidenced by iron speciation and redox-sensitive trace element data) and promoting preservation of OM. Taken together, our results shed light on the critical role of oceanic upwelling on the marine primary productivity on the earliest Cambrian Yangtze Platform. [ABSTRACT FROM AUTHOR]

The early Cambrian witnessed profound environmental changes and biological evolution in Earth’ history. During this period, organic-rich shales were widely distributed over almost the entire Yangtze Block. However, the dominant factor that drove the significant accumulation of organic matter (OM) remains controversial and is still debated. Here, we analyzed TOC, organic carbon isotopes, iron speciation, major and trace elements for the lower Cambrian Niutitang Formation in the upper slope Meiziwan section, to investigate the dominant factor controlling OM accumulation. High contents of TOC and Baxs reveal an OM-enriched feature of the Niutitang Formation, and the coupled relationship between them suggest a strong production control on OM accumulation at Meiziwan. Meanwhile, negative relationships between TOC and chemical index of alteration (CIA) values as well as Al contents suggest that influence of chemical weathering and terrestrial input on OM accumulation were limited. Fairly low CoEF × MnEF values provide strong evidence that the deposition of organic-rich shales was under the control of oceanic upwelling event. The upwelling event would bring nutrient-rich deep waters into surface water, stimulating phytoplankton bloom and primary productivity in surface water and facilitating OM enrichment. Meanwhile, enhanced accumulation of OM would have promoted subsequent bacterial sulfate reduction, leading to the occurrence of occasional euxinia (evidenced by iron speciation and redox-sensitive trace element data) and promoting preservation of OM. Taken together, our results shed light on the critical role of oceanic upwelling on the marine primary productivity on the earliest Cambrian Yangtze Platform.

The Upper Ediacaran to Lower Cambrian of the Sichuan Basin in South China has long been considered to be dominated by shallow‐water deposition. Hydrocarbon exploration, however, has revealed that a NW‐SE trending intraplatform trough formed in the basin during the same period. Although different models have been proposed, the formation and evolution of the trough are still not fully understood. In this study, we investigate both the origin of the intraplatform trough and the formation of the Sichuan Basin by integrating seismic interpretation, well correlation and tectonic subsidence analysis. The seismic and well data clearly show three stages of development of the trough. The first stage, in the early Late Ediacaran, is characterized by considerable thinning of the lower two members of the Upper Ediacaran from the platform margins to the trough. In the second stage, in the late Late Ediacaran, the platform margins backstepped and the extent of the trough expanded significantly to a width of ca. ~400 km. The third stage, in the early Early Cambrian, was dominated by gradual filling of the trough and onlapping of the platform margins. Backstripped tectonic subsidence curves show one, or two closely spaced episodes of linear subsidence starting at ~550 Ma and then decreasing exponentially until ~450 Ma. The shape of the subsidence curves is consistent with formation of the Sichuan Basin by low, and slow amounts of lithospheric stretching of thickened cratonic lithosphere. The tectonic subsidence increases from the centre to the NW of the basin. Interestingly the margins of the trough do not correlate with contoured values of increased tectonic subsidence and we infer that the trough was a palaeogeographic embayment in a large carbonate platform that developed in a broad, ramp‐like area of slow and low subsidence tilting down to the proto‐Tethyan ocean located to the NW of the basin. [ABSTRACT FROM AUTHOR]

Early Cambrian basin in South China has deposited successive sedimentary rocks, which not only recorded geochemical information of the variation of palaeo-ocean environment, but also preserved fossils of most phyla of modern animals, such as the large-body Chengjiang and Qingjiang biotas. Although the relation between gradually oxic Cambrian Ocean and biotic diversity is recognized, the temporal and spatial variations of redox condition, as well as the influence of the variations on the biotic evolution during the early Cambrian, remain unclear. Here, we focus on all of the early Cambrian sedimentary rocks deposited in inner-shelf, outer-shelf, slope, and deep basin in the South China Block. According to the geochemical datasets, such as iron speciation, major- and trace element, Hg contents and Hg isotopic data, we have summarized and discussed the changing of marine redox condition, hydrothermal activity and basin restriction during the Cambrian Terreneuvian to the early Cambrian Age 3. In the early Cambrian Terreneuvian, oxic environment appeared in the inner shelf facies, accompanied by the occurrence of small shelly fossils. Then, hydrothermal activities occurred in the center South China block of the outer-shelf and slope facies with a ferruginous environment in the late Terreneuvian and early Cambrian Age 3. However, the water column in the block margin of this two facies was euxinic during this period. In the early Cambrian Age 3, oxic water column had expanded to the slope and deep basin facies. Based on the compiled data, we propose that strong hydrothermal activities had provided abundant of Fe and other nutrients, and terrigenous inputs which also provided amounts of terrigenous sulfur that led to distinct environments in the slope facies. In the early Cambrian Age 3, the expanding oxic water column and nutrients after the hydrothermal activities stimulated the evolution of the biological diversity together, such as the Qingjiang and Chengjiang biotas. These variations highlight the effects of redox condition and hydrothermal activities on the biotic flourish of the early Cambrian. [ABSTRACT FROM AUTHOR]

The Early Cambrian was a critical interval marked by dramatic oceanic, geochemical, and biological changes in Earth's history. Although numerous studies have documented oceanic redox changes and revealed a heterogeneous pattern from the inner shelf to outer shelf margin in the Yangtze Block, less attention has been paid to their palaeogeographic‐specific occurrences, particularly in the deeper basinal regime during that time. To better understand marine redox variations and organic accumulation across the entire outer shelf‐basin transect, we selected a coeval organic‐rich section (i.e., Guantang section, western Hunan) deposited on a farther seaward submarine sill for a comparison. Integrated geochemical data of iron speciation, redox‐sensitive trace element [i.e., molybdenum (Mo) and uranium (U)] concentrations and total organic carbon (TOC) contents as well as P/Al and Ba/Al ratios, in combination with previously published data from the outer shelf slope‐basin, reveal an overall high TOC content on the sill and further attest to the occurrence of a fluctuating euxinic water wedge overlying the deep ferruginous water on the outer shelf slope to basin transect in the presence of oceanic upwelling system. The fluctuating euxinic watermass could be dynamically advanced upslope and retreated downslope in the pace of sea‐level rise and fall. All these signify the critical roles of palaeogeographic setting and eustatic changes in controlling the spatiotemporal productivity and redox fluctuations in the Yangtze sea during the Early Cambrian. [ABSTRACT FROM AUTHOR]

Abstract Background Our understanding of the ontogeny of Palaeozoic brachiopods has changed significantly during the last two decades. However, the micromorphic acrotretoids have received relatively little attention, resulting in a poor knowledge of their ontogeny, origin and earliest evolution. The uniquely well preserved early Cambrian fossil records in South China provide a great new opportunity to investigate the phylogenetically important ontogeny of the earliest acrotretoid brachiopods, and give new details of the dramatic changes in anatomy of acrotretoid brachiopods during the transition from planktotrophic larvae to filter feeding sedentary juveniles. Results Well preserved specimens of the earliest Cambrian acrotretoid brachiopods Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis (Cambrian Series 2, Shuijingtuo Formation, Three Gorges area, South China) provide new insights into early acrotretoid ontogeny, and have significance for elucidating the poorly understood early phylogeny of the linguliform brachiopods. A more comprehensive understanding of the applied terminology based on new observation, especially in definition of the major growth stages (embryo, planktotrophic larva, post-metamorphically sessile juvenile and adult), is established. The so-called acrotretoid “larval shell” of both valves of Eohadrotreta demonstrates evidence for metamorphosis (shedding of the larval setae and transitions of shell secretion), during the planktotrophic stage. Therefore, it is here termed the metamorphic shell. The inferred early acrotretoid larval body plan included a bivalved protegulum, secreted at the beginning of the pelagic stage, which later developed two pairs of larval dorsal setal sacs and anterior–posterior alignment of the gut during metamorphosis. Conclusion The primary larval body plan of acrotretoid Eohadrotreta is now known to have been shared with most early linguliforms and their relatives (including paterinates, siphonotretoids, early linguloids, the problematic mickwitziids, as well as many early rhynchonelliforms). It is suggested that this type of earliest ontogeny can be considered as plesiomorphic for the Brachiopoda and probably first evolved in stem group brachiopods with subsequent heterochronic changes.

[Display omitted] • In-situ elemental analyses of Fe (oxyhydr)oxides from Zhijin distinguish two types of Fe (oxyhydr)oxides. • Fe-(oxyhydr)oxides showed mainly hydrogenic origin mixed with hydrothermal origin. • Fe (oxyhydr)oxides control the enrichment of REE in francolites via Fe-redox pumping. • REEs were adsorbed by Fe (oxyhydr)oxides in oxic seawater and released in suboxic/anoxic porewaters. Marine phosphorites have shown extraordinary REE potential. Enrichment mechanisms have been investigated as yet, however, the roles of Fe/Mn (oxyhydr)oxides in REE enrichment were poorly known. In this paper, we report new data for early Cambrian phosphorites in South China with a particular focus on REE enrichment and Fe-redox cycling. Phosphorites from the Cambrian Gezhongwu occur in the upper and lower units. The phosphorites contain a large number of of Fe (oxyhydr)oxides, especially those in the upper unit. SEM and laser Raman show that Fe (oxyhydr)oxides have a gray rim and a bright core, attributed to transformation from hematite to goethite during the weathering. In-situ elemental analyses show Fe (oxyhydr)oxides have PAAS-normalized REE patterns depleted in LREE and enriched in HREE with negative to positive Eu anomalies and positive Y anomalies. However, two patterns are clearly distinguished according to Ce anomalies and these are related to the origins of Fe-(oxyhydr)oxides: hydrothermal Fe (oxyhydr)oxides with negative Ce anomalies and hydrogenic ones with positive Ce anomalies. The role of Fe (oxyhydr)oxides played in REE enrichment was controlled by Fe-redox pumping. In this process, Fe (oxyhydr)oxides adsorbed REEs in oxic water columns; after precipitating to suboxic/anoxic conditions, the absorbed REEs were released into porewater. Francolites subsequently formed and captured REEs during early diagenesis. Even though additional evidence is required, our investigations were significant in conforming the origins of Fe/Mn (oxyhydr)oxides and their role in REE enrichment in marine phosphorites. [ABSTRACT FROM AUTHOR]

The early Cambrian was a critical interval in geological history and featured profound oceanic and biotic changes. To unravel the oceanic redox conditions, high-resolution analyses of iron speciation and redox-sensitive trace elements (Mo and U) within the framework of sequence stratigraphy were carried out on the Niutitang Formation (~528–521 Ma). The two examined sections, the Daotuo and Bahuang sections, were respectively located in a mid-upper slope setting and a lower slope to basin settings behind a seaward submerged sill on the middle Yangtze Block, South China. At Daotuo, the Fe py /Fe HR values display moderate positive covariances with the total organic carbon contents (TOC) and the Mo/TOC ratios, notably in the basal part of the Niutitang Formation. Therefore, at this locality the euxinic water mass wedge was developed in association with a high primary organic productivity/burial rate, likely within a high-productivity zone. In contrast, at Bahuang, the Fe py /Fe HR values show weak to negative covariances with the Mo/TOC ratios and TOC contents in the basal part, indicating that the biogeochemical cycles of Fe, Mo with C were decoupled in a lower-productivity, ferruginous deeper basinal setting. These spatial changes in marine redox structures and biogeochemical cycles can be reasonably explained by the existence of oceanic upwelling in the presence of strong offshore currents and a seaward submarine sill, reconciling the oxygen minimum zone (OMZ) in modern oceanic margins to some extent. This spatial redox pattern also accounts well for previous data based on which the euxinic state intermittently invaded upward onto the shelf margin and evolved into a ferruginous-dominant anoxia in the inner shelf subbasins. In addition, the euxinic wedge dynamically fluctuated upslope and downslope along the transect from the ferruginous outer shelf slope to the basin in response to rise and fall of sea-level. Consequently, the temporal evolution of the redox conditions was driven in part by eustatic variations. Moreover, the paleogeographic position of the Yangtze Block in the mid-lower latitudes suggests that the block was associated with relatively strong offshore currents induced by trade winds, which notably enhanced the oceanic upwelling near the transgression maximum. On the other hand, the dominance of deep ferruginous waters with the local occurrence of a euxinic wedge on the outer shelf slope indicates relatively low concentrations of seawater sulfate and atmospheric oxygen during the deposition of the Niutitang black shales. This redox model thus highlights the important roles of paleogeographic, paleoclimatic and eustatic sea-level changes in controlling the spatiotemporal fluctuations in marine redox conditions and biogeochemical cycling in the early Cambrian ocean. [ABSTRACT FROM AUTHOR]

The Neoproterozoic to Early Cambrian period is a very important era in the history of earth evolution. Research on stratigraphic framework, paleo-oceanography, palaro-atmospheric, and paleoclimatology are focuses of international geosciences. As a new stratigraphic dating method, Re-Os isotopic dating was developed rapidly in the last decades. It has dual advantages of dating and tracing the paleoenvironment for the organic-rich black shale deposited at the important timing turning points during Neoproterozoic to Early Cambrian. Taking the Neoproterozoic to Early Cambrian strata as an example, the application of the Re-Os isotopic dating method in chronostratigraphy is described in this paper and the reliability of the Re-Os dating method by comparison with numerous precise zircon U-Pb ages is confirmed. This method has played an important role in the research of the building order in the Doushantuo Formation and the sedimentary time of the Liuchapo Formation. Combining previous Sr isotope curves with the initial 187Os/188Os ratios of Neoproterozoic to Early Cambrian strata, the role of Os isotopes in the interpretation of the paleoenvironment and tracing the source of Ni-Mo-V polymetallic layers is discussed. Sr isotope analysis is difficult to conduct on black shales, while initial 187Os/188Os can complement this defect. Therefore, the mutual combination of these two methods can contribute to the exploration of the paleoenvironment during Neoproterozoic to Early Cambrian. Finally, with the development of Re-Os isotope analysis technology and establishment of more experiment processes, it will produce a significant effect on the study of future chronostratigraphy, especially on the age-determination of the older strata.

The early Cambrian Period was a key interval in Earth history with regard to changes in both ocean chemistry and animal evolution. Although increasing ocean ventilation has been widely assumed to have played a key role in the rapid appearance, diversification and spatial colonization of early animals, this relationship is in fact not firmly established. Here, we report a high-resolution Fe-C-S-Al-Ti geochemical study of the lower Cambrian Wangjiaping section from an outer-shelf setting of the Yangtze Sea of South China. Iron speciation data document a redox transition from dominantly euxinic to ferruginous conditions during Cambrian Age 3 (c. 521–514 Ma). Interpretation of coexisting pyrite sulphur isotope (δ34Spy) records from Wangjiaping reveals relatively high marine sulphate availability at Wangjiaping. Furthermore, Wangjiaping section shows lower δ34Spy (‒2.1±5.3‰) and lower TOC (2.4±1.1%) values but higher positive correlation (R2 = 0.66, p < 0.01) between TOC and Fepy/FeHR relative to deeper sections reported previously, suggesting that euxinia developed at Wangjiaping in response to increasing marine productivity and organic matter-sinking fluxes. Our reconstructed redox conditions and fossils at Wangjiaping in comparison with previously well-studied strata in the inner-shelf Xiaotan and Shatan sections suggest that planktonic and benthic planktonic trilobites with bioturbation appeared in the oxic water columns, whereas only planktonic trilobites without bioturbation occurred within the anoxic (even euxinic) water columns during Cambrian Age 3. This finding indicates that spatial heterogeneity of redox conditions in the shelves had an important effect on early animal distribution in the Yangtze Block. [ABSTRACT FROM PUBLISHER]

Throughout the geological history of the Earth, submarine hydrothermal activity has played an important role in seawater chemistry, biological evolution and enrichment of metals in the Earth crust. However, the prospect of hydrothermal activity for extreme element accumulation during the early Cambrian, a key geological period, in South China has not been well-constrained. This study reports geochemical (e.g. REE and Sr isotope) investigations of a coarse-grained limestone layer and associated calcite veins in Zunyi and Nayong areas, Guizhou Province, to constrain the hydrothermal activity and evaluate the significance of hydrothermal contribution to extreme element accumulation during the early Cambrian, South China. Our results reveal positive Eu anomalies and higher initial 87 Sr/ 86 Sr ratios (0.7083–0.7150) for carbonate samples than those of early Cambrian seawater, indicating the presence of hydrothermal processes. Combined with constraints from the spatial relationships and coincidence with adjacent mineralization, these hydrothermal processes provide the most probable contribution for polymetallic Ni–Mo–PGE mineralization. Furthermore, there are abundant hydrothermal dolomite and barite-calcite veins in the dolostone of the Dengying Formation, indicating the occurrence of a variety of hydrothermal fluids. Overall, multi-stage hydrothermal pulses with different fluid compositions spanned the Ediacaran–Cambrian transition in South China. In particular, these hydrothermal fluids with positive Eu anomalies and enriched radiogenic Sr, originating from Proterozoic mafic/ultramafic rocks, may have flowed through the underlying Precambrian silicate clastic rocks (e.g., Xiajiang, Banxi and Lengjiaxi Groups) and may have been crucial for the marine environment, biological diversity and extreme element accumulation during the early Cambrian, South China. [ABSTRACT FROM AUTHOR]

The Ediacaran–Cambrian transition records distinct evolutionary changes of metazoans. The Ediacaran fossils ( i.e. , Ediacara-type biota and tubular fossils) are contrasting with the diverse small skeletal fossils (SSFs) from the early Cambrian. The apparent dissimilarities hindered studies deciphering their evolutionary relationships. This also led to a popular assumption that there exists a mass extinction of the Ediacara biota and cloudinids at the Precambrian–Cambrian (Pc–C) boundary. Here we report for the first time a transitional fauna which consists of typical elements of Ediacaran, i.e. cloudinids and related tubicolous organisms, together with Cambrian SSFs including protoconodonts, anabaritids and siphogonuchitids from South China and Maly Karatau (Kazakhstan). The sediments yielding the transitional fauna are characterized by siliceous rocks in both regions and were previously considered to be unfossiliferous. Their chronostratigraphic assignment in South China has been debated for years. Based on the new fossil assemblage, the siliceous strata of the Daibu Member (Northeast Yunnan, South China) and the basal Kuanchuanpu Formation (South Shaanxi, South China) can be assigned to the earliest Cambrian SSF biozone ( Anabarites trisulcatus – Protohertzina anabarica Assemblage Zone) and thus can be considered of early Cambrian in age. A new subzone of the earliest SSF zone in eastern Yunnan (South China) is proposed herein defined as Ganloudina symmetrica – Rugatotheca typica Interval Subzone. The new fauna demonstrates that the cloudinids, originally confined to the late Ediacaran, persisted into the Cambrian Fortunian, and no major extinction event occurred at the Pc–C transition. [ABSTRACT FROM AUTHOR]

The early Cambrian radiation of macroscopic animals, commonly referred as the ‘Cambrian Explosion’, is one of the most important biological innovations in Earth history. Because oxygen is a requirement for complex animal life, it is generally accepted that ocean oxygenation may have been the environmental trigger for animal evolution, but direct geochemical evidence supporting oxygen increase prior to the ‘Cambrian Explosion’ remains sparse and insufficient. To better understand the casual relationship between ocean oxygenation and animal evolution, we have carried out a systematic analysis of redox sensitive trace element (RSTE) concentrations, total organic carbon (TOC) contents and pyrite morphology for the early Cambrian Niutitang Formation of the Hejiapu section, northeastern Guizhou Province, South China. The new data, in combination with previously published data from other sections, show strong Mo enrichments (average of 88 ppm) and high Mo/TOC ratios (average of 14 ppm/wt.%) in the euxinic black shales of the lower Niutitang Formation (~ 520 Ma), indicating increase of the oceanic Mo reservoir. In the light of the positive correlation between sediment Mo/TOC and aqueous Mo in modern basins, the early Cambrian oceanic Mo concentration is estimated as ~ 50 nmol, which is about half of the modern ocean Mo concentration (~ 105 nmol) but is significantly higher than that of the Mesoproterozoic oceans (10–20 nmol). The marked increase in oceanic Mo reservoir is therefore interpreted as recording significant rise of atmospheric oxygen and pervasive ocean oxygenation, which may have facilitated the diversification of macroscopic metazoans during the early Cambrian. [ABSTRACT FROM AUTHOR]

Abstract: The origin of radiolarians and their early evolution are of great interest for paleontologists and biologists. The needle-like or spicular construction of radiolarians represented by Order Archaeospicularia has been considered as an ancestor of radiolarians until Braun et al. (2007) discovered some radiolarians with latticed shells from the Early Cambrian in South China. The two radiolarian tests reported here were collected from the Shuijingtuo Formation corresponding to the basal part of Cambrian Series II, in South China. One of them is spherical, latticed, and similar to the radiolarians described by Braun et al. (2007). The other is grouped within the genus Kalimnasphaera formerly reported only from the Ordovician. The result indicates that the oldest known radiolarian fauna does not belong to Order Archaeospicularia. The hypothesis that Archaeospicularia is the representative of the oldest radiolarian group needs to be reexamined. Further investigation of early forms of radiolarians in the Early Cambrian or older strata is warranted. [Copyright &y& Elsevier]

Organic carbon isotope ( δC) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δC values of the late Ediacaran Dengying Formation range from −29‰ to −24‰. In the deep-water Longbizui section, δC values from time-equivalent strata of the Dengying Formation are mostly between −35 and −32‰. These new data, in combination with δC data reported from other sections in South China, reveal a 6‰-8 shallow-to-deep water δC gradient. High δC values (>−30‰) occur mostly in shallow-water carbonate rocks, whereas low δC values (<−32‰) dominate the deep-water black shale and chert. The large temporal and spatial δC variations imply limited buffering effect from a large dissolved organic carbon (DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δC variations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δC values (>−30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δC data and may record the isotope signature of organic matter from primary (photosynthetic) production. In contrast, low δC values (<−32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δC data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform. [ABSTRACT FROM AUTHOR]

Following the Ediacaran metazoan radiation, the 'Cambrian Explosion' set up the major framework of todays' animal phyla as well as modern marine ecosystem. Here, we present a preliminary investigation on the temporal and spatial (from shallow to deep waters) variations of the early Cambrian ocean chemistry in South China through analyzing a Fe-S-C systematic dataset integrated from literature. Our investigation indicates that the early Cambrian deep ocean in South China was still anoxic and Fe-enriched (i.e., ferruginous) although its surface was oxic, and in between a metastable euxinic (anoxic and sulfidic) water zone may have dynamically developed in anoxic shelf waters with an increasing weathering sulfate supply. Furthermore, accompanying marine transgression and regression cycles in the early Cambrian, such a 'sandwich' structure in ocean redox chemistry demonstrates five evolutional stages, which can be well correlated to the spatiotemporal patterns of fossil records in South China. The good correlation between metazoan fossil occurrences and water chemistry in South China suggests that early animals possibly possessed ability to inhabit anoxic but generally not euxinic environments as free HS was fatal to most eukaryotes. This view can well explain why those small shell fauna and sponges disappeared from shelf to slope areas where sulfidic Ni-Mo-rich shales were widely deposited. Thus, we conclude that the spatiotemporal variations of ocean chemistry and its biological effects probably played a key role in the phased animal radiations and 'extinctions' in the early Cambrian. [ABSTRACT FROM AUTHOR]

Abstract: Environmental investigation of the Early Cambrian is assessed through molecular fossils based from three core samples drilled in the Three Gorges area, South China. The core samples record environmental information dating from the earliest part of the Early Cambrian to nearly the end of the Early Cambrian, making this investigation unique from previous investigations because a more far-reaching temporal record is assessed. This record includes unusually high abundance of longer chain n-alkanes (LCA) around nC27, having been recognized in the earliest Cambrian strata. Based on no odd–even preference of the LCA and comparisons to hydrocarbons in cultured cells of microorganisms, the LCA may be derived from hydrocarbons of sulfate reducing bacteria. On the other hand, a phototrophic origin of the SCA is supported by a clear positive correlation with pristane. The relative abundance of LCA to SCA increased in the earliest Cambrian, suggesting predominance of sulfate reducers against phototrophs. This LCA abundance with simultaneous decrease of pristane/phytane may indicate a euxinic condition, where sulfate reducers remineralize organic carbon in the water column. This is consistent with the concomitant decrease of the δ13Ccarb value probably due to a massive reoxidation of organic carbon. The inferred reducing condition is further supported by the enhanced appearance of squalane that could be derived from an Archeal lipid. Our results suggest the euxinia of the Yangtze platform in the earliest Cambrian that may relate to the emergence of the large nektonic animals which require sufficient level of free oxygen. [Copyright &y& Elsevier]

Abstract: The Early Cambrian black shale sequence of the Niutitang Formation in South China hosts a synsedimentary, organic carbon-rich, polymetallic sulfide layer with extreme metal concentrations, locally mined as polymetallic Ni–Mo–PGE–Au ore. In combination with previously reported data, we present Mo isotope, platinum-group element (PGE), and trace and rare-earth element (REE) data for the polymetallic sulfide ores and host black shales from four mine sites (Dazhuliushui and Maluhe in Guizhou Province, and Sancha and Cili in Hunan Province, respectively), several hundred kilometers apart. The polymetallic sulfide ores have consistently heavy δ98/95Mo values of 0.94 to 1.38‰ (avg. 1.13±0.14‰, 1σ, n =11), and the host black shale and phosphorite have slightly more variable δ98/95Mo values of 0.81‰ to 1.70‰ (n =14). This latter variation is due to variable paleoenvironmental conditions from suboxic to euxinic, and partly closed-system fractionation in isolated marine sedimentary basins. Both the polymetallic sulfides and host black shales show PGE distribution patterns similar to that of present-day seawater, but different from those of ancient submarine-hydrothermal deposits and modern submarine hydrothermal fluids. The polymetallic sulfide bed has a generally consistent metal enrichment by a factor of 107 compared to present-day seawater. PAAS-normalized REE+Y patterns of the polymetallic sulfide bed are characterized by a remarkably positive Y anomaly, consistent with an origin of the REE predominantly from seawater. Small positive Eu anomalies in some of the sulfide ores could reflect minor hydrothermal components involved. The Mo isotope, PGE, and trace and rare-earth element geochemical data suggest that metals in the polymetallic Ni–Mo–PGE–Au sulfide ore layer were scavenged mostly from Early Cambrian seawater, by both in-situ precipitation and local re-deposition of sulfide clasts. [Copyright &y& Elsevier]

Abstract: Early Cambrian black shales of South China not only host important sponge, arthropod and other soft-bodied fossils that have helped to trace early metazoan diversification, but also show extreme enrichments of a number of trace metals in particular Ni, Mo and V. In this study, we use a new approach by analyzing rare earth elements in kerogen extracted from the black shales, together with a number of redox-sensitive trace element compositions and total organic carbon (TOC) concentrations in an early Cambrian black shale sequence in Zunyi, Guizhou province, South China, to place better constraints on the oceanic redox conditions and the origin of the extreme metal enrichment. Our data show significant negative Ce anomalies (Ce/Ce* as low as 0.4) occurring in kerogen, which indicate an oxygenated surface environment of primary productivity in consistent with the concept that the organic matter is mainly derived from organisms in the euphotic zone. Mass balance calculation suggests that the kerogen-associated REE can dominate the measured black shale REE budget, while similarity between our measured REE patterns and those of similarly aged phosphorites indicates that the REE content of ancient phosphorites may have also derived initially from organic matter. The redox-sensitive trace elements, such as U, V, Mo, and their ratios of U/Al, V/Al and Mo/Al in black shales show different correlation patterns with TOC contents. The upper black shales show a good metal/TOC correlation, but such a correlation is absent in the lower part. The lower black shales exhibit much higher metal enrichments compared to Black Sea sulphidic (euxinic) sediment. This is taken to indicate the presence of sulphidic bottom waters during the deposition of the lower black shales, including the Ni–Mo ore layer. In contrast, anoxic, non-sulphidic conditions occurred during the deposition of the upper black shales. Taking all these geochemical data together, we suggest that the early Cambrian South China seaway was strongly stratified and stagnant, and that euxinic bottom water conditions may have led to enrichment of the redox-sensitive metals such as U, V and Mo in the lower black shales, and in one case the occurrence of a polymetallic Ni–Mo sulphide ore bed bearing an extraordinarily extreme metal enrichment, which, according to the Mo/TOC and Ni/TOC ratios and much other geochemical evidence, may have been additionally influenced by hydrothermal input of metals within the rift basin as suggested by a number of previous studies. [Copyright &y& Elsevier]

Abstract: Several new archaeocyaths are described from the lower Cambrian Xiannudong Formation of Fucheng, Shaanxi Province, and from the lower Cambrian Mingxisi Formation of Meitan, Guizhou Province (South China). One new genus (Qinlingocyathus nov. gen.) and four new species (Qinlingocyathus astomus nov. gen., nov. sp., Gandinocyathus pinguis nov. sp., Thalamocyathus mui nov. sp., and Dailycyathus aporosus nov. sp.) are erected. These discoveries greatly expand our knowledge of the morphology of key archaeocyathan taxa and of the diversity of early Cambrian archaeocyathan faunas of the Yangtze Platform. [Copyright &y& Elsevier]

The lower Cambrian Niutitang Formation, a widespread black shale deposition, is of geological interest because of its polymetallic beds, Cambrian explosion, depositional ages, dramatic environmental changes and so on. Previous study focused mainly on inorganic geochemistry and few studies have investigated the organic fractions of upper Neoproterozoic-lower Cambrian strata in South China. Here we report a study of biomarkers plus organic carbon isotopes for black shales from Ganziping, Hunan Province (China). All the saturated hydrocarbon fractions have a unimodal distribution of n-alkanes, a high content of short-chain alkanes and maximize at C19 or C20 (C23 for sample Gzh00-1). The C27/C29 sterane ratio ranges from 0.77 to 1.20 and 4-methylsteranes are in low abundance. These parameters indicate that algae and bacteria are the important primary producers. Furthermore, biomarker maturity proxies show the samples to be higher maturity. The low Pr/Ph values (<0.7) suggest that the samples were deposited under anoxic conditions and, likely, under stratified water columns. In addition, 25-norhopanes and gammacerane are present as diagnostic indicators of normal marine salinity and dysoxic to anoxic conditions. During the Early Tommotian, known to coincide with a transgression event, small shelly fossils increased in abundance and diversity. Moreover, positive δ13Corg excursions close to 1.4‰ occur at the base of the Tommotian stage. In summary, the Early Cambrian black shales were deposited under dramatic paleoenvironmental changes, including oceanic anoxia, higher primary productivity and sea-level rise. [ABSTRACT FROM AUTHOR]

Abstract: The early Cambrian (ca. 542–520Ma) strata in South China record two prominent negative carbonate carbon isotope (δ13Ccarb) excursions of early Nemakit–Daldynian (N–D) and early Tommotian ages. Across each of these excursions, carbonate and organic carbon isotopes (δ13Ccarb and δ13Corg) are strongly decoupled. Regional correlation across a shelf-to-basin transect shows lateral heterogeneity of δ13Corg during the early-middle N–D but more homogenized δ13Corg values across the basin during the late N–D and Tommotian. The temporal and lateral variations in δ13Corg suggest that decoupled δ13Ccarb and δ13Corg across the N–D δ13Ccarb excursion were possibly caused by diagenetic alteration of organic matter and/or amplification of detrital organic carbon isotope signature in low-TOC carbonates. In contrast, decoupled δ13Ccarb and δ13Corg of the upper N–D and Tommotian were likely resulted from chemoautotrophic–methanotrophic biomass contribution to TOC in organic-rich black shale and carbonates. The decoupled δ13Ccarb–δ13Corg pattern from the lower N–D strata (ca. 542Ma) shows striking similarities with those from the basal (ca. 635Ma) and upper (ca. 551Ma) Doushantuo Formation. In all three cases, decoupled δ13Ccarb–δ13Corg are seen in organic-poor carbonates (TOC≤0.1‰) and coupled δ13Ccarb–δ13Corg occur in organic-rich black shale and carbonates at the end of the negative δ13Ccarb excursion. These similarities suggest that the shift from decoupled to coupled δ13Ccarb–δ13Corg has no causal link with the terminal oxidation of a large oceanic DOC reservoir. Given the pervasive anoxia/euxinia in Ediacaran–early Cambrian oceans, local DOC-rich environments may have been common, but a large oceanic DOC reservoir capable of buffering the δ13C of marine organic matter requires independent evidence. [Copyright &y& Elsevier]

Abstract: Multi-episodic (three episodes at least) hydrothermal silica chimneys are identified in the black chert successions (up to ∼100 m thick locally), overlain by thick black shales (up to a few 100 m thick), in the Ediacaran–Cambrian boundary successions along the southern marginal zone of the Yangtze Platform, South China. Their occurrences are constrained within the lowermost Cambrian by the SHRIMP U–Pb zircon dating and stable isotopic chemostratigaphic data, and are stratigraphically coincident with the negative isotopic excursions of organic carbon (∼10‰ VPDB in magnitude) and sulfide sulfur (between 0 and 10‰ CDT), i.e., in bases of Nemakit–Daldynian and Tommotian stages in the Lower Cambrian. In this case, a causal link could have existed between which the hydrothermal venting could have released huge amounts of reduced silica-rich hydrothermal fluids with abundant 13C-depleted greenhouse gases (methane) and volcanic-originated H2S into the ocean and/or atmosphere, resulting in an extreme warm climate and oceanic anoxia, oceanic chemical perturbation, and subsequent massive precipitation of silica (chert), and large-magnitude negative isotopic excursions of organic carbon and sulfide sulfur. This scenario provides a unique chance to explore the causes of apparent tectono-depositional, oceanic and geochemical perturbations during this critical interval in South China and elsewhere. [Copyright &y& Elsevier]

Abstract: Carbon isotope chemostratigraphy has been used for worldwide correlation of Precambrian/Cambrian (Pc/C) boundary sections, and has elucidated significant change of the carbon cycle during the rapid diversification of skeletal metazoa (i.e. the Cambrian Explosion). Nevertheless, the standard δ 13C curve of the Early Cambrian has been poorly established mainly due to the lack of a continuous stratigraphic record. Here we report high-resolution δ 13C chemostratigraphy of a drill core sample across the Pc/C boundary in the Three Gorge area, South China. This section extends from an uppermost Ediacaran dolostone (Dengying Fm.), through a lowermost Early Cambrian muddy limestone (Yanjiahe Fm.) to a middle Early Cambrian calcareous black shale (Shuijingtuo Fm.). As a result, we have identified two positive and two negative isotope excursions within this interval. Near the Pc/C boundary, the δ 13Ccarb increases moderately from 0 to +2‰ (positive excursion 1: P1), and then drops dramatically down to −7‰ (negative excursion 1: N1). Subsequently, the δ 13Ccarb increases continuously up to about +5‰ at the upper part of the Nemakit–Daldynian stage. After this positive excursion, δ 13Ccarb sharply decreases down to about −9‰ (N2) just below the basal Tommotian unconformity. These continuous patterns of the δ 13C shift are irrespective of lithotype, suggesting a primary origin of the record. Moreover, the obtained δ 13C profile, except for the sharp excursion N2, is comparable to records of other sections within and outside of the Yangtze Platform. Hence, we conclude that the general feature of our δ 13C profile best represents the global change in seawater chemistry. The minimum δ 13C of the N1 (−7‰) is slightly lower than carbon input from the mantle, thus implying an enhanced flux of 13C-depleted carbon just across the Pc/C boundary. Hence, the ocean at that time probably became anoxic, which may have affected the survival of sessile or benthic Ediacaran biota. The subsequent δ 13C rise up to +5‰ (P2) indicates an increase of primary productivity or an enhanced rate of organic carbon burial, which should have resulted in lowering pCO2 and following global cooling. This scenario accounts for the cause of the global-scale sea-level fall at the base of the Tommotian stage. The subsequent, very short-term, and exceptionally low δ 13C (−9‰) in N2 could have been associated with the release of methane from gas hydrates due to the sea-level fall. The inferred dramatic environmental changes (i.e., ocean anoxia, increasing productivity, global cooling and subsequent sea-level fall with methane release) appear to coincide with or occur just before the Cambrian Explosion. This may indicate synchronism between the environmental changes and rapid diversification of skeletal metazoa. [Copyright &y& Elsevier]

Abstract: South China with its rich fossil record is an important region for studying Early Cambrian metazoan diversity patterns. Compilation of a database of metazoan genera from the traditional Lower Cambrian (Cambrian Series 1–2) of South China allows a quantitative analysis of biodiversity changes. The dataset shows that about 876 genus names have been reported from the Lower Cambrian of South China. Of these only about 582 genera are now considered valid after taxonomic scrutiny. Plotting generic diversity shows a two-part, first-order trend: 1, diversity increase through the most of the Meishucunian and Qiongzhusian stages (South China usage), but punctuated by a modest decline during the upper Meishucunian; followed by 2, diversity decline in the Canglangpuan and Longwangmiaoan stages (South China usage). About 155 genera of small shelly fossils (SSFs) occur in Meishucunian rocks. The Meishucunian fauna is dominated by helcionellids, orthothecimorph hyoliths and other enigmatic fossils and is very consistent with the pretrilobitic ‘Tommotian Fauna’. Diversity increases in the Qiongzhusian stage, reaching 304 genera, the peak value known for the Cambrian. The Qiongzhusian fauna is characterized mainly by the occurrence of dominant pan-arthropods and crown-group brachiopods, representing an initial phase of the Cambrian Evolutionary Fauna. A substantial decline in generic diversity through the Canglangpuan (ca 153 genera) and Longwangmiaoan (ca 69 genera) stages is due partly to a eustatic regression, and it approximates the global pattern, reflecting an extinction event at the end of the Lower Cambrian. The observed regional biodiversity through this interval has both evolutionary and taphonomic influences: the decline in SSF diversity beginning in the upper Meishucunian was not only related to the decimation of SSFs, but also to a reduction in phosphogenesis. An apparent peak in diversity during the Qiongzhusian was somewhat exaggerated by the exceptional Chengjiang Biota. [Copyright &y& Elsevier]

Abstract: Profound geotectonic, climatic and biological changes occurred during the terminal Neoproterozoic and its transition into the Early Cambrian. These changes are reflected in temporal variations of the carbon isotopic composition of seawater, recorded in a sedimentary succession on China''s Yangtze Platform. However, in addition to secular changes in carbon isotopes, the studied succession reflects additional variations in accordance with the deposition in different sedimentary facies, ranging from shallow water platformal to deeper water basinal settings. Likely a consequence of incorporation of variable amounts of 13C depleted bacterial biomass, this regional signal is superimposed on the secular variations of the global carbon cycle. [Copyright &y& Elsevier]

Abstract: A well-preserved benthic, epifaunal assemblage of articulated sponges is described from the stone coal beds of the Lower Cambrian Hetang Formation at Lantian, southern Anhui Province, South China. These sponges are Meishucunian–Qiongzhusian (=Diandongian–early Qiandongian) in age. In Siberian terminology, they are probably Tommotian–Atdabanian, approximately 535–520 Ma. The Hetang sponge fauna is taxonomically diverse and morphologically complex. Eleven species of both demosponges and hexactinellids, including three new taxa (Choia? striata sp. nov., Protospongia gracilis sp. nov., and Lantianospongia palifera gen. et sp. nov.), are described. Two undetermined forms are also illustrated. The Hetang and other Neoproterozoic–Cambrian sponge fossils, at their face value, indicate that that hexactinellids evolved no later than the Nemakit-Daldynian–Tommotian and perhaps in the late Neoproterozoic, and the demosponges and calcareans evolved no later than the Atdabanian. The divergence of sponge classes therefore appears to be part of the Cambrian Radiation event. In comparison with the eumetazoans which probably diverged at ca. 600 Ma, however, the sponges (particularly the demosponges and calcareans) appear to have a missing fossil record in the late Neoproterozoic and earliest Cambrian. The minimum implied gaps (MIGs) of the calcareans and demosponges are substantial (tens of Myrs to perhaps more than 100 Myrs), particularly if the calcareans constitute a sister group of the eumetazoans—a topology supported by currently available molecular evidence. [Copyright &y& Elsevier]

[Display omitted] • Early Cambrian black shales in South China have very high concentrations of Hg from 714 to 6187 ppb. • Volcanic activities may have led the enrichment of Hg in the early Cambrian black shales. • Hg in the early Cambrian black shales is hosted in organic matter and sulfides. The widespread lower Cambrian black shales reflect a significant anoxic event during a global transgression, recording synergistic changes in the marine environment and biological evolution. Extremely high Hg (714–6187 ppb, average is 3380 ppb) contents and Hg to total organic carbon ratios (Hg/TOC: 120–765 ppb/wt.%) were observed in the Danquan section of South China, suggesting that extensive volcanic activities may occur during the early Cambrian. The redox condition of the Danquan section is euxinic as evidenced by multiple redox proxies, which include U/Th, V/Cr, C org /P and authigenic enrichment of redox sensitive elements (Mo EF , U EF). The Co*Mn – Al crossplot revealed that black shale developed in different geographical settings (upwelling setting for the Interval I, and restricted condition for the Interval II). Besides, the sedimentary Hg is hosted in different phases in the two intervals: in organic matter for the Interval I, and in sulfides for the Interval II. Moreover, the anomalous enrichment of Hg in both intervals document the Hg controlled by geographical settings and host phases. Thus, volcanic activities and geographical settings both constrain the evolution of life, as well as redox conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Paleoceanic redox variation controls mineralization gradient of Ni-Mo and V. • The Ni-Mo sulfide ores formed under the iron-limited restricted euxinic basin. • The vanadium ores formed under euxinic to suboxic conditions. 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. [ABSTRACT FROM AUTHOR]

Abundant metazoans phyla made their first appearances in the fossil record during a relatively short time span in the Ediacaran-Cambrian interval (560–520 Ma) and rapidly expanded ecologically during the early Cambrian, leading to the establishment of metazoan-dominated ecosystems accompanied by widespread biomineralization. This unique evolutionary event was known as the 'Cambrian Explosion'. Although a significant amount of work has been undertaken to understand the evolution of life during this time interval, previous studies have mainly focused on fossil records from the Cambrian fine-grained clastic rocks deposited in shallow-water environments. However, fossil records from marlstones and mudstones formed in deep-water basins were poorly understood. In this study, we focus on fossil records from the black thin-bedded marl and calcareous mudstones of the lower Cambrian Shuijingtuo Formation in the Three Gorges area, Hubei Province. A total of 9059 fossil specimens, including both microfossils and megafossils belonging to 45 species and 14 major taxonomic groups were recovered from the Shuijingtuo Formation at the Luojiacun section. Four palaeoecological assemblages were recognized based on fossil composition and ecological characteristics. The oldest assemblage (Assemblage A) features an Isoxys -dominated planktonic community with only one benthic group of bradoriids and exhibits the highest fossil abundance and lowest species diversity. Assemblage B is characterized primarily by highly diverse benthic groups, which in decreasing order of abundance include bradoriids (27.4%), sponge spicules (9.8%), cnidarians (5.7%) and hyoliths (2.0%). The third assemblage (Assemblage C) introduces the predominant role of sponge spicules, which compose 44.8% of the community. In contrast, Assemblage D is predominated by phytoplankton featuring the lowest fossil abundance and low diversity. Integration with geochemical and sedimentary data indicates that these biotic assemblages and their evolution were mainly controlled by environmental redox variations and changes in terrestrial fluxes and water depth, subsequently suggesting a significant role of these factors on biotic evolution in the early Cambrian deep-water environment. This study provides support for the importance of oxygen levels in the evolution of life during the early Cambrian. • Four palaeoecological assemblages were recovered from the Shuijingtuo Formation. • Redox, terrestrial fluxes and sea level change were considered the main controls. • Significance of oxygen content were implied on the evolution of early life. [ABSTRACT FROM AUTHOR]

The early Cambrian was a critical interval for the Earth system, during which a rise in oceanic and atmospheric oxygen levels coincided with the rapid diversification of metazoans. A variety of contrasting models have been proposed for the spatiotemporal redox evolution of the early Cambrian ocean. These include the development of a well-oxygenated deep ocean at the base of Cambrian Stage 3 (commencing at ~521 Ma), or alternatively, persistent and widespread anoxic (ferruginous) conditions throughout the early Cambrian ocean. Here, we present redox sensitive trace element (RSTE), Fe speciation, and N and C isotope (δ15N sed and δ13C org) data for samples from a section (Zhongnancun) of the early Cambrian Niutitang Formation, which was deposited on the outer-shelf of the Yangtze Block, South China. The Fe speciation and RSTE data provide evidence of a transition from euxinic, through ferruginous, to oxic conditions during deposition of the Niutitang Formation. The combination of these new data with existing data from the inner-shelf to basin environment, implies regional redox stratification across the Yangtze Block during Cambrian stages 2 and 3, with oxic shallow waters above ferruginous deep waters, and spatial variability in the degree of mid-depth euxinia. Oxygenation of deeper waters may have occurred by early Cambrian Stage 4 (~514 Ma). A compilation of δ15N values from multiple early Cambrian sections of the Yangtze Block indicate that N 2 fixation dominated the nitrogen cycle during late Cambrian Stage 2. Low δ15N values (<−2‰) preserved in shelf sections can be interpreted to represent partial assimilation of NH 4 +, where NH 4 + was not a limiting nutrient. During the early-middle Cambrian Stage 3, more positive δ15N values (0 to +3‰) are recorded in shelf sections, with lower values (−2 to +2‰) recorded in slope-basin sections. The positive δ15N values observed in shelf sections are likely a consequence of partial denitrification in the water column, whereas coeval deeper water δ15N values of ~0‰ may reflect the dominance of N 2 fixation. The distribution of δ15N values, combined with a gradient in δ13C org values, is consistent with a stratified ocean model. The δ15N values of all sections are lower than those of the modern ocean, which may indicate that the nitrate concentration of the early Cambrian Yangtze ocean was generally low during Cambrian Stage 3. The observed gradient in δ15N values is similar to that observed in records from Mesoproterozoic oceans, suggesting that abundant nitrate availability may have been restricted to shelf environments. We propose that increased nitrogen availability in shelf settings may have contributed to the evolution of large-celled eukaryotic phytoplankton. This provided a positive feedback on ocean oxygenation, allowing for increased complexity in early animal ecosystems on the continental shelf, and ultimately deep water oxygenation. • Geochemical data from Zhongnancun reveals dynamic water column redox. • A distinct gradient in δ15N is observed in the early Cambrian Yangtze ocean. • Abundant nitrate availability may have been restricted to shelf environments. • Nitrate-rich waters may have provided sufficient habitat space for metazoans. [ABSTRACT FROM AUTHOR]