Ediacaran Cambrian Transition Research Articles

Cadmium isotope evidence for near-modern bio-productivity in the early Cambrian ocean

The Ediacaran–Cambrian transition was one of the major transitions in Earth's history, with mass extinctions and radiation of animals. This event has been linked with perturbations in ocean geochemistry, including redox conditions and the oxygenation of Earth's surface environments. Here we present a highly resolved cadmium isotope record (δ114Cd) from shales deposited in a deep-water environment covering the Late Ediacaran to Cambrian Stage 4. Our δ114Cd data vary from −0.70 to +0.35 ‰ and demonstrate the importance of local redox-dependent burial phases on bulk sediment Cd isotope compositions, which are superimposed on global-scale changes in the Cd cycle. The heaviest Cd isotope composition recorded in the early Cambrian Liuchapo Formation is similar to the modern deep ocean, implying a broad similarity in global Cd cycling, perhaps involving similar organic-Cd burial rates. The provision of bio-limiting nutrients by continental weathering with rising oxygen concentrations in Earth's surface environments could have expanded the habitable area of the ocean and contributed to animal diversification.

Seawater barium and sulfide removal improved marine habitability for the Cambrian Explosion of early animals.

An increase in atmospheric pO2 has been proposed as a trigger for the Cambrian Explosion at ∼539-514Ma but the mechanistic linkage remains unclear. To gain insights into marine habitability for the Cambrian Explosion, we analysed excess Ba contents (Baexcess) and isotope compositions (δ138Baexcess) of ∼521-Myr-old metalliferous black shales in South China. The δ138Baexcess values vary within a large range and show a negative logarithmic correlation with Baexcess, suggesting a major (>99%) drawdown of oceanic Ba inventory via barite precipitation. Spatial variations in Baexcess and δ138Baexcess indicate that Ba removal was driven by sulfate availability that was ultimately derived from the upwelling of deep seawaters. Global oceanic oxygenation across the Ediacaran-Cambrian transition may have increased the sulfate reservoir via oxidation of sulfide and concurrently decreased the Ba reservoir by barite precipitation. The removal of both H2S and Ba that are deleterious to animals could have improved marine habitability for early animals.

Priapulid neoichnology, ecosystem engineering, and the Ediacaran–Cambrian transition

AbstractThe evolutionary rise of powerful new ecosystem engineering impacts is thought to have played an important role in driving waves of biospheric change across the Ediacaran–Cambrian transition (ECT; c. 574–538 Ma). Among the most heavily cited of these is bioturbation (organism‐driven sediment disturbance) as these activities have been shown to have critical downstream geobiological impacts. In this regard priapulid worms are crucial; trace fossils thought to have been left by priapulan‐grade animals are now recognized as appearing shortly before the base of the Cambrian and represent some of the earliest examples of bed‐penetrative bioturbation. Understanding the ecosystem engineering impacts of priapulids may thus be key to reconstructing drivers of the ECT. However, priapulids are rare in modern benthic ecosystems, and thus comparatively little is known about the behaviours and impacts associated with their burrowing. Here, we present the early results of neoichnological experiments focused on understanding the ecosystem engineering impacts of priapulid worms. We observe for the first time a variety of new burrowing behaviours (including the formation of linked burrow networks and long in‐burrow residence times) hinting at larger ecosystem engineering impacts in this group than previously thought. Finally, we identify means by which these results may contribute to our understanding of tracemakers across the ECT, and the role they may have had in shaping the latest Ediacaran and earliest Cambrian biosphere.

Dawn of diverse shelled and carbonaceous animal microfossils at ~ 571 Ma

The Ediacaran-Cambrian transition documents a critical stage in the diversification of animals. The global fossil record documents the appearance of cloudinomorphs and other shelled tubular organisms followed by non-biomineralized small carbonaceous fossils and by the highly diversified small shelly fossils between ~ 550 and 530 Ma. Here, we report diverse microfossils in thin sections and hand samples from the Ediacaran Bocaina Formation, Brazil, separated into five descriptive categories: elongate solid structures (ES); elongate filled structures (EF); two types of equidimensional structures (EQ 1 and 2) and elongate hollow structures with coiled ends (CE). These specimens, interpreted as diversified candidate metazoans, predate the latest Ediacaran biomineralized index macrofossils of the Cloudina-Corumbella-Namacalathus biozone in the overlying Tamengo Formation. Our new carbonate U–Pb ages for the Bocaina Formation, position this novel fossil record at 571 ± 9 Ma (weighted mean age). Thus, our data point to diversification of metazoans, including biomineralized specimens reminiscent of sections of cloudinids, protoconodonts, anabaritids, and hyolithids, in addition to organo-phosphatic surficial coverings of animals, demonstrably earlier than the record of the earliest known skeletonized metazoan fossils.

Cyanobacterial and fungi-like microbial fossils from the earliest Cambrian phosphorite of South China

Microbes, as essential components of any ecosystem for their metabolic capabilities of driving energy and matter cycles, must have been omnipresent in the earliest metazoan-dominated marine ecosystem that was initially established during the Ediacaran-Cambrian transition. However, contemporaneous microbial fossils and their ecological roles are rarely known. Here we report an exquisitely preserved microbial fossil assemblage from the ∼535 million years old phosphorite of South China. Ten types of filamentous and spherical fossils have been identified and most are comparable to modern fungi, cyanobacteria, and microalgae at a cellular level. Particularly, mold- and yeast-like morphotypes that are interpreted as fungi provide potential fossil evidence for exploring the early evolution of fungi. This microbial assemblage including fungal and cyanobacterial analogues built symbiotic mats composed of decomposers and producers that were ecologically indispensable for early metazoans. A rapid phosphatization followed by silica-cementation is responsible for the high fidelity of fossil preservation, and hence is an ideal taphonomic window to explore the microbial world in geological past.

Characteristics and sedimentary evolution of late Ediacaran-early Cambrian microbial carbonates, chert, and phosphorite in South China

ABSTRACT The Ediacaran-Cambrian (E-C) transition period witnessed one of the most important environmental changes in Earth’s history. The western margin of the South China Block (SCB), a shallow-water environment, shows a complete sedimentary record of the E-C transition. Section surveys revealed that dolostone, microbial carbonate, chert, and phosphorite predominantly compose the lithologies of the western margin of the SCB during the E-C transition. Petrological analysis indicates that microbial carbonates in this region exhibit stromatolitic, thrombolytic, cavitated, and sporadic microbial structures. Microbial carbonates with stromatolitic, thrombolytic/cavitated, and sporadic structures correspond to strong, moderate, and weak hydrodynamic conditions, respectively. Microcrystalline dolomites crystallize during the syngenesis stage, while meso- to macrocrystalline and sparry dolomites develop during the burial diagenesis stage. Analysis of major elements and rare earth elements anomalies (δCe: 0.580 ~ 0.753; δEu: 1.234 ~ 1.433) suggests that the siliceous component originates from hot-water sources and is transported into shallow-water environments by upwelling. The absence of a δEu anomaly and the morphology of peloids indicate that the phosphatic component is primarily influenced by biological activity and upwelling. During the Tongwan Movement (545–535 Ma) in the E-C transition period, cyclical upwelling transported increased amounts of deep-source materials into shallow platform waters, leading to higher concentrations of siliceous components and widespread microbial mortality. Following the widespread microbial mortality, the phosphatic component became enriched in deep-water environments. Subsequently, the upwelling transported siliceous- and phosphatic-rich fluids back into the shallow-water environments. The phosphatic component was ultimately assimilated by small shelly fauna and deposited in the phosphatic-rich strata of the early Cambrian period. This study elucidates the origins of complex lithological compositions and the sedimentary evolution model in shallow water environments of the E-C transition period in western SCB, offering evidence for global environmental changes during this period.

Morphology shapes community dynamics in early animal ecosystems

The driving forces behind the evolution of early metazoans are not well understood, but key insights into their ecology and evolution can be gained through ecological analyses of the in situ, sessile communities of the Avalon assemblage in the Ediacaran (~565 million years ago). Community structure in the Avalon is thought to be underpinned by epifaunal tiering and ecological succession, which we investigate in this study in 18 Avalon communities. Here we found that Avalon communities form four distinctive Community Types irrespective of succession processes, which are instead based on the dominance of morphologically distinct taxa, and that tiering is prevalent in three of these Community Types. Our results are consistent with emergent neutrality, whereby ecologically specialized morphologies evolve as a consequence of neutral (stochastic or reproductive) processes within niches, leading to generalization within the frond-dominated Community Type. Our results provide an ecological signature of the first origination and subsequent loss of disparate morphologies, probably as a consequence of community restructuring in response to ecological innovation. This restructuring led to the survival of non-tiered frondose generalists over tiered specialists, even into the youngest Ediacaran assemblages. Such frondose body plans also survive beyond the Ediacaran–Cambrian transition, perhaps due to the greater resilience afforded to them by their alternative ecological strategies.

Reconstruction of the paleo-marine environment during the Ediacaran-Cambrian transition: new insights from geochemical proxies, northern Tarim Platform, Northwest China

Reconstruction of the paleo-marine environment during the Ediacaran-Cambrian transition: new insights from geochemical proxies, northern Tarim Platform, Northwest China

Evidence for low sulfate and anoxic deep waters in early Cambrian

The role of environmental change as a significant force driving biological evolution during the Ediacaran–Cambrian transition remains a subject of extensive debate. In this study, we present high-resolution C-isotope data of organic matter (δ13Corg) and multiple S-isotope compositions of pyrite (δ34Spy and Δ33Spy) obtained from a continuous drill core (ZK1505) in South China, spanning the interval from the Ediacaran to Cambrian Stage 2. Our δ13Corg and δ34Spy data, combined with published data from four sections deposited across a shelf-to-basin transect, reveal substantial gradient from shallow to deep water in δ13Corg and sustained positive δ34Spy values in the early Cambrian. The vertical δ13Corg gradient suggests chemical stratification of the early Cambrian ocean, characterized by oxic shallow water and anoxic deep water. The sustained positive δ34Spy values in the same period suggest low seawater sulfate concentrations. Furthermore, we observed intermittent occurrences of negative Δ33Spy values coupled with positive δ34Spy, suggesting a dominant deep-water anoxia during early Cambrian in the Yangtze Block. We propose that protracted deep-water anoxia may have contributed to the extinction of the Ediacaran biota and played a significant role in the evolution of Cambrian animals.

Ediacaran marine animal forests and the ventilation of the oceans

The rise of animals across the Ediacaran-Cambrian transition marked a step-change in the history of life, from a microbially dominated world to the complex macroscopic biosphere we see today.1,2,3 While the importance of bioturbation and swimming in altering the structure and function of Earth systems is well established,4,5,6 the influence of epifaunal animals on the hydrodynamics of marine environments is not well understood. Of particular interest are the oldest "marine animal forests,"7 which comprise a diversity of sessile soft-bodied organisms dominated by the fractally branching rangeomorphs.8,9 Typified by fossil assemblages from the Ediacaran of Mistaken Point, Newfoundland,8,10,11 these ancient communities might have played a pivotal role in structuring marine environments, similar to modern ecosystems,7,12,13 but our understanding of how they impacted fluid flow in the water column is limited. Here, we use ecological modeling and computational flow simulations to explore how Ediacaran marine animal forests influenced their surrounding environment. Our results reveal how organism morphology and community structure and composition combined to impact vertical mixing of the surrounding water. We find that Mistaken Point communities were capable of generating high-mixing conditions, thereby likely promoting gas and nutrient transport within the "canopy." This mixing could have served to enhance local-scale oxygen concentrations and redistribute resources like dissolved organic carbon. Our work suggests that Ediacaran marine animal forests may have contributed to the ventilation of the oceans over 560 million years ago, well before the Cambrian explosion of animals.

Adaptations to changing substrates in diploblastic dinomischids from the early Cambrian

The significant changes in seafloor conditions during the Ediacaran-Cambrian transition presumably bear on the autecology of the benthic animals living in this time interval. However, little evidence shows morphological modifications and life strategy transitions in early animals in response to this substrate revolution. Dinomischidae is a family of diploblastic-grade with a sessile, suspension-feeding lifestyle, which includes Xianguangia, Daihua and Dinomischus. Here, we redescribe Calathites spinalis, a poorly known taxon from the early Cambrian (Epoch 2, Age 3, ∼518 Ma), as a new member of this family. Our new material reveals that the body of C. spinalis consists of a stalked calyx and 18 tentacle-sheath complexes, which is a typical trait of dinomischids. Our phylogenetic analyses recover Calathites as a sister group of Xianguangia and Daihua and support dinomischids being a monophyletic group. Members of dinomischids possess unique attachment structures that vary in shape and size, presenting a unique example of morphological adaptations of early sessile animals during the Cambrian substrate revolution. Comparison with early metazoan fossils reveals that dinomischids and other early basal metazoans lagged behind sessile bilaterians in terms of adaptations to substrate changes; the former tends to persist with attachment strategies adapted to Proterozoic-style seafloor conditions that were relatively firm.

The Paleoecological Environment during the Ediacaran–Cambrian Transition in Central Guizhou Province, China: Evidence from Zn Isotopes

During the Ediacaran–Cambrian transition, a series of stratal continuous and well-preserved siliceous rock and phosphorite assemblages developed in Qingzhen, Guizhou Province, China, facilitating research on the biological evolution, marine chemistry, and palaeoecological environment of this period. Therefore, we investigated the paleontology, trace and rare earth elements, total organic carbon, total sulfur content, and Zn isotopes of the phosphorus-bearing rock series in the Taozichong Formation of the Cambrian period in Qingzhen. Geochemical analysis reveals that the sedimentary rocks in this area were formed in the oxygen-rich seawater environment and were not affected by high-temperature hydrothermal activity. The upwelling ocean current provided abundant rare earth elements and other nutrient elements, as well as conditions for the prosperity of biota in Qingzhen. In addition, the δ66Zn value (−0.21‰–0.41‰ range and 0.17‰ mean) in the Qingzhen phosphorous rock series was much lower than that in seawater, indicating a strong level of biological productivity. The variation trend of δ13C, δ18O, and δ66Zn exhibited four stages and three obvious drift events. The results suggest that climate change during this period led to the intermittent flourishing and extinction of organisms, which triggered the negative drift of δ13C and δ18O in the ocean, resulting in a coordinated response of δ66Zn. The unique ecological environment of the Taozichong Formation in Qingzhen also provides favorable conditions for the population continuation of Ediacaran-type benthic soft-bodied metazoon dominated by discoid fossils, Shaanxilithes, worm fossils, and sponge fossils in the Cambrian strata, as well as participation in the global biological explosion events. The study area provides new insights for rebuilding the global Ediacaran–Cambrian ecosystem during the transition period.

A shift in redox conditions near the Ediacaran/Cambrian transition and its possible influence on early animal evolution, Corumbá Group, Brazil

The Ediacaran–Cambrian transition witnessed some of the most important biological, tectonic, climatic and geochemical changes in Earth’s history. Of utmost importance for early animal evolution is the likely shift in redox conditions of bottom waters, which might have taken place in distinct pulses during the late Ediacaran and early Paleozoic. To track redox changes during this transition, we present new trace element, total organic carbon and both inorganic and organic carbon isotopes, and the first iron speciation data on the Tamengo and Guaicurus formations of the Corumbá Group in western Brazil, which record important paleobiological changes between 555 Ma to < 541 Ma. The stratigraphically older Tamengo Formation is composed mainly of limestone with interbedded marls and mudrocks, and bears fragments of upper Ediacaran biomineralized fossils such as Cloudina lucianoi and Corumbella werneri. The younger Guaicurus Formation represents a regional transgression of the shallow carbonate platform and is composed of a homogeneous fine-grained siliciclastic succession, bearing meiofaunal bilateral burrows. The new iron speciation data reveal predominantly anoxic and ferruginous (non-sulfidic) bottom water conditions during deposition of the Tamengo Formation, with FeHR/FeT around 0.8 and FePy/FeHR below 0.7. The transition from the Tamengo to the Guaicurus Formation is marked by a stratigraphically rapid drop in FeHR/FeT to below 0.2, recording a shift to likely oxic bottom waters, which persist upsection. Redox-sensitive element (RSE) concentrations are muted in both formations, but consistent with non-sulfidic bottom water conditions throughout. We interpret the collected data to reflect a transition between two distinct paleoenvironmental settings. The Tamengo Formation represents an environment with anoxic bottom waters, with fragments of biomineralized organisms that lived on shallower, probably mildly oxygenated surficial waters, and that were then transported down-slope. Similar to coeval successions (e.g., the Nama Group in Namibia), our data support the hypothesis that late Ediacaran biomineralized organisms lived in a thin oxygenated surface layer above a relatively shallow chemocline. The Guaicurus Formation, on the other hand, records the expansion of oxic conditions to deeper waters during a sea level rise. Although the relationship between global biogeochemical changes and the activities of early bioturbators remains complex, these results demonstrate an unequivocal synchronous relationship between oxygenation of the Corumbá basin and the local appearance of meiofaunal bioturbators.

Increased habitat segregation at the dawn of the Phanerozoic revealed by correspondence analysis of bioturbation

The Agronomic Revolution of the early Cambrian refers to the most significant re-structuration of the benthic marine ecosystem in life history. Using a global compilation of trace-fossil records across the Ediacaran-Cambrian transition, this paper investigates the relationship between the benthos and depositional environments prior to, during, and after the Agronomic Revolution to shed light on habitat segregation via correspondence analysis. The results of this analysis characterize Ediacaran mobile benthic bilaterians as facies-crossing and opportunistic, with low levels of habitat specialization. In contrast, the Terreneuvian and Cambrian Series 2 reveal progressive habitat segregation, parallel to matground environmental restriction. This event was conducive to the establishment of distinct endobenthic communities along the marine depositional profile, showing that the increase in styles of animal-substrate interactions was expressed by both alpha and beta ichnodiversity. Habitat segregation at the dawn of the Phanerozoic may illustrate an early extension of the trophic group amensalism at community scale.

New advances on the carbon isotope and rare earth elements chemostratigraphy of the late Ediacaran Tamengo Formation (Corumbá Group, Brazil)

The Tamengo Formation (Corumbá Group, midwest Brazil) is a carbonate-dominated succession of major importance to unravel the environmental and biological changes during the Ediacaran–Cambrian transition in Gondwana. Although it has been extensively studied in terms of sedimentology, isotope geochemistry, biostratigraphy, and geochronology, these studies are constrained to the Corumbá region. This work presents new sedimentological, C and O isotope chemostratigraphy, and rare earth elements (REE) plus yttrium (REY) data of four sections of the Tamengo Formation in the Serra da Bodoquena region, approximately 200 km south of Corumbá, discussing how these new data are related to the type sections. The sections present ooid grainstones, with hummocky, swaley, and wavy structures, interbedded with mudstones and shales, indicating deposition of fine particles by suspension fallout in-between periods of high-energy, with reworking by storm waves. The δ13Ccarb curves show positive plateaus with minor, short-lived negative excursions linked to facies variations, which are related to a large δ13Ccarb depth gradient in a redox-stratified water column. The REY profiles present middle REE (MREE)-bulge patterns and absent to slightly positive Ce anomalies, consistent with MREE adsorption onto Fe–Mn oxyhydroxides in the water column followed by release in pore waters. Subsequently, REY remobilization during anoxic diagenetic stages resulted in the MREE-bulge pattern and overprinted some of the original seawater REY features, including the Ce anomalies. The Tamengo Formation in the Serra da Bodoquena region represents a storm-dominated carbonate ramp, as previously interpreted for this unit in the Corumbá region. Nevertheless, there is a significant shift of 2 ‰ in the peaks of δ13Ccarb data between both localities, which may be related to differences in the overall sector of the carbonate ramp. There are also substantial variations in the δ13Ccarb record of the Tamengo Formation and other coeval Gondwana basins, such as the Nama and Itapucumi groups, with different peak values, which may be related to latitudinal differences on the inorganic carbon isotope composition or to the degree of basin restriction.

Shadows of the past: An Ediacara-style taphonomic window in advent of Cambrian bioturbators

One of the hypotheses, explaining a sudden vanishing of the Ediacaran biota at the Ediacaran-Cambrian transition, correlates this phenomenon with a disappearance of specific taphonomical factors providing fossilization of soft-bodied organisms (‘Cheshire Cat’ hypothesis). The terminal Ediacaran-Lower Cambrian fossil record does reveal a progressive development of bioturbation and associated decrease of microbial mats, which played a crucial role in the late Precambrian ecosystems. But the tempo of the evolution of organism-sediment interactions significantly varied between different zones of marine basins. Hence, if the ‘Cheshire Cat’ hypothesis is correct, the sedimentary environments barely (if any) colonized by the burrowing eumetazoans might provide the unique interplay of conditions for habitat and Ediacara-style preservation of soft-bodied organisms even in the early Cambrian. Herein we present the new carbon isotope data calibrating the Terreneuvian record in the Ura Uplift of South Siberia and also report fossilized soft-bodied organisms morphologically-close to the Ediacaran holdfasts preserved in association with microbially-induced sedimentary structures in the Cambrian Stage 2 carbonate-siliciclastic Nokhtuisk Formation. Although the late Terreneuvian age, this marginal marine succession is poorly bioturbated and hosts the soft-bodied fossils in the intervals undisturbed by the burrowing organisms. Our study demonstrates the possibility of Ediacara-style preservation of soft-bodied organisms in specific habitat and taphonomic conditions until being replaced by the Cambrian endobenthos.

First record of trace fossils in the Ediacaran–Cambrian transition on the northern Gondwana platform (Anti-Atlas, Morocco)

The Anti-Atlas Mountains of Morocco preserve one of the most complete latest Neoproterozoic to Cambrian stratigraphic successions worldwide, with high-resolution chemostratigraphic δ13Ccarb coverage. However, the exact stratigraphic position of the Ediacaran–Cambrian boundary remains unresolved. Until now, no trace fossils or body fossils have been recorded from the terminal Ediacaran strata in the Anti-Atlas Mountains. The only exception is a discoidal body fossil-like structure from the shallow-marine sandstones of the Ediacaran Tabia Member, Adoudou Formation, in the Taroudant Group in the Igherm inlier. For the first time, we document the occurrence of abundant trace fossils from the base of the Adoudou Formation, including Treptichnids, Monomorphichnus isp., Helminthoidichnites isp., Gordia isp., Palaeophycus isp., Planolites isp., and Conichnus isp. The integration of both trace fossils and carbon isotope δ13Ccarb records suggests that the Ediacaran–Cambrian transition in the Sous Basin can be placed at the contact between the Tifnout and Tabia Members, which chemostratigraphically coincides with the large negative δ13Ccarb excursion that had been previously suggested for the Ediacaran–Cambrian boundary (BACE) in the western Anti-Atlas Mountains.

Colonial green algae in the Cambrian plankton.

The fossil record indicates a major turnover in marine phytoplankton across the Ediacaran-Cambrian transition, coincident with the rise of animal-rich ecosystems. However, the diversity, affinities and ecologies of Cambrian phytoplankton are poorly understood, leaving unclear the role of animal interactions and the drivers of diversification. New exceptionally preserved acritarchs (problematic organic-walled microfossils) from the late early Cambrian (around 510 Ma) reveal colonial organization characterized by rings and plates of interconnected, geometrically arranged cells. The assemblage exhibits a wide but gradational variation in cell size, ornamentation and intercell connection, interpreted as representing one or more species with determinate (coenobial) colony formation via cell division, aggregation and growth by cell expansion. An equivalent strategy is known only among green algae, specifically chlorophycean chlorophytes. The fossils differ in detail from modern freshwater examples and apparently represent an earlier convergent radiation in marine settings. Known trade-offs between sinking risk and predator avoidance in colonial phytoplankton point to adaptations triggered by intensifying grazing pressure during a Cambrian metazoan invasion of the water column. The new fossils reveal that not all small acritarchs are unicellular resting cysts, and support an early Palaeozoic prominence of green algal phytoplankton as predicted by molecular biomarkers.

Sedimentary characteristics and evolution of Ediacaran glaciation in western Henan Province, southern North China Craton

The Ediacaran-Cambrian transition is recognized as a warm period. However, glaciations have been reported in many paleocontinents worldwide during this transition, including the Luoquan glaciation in Northern China. The type of glacier remains unclear, and there are still uncertainties regarding the differences and similarities between the Luoquan glaciation and other Ediacaran glaciations, which restrict global comparison of glaciation characteristics. Research on sedimentary characteristics and succession of eight sections of the Luoquan and Dongpo formations in western Henan Province in the southern North China Craton shows that from north to south, the Luoquan and Dongpo formations show three vertical facies associations spatially, indicating an obvious sedimentary environment evolution of subglacial-periglacial-glaciomarine-shallow marine, namely, from the subglacial lodgement to the periglacial outwash fan and permafrost, then to the proximal glaciomarine grounding-line fan and iceberg rainout to the distal glaciomarine ice-rafted deposits, and finally to the normal shallow sea.The spatial evolution of sedimentary environment indicates that the Luoquan glaciation in western Henan Province was a large ice sheet that extended from land to the sea and was not completely frozen but partially melted during warm periods, and temporary open-water conditions existed. Further comparison shows that although the number, frequency and global characteristics have not yet been determined, the global Ediacaran glaciations also show dynamic evolution characteristics from continental to marine glaciers, which is similar to the evolution mechanism of ice sheet during the snowball Earth, and also not indicative of hard snowball Earth pattern of complete ice cover.

Assessing the correlation between Ediacaran-Cambrian units of SW Gondwana: The Tagatiya Guazú (Itapucumi Group, Paraguay) and Tamengo (Corumbá Group, Brazil) formations

The Tagatiya Guazú (PY) and Tamengo (BR) formations are South-American Ediacaran-Cambrian carbonate units that share striking similarities between their sedimentological, paleontological, and geochemical records. The Tagatiya Guazú Formation is mainly composed of grainstone and abundant microbial facies, bearing a diverse fossil content represented by autochthonous to parautochthonous remains of late Ediacaran tubular skeletal taxa, such as Cloudina, Corumbella, and Namacalathus, in addition to complex trace fossils indicative of the Ediacaran-Cambrian transition. The Tagatiya Guazú Formation is interpreted as deposited in a protected inner ramp setting. The Tamengo Formation is constituted by grainstone, packstone, wackestone, mudstone, marl, shale, and microbialite facies, also containing Cloudina and Corumbella as allochthonous and parautochthonous remains, respectively. The possible recognition of hummocky and swaley cross-stratification in the Brazilian unit, led to the interpretation of deposition in a storm-dominated middle to outer ramp setting in slightly deeper conditions than the Tagatiya Guazu Formation. Both units display the characteristic late Ediacaran Positive Carbon Isotope Plateau (EPIP) detected in terminal Ediacaran successions worldwide, commonly interpreted as a result of an intensely redox-stratified ocean, which is probably responsible for a decreasing δ13C trend from inner to outer ramp settings. The existence of this gradient can be inferred between the Tagatiya Guazú Formation and its lateral correlate, the Camba Jhopo Formation. On the other hand, the Tamengo Formation shows an unexpected opposite pattern (i.e., increasing δ13C values from inner to outer ramp settings). The limited size of the putative hummocky and swaley cross-stratifications, together with the problematic occurrence of parautochthonous skeletal fossils in deep (and probably anoxic) waters and the inverse δ13C isotopic gradient, suggest some inconsistencies in the Tamengo Formation depositional model. The reinterpretation of these deep-water facies as deposited in a shallower setting would solve (or reduce) the isotopic gradient problem and provide a better fit between the Tamengo Formation redox model and the ones interpreted for other units worldwide. Finally, multiple lines of evidence indicate that both the Tagatiya Guazú and Tamengo formations represent coeval successions (with the former being slightly younger) deposited in similar, shallow water depths in carbonate ramps opened to the Clymene Ocean.