Ediacaran Cambrian Transition Research Articles

Depositional environment and organic matter enrichment mechanism of the lower cambrian shale in the southern Sichuan Basin

The Ediacaran-Cambrian transition was one of the most important periods in Earth’s history. Based on the analysis of lithofacies, mineral composition, total organic carbon major and trace elements, we have studied the depositional environment and organic matter enrichment mechanism of Qiongzhusi Formation shale in the southern Sichuan Basin. The results show that V/Cr, Ni/Co, U/Th, Mo-EF and U-EF values suggest stronger reducing conditions in black shale compared to grey shale, with anoxic conditions decreasing from the interior of the faulted-sag to its exterior. Mo-TOC crossplots and U-Mo covariation analyses indicate a moderately restricted environment during the deposition of the Qiongzhusi Formation. Ba/Al and biogenic barium (Baxs) suggest that the black shale had higher paleoproductivity than grey shale, and the faulted-sag interior higher paleoproductivity compared to that of the slope and outside faulted-sag. The Ti/Al ratio indicates a stable terrigenous input during deposition. The chemical index of alteration (CIA) values in the Qiongzhusi formation range mostly from 50 to 70, indicating low chemical weathering under a cold and arid climate. The Cd/Mo ratio and Co (μg/g) × Mn (%) plot indicate that upwelling had a minor influence in general. Overall, the organic matter enrichment in the Qiongzhusi Formation was primarily controlled by the redox conditions, which were influenced by the Mianyang-Changning faulted-sag. We proposed two depositional models for the Qiongzhusi Formation shale in the southern Sichuan Basin: (1) OM-enriched black shale, deposited under anoxic-suboxic conditions, experiencing low chemical weathering, cold and arid climate and high paleoproductivity; (2) OM-lean grey shale, deposited under suboxic-oxic conditions, with low chemical weathering, a cold and arid climate and low paleoproductivity.

Spatiotemporal fluctuations of pyrite sulfur isotope with pyrite morphology during the Ediacaran-Cambrian transition

Spatiotemporal fluctuations of pyrite sulfur isotope with pyrite morphology during the Ediacaran-Cambrian transition

Trace fossils from the Meishucun section of South China: revisiting ichnotaxonomy, behavioural diversification and ecosystem engineering from a key Ediacaran–Cambrian succession

AbstractThe Ediacaran–Cambrian transition was a time of profound geobiological revolution and sedimentary change in Earth's history, including arguably the most dramatic evolutionary radiation and a remarkable biotic replacement. The complex feedback between diversification of metazoans and their ecological interactions, especially after the extinction of the latest Ediacaran biota, is still debated. Here, we systematically studied the trace fossil content of the Ediacaran–Cambrian succession from the Meishucun section of South China. A total of 20 ichnogenera comprising 31 ichnospecies have been identified in the succession. The occurrence of Treptichnus pedum in the Lower Phosphate of the Zhongyicun Member reinforces the importance of this ichnotaxon as an indicator of the Ediacaran–Cambrian boundary. Our critical review of the trace fossil record of this key succession shows that increases in ichnodiversity, ichnodisparity, degree of bioturbation, and bioturbation depth were accompanied by increases in modes of life and ecosystem engineering throughout the early Cambrian. The dramatic and stepped increased in ichnodiversity and ichnodisparity in the Ediacaran–Cambrian transition, from simple trace fossil morphologies in the Ediacaran to complex branching in the Fortunian, and finally horizontal looping trails and suspension feeding burrows in Cambrian Stage 2, paralleled the shift from the Cambrian information revolution to the agronomic revolution. The dramatic hike in bioturbation intensity and depth that occurred during Cambrian Stage 2 may have resulted in an increase in bioirrigation levels and was conducive to a further deepening of the redox discontinuity surface.

Integrated chemostratigraphy and detrital zircon geochronology of the Ediacaran–Cambrian transition in the Western Anabar Uplift: Implications for the Neoproterozoic evolution of the northern Siberian Platform

Integrated chemostratigraphy and detrital zircon geochronology of the Ediacaran–Cambrian transition in the Western Anabar Uplift: Implications for the Neoproterozoic evolution of the northern Siberian Platform

Nitrogen cycling and marine redox evolution during the Ediacaran–Cambrian transition

Nitrogen cycling and marine redox evolution during the Ediacaran–Cambrian transition

Subduction-collision and rapid uplift of the NW Yangtze Block during the Ediacaran-Cambrian transition: new evidence from the shoshonite series

ABSTRACT The NW Yangtze Block has long been regarded as a passive continental margin during Ediacaran and Cambrian. However, with the deepening of the regional geological survey and deep oil and gas exploration, the depositional sequence and palaeogeographic patterns of Ediacaran and Cambrian in the Sichuan basin are not consistent with the general sedimentary evolution process of passive margin more and more. To fully understand the tectonic property and palaeogeography of the NW Yangtze Block, a comprehensive study of the whole rock geochemistry, zircon U-Pb chronology and Hf isotopic composition analysis were carried out on the Weimen volcanic rocks, which mainly consist of trachyte, trachy-andesite, andesite, dacite, and rhyodacite in Maoxian area. They have high light rare earth element (LREE) and large ion lithophile element (LILE) concentrations, but are depleted in high field strength elements (HFSE). They are characterized by having high K2O+Na2O (5.60–9.48%), MgO (1.47–4.58%), Ce/Yb (11.67–25.79), and normalized hypersthene (Hy = 2.14–14.17), indicate an unambiguous feature of the shoshonite series. LA-ICP-MS zircon U-Pb dating results show that the volcanic rocks have crystallization ages ranging from 528 Ma to 523 Ma, with positive zircon εHf (t) ranging from + 1.31 to + 5.26. In combination with previous studies, the early Cambrian shoshonite series in Maoxian area are interpreted to be formed in a continental collision setting. Therefore, the discovery of the shoshonite series suggests that the NW Yangtze Block has experienced an important process of continental subduction and collisional orogeny in response to the Gondwana assembly, and consequently provided a large amount of terrigenous sources to rapidly fill into the early Cambrian foreland basin.

In situ Rb–Sr geochronology of slickensides reveals reactivation of cratonic margins post-Gondwana assembly

In situ Rb–Sr geochronology of slickensides reveals reactivation of cratonic margins post-Gondwana assembly

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

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

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

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

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

The Cambrian Terreneuvian Yuertusi Formation of the Tarim Basin, regarded as a potential hydrocarbon source rock (TOC max = 29.8 wt%), preserves the record of the Ediacaran to Cambrian Series 2 transition. This study presents a highresolution multi-proxy investigation of the Sugaitebulake and Yutixi sections of the NW Tarim Basin. Evidence of hydrothermal activity and euxinic conditions in the earliest Cambrian has been delineated by samples from the lower part of the Yuertusi Formation group A, featured by weakly positive Ce anomalies (Ce/Ce* avg. = 0.46), extremely positive Eu anomalies (Eu/Eu* avg. = 26.39), Y/Ho ratios (Y/Ho avg. = 40.48) and several barite layers. Mo and U covariation suggests that the Yuertusi Formation group A and Xiaoerbulake Formation were deposited under sulfidic conditions, whereas the Ediacaran Qigebulake Formation, and Yuertusi Formation groups B and C and D accumulated under suboxic to anoxic conditions. The basin has been weakly restricted and characterized by elevated productivity (Ba-excess avg. = 6,410.42 ppm) during Terreneuvian time. Suboxic conditions in late Ediacaran time (Qigebulake Formation) became euxinic conditions in association with increased sea level and productivity sustained by hydrothermal activity and upwelling, which have been preserved in the organic-rich lower part of the Yuertusi Formation. Suboxic conditions were reestablished during the Cambrian Terreneuvian time as reflected in the geochemistry of the upper part of the Yuertusi Formation. Euxinic conditions were once again established early in Cambrian Series 2 Xiaoerbulake Formation. Euxinic conditions were coincident with transgressions of the South Tianshan Ocean during that interval. The accumulation and preservation of organic matter of the Early Cambrian northern Tarim Platform reflect the interaction of hydrothermal activity, coastal upwelling, and sea level change.

Evidence for low sulfate and anoxic deep waters in early Cambrian

Evidence for low sulfate and anoxic deep waters in early Cambrian

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

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