Ediacaran Cambrian Boundary Research Articles

Cloudina aggregates from the uppermost Dengying Formation, Three Gorges area, South China, and stratigraphical implications

Numerous issues remain concerning palaeobiology, palaeoecology, and biostratigraphy of Cloudina, an iconic tubular organism of the Ediacaran period. In this study, accumulations of Cloudina are described from the topmost bed of the Dengying Formation in the Baiguotang section of the eastern Three Gorges area, South China. The massive occurrence of Cloudina refutes any possibility of setting the Ediacaran-Cambrian boundary (E-CB) beneath the lithological transition between Dengying and Yanjiahe formations in this area. The age of the Cloudina aggregates is younger than 540.6 Ma based on a concordia 206Pb/238U age of 540.6 ± 3.4 Ma obtained from underlying upper part of the Baimatuo Member (Baiguotang section), which is consistent with the 539 Ma E-CB reported from southern Namibia. Combining fossil evidence provided by Asteridium–Heliosphaeridium–Comasphaeridium acritarchs and Anabarites trisulcatus–Protohertzina anabarica small shelly fossil assemblages and the Basal Cambrian Carbon isotope Excursion (BACE), we tentatively place the E-C boundary within the lower part of the Yanjiahe Formation, close to the BACE excursion located in the lower part of the Yanjiahe Formation in the southern Huangling Anticline.

Detrital zircon and apatite U-Pb geochronology of Ediacaran fossil–bearing strata spanning the late Ediacaran–Cambrian boundary in central Iran

The Neoproterozoic successions exposed in the Koushk and Chahmir areas of central Iran include both volcanic, and shallow to deep marine siliciclastic and carbonate units. We obtained detrital zircon dates from tuffaceous shales bearing Ediacaran fossils, including Corumbella, Persimedusites, ?Kimberella, ?Cloudina, and Namalia-like forms, together with a single metazoan holdfast structure from the Dargazin area of the Koushk region that correlates with the upper part of the Wedge succession dated at 555 ± 14 Ma. Zircon grains of the tuffaceous siltstones containing Corumbella, Persimedusites, ?Kimberella, ?Cloudina, Namalia-like structures, ?Spriggina, ?Pteridinium, holdfasts and algae-like branching structures in the Wedge section of the Koushk area, produced an age of 581 ± 9 Ma, whereas pyroclastic sandstones bearing Corumbella, Persimedusites, a possible holdfast, ?Pteridinium, ?Cloudina, algae-like branched structure, possible trace fossils and marine algal remains in the Chahmir area yielded an age of 541 ± 5 Ma. Zircon dates from the Dargazin and Wedge deposits are of late Ediacaran and Chahmir strata are of a late Ediacaran to early Cambrian age. Ages of the Chahmir sedimentary rocks are close to those from Dargazin. Based on our results, the maximum depositional age of the detrital zircons indicates relatively consistent sedimentation throughout the late Ediacaran and the early Cambrian, ranging from 590 to 530 Ma. Farnoosh Farjandi [ ffarjandi@swin.edu.au ], Faculty of Science, Engineering and Technology, Swinburne University of Technology, Melbourne (Hawthorn), Victoria 3122, Australia and Department of Geochemical Exploration, Geological Survey of Iran, Tehran, Iran; Patricia Vickers Rich [ prich@swin.edu.au ], Faculty of Science, Engineering and Technology, Swinburne University of Technology, Melbourne (Hawthorn), Victoria 3122, Australia and School of Earth, Atmosphere and Environment, Monash University, Melbourne (Clayton), Victoria 3800, Australia; Ulf Linnemann [ ulf.linnemann@senckenberg.de ] and Mandy Hofmann [ mandy.hofmann@senckenberg.de ], Senckenberg Naturhistorische Sammlungen Dresden, Sektion Geochronologie, 01109 Dresden, Germany; Massimo Raveggi [ massimo.raveggi@monash.edu ] and Michael Hall [ mike.hall@monash.edu ], School of Earth, Atmosphere and Environment, Monash University, Melbourne (Clayton), Victoria 3800, Australia; Thomas H. Rich [trich@museum.vic.gov.au ], Museum Victoria, Melbourne, Victoria, 3000, Australia

Ediacaran-Cambrian microbialites of the Southern Amazon Craton: relation with the metazoan rise, sea-level changes, and global tectonics

Microbialites are the most abundant life evidence in Precambrian sedimentary rocks. They are produced by microbial interaction activity and sedimentary processes reflecting paleoenvironmental conditions. The Ediacaran-Cambrian carbonate and siliciclastic successions in the Southern Amazon Craton in Central Brazil, provide a key opportunity to understand how the metazoan life coexisted with the microbial communities. The spatial and temporal distribution of microbialites as well as morphological and paleoenvironmental changes have been assessed, reinterpreting previous works and including new data from the Araras-Alto Paraguai and Corumbá basins. The deposition was controlled by subsidence and sea-level changes that affected these basins, considered extensions of epicontinental seas during the Gondwana assembly. The stromatolites are restricted to coastal deposits and experienced thriving flourishment intervals after the Marinoan Glaciation (635 Ma). Post-glacial transgression was marked by microbial colonization in shallow platforms represented by stratiform and giant domical stromatolites in the Araras-Alto Paraguai Basin. The continuity of the transgression generated a moderately deep aragonite sea at about 622 Ma. A progressive sea-level fall caused the implantation of coastal environments under greenhouse conditions with tidal flat and sabkha settings colonized by centimetric-scale stromatolites. The sea retreat was accompanied by progressive uplift, causing a moderate inversion of the basin and erosion of the succession until ~560 Ma with the deposition of the last preserved tidal flat deposits with the occurrence of thrombolites. The subsiding Corumbá Basin was the site of microbially-induced deposition of carbonates in a shallow platform connected to an offshore setting with the proliferation of metazoan straddling the Ediacaran-Cambrian boundary. Microbial communities were restricted to lagoon deposits during the Lower Cambrian transgression in the Araras-Alto Paraguai Basin and the last phase refers to the sea retreat towards southeast, developing a fluvial system connected with the arid and evaporitic tidal flats colonized by microbialites that lasted until the upper Cambrian. Except for the post-glacial stromatolites, the columnar and domal microbialite indicate that the coastal settings dominated the Ediacaran-Cambrian transition. The preservation of microbialites in the post-glacial intervals can be associated with the Mg-Ca-CO3 oversaturation in dolomitic platforms. The rapid calcification and ability to resist the dissolution and replacement have increased the stromatolites’ preservation potential reported here, where its well-preserved occurrence in tidal flats and sabkha occurs due to intense early diagenetic silicification. The change from carbonate accumulation to siliciclastic-rich environments contributed to the demise of microbially-induced strata. In general, the scarce coexistence between coastal stromatolite and metazoan-bearing marine deposits makes it challenging to establish a competitive relationship between these organisms, as previously postulated.

Distribution and correlation ofSabellidites cambriensis(Annelida?) in the basal Cambrian on Baltica

AbstractSabellidites cambriensisis a tubular non-mineralized metazoan that appears as compressed ribbon-shaped imprints with transverse wrinkling, thick walls and an even tube diameter of up to 3 mm. The distribution ofSabelliditesis investigated in three Ediacaran–Cambrian sections on the Digermulen Peninsula in Arctic Norway, spanning the Manndrapselva Member of the Stáhpogieddi Formation and the lower member of the Breidvika Formation. Here, the Ediacaran–Cambrian boundary is located in the lower part of the upper parasequence (third cycle) of the Manndrapselva Member. Specimens ofSabelliditesare rare but consistently present close to the lowest level ofTreptichnus pedumand upsection, whereas the taxon is common and abundant in the lower part of the lower member of the Breidvika Formation, with an upper record atc.55 m above the base. The range is comparable with that of the GSSP section in Newfoundland, Canada, establishingSabelliditesas an index fossil for the lowermost Cambrian. In the Manndrapselva Member,Sabelliditesco-occurs with the acritarchGranomarginata, indicative of the lowermost CambrianGranomarginataZone, whereas in the Breidvika Formation it co-occurs withAsteridium.Sabelliditesis widely distributed in Baltica, through the Rovnian and Lontovan regional stages but confined to the Fortunian global stage. In its lower range,Sabelliditesis associated with aTreptichnus pedumtrace fossil association and a depauperate leiosphaerid acritarch assemblage, followed by aGranomarginataassemblage. In its upper range,Sabelliditesco-occurs with acritarchs of theAsteridium–ComasphaeridiumZone and the tubular foraminiferanPlatysolenites. In Baltica,Sabelliditesis a useful index fossil.

Late Ediacaran occurrences of the organic-walled microfossilsGranomarginataand flask-shapedLagoenaforma collarisgen. et sp. nov.

AbstractNew occurrences of flask-shaped and envelope-bearing microfossils, including the predominantly Cambrian taxonGranomarginata, are reported from new localities, as well as from earlier in time (Ediacaran) than previously known. The stratigraphic range ofGranomarginataextends into the Cambrian System, where it had a cosmopolitan distribution. This newly reported Ediacaran record includes areas from Norway (Baltica), Newfoundland (Avalonia) and Namibia (adjacent to the Kalahari Craton), and puts the oldest global occurrence ofGranomarginatain the Indreelva Member (< 563 Ma) of the Stáhpogieddi Formation on the Digermulen Peninsula, Arctic Norway. AlthoughGranomarginatais rare within the assemblage, these new occurrences together with previously reported occurrences from India and Poland, suggest a potentially widespread palaeogeographic distribution ofGranomarginatathrough the middle–late Ediacaran interval. A new flask-shaped microfossilLagoenaforma collarisgen. et sp. nov. is also reported in horizons containingGranomarginatafrom the Stáhpogieddi Formation in Norway and the Dabis Formation in Namibia, and flask-shaped fossils are also found in the Gibbett Hill Formation in Newfoundland. TheGranomarginata–Lagoenaformaassociation, in addition to a low-diversity organic-walled microfossil assemblage, occurs in the strata postdating the Shuram carbon isotope excursion, and may eventually be of use in terminal Ediacaran biostratigraphy. These older occurrences ofGranomarginataadd to a growing record of body fossil taxa spanning the Ediacaran–Cambrian boundary.

Enigma variations: characteristics and likely origin of the problematic surface texture Arumberia, as recognized from an exceptional bedding plane exposure and the global record

AbstractArumberia is an enigmatic sedimentary surface texture that consists of parallel, sub-parallel or radiating ridges and grooves, most commonly reported from upper Neoproterozoic – lower Palaeozoic strata. It has variably been interpreted as the impression of a small metazoan, a ‘vendobiont’, a physical sedimentary structure formed on a substrate with or without a microbial mat covering, or a non-actualistic microbial community. In this paper we contribute new insights into the origin of Arumberia, resulting from the discovery of the largest contiguous bedding plane occurrence of the texture reported to date: a 300 m2 surface in the lower Cambrian Port Lazo Formation of Brittany, NW France. We compare the characteristic features of Arumberia at this locality with 38 other global records, revealing four defining characteristics: (1) the three-dimensional (3D) morphology of exposed Arumberia lines (either positive relief ‘ridges’ or negative relief ‘grooves’) records fully preserved cords within clay laminae; (2) lines may transition laterally into reticulated patterns; (3) characteristic parallel and sub-parallel Arumberia lines can become modified by desiccation on emergent substrates prior to interment; and (4) Arumberia are streamlined with palaeoflow in successions showing evidence of unidirectional currents, but are organized parallel to ripple crests where strata were sculpted by oscillatory flows. These characteristics indicate that Arumberia records a 3D entity, distinct in material properties from its host sediment, which occurred in very shallow water settings where it was prone to passive reorganization in moving water, and desiccation when water drained. A literature survey of all known Arumberia occurrences reveals that the most reliable examples of the form are stratigraphically restricted to a 40 Ma interval straddling the Ediacaran–Cambrian boundary (560–520 Ma). Together these characteristics suggest that Arumberia records the remains of extinct, sessile filamentous organisms (microbial or algal?) that occupied very shallow water and emergent environments across the globe at the dawn of the Phanerozoic Eon.

Redox-Controlled Ammonium Storage and Overturn in Ediacaran Oceans

As a key nutrient, nitrogen can limit primary productivity and carbon cycle dynamics, but also evolutionary progress. Given strong redox-dependency of its molecular speciation, environmental conditions can control nitrogen localization and bioavailability. This particularly applies to periods in Earth history with strong and frequent redox fluctuations, such as the Neoproterozoic. We here report on chlorophyll-derived porphyrins and maleimides in Ediacaran sediments from Oman. Exceptionally light δ15N values (< –10‰) in maleimides derived from anoxygenic phototrophs point towards ammonium assimilation at the chemocline, whereas the isotopic offset between kerogens and chlorophyll-derivatives indicates a variable regime of cyanobacterial and eukaryotic primary production in surface waters. Biomarker and maleimide mass balance considerations imply shallow euxinia during the terminal Ediacaran and a stronger contribution of anoxygenic phototrophs to primary productivity, possibly as a consequence of nutrient ‘lockup’ in a large anoxic ammonium reservoir. Synchronous δ13C and δ15N anomalies at the Ediacaran–Cambrian boundary may reflect one in a series of overturn events, mixing ammonium and isotopically-light DIC into oxic surface waters. By modulating access to nitrogen, environmental redox conditions may have periodically affected Ediacaran primary productivity, carbon cycle perturbations, and possibly played a role in the timing of the metazoan radiation across the terminal Ediacaran and early Cambrian.

C-Sr isotope stratigraphy of carbonate formations of the late Neoproterozoic - Cambrian Marwar Supergroup, western India

The Ediacaran-Cambrian transition period was one of the most significant intervals in the Earth’s history. Widespread changes in the oceanic and atmospheric conditions characterized this interval, which likely triggered the radiation of metazoans. Much of our knowledge of these changes have come from geochemical studies of marine carbonates deposited during this time. In search of such information and to establish the true geographical extent of these events, we carried out a detailed field and geochemical investigation in the Bilara Group of the Marwar Supergroup, the youngest Proterozoic marine carbonate succession of peninsular India. The variations in the 87Sr/86Sr observed in the limestone formation of the Bilara Group (0.70810 to 0.70916) suggest an early Cambrian (520–539 Ma) depositional age for the Group. We identify two δ13C negative excursions of magnitudes ~6‰ and ~7‰, respectively, in the middle and lower parts of the Bilara Group. Since these δ13C anomalies are observed through-out the Marwar Basin that was connected to global oceans, we believe that they represent temporal changes in the chemistry of seawater. Using the principles of δ13C stratigraphy and the available geochronological information, we correlate the middle Bilara anomaly to the Shiyantou carbon isotope excursion (SHICE). Although, poor age constraints hinder a definitive correlation of the older Bilara anomaly, we speculate that it is the basal Cambrian carbon isotope excursion (BACE). These results place the Ediacaran-Cambrian boundary closer to the basal part of the Bilara Group in the Marwar Supergroup.

Earliest known Cambrian calcimicrobial reefs occur in the Gobi-Altai, western Mongolia: Intriguing geobiological products immediately after the Ediacaran–Cambrian boundary

A dramatic shift in microbial reefs occurred around the Ediacaran–Cambrian boundary. Here we describe changes in the composition, construction, and texture of microbial reefs in the Zavkhan Terrane of Gobi-Altai Province, western Mongolia, during the late Ediacaran and early Cambrian. Stromatolites consisting of peloids, micritic clots, and homogeneous lime mud without calcified microbes (calcimicrobes) are characteristic of the upper Ediacaran (units 9 and 16A of the Zuun-Arts Formation). In contrast, abundant thrombolites with stromatolites occur in the lowest Terreneuvian (units 17A and 17 of the Bayan Gol Formation). These Cambrian microbial reefs are made up of micritic clots and homogeneous lime mud in close association with calcimicrobes including Korilophyton, Renalcis, and Tarthinia. The thrombolites and calcimicrobial reefs studied herein occur directly stratigraphically above strata that record a strong negative shift in δ13C values and are dominated by small shelly fossils; these are the earliest known calcimicrobial reef representatives of the Phanerozoic. These microbial reefs changed almost simultaneously with drastic fluctuations in environmental conditions (e.g., seawater chemistry, Ca concentration, carbonate saturation, and oxygen level). These changes would have been influenced by the evolution of calcimicrobes and skeletal metazoans across the Ediacaran–Cambrian boundary. The present work provides crucial geobiological information on substantial shifts at the Ediacaran–Cambrian boundary and how calcimicrobes and related textures appeared in tandem with the innovation of biomineralisation.

Geology and palaeontology of the Codos anticline, eastern Iberian Chains, NE Spain: age constraints for the Ediacaran–Cambrian boundary in the Iberian Chains

AbstractThe two major structural elements of the Iberian Chains, the Datos and Jarque thrust faults, have been described as occurring in proximity in the area around the village of Codos. The purported Jarque fault corresponds to the axial plane of an anticline known as the Codos anticline, which exposes the oldest stratigraphic unit in this area, i.e. the Codos Bed, a limestone bed bearing skeletal fossils of putative Ediacaran or earliest Cambrian age. Details of the geology of the area and the age of the known fossils are poorly understood or not universally agreed upon. New investigations in the anticline revealed the presence of a normal fault, introduced as the Codos fault, which cross-cuts the course of the alleged Jarque fault. The vertical displacement along the axial plane of the anticline appears to be insignificant, challenging the traditional interpretation of the plane as an equivalent of the Jarque thrust fault. Reinvestigation of the Codos Bed revealed previously unknown skeletal fossils of early Cambrian age, confirming notions that the bed is younger than traditionally assumed. In particular, two helcionelloid molluscs, Anabarella cf. plana and Igorella? sp., allow correlations with Terreneuvian strata of central Spain (Pusa and Fuentepizzara formations) and support previously suggested connections with the Heraultia Limestone of southern France. Former identification of the terminal Ediacaran index-fossil Cloudina in the bed is erroneous. Consequently, the Ediacaran–Cambrian boundary in the Iberian Chains, which has been placed at the top of the formation overlying the Codos Bed, is stratigraphically distinctly lower.

Carbon isotope chemostratigraphy of a Gondwanan Ediacaran–Cambrian transition, Soltanieh Mountains, northern Iran

Mixed siliciclastic-carbonate rocks of the Kahar and overlying Soltanieh formations from Chopoghlu, in the Soltanieh Mountains, northern Iran, preserve an expanded stratigraphic record of Ediacaran to lower Cambrian life and environments. To date, regional placement of the Ediacaran-Cambrian boundary has been based on biostratigraphy, with contrasting views of its stratigraphic position. Here we present a high-resolution carbon isotope chemostratigraphic record to provide an independent assessment of correlation to other successions worldwide. Facies analysis indicates that Kahar siliciclastic sediments accumulated in fluvial to tidal environments, with thin microbial carbonates in tidally influenced facies. The overlying Soltanieh Formation records peritidal carbonate deposition followed by significant deepening and deposition of interlaminated mudstones and thin sandstones. δ13C values for Kahar carbonates, with a maximum age of about 550 Ma, lie within a narrow range of 0 to −2‰, save for a moderate positive excursion near the top of the formation. Above this, carbonates of the Lower Dolomite Member, Soltanieh Formation, show consistently negative δ13C values. The overlying Middle Dolomite, Middle Shale and basal part of the Upper Dolomite members feature high amplitude δ13C variations, which rise to values as high as 4‰ towards the top. Considering the integrated chemostratigraphic and paleontological records, Chopoghlu deposition appears to have begun during the late Ediacaran Period. The stratigraphically variable δ13C record of Soltanieh carbonates closely resembles lowermost Cambrian successions elsewhere, supporting placement of the E-C boundary near the top of the Lower Dolomite Member (LDM) and a Terreneuvian age for overlying Soltanieh beds. This is consistent with the presence of Protohertzina sp. and Hyolithellus sp. in or just above uppermost LDM carbonates and the lowermost occurrence of Treptichnus pedum above this. Together, bio- and chemostratigraphy help to integrate the Chopoghlu section into the global record of biospheric change as the Phanerozoic Eon began.

The role of tectonic inheritance in the development of a fold-thrust belt and superimposed rift: An example from the São Francisco basin, eastern Brazil

The São Francisco intracratonic basin, eastern Brazil, hosts multiple sedimentary successions younger than 1.8Ga that witnessed major plate reorganizations since its generation. During the Brasiliano/Pan-African Gondwana amalgamation, in the Ediacaran-Cambrian boundary, the western half of the basin was involved in the Brasília foreland fold-thrust belt, which propagated eastwards deforming its Proterozoic-Paleozoic strata. Lately, as the South Atlantic rift evolved giving rise to the Brazilian continental margin, the basin was affected by extensional tectonism leading to the development of the Cretaceous Abaeté half-graben. During this episode, elements related to a prominent structure of the Brasília foreland fold-thrust belt, the João Pinheiro fault, were reactivated controlling the graben architecture. Aiming to unravel the evolutionary history of the fault zone since its nucleation until its Cretaceous extensional reactivation, we conducted a structural investigation based on field, well-tied 2D seismic data, and paleostress analysis. Our study revealed that the fault represents an emergent ramp of the basal detachment of the Brasilia foreland fold-thrust belt. Formed under EW-oriented shortening, the structure exhibits a gentle recces-salient geometry in map view, which reflects variations in the detachment depth influenced by pre-orogenic rift structures. Trains of NS-trending chevron folds are abundant and their high symmetry and dominance are expressions of the control exerted by mechanical stratigraphy and low-friction basal detachment. The superposed Abaeté half-graben developed under an extensional regime with a NE-trending maximum extension. Under this condition, a stratigraphic interface along the João Pinheiro fault zone was reactivated giving rise to the graben border fault. NS- to NW-oriented preexisting bedding planes were activated as small normal faults. Although newly-formed faults are present, reactivation is the most effective mechanism of extension accommodation. Therefore, this study provides insights on the influence of tectonic inheritance on the architecture, deformation pattern, strain/stress partitioning and evolution of both extensional and compressional intracontinental systems.

Detrital zircon U-Pb and Hf constraints on provenance and timing of deposition of the Mesoproterozoic to Cambrian sedimentary cover of the East European Craton, part II: Ukraine

We present the U-Pb geochronology and Hf isotope analysis of detrital zircons from the Ediacaran/Cambrian sediments of Podillya and south Volyn in western Ukraine, supplemented by the bulk rock XRD mineralogy of the host rocks. Such a combined analytical approach allows for identifying the source areas supplying detritus to sediments and for constraining an age of deposition. Our provenance analysis is based on fourteen samples collected from six exposures, mostly in the valley of the Dniester river. 84 mudstone samples were also examined by the XRD method. U-Pb dating of detrital zircons yielded two sets of maximum depositional ages: 578–546 Ma and 547–523 Ma, for the Mohyliv-Podilsky and Kanyliv Series, respectively. This suggests that the Ediacaran-Cambrian boundary in Podillya coincides with a major erosional gap, with a major change in provenance, and the disappearance of the Ediacaran fauna at the base of the Kanyliv Series, with implications for the stratigraphy and paleogeography of the entire East European Platform. Zircon U-Pb age spectra from the lower part of the Mohyliv-Podilsky Series include a large quantity of 2.2 to 1.9 Ga grains that reveal predominantly negative to nearly chondritic ɛHf values, jointly suggesting detritus supply from the crystalline basement of Sarmatia. Both U-Pb and mineralogical data also indicate a major contribution of volcanic detritus from the Volyn flood basalts. The younger Nagoryany rocks yielded zircon age spectra with peaks at c. 1.80 and 1.49 Ga, implying a shift of the catchment area to Fennoscandia. Above an erosional gap, the zircon age spectra in the Kanyliv and Baltic Series are dominated by peaks at 560–535 Ma. These data and ɛHf values ranging from negative to chondritic and juvenile suggest, in line with the mineralogical data, detritus supply from a continental magmatic arc and collisional orogen. Thus, we interpret the Kanyliv Series as infill of an early Cambrian foreland basin that was established in front of the Scythides and Santacrucides orogens, overriding the SW margin of Baltica.

Mercury anomalies across the Ediacaran–Cambrian boundary: Evidence for a causal link between continental erosion and biological evolution

Significant perturbations in ocean redox states have been thought to be one of the major causes for the biological evolution from the Ediacara biota to the Cambrian fauna. However, the driving force of these changes in the ecosystem across the Ediacaran–Cambrian (E–C) boundary is still a matter of debate. In this study, we reported stratigraphic variations of Hg content and Hg isotopes in E–C sediments from the Mussoorie section, Lesser Himalaya, India. Dolomites from the late Ediacaran Krol D and E members have increasing Hg/Al ratios and decreasing Δ199Hg values from the base upwards, which were most likely originated from a prolonged and enormous terrestrial-Hg input into the E–C ocean. The overlying cherty phosphorites in the lowest Tal Group are highly rich in Re relative to Mo, indicative of a suboxic-anoxic depositional environment on the earliest Cambrian shelf that was coincident with the disappearance of the Ediacara biota. Such an O2-deficient condition was possibly led by a consumption of atmospheric O2 during the oxidative weathering of sedimentary organic matter and shallow-water eutrophication, with evidence from the coupled δ202Hg and δ13C values in late-Ediacaran dolomites and high Cd/TOC and Zn/TOC ratios in early Cambrian phosphorites. The overlying black shales have decreasing contents of Hg and micronutrients (Cd and Zn) and increasing values of Δ199Hg, representing notable declines of terrestrial flux and seawater nutrient at the Cambrian Stage 2 before the major phase of Cambrian diversification. Therefore, we conclude that enhanced continental erosion and pervasive upwelling had led to major changes in redox state and nutrient content in shallow seawaters on shelves, which in turn had promoted the biological evolution across the E–C boundary.

The excursion of Nitrogen and Carbon isotope in the Lower Cambrian of Tarim Basin: Implication for the transformation of anaerobic and euxinic settling

The Nitrogen (N) along with Carbon (C) isotopes, Rear earth elements (REEs) and trace elements (TEs) were detected in the lower Cambrian Yuertusi formation, Northwest Tarim Basin. Two sections of obvious isotopic and elemental cycles have been investigated, which reflects the sedimentary transformation from lower euxinic to upper anaerobic setting. The extreme negative excursion of kerogen δ13C emerged in the bottom layer silica shales. The lowest value of kerogen δ13C is up to -37‰, which is similar with the global “Base” excursion in the bottom of lower Cambrian. It could be inferred the deposition of Yuertusi formation might be occurred after the Neoproterozoic glacial. Consequently, the kerogen δ13C of upper Yuertusi formation mostly exceeded -32‰, which could inferred the rapid rise of atmosphere and oceanic temperature. Based on the typical theory of isotope fraction, the δ13C of bitumen should be lower than that of kerogen. The bitumen δ13C in the upper Yuertusi formation varies from -34‰ ∼ 31.5‰, which is lighter than the δ13C of kerogen. Nevertheless, the δ13C of bitumen in the bottom layer silica shales is heavier than the δ13C of kerogen. This phenomenon has been noticed by previous researchers. The nadir negative excursion of bulk δ15N occurred in the bottom of Yuertusi formation, which has been indicated the sedimentary environment of Euxinic photic zone. The assimilation of bioavailable nutrient N (either in form of NH4 + or NO3-) may have contributed to the persistently low δ15C values. Ammonium may have served as the dominant nutrient N source for this period. The recycled NH4+ can be used directly or was sometimes oxidized to nitrate and then quantitatively used by phytoplankton to fuel primary productivity, which finally resulted into accumulation of high TOC and negative excursion of carbon isotope in the bottom of Yuertusi formation. It could be alternative interpretation for the depositional conditions of TOC-rich shales around Ediacaran-Cambrian boundary.

Treptichnus pedum in the Três Marias Formation, south-central Brazil, and its implications for the Ediacaran-Cambrian transition in South America

The index ichnofossil Treptichnus pedum Seilacher 1955 and four types of Microbially Induced Sedimentary Structures (MISS) - variants of wrinkle and elephant skin structures – are the first fossils described from the Três Marias Formation, topmost unit of the Bambuí Group in south-central Brazil. Despite the stratigraphic importance and widespread occurrence of this unit in the São Francisco Craton and adjacent Brasília Fold Belt, its age is still poorly constrained between the Neoproterozoic and Early Paleozoic. This discovery is significant because the first appearance datum (FAD) of T. pedum has been used to define the Ediacaran-Cambrian boundary. Hence, the presence of T. pedum indicates an early Paleozoic age for the uppermost Bambuí Group and means that the Ediacaran-Cambrian limit must lie deeper in the Bambuí Group, somewhere between this level and that of the terminal Ediacaran index-fossil Cloudina sp. in the Sete Lagoas Formation, near the base of the group. Available age determinations on detrital zircon grains and chemostratigraphic correlations are consistent with an Ediacaran to Cambrian age for this group. Detailed paleontological prospection throughout the group may now proceed within a better constrained time frame and hopefully reveal additional fossil evidence of Cambrian as well as Ediacaran life in the Bambuí Group.

A link between rift-related volcanism and end-Ediacaran extinction? Integrated chemostratigraphy, biostratigraphy, and U-Pb geochronology from Sonora, Mexico

AbstractWe present chemostratigraphy, biostratigraphy, and geochronology from a succession that spans the Ediacaran-Cambrian boundary in Sonora, Mexico. A sandy hematite-rich dolostone bed, which occurs 20 m above carbonates that record the nadir of the basal Cambrian carbon isotope excursion within the La Ciénega Formation, yielded a maximum depositional age of 539.40 ± 0.23 Ma using U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry on a population of sharply faceted volcanic zircon crystals. This bed, interpreted to contain reworked tuffaceous material, is above the last occurrences of late Ediacaran body fossils and below the first occurrence of the Cambrian trace fossil Treptichnus pedum, and so the age calibrates key markers of the Ediacaran-Cambrian boundary. The temporal coincidence of rift-related flood basalt volcanism in southern Laurentia (>250,000 km3 of basalt), a negative carbon isotope excursion, and biological turnover is consistent with a mechanistic link between the eruption of a large igneous province and end-Ediacaran extinction.

Marine carbon and sulfur cycling across the Ediacaran-Cambrian boundary in Tarim Block and its implications for paleoenvironmental changes

The Ediacaran-Cambrian transition was a critical period of Earth history because of the coincidence among climate change, bioevolution, and tectonic activity. The nature of carbon and sulfur-cycling processes spanning the Ediacaran-Cambrian boundary remains debatable. This study considers high-resolution stable isotope chemostratigraphy of the Sugetbrak and Yutixi sections of the Keping area of the Tarim Basin, northwestern China. The sections display a pronounced negative δ13Ccarb shift (the Basal Cambrian Carbon Isotope Excursion) that spans the Ediacaran-Cambrian boundary. Deposits overlying the negative excursion are characterized by heavier δ13Ccarb values ranging from −7.8‰ to +4.1‰. The biostratigraphically calibrated δ13Ccarb profiles likely reflect widespread perturbation of the global oceanic carbon cycle and associated mass extinction and subsequent biological evolution. The δ34Spy profiles of the studied sections display a wide distribution of values, ranging from −16.4‰ to 23.8‰. The variable δ34Spy signatures of these sections are partially controlled by microbial sulfate reduction rates, which are sensitive to local sulfate concentration, availability of organic matter and the marine redox state. The strong coupling between enhanced organic carbon and pyrite burial, the appearance of Cambrian-type animals and a shift to a more oxidizing marine environment might suggest a temporal causality of higher oxygen levels of the atmosphere-ocean realm with bio-events during the Ediacaran–Cambrian transition. The development of ecosystems across the Ediacaran–Cambrian transition may have significantly impacted the oceanic carbon and sulfur biogeochemical processes in Earth's history and, in turn, affecting themselves.

Tracking the evolution of seawater Mo isotopes through the Ediacaran–Cambrian transition

The rapid increase in animal diversity and abundance over the Ediacaran–Cambrian boundary has been attributed to, among other parameters, pronounced Earth surface oxygenation at that time. However, whether atmospheric oxygen elevated to modern levels, or whether the extent was smaller and oxygen remained relatively muted until the late Paleozoic is currently debated. Here, we present new Mo isotope datasets of drill core material for two key black shale units (the Ediacaran Doushantuo Formation and the lower Cambrian Jiumenchong Formation), located on the Yangtze Block. These data capture a 0.7‰ isotope fractionation between sediments experiencing quantitative and non-quantitative Mo removal. By identifying this offset, we provide integrated estimates for global seawater Mo isotope compositions during the deposition of these two formations (~1.2–1.4‰ and ~1.8–2‰, respectively). When paired with a mass balance model, our results indicate that a large proportion of the early Cambrian ocean was bathed under oxic waters. We also discuss how recent discoveries have provided insights into the geochemical processes underpinning the oxygenation of the oceans.

U–Pb SHRIMP zircon ages of Ediacaran-Cambrian granitic bodies in central Brazil: Implications for the tectonic evolution of the Araguaia belt

The small granitic plutons of the Ramal do Lontra, Presidente Kennedy, Barrolândia and Santa Luzia are emplaced into the Estrondo Group metasedimentary rocks of the Neoproterozoic Araguaia Belt. They have similar petrographic and compositional characteristics and are coeval with the medium-grade regional metamorphism of the belt. New U–Pb SHRIMP zircon data yielded quite robust ages. These results fall within the experimental error, and the integrated Concordia age of 542 ± 3 Ma was chosen as representative of their magmatic crystallization. A relevant number of inherited grains are present, showing overgrowth rims with high-U content, formed during the later stage of magmatic crystallization. LA-ICP-MS U–Pb dating on detrital zircon grains from a metasedimentary rock of the Estrondo Group yielded several Paleoproterozoic ages and a few Neoarchean ages, but the major peak in the histogram was between 1000 and 1200 Ma. The youngest grains indicate an Ediacaran age near 600 Ma, which is considered a reasonable upper limit for the deposition of the Estrondo Group. The Araguaia Belt started as an intracontinental system, segmenting the Amazonian Craton, formed by rifting during the breakup of Rodinia and was after occupied by a sedimentary basin which evolved to form an oceanic lithosphere. In the Ediacaran, the huge continental collisions that formed the West Gondwana Orogen most likely caused its remote activation, producing tectonic inversion with vergence to the west. Granitic plutons were emplaced in the last stages of the evolution of the Araguaia Belt, during the Brasiliano-PanAfrican orogenesis, at the Ediacaran-Cambrian boundary.