Precambrian Cambrian Transition Research Articles

A chemical weathering and paleoclimatic reconstruction of the early Cambrian environment of the Wyoming Craton from the Wind River Canyon, WY paleosol on the Great Unconformity

The weathering of continental rocks during the Precambrian-Cambrian transition contributed to the carving of the Great Unconformity and coincided with increasing biodiversity during the early Cambrian world. We use an early Cambrian paleosol exposed in the Wind River Canyon of Wyoming where it is buried by the overlying mid-Cambrian Flathead Sandstone to assess the paleo-weathering environment and constrain paleoclimate conditions on the Wyoming Craton in early Cambrian. The ∼ 3 m thick paleo-weathering profile is characterized by complete plagioclase weathering and moderate alkali feldspar weathering with maximum CIA (Chemical Index of Alteration) and PIA (Plagioclase Index of Alteration) reaching 69 and 98, respectively. Higher K concentrations toward the upper part of the profile associated with the authigenic precipitation of sericite clay minerals suggest that modest post-burial K-metasomatism may have overprinted the weathering profile. Our results suggest that the saprolite was developed under a mean annual paleoprecipitation and paleotemperature comparable to modern temperate environments. Geochemical characteristics of this paleosol formed during the final chapter of the Precambrian suggest intensive chemical weathering in the presence of elevated atmospheric CO2 concentration, forming a saprolite profile similar to other, previously described Cambrian and Precambrian-aged paleosols from the North American Craton.

Rare earth element enrichment in sedimentary phosphorites formed during the Precambrian–Cambrian transition, Southwest China

Numerous sedimentary phosphorites in Southwest China were formed around the Precambrian–Cambrian transition (PC–C), including the upper Ediacaran Doushantuo Formation and lower Cambrian Gezhongwu Formation. The Gezhongwu phosphorites in Zhijin exhibit marked rare earth element (REE) enrichment (>1000 ppm), and may represent new REE resources. Although the main characteristics of the Gezhongwu phosphorites have been well constrained, the REE enrichment mechanisms remain unclear. We undertook a comparative study of three typical sedimentary phosphorites with variable REE contents formed at the PC–C transition in central Guizhou Province, Southwest China. These include sections A and B of the Doushantuo phosphorites (560 ± 8 Ma) from the Weng’an area (i.e., WA-A and WA-B), and the Gezhongwu phosphorites (527 ± 24 Ma) from the Zhijin area (ZJ). The phosphorites were investigated with state-of-the-art macroscale to nanoscale analytical techniques. In contrast to the extraordinary REE enrichment in the ZJ phosphorites (average ΣREE = 1157 ppm), the phosphorites in WA-A (average ΣREE = 234 ppm) and WA-B (average ΣREE = 114 ppm) are REE-poor. Elemental mapping by laser ablation–inductively coupled plasma–mass spectrometry, along with transmission electron microscopy analyses, showed the REEs in the studied phosphorites are hosted in nanoscale francolites. The 87Sr/86Sr and Y/Ho ratios of the francolite grains indicate that greater terrigenous input may have led to more REE enrichment in the WA-A than WA-B phosphorites, but this cannot explain the extraordinary REE enrichment in the ZJ phosphorites. The F/P2O5 values of the francolite grains in the ZJ phosphorites (∼0.097) are higher than those in the WA-A (∼0.084) and WA-B (∼0.084) phosphorites, and the grain size of the francolite in the ZJ phosphorites (∼89.9 nm) is larger than those in the WA-A (∼56.6 nm) and WA-B (∼57.4 nm) phosphorites, indicative of more intense reworking of the ZJ than WA phosphorites during early diagenesis. A plot of Nd concentration versus Ce/Ce* reveals that lower sedimentation rates characterized the ZJ phosphorites. Therefore, intense sedimentary reworking during early diagenesis resulted in more REEs being sequestered by the marine phosphates from seawater and pore waters at a lower sedimentation rate, which resulted in the extraordinary REE enrichment in the ZJ phosphorites. Our findings highlight the multiple factors that controlled formation of sedimentary phosphorites around the PC–C transition (especially the intense reworking and redox conditions of the overlying seawater), and provide further insights into REE enrichment in sedimentary phosphorites worldwide.

A macroscopic free-swimming medusa from the middle Cambrian Burgess Shale.

Cnidarians are regarded as one of the earliest-diverging animal phyla. One of the hallmarks of the cnidarian body plan is the evolution of a free-swimming medusa in some medusozoan classes, but the origin of this innovation remains poorly constrained by the fossil record and molecular data. Previously described macrofossils, putatively representing medusa stages of crown-group medusozoans from the Cambrian of Utah and South China, are here reinterpreted as ctenophore-grade organisms. Other putative Ediacaran to Cambrian medusozoan fossils consist mainly of microfossils and tubular forms. Here we describe Burgessomedusa phasmiformis gen. et sp. nov., the oldest unequivocal macroscopic free-swimming medusa in the fossil record. Our study is based on 182 exceptionally preserved body fossils from the middle Cambrian Burgess Shale (Raymond Quarry, British Columbia, Canada). Burgessomedusa possesses a cuboidal umbrella up to 20 cm high and over 90 short, finger-like tentacles. Phylogenetic analysis supports a medusozoan affinity, most likely as a stem group to Cubozoa or Acraspeda (a group including Staurozoa, Cubozoa and Scyphozoa). Burgessomedusa demonstrates an ancient origin for the free-swimming medusa life stage and supports a growing number of studies showing an early evolutionary diversification of Medusozoa, including of the crown group, during the late Precambrian-Cambrian transition.

How did Reefs Evolve during the Precambrian-Cambrian Transition?

How did Reefs Evolve during the Precambrian-Cambrian Transition?

Precambrian–Cambrian Transition at the Igarka Uplift (Northwestern Siberian Platform)

Abstract —The Precambrian–Cambrian transition is one of the most fundamental evolutionary turnovers in the Earth’s history. However, in many cases precise identification and correlation of this crucial boundary in lithologically contrasting sections is complicated, even if the whole spectrum of biostratigraphic and chemostratigraphic methods is applied. The Precambrian–Cambrian transitional strata of the Igarka Uplift (Sukharikha Formation) perfectly illustrate this problem. This unit has arguably one of the most detailed carbonate carbon isotope curves for this time interval (at least within the Siberian Platform). However, an extremely poor paleontological record previously reported from the Sukharikha Formation makes identification of major stratigraphic boundaries highly debatable. We present a detailed study of the Sukharikha and overlying Krasnyi Porog formations in three sections (Sukharikha River, Kulyumbe River, and Khantaisko-Sukhotungusskaya-1 well). Our chemostratigraphic and biostratigraphic data provide a correlation basis for these sections and identify the Cambrian boundary in terms of both the International Chronostratigraphic Chart and General Stratigraphic Scale of Russia. We show that lithologic boundaries and levels of the local first appearance of Tommotian small skeletal fossils are diachronous within the Igarka paleobasin. The latter is putatively caused by paleoecology of the early Cambrian biota and by taphonomic factors. Our data specify the location of the Tommotian Stage (sensu lato)/Cambrian Stage 2 boundary in other key Precambrian–Cambrian transitional sections of the Siberian Platform.

Proterozoic-Mesozoic development of the Quanji block from northern Tibet and the cratonic assembly of eastern Asia

Unraveling the timing, location, and mechanisms of cratonic aggregation in Earth9s continental jigsaw puzzle is a key factor for plate tectonic reconstructions. The Quanji Block (QB) is a sliver of anomalously old and well-preserved continental crust embedded within the Paleozoic-Mesozoic tectonic collage of the northeastern Tibetan Plateau and has played a critical role in Proto-/Paleotethys paleogeographic reconstructions. New geological mapping, stratigraphic logging, and geochronological analysis lead to a refined understanding of QB9s history from Paleoproterozoic to present. Deposited atop a largely Paleoproterozoic basement, the Quanji Group records rifting and epicratonic cover at 1.7 to 1.6 Ga. The Quanji Group is unconformably overlain by the Xiaogaolu Group, which preserves black shale, Ediacaran-type <i>Charnia</i>, ribbon-shaped fossils and a late Ediacaran glaciation. U-Pb detrital zircon ages from Cambrian Olongbuluke Group marine platform deposits are quite different from ages in underlying units, with a minor component of Neoproterozoic (880–815 Ma) ages. The apparent change in detrital zircon sources coincides with a regionally expressed Great Unconformity during the Precambrian–Cambrian transition. The new stratigraphy and U-Pb geochronology of QB suggest that the late Paleoproterozoic to Cambrian history of QB has a remarkable similarity to that of the southern margin of North China Block (NCB), indicating that the QB has been displaced dextrally from an initial location adjacent to NCB. The transform motion occurred in stages between <i>ca.</i> 350 and 200 Ma, which suggests that transform tectonism appears to be an essential element of any viable model for kinematic development of the Paleo-Tethyan oceanic domains and the ultimate cratonic assembly of eastern Asia.

Exploration targeting of copper deposits using staged factor analysis, geochemical mineralization prospectivity index, and fractal model (Western Anti-Atlas, Morocco)

The present work aims to identify the geochemical signatures of copper deposits in the Igherm area (Anti-Atlas, Morocco) using stream sediment data. We applied factor analysis on the data normalized by log transformation. This approach identified three geochemical data associations (F1, F2, and F3) which were then used to calculate the values of the geochemical prospectivity index (GPMI). The results indicated that the large GMPI Cu values correlate strongly with the surface indices. A concentration-surface (C-A) fractal classification was applied to separate the geochemical anomalies from the regional background. To recognize and geo-position the significant geochemical signatures of the deposit, we applied the prediction zone (P-A) plot. A geographic information system (GIS) was used to define the spatial distribution of the anomalies at the sub-watershed level. The results represent the first approach to evaluating potential mineral resources at the scale of the Anti-Atlas: 90% of the anomalous sub-catchments mapped are located along the Precambrian-Cambrian transition (Base Series), 67% are traversed by major tectonic faults, and 40% contain copper showings and/or deposits. The remaining 60% are devoid of any visible surface copper occurrences. They constitute promising areas for future exploration of Cu deposits hidden under the Cambrian cover.

Marine biomass changes during and after the Neoproterozoic Marinoan global glaciation

The late Neoproterozoic (645–535 Ma) represents a key geological period, when a climate shift and revolutionary biological innovations occurred during the Precambrian–Cambrian transition. The Marinoan glaciation impacted the climate and chemical composition of the oceans, restraining the evolution of early life. To elucidate evolutionary processes during the Marinoan–Ediacaran transition, this study presents biomarker evidence (molecular fossils) from the Nantuo Formation and the lower part of the Doushantuo Formation at the Jiulongwan section, China. Our results reveal possible photosynthetic activity (n-C17 + n-C19 [algae] and pristane + phytane [photosynthesis]) during the Marinoan glaciation, followed by low productivity during the early period of cap dolostone precipitation, and recovery of photosynthetic organisms (n-C17 + n-C19: from 0.00 to 0.96 μg/g TOC; pristane + phytane: from 0.00 to 3.6 μg/g TOC) or other bacteria (hopanes: from 0.00 to 0.07 ng/g TOC) during late cap carbonate deposition, with the expansion of eukaryotes (steranes/[hopanes + steranes]: from 0.0 to 0.6) after cap carbonate deposition in the early Ediacaran.

Signals of combined chromium–cadmium isotopes in basin waters of the Early Cambrian – Results from the Maoshi and Zhijin sections, Yangtze Platform, South China

The combination of authigenic chromium and cadmium stable isotopes in marine sedimentary archives enables reconstruction of the redox conditions and levels of primary productivity in past oceans and basins. The Ediacaran to Cambrian Nanhua Basin (Yangtze Platform, South China) and its sedimentary sequences have been studied intensively in the past with respect to investigating the effects of post-Snowball Earth glaciations on the oxygenation of Earth's atmosphere, and intimately related, on the consequences of climate changes on the evolution of the ocean environment and of life during the Precambrian-Cambrian transition. Following recent propositions of using cadmium isotopes as a novel proxy for reconstructing the extent of primary productivity in paleooceans, we here present results from a combined cadmium (Cd)‑chromium (Cr) isotope study on metalliferous black shale, black shale and phosphorite sediments deposited during the Early Cambrian Niutitang Formation at the Maoshi and Zhijin sections. Results point toward strongly oxidized and highly productive surface water layers in the Nanhua Basin during this period. The samples have strongly positively fractionated δ114Cd and δ53Cr values up to +0.6‰ and + 1.4‰, respectively, similar to such values from carbonaceous sediments in modern oceans. The association of Cd with sulfides in the sediments, together with high Cd/TOC ratios particularly in the metalliferous black shales, implies a sulfide-controlled enrichment of Cd under restricted euxinic conditions. The lower Cd/TOC values in phosphorites and barren black shales and negatively fractionated δ114Cd values down to −0.33‰ point to less effective precipitation of Cd under more suboxic conditions as likely prevailing in an open shallow shelf. The Cr content in the sediments is mostly close to average shale, and the δ53Cr values are little fractionated in the 0–0.2‰ range, corroborated by flat shale-like rare earth element + yttrium (REE + Y) patterns. However, some metal-rich shale samples have clearly authigenic Cr isotope signatures with up to +1.4‰ δ53Cr, and the authigenic Cr component in the phosphorite samples is around 1‰ δ53Cr, indicating oxic surface waters. The latter sediments also depict seawater-like, middle REE enriched REE + Y patterns with distinct negative Ce and positive Y anomalies indicative of an oxygenated surface water layer, and with positive Eu anomalies which they likely received during phosphatogenesis in the euxinic bottom water environment. In accord with previous studies reporting on extreme trace metal enrichments (Mo, Ni, V, Hg, etc.) in these Early Cambrian sediments, our Cd and Cr isotope signatures favor a scenario of a strongly stratified basin in which high primary productivity thrived in an oxidized surface water layer, potentially as a result of increased nutrient supply from a strongly weathering continental hinterland in the aftermath of the last Snowball Earth glaciations in the Ediacaran.

Siliceous sponge expansion and phosphogenesis in a shallow water environment in the Malyi Karatau Range (Kazakhstan) during the Precambrian-Cambrian transition

Ocean oxygenation during the Precambrian-Cambrian transition is thought to be one of the key factors for the rapid diversification of animals, known as the ‘Cambrian Explosion’. Siliceous sponges inhabiting continental slope settings were capable of shifting the respiratory oxygen demand in the water column to depth, resulting in an increase in oxygen levels of shallow water. To assess the global importance of the impact of ecosystem engineering by sponges, we evaluated the palaeo-environmental conditions that confine sponge expansion using multiple geochemical proxies including silicon and carbon stable isotopes as well as major- and trace elements in cherts and siliceous sedimentary rocks that were deposited in shallow water around the Precambrian-Cambrian transition. We investigated three lithosections in the Malyi Karatau Range in southern Kazakhstan. The lowermost chert samples of these sections are typically characterized by comparably low δ30Si values and exhibit molds of siliceous sponge spicules. Upsection the occurrence of cross-bedding structures and granular phosphorites document the shallowing of the depositional environment, where overall increasing δ30Si values and increasing Al/Si indicate less settlement of sponges and higher suspended particle loads. Corresponding carbon isotope records of the carbonate fraction decrease from near-zero to negative carbon isotope values averaging −5.9‰. The δ13Ccarb values overall coincide with Y/Ho, the redox indicator Ce/Ce*, and mass fractions of carbonate fluorapatite, suggesting that these ratios are predominantly controlled by the mineralization of organic matter in phosphatic sediment. The studied sections document the ecological requirements for sponge settlement and proliferation, such as the presence of a hard substrate and sufficiently low suspended particle concentrations and/or low wave energy. This work suggests that the widespread appearance of karsted surfaces around the Precambrian-Cambrian transition that were subsequently submerged during the Cambrian transgression would have created permissive habitats for sponges and large phosphate deposits. The capability of sponges to oxygenate shallow seawater and extensive phosphorous burial in epicontinental basins are thus feasible mechanisms for the seawater oxygenation around the Precambrian-Cambrian transition.

Sterol preservation in hypersaline microbial mats

Abstract. Microbial mats are self-sustaining benthic ecosystems composed of highly diverse microbial communities. It has been proposed that microbial mats were widespread in Proterozoic marine environments, prior to the emergence of bioturbating organisms at the Precambrian–Cambrian transition. One characteristic feature of Precambrian biomarker records is that steranes are typically absent or occur in very low concentrations. This has been explained by low eukaryotic source inputs, or degradation of primary produced sterols in benthic microbial mats (“mat-seal effect”). To better understand the preservational pathways of sterols in microbial mats, we analyzed freely extractable and carbonate-bound lipid fractions as well as decalcified extraction residues in different layers of a recent calcifying mat (∼1500 years) from the hypersaline Lake 2 on the island of Kiritimati, central Pacific. A variety of C27–C29 sterols and distinctive C31 4α-methylsterols (4α-methylgorgosterol and 4α-methylgorgostanol, biomarkers for dinoflagellates) were detected in freely extractable and carbonate-bound lipid pools. These sterols most likely originated from organisms living in the water column and the upper mat layers. This autochthonous biomass experienced progressive microbial transformation and degradation in the microbial mat, as reflected by a significant drop in total sterol concentrations, up to 98 %, in the deeper layers, and a concomitant decrease in total organic carbon. Carbonate-bound sterols were generally low in abundance compared to the freely extractable portion, suggesting that incorporation into the mineral matrix does not play a major role in the preservation of eukaryotic sterols in this mat. Likewise, pyrolysis of extraction residues suggested that sequestration of steroid carbon skeletons into insoluble organic matter was low compared to hopanoids. Taken together, our findings argue for a major mat-seal effect affecting the distribution and preservation of steroids in the mat studied. This result markedly differs from recent findings made for another microbial mat growing in the nearby hypersaline Lake 22 on the same island, where sterols showed no systematic decrease with depth. The observed discrepancies in the taphonomic pathways of sterols in microbial mats from Kiritimati may be linked to multiple biotic and abiotic factors including salinity and periods of subaerial exposure, implying that caution has to be exercised in the interpretation of sterol distributions in modern and ancient microbial mat settings.

Earthquake-induced soft sediment deformation (SSD) structures from the Bilara limestone formation, Marwar basin, India

The Neoproterozoic Bilara limestone Formation of the Marwar Group, Rajasthan, India exposes metres-thick layers of soft sediment deformation (SSD) structures at different stratigraphic levels which could be traced over hundreds of metres on the outcrop scale. The SSD structures include disharmonic folds, low-angle thrusts, distorted laminae, fluidisation pipes, slump and load structures, homogeneities, diapirs, etc. Whereas SSD structures suggesting tensional stress, viz., intrastriatal graben, fluidisation, slump, etc. dominate in the lower part of the Bilara succession, features implicating compression, viz., folds, low-angle thrust are prevalent in the uppermost part. Since SSD structures are mostly confined within the algal laminites, we interpret that enhanced micritic fluid pressure below early cemented algal carbonate played a major role in laminae deformation. Depending on the degree of lithification and pore-water pressure, deformation features formed either plastically or led to diapiric injection at enhanced pore water pressure. Separated by near-horizontal underformed strata, the SSD layers, traceable over hundreds of metres, are interpreted as products of seismic shacking. Considering the time frame of the Marwar basin, i.e., the Precambrian–Cambrian transition, the SSD horizons present within the Bilara succession may hold the potential for the correlation with SSD structures reported from the time-correlative stratigraphic successions present in erstwhile adjoining tectonic terrains, e.g., China, Siberia, etc.

A new macroalgal assemblage from the Xiaoshiba Biota (Cambrian Series 2, Stage 3) of southern China

Exquisitely preserved Burgess Shale-type fossils from the Xiaoshiba Lagerstätte in the Cambrian (Series 2) Hongjingshao Formation of southern China provide remarkable perspective on early metazoans but have never been reported with plants. Here we first recognize a new assemblage of highly mineralized macroalgae from this biota. Based on morphological differences, there are three new genera and four new species identified: Doulia rara gen. et sp. nov., Eolaminaria simigladiola sp. nov., Singulariphyca ramosa gen. et sp. nov. and Rugophyca longa gen. et sp. nov. Relative to the other three species, S. ramosa is widely distributed among the beds, which may indicate a dominat species in this community. This new discovery fills part of the long gap in the detailed record of macroalgae between the Chengjiang Biota and the Kaili Biota, representing a localized macroalgal flourish at the end of Cambrian Stage 3. The assemblage that shows morphological characters similar to some macroalgae found in the Precambrian and primitive to those of the Cambrian Stage 5 lays the basis for revealing the evolutionary biology of early macroalgae and understanding the Precambrian-Cambrian transition questions. Taphonomic study of this fossiliferous bed indicates that some well-preserved macroalgae are preserved within their original habitats. Through analyzing the depositional environments of the known Cambrian macroalgae, we suggest that macroalgae tend to be preserved in relatively deep marine environments with less disturbance.

Extensive oxidative weathering in the aftermath of a late Neoproterozoic glaciation – Evidence from trace element and chromium isotope records in the Urucum district (Jacadigo Group) and Puga iron formations (Mato Grosso do Sul, Brazil)

The massive Fe and Mn deposits of the Urucum district (Banda Alta Formation) and the iron formations from Fazenda São Manoel (Puga Formation) in Mato Grosso do Sul, Brazil, are associated with glacigenic deposits and represent the youngest and largest sedimentary Fe and associated Mn formation (IF;MnF) deposits of Cryogenian age in the world. The Urucum district IFs studied are predominantly composed of pure classical plane-parallel and stratified hematite–chert–iron - and intercalated manganese (cryptomelane) micro- and mesobands, whereas the IFs at Fazenda São Manoel are closely associated with diamictites and shales. Although the precise depositional age is unknown, maximum age constraints for the Puga IFs are defined by the youngest detrital zircon with an U-Pb age of 706±9Ma (Babinski et al., 2013), a result which we here confirm by a U-Pb age of the youngest zircon of 695±17Ma from within shaly beds in the Urucum district IF succession. Redox-sensitive trace element signatures and tendency to hump-shaped Rare Earth Element+Yttrium (REY) patterns with negative Ce- but without Eu anomalies support the presence of an oxic surface water layer, fertilized by low temperature hydrothermal fluids injected from submarine thermal springs and/or, alternatively, by significant fresh water input directly derived from glacial meltdown. Strongly positively fractionated, authigenic chromium isotope signatures (average δ53Cr=1.10±0.4‰; n=16; 2σ) prevailing throughout the entire stratigraphic section indicate that riverine supply of continentally-derived Cr, remained more or less constant throughout the glacier meltdown and IF depositional period. Cycling across a redoxcline and predominant deposition of the IF in anoxic deeper seawater of the Jacadigo basin is supported by the peculiar presence of subchondritic Y/Ho ratios, by decreased negative Ce anomalies and by shifts of LREE patterns towards higher values in the Urucum district IFs that are independent of detrital contamination.The strongly positively fractionated Cr isotope signatures measured in these iron formations are compatible with those from other iron formations and black shales deposited during the late Neoproterozoic and Precambrian-Cambrian transition worldwide and are in support of prevailing high atmospheric O2 levels following the late Neoproterozoic glaciations that accompanied the evolution of macroscopic multicellular organisms.

Magmatic evolution of the Jbel Boho alkaline complex in the Bou Azzer inlier (Anti-Atlas/Morocco) and its relation to REE mineralization

The Jbel Boho complex (Anti-Atlas/Morocco) is an alkaline magmatic complex that was formed during the Precambrian-Cambrian transition, contemporaneous with the lower early Cambrian dolomite sequence. The complex consists of a volcanic sequence comprising basanites, trachyandesites, trachytes and rhyolites that is intruded by a syenitic pluton. Both the volcanic suite and the pluton are cut by later microsyenitic and rhyolitic dykes.Although all Jbel Boho magmas were probably ultimately derived from the same, intraplate or plume-like source, new geochemical evidence supports the concept of a minimum three principal magma generations having formed the complex. Whereas all volcanic rocks (first generation) are LREE enriched and appear to be formed by fractional crystallization of a mantle-derived magma, resulting in strong negative Eu anomalies in the more evolved rocks associated with low Zr/Hf and Nb/Ta values, the younger syenitic pluton displays almost no negative Eu anomaly and very high Zr/Hf and Nb/Ta. The syenite is considered to be formed by a second generation of melt and likely formed through partial melting of underplated mafic rocks. The syenitic pluton consists of two types of syenitic rocks; olivine syenite and quartz syenite. The presence of quartz and a strong positive Pb anomaly in the quartz syenite contrasts strongly with the negative Pb anomaly in the olivine syenite and suggests the latter results from crustal contamination of the former. The late dyke swarm (third generation of melt) comprises microsyenitic and subalkaline rhyolitic compositions. The strong decrease of the alkali elements, Zr/Hf and Nb/Ta and the high SiO2 contents in the rhyolitic dykes might be the result of mineral fractionation and addition of mineralizing fluids, allowing inter-element fractionation of even highly incompatible HFSE due to the presence of fluorine. The occurrence of fluorite in some volcanic rocks and the Ca-REE-F carbonate mineral synchysite in the dykes with very high LREE contents (Ce ∼720 ppm found in one rhyolitic dyke) suggest the fluorine-rich nature of this system and the role played by addition of mineralizing fluids.The REE mineralization expressed as synchysite-(Ce) is detected in a subalkaline rhyolitic dyke (with ΣLREE = 1750 ppm) associated with quartz, chlorite and occasionally with Fe-oxides. The synchysite mineralization is probably the result of REE transport by acidic hydrothermal fluids as chloride complex and their neutralization during fluid-rock interaction.The major tectonic change from compressive to extensional regime in the late Neoproterozoic induced the emplacement of voluminous volcaniclastic series of the Ediacran Ouarzazate Group. The alkaline, within-plate nature of the Jbel Boho igneous complex implies that this extensional setting continued during the early Cambrian.

Earth's oldest jellyfish strandings: a unique taphonomic window or just another day at the beach?

AbstractDiscoidal macrofossils reported herein from the lower Cambrian Zabriskie Quartzite (Great Basin, western United States) record the oldest Phanerozoic medusozoan body fossils, as well as the oldest medusozoan stranding event on record. Moreover, these fossils provide evidence of a significant shift in the taphonomic mode characteristic of preservation of nonmineralized taxa in coarse-grained siliciclastic successions near the onset of the Phanerozoic. Taphonomic and sedimentological evidence recorded by these and younger examples of stranded Cambrian medusae is consistent in suggesting that several of the requirements for preservation of these fossils were holdovers from the Ediacaran Period, including the presence of microbial mats and a lack of carcass disturbance by scavenging and/or bioturbating taxa. To shed further light upon the taphonomic factors necessary for the preservation of Cambrian medusae, we compared the biostratinomy and sedimentology of Cambrian medusa strandings to those of Ediacara Biota assemblages from lithologically similar successions. We find key secular disparities in the taphonomic histories of these two types of fossil assemblage. Inconsistencies between the preservational styles characteristic of fossil assemblages preserved in sandstone lithofacies on each side of the Precambrian–Cambrian boundary are explained by a considerable change in the preferred depositional setting in which these macrofossil assemblages are preserved. Thus, rather than documenting a single taphonomic continuum through the Precambrian–Cambrian transition, the Zabriskie and younger medusozoan body fossil assemblages record the advent of an entirely new, yet still very rarely exploited, taphonomic window exclusive to the Cambrian Period.

Protracted development of bioturbation through the early Palaeozoic Era

Mobile organisms first appeared in the fossil record prior to the Precambrian–Cambrian transition. Sediment textures indicate that the degree of sediment mixing by animal activity remained low for 120 million years following the transition.

Coupled sulfur, iron and molybdenum isotope data from black shales of the Teplá-Barrandian unit argue against deep ocean oxygenation during the Ediacaran

The Earth’s atmosphere and hydrosphere changed from an Archean anoxic to a modern oxygenated world in two major steps, the Paleoproterozoic Great Oxidation Event (2.4–2.3 billion years ago) and the Neoproterozoic Oxidation Event (0.8–0.5 billion years ago). Both events had a strong influence on the availability of redox sensitive and bio-essential metals within the ocean and are, thus, strongly linked to fundamental biological innovations and diversification. Biological diversification during the Precambrian–Cambrian transition between 555 and 540 million years ago may have been driven by ocean–atmosphere oxygenation. The exact timing and the extent of (deep) ocean oxygenation within this time period remains unresolved though.Here we present major and trace element compositions as well as Mo, S and Fe isotopic data of organic-rich black shales from the Teplá-Barrandian unit, Czech Republic. New in situ zircon U–Pb ages provide a maximum depositional age of 559.8±3.8 million years. Black shales with strong metal enrichment show low δ56Fe values due to the dominance of authigenic pyrite-Fe with δ56Fe values around −0.6‰ over detrital Fe with δ56Fe values around 0.1‰. Samples with lower authigenic metal enrichment show relatively low Mo/TOC ratios and increasing δ34S values, which is interpreted to reflect basinal restriction and longer seawater renewal times. In analogy to the modern Black Sea, the accompanied depletion of basinal Moaq due to near quantitative Mo removal might have led to the preservation of the seawater δ98Mo in the respective black shales. Our best estimate for this seawater Mo isotopic composition <560 million years ago is +1.2‰ in δ98Mo, which is nearly identical to seawater δ98Mo values inferred from Mid-Proterozoic black shales. The lack of higher δ98Mo values in black shales (and seawater) argues against contemporaneous Mn oxide formation in well oxygenated deep sea settings, which would preferentially adsorb isotopically light Mo leaving behind an isotopically heavy ocean. By contrast, the deep ocean might have remained ferruginous with the hydrothermal Fe still outbalancing surficial oxygen production. Our results therefore contribute to a growing data set, which suggests limited deep water oxygenation during major biological innovations in the late Ediacaran period.

Widespread delayed mixing in early to middle Cambrian marine shelfal settings

The radiation of burrowing metazoans in the early Phanerozoic dramatically altered the properties of marine sediment, an event commonly referred to as the “Cambrian substrate revolution” or “agronomic revolution.” The advent of infaunalization, and especially biogenically-mediated sediment mixing, profoundly impacted the development of Phanerozoic biogeochemical cycling, including nutrient fluxes, organic carbon burial, seafloor oxygenation and sediment ecology. However, the timing of the development of mixed seafloor sediment has, historically, not been well constrained. Mixing has been assumed, in the absence of data, to occur at the Precambrian–Cambrian boundary with the appearance of the index fossil and three-dimensional burrow Treptichnus pedum. Herein we present new ichnological, stratigraphic and taphonomic data suggesting that, although significant developments in infaunalization—paleobiologically complex animal–substrate interactions, particularly burrow construction—occurred during the early and middle Cambrian, mixing remained suppressed throughout this interval. We demonstrate, using a novel multi-proxy approach, that shelfal sediment in the earliest Cambrian was essentially unmixed. Moreover, our findings indicate that even as late as the middle Cambrian—30millionyears after the Precambrian–Cambrian transition and well after the appearance of supposedly deposit-feeding trilobites—seafloor heterolithic shelfal sediment remained largely unmixed on a global scale. These findings challenge the current assumption that mixing occurred with the first appearance of three-dimensional burrows and hold important implications for the advent and development of modern-style biogeochemical cycling.

Diversity and species abundance patterns of the Early Cambrian (Series 2, Stage 3) Chengjiang Biota from China

Lagerstättenfrom the Precambrian–Cambrian transition have traditionally been a relatively untapped resource for understanding the paleoecology of the “Cambrian explosion.” This quantitative paleoecological study is based on 10,238 fossil specimens belonging to 100 animal species, 11 phyla, and 15 ecological categories from the lower Cambrian (Series 2, Stage 3) Chengjiang biota (Mafang locality near Haikou, Yunnan Province, China). Fossils were systematically collected within a 2.5-meter-thick sequence divided into ten stratigraphic intervals. Each interval represents an induced time-averaged assemblage of various event (obrution) beds of unknown duration. Overall, the different fossil assemblages are taxonomically and ecologically similar, suggesting the presence of a single community type recurring throughout the Mafang section. The Mafang community is dominated by epibenthic vagile hunters or scavengers, sessile suspension feeders, and infaunal vagile hunters or scavengers represented primarily by arthropods, brachiopods, and priapulids, respectively. Most species have low abundance and low occurrence frequencies, whereas a few species are numerically abundant and occur frequently. Overall, in structure and ecology the Mafang community is comparable to the Middle Cambrian (Series 3, Stage 5) Burgess Shale biota (Walcott Quarry, Yoho National Park, British Columbia, Canada). This suggests that, despite variations in species identity within taxonomic and ecological groups, the structure and ecology of Cambrian Burgess Shale-type communities remained relatively stable until at least the Middle Cambrian (Series 3, Stage 5) in subtidal to relatively deep-water offshore settings in siliciclastic soft-substrate environments.