Ediacaran Cambrian Boundary Research Articles

Mafic dykes intrusive into Pre-Cambrian rocks of the São Luís cratonic fragment and Gurupi Belt (Parnaíba Province), north–northeastern Brazil: Geochemistry, Sr–Nd–Pb–O isotopes, 40Ar/39Ar geochronology, and relationships to CAMP magmatism

Dykes of diabase and microgabbro intruded into Pre-Cambrian rocks of the São Luís cratonic fragment and Gurupi Belt, which are tectonic and erosive windows of the Parnaíba Basin in north–northeastern Brazil. Ar–Ar ages were determined, and major, trace element, and Nd–Sr–Pb–O isotopic compositions of these dykes were measured to provide insights into their age, and into the nature of their mantle sources and petrogenetic processes. The data have also been used to compare the chemical and isotopic signatures of the dykes with those of the Central Atlantic Magmatic Province (CAMP). Four chemical groups of mafic dykes have been identified. These comprise two subtypes of high-Ti rocks (i) HTi-1 (TiO2<2.3wt.%; SiO2>47 wt.%), (ii) HTi-2 (TiO2>2.7wt.%; SiO2>47wt.%), in addition to (iii) evolved high-Ti (TiO2>4wt.%; SiO2 of ~46wt.%) and (iv) low-Si (TiO2>2.2wt.%; SiO2<45wt.%) rocks. 40Ar/39Ar geochronology of plagioclase returned ages of 201±4 Ma and 193±10 Ma for the HTi-2 subtype, and of 201±2 Ma and 207±9Ma for the evolved high-Ti group. The HTi-1 and low-Si groups presented highly disturbed age spectra, and did not allow the definition of their emplacement ages. The Argon data indicate an age >200Ma for the low-Si group and are dubious with respect to the age of theHTi-1 subtype, if coeval with (i.e., ~200Ma), or older than, the HTi-2 and evolved high-Ti types. All groups present δ18O values of pyroxene that are compatible with uncontaminated mantle-derived magmas. The HTi-1 subtype (average 143Nd/144Nd200=0.512644; 87Sr/86Sr200=0.7035; 206Pb/204Pb of 17.86) shows the less enriched and less fractionated (more primitive) trace element distribution of all groups. The HTi-2 subtype shows enriched trace element pattern and depleted Nd–Sr signature (143Nd/144Nd200=0.512610; 87Sr/86Sr200=0.7037) and average 206Pb/204Pb ratios of 17.23. The evolved high-Ti chemical group shows average ratios of 143Nd/144Nd200=0.512558, 87Sr/86Sr200=0.7035, and 206Pb/204Pb of 16.88, and the more enriched trace-elements signature among the four groups. The chemical and isotopic compositions and trends of the HTi-1/HTi-2/EHTi types are consistent with their derivation from an asthenosphere-derived parental magma further modified by differentiation and minimal crust contamination (higher in the HTi-2 and EHTi types), and by the derivation of one type from another via fractional crystallization. These high- and evolved high-TiO2 types show ages and some chemical and isotopic features that are consistent with those of the CAMP magmatism. Some differences found are ascribed to petrogenetic processes, such as magma differentiation. A combination of warming of the mantle and edge-driven convection beneath the Pangea supercontinent after the closure of the Neoproterozoic (Brasiliano/Pan-African) orogenies in the Ediacaran–Cambrian boundary might have triggered the magmatic event. The low-Si type shows paired Ta–Nb and Zr–Hf depletions, and depleted Sr–Nd (average 143Nd/144Nd200=0.512687; 87Sr/86Sr200=0.703) and enriched Pb (206Pb/204Pb of 18.66) isotopic compositions that may be interpreted to result either from interaction of a subcontinental lithospheric mantle with products of an earlier subduction or by contamination of the mantle-derived magma during ascent and emplacement in the continental crust. It is hypothesized that these dykes were emplaced in the Ediacaran–Cambrian boundary, after the Neoproterozoic orogeny that built up the Gurupi Belt and in the early extensional stages that preceded the formation of the Parnaíba Basin.

Environmental tolerance and range offset of Treptichnus pedum: Implications for the recognition of the Ediacaran-Cambrian boundary

The Ediacaran-Cambrian boundary, arguably the most important in the stratigraphic column, is based on the first appearance of the ichnospecies Treptichnus pedum . However, most trace fossils have long temporal ranges and occur in a narrow range of facies, and are typically of little use in biostratigraphic studies. Therefore, understanding the environmental tolerance and range offset of T. pedum is essential to evaluate the reliability of the Ediacaran-Cambrian boundary. Our study in Fortunian units in the Vanrhynsdorp Group of South Africa shows a broad environmental tolerance for the T. pedum producer in shallow-marine clastic settings. This ichnotaxon is not only present in low-energy offshore wave-dominated marine settings, but it also occurs at considerably shallower water in intertidal and shallow-subtidal zones of tide-dominated systems. T. pedum seems to have high values of peak abundance in the upper offshore and lower intertidal sand flats. In many sections, the Ediacaran-Cambrian transition occurs in successions containing a sequence boundary due to incision of valleys that were filled with coarse-grained deposits of fluvial or estuarine origin, both representing facies that are unsuitable for T. pedum . The range offset of this ichnotaxon is typically greater above sequence boundaries and within transgressive systems tracts, providing some constrains on its use. However, the broad environmental tolerance of T. pedum in shallow-marine clastic settings supports evolutionary innovations rather than facies controls as the main mechanism underlying the observed vertical pattern of distribution of this ichnospecies in the relatively continuous succession of the Ediacaran-Cambrian boundary Global Standard Stratotype Section and Point.

Reproductive strategy of the bradoriid arthropod Kunmingella douvillei from the Lower Cambrian Chengjiang Lagerstätte, South China

The bivalved bradoriid arthropod Kunmingella douvillei (Mansuy, 1912) is the most common species in the Lower Cambrian Chengjiang Lagerstätte. Its soft anatomy has been reported based on well-preserved specimens. However, as with other Cambrian arthropods, its reproductive behavior is poorly documented. Rare specimens of K. douvillei preserved with eggs have been reported. Our new collections confirm that the eggs were directly attached to the last three pairs of biramous appendages and verify the head–trunk boundary suggested by previous work. The brooding behavior of K. douvillei was most probably related to its small body size, as in living crustaceans. The K. douvillei female could carry 50–80 eggs ranging from 150–180μm in diameter, possibly indicating a K-reproductive strategy; its ontogenetic and reproductive mode might have been an adaptation in response to increasing predatory pressure across the Ediacaran–Cambrian boundary. All of the three main directions of development in the life history of living marine invertebrates, namely planktotrophic larvae, lecithotrophic larvae, and brood care, most likely evolved by Stage 3 of the Cambrian.

Flashing light in sponges through their siliceous fiber network: A new strategy of “neuronal transmission” in animals

Sponges (phylum Porifera) represent a successful animal taxon that evolved prior to the Ediacaran-Cambrian boundary (542 million years ago). They have developed an almost complete array of cell- and tissue-based interaction systems necessary for the establishment of a functional, multicellular body. However, a network of neurons, one cell/tissue-communication system is missing in sponges. This fact is puzzling and enigmatic, because these animals possess receptors known to be involved in the nervous system in evolutionary younger animal phyla. As an example, the metabotropic glutamate/GABA-like receptor has been identified and cloned by us. Recently, we have identified a novel light transmission/light responsive system in sponges that is based on their skeletal elements, the siliceous glass fibers, termed spicules. Two classes of sponges, the Hexactinellida and the Demospongiae, possess a siliceous skeleton that is composed of spicules. Studying the large spicules from hexactinellid sponges (>5 cm) revealed that these spicules are effective light-collecting optical fibers. Now we can report that the demosponge, Suberites domuncula, has a biosensor system consisting of the (organic) light producing luciferase and the (inorganic) light transducing silica spicules. The light transmission features of these smaller spicules (200 μm) has been demonstrated and the ability of the sponge tissue to generate light had been proven. Screening for a luciferase gene in S. domuncula was successful. In the next step, we searched for a protein potentially involved in light reception. Such a protein was identified, cloned and recombinantly expressed from S. domuncula. The protein sequence displays two domains characteristic of a cryptochrome, the N-terminal photolyase-related region and the C-terminal FAD-binding domain. The experimental data indicate that sponges may employ a network of luciferase-like proteins, a spicular system and a cryptochrome as the light source, optical waveguide and photosensor, respectively. Finally, we have identified a potential transcription factor involved in the control of the expression of luciferase and cryptochrome, a SOX-related protein. We assume that a flashing light signaling circuit exists, which may control the retinoic acid-induced differentiation of stem cells into pulsating and contracting sponge cells, and into pinacocytes and myocytes. Such a “nervous”-like signal transduction system has not been previously described.

DEEP-WATER INCISED VALLEY DEPOSITS AT THE EDIACARAN-CAMBRIAN BOUNDARY IN SOUTHERN NAMIBIA CONTAIN ABUNDANT TREPTICHNUS PEDUM

Valley-filling deposits of the Nama Group, southern Namibia, record two episodes of erosional downcutting and backfill, developed close together in time near the Ediacaran-Cambrian boundary. Geochronological constraints indicate that the older valley fill began 539.4 ± 1 Ma or later; the younger of these deposits contains unusually well-preserved populations of the basal Cambrian trace fossil Treptichnus pedum. Facies analysis shows that T. pedum is closely linked to a nearshore sandstone deposit, indicating a close environmental or taphonomic connection to very shallow, mud-draped sandy seafloor swept by tidal currents. Facies restriction may limit the biostratigraphic potential of T. pedum in Namibia and elsewhere, but it also illuminates functional and ecological interpretation. The T. pedum tracemaker was a motile bilaterian animal that lived below the sediment-water interface—propelling itself forward in upward-curving projections that breached the sediment surface. The T. pedum animal, therefore, lived infaunally, perhaps to avoid predation, surfacing regularly to feed and take in oxygen. Alternatively, the T. pedum animal may have been a deposit feeder that surfaced largely for purposes of gas exchange, an interpretation that has some support in the observed association of T. pedum with mud drapes. Treptichnus pedum provides our oldest record of animals that combined anatomical and behavioral complexity. Insights from comparative biology suggest that basal Cambrian T. pedum animals already possessed the anatomical, neurological, and genetic complexity needed to enable the body plan and behavioral diversification recorded by younger Cambrian fossils.

Evolution from an anoxic to oxic deep ocean during the Ediacaran–Cambrian transition and implications for bioradiation

The Ediacaran–Cambrian transition, one of the most critical intervals in Earth's history, is marked by dramatic biological, oceanic and geochemical turnovers. Here high-resolution carbon and sulfur isotopic data respectively for organic carbon and pyrite, and iron speciation data are presented from the deep-water Liuchapo and Niutitang Formations on the Yangtze block, South China. The carbon isotopic data, together with biostratigraphic and radiometric dating, offer the compelling evidence for the placement of Ediacaran–Cambrian boundary within the Liuchapo Formation (chert succession), and for its correlation with shallow-water equivalents elsewhere. In this context, iron speciation and sulfur isotopic data further suggest a predominant anoxic and ferruginous deep ocean over the transitional time until the middle Early Cambrian (Atdabanian or Stage 3) when the deep ocean was rapidly oxygenated. Coincidently, during this interval, large-body metazoans (i.e., sponges) abruptly appeared in the deep ocean, which was temporally associated with the highly diversified large-body skeletonized animals (i.e., Chengjiang Biota) which colonized in shallow-water niches particularly in southwestern China. This scenario suggests a causal link between deep oceanic oxygenation and the explosive diversification of large-body skeletonized organisms in the Early Cambrian.

New U–Pb age from the basal Niutitang Formation in South China: Implications for diachronous development and condensation of stratigraphic units across the Yangtze platform at the Ediacaran–Cambrian transition

The black shale dominated Niutitang Formation covers a large portion of the early Cambrian Yangtze platform in South China. The base of this unit has been traditionally taken as the Ediacaran–Cambrian boundary (ca. 542Ma). Recent radiometric ages from the basal Niutitang Formation and its correlative units, however, are significantly younger than 542Ma and raise questions about the diachrony of stratigraphic units across the Yangtze platform at the Ediacaran–Cambrian transition. Here we report a new U–Pb sensitive high-resolution ion microprobe (SHRIMP) age of 522.7±4.9Ma from a tuffaceous bed of the basal Niutitang Formation in a deep-water section in Taoying, Guizhou Province of South China. This age, in combination with existing ages and biostratigraphic data, suggests that the base of the Niutitang Formation is of Tommotian or upper Meishucunian age and the Ediacaran–Cambrian boundary is most likely within the Liuchapo Formation. The depositional age of the unusual metal-enriched (Ni–Mo–PGE) sulfide deposits, which are approximately 1m above the ca. 523Ma tuffaceous bed, should be early Tommotian (upper Meishucunian) rather than early Nemakit-Daldynian (lower Meishucunian). Regional correlation confirms the diachronous development and condensation of stratigraphic units across the early Cambrian Yangtze platform and provides important information for understanding the paleogeography and paleoceanographic events at the Ediacaran–Cambrian transition in South China.

Cerium anomaly variations in Ediacaran–earliest Cambrian carbonates from the Yangtze Gorges area, South China: Implications for oxygenation of coeval shallow seawater

The late Neoproterozoic ocean witnessed the naissance of animals which is believed to have been stimulated by an increase in oxygen levels in the ocean. However, apart from the emergence of animal fossils, little supporting evidence has been found for the rise of oxygen in shallow seawater. Here we present Ce anomaly data, a redox proxy with a higher reduction potential than many other redox proxies, for carbonates from well-preserved marine successions (the Doushantuo Formation, Dengying Formation and lower Yanjiahe Formation of the Ediacaran Period, and the upper Yanjiahe Formation of early Cambrian) on the Neoproterozoic Yangtze Platform in the Yangtze Gorges area, in order to constrain the redox evolution of the shallow marine environment. Calculated Ce anomalies were screened, using the following criteria: Al<0.35% and Fe<0.45% in bulk rock, Th<0.5ppm, Sc<2ppm, ΣREE<12ppm and Y/Ho>36 in acetic-acid-leached carbonate, to target samples that preserve primary seawater REE features and Ce anomalies. The samples satisfying this screening show seawater-like REE distribution patterns in leached carbonates and may have recorded Ce anomalies of the seawater from which the carbonates precipitated. These data show that the Doushantuo Formation has Ce/Ce* values between 0.92 and 0.71, the Dengying Formation between 0.90 and 0.40, and the Yanjiahe Formation between 0.52 and 0.72. The Ce/Ce* values of the Doushantuo Formation suggest that shallow waters during the Doushantuo stage (635–551Ma) were anoxic to suboxic. From the bottom to the top of the Dengying Formation, Ce/Ce* values decrease systematically, which cannot be explained by a change in depositional depth of the carbonate but suggests that the shallow waters became more oxygenated during the Dengying stages (from 551Ma to Ediacaran–Cambrian boundary). These results may provide direct evidence for an increase in oxygen levels in the shallow marine environment during the Ediacaran Period.

A δ34SSO4 approach to reconstructing biogenic pyrite burial in carbonate-evaporite basins: An example from the Ara Group, Sultanate of Oman

Carbonate-associated sulfate (CAS) has the potential to generate high-resolution records of seawater sulfate (δ^(34)S_(SW)) that improve upon existing evaporite-based records. With improved resolution, however, significant isotopic offsets have become apparent between the evaporite and CAS proxies. Here we present high-resolution δ^(34)S measurements of both these proxies from the Ara Group, Sultanate of Oman, a series of six carbonate-evaporite sequences deposited ca. 547–540 Ma. The δ^(34)S_(CAS) from Ara carbonates show little scatter and provide our estimate of δ^(34)S_(SW). Repeated enrichments (as much as 4‰ relative to δ^(34)S_(CAS)) were observed in floor and roof anhydrite units (δ^(34)S_(EVAP)) bounding the Ara carbonates. These enrichments cannot be explained by secular variation or from isotopic fractionation during evaporite deposition and require the existence of an additional ^(34)S-depleted sink, which we attribute to H_(2)S production via ongoing bacterial sulfate reduction during evaporite deposition. To preserve this isotope signature, the resulting sulfide must be sequestered as pyrite. The magnitude of the resulting δ^(34)S_(EVAP) offset (relative to CAS) is a function of local pyrite burial (f_(pyr)). In periods of low pyrite burial, it is possible for δ^(34)S_(EVAP) to be depleted relative to δ^(34)S_(SW), whereas during episodes of substantial pyrite burial (f_(pyr) > 0.05), δ^(34)S_(EVAP) can be strongly enriched relative to δ^(34)S_(SW). Our data suggest that local fpyr of ~0.13 is consistent with the observed δ^(34)S_(EVAP) enrichments found in the Ara Group anhydrites. Such elevated pyrite burial in evaporitic settings requires a substantial iron source, supporting possible ferruginous ocean conditions across the Ediacaran-Cambrian boundary. Isotopic offsets between paired δ^(34)S_(CAS)-δ^(34)S_(anhydrite) data may thus serve as an independent proxy for marine redox through time, in addition to quantifying the importance of microbial activity in a setting where direct evidence (e.g., total organic carbon or biomarkers) may be scarce and physical processes are thought to dominate.

The hypothesis of basic phenotype modules

AbstractThe evolutionary events that allowed the rapid occurrence of metazoa are still enigmatic. The presumably oldest metazoan fossils are microscopic and occur just above 635 Ma, at the beginning of the Ediacaran period. Upon condition that the lack of macrofossils in the lower Ediacaran strata is real, the assumption of a sudden appearance of already complex, but still small animals that flourished during the first half of the Ediacaran is a reasonable option. Consequently, the emergence of the first macrofossils with metazoan affinity in mid Ediacaran strata would indicate a second leap in animal evolution. Here, these apparent leaps are explained in terms of a new concept of evolvability that is based on well definable developmental modules: A system based on blast cell-induced cell division modules has paved the way for rapid evolution to small multicellular animals. The second module, an ancestral form of segments, allowed the construction of a new sort of metameric body plans that appeared some tenth of million years later and at a larger scale. Based on this new model of basic radiations, the lower Ediacaran strata are predicted to contain a sequence of exotic embryos that is followed by an explosion of small metazoan diversity. The upper Ediacaran biota are interpreted as representatives of an evolutionary succession that culminated in a maximally evolvable and segmented ancestor with an already complex archetype body plan. Then, around the Ediacaran-Cambrian boundary branching of most modern animal phyla would have taken place, with extensive secondary simplifications in many groups. This radically new hypothesis allows the formulation of clear predictions that are in agreement with available fossils and geochemical data from that period, however, it calls for a precise and chronologically well-definable sequence of Ediacaran events that can only be confirmed through future investigations.

New carbon isotope stratigraphy of the Ediacaran–Cambrian boundary interval from SW China: implications for global correlation

AbstractThe Yangtze Platform preserves relatively thick carbonate successions and excellent fossil records across the Ediacaran–Cambrian boundary interval. The intensely studied Meishucun section in East Yunnan was one of the Global Stratotype Section candidates for the Precambrian–Cambrian boundary. However, depositional breaks were suspected in the section and the first appearance of small shelly fossils could not be verified. The Laolin section located in NE Yunnan is more continuous and shows great potential for global correlation of carbon isotope features across the Precambrian–Cambrian boundary. However, the stratigraphic framework and correlations were controversial. We studied and systematically sampled the Laolin section and present here new carbon isotope data for this section. The Laolin section consists of, in ascending order, the Baiyanshao dolostone of the Dengying Formation, the Daibu siliceous dolostone, Zhongyicun dolomitic phosphorite, lower Dahai dolostone and upper Dahai limestone of the Zhujiaqing Formation, and the black siltstone of the Shiyantou Formation. Our data reveal a large negative δ13C excursion (−7.2‰, L1′) in the Daibu Member, which matches the previously published data for the Laolin section, and a large positive excursion (+3.5‰, L4) in the Dahai Member, which was not shown in the published data. The excursion L1′ correlates well with the similarly large negative excursion near the first appearance of small shelly fossils in Siberia and Mongolia. Similar magnitude excursions are also known from Morocco and Oman, for which there are no robust fossil constraints but from where volcanic ash beds have been dated precisely at 542 Ma, thus confirming a global biogeochemical event near the Ediacaran–Cambrian boundary. Our data also indicate that deposition was more continuous at the Laolin section compared with the Meishucun section, where there are no records of a comparable negative excursion near the Ediacaran–Cambrian boundary, nor any comparable positive excursion in the Dahai Member. Therefore, the Laolin section has proven potential to be a supplementary Global Stratotype Section for the Ediacaran–Cambrian boundary on the Yangtze Platform.

Hydrothermal venting activities in the Early Cambrian, South China: Petrological, geochronological and stable isotopic constraints

Multi-episodic (three episodes at least) hydrothermal silica chimneys are identified in the black chert successions (up to ∼ 100 m thick locally), overlain by thick black shales (up to a few 100 m thick), in the Ediacaran–Cambrian boundary successions along the southern marginal zone of the Yangtze Platform, South China. Their occurrences are constrained within the lowermost Cambrian by the SHRIMP U–Pb zircon dating and stable isotopic chemostratigaphic data, and are stratigraphically coincident with the negative isotopic excursions of organic carbon (∼ 10‰ VPDB in magnitude) and sulfide sulfur (between 0 and 10‰ CDT), i.e., in bases of Nemakit–Daldynian and Tommotian stages in the Lower Cambrian. In this case, a causal link could have existed between which the hydrothermal venting could have released huge amounts of reduced silica-rich hydrothermal fluids with abundant 13C-depleted greenhouse gases (methane) and volcanic-originated H 2S into the ocean and/or atmosphere, resulting in an extreme warm climate and oceanic anoxia, oceanic chemical perturbation, and subsequent massive precipitation of silica (chert), and large-magnitude negative isotopic excursions of organic carbon and sulfide sulfur. This scenario provides a unique chance to explore the causes of apparent tectono-depositional, oceanic and geochemical perturbations during this critical interval in South China and elsewhere.

A paired sulfate–pyrite δ 34S approach to understanding the evolution of the Ediacaran–Cambrian sulfur cycle

An anomalous enrichment in marine sulfate δ 34 S SO 4 is preserved in globally-distributed latest Ediacaran–early Cambrian strata. The proximity of this anomaly to the Ediacaran–Cambrian boundary and the associated evolutionary radiation has invited speculation that the two are causally related. Here we present a high-resolution record of paired sulfate ( δ 34 S SO 4 ) and pyrite (δ 34S pyr) from sediments spanning ca. 547–540 million years ago (Ma) from the Ara Group of the Huqf Supergroup, Sultanate of Oman. We observe an increase in δ 34 S SO 4 from ∼20‰ to ∼42‰, beginning at ca. 550 Ma and continuing at least through ca. 540 Ma. There is a concomitant increase in δ 34S pyr over this interval from ∼ −15‰ to 10‰. This globally correlative enrichment, here termed the Ara anomaly, constitutes a major perturbation to the sulfur cycle. The absolute values of δ 34S pyr reported here and in equivalent sections around the world, require the isotopic composition of material entering the ocean (δ 34S in) to be significantly more enriched than modern (∼3‰) values, likely in excess of 12‰ during the late Ediacaran–early Cambrian. Against this background of elevated δ 34S in, the Ara anomaly is explained not by increased fractionation between sulfate and pyrite (Δδ 34S), but by an increase in pyrite burial ( f pyr), most likely driven by enhanced primary production and sequestration of organic carbon, consistent with earlier reports of elevated organic carbon burial and widespread phosphorite deposition.

Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman

The Huqf Supergroup, Sultanate of Oman, contains an important record of Neoproterozoic history, including evidence for two glaciations, a massive reorganization of the global carbon cycle, and the Ediacaran-Cambrian transition. New U-Pb geochronologic data provide precise constraints on the age of several key stratigraphic levels in the Neoproterozoic Huqf Supergroup and its subjacent crystalline basement rocks. The basement ages constrain an interval of felsic magmatism to have occurred from at least 840 Ma to approximately 810 Ma. Detrital zircons from several stratigraphic levels within the Huqf Supergroup yield ages in excess of 2.5 Ga, suggesting proximity of Archean crust during the Neoproterozoic evolution of the eastern Arabian Peninsula. Volcanic ash beds intercalated within the Huqf Supergroup were dated in the Oman Mountains, and in several subsurface wells (South Oman Salt Basin). Glacial deposits of the Abu Mahara Group in the Oman Mountains (Ghubrah Formation) contain volcaniclastic rocks that are approximately 713 Ma; overlying syn-glacial turbiditic sandstones of the Fiq Formation yield a suite of detrital zircon dates ranging from 920 to 664 Ma so that deposition of at least the upper Fiq must have post-dated 664 Ma. In the South Oman Salt Basin, volcaniclastic deposits intercalated within glaciogenic strata of the Fiq Formation yielded zircons, the youngest of which is about 645 Ma. These data indicate two distinct episodes of glaciation at approximately 713 and &lt;645 Ma. The uppermost Ara Group of the Huqf Supergoup contains multiple ash beds within its carbonate strata, where an age of roughly 547 Ma is reported for rocks that occur above strata marked by a pronounced negative (-12‰) to positive (+4‰) excursion in carbon isotope composition. Higher in the Ara sequence, three distinct ash beds dated at about 543 Ma, 542 Ma, and 541 Ma closely approximate the Ediacaran-Cambrian boundary in Oman. The dramatic carbon isotope excursion of ∼16 permil in the Shuram Formation (middle Nafun Group) has a firm maximum age of approximately 620 Ma as provided by detrital zircon ages from the base of the formation. Interpolation and downward extrapolation from the Ara Group ages, coupled with correlation to other global strata, suggests the base of the Shuram C-isotope excursion to be on the order of 560 Ma, with an estimated duration of approximately 5 to 11 m.y. This excursion is inferred to post-date the last well-documented Neoproterozoic glaciation (about 582 Ma) and is broadly coincident with the appearance of complex organisms in the fossil record.

The unique skeleton of siliceous sponges (Porifera; Hexactinellida and Demospongiae) that evolved first from the Urmetazoa during the Proterozoic: a review

Abstract. Sponges (phylum Porifera) had been considered as an enigmatic phylum, prior to the analysis of their genetic repertoire/tool kit. Already with the isolation of the first adhesion molecule, galectin, it became clear that the sequences of sponge cell surface receptors and of molecules forming the intracellular signal transduction pathways triggered by them, share high similarity with those identified in other metazoan phyla. These studies demonstrated that all metazoan phyla, including Porifera, originate from one common ancestor, the Urmetazoa. The sponges evolved prior to the Ediacaran-Cambrian boundary (542 million years ago [myr]) during two major "snowball earth events", the Sturtian glaciation (710 to 680 myr) and the Varanger-Marinoan ice ages (605 to 585 myr). During this period the ocean was richer in silica due to the silicate weathering. The oldest sponge fossils (Hexactinellida) have been described from Australia, China and Mongolia and are thought to have existed coeval with the diverse Ediacara fauna. Only little younger are the fossils discovered in the Sansha section in Hunan (Early Cambrian; China). It has been proposed that only the sponges possessed the genetic repertoire to cope with the adverse conditions, e.g. temperature-protection molecules or proteins protecting them against ultraviolet radiation. The skeletal elements of the Hexactinellida (model organisms Monorhaphis chuni and Monorhaphis intermedia or Hyalonema sieboldi) and Demospongiae (models Suberites domuncula and Geodia cydonium), the spicules, are formed enzymatically by the anabolic enzyme silicatein and the catabolic enzyme silicase. Both, the spicules of Hexactinellida and of Demospongiae, comprise a central axial canal and an axial filament which harbors the silicatein. After intracellular formation of the first lamella around the channel and the subsequent extracellular apposition of further lamellae the spicules are completed in a net formed of collagen fibers. The data summarized here substantiate that with the finding of silicatein a new aera in the field of bio/inorganic chemistry started. For the first time strategies could be formulated and experimentally proven that allow the formation/synthesis of inorganic structures by organic molecules. These findings are not only of importance for the further understanding of basic pathways in the body plan formation of sponges but also of eminent importance for applied/commercial processes in a sustainable use of biomolecules for novel bio/inorganic materials.

Evidence for anoxia at the Ediacaran–Cambrian boundary: the record of redox-sensitive trace elements and rare earth elements in Oman

The Ediacaran–Cambrian boundary in Oman is characterized by a globally correlative negative δ 13 C carb excursion in platform carbonate rocks, and deposition of basinal organic-rich rocks. These observations may relate to the development of local, and possibly global, anoxia. We have studied redox-sensitive trace elements (TE) and REE from basinal and platform successions. Detrital input and authigenic element enrichment were the main factors controlling the record of TE and REE. Trace elements in the basin are enriched relative to crustal concentrations. In siliceous rocks, TE enrichment was decoupled from detrital input, suggesting authigenic concentration of TE under anoxic (possibly sulphidic) water column conditions. Trace elements and REE indicate higher detrital input in shales. Element enrichment was minimally influenced by detrital input in the platform carbonate rocks; it is strongest in the basal platform section, coincident with the carbon isotope excursion. The results suggest development of anoxia in a stagnant basinal water mass. Overturn or upward expansion of the deep water charged with TE and isotopically light C produced anoxia on the platform, and was probably related to development of the isotope excursion. These results are consistent with anoxia in boundary strata of Iran, and with the hypothesis of global anoxia at this time.

Ediacaran δ13C chemostratigraphy of South China

We report new δ 13C chemostratigraphic data of the Ediacaran System in South China. Using litho- and biostratigraphic markers as independent calibrators, we combine new and previously published δ 13C data to construct a composite δ 13C profile for the Ediacaran System in South China. The composite curve shows, in chronostratigraphic order, 1) a negative δ 13C excursion (EN1) in the Doushantuo cap carbonate that overlies the Ghaub-age Nantuo diamictite; 2) a pronounced positive δ 13C excursion (EP1) in the lower Doushantuo Formation where Doushantuo-type acanthomorphic acritarchs, red algae, and animal embryos first appear; 3) a mild negative δ 13C excursion (EN2) in the middle Doushantuo Formation; 4) a positive δ 13C excursion (EP2) in the upper Doushantuo Formation where Doushantuo-type acanthomorphic acritarchs, micrometazoans, and stem group florideophyte red algae become abundant and diverse; 5) a pronounced negative δ 13C excursion (EN3) near the uppermost Doushantuo Formation; 6) a strongly positive δ 13C excursion (up to + 6.3‰ PDB; EP3) in the lower Dengying Formation; 7) a stable δ 13C plateau (ca. + 2.5‰ PDB; EI) in the middle and upper Dengying Formation where abundant vendotaenid algae, macroscopic Ediacaran fossils, biomineralized animal tubes, and mostly horizontal animal traces occur; and 8) a − 10‰ negative δ 13C excursion (EN4) immediately below the Ediacaran–Cambrian boundary. The general pattern of this composite δ 13C curve is robust, despite a considerable amount of variation among the δ 13C profiles of different sections. This composite δ 13C curve provides a first-order chemostratigraphic framework for the subdivision and correlation of the Ediacaran System in South China.

Multi-stage Ediacaran ocean oxidation and its impact on evolutionary radiation

High-resolution paired carbon (δ^(13)C_(carb) and δ^(13)C_(org)) and sulfur (δ^(34)S_(SO4) and δ^(34)S_(pyrite)) isotope records of the Ediacaran strata of the Huqf Supergroup, Sultanate of Oman record a three-stage oxidation of the Ediacaran ocean. Following the Marinoan glaciation, stage I oxidation lasts from ∼635 to 610 million years ago (Myr) and records an increase in marine sulfate concentrations above ∼200 μM, likely due to a rise in atmospheric oxygen. Stage II oxidation, from ∼575 to 550 Myr, coincides with the Shuram excursion, ɑ > 13‰ negative excursion in δ^(13)C_(carb)Burns and Matter, 1993 created by the oxidation of a large reservoir of deep ocean dissolved organic carbon (DOC). The onset of stage II oxidation coincides with the evolution of acanthomorph acritarchs in Australia and China Grey, 2005, and the appearance of the first Ediacaran metazoa, Charnia-type segmented fronds Narbonne, 2005, while the first motile macroscopic metazoa (e.g., Kimberella) appear at ∼555 Myr Narbonne, 2005 as the Shuram excursion draws to a close. Stage III oxidation, from ∼550 to 547 Myr, is marked by presence of sulfur disproportionating metabolisms and the onset of co-variation in δ^(13)C_(carb) and δ^(13)C_(org), which is absent in older strata. Coincident with the onset of stage III oxidation, we see the evolution of macroscopic multicellular algae Xiao, 2004, an increase in the diversity of the acanthomorph acritarchs Burns and Matter, 1993, and the appearance of the weakly calcifying metazoa Cloudina and Namacalathus Narbonne, 2005. Following stage III oxidation, there is a period of quiescence characterized by little perturbation in the carbon cycle and an absence of significant evolutionary events. At ∼542 Myr, anoxia at the Ediacaran–Cambrian boundary causes the extinction of many Ediacaran organisms, setting the stage for the Cambrian radiation.

A complex trace fossil from the Spitskop Member (terminal Ediacaran–? Lower Cambrian) of southern Namibia

Streptichnus narbonnei igen. et isp. nov., a new trace fossil from the upper part of the Spitskop Member of the Urusis Formation, southern Namibia, consists of clusters of unidirectionally curved radial elements, in which individual elements typically are composed of imbricated sickle–shaped segments somewhat comparable to those of Treptichnus pedum. Such complex trace fossils generally are found only in Cambrian or younger strata. This opens to question the position of the Ediacaran–Cambrian boundary in the Nama Group, suggesting that it may locally be within the uppermost part of the Urusis Formation, rather than at the base of the Nomtsas Formation.