Early Cambrian Niutitang Formation Research Articles

Origin of organic matter and depositional characteristics of early Cambrian Niutitang Formation from South China: New insights using molecular fossils

The early Cambrian Niutitang Formation is broadly developed in South China, especially in the Yangtze region. This research explains the nature of hydrocarbons through bitumen fractions of the early Cambrian sediments of the Niutitang Formation. For this purpose, source rock samples from two diverse outcrop sections are utilized for total organic carbon content (TOC), solvent extraction, and gas chromatography–mass spectrometry (GC–MS). Results show a lower quantity of extractable organic matter in these older sediments because of higher maturation history. The presence of monomethyl alkanes suggests the contribution of prokaryotic organisms, whereas the presence of hopanoids and C27–C29 steranes shows the occurrence of eukaryotic algal sources in organic matter. Likewise, C30‐4‐methyl sterane, dinosteranes, and ß‐carotene also suggested incorporating prokaryotic and eukaryotic sources during the early Cambrian period. Various hopanoid parameters, alkylated aromatic isomers, triaromatic steroids, and vitrinite reflectance via methyl‐phenanthrene index reveal mature to higher thermal evolution levels of these lower Palaeozoic sediments. Similarly, a higher abundance of polyaromatic pyrene and fluoranthene indicates mature organic matter, and their presence could be due to hydrothermal action. The higher gammacerane index, lower pristine/phytane, and the presence of C30 dimethyl steranes indicate the anoxic saline sedimentary condition throughout the deposition of these lower Palaeozoic Niutitang Formation. The early Cambrian Niutitang Formation is deposited in an oxygen‐deficient environment and contains an adequate amount of mature‐high mature organic matter that could have the potential to produce gaseous hydrocarbons.

Sedimentary geochemistry of the Early Cambrian Niutitang Formation to reconstruct the palaeo‐depositional environments and to evaluate the organic matter enrichment mechanism from the Yangtze Block, South China

In the Yangtze Block, the Early Cambrian Niutitang Formation is mainly composed of mudstone, shale, and carbonates which are important for investigating the depositional environment and evolution of the Early Cambrian rocks. The Niutitang Formation in the study area has a greater geologic interest due to its polymetallic beds, depositional age, variation in environmental conditions, Cambrian explosion, organic matter (OM) enrichment, algal boom, etc. This research represents the sedimentary geochemistry of the Early Cambrian Niutitang Formation to reconstruct the palaeo‐depositional environment and to evaluate the OM enrichment mechanism by means of total organic carbon (TOC), biomarkers, carbon isotopes, mineralogy, scanning electron microscope, etc. Based on the variation in TOC content, the Niutitang Formation is divided into three parts (upper, middle, and lower). The majority of the samples from the middle part of the Niutitang Formation exhibit an excellent source of hydrocarbon (TOC > 4.0 wt%) relative to the upper and the lower part. The lighter carbon isotopic composition (<−30.7%) in these sediments reveals the presence of the I‐amorphous kerogen group. Moreover, these lighter δ13Corg values suggest the presence of type‐I oil‐prone kerogen. Saturated hydrocarbon in these rocks showed the dominance of short‐chain n‐alkanes maximizing at C18. The predominance of these n‐alkanes represents that the OM is chiefly derived from algal/bacterial input. Similarly, the dissymmetric V shape of C27‐C28‐C29 steranes with a predominance of C27 and the higher values of in all three parts reflects that the OM in these rocks is chiefly originated from the lower aquatic marine organisms. Based on the Pr/Ph ratio, it is predicted that the middle part of the Niutitang Formation was deposited in extreme anoxic conditions (Pr/Ph < 0.5), whereas the upper and lower parts were deposited in relatively less anoxic conditions. Some biomarkers have more stable stereochemistry, which cannot be affected by diagenetic processes. These stable configurations are utilized to measure the maturity of OM, that is, Ts/(Ts + Tm), C29ββ/(ββ + αα), C29αα20S/(20S + 20R) steranes, and homohopane C31 22S/(22S + 22R). These geochemical indices reveal that the Early Cambrian Niutitang Formation in the studied area is mature to the post‐mature in the gas generation phase. Moreover, the hydrothermal fluids rich in metallic elements (e.g., Mo, Zn, V, and U) from the deeper part of the Earth owing to elongational forces among Yangtze and Cathaysian plates over Early Cambrian time entered the oceanic basin via remnant features (fissure and cracks) and through upwelling phenomena interacted with shelf sediments. At the ocean's surface, these nutrient‐rich fluids enhanced the breeding and evolution of marine life (bio‐productivity), which created hypoxic water conditions suitable for the preservation of OM in these rocks.

Chemically Oscillating Reactions during the Diagenetic Formation of Ediacaran Siliceous and Carbonate Botryoids

Chemically oscillating reactions are abiotic reactions that produce characteristic, periodic patterns during the oxidation of carboxylic acids. They have been proposed to occur during the early diagenesis of sediments that contain organic matter and to partly explain the patterns of some enigmatic spheroids in malachite, phosphorite, jasper chert, and stromatolitic chert from the rock record. In this work, circularly concentric self-similar patterns are shown to form in new chemically oscillating reaction experiments with variable mixtures of carboxylic acids and colloidal silica. This is carried out to best simulate in vitro the diagenetic formation of botryoidal quartz and carbonate in two Ediacaran-age geological formations deposited after the Marinoan–Nantuo snowball Earth event in South China. Experiments performed with alkaline colloidal silica (pH of 12) show that this compound directly participates in pattern formation, whereas those with humic acid particles did not. These experiments are particularly noteworthy since they show that pattern formation is not inhibited by strong pH gradients, since the classical Belousov–Zhabotinsky reaction occurs in solution with a pH around 2. Our documentation of hundreds of classical Belousov–Zhabotinsky experiments yields a number of self-similar patterns akin to those in concretionary structures after the Marinoan–Nantuo snowball Earth event. Morphological, compositional, and size dimensional comparisons are thus established between patterns from these experiments and in botryoidal quartz and carbonate from the Doushantuo and Denying formations. Selected specimens exhibit circularly concentric layers and disseminations of organic matter in quartz and carbonate, which also occurs in association with sub-micron-size pyrite and sub-millimetre iron oxides within these patterns. X-ray absorption near edge structure (XANES) analyses of organic matter extracted from dolomite concretions in slightly younger, early Cambrian Niutitang Formation reveal the presence of carboxylic and N-bearing molecular functional groups. Such mineral assemblages, patterns, and compositions collectively suggest that diagenetic redox reactions take place during the abiotic decay of biomass, and that they involve Fe, sulphate, and organic matter, similarly to the pattern-forming experiments. It is concluded that chemically oscillating reactions are at least partly responsible for the formation of diagenetic siliceous spheroids and concretionary carbonate, which can relate to various other persistent problems in Earth and planetary sciences.

The influence of syngenetic hydrothermal silica fluid on organic matter preservation in lower Cambrian Niutitang Formation, South China

Early Cambrian shale-chert couplets are widely distributed in South China, within which abundant organisms have been detected. Cherts in early Cambrian are known to be influenced by hydrothermal activity. However, the influence of syngenetic hydrothermal activity on organism preservation has not yet been well evaluated. Litho-, chemo- and bio-stratigraphic data have documented that the syngenetic hydrothermal silica fluid activity from the Majiang-Yangtiao section of the early Cambrian Niutitang Formation in Guizhou province, SW China.Geochemical compositions [Fe/Ti, Al/(Al + Fe), Al–Fe–Mn ternary diagram and Fe/Ti vs. Al/(Al + Fe + Mn) diagram] indicated that the chert, siliceous shales and mudstones were strongly, moderately, and weakly influenced by hydrothermal silica fluids, respectively. Mudstones contained much more terrigenous elements (e.g. Al and Zr). The total organic carbon (TOC) content and concentrations of nutrient elements (Cu, Zn, P, Ba) were highest in siliceous shales, followed by mudstone and lowest in chert. This could be due to a high silica dilution from hydrothermal fluid in chert. Compared with the contiguous siliceous shales, chert was characterized by a high content of aerobic zooplankton (e.g. sponges and pogonophoras) with low V/Cr ratios. This represented a sedimentary facies altered by hydrothermal fluid input in the marine environment. Organisms were well preserved in chert with delicate ornamentation on the surfaces of spherical acritarch cells, and to a lesser extent in silica-rich siliceous shales, but less in mudstones. This suggested that hydrothermal silica fluid with rapid sedimentation provided a good environment for organism preservation. These well preserved organisms could provide organic carbon content with little or no hydrocarbon generation in the rocks, because the morphology were not changed and no solid bitumen were found surrounding these organisms. The study provided a clue for careful consideration in the context of hydrocarbon generation evaluation in early Cambrian source rock.

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.

Spatio-temporal evolution of ocean redox and nitrogen cycling in the early Cambrian Yangtze ocean

The early Cambrian was a critical interval for the Earth system, during which a rise in oceanic and atmospheric oxygen levels coincided with the rapid diversification of metazoans. A variety of contrasting models have been proposed for the spatiotemporal redox evolution of the early Cambrian ocean. These include the development of a well-oxygenated deep ocean at the base of Cambrian Stage 3 (commencing at ~521 Ma), or alternatively, persistent and widespread anoxic (ferruginous) conditions throughout the early Cambrian ocean. Here, we present redox sensitive trace element (RSTE), Fe speciation, and N and C isotope (δ15Nsed and δ13Corg) data for samples from a section (Zhongnancun) of the early Cambrian Niutitang Formation, which was deposited on the outer-shelf of the Yangtze Block, South China. The Fe speciation and RSTE data provide evidence of a transition from euxinic, through ferruginous, to oxic conditions during deposition of the Niutitang Formation. The combination of these new data with existing data from the inner-shelf to basin environment, implies regional redox stratification across the Yangtze Block during Cambrian stages 2 and 3, with oxic shallow waters above ferruginous deep waters, and spatial variability in the degree of mid-depth euxinia. Oxygenation of deeper waters may have occurred by early Cambrian Stage 4 (~514 Ma). A compilation of δ15N values from multiple early Cambrian sections of the Yangtze Block indicate that N2 fixation dominated the nitrogen cycle during late Cambrian Stage 2. Low δ15N values (<−2‰) preserved in shelf sections can be interpreted to represent partial assimilation of NH4+, where NH4+ was not a limiting nutrient. During the early-middle Cambrian Stage 3, more positive δ15N values (0 to +3‰) are recorded in shelf sections, with lower values (−2 to +2‰) recorded in slope-basin sections. The positive δ15N values observed in shelf sections are likely a consequence of partial denitrification in the water column, whereas coeval deeper water δ15N values of ~0‰ may reflect the dominance of N2 fixation. The distribution of δ15N values, combined with a gradient in δ13Corg values, is consistent with a stratified ocean model. The δ15N values of all sections are lower than those of the modern ocean, which may indicate that the nitrate concentration of the early Cambrian Yangtze ocean was generally low during Cambrian Stage 3. The observed gradient in δ15N values is similar to that observed in records from Mesoproterozoic oceans, suggesting that abundant nitrate availability may have been restricted to shelf environments. We propose that increased nitrogen availability in shelf settings may have contributed to the evolution of large-celled eukaryotic phytoplankton. This provided a positive feedback on ocean oxygenation, allowing for increased complexity in early animal ecosystems on the continental shelf, and ultimately deep water oxygenation.

Surface water oxygenation and bioproductivity – A link provided by combined chromium and cadmium isotopes in Early Cambrian metalliferous black shales (Nanhua Basin, South China)

The intrinsic link between atmospheric oxygenation, surface seawater redox and bioproductivity in the upper water column of oceans offers to combine cadmium with chromium isotopes in marine archives to deduce information on past controls of biological metal and nutrient cycling. We here use a novel approach in combining chromium and cadmium isotope signatures in metalliferous and organic matter (OM) rich shales from the Early Cambrian Niutitang formation (south China) to reconstruct the surface water redox and cycling of bioessential metals, in a period directly following the Cambrian animal evolution. The inventory of authigenic Cd in the black shales is controlled by their sulfide abundance, whereas authigenic Cr is dominantly controlled by iron(oxy-)hydroxides and OM. Cadmium isotopes measured on bulk sediments and 3 N HNO3 leachates exhibit ε114Cd values from −1.5 to +4.5, with MoNi rich sulfidic shales revealing elevated values > + 2.5. Chromium isotopes define δ53Cr values from +0.4 to +1.7‰, with V-rich non-sulfidic shales yielding more positively fractionated signatures than MoNi rich sulfidic shales. The negative δ53Cr-ε114Cd correlation trend recorded in the black shales suggests a high-ε114Cd sulfide endmember of authigenic Cd preserved in anoxic bottom conditions. The other endmember is constituted by OM with the isotopically heavy Cr and light Cd signals reflecting efficient phytoplankton uptake in the photic zone. The endmembers constrain the Cd and Cr isotope signatures of surface waters in the Early Cambrian Nanhua Basin to ε114Cd of ~ +7 to +17, and to δ53Cr of ~ + 0.7 to +1.9‰. These values are compatible with those of modern ocean surface waters and attest to a Cambrian surface ocean water sustaining elevated primary bioproductivity in the aftermath of late Ediacaran snowball Earth glaciations. Our study emphasizes the potential of the CrCd double tracer in studies aimed at the reconstruction of paleoproductivity and (bio)geochemical metal cycling in marine paleo-basins.

Cd isotopes trace periodic (bio)geochemical metal cycling at the verge of the Cambrian animal evolution

Abstract Cadmium (Cd) isotopes are an emerging proxy for biological controlled metal and nutrient cycling in the modern oceans, but its potential as a geochemical proxy in ancient environments is still uncertain yet. Sequential leaching experiments of organic matter (OM)-rich shales from the early Cambrian Niutitang Formation (Fm.) were performed in order to understand the cycling of bioessential metals short after the Cambrian animal evolution. Carbonate, OM, sulphide and silicate leachates of OM-rich shales show an overall Cd isotope variation of 6 e112Cd/110Cd (e112Cd) units, indicating preferential incorporation of light Cd isotopes in the order sulphide > OM > carbonate > silicate. Carbonate leachates not only show negative correlations of e112Cd with bulk-rock total organic carbon (TOC) and δ13Corg but also show co-variations with redox-sensitive elements and bioessential metal concentrations, indicating a combined redox and primary productivity evolution of the early Cambrian Nanhua Basin on the Yangtze. Together with increased Cd/Zn ratios and decreasing total organic carbon (TOC) and redox-sensitive elements (RSE) concentrations in the upper Niutitang, this argues for an increase in essential metal availability for primary producers. Coinciding with the transition from a highly unstable ecosystem shortly after the Precambrian/Cambrian boundary to a more habitable environment increased nutrient uptake, oxygen availability and enhanced dissolved organic carbon (DOC) recycling match with the diversification of early metazoan fossil findings at the studied local. Our findings demonstrate that Cd isotopes in combination with trace metals can be used to infer changes in biogeochemical metal cycling in paleoenvironments and further allow establishing Cd isotope systematics as a reliable paleoproductivity proxy in the search for Earth’s earliest phototrophic life.

Depositional environment and organic matter enrichment of the lower Cambrian Niutitang shale in western Hubei Province, South China

Abstract It is important to consider the influences of depositional environment on lithofacies characteristics, lithofacies distribution patterns, and organic matter enrichment when defining reservoir properties and predicting “favorable reservoirs” and “sweet spots”. In this study, we examined the Niutitang shale of the early Cambrian Niutitang Formation in western Hubei Province, middle Yangtze Block, to understand how depositional environment controlled organic matter accumulation in the Niutitang shale. Through a combination of detailed sedimentologic investigations with geochemical analyses, we identified four primary lithofacies, siliceous shale, siliceous-clay mixed shale, calcareous shale, and argillaceous shale, from the Y1 drill core. The sedimentological analyses, including systematic observations of the color of fresh rocks, grain size, sedimentary textures and structures, biogenic and diagenetic (e.g., veins and pyrite) characteristics in cores and thin sections with mineralogical analyses by X-ray diffraction (XRD), and changes in geochemical proxies, including TOC (wt.%), Mo EF , U EF , Ni/Al and Zn/Al, suggest that siliceous shale and siliceous-clay mixed shale were deposited in euxinic and anoxic deep water with high surficial paleoproductivity which primarily controlled by long term global sea level changes, and that the calcareous shale and argillaceous shale were deposited in shallow and oxygen-rich water conditions with low energy and low paleoproductivity in the surface water. The Niutitang shale changed from the siliceous shale-dominated lithofacies association (LA1), to the argillaceous shale-dominated lithofacies association (LA2), and then back to the siliceous shale-dominated lithofacies association (LA1), suggesting a variation of sedimentary environment from euxinic–anoxic deep-water environment (highstand), to relatively oxic and shallow water environment (lowstand), and then back to euxinic–anoxic deep-water environment (highstand). This may reflect changes in sea level and mechanisms of organic matter enrichment during sea level highstand and lowstand. The low Mo/TOC values of the siliceous shale and siliceous-clay mixed shale lithofacies are similar to the Mo/TOC values of marine sediments in Framvaren Fjord, suggesting that the study area was located in a moderately restricted basin during deposition of the Niutitang shale. The Fe/Ti and Al/(Al + Fe) ratios and the Al–Fe–Mn diagram combined with our detailed petrographic studies and the good correlation between TOC and quartz contents are indicative of a mainly biogenic origin with a minor detrital source for the silica in the Niutitang shale. TOC is well correlated to Mo EF , U EF , Ni/Al and Zn/Al values in LA1, but it is only moderately correlated to Mo EF and U EF values and weakly correlated to Ni/Al and Zn/Al values in LA2. The correlation relationships suggest that good preservation in euxinic–anoxic water conditions and high primary paleoproductivity were the major controlling factors of organic matter enrichment, whereas organic matter accumulation during sea level lowstand was mainly controlled by water redox conditions because primary paleoproductivity remained persistently low in the more restricted basin with shallow water and limited nutrient upwelling. The correlations between TOC content and clay minerals within the different lithofacies associations suggest that adsorption of carbon components by clay minerals promoted the accumulation of organic matter to various extents in LA2 but probably did not impact organic matter enrichment in LA1.

Paleo-environmental conditions of the Early Cambrian Niutitang Formation in the Fenggang area, the southwestern margin of the Yangtze Platform, southern China: Evidence from major elements, trace elements and other proxies

The Precambrian/Cambrian transition was a key time in Earth history, especially for marine biological evolution and oceanic chemistry. The redox-stratification with oxic shallow water and anoxic (even euxinic) deeper water in the Early Cambrian Yangtze Sea, which gradually became completely oxygenated, has been suggested as a possible trigger for the “Cambrian explosion” of biological diversity. However, for some areas in northern Guizhou where the exploration and research are lacking, identifying this pattern of redox-stratification by paleo-environmental analysis from borehole data is still in need. Here, we report a remarkable variation range in trace elements (Mo, V, U, Ni, Th, Co, Sc, Zn and Cu), molar Corg:P ratios and pyrite morphology from 27 core samples from one new drill hole (XY1, located in the Fenggang area, northern Guizhou) on the Yangtze Platform, South China.High levels of Ba (from 3242 ppm to 33,800 ppm) and total organic carbon (TOC; from 4% to 9.36%) in 15 core samples in the Lower Member (LM) of the Niutitang Formation indicated elevated primary productivity in the study area. Redox change was recorded based on enrichment factors (EFs) for RSTEs (Mo, U, and V), redox proxies (V/(V + Ni), Ni/Co, V/Sc and Th/U), Corg:P ratios and particle size of framboidal pyrite. These signatures demonstrate that the LM was deposited under anoxic conditions with sulfidic episodes, whereas the Upper Member (UM) of the Niutitang Formation was deposited under suboxic/oxic conditions with intermittently anoxic episodes. Mo/TOC ratios (from 3.72 to 39.86, mean 18.76) suggest weak-moderate water mass restriction. Mo-U covariation patterns (strong but variable enrichment of Mo and U; MoEF ranging from 31.45 to 257.97; UEF ranging from 4.68 to 39.07) in the LM show alternation of particulate shuttling and redox conditions occurred in the Early Cambrian Yangtze Sea, whereas Mo-U covariation patterns (moderate Mo enrichment but depletion or non-enrichment of U; mean MoEF: 7.29; mean UEF: 0.95) in the UM may indicate the combined influence of particulate shuttling and diagenetic diffusion of U via bioactivities, which result in low U values and an anoxic signature from frambiodal pyrite particle size (mean: 4.556 μm; median: 4.41 μm). Additionally, excess Ba (Baxs) concentration (33,800 ppm and 32,500 ppm) and association patterns of trace-metal enrichment in the LM indicate the existence of submarine hydrothermal events. In addition, during deposition of the UM, bioactivities indicated by Mo-U systematics and oxic conditions indicated by redox sensitive trace elements (RSTEs) and multiple-proxies, may be a cause of biological diversification recorded in the Early Cambrian. Finally, data in this record a progressive transition from anoxic bottom waters with euxinic episodes to overwhelming oxic conditions during Early Cambrian.

Increase of seawater Mo inventory and ocean oxygenation during the early Cambrian

The early Cambrian radiation of macroscopic animals, commonly referred as the ‘Cambrian Explosion’, is one of the most important biological innovations in Earth history. Because oxygen is a requirement for complex animal life, it is generally accepted that ocean oxygenation may have been the environmental trigger for animal evolution, but direct geochemical evidence supporting oxygen increase prior to the ‘Cambrian Explosion’ remains sparse and insufficient. To better understand the casual relationship between ocean oxygenation and animal evolution, we have carried out a systematic analysis of redox sensitive trace element (RSTE) concentrations, total organic carbon (TOC) contents and pyrite morphology for the early Cambrian Niutitang Formation of the Hejiapu section, northeastern Guizhou Province, South China. The new data, in combination with previously published data from other sections, show strong Mo enrichments (average of 88ppm) and high Mo/TOC ratios (average of 14ppm/wt.%) in the euxinic black shales of the lower Niutitang Formation (~520Ma), indicating increase of the oceanic Mo reservoir. In the light of the positive correlation between sediment Mo/TOC and aqueous Mo in modern basins, the early Cambrian oceanic Mo concentration is estimated as ~50nmol, which is about half of the modern ocean Mo concentration (~105nmol) but is significantly higher than that of the Mesoproterozoic oceans (10–20nmol). The marked increase in oceanic Mo reservoir is therefore interpreted as recording significant rise of atmospheric oxygen and pervasive ocean oxygenation, which may have facilitated the diversification of macroscopic metazoans during the early Cambrian.

Trace and rare earth element geochemistry of black shale and kerogen in the early Cambrian Niutitang Formation in Guizhou province, South China: Constraints for redox environments and origin of metal enrichments

Early Cambrian black shales of South China not only host important sponge, arthropod and other soft-bodied fossils that have helped to trace early metazoan diversification, but also show extreme enrichments of a number of trace metals in particular Ni, Mo and V. In this study, we use a new approach by analyzing rare earth elements in kerogen extracted from the black shales, together with a number of redox-sensitive trace element compositions and total organic carbon (TOC) concentrations in an early Cambrian black shale sequence in Zunyi, Guizhou province, South China, to place better constraints on the oceanic redox conditions and the origin of the extreme metal enrichment.Our data show significant negative Ce anomalies (Ce/Ce* as low as 0.4) occurring in kerogen, which indicate an oxygenated surface environment of primary productivity in consistent with the concept that the organic matter is mainly derived from organisms in the euphotic zone. Mass balance calculation suggests that the kerogen-associated REE can dominate the measured black shale REE budget, while similarity between our measured REE patterns and those of similarly aged phosphorites indicates that the REE content of ancient phosphorites may have also derived initially from organic matter.The redox-sensitive trace elements, such as U, V, Mo, and their ratios of U/Al, V/Al and Mo/Al in black shales show different correlation patterns with TOC contents. The upper black shales show a good metal/TOC correlation, but such a correlation is absent in the lower part. The lower black shales exhibit much higher metal enrichments compared to Black Sea sulphidic (euxinic) sediment. This is taken to indicate the presence of sulphidic bottom waters during the deposition of the lower black shales, including the Ni–Mo ore layer. In contrast, anoxic, non-sulphidic conditions occurred during the deposition of the upper black shales.Taking all these geochemical data together, we suggest that the early Cambrian South China seaway was strongly stratified and stagnant, and that euxinic bottom water conditions may have led to enrichment of the redox-sensitive metals such as U, V and Mo in the lower black shales, and in one case the occurrence of a polymetallic Ni–Mo sulphide ore bed bearing an extraordinarily extreme metal enrichment, which, according to the Mo/TOC and Ni/TOC ratios and much other geochemical evidence, may have been additionally influenced by hydrothermal input of metals within the rift basin as suggested by a number of previous studies.

Re-Os Age of Polymetallic Ni-Mo-PGE-Au Mineralization in Early Cambrian Black Shales of South China--A Reassessment

The Early Cambrian Niutitang Formation in South China consists of a black shale sequence with a synsedimentary organic carbon-rich polymetallic sulfide layer with extreme metal enrichment locally mined as Ni-Mo-PGE-Au ore. We report a new composite Re-Os isochron age of the sulfide ore layer of 521 ± 5 Ma (n = 14) from three mine sites (Dazhuliushui and Maluhe in the Guizhou province and Sancha in the Hunan province) several hundred kilometers apart. This age corresponds to the biostratigraphic Tommotian age, and is in agreement with recent interpolated Pb-Pb and SHRIMP zircon U-Pb ages from an underlying tuff unit. The initial 187Os/188Os value of the sulfide ore is 0.87 ± 0.07, which is equal to the initial 187Os/188Os ratio of ~0.80 of the black shale host rock, consistent with the seawater origin of the metals under depositional conditions of very low clastic input.

Submarine hydrothermal/hot spring deposition of Early Cambrian Niutitang Formation in South China

INTRODUCTION The relationship between life and environmental processes in critical geological periods is one of the significant geological issues and remains a major concern. Not only environment events led to life evolution, but also biological processes changed the environment and climate which in turn influenced sedimentation and ore forming. In Early Cambrian, a diversity of phyla of skeletal metazoan synchronously appeared, and important phosphorite, Ni-Mo polymetallic ores and hydrocarbon source rocks were formed. Investigations on the Early Cambrian sedimentary rocks can provide an insight into the dynamics of life origin and the Cambrian explosion, and the sedimentary settings of the formation of metallic ores and hydrocarbon source rocks. In South China, Early Cambrian Niutitang Formation, extending more than 1 600 km, is characterized by the presence of black rock series. These rocks witnessed the Cambrian life explosion, and are abun-

New Bradoriid Arthropods from the Early Cambrian Balang Formation of Eastern Guizhou, South China

Abstract:The Early Cambrian Balang Formation is comprised of mudrock and shale, which was deposited in a shelf environment in the eastern part of Guizhou, south China. The Balang Fauna, which consists of seven phyla, occurs in the middle and upper parts of the Balang Formation. Arthropods are important constituents of the Balang Fauna and include a great number of trilobites, large bivalved arthropods, and newly‐discovered well‐preserved bradoriid fossils. The bradoriids present include three genera and four species: Comptaluta inflate (Cheng, 1974) emend Hou et al., 2002; Comptaluta kailiensis sp. nov, and Alutella elongeta sp. nov, Aluta sp. This faunal assemblage in the Balang Formation is distinguished from the Tsunyiella Chang, 1964, Songlinella Yin, 1978 and Kunmingella Hou, 1956 assemblage which occurs in the Niutitang and Mingxinsi formations of the Yangtze Platform in middle region of Guizhou and which is earlier than the Balang Formation in age. However, this assemblage resembles the ComptalutaÖpik, 1968 assemblage from the Early Cambrian Heilinpu Formation in Wuding County, Yuanan Province and from the Ordian Stage of the Cambrian of Australia. The great abundance of ComptalutaÖpik, 1968 and overall taxonomic diversity of the ComptalutaÖpik, 1968 assemblage set it distinctly apart from the Alutella Kobayashi et Kato, 1951 and Aluta Hou, 1956 assemblages of the Balang Formation. Alutella Kobayashi et Kato, 1951 and Aluta Hou, 1956 also occur in the Early Cambrian Niutitang Formation of the Yangtze Platform of Guizhou. Individual Bradoriids from the Balang Formation are characterized by large size (&gt;3 mm). The discovery of new Bradoriid assemblages not only expands the group's geographical range and assemblage affinities, but also indicates that Bradoriids migrated eastward from shallow‐water to deeper‐water environments during the Early Cambrian, indicating that they were capable of life in deeper‐water, and adaptation to a new ecological setting.

Insights from stable S and O isotopes into biogeochemical processes and genesis of Lower Cambrian barite–pyrite concretions of South China

Pyrite bearing barite concretions are abundant in black shales of the Early Cambrian Niutitang Formation in China. Barite in these concretions exhibits greatly elevated δ 34S and δ 18O values, averaging 68 ± 5‰ and 20 ± 1‰, respectively. Such high values indicate precipitation of barite during diagenesis at an advanced stage of bacterial sulphate reduction forming at the “barite front”. Pyrite sulphur has a relatively constant isotopic composition of ∼10‰ in both concretions and host rock and likely formed prior to barite. Fluctuating methane flux into the zone of anaerobic methane oxidation led to the consumption of sulphate and release of barium into higher sediment layers. During radial concretion growth Ba 2+ interacted with SO 4 2 - bearing fluids of various concentrations and isotopic compositions over a prolonged period, resulting in a lack of any systematic correlation between sulphate δ 34S and δ 18O values throughout the concretion.