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An assemblage of microfossils of moderate diversity, with the remarkable occurrence of ECAP acritarchs, is reported from the Ediacaran Krol ‘A’ (= the Mahi Formation) succession of Lesser Himalaya, India. Microfossils occur in the chert nodules exposed in Solan district, Himachal Pradesh. Two microfossils, Barogophycus symmetricus n. gen. n. sp. and Botominella lineata are new to the well-established Krol assemblage. The paper concentrates predominantly on fossil eukaryotic filamentous and coccoidal micro-organism of simple morphology. The assemblage is dominated by remains of prokaryotic cyanobacteria and demonstrates the diversity achieved by microorganisms at the beginning of the Ediacaran Period in the aftermath of the Marinoan glaciation. Filamentous and coccoidal microorganisms differ in taxonomic composition from the Mesoproterozoic microbiotas. The Krol microfossils assemblage has biostratigraphic potential and usefulness in broadly demarcating the different levels of the Ediacaran strata. Because of the presence of ECAP acritarchs, along with the appearance of other eukaryotic microorganisms of filamentous and coccoidal morphology, the general level of the Krol ‘A’ assemblage is considered as Ediacaran.

The transition from the Neoproterozoic to the early Cambrian world was a critical time in the co-evolution of life and the environment, when dramatic changes in the environment, including global glaciations and rising atmospheric oxygen levels precipitated the conditions in which variety of complex life emerged, diversified, and proliferated. However, questions persist regarding the timing and duration of those environmental events, and thus the extent of their direct effect on the evolution of life. These changing environmental conditions also produced unique taphonomic conditions that preserved a wide variety of organisms, and also produced unusual early diagenetic features that can be difficult to distinguish from fossilized organisms. In order to better resolve the link between life and the environment during the Neoproterozoic to Cambrian transition I present three chapters. The first chapter of this dissertation examines the carbonate clasts from a glacial diamictite deposited at the end of the Cryogenian Period in South China. By determining the source locality of those carbonate clasts using stable carbon and oxygen isotope compositions, we can better understand the extent of the Marinoan Glaciation and the intensity of its impact on the Earth's surface. In the second chapter of this dissertation, I investigate the changing chemical conditions of ocean waters following both the Marinoan glaciation and potentially one of the most intense disruptions to the carbon cycle in Earth History, the Ediacaran Shuram carbon isotope anomaly. I use Hg concentrations and stable isotope compositions to determine changes in sediment sourcing along with changes in marine redox, such as the development of photic zone euxinia. In the third chapter, I investigate the identity of Brooksella alternata, a purportedly cnidarian fossil that was later suggested to be a hexactinellid sponge fossil. My morphological and compositional analysis reveals B. alternata to be a concretion and thus a pseudofossil. Doctor of Philosophy The Earth experienced major changes between 635 and ~514 million years ago; the last global scale glaciation in the history of Earth ended, early animals arose, primary producers such as algae grew larger and more complex, and the first animals that were mobile and with hard skeletal parts evolved and diversified tremendously. Concomitantly, the environment on the Earth changed dramatically: the carbon cycle experienced one of the greatest disruptions in all of Earth history and the oceans and atmosphere gradually became more oxygen-rich, though areas with low levels of dissolved molecular oxygen and high concentrations of dissolved iron or hydrogen sulfide persisted, which may have impeded the diversification of complex animals. The exact timing and intensity of these changes are not fully resolved, and by investigating this time interval in the geologic record, we can better understand how the world changed and how life at the time responded. These changing environments also produced unique conditions which led to fossils being preserved in unique ways, though these conditions also produced non-biological structures that superficially resemble fossils. The first chapter of this dissertation investigates the source of clasts made of carbonate rock from a glacial diamictite (the poorly sorted mixture of the fine-grained and coarse-grained deposits left behind as glaciers recede) in South China deposited at the end of the last global scale glaciation about 635 million years ago. By determining the source, we can evaluate how far glacial activity transported sediments. I found that the previously suggested source of these clasts does not match the geochemical fingerprint of the carbonate clasts. I propose that these carbonate clasts may have come from a source that has since become "extinct" because of glacial erosion, or they may have come from another continent (for example, India). These scenarios highlight the magnitude of the global glaciation. In the second chapter, I investigated the mercury concentrations and stable isotope compositions from organic rich shales deposited ~550 million years ago. Mercury is an emerging tool for evaluating the level of dissolved molecular oxygen in ancient oceans. In this study, I found evidence of locally complex marine oxygen levels, including evidence for photic zone euxinia, meaning waters where oxygen was absent and hydrogen sulfide was present. Such conditions are toxic to animals and may have delayed the diversification of complex animal life. In the final chapter, I analyzed the ~503 million-year-old fossil Brooksella alternata. It was first described more than 100 years ago as a jelly-fish fossil. More recently it was suggested to be a sponge fossil (though it has been ascribed many other identities). Based on morphological analysis of a large collection, I determined that Brooksella alternata is not a fossil but rather a concretion with unusual shape.

Geological evidence indicates that the deglaciation of Marinoan snowball Earth ice age (~635 Myr ago) was associated with intense continental weathering, recovery of primary productivity, transient marine euxinia, and potentially extensive CH4 emission. It is proposed that the deglacial CH4 emissions may have provided positive feedbacks for ice melting and global warming. However, the origin of CH4 remains unclear. Here we report Ni isotopes (δ60Ni) and Yttrium-rare earth element (YREE) compositions of syndepositional pyrites from the upper most Nantuo Formation (equivalent deposits of the Marinoan glaciation), South China. The Nantuo pyrite displays anti-correlations between Ni concentration and δ60Ni, and between Ni concentration and Sm/Yb ratio, suggesting mixing between Ni in seawater and Ni from methanogens. Our study indicates active methanogenesis during the termination of Marinoan snowball Earth. This suggests that methanogenesis was fueled by methyl sulfides produced in sulfidic seawater during the deglacial recovery of marine primary productivity., The deglaciation of Marinoan snowball Earth (~635 Myr ago) has been associated with potentially extensive CH4 emissions in relation to transient marine euxinia. Here, the authors find that active methanogenesis occurred during the termination of Marinoan snowball Earth, fueled by methyl sulfide production in sulfidic seawater.

The Ottfjället Dyke Swarm (ODS) is a prominent component of the Ediacaran mafic magmatism associated with opening of the Iapetus ocean, and hosted in the Särv Nappe, Middle Allochthon of the Scandinavian Caledonides. A U-Pb baddeleyite age of 596.3 ± 1.5 Ma for a thick, well preserved, plagioclase-phyric dolerite dyke in Härjedalen, Sweden, dates emplacement of the swarm. The age represents a robust, inheritance-free reference age for variably deformed and metamorphosed tholeiitic dykes in sandstone-dominated sequences of the lower part of the Middle Allochthon, representing the proximal, rifted Baltoscandian margin preceding the opening of Iapetus. The new age is within the narrow time span between 610 and 595 Ma defined by the most reliable age estimates for mafic dykes in structurally higher nappes (upper part of Middle Allochthon), representing the distal margin during the opening of Iapetus. The Ottfjället Dyke Swarm cuts the Tossåsfjället Group succession, which includes sabkha-related carbonate platform and diamictite couples, one of several correlated Neoproterozoic glaciogenic successions in Scandinavia. The intrusion age of ca. 596 Ma therefore sets a minimum age for the glaciogenic successions. It implies that Neoproterozoic glaciations in Scandinavia predate the ca. 580 Ma Gaskiers glaciation event and are probably part of the ca. 635 Ma Marinoan “Snowball-Earth”-type glaciation.

Palaeoclimatic data (thickness variations in the Elatina Formation periglacial varves in the Flinderes Range, South Australia) with a spatial resolution of about one year and a time period of about 1334 years dated to the Marinoan glaciation, 680 Myears ago (late Precambrian), were studied in order to reveal quasiperiodic variations in climate processes in the past. Seven hidden periodicities, i.e., 12, 22, 24, 26, 104, 157, and 316 years, were revealed with a significance level of amplitudes in the framework of the polyharmonic model from 4σ up to 16σ. It was found that some of the detected periodicities were similar to the current solar activity cyclicity.

Zhou et al. (2004) reported a zircon U-Pb isotope dilution – thermal ionization mass spectrometry (ID-TIMS) age of 662.9 ± 4.3 Ma from the basal Datangpo Formation of the Cryogenian System in South China. It was the first zircon U-Pb age to constrain both the termination of Sturtian glaciation and the onset of Marinoan glaciation, despite its much larger uncertainty expected for modern ID-TIMS analysis. In this study, we carried out U-Pb chemical abrasion – isotope dilution – isotope ratio mass spectrometry (CA-ID-IRMS) analysis on the same sample, yielding a precise 206Pb/238U date of 659.96 ± 0.46 Ma. Bayesian eruption age estimate indicates that the ash layer was most likely deposited at 659.23 ± 1.24 Ma. The present study suggests that the Sturtian glaciation terminated before ∼660 Ma. Together with previously reported radiometric dates, the precise U-Pb age presented herein helps refine the current Cryogenian chronology.

Death Valley (California, USA) hosts iconic Cryogenian snowball Earth deposits, but the lack of direct geochronological constraints has permitted a variety of correlations and age models. Here, we report two precise zircon U-Pb isotope dilution–thermal ionization mass spectrometry dates for the Kingston Peak Formation: a volcanic eruptive age of 705.44 ± 0.28 Ma from the synglacial Limekiln Spring Member, and a maximum depositional age of 651.69 ± 0.64 Ma from the nonglacial Thorndike submember, which is below the Wildrose diamictite. These dates confirm that the Limekiln Spring and Surprise Members were deposited during the Sturtian glaciation, while the Wildrose submember is a Marinoan glacial deposit, and the overlying Sentinel Peak Member of the Noonday Formation is a Marinoan cap carbonate. Additionally, the age from the Thorndike submember supersedes existing radioisotopic ages from the Datangpo Formation in South China as the youngest constraint on the onset of the Marinoan glaciation, demonstrating that the Cryogenian nonglacial interlude lasted for at least 9 m.y. and the Marinoan glaciation was

Along the preserved southeast border (i.e., the Sete Lagoas High) of the Bambuí basin, the Pedro Leopoldo Member, basal succession of the Sete Lagoas Formation, unconformably overlies the Archean basement, and mostly includes carbonates with thin pelite intercalations and rare ruditic deposits. One of these, the so-called Carrancas conglomerate in its type-section, has been considered one of the lowermost rudite deposits of the Bambuí basin, being frequently ascribed to a Neoproterozoic glaciation. However, our detailed study, based on facies analysis, reveals that the Carrancas conglomerate was deposited by sediment gravity flow currents within the basal Pedro Leopoldo Member. Two outcrop sections in the São José de Lapa and Vespasiano areas, including thirteen abandoned quarry-cut and cliff outcrops, display eight distinct lithofacies (LF1 to LF8) forming a shallowing-upward carbonate ramp succession. It is composed, from the base to the top, by a mixed siliciclastic-carbonate outer ramp distally bounded by a slope-outer ramp system with sediment gravity flow deposits, a deep outer ramp developed below storm-wave base environments, an outer-middle ramp with aragonite pseudomorph crystal fans developed in a CaCO3 oversaturated below storm-wave base environment in suboxic/anoxic conditions. According to published isotope data correlating δ13C trends and values in basal carbonates of the Pedro Leopoldo Member of the Sete Lagoas and Januária highs, the relatively deep outer-slope ramp mixed siliciclastic-carbonate units of the study area appear to be coeval in age with the cap dolostone of shallow-water inner ramp of the Januária High. The absence of a cap dolostone in the Sete Lagoas High could be explained by lack of accommodation space or a regional erosion due to the tectonically driven forebulge uplift of the Sete Lagoas High. The Pedro Leopoldo Member in the Sete Lagoas High was accumulated in a brine-seawater bottom waters under suboxic/anoxic conditions in the relatively deeper portion of the carbonate ramp system, whilst the cap dolostone in the Januária High was developed in oxygenated mixed layer and intermediate water near the surface water in the shallower portion of the carbonate ramp system. Such lateral variations of facies and δ13C fluctuations are similar to those recorded by the cap carbonate sequences of the basal Schisto-Calcaire/Lukala (Sub)Group in the West Congo Belt and the basal Araras Group in the Paraguay Belt, which display a similar wide range of sub-environments like the studied part of the Pedro Leopoldo Member.

Petrographic studies, geochemistry, δ13C and δ18O isotopes have been used to characterize the well-preserved carbonate depositional environments of the Bobassa and Ndimba unit in the 640-565 Ma Ombella-M'poko Formation, Central African Republic. The carbonate facies are mainly composed of calcite, dolomite and quartz with some accessory minerals (opaque minerals, muscovite and pyrite). The Ca/Mg ratios show that the Bobassa carbonates are slightly metamorphosed dolomitic limestones, while those of Ndimba correspond to slightly metamorphosed highly dolomitic limestones and cipolins. The Bobassa unit recorded a depositional evolution from fluvial to peritidal environments in a possible tectonically active platform as evidenced by changes of lithology and slump structures. The Ndimba was deposited in a tidal-influenced fluvio-lacustrine environment under dry conditions. Geochemistry revealed that the Bobassa and Ndimba units recorded variable redox conditions from anoxic to oxic. The PAAS-normalized REE + Y distributions indicate that the both units were slightly altered by freshwaters and detrital materials in reduced and acidic environments. The lowest δ18O and highest δ13C values suggest warm surface waters and deeper water column respectively. The carbon isotope profiles of Bobassa and Ndimba units show values varying between −0.89 and −0.34‰ that are not stratigraphic coeval with the cap carbonates δ13C profiles deposited in the aftermath Marinoan Glaciation, but rather are correlated to the middle to upper part of the Schisto-Calcaire Group in Gabon and Republic of Congo and the time-equivalent Lukala Subgroup in DRC.

Measurements of triple oxygen isotope ratios in barite horizons within post-Marinoan cap carbonates have provided some of the most compelling evidence that the Marinoan glaciation was a Snowball Earth event. However, the origin of these barite horizons remains unresolved. To constrain the Ba sources, and thus formation mechanisms of these horizons, we analyzed the Ba isotope composition of post-Marinoan barite deposits from Northwest Canada, Northern Norway, Brazil and South China. We augment these analyses with a Ba isotope survey of almost 100 modern and ancient additional barite measurements, including samples from pelagic (or ‘marine’), hydrothermal, terrestrial, Proterozoic stratiform and cold seep environments. Unlike modern cold seep or terrestrial barites, we find that globally-distributed post-Marinoan barites exhibit a relatively narrow isotopic range, suggesting a well-mixed, effectively limitless Ba source. Moreover, post-Marinoan deposits exhibit a similar mean Ba isotope composition to modern marine barites, which we interpret as evidence of a marine Ba source. Considered alongside existing geochemical, geological, and new Ba isotope survey data, we conclude that Ba in barite horizons was sourced from a well-mixed, Ba-replete but SO4-poor reservoir that accumulated during the Marinoan Snowball Earth interval. This deep Ba reservoir was then transported upward—either by ocean circulation or dolomitization of underlying cap carbonates—and was brought into contact with continental weathering-derived sulfate in a post-glacial meltwater surface layer. Thus, in addition to providing a plausible mechanism for generating globally-synchronous deposition of post-Marinoan barite horizons that reconciles all existing geochemical and geological data, our results demonstrate the utility of Ba isotopes to interrogate the origin of enigmatic barite deposits throughout the sedimentary record.

Radiometric calibration of the Cryogenian glaciations is critical in understanding the nature of the climatic, geochemical and biological events in the Cryogenian period. However, the timing of termination of the Sturtian glaciation remains unsettled. Here we carried out new SIMS U-Pb zircon dating from two tuff layers at the non-glacial Datangpo Formation at the Jiangjunshan section in Guizhou, South China. The SIMS U-Pb zircon analyses yield U-Pb concordia ages of 659.3 ± 2.4 Ma and 657.3 ± 3.2 Ma for the two tuff layers, respectively. The age of 659.3 ± 2.4 Ma obtained from the tuff layer occurring at the boundary between Sturtian diamictites and the non-glacial Datangpo Formation is regarded as the best age estimate for the termination of Sturtian glaciation in South China. This new U-Pb age is well consistent with those from the previous studies, suggesting a globally synchronous termination of the Sturtian glaciation at ca. 659 Ma. The onset of the Marinoan glaciation has been estimated to occur at ca. 649 Ma, based on the radiometric age obtained in this study and previous cyclostratigraphic results. Thus it is argued that the Marinoan glaciation was a long-lived glaciation and spanned approximately 14 million years.

Neoproterozoic cap dolostones, which ubiquitously overlie Marinoan glacial diamictites, may record marine and climatic paleo-environmental conditions at the termination of the largest glacial epoch in Earth's history. Many geochemical indices have been used to interpret cap dolostone formation in the context of extreme climate change in the aftermath of the Marinoan glaciation. However, there are significant debates about whether these geochemical data represent global signals or regional sedimentary and/or diagenetic processes. Here we analyzed cap dolostones from three different continental cratons for their Ca-isotope, Sr-isotope and trace element compositions in order to obtain new insights into formation of the Marinoan cap dolostone and post-Marinoan paleo-environmental conditions ( δ 44 Ca excursion (∼0.6‰), coupled to a positive 87Sr/86Sr excursion, is recorded in the lower part of these cap dolostone successions. In the context of a relatively short duration for Marinoan cap dolostone (∼104 year timescale), we propose that the preservation of both the large negative δ 44 Ca excursion and the positive 87Sr/86Sr excursion in three widely-separated stratigraphic sections was not caused by globally uniform changes in isotopic compositions of the whole marine Ca and Sr reservoir. Instead, these excursions may have been caused by the addition of terrestrial meltwater and later deglacial runoff, carrying large amounts of Ca and Sr sourced from continental weathering into shallow seawater. A combined diagenetic-mixing model is used to track the coupled δ 44 Ca and 87Sr/86Sr variations in the cap dolostones. Large inputs of terrestrial meltwater and deglacial runoff under high CO2 atmospheric conditions in the aftermath of Marinoan glaciation is likely to have supplied abundant Ca, Mg and bicarbonate to shallow shelf seawater, helping facilitate cap dolostone deposition and contributing to the recorded negative Ca isotope excursion.

Cryogenian Sturtian and Marinoan glacial and associated strata record the most severe paleoclimatic events in Earth history and are the main test ground of the snowball Earth hypothesis (SEH). The SEH predicts that both Sturtian and Marinoan deglaciations were globally synchronous, rapid, and catastrophic. The sharp transition from glacial diamictite to the overlying cap dolostone represents a key event during these deglaciations. Thus, a positive test for globally synchronous deglaciation requires high-precision radiometric dates to tightly bracket this transition on each paleocontinent and within every substantial sedimentary basin; however, such high-resolution dates are scarce and have limited stratigraphic and paleogeographic coverage. Here, we report two high-precision U-Pb zircon chemical abrasion-isotope dilution-thermal ionization mass spectrometry ages of 658.80 +/- 0.50 Ma and 634.57 +/- 0.88 Ma from tuffaceous layers that occur, respectively, within the cap dolostone atop the Tiesi'ao diamictite (Sturtian age) and at the topmost Nantuo diamictite (Marinoan age) in South China. The 658.80 +/- 0.50 Ma age represents a high-precision minimum age constraint on the termination of the Sturtian-age glaciation. The 634.57 +/- 0.88 Ma age and a previously published age of 635.23 +/- 0.57 Ma from the topmost cap dolostone are indistinguishable within uncertainty, and together they provide tight constraints on the termination of the Marinoan glaciation in South China at ca. 635 Ma and directly bracket the duration of the cap dolo-stone to be

This study investigated the origin and the depositional age of phosphate stromatolites contained in the Neoproterozoic Salitre Formation, Brazil. The stromatolites exhibited columnar shapes and were intercalated with laminated dolostones. Their depositional environment was interpreted to have been an evaporitic ramp where erosion and reworking by waves prevailed. Laser ablation-inductively coupled plasma-mass spectrometry measurements of the stromatolites yielded a U Pb age of 616 ± 32 Ma, suggesting that they were deposited after the Marinoan glaciation. Several types of filamentous structures were recognized in the stromatolites; many of them were “pseudofossil” ambient inclusion trails, but some were possibly microfossils. To consider the general presence of photosynthetic microorganisms on the surface of shallow-water stromatolites, the involvement of microbial photosynthesis in phosphate stromatolite formation was expected. Numerical calculations determined that photosynthesis could have induced phosphate mineral precipitation at the time of deposition if the phosphorus concentration was above ca. 5 μM. To achieve such high concentration at the shallow ocean, globally elevated phosphorus concentration was considered to be the ultimate factor in addition to the local process(es) including upwelling and/or evaporation. Therefore, the phosphate stromatolites in the Salitre Formation are evidence of the development of a phosphorus-rich ocean after the Marinoan glaciation.

Microfossil assemblages preserved in the Ediacaran Doushantuo Formation phosphorites (i.e., the Weng’an biota) and chert nodules in South China provide key evidence for the diversification of marine eukaryotes in the aftermath of Cryogenian global glaciation. Two biozones (i.e., the lower Tianzhushania spinosa biozone and the upper Hocosphaeridium anozos biozone) have been proposed on the basis of acanthomorphic acritarchs preserved in the Doushantuo Formation chert nodules in the Yangtze Gorges area. Strata containing the Weng’an biota at Weng’an have been correlated either with the upper acritarch biozone based mainly on chemostratigraphic data, or with the lower acritarch biozone according to the occurrence of Tianzhushania spinosa in both assemblages. These alternative correlations need to be resolved in order to improve our understanding of the evolutionary pattern of microscopic eukaryotes after the Marinoan glaciation. To address this problem, we carried out an integrated litho-, bio-, and chemostratigraphic study of the Doushantuo Formation in the Zhangcunping area, where the microfossil assemblage resembles the Weng’an biota at Weng’an in both taxonomic composition and biostratigraphic ranges. Our study supports the litho- and chemostratigraphic correlation of the Doushantuo Formation between the Zhangcunping and Yangtze Gorges areas. Thus, the Doushantuo Formation at Zhangcunping provides a link between the Weng’an and Yangtze Gorges areas. Together with published paleontological and stratigraphic data, the present study favors a correlation between the fossiliferous strata at Zhangcunping and the upper part of the T. spinosa biozone recognized in the Yangtze Gorges area. The strata yielding the Weng’an biota at Weng’an can be at least partially correlated with the T. spinosa biozone, supporting the idea that the Weng’an microfossil assemblage is biostratigraphically transitional between the lower and upper acanthomorphic assemblages. This correlation is consistent with the fact that several taxa that used to be regarded characteristic of the H. anozos biozone may actually range downsection into the T. spinosa biozone, thus supporting an earlier diversification of Doushantuo acanthomorphs. These results call for more detailed investigation on the taxonomic composition and biostratigraphic ranges of acanthomorphs in the T. spinosa biozone, and re-consideration of acanthomorphic biozonation of the Doushantuo Formation in the Yangtze Gorges area.

We present the synthesis of new data on detrital zircon geochronology of the Neoproterozoic strata of the southern part of the Siberian craton as well as a comprehensive analysis of previously published stratigraphic, sedimentological and geochronological (LA-ICP-MS) data obtained for key sections in this area that allows us to trace the process of birth and early stages of development of the Paleo-Asian Ocean (PAO). Before the break-up of Rodinia and opening of PAO, Tonian – Cryogenian intracontinental sedimentary basin existed between southern Siberia and northern Laurentia. The detachment of the southern flank of the Siberian craton from northern Laurentia and opening of the PAO between these cratons took place in Cryogenian. The detrital zircon ages from lower parts of Neoproterozoic successions suggest the Siberian craton as the sole provenance area right after the opening of the PAO. The age constraints on the lower parts of the studied Neoproterozoic successions, which are based on correlation of their tillite horizons with the Marinoan glaciation, suggest the late Cryogenian age for these sedimentary rocks. A clear change in the age spectra of detrital zircons from “unimodal” (Early Precambrian only) in older sedimentary rocks to “bimodal” (Early Precambrian as well as Neoproterozoic) in younger sequences of the studied successions marks the next stage of the PAO evolution. The abundance of youngest (630–610 Ma) detrital zircons in the upper parts of the studied sequences reflects a shrinkage of the oceanic basin as a result of the convergence of the craton with the microcontinents and island arcs within the Paleo-Asian Ocean. We suggest that a passive oceanic margin along the southern margin of the Siberian craton has been transformed into a series of foreland basins at ~610 Ma.

Neoproterozoic evolution of the Arabian-Nubian Shield (ANS) and East African Orogen (EAO), 870–541 Ma, spanned revolutionary changes in Earth Systems, including a supercontinent cycle (Rodinia break-up, opening/closing of the Mozambique Ocean, Gondwana assembly), extreme climate fluctuations between long-lived glacial episodes as postulated by the Snowball Earth Hypothesis (1992–2002), marked increases in oceanic and atmospheric oxygen levels, and expansion of the biosphere from simple microbial life to the inclusion of larger and more diverse multicellular organisms. Understanding of these Earth System transitions has advanced tremendously over the past two decades through the integration of global studies of Neoproterozoic sedimentary successions with refined geochronologic techniques. This approach applied to Neoproterozoic Snowball Earth (NSE) localities now indicates that Neoproterozoic glaciations included two global-scale (panglacial) episodes, the ~717–659 Ma Sturtian and ~650–640 to 635 Ma Marinoan glaciations—which together comprise the Cryogenian Period, and more regional episodes during the Ediacaran Period and possibly, albeit controversially, during the earlier Tonian Period. Recent geochronologic, geochemical, and sedimentologic studies of low metamorphic grade ANS successions substantially contribute to the global Neoproterozoic dataset and, along with recently revised age constraints for the Cryogenian Period, facilitate an updated assessment of how Neoproterozoic glaciations may have influenced the sedimentary record of the ANS during its development. Tonian and Sturtian glaciations would have occurred following Rodinian break-up and major phases of juvenile crust formation in arc/island arc settings of the Mozambique Ocean (870–690 Ma), while latent terrane accretion and magmatism were still active. Paleogeographic reconstructions for these intervals generally place the ANS at tropical latitudes, where chemical weathering rates of juvenile crust terranes would have been high. Evidence supporting Tonian glaciation in the ANS is unresolved, with banded iron formation (BIF) and possible glacial diamictite scattered over the Central Eastern Desert (CED) of Egypt, NW Arabia, and possible correlative units in NE Sudan, as strongest candidates based on available age control (~780–740 Ma) and lithologic compatibility with NSE episodes. New age constraints for some of these localities (i.e., Atud diamictite and Um Nar, El-Hadid, Um Ghamis, and Wadi Kareim BIF localities in the CED) now demonstrate that deposition coincided with the Sturtian panglacial interval. Strong evidence of Sturtian glaciation in the ANS also occurs at the top of the Tonian-early Cryogenian Tambien Group in Northern Ethiopia, where polymict diamictite (

International audience; The Neoproterozoic Schisto-Calcaire Group (630 to ca. 580 Ma) was deposited on an extensive carbonate shelf in the margin of the Congo Craton in the Niari-Nyanga and Comba subbasins (Gabon and Republic of the Congo). It consists of three carbonate-dominated subgroups (SCI to SCIII, up to 1300 m-thick) recording relative sea-level changes. The SCIc Formation, at the upper part of the SCI Subgroup, is a carbonate succession of meter-scale shallowing-upward cycles, composed of a standard sequence of 7 facies grouped in 5 facies associations recording the evolution of a marine ramp from distal carbonate muds and giant stromatolitic bioherms (F1–F2) and extensive ooid shoals (F3), to proximal settings submitted to evaporation near a sabkha (F7). Fifth-order ‘meter-scale’ (or elementary parasequences) packages are grouped into fourth-order sequences (parasequence sets), which are not correlative in the whole basin. Two categories of fifth-order elementary parasequences are recognized, on the basis of physical bounding surfaces: (i) subtidal cycles bounded by marine flooding surfaces across which subfacies deepen; and (ii) peritidal cycles bounded by subaerial exposure surfaces. These cycles are the result of the interplay of relative sea-level changes due to eustatic variations related to periodic extensional tectonic events affecting the whole basin. The Niari-Nyanga and Comba subbasins experienced basin tectonics in the general context of the rifting of Rodinia creating changes of relative sea-level in the different parts of the shelf. The SCIc cycles are enclosed into a third-order sequence with two major transgressive-regressive phases, related to the deposition of the SCI Subgroup. The most typical sedimentologic feature of the SCIc Formation is the deposition of giant stromatolitic bioherms (stacked up to 20 m) topped by ooid shoals (up to 75 m thick) deposited during high systems tract prograding and forced regressive systems tract phases that ended with a lowstand systems tract phase with evaporitic and karstic conditions at the top of the SCIc Formation. The elementary parasequences and parasequence sets are probably the result of the migration of lateral environments related to the variation of the energy in relation to tectonic setting. As a result, a regional sea-level increase is for the first time highlighted in transgressive systems tract phase (composed of microbial induced sedimentary structures Facies) in the lower part of the SCIc Formation. The third-order succession can be followed more than 100 km in the Republic of the Congo and several hundred meters from South of Gabon to the Lower Congo in the Democratic Republic of the Congo. Tentative detailed sequence stratigraphy correlations between both Congo's highlight the role of tectonics affecting both areas.

Nearly 90% of the Ediacaran Period (635-541 Ma) of the Neoproterozoic is represented by the Doushantuo Formation (DST Fm) in South China. Its lowest Member I is a 3.7 m-thick cap carbonate deposited at the termination of the Cryogenian Marinoan glaciation. The DST Fm consists of alternating organic-rich black shale and thinly bedded dolostone, and it contains some of the oldest records of multi-cellular life and three pronounced negative carbon isotope excursions. The Jiulongwan (JLW) section is a well-studied reference section for these Ediacaran events. Spectral analysis of geochemical data through the lower DST Fm (22.3 m) shows 27 predominant ∼90 cm sedimentary cycles that correspond to 405-ka long eccentricity cycles. The power spectra of the 405-ka tuned Ca and Fe/Ti series show significant peaks at ∼1.2-Ma, 405-ka, 133-ka, 128-ka, 100-ka, 82-ka, ∼31-ka and 29-ka periods, respectively. A 11.16 Ma-long astronomical time scale has been constructed for the lower DST Fm and provide a duration of 1.6 Ma for the cap carbonate (Member I) based on the 405-ka long eccentricity cycle tuning. Using the U-Pb age of 635.2 ± 0.6 Ma for the volcanic ash bed at the Member I/II boundary, we proposed a 636.8 Ma age for the base of the DST Fm. These ages and astronomical timescale provide important new constraints on the subdivision of Ediacaran strata, and have implications for understanding the character of the first negative δ 13 C excursion (EN1). Orbital forcing may have been played an important role for the climate changes and the evolution of Ediacaran multi-cellular life and the carbon cycle variations.

Recent studies show that zinc isotopes could provide valuable clues to environmental change and biogeochemical cycle of the past oceans. This study reports a modified procedure for leaching the carbonate fractions in sedimentary rocks, a thorough evaluation of diagenetic effects, and systematic variations of Zn and Sr isotope ratios in lower part of the Ediacaran stratigraphic unit deposited in the aftermath of the Marinoan glaciation in South China. The influence of post-depositional diagenesis on Zn isotope compositions of the studied samples is assessed by comparing δ66Zn to other geochemical indexes (87Sr/86Sr, δ13C, δ18O and Mn/Sr ratios). In the five studied cap carbonate sections (Member I of the Doushantuo Formation), dolostones from four sections have δ66Zn values positively correlated with δ18O values and negatively correlated with 87Sr/86Sr ratios (0.7081–0.7204). These correlations suggest that these cap dolostone samples have been modified by post-depositional diagenesis. The light δ66Zn value (−0.02‰) suggests that initial Zn isotope ratios of cap dolostones could have been reset by hydrothermal fluids with relatively high Zn concentration and low δ66Zn values. By contrast, carbonates from Member II of the Doushantuo Formation above cap dolostones are relatively pristine based on their low 87Sr/86Sr ratios (0.7079–0.7086) being indistinct from the proposed early Ediacaran seawater 87Sr/86Sr values. Chemical and isotopic variations in these samples are interpreted to reflect primary signals that record paleo-environmental changes of the early Ediacaran ocean. A rapid increase of δ66Zn from ∼0.3‰ to 1.1‰ occurs in the middle part of Member II, accompanying by relatively invariant 87Sr/86Sr ratios that imply insignificant changes in input from continental weathering. Considering the limited change in atmospheric oxygen during this period, the rapid δ66Zn raise indicates an increase in buried organic matters, which is consistent with the coupled positive shift of δ13Ccarb, as well as the fossil records found in the same strata. These results provide insights into Zn cycling in the post-Marinoan oceans and facilitate the application of Zn isotopes in carbonates as a proxy for the fate of marine organic matter.

The Cuiaba Group is the basal part of the sequence of passive margin sediments that unconformably overly the Amazonian Craton in central Brazil. Despite these rock's importance in understanding Brazil's path in the supercontinent cycle from Rodinia to Gondwana and their potential record of catastrophic glaciation their internal stratigraphy and relationship to other units is still poorly understood. The timing of deposition and source areas for the subunits of the Cuiaba Group sedimentary rocks are investigated here using integrated U-Pb and Sm-Nd isotope data. We sampled in the northern Paraguay Belt, a range that developed in response to the collision between the Amazonian Craton, the Rio Apa Block, the Sao Francisco Craton and the Paranapanema Block. 1125 detrital zircon LA-ICPMS U-Pb ages were calculated and 22 whole rock samples were used for Sm-Nd isotope analysis. The U-Pb ages range between Archean and Neoproterozoic and the main source is the Sunsas Province. Moving up stratigraphy there is a subtle increase in slightly younger detritus with the youngest grain showing an age of 652 ± 5 Ma, found at the top of the sequence. The age spectra are similar across each of the sampled units and when combined with the Sm-Nd data, indicate that the source of the detritus was mostly similar throughout deposition. This is consistent with the analysis here that indicates sedimentation occurred in a passive margin environment on the southern margin of the Amazonian Craton. The maximum depositional age of 652 ± 5 Ma along with the age of the overlying cap carbonate of the Mirassol d’Oeste Formation suggests that part of this section of sediments were deposited in the purportedly global ∼636 Ma Marinoan glaciation, although we give no sedimentological evidence for glaciation in the study area. Compared to the southern Paraguay Belt where no direct age constraints exist, the glacial epoch could be either Cryogenian or Ediacaran. In addition, available data in the literature indicates a diachronous evolution between the northern and southern arms of the Paraguay Belt in the final stages of deposition and deformation.

The Ediacaran cap dolostones overlying the Marinoan Nantuo diamictite of the Doushantuo Formation in South China contain abundant enigmatic structures. Two generations of carbonate cement (named C1 and C2, respectively) can be observed in these structures at Wuhe of the Guizhou Province. C1 is characterized by dolomite cements that line the wall of the structures, and C2 by equant calcite spars that distribute in the center. The REE + Y patterns and the range of δ13C and δ18O values of C1 are similar to those of the micritic matrix, indicating that C1 precipitated from the same fluid as the matrix. The REE + Y patterns of C1 are characterized by minor MREE-enrichment, positive Ce and Eu anomalies, large variations in Y/Ho ratios. These patterns reflect normal marine origin with the release of Fe-Mn oxides in pore water during early diagenesis. Positive Ce anomalies and Th/U ratios of matrix suggest the anoxic depositional conditions. C2 is characterized by MREE-enrichment, positive Eu anomalies, high Y/Ho ratios and unobvious Ce anomalies, which is different from C1 and matrix. The Eu anomalies indicate that the precipitation fluids of C1 and C2 may have involved in different contribution of high-temperature hydrothermal fluids. The precipitating temperatures of C2 would be from 105 °C to 120 °C based on the δ18O variations of C1 and C2. The positive correlation between δ18O and δ13C values of matrix, C1 and C2 may attribute to the alteration by hydrothermal fluids. However, the negative correlation between δ13C and δ18O values of C2 may result from the oxidation of organic matter and methane seeps. The geochemical features of different microfacies of the cap carbonates reflect enigmatic fluid chemistry after the global Marinoan glaciation.

In order to investigate the geochemical responses of Earth's oceans and atmosphere after a Snowball glaciation, we studied an Ediacaran cap carbonate profile pertaining to the base of the Bambui Group (Sete Lagoas Formation) in the central portion of the Sao Francisco basin, east central Brazil, for trace elements (including REE + Y) and C, O, Sr, Nd, Pb and Cr isotopes. The section can be divided in three portions, from bottom to top: (i) a two-meter thick cap dolostone preserving a drop of δ13C values from −3.8 to −4.2‰, overlain by (ii) a ca. 123 meters-thick, reddish limestone rhythmite deposited in a tidal flat, with an important isotopic shift observed at 60 m, with δ13C values below averaging −5‰ and above shifting abruptly to around −0.5‰; and (iii) a topmost ca. 2 meters thick black calcarenite with positive δ13C (+0.9‰) and 87Sr/86Sr = 0.7076. A new, tightly constrained Pb-Pb whole rock isochron of 608 ± 19 Ma provides additional support for deposition during the early Ediacaran. We put forward a model for the deposition of the Sete Lagoas cap carbonate which starts with the glacial aftermath, when the thin cap dolostone and the basal reddish limestone were deposited due to glacier melting and subsequent sea level rise, with important mixing of glacial meltwaters with anoxic bottom waters. A decreasing trend of eNd(630Ma) values, from −6 at the base to ca. −8 at the 60 meters-mark, indicates continued input of continentally-derived detrital particles to the basin. During this phase, intermittent subaerial exposure was responsible for meteoric diagenetic alteration, which shifted 87Sr/86Sr to higher values (>0.710) and δ18O towards lower values. Towards the top of the sequence, the late postglacial succession is marked by marine-like REE + Y (Y/Ho = ca. 46) and lower Al and Mn contents, reflecting the diminishing of glacial meltwater input, a renewed sea level rise, and a change to more distal depositional environments, away from the coastal lines. At this moment, a vigorous input of U(VI) and 53Cr-rich Cr(VI), reflecting in a sharp rise of U/Th ratios and positive δ53Cr, suggests that fully oxygenated conditions finally predominated in post-glacial surface waters in the middle to late Ediacaran. Our results add to a growing body of data for other Ediacaran cap carbonate sequences around the world (e.g. Otavi Group, Namibia and Doushantuo Formation, China) that suggest that after ca 1 Ga of anoxia (the “Boring Billion”) following the first Great Oxygenation Event, the oceans became increasingly oxygenated after the end of the Marinoan glaciation ca. 635 Ma ago, ultimately leading to the explosion of complex macroscopic life that followed this Neoproterozoic Oxygenation Event.

Constructing an accurate timeline is critical for reconstructing the Earth systems through critical transitions in climate, geochemistry, and life. Existing dates constrain synchronous initiation (ca. 717 Ma) and termination (ca. 660 Ma) of the Sturtian glaciation from multiple continents. The termination of the younger Marinoan glaciation is also well dated at ca. 635 Ma, but the onset of this glaciation is only roughly constrained as ≤ ca. 654 Ma (South China) and ≥ ca. 639 Ma (Namibia). To test if the Marinoan glaciation started close to ca. 654 Ma or ca. 639 Ma, we have conducted a cyclostratigraphic study on the Cryogenian non-glacial Datangpo Formation that conformably overlies and underlies Sturtian and Marinoan glacial diamictites, respectively, in a deep-water basin section in South China. A total of 28,765 magnetic susceptibility (MS) measurements from a drillcore of the 292-m-thick, muddy siltstone- and shale-dominated Datangpo Formation are used for cyclostratigraphic analysis. The results reveal significant decameter- to meter-scale sedimentary cycles of 16–12 m, 3.6–3.0 m, 1.0–0.8 m, and 0.6–0.4 m. The ratios of these cycle wavelengths match well with those of the Milankovitch cycles calibrated for the Cryogenian Period. The established astrochronologic time scale suggests that the duration of the Datangpo Formation is about 9.8 million years. Together with the radiometric age of ca. 660 Ma for the termination of the Sturtian glaciation, the cyclostratigraphic data suggest that the Nantuo (Marinoan) glaciation in South China initiated at ca. 650 Ma, which is slightly younger than but consistent with the ca. 654 Ma U–Pb age from the top of the Datangpo Formation in shelf sections. This age, however, is significantly older than the ages obtained from Marinoan-age glacial diamictites in South China (ca. 636 Ma) and Namibia (ca. 639 Ma). Given that most of the shelf sections may have suffered from glacial erosion, obtaining the onset age of the Marinoan glaciation should focus on relatively complete, deep-water successions.

The aftermath of the Marinoan glaciation is an important period in the evolutionary history of Earth. Here we report on the dynamically biogeochemical cycling and organic mineralization processes of Mn in the Doushantuo Formation, Yangtze Block, South China, which was sourced hydrothermally during this time. This process occurred in an extensional tectonic setting as the water depth became shallower. Mn(+2) ascended with deep reducing hydrothermal fluids and accumulated in the form of Mn(+4) near the redox interface of locally oxidized water, which may be associated with metabolism by autotrophic microorganisms. Mn(+4) was reduced to Mn(+2), with organic matter acting as an electron donor during its degradation, which may be mediated by heterotrophic microorganisms. Mn(+2) in the water combined with CO32− generated by the oxidation and degradation of organic matter to form rhodochrosite. Algae and bacteria provided nucleation sites for the crystallization of rhodochrosite. Finally, spherical and ellipsoidal rhodochrosite with a biological structure and negative carbon isotope values (average δ13CPDB = −7.04‰) under the influence of Mn bacteria was formed. The input of hydrothermal material, mineralization involving the participation of organic matter, and mixed enrichment during syngenesis and syngenetic–early diagenetic stage were unique factors in the formation of Mn deposits in the Doushantuo Formation along the northern margin of the Yangtze Block. This represents the first report of Mn mineralization in carbonate form in the Ediacaran globally (Doushantuo Formation), and provides new insights into the biogeochemical cycling of Mn in the Ediacaran.

Global abrupt climate change from Marinoan snowball Earth to greenhouse Earth, recorded as cap carbonate overlain on diamictite, had shed the first light on Cambrian bio-radiation. The most documented cap carbonate sections are typical with comprehensive δ13C negative values and ubiquitous sedimentary structures, such as tepee-like, sheet-crack etc., which are associated with successive glacial eustatic variation caused by isostatic rebound in shallow-water facies. Here we report a deep-water basinal cap carbonate section with strong negative δ13C values in the southern margin of the Qinling Orogen, Heyu, Chengkou County, Chongqing in China, which consists of massive dolostone with abundant carbonaceous laminae. However, it lacks the sedimentary structure as mentioned above and is overlain by thin-bedded silicious shales and cherts. A K-bentonite bed was discovered within the base of cap carbonates, about 0.7 m above the top of the Marinoan diamictite. Magmatic zircons that were separated from the K-bentonite bed yield a SIMS concordia U–Pb age of 634.1 ± 1.9 Ma (1σ, MSWDCE = 0.31, ProbabilityCE = 1.000, n = 20). The age is in good agreement with previously reported TIMS U–Pb ages for the termination of Marinoan glaciation and provides a geochronological constraint for the Ediacaran successions in the Qinling Orogen.

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.

Cyclostratigraphy is a powerful tool in chronostratigraphic studies, and has shown its potential in Precambrian time. The Cryogenian Period is one of the most dynamic and intriguing intervals in Earth history and it is a witness to the most extreme climate changes on the planet, known as two Snowball Earth events. However, a robust chronostratigraphic framework is still lacking especially for the younger Marinoan Snowball Earth event, hampering the stratigraphic correlation and comparison among geographically separated records, as well as the understanding of the initiation and termination of this global-scale glaciation. The Cryogenian Fiq Formation in Oman is thought to be Marinoan-equivalent. The duration of the nonglacial units in the Fiq Formation could provide a conservative age constraint on the previously not well-determined onset age of the Marinoan glaciation. In this work, a cyclostratigraphic study has been conducted on three drill cores of the Fiq Formation. The multi-taper method (MTM) spectral analysis, the correlation coefficient (COCO), and the evolutionary correlation coefficient (eCOCO) analyses were performed on the high-resolution gamma-ray (GR) data from the three cores. A hierarchy of Milankovitch cycles (eccentricities, obliquities, and precessions) has been identified, which was used to estimate the optimal sedimentary accumulate rates and the duration of the Fiq Formation in each core. Based on the cyclostratigraphic results, the time scale of the Fiq Formation is at least ~6 Myr. Integrating the radiometric ages and the cyclostratigraphic data from other continents, the onset age of the Marinoan glaciation is suggested to be 650–641 Ma. This new result helps improve the chronostratigraphic framework for the Cryogenian Period and also provides critical temporal constraints for the geological and numerical models that explore the possible mechanisms for the Snowball Earth events.