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The Neoproterozoic Era (1000–541 Ma) is characterized by the largest negative carbon isotope excursions in Earth history. Younger Neoproterozoic excursions are well-documented on multiple continental margins and associated with major events including snowball Earth glaciations, ocean-atmosphere oxygenation, and the evolution of animal multicellularity. However, due to a large gap in carbon isotopes before 900 Ma, the onset of early Neoproterozoic carbon cycle volatility has heretofore been obscure. Here, we present a mid-amplitude, negative carbon isotope excursion with δ13C values reaching a nadir of ~ − 6‰ from the Majiatun Formation in the Dalian Basin of the North China Craton dated between 950 and 920 Ma. The whole-rock 87Sr/86Sr ratios of limestone and dolostone are as low as ~0.7055, which is compatible with the Tonian age of the global carbonate curve. Identification of the Majiatun anomaly thus fills the critical gap in carbon isotopes and implies the onset of Neoproterozoic carbon cycle volatility ~130 m.y. earlier than previously thought. Now 5 negative carbon isotope excursions occur with increasing amplitude throughout Neoproterozoic time, implying deeper roots for the biogeochemical processes that may have causally led to late Neoproterozoic snowball glaciation, oxygenation, and biological innovation.

A sea level model which is normally used to calculate sea level changes during glacial-interglacial cycles is modified to solve the approximate nonlinear Liouvelle equation, in order to calculate ‘large’ true polar wander (TPW) that may be induced by ice sheet loading. The purpose is to understand when the TPW will be too large to be solved properly by the linear model, and to properly calculate the TPW induced by snowball Earth events. It is found that the relative error for TPW calculated with a linear model will be >10% when the TPW exceeds ∼2° for ice sheets that develop near the poles, but remains 10° for certain viscosity profiles of the Earth.

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.

Large-scale silicic volcanism has occurred frequently in regions mantled by thin to very thick glacial ice cover, with several notable examples during the Pleistocene (Yellowstone, Long Valley, Iceland), including high altitude activity during the most recent glacial (e.g. Cascades, Kamchatka, Andes). More ancient caldera-forming events must have occurred during episodes of long-lasting (ca. 10–50 Ma) snowball Earth glaciation in the Neo- and Paleo-proterozoic, and many extraterrestrial eruptions on other ice-covered planets and moons are also of this general form. Recent work suggests that the process of caldera collapse typically lasts hours to weeks, during which the caldera floor drops by several hundreds to thousands of meters and is covered by hot tephra. With this scenario in mind, we investigate glaciovolcanic interactions immediately following the deposition of thick, hot ash layers on ice and consider the destiny of buried tephra and melting ice inside calderas. Our focus is drawn in particular to the post-emplacement hydration of volcanic glasses, with the goal of assessing whether the δ18O, δ17O and δD signatures in ancient deposits might be used to infer the syn-eruptive climate state. Scaling arguments, augmented by an idealized 1D model, suggest that ice should often survive for several decades or centuries as an active meltwater source to the overlying cooling intracaldera tuff. As liberated glacial water (both liquid and vapor) infiltrates and interacts with the tephra layer, volcanic glasses can become fully hydrated to water saturations of several wt.%. Our theoretical treatment is motivated in part by our recent measurements of lower than modern δD values in products of several Pleistocene eruptions in the western U.S. occupying regions that were likely glaciated immediately prior to the emplacement of volcanic products. We discuss how δ18O–δD and δ18O–Δ17O systematics can be used to recognize syn-glacially hydrated intracaldera tephra, potentially including samples that have been buried, altered and subsequently exposed either by fault uplift and erosional exposure or by drilling operations, such as those being performed currently in the Central Snake River Plain near Yellowstone.

Gondwana changed its high latitude location during the late Paleozoic (338–265 Ma), relative to the South Pole, and the style of glaciation evolved from localized alpine glaciers and ice fields to ~30 small ice sheets across the supercontinent. We report the analysis of heavy mineral populations (n = 2217) and the ages of detrital zircons (n = 2920 U-Pb LA-ICPMS results) from Gondwana diamictite deposits from eight landmasses: Africa (5 samples), Antarctica (5), Australia (8), the Ellsworth Mountains terrane (1, Antarctica), the Falkland Islands (2, diamictite plus U-Pb SHRIMP ages on granite clasts), India (1), Madagascar (1), Oman (3), the equatorial Lhasa terrane (2), the equatorial North Qiantang terrane (2) and South America (10). Heavy mineral separations (SEM-WDS analysis) identified one anomaly, pyrope garnets present only in Dwyka Group and Dwyka-equivalent samples suggesting an ultramafic Antarctic source. Statistical analysis of detrital zircon age distributions support the inference of local transport of sediment from many small ice centers with five examples of far-field ice transport (>1000 km; four with ice flow >2000 km), and three from ice fields located along coastal Antarctica. We propose that ice was distributed from five main ice-caps of different ages in southern Gondwana with ice flow away from central Gondwana. We also confirm that the Permo-Carboniferous detrital zircon populations of Euramerica (eolian and fluvial) and Gondwana (ash, detrital-glacial) are not mixed across the equator or seaway and ponder the possibility of a late Paleozoic snowball Earth.

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.

The Archean-Proterozoic transition was among the most important in geological history for it includes evidence of establishment of ‘modern-style’ plate tectonics, unprecedented paleoclimatic upheavals, and oxygenation of the atmosphere. The early Paleoproterozoic sedimentary record includes evidence of the world's first widespread glacial episodes, which have come to be known as the ‘Huronian Glacial Event’. None of these important changes coincides precisely with the accepted date of 2.5 Ga for the ‘boundary’ between the two great Precambrian eons. Rather, the geological record contains evidence of gradual transitions over many millions of years. For example the Archean sedimentary record in areas such as South Africa includes evidence of stable conditions (e.g. the Pongola Supergroup) that were not achieved in the Laurentian craton until much later during the Paleoproterozoic Era. The Pongola Supergroup in South Africa contains some of the world's oldest (c. 2.9 Ga) but locally developed glacial deposits. Many of these important changes are now considered to have been gradual and oscillatory in nature, including evidence of ‘whiffs of oxygen’ in Archean rocks, long before the Paleoproterozoic Great Oxidation Event. Oxidation of the oceans was also a long and extremely complex process, the details of which are still poorly understood. Glaciations near the beginning of the Paleoproterozoic Era have been considered by some to have been world-spanning ‘snowball Earth’ events. Repeated Huronian glaciations were probably brought about by weathering of Lauroscandia, the world's first ‘supercraton’, controlled by episodic rift-related uplifts during its disintegration. Among these glaciations only the third, represented by the Gowganda Formation and equivalents, was widespread throughout Lauroscandia. Because the two older glaciogenic units are known from only two locations in North America, their formation and preservation were probably controlled by local tectonic events. In like fashion, Paleoproterozoic glaciogenic units in South Africa and Western Australia appear to be local deposits from mountain glaciers formed during periods of tectonically generated (compressional?) uplift. The restricted distribution and diachronous nature of such tectonic events, and associated glaciogenic deposits, cast doubt on the viability of attempts at global correlations of individual Paleoproterozoic glaciogenic formations and on the existence of a Paleoproterozoic (or Neoproterozoic?) snowball Earth. This article is part of a special issue entitled: Archean Earth Processes; Edited by: Rajat Mazumder and Patrick Eriksson.

Over the past several decades, a number of studies have addressed the record of eukaryotic species richness in the Proterozoic, each making quite clear that during the Neoproterozoic Era, in particular, tremendous changes occurred in Earth’s biota. The relative scarcity of radiometric age constraints for rocks of this interval, however, have necessitated the use of coarse time bins (∼100 Ma) and the omission of fossiliferous but poorly dated units, resulting in low resolution of eukaryotic richness trends. Here we present a new estimate of early to middle Neoproterozoic (Tonian and Cryogenian) eukaryotic richness developed by use of the CONOP seriation algorithm; this approach permits inclusion of poorly dated and un-dated units and allows for greater resolution. The CONOP ( con strained op timization) algorithm operates by evolutionary ordination—considering evidence of stratigraphic order from all locations simultaneously and starting from a random ordinal sequence that improves by mutations retained or removed according to best-fit rules. This program has been applied successfully to biochronologic and biostratigraphic problems throughout the Phanerozoic geologic record. Here we apply this objective approach to a new compilation of taxonomically well-constrained organic-walled microfossil occurrences as well as geochemical, sedimentological and geochronological data from more than 160 formations from 60 groups in paleogeographically distant successions. From this dataset was developed a high-resolution eukaryotic species richness record for the early to middle Neoproterozoic Era (∼1000 to 635 Ma). This new estimate of eukaryotic species richness indicates an increase in richness began ∼800 Ma and continued towards a peak ∼770 Ma when it declined with the losses of many long-lived acritarch taxa. The overall decline is punctuated by a sharp richness increase with a ∼738 Ma peak due to the iconic and short-lived Tonian vase-shaped microfossil taxa such as Cycliocyrillium simplex . These VSM taxa were lost ∼733 Ma and richness continued to decline until flat-lining well in advance of the ∼720 Ma onset of the Cryogenian Snowball Earth glaciations. Eukaryotic species richness did not rebound until after the termination of the second Cryogenian glaciation when a new suite of acritarch taxa appeared in the Ediacaran Period. Use of CONOP with this dataset also permitted assessment of fossil taxa that had previously been suggested as Neoproterozoic biostratigraphic index taxa. Our results provide particular support for biostratigraphic use of the acritarch Cerebrosphaera globosa (= C. buickii ) and for species of vase-shaped microfossils.

In the Neoproterozoic Era there was widespread deposition of iron formations in close association with global or near glaciations. These ‘Snowball Earth’ glaciations likely played a key role in iron formation distribution and deposition. However, the environmental conditions, Fe sources, and formation mechanisms remain debated. Here we present the rare earth element geochemistry and Fe isotope composition of the synglacial iron formation within the Neoproterozoic Fulu Formation, South China. The Fulu iron formation consists of layers of authigenic minerals (mainly hematite) and detrital components (quartz, feldspars, Fe chlorite, and minor biotite). Positive Eu anomalies in one of the Fulu localities indicate a hydrothermal influence, suggesting that Fe was mainly sourced from distal hydrothermal systems. The bulk-rock Fe isotope composition of the Fulu iron formation shows a large range, with δ56Fe from −0.23 to +1.78‰. Correlation between bulk-rock δ56Fe values and Al/Fe ratios demonstrates that δ56Fe variability reflects, in part, varying proportions of authigenic versus detrital components. The Fe isotope composition of authigenic hematite is calculated by a linear regression and shows δ56Fe between +0.83 and +2.21‰, with an average at +1.54 ± 0.50‰ (2σ, n = 41). Using a dispersion-reaction model, the high δ56Fe values of hematite constrain local dissolved O2 concentrations of the ocean to less than 0.4 nmol/L, even in the shallow part of the water column. This relationship is consistent with highly reducing conditions in the Neoproterozoic oceans favored by isolation from the atmosphere by a sea ice. We attribute the extremely positive values to partial iron oxidation in waters that were cold relative to modern surface oceans. The dominant occurrence of hematite supports an abiotic precipitation pathway, given that biological activity would have introduced organic matter to the sediments and led to partial reduction of Fe(III) oxides and subsequent formation of magnetite and/or siderite, as is observed in Archean and Paleoproterozoic iron formations. Oxic glacial meltwater and/or O2 vertical transfer from the atmosphere to the upper ocean linked to ice dynamics is likely to have mediated the abiotic oxidation. We propose that vertical transfer of O2 resulted from the deposition of snow that trapped air bubbles at the top of the glacier coupled to a melting of the bottom of the glacier, which in combination delivered a limited but continuous amount of O2 to the ocean.

Hydrothermally-altered rocks with ultra-low-δ18O and δD were discovered in Karelia, Russia, over 500 km along the Belomorian Belt of the Baltic Shield. The ages of low δ18O and δD values are 2.41 Ga and 2.29 Ga, corresponding to the first and third Paleoproterozoic Snowball earth glacial episodes. Paleogeographic reconstruction suggests low latitudes for Baltica throughout Paleoproterozoic ice ages, requiring an effective distillation of water isotopes over short transport distances. Here we summarize how to use hydrothermally altered rocks to infer δ18O value of altering meteoric waters, then use an idealized General Circulation Model to test climate states with variable amount of ice cover and to check the hydrologic possibility of ultra-low-δ18O precipitation at low latitudes. Our model results suggest that the low δ18O values in precipitation is best achieved if the area of open water is quite narrow around the equator between latitudes 5°S and 5°N, or the Jormungand climate state. The δ18O of precipitation is high (>−15‰) over open water where high-δ18O vapor from ocean mixes with low-δ18O atmospheric water vapor. As such, a position of about 5–10° north or south of the equator is needed in order to achieve low δ18O of precipitation from an open water source. Our results suggest a slushball climate state with wide areas of open water is less likely to produce ultra-low-δ18O rocks for the low latitude position of Baltica.

The Cryogenian Period brackets an interval of profound glaciation, commonly referred to by the evocative epithet ‘Snowball Earth’. According to its recent redefinition, the Cryogenian began before the onset of low-latitude glaciation ca. 717 Ma and ended during its final and catastrophic melting phase ca. 635.5 Ma. Pending future ratification of a Global Stratotype Section and Point or GSSP, the onset of the Cryogenian Period is estimated at about 720 Ma. However, no consensus has yet been reached concerning the best section or criteria for its definition. The articles in this special issue represent the state-of-the-art in Tonian-Cryogenian transition stratigraphy around the world and will contribute to the selection of the future GSSP.

Here we present a new U-Pb baddeleyite ID-TIMS age of 713.7 ± 0.9 Ma on the shoshonitic Tatonduk Suite near the Yukon-Alaska border in the Proterozoic Tatonduk inlier, which occur as mafic and intermediate dykes and a sequence of basaltic to andesitic volcanic flows immediately beneath iron formation-bearing Sturtian glacial deposits correlated with the early Cryogenian Rapitan Group. Considering the age overlap with what is inferred to represent the Franklin Large Igneous Province (LIP) of northern Canada, the Tatonduk Suite might logically be considered a far-flung component of the Franklin Large Igneous Province. However, the Tatonduk Suite was emplaced in an actively extending environment >1200 km from the plume head on Melville Island and does not obviously align with the Franklin dyke trend. Furthermore, major, trace and isotopic data on the Tatonduk Suite reveal a composition that is unique from the Franklin LIP and most likely reflects melting of potassic and trace element enriched sub-continental lithospheric mantle at pressures of less than 10 kb. These volcanics and dykes highlight the complexity in defining what constitutes a LIP. The Tatonduk Suite either represent a non-plume end-member composition of the Franklin LIP, or more likely, rift volcanics associated with extension along the actively extending margin of northwestern Laurentia. Although the plume and rifting may be geodynamically linked, this interpretation suggests that this new age for the Tatonduk Suite does not constrain the timing of Franklin magmatism. The Tatonduk Suite was emplaced during the Sturtian Snowball Earth event and contributes to the growing body of geochronological data that indicates the onset of glaciation at ca. 717 Ma. Furthermore, this age provides direct evidence for the syn-glacial origin of Sturtian iron formation in the Tatonduk inlier and further links iron formation to widespread mafic magmatism at this time.

A central prediction of the Snowball Earth hypothesis is that glacial onset should be synchronous at low latitudes, and its termination should be rapid and synchronous globally. High precision U/Pb zircon ages provide supporting evidence for the synchronous onset (within error) of the Sturtian glaciation (ca. 716 Ma) on multiple continents. Successful application of Re-Os techniques on organic rich shales and carbonates allow for the possibility of a globally synchronous Sturtian deglaciation (ca. 660 Ma), but the sparse isotopic age constraints leave this open to debate. Here we report the first high precision U-Pb zircon age of 663.03 ± 0.11 Ma (2σ) for the end Sturtian recorded in the Wilyerpa Formation of South Australia. This age supports previously published ages and is permissive with a globally synchronous deglaciation. In conjunction with the timing of glacial onset, this age reinforces the ca. 58 Myr duration of the Sturtian Snowball.

The seawater chemistry and oceanographic information associated with Snowball Earth are commonly inferred from the geochemistry of cap carbonates deposited on continental margins during and after deglaciation. However, interpretation of such records can be complicated by carbonate diagenesis and contamination from siliciclastic components. In an attempt to disentangle these effects, we studied the geochemistry of the post-Marinoan cap carbonate sequence from Mongolia using a step-leaching procedure, which revealed that most samples are heterogeneous with respect to multiple geochemical signatures, including trace element concentrations, Sr, Mg, C and O isotopic signatures, raising questions to previous studies applying carbonate bulk-rock geochemistry for paleoenvironment reconstructions. Such sample heterogeneity can be explained by contamination from non-carbonate phases and carbonate alteration. After stepped leaching, the least-altered/contaminated geochemical signatures for each sample were identified and the influences of carbonate diagenesis were evaluated. Our data indicate that mixing of glacial meltwater persisted to the maximum flooding surface within the cap carbonate sequence, below which carbonates record significant Mg and Sr isotope fluxuations that are most readily interpreted in the context of the mixing of water masses having distinct isotopic compositions. Only limestones deposited above the maximum flooding surface formed in a well-mixed ocean and exhibit Mg and Sr isotope values that record the integrated effects of Snowball Earth on ocean chemistry. Our study cautions against interpreting the geochemistry of cap carbonates in terms of whole ocean geochemical cycles.

The northern margin of the West African Craton preserved in the Anti-Atlas area of southern Morocco, represents an outstanding geological archive covering parts of the Neoproterozoic and the Paleozoic. Despite isolated mentions in the literature, relics of Neoproterozoic glaciations have hitherto not received much attention in this area and tight geochronological constraints had been lacking. This paper documents the sedimentology and stratigraphic context of several occurrences from the eastern and central Anti-Atlas. Based on outcrop-scale sedimentological and microsedimentological investigations, different types of diamictites are distinguished and their origin with reference to modern glacial systems is discussed. The sedimentary record is explained by one single glacial period (albeit interrupted by interglacial phases) affecting both mountainous terrestrial and marine realms. Sedimentary facies and preserved paleotopographies are best accounted for by invoking a fjord-like depositional system with steep-sided valleys being repeatedly drowned by marine water or occupied by valley glaciers. High-precision geochronological constraints suggest that this glacial period must have taken place sometimes between 592 Ma and 579 Ma. Hence it is suggested that this is the first temporally constrained report of the Ediacaran Gaskiers glaciation on the West African Craton (WAC). Existing Ediacaran paleomagnetic data from the WAC are sparse and allow different interpretations regarding paleolatitude of this glaciation. The latter is finally discussed vis-a-vis potential rapid true polar wander events and the possibility that the Gaskiers glaciation was more global in extent than hitherto assumed and might even have brought Earth close to a true pan-glacial state.

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.

The Marinoan Snowball Earth glaciation (~650–635 Ma) was followed by an increase in atmospheric O2 level and the diversification of eukaryotes. These dramatic changes have been assumed to have resulted from the rapid recovery of primary productivity during deglaciation, but this hypothesis has not yet been tested. Carbonate carbon isotope (δ13Ccarb), closely related to organic matter production and burial, has the potential to provide with the most straightforward evidence for the perturbation in marine carbon cycle. However, individual δ13Ccarb record might be less well-suited to test this hypothesis than a large data compilation. Here, we analyze 1590 globally-distributed carbonate carbon isotope data from the Marinoan cap carbonate. The least square sum analysis of the shallow-water cap carbonate δ13Ccarb data indicates that the carbon isotopic composition of seawater dissolved inorganic carbon (DIC, δ13CDIC) was −3.01‰ during cap carbonate precipitation. This result sharply contrasts with the assumed synglacial seawater δ13CDIC value of −5‰, which was verified with a binary mixing model. The 2‰ increase in the seawater δ13CDIC can be attributed to intensive organic matter production and burial during the deglaciation in less than 106 years. Massive organic carbon burial in the wake of the Marinoan Snowball Earth glaciation contributed a postglacial increase of atmospheric O2 level by 3.8% to 13.4% PAL (present atmospheric level) and thus further facilitated diversification of eukaryotes.

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 Neoproterozoic Cryogenian (‘Marinoan’) Ghaub Formation of northwestern Namibia represents an important founding pillar of the Snowball Earth hypothesis and its derivative, the Panglacial Earth hypothesis. These hypotheses assume oceans and continents covered by thick ice, even in the tropics, which caused a very distinct drop in eustatic sea-level. Over time, strongly increased CO2 contents of the atmosphere led to sudden ice melting, very substantial sea-level rise, and strong weathering on the continents associated with the deposition of cap carbonates in the newly ice-free oceans. The ongoing controversy about Snowball-type glaciations in Namibia and elsewhere is reviewed, and other hypotheses (Slushball Earth, Waterbelt Earth, Jormungand state of the Earth, Thin Ice state of the Earth, Zipper-Rift Earth, High-Obliquity Earth) are discussed. We prefer the term ‘Waterbelt Earth’ instead of the originally proposed ‘Waterbelt state’ because of the clearer contrast with ‘Snowball Earth’. Because a great deal of information related to Cryogenian glaciations comes from the Ghaub Formation of northwestern Namibia, these hypotheses should be tested independently based on a time-equivalent depositional system. This analogue was found in the carbonate-dominated successions of the Otavi Mountainland (OML), northeastern Namibia, and is highly comparable with the successions in the well-investigated northwest of the country. An extreme eustatic sea-level drop caused by a global glaciation of oceans and continents and imposed on a carbonate platform or ramp such as the one in the OML would have led either to glacial cover or widespread subaerial exposure and extensive erosion, including deeply incised valleys. The presence of such features would strongly support the Snowball Earth hypotheses if tectonic effects did not play a major role. During the postglacial transgression, distinct reworking of the carbonate platform/ramp surface would have occurred, leaving behind lag deposits, as well as infills of incised valleys with fluvial, reworked glacial, and marine deposits. The main objective of our research was to weigh and investigate the strengths and weaknesses of the proposed Snowball Earth model of glacially induced large-amplitude sea-level changes during Ghaub time, and to compare different models to obtain a rough estimate of the amount of glaciation. The study area in the OML includes two different, age-equivalent facies realms: platform sedimentation in the Southern area without diamictites, and slope deposits, including Ghaub diamictites, in the Northern area. The southern, continuously shallow-marine area shows a shallowing-upward succession from the pre-glacial lower Auros Formation, often varve-like laminated shales formed below wave base, to metre-high columnar stromatolites and microbial mat-related carbonates with intervals of vertical tubes (degassing features) of the upper Auros Formation, overlain by cap carbonates of the Maieberg Formation. The columnar stromatolites and the microbial tubestone lithotypes were clearly deposited in the euphotic zone. Indications for tidal conditions or subaerial exposure were not recorded in this platform succession without unconformities. Neither dropstones, nor incised channels, nor transgressive lag deposits were observed. The facies changes from below storm wave base to the photic zone and finally a shallow subtidal zone is explained by a prolonged, modest sea-level fall, partly counterbalanced by subsidence, followed by a slow transgression. In contrast, coarse-grained sedimentary rocks (e.g., oolites, debrites) characterise the time-equivalent successions in the Northern area. Starting with laminated shales at the base, similar to the Southern area, the overlying redeposited oolites and breccias of the Auros Formation show distinct lateral and vertical inhomogeneities and thickness changes, which indicate long-lasting synsedimentary tectonism. The same phenomenon is observed in the overlying diamictites of the Ghaub Formation. Their variable clast content indicates erosion of a strongly uplifted local source area formerly covered by a thick carbonate succession, which was downstripped to the crystalline basement. The prograding diamictite succession with repeatedly intercalated silt-stringers is interpreted as periglacial debris flows into a marine environment. Sparse striated clasts in the diamictites and very rare dropstones (much less common than in northwestern Namibia) are indicators of glaciations somewhere in the area. However, compared with other glacial sequences, e.g. Quaternary periglacial sediments at the forefront of continental ice, dropstones and striated clasts would be expected to be much more common and more uniformly distributed if the entire area was covered by melting continental ice, as proposed in the Snowball/Panglacial Earth scenario. In the Southern area, dropstones would be expected to occur on the flooded platforms/ramps as well, even when diamictites are absent. Both the relatively moderate sea-level change and the less common, irregular distribution of locally concentrated glacial rainouts provide strong evidence against the presence of a thick, laterally continuous ice cover over oceans and continents extending to equatorial areas. The oceans possibly corresponded to the scenario of a Waterbelt Earth or High-Obliquity Earth; evidence of open oceanic water exists, which would have enabled the continued evolution of biota. Glacial ice was present on tropical continents, but its occurrences may have been regional in patches, sourced from mountainous areas, and ice streams would have reached the oceans only locally, unrelated to a thick continental ice cover.

International audience; The biomarker distributions of mid-Neoproterozoic sediments (800–717 million yr, Ma) from various locations are characterized by the occurrence, besides cholestane isomers, of a novel series of C28 steranes, whereas classical C-24 alkylated steranes are absent. These unusual C28 steranes, termed cryostanes, seem to be restricted to pre-Snowball Earth sediments. We report their conclusive identification as 26-methyl cholestanes based on comparison of gas chromatographic (GC) behavior and mass spectrum of the last eluting cryostane isomer with those of a 26-methyl 5α(H),14α(H),17α(H),20R-cholestane standard obtained by way of synthesis. Sterols methylated at C-26 seem to be restricted to some demosponges and, based on molecular clock estimates, demosponges may have emerged in the time interval 800–700 Ma. A sponge origin of cryostanes is thereby conceivable, making cryostanes the oldest molecular markers for animals. However, other biological sources need to be explored, including the wide variety of eukaryotic protists.

The primary mineralogy of marine carbonate precipitates has been a crucial factor in constraining the major element composition of ancient oceans. Secular changes in Phanerozoic marine chemistry, including Mg/Ca, have been well-documented using the original carbonate mineralogy of ooids, marine cements and biominerals. However, the history of Precambrian seawater chemistry is not as well constrained, partially due to the prevalence of dolomitisation in the Precambrian geological record. The Neoproterozoic (~ 1000 Ma to ~ 541 Ma) record of primary carbonate mineralogy is documented here using a combination of literature data and new analysis of marine carbonate precipitates from the Otavi Fold Belt, Namibia, the Death Valley succession, USA and the Adelaide Fold Belt, Australia. These data suggest that the last ~ 460 million years of the Proterozoic were dominated by aragonite and high-Mg calcite precipitation in shallow marine settings. In contrast, low-Mg calcite has only been recognised in a small number of formations. In addition to aragonite and calcite precipitation, marine dolomite precipitation was widespread in Neoproterozoic oceans, including mimetic (syn-sedimentary) dolomitisation and primary dolomite marine cementation. The combination of marine aragonite, high Mg-calcite and dolomite precipitation during the Neoproterozoic suggests extremely high seawater Mg/Ca conditions relative to Phanerozoic oceans. Marine dolomite precipitation may also be linked to widespread marine anoxia during this time.

A study of the K-bentonites in the Doushantuo cap dolostones on the Yangtze Platform in South China would facilitate understanding the origin of the Marinoan cap dolostones and testing the snowball Earth hypothesis. To date, sporadic work has been conducted on the whole-rock geochemistry of the K-bentonites from the platform and slope of the Yangtze Platform, to preliminarily understand the genesis of the K-bentonites. However, a study of the whole-rock geochemistry of the K-bentonites from the basin and on zircon geochemistry of the K-bentonites from the entire Yangtze Platform has not been carried out. Therefore, the nature of the primary magmas, source and tectonic setting of the source volcanoes, and the episodes of source volcanic eruptions of the K-bentonites are unknown, which hinders unraveling the possible link between the sedimentary model of the Marinoan cap dolostone and the K-bentonites. Here, we perform a systematic study of the whole-rock and zircon geochemistry of the K-bentonites. The major and trace element concentrations of the K-bentonites and trace element contents in their volcanic zircons suggest that the primary magmas of the K-bentonites from the Jiuqunao and Heyu sections are different in nature. The same geochemical results indicate that the source volcanoes of all the K-bentonites may occur in the extensional setting on the northern margin of the Yangtze Block. Hf isotopic compositions of the volcanic zircons in the K-bentonites imply that the source magmas of the K-bentonites are mainly mantle-derived, and may have mixed with the crustal materials from the northern margin of the Yangtze Block. Based on the difference in whole-rock and zircon geochemistry of the K-bentonites from Heyu and Jiuqunao, two episodes of the source volcanic eruptions of the K-bentonites could be identified. The K-bentonite at Heyu most probably represents the record of the old episode, while that at Jiuqunao represents the record of the young one. The here proposed dual episodic deposition of K-bentonites within the Doushantuo cap carbonates holds potential significance to constrain 1) the duration of the cap carbonate deposition and 2) their genesis in potential slushball or snowball Earth environments.

A simulation of Earth's climate system through geological time was produced with a view to estimating the sun's continuously habitable zone (CHZ). The model consists of four boxes (air, land, ocean, and rock) and parameterizes the carbonate-silicate cycle as a five-step process, comprising volcanism, dissolution of airborne carbon dioxide in seawater, continental weathering, neritic deposition of carbonates on the sea floor, and seafloor weathering. Weathering, surface albedo, and planetary albedo are affected by the growth of continents over time, and by changes in cloud cover with planetary rotation rate. Radiative transfer is handled with a semigray method based on conditions on preindustrial Earth and present-day Mars. The model includes a process for recovering from Snowball Earth events, involving changing surface albedo due to volcanic soot deposition. The simulation produces a runaway greenhouse effect inside 0.982 AU and “limit cycling” between temperate and freezing temperatures outside 1.18 AU. Because continental weathering is less reactive to temperature than previously thought, the results indicate that Earth is more vulnerable to catastrophic climate runaways at the outer CHZ boundary than foreseen earlier. The narrow spacing ratio (R ​= ​1.20) implies that habitable planets will be less frequent than previously expected.

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.

Snowball Earth glaciations are the most extreme climate perturbations recorded in Earth’s history. It has been argued that the termination of these events was characterized by a single rapid transition from near-global ice coverage to an ice-free greenhouse climate state. Notably, this deviates with more extended transition periods of ice sheet waxing and waning typical of Phanerozoic glaciations. Using a coupled mineralogical and Mg and Li isotopic approach, we explore the role that authigenic clay formation within the seafloor may have played on Earth’s climate during deglaciation of the Marinoan Snowball Earth event. Marine authigenic clay formation—a process referred to as reverse weathering—recycles carbon within the ocean–atmosphere system and acts to elevate atmospheric CO2 levels. The results indicate a shift towards more extensive reverse weathering within the uppermost portion of the glaciogenic Nantuo Formation in South China. Carbon cycle modeling indicates that widespread reverse weathering could have driven a protracted (millions of years) carbon dioxide drawdown following high carbon dioxide levels expected during deglaciation.

Neoproterozoic sedimentary records reveal a poorly documented, ∼150 m.y. time interval, between ∼1000 and 850 Ma, that limit our understanding of C and Sr isotope variations in seawater and thus operation of the biogeochemical carbon cycle, changes in surface redox state, and fluctuations in continental vs. hydrothermal fluxes to the oceans following assembly of the supercontinent Rodinia. Carbon and Sr isotope data for carbonates from the Karatau Group of the Southern Ural Mountains in Russia provides a record partially covering the younger portion of this time interval. The lower part of the Karatau Group (LKG) contains well-preserved carbonate strata of the Katav, Inzer, and Min’yar formations that are focus of this study. Pb-Pb isochron ages for carbonates from the Inzer and Min’yar formations are 844 ± 24 and 820 ± 77 Ma, respectively, establishing an early Tonian age. Carbon isotope data for unaltered carbonates in the LKG show a range of moderately positive to negative values from −2.8 to +5.9‰, with the majority below +3.0‰. 87Sr/86Sr values range from 0.70522 to 0.70534 in the Lower Inzer Member, increasing in the overlying Upper Inzer Member and Min’yar Formation, from 0.70555 to 0.70600. The Sr isotope range of the Lower Inzer Member is similar to that typical for the 1.03–0.95 Ga seawater (0.70519–0.70554), suggesting that the Sr isotope composition of seawater in the aftermath of Rodinian assembly was unradiogenic for 150 m.y. This pattern indicates that supercontinent-scale orogenic events do not result in an enhanced, long-term flux of radiogenic Sr from continents once the supercontinent assembled. To account for the prolonged period characterized by unradiogenic Sr seawater composition and moderate-amplitude C isotope variations, we suggest a supercontinent configuration ringed by continental arcs and with predominantly internal runoff. In this model, accompanying supercontinent blanketing resulted in thermal perturbation in the mantle, emplacement of multiple Large Igneous Provinces, and high continental freeboard. Weathering of juvenile arcs would have provided unradiogenic Sr and a high flux of sediments to the oceans, enhancing organic carbon burial and progressive oxygenation of surface environments and contributing to instability in the biogeochemical carbon cycle. Mass anomalies induced by protracted mantle plume activity led to True Polar Wander and brought the supercontinent, covered with juvenile mafic volcanics and segmented by failed rift systems, to low latitudes thus enhancing chemical weathering, atmospheric CO2 consumption, and eventually ushering in Snowball Earth conditions. Comparison with the records for early Paleoproterozoic strata indicates that a similar sequence of events bracketed both ends of the Proterozoic, highlighting fundamental relationships among plate tectonics, mantle dynamics, biogeochemical carbon cycling, surface oxygenation, and climate change.

Extremely negative δ 13 C values (as low as −48‰) of blocky calcite cements filling cavities, sheet-cracks, and fractures in the basal Ediacaran cap dolostone of the Doushantuo Formation in South China have been considered as evidence for methane release possibly associated with gas hydrate destabilization in the aftermath of the terminal Cryogenian snowball Earth glaciation. Alternatively, these extremely negative δ 13 C values may have resulted from oxidation of thermogenic methane derived from localized hydrothermal activity long after the deposition of the Doushantuo cap dolostone. The latter interpretation was based on carbonate clumped isotope thermometry suggesting that the precipitation of extremely 13 C-depleted calcite occurred at temperatures up to 476 °C (mean = 378 °C, n = 4) whereas carbonate fabrics with moderately negative δ 13 C values (generally > −10‰) were formed at much lower temperatures (mean = 112 °C, n = 4). To assess these competing hypotheses, we systematically investigated the petrography, carbonate carbon and oxygen isotopes, and carbonaceous material (CM) Raman spectroscopy of the Doushantuo cap carbonate. Raman geothermometry suggests that CM associated with different microfabrics and δ 13 C signatures—including sedimentary fabrics with moderately negative δ 13 C values (e.g., peloids, stromatolite laminae, microbial mat fragments), diagenetic fabrics with moderately negative δ 13 C values (e.g., some void-filling or crack-filling cements), and diagenetic fabrics with extremely negative δ 13 C values (e.g., void-filling or crack-filling blocky and bladed calcites)—experienced temperatures generally 13 C signatures and paleotemperatures. Whereas the origins of these extremely negative δ 13 C values remain unresolved, the Raman geothermometric data challenge the previous interpretation that they were derived from thermogenic methane related to localized hydrothermal activity at temperatures up to almost 500 °C that only affected the thermal history of the calcite with extremely negative δ 13 C values. Instead, the new data suggest that the analyzed microfabrics experienced the same peak temperatures

U-Pb dating of detrital zircons was undertaken in the matrix component of the Swartleikrans Bed (Bloupoort Formation, Gifberg Group), located in the Vredendal Outlier of western South Africa. Results from this study indicate detrital zircons were derived from both local and regional (or distal) sources. The deposit is reportedly Neoproterozoic in age, but convincing age constraints are absent. These meta-diamictitic rocks were interpreted as glacial and are correlated with other diamictites in SW Gondwana, in the wake of the snowball Earth hypothesis. While sedimentological data is interpreted as glacial, geochemical proxies (Mo, Cr, U/Th, Ce*, V) point to an oxygenated depositional environment. Thus, if the deposit was related to a snowball Earth glacial event, then deposition must have been distal to the ice sheets. Nearly 50% of the dated detrital zircons (n = 68) have crystallization ages that cannot be correlated with known sources in the locally exposed basement. These predominantly Mesoproterozoic grains were either derived from distal sources or are reworked from unknown, now eroded, sedimentary rocks. Only 14% of all concordant zircon have an Early Mesoproterozoic to Late Palaeoproterozoic age, and a single grain relates to an older Paleoproterozoic event. Results are similar to other glacial deposits in the region (further north, in Namibia). In total, sedimentological observations and detrital zircon age populations point to a large catchment area, characteristic of glacial events of regional significance. However, direct regional correlation is still not possible using maximum depositional ages indicated by detrital zircons.

Sturtian cap carbonate deposits, previously unknown in South China, are documented in this study from the base of the Datangpo Formation in the Nanhua Basin (Guizhou Province). The age of these deposits is constrained by an LA-ICP-MS U-Pb zircon age of 662.7 ± 6.2 Ma from the overlying Mn-shale layer, which is approximately coeval with the termination of the Sturtian glaciation globally. The newly discovered cap carbonates are composed of massive, laminated, pisolitic, and sandy dolomites, and formed subtidally on the summits of horsts within the Nanhua Basin. Two δ13Ccarb profiles (from the JJS-1 and ZK01 sections) show positive shifts of 4–5‰ from the base to the top of the cap carbonates with a ∼2‰ offset between the profiles. These features suggest both a large global perturbation as well as local influences on carbonate carbon isotopes during the Sturtian deglaciation, which is consistent with aspects of both the Snowball Earth and gas hydrate destabilization hypotheses for the origin of cap carbonates. The newly discovered cap carbonates are inferred to be correlative with rhodochrosite (Mn-carbonate)-bearing deposits of the basal Datangpo Formation in adjacent grabens of the Nanhua Basin. This correlation is based on (1) the equivalent stratigraphic positions of the cap carbonates and Mn-deposits, both of which are found at the base of the Datangpo Formation, and (2) consistent U-Pb dates of 667–663 Ma for tuff layers interbedded with these deposits at two locales. The Mn-deposits show significantly higher TOC content (1–3% vs

A global rise in oxygen levels has been proposed to coincide with the Ediacaran to Cambrian radiation of animals, yet the precise timing and nature of this change remains unresolved. One hypothesis is that the ocean/atmosphere system became temporarily well-oxygenated in the earliest Ediacaran, directly following the Marinoan Snowball Earth glaciation (~ 635 Ma). The evidence for oxygenation is based on large enrichments of redox-sensitive trace elements in black shale from South China presumably deposited under a euxinic water column. These enrichments meet or exceed the bulk concentrations of redox-sensitive trace elements found in Phanerozoic shale deposited under euxinic water columns. Here we test the early Ediacaran post-Snowball oxygenation hypothesis with new data from a high-resolution, multi-proxy geochemical and sedimentological study of three stratigraphic sections in the earliest Ediacaran Sheepbed Formation of the Mackenzie and Wernecke Mountains, NW Canada. Iron speciation data from all sections suggest that the local water column was dominantly ferruginous, with a notable exception of probable euxinic conditions recorded in part of one section. Redox-sensitive elements show no appreciable enrichments in the basal Sheepbed Formation, with maximum concentrations of molybdenum, vanadium, uranium and chromium only slightly above world average shale values. The lack of substantial elemental enrichments within the Sheepbed Formation is consistent with results from coeval ferruginous strata in Svalbard. The dominance of local ferruginous conditions in NW Canada cannot alone account for the muted redox-sensitive element enrichments as V, U, and Cr are thought to be enriched under these conditions, and samples deposited under euxinic conditions also lack Phanerozoic-style enrichments. These contrasting results from different localities highlight a need to further investigate the veracity of trace metal enrichments in all localities as representing global redox conditions as the results have important implications for the timing of oxygenation with respect to early animal evolution.

The Tonian Bitter Springs Group (∼830–750 Ma), within the Amadeus Basin in central Australia consists of thick halite and sulphate evaporite accumulations. The deposition of halite occurred in shallow marine, lagoon (salina) environment (Gillen Formation), and developed into sulphate-dominated supratidal sabkha during sea level regression (Johnnys Creek Formation). The overall regression was interrupted by a transgressive phase lasting at least 20 Ma and leading to deposition of basin-wide stromatolitic dolostone (Loves Creek Formation). The salinity and high evaporation is reflected in positive δ 13 C in carbonates (+4 to +6‰ VPDB) of the evaporitic units, while the shallow marine incursion of the Loves Creek Formation (−2‰ δ 13 C) show typical marine carbonate isotopic values. The Bitter Springs Group chemostratigraphy has been correlated globally and the negative excursion was named previously after this unit (Bitter Springs Stage anomaly). However, the mechanism of evaporation-driven fractionation of δ 13 C is different from the commonly proposed inorganic-organic carbon fractionation, and challenges the views on interpreting global chemostratigraphic anomalies or excursion and their cause. Evaporite basins covered vast regions worldwide prior to the Sturtian glaciation, e.g. the Australian evaporites would have covered a third of the continent. The light surface of evaporites and associated carbonates would have had a high albedo and effectively cause less surface heat absorption. This subsequently would have triggered temperature decrease on a continental and possibly global scale. This paper hypothesises that the deposition of evaporites worldwide would have contributed to global cooling starting ∼100 Ma prior to Snowball Earth and would have played an important role in global glaciation.

The Francevillian Group in Gabonese Republic was recently established as a typical sedimentary sequence for the Paleoproterozoic. However, its age is rather poorly constrained, mainly based on Rb-Sr and Nd-Sm datings. This study reports new zircon data obtained from Chaillu massif and N'goutou complex, which constrain the protolith age of the basement orthogneisses and the igneous age of an intrusive granite, respectively. Most zircons from the orthogneisses are blue and exhibit oscillatory zoning in cathode-luminescence images. Zircons with lower common lead abundances tend to be distributed close to the concordia curve. Two age clusters around 2860 Ma and 2910 Ma are found in zircons plotted on the concordia curve. Based on the Th/U ratios of zircons, these ages correspond to the protolith ages of the orthogneisses, and the zircons are not metamorphic in origin. Syenites and granites were collected from the N'goutou complex that intrudes into the FA and FB units of the Francevillian Group. The granitoids exhibit chemical composition of A-type granite affinity. Half of zircons separated from the granite are non-luminous, and the remaining half exhibit obscure internal textures under cathode-luminescence observation. All zircon grains contain significant amounts of common lead; the lead isotopic variability is probably attributed to the mixing of two components in the zircons. The zircon radiogenic 207 Pb/ 206 Pb ratio is 0.13707 ± 0.0010, corresponding to a 207 Pb/ 206 Pb age of 2191 ± 13 Ma. This constrains the minimum depositional age of the FA and FB units. Furthermore, the FB unit consists of manganese-rich carbonate rocks and organic carbon-rich black shales with macroscopic fossils. Based on our age constraints, these organisms appeared in the study area just after the last Paleoproterozoic Snowball Earth event, in concert with global scale oxidation event encompassing the Snowball Earth.

In the Neoproterozoic Era, the Earth experienced two broad intervals of global glaciation, commonly known as Snowball Earth. There was also a rapid diversification of life, with the evolution of most of the eukaryotic lineages. Here, salient evidence for the Neoproterozoic global glaciations, including the carbon isotope record, is reinterpreted and an alternative explanation for the causes of glaciation is provided. The proliferation of life could have led to increases in atmospheric O2 levels and concomitant decreases in CO2 and CH4. Coupled biochemical and geochemical changes would have led to global cooling and glaciation. This so-called biotic hypothesis of the Snowball Earth is consistent with the most salient features of the reported evidence and explains the consecutive global glaciations.

This work was supported by a grant from the Simons Foundation (SCOL 611878, ASCA) and the Carlsberg Foundation to ASCA. ASCA and CJB also acknowledge support from the Danish National Research Foundation (Grant No. DNRF53). ACM and CVR acknowledge support from NSF (EAR-0842946) for funding fieldwork on the Trezona Formation in South Australia. JAH acknowledges support from NSF (IES-1410317) and from NSF OCE CAREER Grant (1654571). The Trezona carbon isotope excursion is recorded on five different continents in platform carbonates deposited prior to the end-Cryogenian Marinoan glaciation (>635 Ma) and represents a change in carbon isotope values of 16–18‰. Based on the spatial and temporal reproducibility, the excursion previously has been interpreted as tracking the carbon isotopic composition of dissolved inorganic carbon in the global ocean before the descent into a snowball Earth. However, in modern restricted shallow marine and freshwater settings, carbon isotope values have a similarly large range, which is mostly independent from open ocean chemistry and instead reflects local processes. In this study, we combine calcium, magnesium, and strontium isotope geochemistry with a numerical model of carbonate diagenesis to disentangle the degree to which the Trezona excursion reflects changes in global seawater chemistry versus local shallow-water platform environments. Our analysis demonstrates that the most extreme carbon isotope values (∼-10‰ versus +10‰) are preserved in former platform aragonite that was neomorphosed to calcite during sediment-buffered conditions and record the primary carbon isotope composition of platform-top surface waters. In contrast, the downturn and recovery of the Trezona excursion are recorded in carbonates that were altered during early fluid-buffered diagenesis and commonly are dolomitized. We also find that the nadir of the Trezona excursion is associated with a fractional increase in siliciclastic sediments, whereas the recovery from the excursion correlates with a relative increase in carbonate. This relationship suggests that the extreme negative isotopic shift in platform aragonite occurred in concert with periods of increased input of siliciclastic sediments, changes in water depth, and possibly nutrients to platform environments. Although the process for generating extremely negative carbon isotope values in Neoproterozoic platform carbonates remains enigmatic, we speculate that these excursions reflect kinetic isotope effects associated with CO2 invasion in platform waters during periods of intense primary productivity. Publisher PDF

Otavi Group is a 1.5–3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran−Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S−N crustal stretching (770–655 Ma), followed by post-rift thermal subsidence (655–600 Ma). Rifting under southern Swakop Group continued until 650–635 Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S−N plate divergence. Rift-related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746 Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770–717 Ma; (2) glacial early Cryogenian/Sturtian, 717–661 Ma; (3) non-glacial middle Cryogenian, 661–646 ± 5 Ma; (4) glacial late Cryogenian/Marinoan, 646 ± 5–635 Ma; and (5) non-glacial early Ediacaran, 635–600 ± 5 Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585 measured sections (totaling >190 km of strata) and > 8764 pairs of δ13C/δ18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5–9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3–8.8 m Myr−1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ≤1.6 km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9 m regional average thickness, n = 140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ≤90 m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ≥0.6 km of paleobathymetric relief. Late Tonian shallow-neritic δ13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770–760 Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737–717 Ma) in Swakop Group. Devede Fm is isotopically heavy, +4–8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746 Ma volcanics and shows two negative excursions bridged by heavy δ13C values. The negative excursions could be correlative with Russoya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic δ13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling-stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0–3.5‰. Early Ediacaran neritic δ13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (≤ + 11‰) with extreme point-to-point variability (≤10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform δ13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9 ± 1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60–0.58 Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ≥1.85 km of megakarstic relief, and km-thick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.

Cryogenian low-latitude glaciations mark the ‘Snowball Earth’ events and were thought to be dynamic on the basis of sedimentary features, especially glacial cycles. Although the glaciation dynamics and associated environmental changes can be investigated through the sedimentary records, there are sparse sedimentary studies. Studies from the Marinoan deposits in South China provide the basis for establishing a sequence stratigraphic framework that includes two complete glacial cycles across a basin-to-platform transect. Both cycles are characterized by symmetrical facies patterns, with a lower ice-advance succession and an upper ice-retreat succession. Each ice-advance succession consists primarily of a massive diamictite lithofacies association, and is dominated by proximal glaciomarine deposits. The gravel abundance and size of the deposits increase upward gradually, suggesting a progradational sequence. Each ice-retreat succession is composed of both a massive diamictite lithofacies association and a fine-grained lithofacies association, representing a mix of distal glaciomarine, proximal glaciomarine, and non-glacial marine deposits. The gravel abundance and size of these varied deposits decrease upward gradually, interpreted to reflect a retrogradational sequence. This sequence stratigraphic framework, interpreted to reflect gradual alternation from ice advance to ice retreat, indicates dynamic co-evolution of the sedimentary environments, paleoclimate, and sea level during the Marinoan glaciation. The glaciation dynamics, as they are reflected globally, highlight a gradual transition from the Marinoan glaciation to the Ediacaran greenhouse, and indicate a time lag between the onset of the deglaciation and the formation of the cap carbonates.

Rapid diversification of eukaryotes immediately after the Marinoan Snowball Earth glaciation suggests the possible linkage between the extreme icehouse climate and biological evolution. To unravel how the severe ice-age had triggered biological evolution, it is essential to retrieve the postglacial environmental background. Cap carbonate that directly overlies the glacial deposit represents the earliest chemical deposition after the Snowball Earth glaciation, and thus records the postglacial seawater compositions. In the Yangtze Block, South China, the Nantuo Formation represents the deposition of the Marinoan Snowball Earth glaciation, and the overlying cap carbonates of the basal Doushantuo Formation have nearly identical thickness and similar lithological sequence and widely outcropped from the shelf to basin environments. In this study, we report carbonate associated Fe (Fecarb) and carbonate associated Mn (Mncarb) contents of the Doushantuo (Marinoan) cap carbonate samples collected from 13 outcrop sections and 2 drill cores, spanning from the shelf to basin environments of the Yangtze Block. In addition, we also analyzed Fe and Mn contents in siliciclastic deposits (Feclast and Mnclast) of the upper Nantuo Formation. The Doushantuo cap dolostone have significantly higher Mncarb but comparable Fecarb values, and thus show extremely high Mncarb/Fecarb ratios, compared with dolostone of other ages. This observation suggests precipitation of the Doushantuo cap dolostone from Mn-enriched yet Fe-lean seawater after the Marinoan Snowball Earth glaciation. The Mn-enrichment in the postglacial ocean might be attributed to a continuous hydrothermal Mn2+ flux for ~15 million years during the Snowball Earth. The hydrothermal-derived Mn2+ could be accumulated in the Snowball Earth ocean due to the absence of synglacial carbonate precipitation and the persistent reduced condition during the Snowball Earth glaciation. In addition, the postglacial remobilization of MnO2 from glacial sediments might have been fueled by the recovery of surface ocean productivity, contributing to the postglacial seawater Mn2+ inventory. MnO2 remobilization is supported by the lower Mnclast values of the top of Nantuo Formation. Our study supports the delayed cap carbonate precipitation after the melting of the Marinoan Snowball Earth glaciation, and the postglacial seawater was suboxic and enriched in Mn2+ but not Fe2+ throughout the Yangtze Block. Thus, the Ediacaran ocean oxidation might have predated the evolution of eukaryotes or the marine redox condition during cap carbonate precipitation might have been oxic enough for the evolution of eukaryotes, though there is a slightly delayed appearance of Doushantuo-type fossils in the stratigraphic record.

Increasing sedimentary records and non-glacial facies reported from various regional glaciations suggest diverse interpretation against the ‘hard’ snowball Earth. At Shennongjia, in northern Yangtze Craton, glaciogenic strata of the Nantuo Formation (~649–635 Ma) are widely outcropped, which represents the Marinoan glacial record. In this study, sedimentary logging of eight high-quality exposed sections of the Nantuo Formation were systematically conducted. Four facies associations were recognized: (i) diamictites, (ii) heterolithics, (iii) lonestone-bearing siltstone, and (iv) clast-free siltstone. The succession reveals significant spatial and lateral facies variation, therefore, the stratigraphic framework of Nantuo Formation could be compartmentalized into western, central and eastern types. Based on the facies associations analysed, the sequence stratigraphic method was applied to determine ice advance-retreat cycles in the region. Our data suggest the ice dynamic evolution is three-fold, comprising: (i) ice sheet moved from continental environment from eastern to the subglacial grounding zone in the western Shennongjia features first ice advance-retreat cycle, which is recorded by the lower member of Nantuo Formation; (ii) the second glacial episode is illustrated by glaciolacustrine and non-glacial lacustrine deposits in periglacial environment, which is indicated by the middle member of Nantuo Formation in western Shennongjia, though situation in rest parts of Shennongjia is unclear; (iii) the appearance of ice rafted deposits (IRD) in both western and eastern Shennongjia suggests ice-distal environments, alongside the siltstone on the top Nantuo Formation, representing final deglaciation period. Altogether three glacial advance systems tracts (GAST) were identified, separated by three glacial retreat systems tracts (GRST). Meanwhile, a general east–west direction ice sheet movement could also be reconstructed. Besides, based on sedimentology, notably the stacking patterns and clast character, we tentatively interpret the Nantuo glacier as the transitional type between temperate and polythermal glacier.

Giant domal stromatolites associated with vertical tubestone are reported for the first time in the Marinoan (635 Ma) Puga cap dolostone on the southern Amazon Craton, Brazil. The well-preserved giant domal mounds reach 12 m in diameter and 10 m in height, forming laterally continuous biostromes of at least 200 m. The giant mounds occur mainly on the top cap carbonate succession overlying stratiform stromatolites directly underlying glacial diamictites marking the Cryogenian-Ediacaran boundary. We discuss the sedimentological and biological conditions during the greenhouse period that led to the development of these exceptional stromatolitic features. The microbial communities primordially colonized the diamictite substrate, forming stratiform bioconstructions in a shallow platform under hypersaline calm water conditions. The transition from stratiform to domal morphology reflects the progressive deepening of the platform, concomitant with increases in microbial activity. The high production and pre-lithification of carbonate muds, that formed a rigid load-resistant framework, allowed for the accumulation by vertical accretion, increasing the potential for preservation of giant mounds. The progressive generation of accommodation space related to sea-level rises and glacial isostatic adjustment (GIA) caused subsidence in the coastal zone, as proposed by the Snowball Earth hypothesis. The maximum flooding was associated with an increase in siliciclastic inflow, causing burial of microbial communities and the demise of giant stromatolites. The succession was succeeded by the deposition of long-term transgressive lime mud in a CaCO3-oversaturated sea. The giant stromatolites consist of a new element of the Marinoan Puga cap carbonate and provide essential insights to enlarge the discussion about the biological and sedimentological interaction that occurred on post-glacial carbonate platforms worldwide.

Zinc and cadmium are important biologically cycled nutrients in the modern ocean. Recent analytical advances and sampling efforts such as GEOTRACES have led to measurements of the global distribution of Zn and Cd and their stable isotope ratios (δ66Zn and δ114Cd), providing an understanding of how biological processes cycle these elements and their isotopes in the modern ocean. This opens the door to better application of δ66Zn and δ114Cd as tracers of biogeochemical processes archived in the geological record. Here we present new measurements of [Cd] and δ114Cd on samples from the Nuccaleena cap dolostone in South Australia, which was deposited during deglaciation of the ca. 635 Ma old Marinoan snowball Earth. Our new data are combined with previously measured δ66Zn data from the same samples. The combined record of δ114Cd and δ66Zn is used to explore various hypotheses about the sources and sinks of Cd and Zn in the post-Marinoan ocean. Key features of this record are that [Zn] and [Cd] remain relatively similar throughout, that δ66Zn and δ114Cd change at the same stratigraphic level and in the same direction, and that δ66Zn changes are about four times larger than δ114Cd changes even though both quantities reflect a similar fractional mass difference. We suggest that both Zn and Cd concentration and isotopic data are consistent with a change in the sink by which Zn and Cd are removed from the oceans. We propose that no isotope fractionation is expressed during removal of Zn and Cd as sulfides, while Zn and Cd removed with biological material are about 0.6‰ and 0.2‰ lighter than deep ocean δ66Zn and δ114Cd, respectively. Many previous studies attribute the burial of isotopically light biological Zn to preferential assimilation of isotopically lighter Zn by phytoplankton. In contrast, we propose that lighter biological δ66Zn occurs mostly because isotopically heavy Zn was scavenged by organic matter, which causes dissolved δ66Zn in the surface ocean to be lighter than deep ocean δ66Zn. The organic matter originated from primary producers that thrived in the surface ocean during deglaciation. For Cd, whose isotope cycling in the modern ocean is dominated by preferential assimilation of isotopically lighter Cd by phytoplankton, much less biological fractionation is expressed. The combined analyses of δ66Zn and δ114Cd provide insight into the biogeochemical processes which occurred after the Marinoan snowball Earth glaciation, and may be a useful new tool to explore the history of life on Earth in other geological records as well.

The appearance and radiation of animals are commonly attributed to Neoproterozoic oceanic oxygenation, yet independent geochemical evidence for such an event remains equivocal. Strata deposited between the Sturtian and Marinoan Snowball Earth glaciations (660 to 640 Ma) contain the earliest animal biomarkers and possible body fossils. To quantify the extent of seafloor oxygenation during this critical interval, we present uranium isotope ratios (238U/235U denoted as δ 238 U ) from limestone of the Taishir Formation in Mongolia through two stratigraphic sections that are separated by ∼75 km within the same depositional basin. Above the Sturtian glacial deposits, through ∼150 m of stratigraphy, δ 238 U compositions have a mean value of −0.47‰. This interval is followed by a ∼0.3‰ decrease in δ 238 U , coincident with the Taishir negative carbon isotope excursion. Thereafter, δ 238 U values remain relatively low until the erosional unconformity at the base of the Marinoan glacial deposits. Using a box model, we show that the best explanation for the higher δ 238 U values of the post-Sturtian limestones is extensive—but temporary—oxygenation of the seafloor, and is inconsistent with a scenario involving only increased delivery of uranium to the oceans due to post-Snowball weathering. The decline in δ 238 U in overlying strata, coincident with the Taishir negative δ 13 C excursion, indicates a subsequent decrease in seafloor oxygenation. The U isotopic data, combined with modeling results, challenge the notion of a simple, unidirectional oxygenation of Neoproterozoic oceans.

The upper carbonate-rich parts of the West Congo Supergroup (~ 1000–560 Ma) from the Democratic Republic of the Congo have hitherto been considered as a record of abrupt eustatic and climatic events accompanying glaciation and deglaciation of a Snowball Earth-type Marinoan ice age that was of global extent. These strata have however never been investigated in detail. Results of new sedimentological work at key outcrops over a 1300 km outcrop belt show that pre- and post-Marinoan carbonates are respectively, storm-influenced sediments deposited principally in a mid/outer-ramp setting, and deep-water slope carbonates (calicturbidites) representing a lobe-fringe or levee-overbank setting. The Upper Diamictite Formation held previously by some to be a subglacial tillite, comprises gravity flows (debrites) deposited in deep water below wave base along the unstable margins of a carbonate ramp. A direct glacial influence on sedimentation for diamictites or any accompanying facies cannot be readily identified. Sedimentary facies reported here primarily record the presence of deep-water submarine to alluvial fan systems related to extensional tectonic processes of the central-southern Macaubas Basin (now located in Brazil) between 700 Ma and 660 Ma followed by the 630-Ma onset of the pre-collisional magmatic arc in the Aracuai-West Congo Orogen. No extreme short-lived climatic or eustatic events of a Snowball Earth-type ice age are recorded in the studied succession, which primarily reflects long-term overriding regional tectonic controls resulting in diachronous sedimentation along the western margin of the Congo Craton.

The tides of an ice-covered ocean are examined using a Cartesian representation of the elastic and fluid equations. Although unconstrained by any observations, the ocean tides of a Neoproterozoic “snowball” Earth could have been significantly larger than they are today. Time-mean tidal-residual circulations would then have been set up that are competitive with the circulation driven by geothermal heating. In any realistic configuration, the snowball Earth would have had an ice cover that is in the thin shell limit, but by permitting the ice thickness to become large, more interesting ice tidal response can be found, ones conceivably of application to bodies in the outer Solar System or hypothetical exoplanets. Little can be said concerning a reduction in tidal dissipation necessary to avoid a crisis in the history of the lunar orbit.

Atmospheric CO2 levels and global climate are regulated on geological timescales by the silicate weathering feedback. However, this thermostat has failed multiple times in Earth's history, most spectacularly during the Cryogenian (c. 720–635 Ma) Snowball Earth episodes. The unique middle Neoproterozoic paleogeography of a rifting, low-latitude, supercontinent likely favored a globally cool climate due to the influence of the silicate weathering feedback and planetary albedo. Under these primed conditions, the emplacement and weathering of extensive continental flood basalt provinces may have provided the final trigger for runaway global glaciation. Weathering of continental flood basalts may have also contributed to the characteristically high carbon isotope ratios ( δ 13 C ) of Neoproterozoic seawater due to their elevated P contents. In order to test these hypotheses, we have compiled new and previously published Neoproterozoic Nd isotope data from mudstones in northern Rodinia (North America, Australia, Svalbard, and South China) and Sr isotope data from carbonate rocks. The Nd isotope data are used to model the mafic detrital input into sedimentary basins in northern Rodinia. The results reveal a dominant contribution from continental flood basalt weathering during the ca. 130 m.y. preceding the onset of Cryogenian glaciation, followed by a precipitous decline afterwards. These data are mirrored by the Sr isotope record, which reflects the importance of chemical weathering of continental flood basalts on solute fluxes to the early–middle Neoproterozoic ocean, including a pulse of unradiogenic Sr input into the oceans just prior to the onset of Cyrogenian glaciation. Hence, our new data support the hypotheses that elevated rates of flood basalt weathering contributed to both the high average δ 13 C of seawater in the Neoproterozoic and to the initiation of the first (Sturtian) Snowball Earth.

Paleogeographic data indicate that the Neoproterozoic Rapitan Group accumulated in two spatially related, silled basins during the first mid-Neoproterozoic glacial episode (ca. 711 Ma), yet the controls on glaciation, reason for basin development, spatio-temporal relationships among stratigraphic units, and constraints on iron formation deposition and distribution in this region and globally at this time have not been adequately explained. The lack of a coherent understanding of the group's regional stratigraphy has been a significant barrier to deciphering the geologic history of this critically important succession. Based on new stratigraphic data, and incorporating the results of a related geochemical study, the Rapitan Group accumulated in tectonically complex sub-basins of a young rift zone. The spatial and temporal dynamics of these sub-basins controlled the grain size, volume, and distribution of detrital sediment (predominantly turbidites and diamictites) deposited during two glacially influenced episodes with ice cover. During an interglacial episode, the rotated, exposed upper surfaces of newly displaced rift blocks directed drainage away from basin margins, precluding significant clastic sediment delivery, and subaqueous margins of outboard tilted fault blocks acted as sills that isolated local bottom water, allowing the accumulation of an entirely hydrogenous succession (jaspilitic and hematitic iron formation). Two strike-parallel normal faults that strongly controlled basin-margin development are prominent thrust faults in the present-day orogen. Syndepositional activity of strike-normal transfer faults segmented the basins along their length, as reflected in pronounced along-strike variations in the presence/absence, thickness, and composition of the three stratigraphic units (Mt. Berg, Sayunei, and Shezal formations), and in the presence or absence of iron formation (locally >100 m thick; top of Sayunei Formation). Differential subsidence in actively rifting basins was essential for deposition of Rapitan-type iron formation, but glaciation, although coeval with Rapitan Group accumulation, was not a strong control. The timing of the onset of glacially influenced sediment deposition in the closely spatially related Rapitan (similar to 711 Ma) and Mount Harper groups (similar to 717 Ma) in northwestern Laurentia differs by 6 m.y., arguing for diachronous Sturtian glaciation, both locally and globally. (C) 2016 Elsevier B.V. All rights reserved.

We present stable isotope and chemical data for 206 Precambrian bulk shale and tillite samples that were collected mostly from drillholes on all continents and span the age range from 0.5 to 3.5 Ga with a dense coverage for 2.5–2.2 Ga time interval when Earth experienced four Snowball Earth glaciations and the irreversible rise in atmospheric O2. We observe significant, downward shift of several ‰ and a smaller range of δ 18 O values (7 to 9‰) in shales that are associated with the Paleoproterozoic and, potentially, Neoproterozoic glaciations. The Paleoproterozoic samples consist of more than 50% mica minerals and have equal or higher chemical index of alteration than overlying and underlying formations and thus underwent equal or greater degrees of chemical weathering. Their pervasively low δ 18 O and δ D (down to − 85 ‰ ) values provide strong evidence of alteration and diagenesis in contact with ultra-low δ 18 O glacial meltwaters in lacustrine, deltaic or periglacial lake (sikussak-type) environments associated with the Paleoproterozoic glaciations. The δ D silicate values for the rest of Precambrian shales range from −75 to − 50 ‰ and are comparable to those for Phanerozoic and Archean shales. Likewise, these samples have similar ranges in δ 13 C org values (−23 to − 33 ‰ PDB) and Corg content (0.0 to 10 wt%) to Phanerozoic shales. Precambrian shales have a large range of δ 18 O values comparable to that of the Phanerozoic shales in each age group and formation, suggesting similar variability in the provenance and intensity of chemical weathering, except for the earliest 3.3–3.5 Ga Archean shales, which have consistently lower δ 18 O values. Moreover, Paleoproterozoic shales that bracket in age the Great Oxidation Event (GOE) overlap in δ 18 O values. Absence of a step-wise increase in δ 18 O and δ D values suggests that despite the first-order change in the composition of the atmosphere, weathering cycle was not dramatically affected by the GOE at ∼2.4–2.3 Ga. Shales do not show comparable δ 18 O rise in the early Phanerozoic as is observed in the coeval δ 18 O trends for cherts and carbonates. There is however a sharp increase in the average δ 18 O value from the Early Archean to the Late Archean followed by a progressively decelerating increase into the Phanerozoic. This decelerating increase with time likely reflects declining contribution of mantle-extracted, normal- δ 18 O crust and lends support to crustal maturation and increasing 18O sequestration into the crust and recycling of high- δ 18 O (and 87Sr/86Sr) sedimentary rocks. This secular increase in the δ 18 O composition of the continental crust could have also had a mild effect on seawater δ 18 O composition.

High concentrations of extraterrestrial iridium have been reported in terminal Sturtian and Marinoan glacial marine sediments and are used to argue for long (likely 3–12 Myr) durations of these Cryogenian glaciations. Reanalysis of the Marinoan sedimentary rocks used in the original study, supplemented by sedimentary rocks from additional terminal Marinoan sections, however, does not confirm the initial report. New platinum group element concentrations, and 187Os/188Os and 3He/4He signatures are consistent with crustal origin and minimal extraterrestrial contributions. The discrepancy is likely caused by different sample masses used in the two studies, with this study being based on much larger samples that better capture the stochastic distribution of extraterrestrial particles in marine sediments. Strong enrichment of redox-sensitive elements, particularly rhenium, up-section in the basal postglacial cap carbonates, may indicate a return to more fully oxygenated seawater in the aftermath of the Marinoan snowball earth. Sections dominated by hydrogenous osmium indicate increasing submarine hydrothermal sources and/or continental inputs that are increasingly dominated by young mantle-derived rocks after deglaciation. Sedimentation rate estimates for the basal cap carbonates yield surprisingly slow rates of a few centimeters per thousand years. This study highlights the importance of using sedimentary rock samples that represent sufficiently large area–time products to properly sample extraterrestrial particles representatively, and demonstrates the value of using multiple tracers of extraterrestrial matter.

It has been long recognized that glacial episodes can affect the δ 18 O value of ocean water, where preferential storage of 16O in ice changes the 18O/16O ratio of the ocean. However, these effects are generally thought of as transient, as Cenozoic glaciation has had neither the magnitude or duration to cause long-term change with ocean water buffered to values close to 0±2‰ VSMOW by tectonic processes. The Snowball Earth glaciations of the Cryogenian have the potential to cause much larger changes in ocean water δ 18 O values due to their increased ice volume and long duration relative to Cenozoic glaciation, but these effects have not been previously investigated. Here, I use a numerical box model to investigate ocean water δ 18 O values over the Proterozoic and Phanerozoic. The model simulates various temperature and tectonics dependant fluxes of 18O, while also incorporating a zero-dimensional climate model and ice volume component to model glacial cycles. Monte Carlo simulations of the Sturtian and Marinoan glaciations reveal that these had the potential to alter ocean water δ 18 O values for hundreds of millions of years after the termination of glaciation, providing a mechanism for secular change in the δ 18 O value of ocean water. This occurs as a very large volume of ice (presumably, but not necessarily 18O depleted) is sequestered from the ocean, causing the ocean to become enriched enough in 18O for exchange at mid-ocean ridges to remove 18O from the ocean and slowly change the overall ocean water δ 18 O value. If Snowball Earth ice volumes were as large as proposed (∼28-32% of ocean volume), present day values of ice δ 18 O would cause significant secular change in ocean water δ 18 O extending into the Phanerozoic. An additional finding of this work is that the duration of the Sturtian glaciation required a very low CO2 degassing rate on the order of ∼2 Tmol/year, significantly less than that estimated from most other mass balance approaches for the Phanerozoic.