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• First convincing evidence of the glacial diamictite-cap carbonate couplet from the Western Himalaya. • The glacial diamictite-cap carbonate succession were deposited in a rift basin rather than a foreland basin. • New understanding of the late Neoproterozoic stratigraphic framework and correlation between the Western and Central Himalaya. One of the central tenets of the snowball Earth hypothesis is the global recognition of the glacial diamictite-cap carbonate couplets in the Cryogenian sedimentary record. However, the scarcity of such peculiar stratigraphic elements in the Neoproterozoic successions of the Himalaya brings into question the severity and global extent of the snowball Earth event and likewise hampered glacio-stratigraphic correlation within the Himalaya and elsewhere. This study provides the first convincing evidence of the glacial diamictite-cap carbonate couplet from the Tanakki Member of the basal Kakul Formation, previously unknown from the Western Himalaya in North Pakistan. Detailed sedimentological analysis of the diamictite from the Tanakki Member reveals deposition in glacially-influenced proximal to distal subaqueous debris apron. The presence of glaciogenic clasts (striated, facetted and bullet-shaped) together with evidence of the ice-rafted dropstones in pervasive facies association provides credence to the glaciogenic affinity. The thin cap carbonate (herein referred to as Tanakki-cap dolomite; TCD) overlying the glacial diamictite record deposition in a deeper shelf (offshore) setting. The lithological, depositional and persistent negative C-isotope characteristics (ca. −3.2 to −5.8‰) combined with regional stratigraphic and available geochronological data allow us to interpret TCD as a 'Marinoan' cap carbonate and the underlying diamictite as an expansion of the terminal-Cryogenian (Marinoan) glaciation in the Western Himalaya. Moreover, the analyses of the tectonic and depositional history of the Tanakki Member coupled with the Neoproterozoic paleogeographic evolution of the northern margin of the Indian Plate argue against a previous interpretation of the culminating foreland basin orogeny and instead support deposition in an extensional fault-controlled rift basin. Finally, this study permits us to revise the Neoproterozoic stratigraphic framework of the Western Himalaya by describing the Cryogenian-Ediacaran boundary interval in the region that ultimately helps to overcome the previous glacio-stratigraphic discrepancies in the Neoproterozoic record of the Himalaya. [ABSTRACT FROM AUTHOR]

During the Neoproterozoic snowball Earth events, the climate was cold and the oceans were covered by marine ice of ~1000 m thick (sea glacier). Extremely high CO 2 level was required in order to trigger the deglaciation these events. It is unclear how long it would take for the sea-ice cover to be completely ablated after the deglaciation started, and what the physical state of the ocean looked like. Here we use a fully coupled general circulation model, CCSM3, to evaluate the rate of poleward retreating of the marine ice and estimate an upper-bound timescale for the marine deglaciation. It is found that the deglaciation will take at most ~300 to 1500 years, depending on the surface albedo of ice; as the albedo decreases, the deglaciation will be prolonged due to the lower CO 2 level required to start the deglaciation. Such fast calving/melting of sea ice allows a fresh water lid of ~800 m to develop at the end of the deglaciation. • The sea-glacier retreating of snowball Earth is studied using a climate model • The complete ablation of the thick marine ice takes at most ~300 yr to 1500 yr • The elapsed time is longer if the sea-ice albedo is lowered, e.g., by dust • A fresh water lid of hundreds of meters thick would form upon completion [ABSTRACT FROM AUTHOR]

• A numerical model is applied to constrain seawater sulfate levels. • Low seawater sulfate concentrations at the onset of Marinoan glaciation. • Weak oxidative weathering is responsible for sulfate deficient ocean. The microbial sulfate reduction plays a significant role in regulating marine organic matter degradation. Thus the variation of ancient marine sulfate levels is closely coupled with the changes in global organic carbon burials and Earth's surface redox evolution in geological time. It is widely assumed that seawater sulfate concentration was deficient during the anoxic Cryogenian Marinoan glaciation (ca.650–635 Ma) which represents the most extreme icehouse condition in Earth's history. However, such a low seawater sulfate level has not been explicitly confirmed. To address this, here we carried out pyrite content and sulfur isotope (δ34S py) analysis of the Cryogenian non-glacial (the Datangpo Formation) and the Marinoan glacial successions (the Nantuo Formation) in South China. The Datangpo and Nantuo Formation show relatively high values of δ34S py , ranging from +19.9 ‰ to +32.9 ‰, and low values of pyrite content (mean = 0.72 wt%). These pyrites are dominated by euhedral crystals with a rare occurrence of framboids, indicating an early diagenetic origin, i.e., pyrite formed in sediments with sulfate supplied from seawater. Using and a one-dimensional diffusion–advection-reaction (DAR) model, we calculate marine sulfate levels. The modeling results show that the seawater sulfate concentration is <1 mM during the transition from the top Datangpo to the basal Nantuo Formation, suggesting a shrink of the marine sulfate reservoir at the onset of Marinoan glaciation. We propose that the low marine sulfate concentration was resulted from a decrease in continental chemical weathering. This study provides a quantitative constraint on marine sulfate levels during the Cryogenian snowball Earth event, and it may shed new light on our understanding of the marine biogeochemical changes in the Neoproterozoic. [ABSTRACT FROM AUTHOR]

Citation of the inferred glacial diamictite-cap carbonate association, as the unequivocal evidence of the Snowball Earth hypothesis, fails to explain the absence of the sedimentary record of the intermediate period between the onset of thawing and the restoration of the optimum temperature for carbonate deposition. The Krol Sandstone-black shale association of the Lesser Himalayan Neoproterozoic succession, represents the initial phase of that hothouse aftermath. The black shale is the product of oceanic anoxia developed during the initial thawing phase of late Cryogenian glaciation. As the melting process continued, remobilization of some pre-existing aeolian sand sheet deposit, lying on the melting ice sheet in the upper shelf region, led to the accumulation of deeper shelf fans, the Krol Sandstone, over this black shale. The petrographic composition, detrital zircon age, and the palaeoflow directions, in conjunction with the available reconstructed paleogeography, suggest that the constituent sand particles of Krol Sandstone were possibly derived from a Late Neoproterozoic aeolian sand sheet of South Australia formed during the coldest phase of the late Cryogenian Elatina glaciation. These findings not only document the sedimentary record of the initial phase of the hothouse aftermath, but also open up a new line of evidence for paleogeographic reconstruction. • Neoproterozoic Krol succession constitutes the MBT sheet in the Lesser Himalaya of Himachal Pradesh. • Krol Sandstone and Pyritic black shale constitute a major part of Krol succession. • Black shale is the product of oceanic anoxia developed during the initial thawing phase of late Cryogenian glaciation. • Krol Sandstone represents a sandy turbidite succession derived from the aeolian Whyallah Sandstone of South Australia. • The Krol Sandstone-black shale association represents the initial phase of that hothouse aftermath. [ABSTRACT FROM AUTHOR]

Highlights: [•] New carbonate sections deposited in the outer platform of the Ediacarian Araras platform are described. [•] Facies analyses at the platform scale indicate a transgressive systems track deposition of the lower Ediacarian. [•] Measurements of trace metals, pyrite and organic carbon contents are compatible with a dominantly oxygenated post-Marinoan ocean. [Copyright &y& Elsevier]

Abstract: Existing correlation schemes between early Paleoproterozoic successions divorce the low-latitude Makganyene glaciation in southern Africa from the Great Oxidation (GO), as recorded by the disappearance of mass-independent fractionation of S-isotopes (MIF-S) in sedimentary sulfide and sulfate minerals. They also suggest a younger age for the GO in southern Africa (~2.3Ga) compared with North America (~2.4Ga), which is physically implausible. A new correlation scheme is proposed in which the Makganyene glaciation is temporally linked to the GO and to the second of three Huronian glaciations in North America, the one with postglacial cap-carbonate. In the new scheme, only three glacial epochs are needed globally, all three are represented in southern Africa, and the second was a circa 2.40Ga snowball Earth coincident with the GO. [Copyright &y& Elsevier]

Abstract: An experimental study was carried out to assess the abiotic natural reactions associated with carbonate precipitation leading to the formation of Neoproterozoic cap carbonate sequences. For this purpose, synthetic seawater solutions were prepared reflecting the presumed composition of Neoproterozoic seawater. These solutions were heated to 40°C, undergoing degassing and/or evaporation, while monitoring their chemical composition (sulphate concentration, Mg/Ca molar ratio) and identifying the precipitated minerals. The experimental results are in good agreement with previous studies, allowing us to constrain the type of carbonate precipitates formed in the ocean during the Snowball Earth aftermath. Among the phases precipitated during heating of the solutions, Mg-bearing carbonate minerals (i.e. Mg-calcite, magnesite and dolomite) remain of minor importance compared to Ca-carbonates (especially aragonite). Consequently, the basal dolostone units of the cap carbonates (i.e. cap carbonates s.s.) were originally in the form of CaCO3, and evolved spontaneously toward dolomite. Cap carbonates s.l. (i.e. cap limestones) containing aragonitic cement could result from the degassing of the whole ocean and the concomitant precipitation of aragonite. The calculated thicknesses of carbonates formed in this way are of the order of several metres, in line with the field observations. Although this process of abiotic carbonate precipitation does not rule out biologically-mediated carbonate formation, it may represent an alternate route to produce carbonate, particularly in environments which are not yet colonized by bacteria after the global ice melting. [Copyright &y& Elsevier]

Highlights: [•] Sr isotope stratigraphy is analyzed for the Ediacaran mid-oceanic paleo-atoll limestone in Siberia. [•] 87Sr/86Sr ratios vary mostly between 0.7069 and 0.7079. [•] The basal Baratal limestone was deposited in the Early Ediacaran immediately after Marinoan snowball Earth event. [Copyright &y& Elsevier]

Abstract: The rise in atmospheric oxygen between 2.45 and 2.2 Ga has been linked to the demise of a methane-rich atmosphere and the onset of multiple glaciations, culminating in a possible Snowball Earth. The glacial deposits represent possible global marker horizons, however, their correlation is uncertain because key sedimentary successions are undated, hampering paleoenvironmental reconstructions. Three potentially global glaciations have been proposed based on the correlation of glacial deposits in southern Africa with those in North America. However, it has also been suggested that there were four glaciations and that the youngest was a Snowball Earth event recorded only in South African successions. In situ U–Pb zircon ages for tuffs in southern African and North American successions establish the existence of four glaciations between 2.45 and 2.22 Ga. Geochronological and stratigraphic data demonstrate that the three oldest glaciations predate ∼2.31 Ga and that the final glaciation, which is only recognized in South Africa, occurred between 2.26 and 2.22 Ga. The new age-calibrated correlations show that a rise in atmospheric oxygen inferred from sulfur isotope data occurred between the second and third glaciations. At 2.31 Ga, the first marine sulfate evaporites were deposited contemporaneously with 13C-enriched carbonates, indicating a direct link between perturbations of the carbon and sulfur cycles and rising atmospheric oxygen. The appearance of “red beds” and oxidized palesols after the third glaciation signals a major increase in atmospheric oxygen levels, which culminated with the fourth glaciation between 2.26 and 2.22 Ga, after which the atmosphere remained irreversibly oxygenated. [Copyright &y& Elsevier]

The ~635 Ma Marinoan glaciation is marked by dramatic Earth system perturbations. Deposition of nonmass-dependently 17O-depleted sulfate (SO42-) in worldwide postglacial sediments is, thus far, unique to this glaciation. It is proposed that an extremely high-pCO2 atmosphere can result in highly 17O-depleted atmospheric O2, or the Marinoan Oxygen-17 Depletion (MOSD) event. This anomalous 17O signal was imparted to sulfate of oxidative weathering origin. However, 17O-depleted sulfate occurs in limited sedimentary intervals, suggesting that Earth surface conditions conducive to the MOSD had a finite duration. An MOSD duration can, therefore, provide much needed constraint on modeling Earth system responses at that time. Unfortunately, the sulfate 17O record is often sparse or lacks radiometric dates. Here, we report 11 barite layers from a post-Marinoan dolostone sequence at Wushanhu in the South China Block. The 17O depletion fluctuates in magnitude in lower layers but is persistently absent up section, providing the most confident first and last sedimentary appearance of the anomaly. δ13C chemostratigraphy is used to correlate the Wushanhu section to two proximal sections on the same shallow platform that lack barite layers but have published U-Pb dates that occur in dolostone and shale. Assuming a similar pattern and rate for carbonate and shale deposition among the different sections, we estimate the MOSD duration at 0-0.99 My. This number can be further constrained by new radiometric dates from equivalent sequences worldwide, thus underpinning models on the nonsteady-state Earth system response in the immediate aftermath of the Marinoan meltdown. [ABSTRACT FROM AUTHOR]

Ice-albedo feedback is a potentially important destabilizing effect for the climate of terrestrial planets. It is based on the positive feedback between decreasing surface temperatures, an increase of snow and ice cover, and an associated increase in planetary albedo, which then further decreases surface temperature. A recent study shows that for M stars, the strength of the ice-albedo feedback is reduced due to the strong spectral dependence of stellar radiation and snow/ice albedos; that is, M stars primarily emit in the near IR, where the snow and ice albedo is low, and less in the visible, where the snow/ice albedo is high. This study investigates the influence of the atmosphere (in terms of surface pressure and atmospheric composition) on this feedback, since an atmosphere was neglected in previous studies. A plane-parallel radiative transfer model was used for the calculation of planetary albedos. We varied CO2 partial pressures as well as the H2O, CH4, and O3 content in the atmosphere for planets orbiting Sun-like and M type stars. Results suggest that, for planets around M stars, the ice-albedo effect is significantly reduced, compared to planets around Sun-like stars. Including the effects of an atmosphere further suppresses the sensitivity to the icealbedo effect. Atmospheric key properties such as surface pressure, but also the abundance of radiative trace gases, can considerably change the strength of the ice-albedo feedback. For dense CO2 atmospheres of the order of a few to tens of bar, atmospheric rather than surface properties begin to dominate the planetary radiation budget. At high CO2 pressures, the ice-albedo feedback is strongly reduced for planets around M stars. The presence of trace amounts of H2O and CH4 in the atmosphere also weakens the ice-albedo effect for both stellar types considered. For planets around Sun-like stars, O3 could also lead to a very strong decrease of the ice-albedo feedback at high CO2 pressures. [ABSTRACT FROM AUTHOR]

Abstract: This review paper presents a set of revised global palaeogeographic maps for the 825–540Ma interval using the latest palaeomagnetic data, along with lithological information for Neoproterozoic sedimentary basins. These maps form the basis for an examination of the relationships between known glacial deposits, palaeolatitude, positions of continental rifting, relative sea-level changes, and major global tectonic events such as supercontinent assembly, breakup and superplume events. This analysis reveals several fundamental palaeogeographic features that will help inform and constrain models for Earth’s climatic and geodynamic evolution during the Neoproterozoic. First, glacial deposits at or near sea level appear to extend from high latitudes into the deep tropics for all three Neoproterozoic ice ages (Sturtian, Marinoan and Gaskiers), although the Gaskiers interval remains very poorly constrained in both palaeomagnetic data and global lithostratigraphic correlations. Second, continental sedimentary environments were dominant in epicratonic basins within Rodinia (>825Ma to ca. 750Ma), possibly resulting from both plume/superplume dynamic topography and lower sea-level due to dominantly old oceanic crust. This was also the case at ca. 540Ma, but at that time the pattern reflects widespread mountain ranges formed during the assembly of Gondwanaland and increasing mean age of global ocean crust. Third, deep-water environments were dominant during the peak stage of Rodinia break-up between ca. 720Ma and ca. 580Ma, likely indicating higher sea level due to increased rate of production of newer oceanic crust, plus perhaps the effect of continents drifting away from a weakening superplume. Finally, there is no clear association between continental rifting and the distribution of glacial strata, contradicting models that restrict glacial influence to regions of continental uplift. [Copyright &y& Elsevier]

Abstract: The trigger mechanism of the Snowball Earth events at 2.2–2.4Ga and 0.55–0.77Ga in the Proterozoic eon remains unknown despite intensive study over the last decade. We present the starburst model of the Snowball Earth. During a starburst of the Milky Way Galaxy, frequent and prolonged encounters with dark clouds and supernova remnants occur. The increased flux of cosmic dust particles and cosmic rays during the nebula encounters lead to a global super-cool climate, a Snowball Earth event. The individual nebula encounters may correspond to the substructures of super-cool/super-warm cycles in a Snowball Earth event. The starburst periods deduced from the ages of stars and star clusters coincide well with the Snowball Earth events reconstructed from geological records. We comprehensively evaluate the effects of cosmic rays, UV radiation, and cosmic dust particles during nebula encounters for the first time, and found that the starburst model of Snowball Earth events can adequately explain the triggering and occurrence pattern of Snowball Earth events. The direct evidence of nebula encounters can be obtained from deep-sea sediments deposited during the Snowball Earth events. [Copyright &y& Elsevier]

Abstract: The middle to late Neoproterozoic records the transition from the dispersal of supercontinent Rodinia to the subsequent assembly of supercontinent Gondwana. Determining paleogeographic positions of the major cratons during this transition is crucial to understand the displacement histories of these blocks. However, high-quality paleomagnetic results from well-dated rocks during this period remain sparse for the cratons. In this paper, we have carried out a paleomagnetic study of the succession of the Cryogenian Qiaoenbrak formation (Fm) in the Aksu area, NW China, which contains diamictites that are of probable glacial origin. The age of the studied section is constrained at around 730Ma based on the stratigraphy and recent zircon U–Pb chronologic data. Stepwise alternating field (AF) and thermal demagnetizations generally reveal two-component magnetizations. The low temperature (coercivity) component was removed by ∼300°C (∼30mT) and the high temperature (coercivity) component decayed toward the origin, defining the characteristic remanent magnetization (ChRM). Rock magnetic data indicate that magnetite is the major remanence carrier. A negative fold test for an isoclinal soft-sediment fold, the presence of both normal and reversed polarities, and positive conglomerate tests in conjunction with the presence of pristine sedimentary features by microscopic inspections of thin sections suggest that the remanence was acquired shortly after the deposition, i.e. post-depositional remanent magnetization (pDRM), which is very close to the magnetic field at the time of deposition. Fourteen out of 17 sites yield stable ChRMs with a mean direction of D s =246.1°, I s =30.4°, k s =14.1, α 95 =11.0° in stratigraphic coordinates, corresponding to a paleopole (Q =6) at λ =6.3°S, φ =17.5°E, A 95 =9.1°. Together with geologic constraints, the new paleomagnetic results place the Tarim Block at 16.3±5.6°N adjacent to either the west or east of Australia (WOA or EOA) at around 730Ma, but the WOA position is preferred based on the overall relatively favorable evidence. In addition, since the studied diamictites of the Qiaoenbrak Fm that are of probable glaciogenic origin yield a low paleolatitude, this study may lend support to the “Snowball Earth” hypothesis during the Neoproterozoic times. [Copyright &y& Elsevier]

Abstract: The precipitation of Cryogenian ironstones has been attributed to a spectrum of mechanisms ranging from virtually instantaneous “rusting of the seas” in response to post-snowball Earth ice meltback, to localised hydrothermal activity during fragmentation of Rodinia. The former model presupposes that ironstone deposition took place following peak glaciation. In the Chuos Formation of the Otavi Mountain Land, northern Namibia, ironstone facies precede, and are vertically gradational into, diamictites. Evidence for glaciation in the diamictites includes 1) dropstone textures, 2) subglacially deformed and attenuated, thinly stratified diamictites, supported by 3) the co-occurrence of soft-sediment striations. The underlying ironstones contain evidence for tractional processes (large-scale cross bedding) and biogenic growth (stromatolites) in strata rich in magnetite and hematite. Using the analogy of acidophile biomats in modern acid mine drainage environments, where photosynthetic bacteria construct stromatolites, fixing CO2 and Fe intracellularly, it is suggested that Cryogenian acidophile biomats did likewise, triggering ironstone precipitation and local CO2 drawdown, thereby facilitating concomitant glaciation. [Copyright &y& Elsevier]

Abstract: Neoproterozoic ‘Avalonian-Cadomian’ volcanic arc basins once lay peripheral to Gondwana and are now found around the North Atlantic Ocean in New England, Atlantic Canada and northwestern Europe as ‘peri-Gondwanan terranes.’ Their thick (up to 9km) marine fills are dominated by turbidites, debrites (diamictites and variably graded conglomerates), slumps and olistostromes recording the dominance of mass flow processes in arc basins oversupplied with volcaniclastic sediment. Several diamictite horizons in these basins were identified as glacial tillites more than one hundred years ago on the basis of poor textural sorting, and the lack of any understanding of mass flow processes. An association with thin-bedded turbidite facies, then interpreted as glaciolacustrine varvites, was seen as evidence for widespread glacial conditions which is still the basis today of a near global ‘Gaskiers glaciation’ at c. 580Ma, despite classic sedimentological work which shows that the ‘tillites’ and ‘varvites’ of these basins are deep marine sediment gravity flow deposits. Only in two basins (Gaskiers Formation, Avalon Peninsula in Newfoundland, and the Konnarock Formation of Virginia) is a distal and regionally-restricted glacial influence on marine sedimentation identified from ice-rafted, striated dropstones in turbidites but terrestrial ‘ice-contact’ facies are absent. As revealed in this study, terrestrial glacial facies may not have survived frequent volcanic activity such as seen today on glaciated active plate margin volcanoes such as Mount Rainier in Washington USA, and Cotopaxi Volcano in Ecuador where primary glacial sediment is frequently reworked by lahars, pyroclastic flows, debris avalanches and outburst floods. The weight of evidence presented in this study indicates that ice covers during the Gaskiers glaciation were not widespread across the Avalonian-Cadomian back arc basins; the deep marine Grenada Basin (Caribbean Sea) filled with turbidites, debrites (lahars) and debris avalanches from the adjacent Lesser Antilles Arc is identified here as a modern analogue for these ancient basins. [Copyright &y& Elsevier]

Abstract: Neoproterozoic successions in NE Svalbard and East Greenland host a pair of glacigenic formations, the younger of which are correlated with the terminal Cryogenian (Marinoan) glaciation based on their lithologically and isotopically diagnostic cap dolostones. A deep negative carbon isotope excursion (CIE) occurs stratigraphically beneath the older glacigenic formations in both areas, but no CIE is preserved beneath the younger glacial horizon in either area. This led to uncertainty over the age of the CIE and the paleoenvironmental significance of the units separating the glacigenic formations. 87Sr/86Sr ratios in Sr-rich limestone associated with the CIEs are ≤0.7064 in East Greenland and ≤0.7068 in NE Svalbard, consistent with early Cryogenian values globally and inconsistent with late Cryogenian ratios, which are exclusively ≥0.7071. The CIEs are tentatively correlated with the Islay excursion in the Scottish and Irish Caledonides, and potentially correlative subglacial CIEs in northwestern Canada, northwestern Tasmania and the southwestern United States. In NE Svalbard, newly-acquired δ 13Corg data covary with δ 13Ccarb across the CIE, but the organic excursion is roughly half the amplitude of the inorganic excursion. δ 13Ccarb and δ 18Ocarb do not covary, nor does δ 13Corg vary as a function of total organic carbon content. A primary origin for the CIE is supported and the accompanying proliferation of stromatolites suggests rising carbonate saturation (falling pCO2) prior to glaciation. New data suggest that the hiatus responsible for the missing Trezona CIE occurs at the top, not the bottom, of the Bråvika member sandstone in NE Svalbard. A dramatic decline in regional average thickness of older Cryogenian glacial deposits from the paleomagnetically-determined subtropics (British Isles) to the deep tropics (Svalbard) is consistent with meteoric net accumulation and sublimation zones on a snowball Earth, and inconsistent with the Jormungand climate zonation. [Copyright &y& Elsevier]

Abstract: This paper reviews the state of our current knowledge of the Cryogenian glacial and deglacial sedimentary records of South Australia. Two, distinct, glacial intervals are recognised within rocks of Cryogenian (850–630Ma) age, separated by an interglacial succession that by definition lacks glacial facies. The glacial successions were laid down during the Sturt and Elatina glaciations. Each of these glaciations is arguably of global extent under the Snowball Earth hypothesis (Hoffman and Schrag, 2002). Evidence for glaciation has been known for over one hundred years from the Sturt Tillite (Howchin, 1900). When compared like-for-like in a systematic facies analysis, both glacial successions show some similarities, notably the occurrence of diamictites representing glacial deposition and slope reflux, and dropstone-bearing intervals, but also show some dramatic differences. Notably, deposition of the Sturt succession was strongly influenced by extensional tectonics, for which evidence is provided by dramatic thickness variations either side of major faults and palaeocurrent data. Sub-basins formed during regional extension contain diamictites 4–6000m thick (e.g. the Yudnamutana and Baratta “troughs” in the northern and central Flinders Ranges respectively). Deposition of the younger, Elatina glacial deposits was locally influenced by diapirism in the central Flinders Ranges, but otherwise these younger deposits seem to maintain a relatively constant thickness of <100m, although exceptionally, thicknesses of up to 1500m occur in basinal sections (Williams et al., 2008). The emergence of South Australia from the Sturt and Elatina glaciations culminated in black, silty shale and rather complex “cap-dolostones” respectively- two contrasting styles of deglaciation. [Copyright &y& Elsevier]

Abstract: Paleomagnetic data suggest that the early Cryogenian (Sturtian) glaciation extended to sea level at low latitude. The impact of this dramatic environmental change on biota, and the composition of ecosystems in the immediate aftermath of the Sturtian glaciation remain virtually unknown. Here we report the discovery of abundant agglutinated tests in organic-rich carbonates directly overlying Sturtian glacial deposits from two different paleocontinents: the Rasthof Formation of the Congo craton in northern Namibia and the Tsagaan Oloom Formation of the Dzabkhan terrane in Mongolia. The most abundant tests preserve morphological and compositional characters consistent with those found in at least two different families of modern lobose testate amoebae (Amoebozoa), a group of heterotrophic microbial eukaryotes. The presence of spatially and compositionally variable clay minerals, quartz and microcline on the test walls is a signature of widespread biological agglutination. The post-glacial fossil assemblages differ from the most common pre-Sturtian vase-shaped fossil testate amoebae, perhaps as a result of different preservational mechanisms or of the appearance of new forms after the glaciation. The apparent local abundance of eukaryotic body fossils in the post-Sturtian carbonates suggests that the Cryogenian limestones and dolostones may host a currently unexplored fossil record of modern eukaryotes. [Copyright &y& Elsevier]

Abstract: The Cambrian explosion, c. 530–515Ma heralded the arrival of a diverse assembly of multicellular life including the first hard-shelled organisms. Fossils found in Cambrian strata represent the ancestors of most modern animal phyla. In contrast to the apparent explosiveness seen in the Cambrian fossil record, studies of molecular biology hint that the diversification observed in Cambrian strata was rooted in ancestry extending back into the Ediacaran (635–542Ma). Fossil evidence for this mostly cryptic phase of evolution is derived from the soft-bodied fossils of the Ediacaran biota found throughout the world and bilaterian embryos found in the Doushantuo lagerstätte in South China. The first appearance of Ediacara fauna is thought to have followed the last of the ~750–635Ma Neoproterozoic glacial episodes by 20–30million years. In this paper, we present evidence for the oldest discovery of the ‘Ediacara’ discoidal fossils Nimbia occlusa and Aspidella terranovica (?) that predate the early Cryogenian glaciations by more than fifty million years. There is considerable disagreement over the significance of discoidal Ediacaran fossils, but our findings may support earlier suggestions that metazoan life has roots extending deeper into the Proterozoic Eon. We also confirm the presence of a Late Cryogenian (e.g. “Marinoan”) glaciation on the Lesser Karatau microcontinent including dropstones and striated clasts within the glacial strata. [Copyright &y& Elsevier]

Abstract: Volcanoes with silica-rich and highly viscous magma tend to produce violent explosive eruptions that result in disasters in local communities and that strongly affect the global environment. We examined the timing of 11 eruptive events that produced silica-rich magma from four volcanoes in Japan (Mt. Fuji, Mt. Usu, Myojin-sho, and Satsuma-Iwo-jima) over the past 306years (from AD 1700 to AD 2005). Nine of the 11 events occurred during inactive phases of solar magnetic activity (solar minimum), which is well indexed by the group sunspot number. This strong association between eruption timing and the solar minimum is statistically significant to a confidence level of 96.7%. This relationship is not observed for eruptions from volcanoes with relatively silica-poor magma, such as Izu-Ohshima. It is well known that the cosmic-ray flux is negatively correlated with solar magnetic activity, as the strong magnetic field in the solar wind repels charged particles such as galactic cosmic rays that originate from outside of the solar system. The strong negative correlation observed between the timing of silica-rich eruptions and solar activity can be explained by variations in cosmic-ray flux arising from solar modulation. Because silica-rich magma has relatively high surface tension (~0.1Nm−1), the homogeneous nucleation rate is so low that such magma exists in a highly supersaturated state without considerable exsolution, even when located relatively close to the surface, within the penetration range of cosmic-ray muons (1–10GeV). These muons can contribute to nucleation in supersaturated magma, as documented by many authors studying a bubble chamber, via ionization loss. This radiation-induced nucleation can lead to the pre-eruptive exsolution of H2O in the silica-rich magma. We note the possibility that the 1991 Mt. Pinatubo eruption was triggered by the same mechanism: an increase in cosmic-ray flux triggered by Typhoon Yunya, as a decrease in atmospheric pressure results in an increase in cosmic-ray flux. We also speculate that the snowball Earth event was triggered by successive large-scale volcanic eruptions triggered by increased cosmic-ray flux due to nearby supernova explosions. [Copyright &y& Elsevier]

The Neoproterozoic is a time of transition between the ancient microbial world and the Phanerozoic, marked by a resumption of extreme carbon isotope fluctuations and glaciation after a billion-year absence. The carbon cycle disruptions are probably accompanied by changes in the stock of oxidants and connect to glaciations via changes in the atmospheric greenhouse gas content. Two of the glaciations reach low latitudes and may have been Snowball events with near-global ice cover. This review deals primarily with the Cryogenian portion of the Neoproterozoic, during which these glaciations occurred. The initiation and deglaciation of Snowball states are discussed in light of a suite of general circulation model simulations designed to facilitate intercomparison between different models. Snow cover and the nature of the frozen surface emerge as key factors governing initiation and deglaciation. The most comprehensive model discussed confirms the possibility of initiating a Snowball event with a plausible reduction of CO2. Deglaciation requires a combination of elevated CO2 and tropical dust accumulation, aided by some cloud warming. The cause of Neoproterozoic biogeochemical turbulence, and its precise connection with Snowball glaciations, remains obscure. [ABSTRACT FROM AUTHOR]

Abstract: Two Cryogenian glacial sequences, the Sturt and Elatina, occur within the Adelaide Rift Complex, S Australia, and are part of an ∼8km thick succession recording Rodinia fragmentation and subsequent passive margin development. We compare and contrast these glacial successions, presenting new data, including detailed logged sections, for both glacial sequences. Facies analysis reveals a total of six facies associations, recording a range of glacial and glacially related processes. These are a diamictite facies association (ice-proximal debris flows), a sheet sandstone facies association (fluvial deposits), a flaser and rhythmically bedded facies association (tidal flat), a hummocky cross-stratified sandstone facies association (shoreface to offshore transition zone deposits) and an underflow facies association (recording offshore deposition). A sixth facies association, a ferruginous lonestone-bearing siltstone, is restricted to the top of the Elatina Formation and records rapid environmental amelioration and emergence from glaciation. In the central Flinders Ranges, the Sturt succession commences with a glacial erosion surface (GES), which in agreement with previous workers, was probably enhanced by rifting. This GES is overlain, in ascending order, by the Pualco Tillite, Holowilena Ironstone and Wilyerpa Formation, with the Warcowie Dolomite Member at the base of this latter formation. A second GES is preserved within this latter unit, and is overlain by a stratigraphic motif that records an alternation of offshore transition zone and offshore deposits. Significant base level falls within the Wilyerpa Formation are identified by fluvial deposits sitting disconformably upon offshore deposits. In contrast, within the Elatina Formation, a single GES is proposed, which defines the base of the formation. The Elatina Formation is split into two informal members on the basis of analysis of sediment stacking patterns and the recognition of a sharp stratigraphic surface that divides the formation. Apart from a locally preserved basal diamictite, the lower member consists of fluvial deposits, whereas the upper member is dominated by a mixture of tidal flat sediments and debris flow deposits. The boundary between these units is therefore interpreted as a hybrid surface recording tidal ravinement in some areas and a glacial advance in others. Deglaciation left contrasting deposits: the Sturt deglaciation culminated in deposition of black shale of the Tindelpina Shale Member of the Tapley Hill Formation, seemingly as a sheet-like deposit over the Adelaide Rift Complex and beyond into the Amadeus Basin of central Australia. The Elatina deglaciation, meanwhile, began first with deposition of the sixth facies association (ferruginous dropstone-bearing siltstone), then proceeded with deposition of cap dolostone of the Nuccaleena Formation. [Copyright &y& Elsevier]

The Witputs section of the Gariep Belt (S Namibia) comprises a sequence of clastic and chemical sediments, which have been interpreted as representative of a Late Neoproterozoic global or near-global ice age event, and recent biostratigraphic work in the upper rocks of the Witputs suggest a Late Ediacaran age. To further characterise this sequence and provide additional age constraints, a detailed sedimentological and detrital zircon study has been carried out. The petrographic, sedimentological and geochemical characteristics of the Witputs diamictite determined in this study are homogenous and indicative of debris flow or palaeo-valley infill sediments, deposited in an oxic environment with no glaciogenic evidence. This homogeneity is also reflected in the detrital zircon age spectra with most ages falling between 1.0 and 1.3 Ga, representing the local geology, with the youngest grain at 1030.2 ± 10.9 (2σ) ( n = 92 <10% discordance), despite the fact that mid and Late Neoproterozoic volcanic activity is known in the local region. The overlying carbonate rocks, often considered to be 'cap carbonates', show high Mn (up to 60% MnO), with base metal precipitation (Zn, V, Co), and are recrystallised. Their δC isotope ratios are homogeneous at around-3. Major and trace element ratios reach values which indicate that C-O isotopes may be disturbed and might not reflect primary global seawater composition, thus questioning their use for global correlation and comparison with composite chemostratigraphic curves. The contact to the overlying Late Ediacaran Sanddrif Member is not exposed, and the rocks dip in a different direction than the underlying carbonate rocks. The c. 40-m-thick section is characterised by rapid lithology changes including shales, calcareous sandstones and wackes, fine-grained conglomerates and rare clean quartz-rich sandstones, all of which have strikingly similar detrital zircon populations, and the youngest zircon is dated at 1082.8 ± 10 Ma (2σ errors, from 72 grains with <10% discordance). Acritarchs earlier found in the Sanddrif Member, however, indicate a post-570 Ma depositional age. If the diamictites are glacio-marine deposits, then an interesting conclusion is that the clastic sediments can display a very immature geochemical signature, indicating a localised provenance, with derivation purely from the local basement rocks, which is also reflected in the detrital zircon populations. However, we would hesitate to assign a glacial origin to the deposits as no glacial indicators, other than a diamictitic texture, were observed. Clearly, far more work on the detailed mapping and sedimentology of the Neoproterozoic Gariep Belt deposits is required, particularly as many are currently used for global correlation. Age constraints derived from extensive detrital zircon work can only constrain the deposits as being post 1.03 Ma with the detritus being purely locally derived. [ABSTRACT FROM AUTHOR]

Abstract: A palaeomagnetic study was carried out on Neoproterozoic samples from seven sites of the sub-basins of Gourma and Bobo Dioulasso, which include a Marinoan glaciogenic deposit. Magnetic mineralogy is represented essentially by magnetite and hematite. The mean directions of the sites are calculated on the high temperature component (500–670°C). Two locations provide data constrained by statistical reversal and fold tests and determining Neoproterozoic virtual geomagnetic poles. The palaeolatitudes display very low values which place the West-African craton in the sub-equatorial position during the Marinoan glaciation. This result enhances the Snowball Earth hypothesis, which places most of the continental landmasses, and notably Africa, at low latitudes during the Neoproterozoic. [Copyright &y& Elsevier]

Abstract: Two cap carbonates overlying glaciogenic diamictites crop out extensively in the eastern Vaza Barris Domain of the Sergipano Belt, northeastern Brazil. They are represented by carbonates of the Jacoca Formation, resting on top of diamictite of the Ribeiropolis Formation, and by the Olhos D’Agua Formation (carbonates, organic-rich towards the top), which overlies diamictite of the Palestina Formation. These two sequences were deformed and metamorphosed at sub-greenschist facies-conditions during the Brasiliano cycle (650–600 Ma). In the western Vaza Barris Domain, dolostone of the Acauã Formation rest, in sharp contact, on diamictites of the Juetê Formation and are much less deformed than the Olhos D’Agua Formation. Values of (13C for the Jacoca and Acauã Formations cluster around −4 to −5‰. In the Olhos D’Agua Formation, however, negative values (around −5‰) in the shallow-marine base of the sequence are replaced up section by values close to zero and, abruptly, by positive values between +8 and +10‰ at the top. Strontium-isotope ratios for carbonates of these three formations are within the range of late Neoproterozoic seawater (0.7060–0.7090). The Acauã Formation displays 87Sr/86Sr ratios from 0.7072 to 0.7073, while the Jacoca and Olhos D’Agua formations display values within the range from 0.7077 to 0.7081. Strontium-isotopes coupled with available C-isotopes, detrital zircon U–Pb ages and structural data suggest that the Jacoca Formation and Acauã are probably correlative and mid Cryogenian in age, and that the Olhos D’Agua Formation is lowermost Ediacaran (Marinoan) in age. Mercury concentrations in carbonates of the Jacoca, Olhos D’Agua and Acauã formations are usually much higher than those observed in carbonates deposited not concomitantly with volcanic activities (<3ngg−1). They are, in fact, similar to those in carbonates deposited during volcanic activity (e.g. Tertiary Punta Rocallosa carbonates in Chile), with concentration values between 20 and 80ngg−1, suggesting that CO2 in the basal portion of these carbonate formations is mostly mantle-derived, accumulated in the atmosphere during Cryogenian-Ediacaran glacial events. [Copyright &y& Elsevier]

We use the coupled atmosphere-ocean general circulation model ECHAM5/MPI-OM to investigate the transition from the present-day climate to a modern Snowball Earth, defined as the Earth in modern geography with complete sea-ice cover. Starting from the present-day climate and applying an abrupt decrease of total solar irradiance (TSI) we find that the critical TSI marking the Snowball Earth bifurcation point is between 91 and 94% of the present-day TSI. The Snowball Earth bifurcation point as well as the transition times are well reproduced by a zero-dimensional energy balance model of the mean ocean potential temperature. During the transition, the asymmetric distribution of continents between the Northern and Southern Hemisphere causes heat transports toward the more water-covered Southern Hemisphere. This is accompanied by an intensification of the southern Hadley cell and the wind-driven subtropical ocean cells by a factor of 4. If we set back TSI to 100% shortly before the transition to a modern Snowball Earth is completed, a narrow band of open equatorial water is sufficient for rapid melting. This implies that for 100% TSI the point of unstoppable glaciation separating partial from complete sea-ice cover is much closer to complete sea-ice cover than in classical energy balance models. Stable states can have no greater than 56.6% sea-ice cover implying that ECHAM5/MPI-OM does not exhibit stable states with near-complete sea-ice cover but open equatorial waters. [ABSTRACT FROM AUTHOR]

Abstract: The end of the Neoproterozoic era is punctuated by two global glacial events marked by the presence of glacial deposits overlaid by cap carbonates. Duration of glacial intervals is now consistently constrained to 3–12million years but the duration of the post-glacial transition is more controversial due to the uncertainty in cap dolostone sedimentation rates. Indeed, the presence of several stratabound magnetic reversals in Brazilian cap dolostones recently questioned the short sedimentation duration (a few thousand years at most) that was initially suggested for these rocks. Here, we present new detailed magnetostratigraphic data of the Mirassol d''Oeste cap dolostones (Mato Grosso, Brazil) and “bomb-spike” calibrated AMS 14C data of microbial mats from the Lagoa Vermelha (Rio de Janeiro, Brazil). We also compile sedimentary, isotopic and microbiological data from post-Marinoan outcrops and/or recent depositional analogues in order to discuss the deposition rate of Marinoan cap dolostones and to infer an estimation of the deglaciation duration in the snowball Earth aftermath. Taken together, the various data point to a sedimentation duration in the range of a few 105 years. [Copyright &y& Elsevier]

Abstract: Sedimentologically and geochemically distinctive carbonate sequences consistently drape the glacial deposits associated with the younger Cryogenian ice age. The presence of ice-rafted debris in the basal dolostone implies that at least the lower portion of the cap sequence records deglaciation. An isochronous model proposes that cap dolostones were deposited synchronously around the world regardless of water depth, whilst a diachronous model proposes that deposition tracked glacioeustatic flooding during deglaciation. The Nuccaleena Formation of the Adelaide Rift Complex (ARC), South Australia, exhibits many of the unique sedimentary features observed in other younger Cryogenian cap dolostones around the world. Some bedforms are the product of wind-driven oscillatory flow, thus constraining post-glacial dolostone deposition to <400m. These sedimentary features are absent in the deepest basinal facies in the northern ARC, suggesting that this region was below storm wave base even at the glacial sea level lowstand. In the north Flinders Ranges, there is a distinct relationship between lateral facies variability in the pre-, syn- and post-glacial sediments and the axes of 50km scale structural folds. The northern fold limbs are characterised by basinal facies, whilst the southern limbs are associated with upper-slope facies. We interpret the abrupt facies transitions to reflect lower slope to upper slope/shelf breaks across a series of linked, south-facing half-grabens. The majority of cap dolostone carbon isotope records show monotonic declines in δ 13C of <2‰, with starting points between −0.5 and −3.5‰. An isochronous model implies a ∼3.0‰ lateral gradient from platform to lower slope that varies dramatically on a short spatial scale and non-systematically with palaeobathymetry. If pre- and syn-glacial facies are used to infer palaeobathymetry, and cap dolostones are deposited diachronously as sea level rises during deglaciation, then δ 13C values becomes progressively lighter with time, implying that cap dolostone deposition tracked the glacioeustatic sea-level rise over a series of half-grabens that deepened to the north. The carbon isotope dataset cannot rule out uniquely isochronous or diachronous models. Given the high frequency spatial variability of δ 13C values, temperature cannot be the dominant control on the isotopic variability of the cap dolostone. [Copyright &y& Elsevier]

The current state of knowledge suggests that the Neoproterozoic snowball Earth is far from deglaciation even at 0.2 bars of CO2. Since understanding the termination of the fully ice-covered state is essential to sustain, or not, the snowball Earth theory, we used an Atmospheric General Climate Model (AGCM) to explore some key factors which could induce deglaciation. After testing the models’ sensitivity to their parameterizations of clouds, CO2 and snow, we investigated the warming effect caused by a dusty surface, associated with ash release during a mega-volcanic eruption. We found that the snow aging process, its dirtiness and the ash deposition on the snow-free ice are key factors for deglaciation. Our modelling study suggests that, under a CO2 enriched atmosphere, a dusty snowball Earth could reach the deglaciation threshold. [ABSTRACT FROM AUTHOR]

Abstract: The Neoproterozoic–Cambrian Dalradian Supergroup of Scotland contains three glaciogenic sedimentary units, the Port Askaig Formation, the Stralinchy-Reelan Formation, and the Loch na Cille Boulder Bed, in ascending order. Although the idea, that relatively cold climates occurred during the latest Proterozoic, is widely accepted, the cause of the onset of glaciation is still controversial; we use new carbonate 87Sr/86Sr data to better constrain this environmental change. Some Dalradian limestones have high strontium contents (>1000ppm), low manganese contents and Mn/Sr ratios (<0.2) and, according to their silica, aluminum and potassium contents and Rb/Sr ratios, they suffered insignificant contamination from siliciclastic minerals, thus they are suitable for estimating the 87Sr/86Sr values of paleoseawater in the Neoproterozoic. The 87Sr/86Sr values of limestones of the Islay Formation (ca. 0.7067), sampled on the island of Islay, are similar to the values of previous studies. However, the 87Sr/86Sr values of limestones just below the Port Askaig Formation (ca. 0.7064), collected on the island of Garbh Eileach, have the lowest values recorded in the Dalradian Supergroup. Comparing these 87Sr/86Sr values with coeval 87Sr/86Sr values of other regions in the world, such low 87Sr/86Sr (<0.7065) has only been reported from successions pre-dating the older Cryogenian (Sturtian) glaciation, which supports the idea that the Port Askaig Formation was deposited during the older Cryogenian glaciation. We have reconstructed the variations in the i87Sr/86Sr ratio from the Tonian through the Cryogenian and Ediacaran to the earliest Cambrian. The results show that the 87Sr/86Sr ratio of seawater decreased just before the older Cryogenian glaciation, that it increased by two steps in the early and middle Ediacaran, and that there were at least two abrupt positive excursions in the middle Ediacaran and at the Precambrian–Cambrian boundary. The decline of the 87Sr/86Sr ratio of seawater just before the older Cryogenian Sturtian glaciation suggests that enhancement of weathering, related to break-up of the Rodinia supercontinent and continental flood basalt volcanism, was a likely driver for generating global cooling for the Snowball Earth conditions (). [Copyright &y& Elsevier]

The diamictites of the Neoproterozoic Kapp Lyell Sequence in northern Wedel Jarlsberg Land, southwest Spitsbergen, have long been recognized as ancient glacial deposits, but their place within the global stratigraphic framework of 'snowball Earth' has remained unclear, owing to the complexity of superimposed Caledonian deformation and to the relatively inaccessible terrain in which they occur. Recently deglaciated exposures of the rocks now provide a more complete picture of the changing environment in which the diamictites were deposited, and new understanding of regional correlations help constrain their place in the global chronostratigraphy of the Cryogenian Period. The 2500 m thick Kapp Lyell Sequence consists of three distinct types of glaciomarine diamictite. The succession begins with about 1000 m of finely laminated diamictite containing abundant lonestones. Themillimetre- to centimetre-scale laminae, apparent suspension deposits, consist of sand- to silt-sized particles of quartz and dolomite alternating with thin films of graphitic phyllite. The laminated unit gives way abruptly to 500-1000 m of unsorted, unlayered diamictite that alternates and interfingers with graded beds of conglomerate to sandstone. These apparent turbidite deposits become increasingly prevalent toward the top of the exposed section. Regional lithostratigraphic relationships suggest that the Kapp Lyell sequence corresponds to the second major stage of Neoproterozoic glaciation at c. 635 Ma. The graphitic material in the laminated unit yields δ13C values in the range of -20 to -22 ‰, pointing to a biogenic origin and an active marine biosphere at the time of deposition. The preservation of organic carbon and unusually large ratios of highly reactive Fe to total Fe suggest that low oxygen conditions prevailed in the deep basin that received these sediments. The transition from laminated, to unsorted, to graded diamictites may represent change from (1) a stable ice margin that released rare icebergs into a deep, quiet basin to (2) a collapsing ice sheet that unleashed flotillas of icebergs and large volumes of sediment to (3) submarine landslides that triggered turbidity flows from the rapidly deposited, gravitationally unstable sediments. The Kapp Lyell diamictite sequence appears to chronicle the demise of a large ice mass in this part of the Neoproterozoic world. [ABSTRACT FROM AUTHOR]

Abstract: The Cryogenian Grand Conglomerat Formation (<765 and >735Ma in age and with considerable thickness variations up to a maximum thickness of about 950m) is an association of interbedded glaciogenic, clastic, periglacial and non-glacial deposits topped by the Kakontwe Limestone cap carbonate. These units occur within the Nguba Group of the Katanga Supergroup, which was deposited in a rift basin and subsequently deformed as part of the Pan African Lufilian orogenic belt that separates the Congo and Kalahari cratons. Both, correlation of regional unconformities and facies distribution suggest that the Grand Conglomerat sediments were deposited (during and after eruption of flood basalts) in an asymmetrical rift, with a strongly uplifted southern shoulder, and a graded shelf defining the northern margin. Glaciomarine sedimentary rocks along the southern margin of the Katangan rift are preserved within fan delta conglomerates supplied from an elevated rift shoulder. By contrast, the northern margin of the rift was the site of continental glaciation with cross-bedded, glaciofluvial and marginal marine sandstones and conglomerates, associated with massive tills (glaciogenic diamictites) that pass laterally towards the south into glaciomarine mixtite interlayered with wedges of dolomitic sandstone. The Kakontwe Limestone cap carbonate is present only in the distal parts of the basin. Its absence in proximal regions is considered to reflect very high rates of sedimentation of fine-grained, glaciogenic debris derived from deglaciated source areas. Palaeomagnetic data indicates that the Grand Conglomerat glaciogenic sediments were deposited close to the Equator during the Cryogenian. This low latitude setting coupled with the absence of a topographical trigger would suggest that glaciation was related to global atmospheric cooling. However, the presence of water-borne, glaciogenic, on-shore sediments and offshore sediments derived from floating glaciers suggests that the ocean during this part of the Cryogenian was not completely frozen. Associations of glaciogenic facies with non-glaciogenic sedimentary rocks imply glaciation with interglacial periods and gradual deglaciation, instead of severe conditions of permanent sea ice cover and rapid change to the greenhouse environment. [Copyright &y& Elsevier]

Abstract: The distributions of 77 Neoproterozoic glacigenic formations are shown on global palaeogeographic maps for 715 Ma (Sturtian), 635 Ma (Marinoan) and 580 Ma (Ediacaran), constructed on grounds independent of palaeoclimatic indicators. The meridional distribution of Sturtian and Marinoan deposits is biased in favour of low palaeolatitudes, whereas Ediacaran deposits are biased in favour of high palaeolatitudes. All carbonate-hosted glacigenic formations (n =22) fall within 35° of the palaeoequator. Most (6 of 8) examples of periglacial polygonal sand-wedges occur at palaeolatitudes greater than 30°, whereas most (8 of 9) occurrences of large syn-glacial Fe and Fe–Mn deposits lie within 30° of the palaeoequator. Marinoan syn-deglacial cap dolostones (n =24) decline in maximum thickness with palaeolatitude, consistent with poleward ice retreat, normal meridional temperature gradients and a small-obliquity orbit. Meridional (N–S) mean orientations of giant wave ripples in Marinoan cap dolostones from different regions (n =10) and absence of zonal (W–E) orientations are consistent with zonal wind-driven waves and not with hurricanes. In general, the results support the validity of the palaeogeographic reconstructions and the pan-glacial character of Sturtian and Marinoan ice ages. [Copyright &y& Elsevier]

• An asteroid impact during snowball Earth may have caused temporary warming. • A well-dated impact crater is coeval with tuffs that were deposited in open ocean. • Basins indicating meltback were positioned in restricted seaways with thin sea ice. The ca. 717 Ma low-latitude Sturtian "snowball Earth" glaciation lasted ∼56 Myr. However, sedimentological evidence for transient, open ocean conditions during the glaciation appears to contradict the concept of a global deep freeze. We demonstrate multiple lines of geologic evidence from five continents for a temporary, localized sea-ice retreat during the middle of the Sturtian glaciation, which coincides with one, perhaps two, asteroid impacts, and arguably more terrestrial impacts as inferred from the lunar impact record. The well-dated Jänisjärvi impact (ca. 687 Ma) is synchronous with repeated volcanic ash falls whose deposition is most parsimoniously interpreted to indicate a partially ice-free ocean. Temporary greenhouse warming caused by the vaporization of sea ice can explain localized glacial retreat within restricted seaways between these continents, where ice flow would have been constricted and sea ice thinnest before impact. [ABSTRACT FROM AUTHOR]

The rotation model for the opening of the Canada Basin of the Arctic Ocean predicts stratigraphic links between the Alaskan North Slope and the Canadian Arctic islands. The Katakturuk Dolomite is a 2080-m-thick Neoproterozoic carbonate succession exposed in the northeastern Brooks Range of Arctic Alaska. These strata have previously been correlated with the pre-723 Ma Shaler Supergroup of the Amundson Basin. Herein we report new composite δ13C profiles and detrital zircon ages that test this connection. We go further and use stratigraphic markers and a compilation of δ13C chemostratigraphy from around the world, tied to U-Pb ages, to derive an age model for deposition of the Katakturuk Dolomite. In particular, we report the identification of ca. 760 Ma detrital zircons in strata underlying the Katakturuk Dolomite. Moreover, a diamictite present at the base of the Katakturuk Dolomite is capped by a dark-colored limestone with peculiar roll-up structures. Chemostratigraphy and lithostratigraphy suggest this is an early-Cryogenian glacial diamictite-cap carbonate couplet and that deposition of the Katakturuk Dolomite spanned much of the late Neoproterozoic. Approximately 500 m above the diamictite, a micropeloidal dolomite, with idiosyncratic textures that are characteristic of basal Ediacaran cap carbonates, such as tubestone stromatolites, giant wave ripples, and decameters of pseudomorphosed former aragonite crystal fans, rests on a silicified surface. Chemostratigraphic correlations also indicate a large increase in sedimentation rate in the upper ∼1 km of the Katakturuk Dolomite and in the overlying lower Nanook Limestone. We suggest that the accompanying increase in accommodation space, along with the presence of two low-angle unconformities within these strata, are the product of late Ediacaran rifting along the southern margin of the North Slope subterrane. There are no strata present in the Amundson Basin that are potentially correlative with the late Neoproterozoic Katakturuk Dolomite, as the Cambrian Saline River Formation rests on the ca. 723 Ma Natkusiak Formation. Detrital zircon geochronology, chemostratigraphic correlations, and the style of sedimentation are inconsistent with both a Canadian Arctic origin of the North Slope subterrane and a simple rotation model for the origin of the Arctic Ocean. If the rotation model is to be retained, the exotic North Slope subterrane must have accreted to northwest Laurentia in the Early to Middle Devonian. [ABSTRACT FROM AUTHOR]

Abstract: Carbonates capping Neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball Earth aftermath. It is commonly assumed that post-snowball climate dominated by CO2 partial pressures several hundred times greater than modern levels, would be characterized by extreme temperatures, a vigorous hydrological cycle, and associated high continental weathering rates. However, the climate in the aftermath of a global glaciation has never been rigorously modelled. Here, we use a hierarchy of numerical models, from an atmospheric general circulation model to a mechanistic model describing continental weathering processes, to explore characteristics of the Earth system during the supergreenhouse climate following a snowball glaciation. These models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse. Indeed, constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation. Even at 400 times the present day pressure of atmospheric CO2, continental runoff is only 1.2 times the modern runoff. Under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event, the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux. Consequently, it takes millions of years for the silicate weathering cycle to reduce post-snowball CO2 levels to background Neoproterozoic levels. Regarding the origin of the cap dolostones, we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume. [Copyright &y& Elsevier]

Abstract: Analysis of a Cryogenian interglacial platform margin in the Adelaide Geosyncline reveals a strong carbonate δ 13C-facies relationship. Detailed chronostratigraphic correlation between sections ranging from shallow platform facies to deep basinal facies indicates the presence of a carbon isotopic gradient of between 8 and 11‰ in time-equivalent strata. Shallow-water back-reef facies have δ 13C values up to 8.2‰, while equivalent basinal sediments have δ 13C values between −3 and 0‰. Allochthonous blocks that have been transported from the platform margin into basinal environments retain their heavy δ 13C values (4–9‰) and are surrounded by basinal calcareous shales with light δ 13C values (ave. 0.8‰). The regional and stratigraphic consistency of these δ 13C trends suggests a primary marine origin. We interpret this δ 13C-facies correlation to be the result of ocean stratification/stagnation that significantly reduced the rate of deep-ocean ventilation and produced a large deep-water, organically derived carbon reservoir. We suggest that stratification may have been a persistent feature of the global ocean throughout much of Neoproterozoic time. Ocean stratification may explain many of the unusual features that characterise the sedimentary record of this era, including large-scale δ 13C variation, extreme climatic fluctuations, and the presence of cap carbonates. A highly variable climatic regime would be expected with the development of a large deep-water carbon reservoir. Small changes in ocean circulation could rapidly transfer or remove large volumes of carbon to and from the surface-ocean and atmospheric reservoirs, leading to intense greenhouse or icehouse conditions respectively. [Copyright &y& Elsevier]

Abstract: U-Pb ages of magmatic zircons in tuff horizons in the Chhattisgarh and the Vindhyan Supergroups in India, backed up by paleomagnetic data, suggest that most Proterozoic basins in India are about 500Ma older than the current consensus. The issue is hotly debated including questions about the stratigraphic positions of the tuff horizons. Thus, the geologic significance of the ∼1000Ma age of the rhyolitic tuff near Sukhda and Sapos villages in the Chhattisgarh Supergroup in central India hinges on its proper stratigraphic placement. If the tuff is near the top of the Chhattisgarh Supergroup, then the Chhattisgarh and its equivalent sediments were deposited in the Mesoproterozoic and not, as has been the general notion, in the Neoproterozoic. The tuff lies conformably on the platform-facies Saradih Limestone of the Raipur Group (upper Chhattisgarh) and not on an Archean-Proterozoic basement; it is overlain by fluvial volcaniclastic conglomeratic lithologies of the Sarnadih Sandstone. Had this sandstone belonged to the basal Lohardih Formation deposited in fan-deltas of an opening basin, as is currently believed, it would have been overlain by prodelta deposits including mature quartz arenites and floored by crystalline rocks. Formation-mapping, facies analysis, and petrologic considerations place the Sarnadih Sandstone, and thus the ∼1000Ma Sukhda Tuff, near the top of the sedimentary sequence of the Chhattisgarh basin. Consequently, rocks below the Sukhda Tuff must be pre-Neoproterozoic in age. Hence, rocks and tectonics of these Proterozoic basins are irrelevant to arguments about the Cryogenian or break-up of Rodinia, but are related more to the assembly of Rodinia and the break-up of Nuna. Metazoan and animal life forms, reported from the Chhattisgarh and equivalent basins, must also have originated and evolved in pre-Neoproterozoic time. [Copyright &y& Elsevier]