Snowball Glaciation Research Articles

Three-stage formation of cap carbonates after Marinoan snowball glaciation consistent with depositional timescales and geochemistry

At least two global “Snowball Earth” glaciations occurred during the Neoproterozoic Era (1000-538.8 million years ago). Post-glacial surface environments during this time are recorded in cap carbonates: layers of limestone or dolostone that directly overlie glacial deposits. Postulated environmental conditions that created the cap carbonates lack consensus largely because single hypotheses fail to explain the cap carbonates’ global mass, depositional timescales, and geochemistry of parent waters. Here, we present a global geologic carbon cycle model before, during, and after the second glaciation (i.e. the Marinoan) that explains cap carbonate characteristics. We find a three-stage process for cap carbonate formation: (1) low-temperature seafloor weathering during glaciation generates deep-sea alkalinity; (2) vigorous post-glacial continental weathering supplies alkalinity to a carbonate-saturated freshwater layer, rapidly precipitating cap carbonates; (3) mixing of post-glacial meltwater with deep-sea alkalinity prolongs cap carbonate deposition. We suggest how future geochemical data and modeling refinements could further assess our hypothesis.

Climatic Effects of Ocean Salinity on M Dwarf Exoplanets

Ocean salinity is known to dramatically affect the climates of Earth-like planets orbiting Sun-like stars, with high salinity leading to less ice and higher surface temperature. However, how ocean composition impacts climate under different conditions, such as around different types of stars or at different positions within the habitable zone, has not been investigated. We used ROCKE-3D, an ocean-atmosphere general circulation model, to simulate how planetary climate responds to ocean salinities for planets with G-star versus M dwarf hosts at several stellar fluxes. We find that increasing ocean salinity from 20 to 100 g kg−1 in our model results in nonlinear ice reduction and warming on G-star planets, sometimes causing abrupt transitions to different climate states. Conversely, sea ice on M dwarf planets responds more gradually and linearly to increasing salinity. Moreover, reductions in sea ice on M dwarf planets are not accompanied by significant surface warming as on G-star planets. High salinity can modestly bolster the resilience of M dwarf planets against snowball glaciation and allow these planets to retain surface liquid water further from their host star, but the effects are muted compared to G-star planets that experience snowball bifurcation and climate hysteresis due to the ice-albedo feedback.

The Mall Bay Formation (Ediacaran) and the protracted onset of the Gaskiers glaciation in Newfoundland, Canada

The Gaskiers is the oldest Ediacaran glaciation and thus represents Earth’s first icehouse since the end of the Snowball glaciations that characterized the Cryogenian world.Age bracketing of the 300-m-thick marine tillites of the Gaskiers Formation to 581–580 Ma has previously been taken as an indication that the Gaskiers glaciation was short-lived. New studies of the underlying ∼ 1000-m-thick Mall Bay Formation on islands offshore of Newfoundland, Canada, reveal a hitherto unrecognized protracted onset to the Gaskiers glaciation that nearly triples its known stratigraphic thickness and elucidates progressive stages in the development of an Ediacaran glaciation.The Mall Bay Formation consists predominantly of mudstone and fine-grained sandstone deposited in a deep-water, SW-NE trending, turbidite-dominated axial basin with abundant input of volcanic ash.No evidence of glacial activity was observed in the lower half of the Mall Bay Formation.Cold-climate proxies first appear approximately 550 m below the top of the formation in the form of glendonites (stellate pseudomorphs of the cold-water carbonate ikaite) that attest to cold bottom waters and hypothesized downwelling, while the preservation of outsized clasts, frozen aggregates (till pellets), and coarse sandstone lags near the same stratigraphic level imply the presence of icebergs floating above the depositional environment.Evidence for cold bottom waters and an influx of sediment from floating ice persists to the top of the Mall Bay Formation. There is a progressive stratigraphic increase in the abundance and size of iceberg-rafted debris in the formation that implies increasing proximity of glaciers to the study area.Pebbles showing dropstone features first appear 120 m below the top of the formation, and the lowest bed of pebble diamictite occurs 4 m below the top of the formation.The overlying cobble to boulder diamictite of the Gaskiers Formation likely represents the Gaskiers glacial maximum when glaciers reached the local shelf edge and glacially derived debris poured down the slope into the axial, turbidite-dominated Avalon Basin.The Ediacaran Gaskiers glaciation shows little similarity to the older panglacial Snowball glaciations of the Cryogenian but exhibits paraglacial features similar in many respects to the Paleozoic glaciations of Gondwana that followed it.

Dynamic deep-water oxygenation of marginal seas in the aftermath of the Sturtian Snowball Earth: Insights from redox-hydrological reconstructions of the Nanhua basin (South China)

The Cryogenian interglacial period, separated by the Sturtian and Marinoan icehouses, witnessed early radiations of eukaryotic algae and the first emergence of putative metazoans. To date, the detailed oceanic redox state in the aftermath of the Sturtian glaciation remains obscure, limiting our understanding of the causal linkage, if any, between environmental changes and biological evolution during this critical interval. In order to gain further insights into the marine redox framework following the Sturtian Snowball glaciation, here we present high-resolution geochemical results of an integrated study on the basal Datangpo Formation from the Daotuo area (South China), and also provide a systematically compiled dataset of counterparts in other profiles from the Nanhua Basin. The basal Datangpo Formation is characterized by black shales intercalated with Mn carbonate ore layers in the lower (Member I), and only black shales in the upper (Member II). The Member I black shales have relatively high abundances of Mo and Re, and display a MoEF-UEF pattern typical of Mn-oxide shuttle, indicating that the Nanhua basin was redox-stratified with the oxic-euxinic chemocline fluctuating above the deep-basin floor. Multiple lines of evidence, including positive PAAS-normalized Ce anomalies, negative δ13Ccarb values, and marked authigenic enrichments of manganophile elements (Co, Ni, Cu), imply that the Mn carbonate ore beds were generated through the diagenetic transformation of Mn oxides originally deposited under oxic bottom waters. Therefore, these Mn ore layers hosted by the Member I black shales reflect episodic benthic oxygenation. By contrast, the Nanhua basin was more stagnant during deposition of Member II black shales, as supported by their rather low Mo/TOC ratios, as well as the relatively lower abundances of Mo and Re compared to Member I black shales. Moreover, the Mo isotope data of the basal Datangpo Formation indicate that the global ocean remained pervasively anoxic. Taken together, this study reveals a more holistic redox landscape in the wake of the Sturtian Snowball Earth—that is, extensive anoxia in the global ocean with episodic deep-water oxygenation in the marginal seas such as the Nanhua Basin. These epicratonic seas with dynamic redox variations might promote the ecological revolution during the interglacial period immediately after the Sturtian glaciation.

Biological diversification linked to environmental stabilization following the Sturtian Snowball glaciation.

The body fossil and biomarker records hint at an increase in biotic complexity between the two Cryogenian Snowball Earth episodes (ca. 661 million to ≤650 million years ago). Oxygen and nutrient availability can promote biotic complexity, but nutrient (particularly phosphorus) and redox dynamics across this interval remain poorly understood. Here, we present high-resolution paleoredox and phosphorus phase association data from multiple globally distributed drill core records through the non-glacial interval. These data are first correlated regionally by litho- and chemostratigraphy, and then calibrated within a series of global chronostratigraphic frameworks. The combined data show that regional differences in postglacial redox stabilization were partly controlled by the intensity of phosphorus recycling from marine sediments. The apparent increase in biotic complexity followed a global transition to more stable and less reducing conditions in shallow to mid-depth marine environments and occurred within a tolerable climatic window during progressive cooling after post-Snowball super-greenhouse conditions.

Mid-latitudinal habitable environment for marine eukaryotes during the waning stage of the Marinoan snowball glaciation

During the Marinoan Ice Age (ca. 654–635 Ma), one of the ‘Snowball Earth’ events in the Cryogenian Period, continental icesheets reached the tropical oceans. Oceanic refugia must have existed for aerobic marine eukaryotes to survive this event, as evidenced by benthic phototrophic macroalgae of the Songluo Biota preserved in black shales interbedded with glacial diamictites of the late Cryogenian Nantuo Formation in South China. However, the environmental conditions that allowed these organisms to thrive are poorly known. Here, we report carbon-nitrogen-iron geochemical data from the fossiliferous black shales and adjacent diamictites of the Nantuo Formation. Iron-speciation data document dysoxic-anoxic conditions in bottom waters, whereas nitrogen isotopes record aerobic nitrogen cycling perhaps in surface waters. These findings indicate that habitable open-ocean conditions were more extensive than previously thought, extending into mid-latitude coastal oceans and providing refugia for eukaryotic organisms during the waning stage of the Marinoan Ice Age.

Tonian Low‐Latitude Marine Ecosystems Were Cold Before Snowball Earth

AbstractPrecambrian marine carbonate strata are commonly assumed to have formed in warm‐water carbonate factories due to the temperature dependence of non‐skeletal carbonate precipitation rates. However, some climate models and geological observations suggest that global climate was cool for tens of millions of years prior to the onset of Snowball Earth glaciation at ∼717 Ma, in conflict with common interpretations of pre‐glacial carbonates as warm‐water carbonate factories. We report the occurrence of guttulatic microfabric—a petrographic fingerprint of ikaite, a carbonate mineral that only forms in cold sedimentary environments—in the Beck Spring Dolomite, a carbonate succession deposited in a low‐latitude shallow marine environment between ∼780 and 730 Ma. This interpretation of pre‐glacial carbonate factories aligns cold conditions with vase‐shaped microfossils, possible algal fossils, and molecular clock dates for crown‐group metazoans. Our observations indicate that these marine ecosystems were able to thrive in cold low‐latitude environments millions of years before the Snowball glaciations.

Sequence stratigraphy and paleoenvironmental significance of the Neoproterozoic Bambui Group, Central Brazil

The Bambui Group is a ca. 2,000 m-thick mixed biochemical-siliciclastic sedimentary succession that accumulated in the Sao Francisco Basin, central Brazil. Its age is contentious, but deposition during the Cryogenian-Ediacaran (ca. 635–550 Ma) is consistent with a new sequence stratigraphic model presented herein and previously published geochemical and structural data. Six stratigraphic sequences are recognized that record depositional evolution from an epeiric sea to foreland basin during the Marinoan snowball glaciation (ca. 635 Ma) to shortly after the Gaskiers (ca. 580 Ma) glaciation. Although this stratigraphic architecture differs from the classic lithostratigraphic and more recent chemostratigraphic and sequence stratigraphic interpretations of the Bambui Group, it is supported by correlation to other Cryogenian-Ediacaran successions globally. Collectively, these six sequences preserve a robust record of seawater redox structure leading up to the oxygenation threshold that triggered the evolution of multicellular animals in the Ediacaran.Sequence 1 is composed of Marinoan glacial deposits and an overlying cap carbonate with aragonite fan pseudomorphs and barite. Sequence 2 is composed of lime-mudstone that changes laterally into loess-rich peritidal phosphorites, suggesting oxygen stratification in shallow paleoenvironments. Sequence 3 is a carbonate succession with abundant stromatolites implying widespread oxygen production. The top of Sequence 3 is marked by a regional subaerial unconformity developed during uplift associated with the onset of the Brasiliano-Pan African Orogeny. Sequence 4 signals a return to loess derived sedimentation. Distal deposits contain abundant organic matter with framboidal pyrite reflecting bacterial sulfate reduction in low sulfate Ediacaran seawater. Sequence 5 is composed of well-developed aggradational parasequences composed of alternating deep subtidal organic-rich siltstones and shallow-water carbonate grainstones. Their well-defined cyclicity is interpreted to record extreme sea level fluctuations during the onset of the Gaskiers glaciation. Stromatolites suggest an oxygen-stratified water column. Sequence 6 marks the change from an epeiric sea to a foreland basin. Interbedded basinal glauconitic siltstone and shale loess-rich are correlative with shoreface sandstones. Authigenic glauconite requires suboxic bottom waters to precipitate and thus reflects oxygenation of distal settings. Most of the deposition ended by ca. 560–550 Ma when the entire Bambui Group was deformed during the assembly of West Gondwana.In sequences 1, 2, 3, 4, 5, and the lower portion of 6, the lateral continuity of lithofacies, predictable layer-cake stratigraphy, and lack of fluvial coarse siliciclastic deposits are characteristics of epeiric sea successions. The appearance of immature sandstone, poorly sorted conglomerate, braided fluvial deposits, and synsedimentary deformation in the upper part of Sequence 6 are interpreted as the onset of foreland basin development. Stacking relationships of redox sensitive, authigenic lithofacies in both the epeiric sea and foreland phases support recent research suggesting seawater redox conditions were in a state of flux during the apex of the purported Neoproterozoic Oxygenation Event. Organic matter and authigenic pyrite in Sequences 4 and 5 reflect increased primary productivity in surface waters fertilized by nutrients such as P and Fe adsorbed on loess particles. This mechanism could be expanded to a global scale and have contributed to the increase in atmospheric and oceanic oxygen levels. These findings redefine and highlight the significance of the Bambui Group for Neoproterozoic Earth history.

The Effect of Ocean Salinity on Climate and Its Implications for Earth's Habitability

The influence of atmospheric composition on the climates of present‐day and early Earth has been studied extensively, but the role of ocean composition has received less attention. We use the ROCKE‐3D ocean‐atmosphere general circulation model to investigate the response of Earth's present‐day and Archean climate system to low versus high ocean salinity. We find that saltier oceans yield warmer climates in large part due to changes in ocean dynamics. Increasing ocean salinity from 20 to 50 g/kg results in a 71% reduction in sea ice cover in our present‐day Earth scenario. This same salinity change also halves the pCO2 threshold at which Snowball glaciation occurs in our Archean scenarios. In combination with higher levels of greenhouse gases such as CO2 and CH4, a saltier ocean may allow for a warm Archean Earth with only seasonal ice at the poles despite receiving ∼20% less energy from the Sun.

A transient peak in marine sulfate after the 635-Ma snowball Earth

SignificanceEarth system's response to major perturbations is of paramount interest. On the basis of multiple isotope compositions for pyrite, carbonate-associated sulfate, carbonates, and organics within, we inferred that the much-debated, enigmatic, extremely 13C-depleted calcite cements in the ∼635-Ma cap carbonates in South China preserve geochemical evidence for marine microbial sulfate reduction coupled to anaerobic oxidation of methane. This interpretation implies the existence of a brief interval of modern-level marine sulfate. We determined that this interval coincides with the earliest Ediacaran 17O-depletion episode, and both likely occurred within ∼50 ky since the onset of the 635-Ma meltdown, revealing an astonishing pace of transformation of the Earth system in the aftermath of Earth's latest snowball glaciation.

Oldest-known Neoproterozoic carbon isotope excursion: Earlier onset of Neoproterozoic carbon cycle volatility

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.

Weak tides during Cryogenian glaciations

The severe “Snowball Earth” glaciations proposed to have existed during the Cryogenian period (720 to 635 million years ago) coincided with the breakup of one supercontinent and assembly of another. Whereas the presence of extensive continental ice sheets predicts a tidally energetic Snowball ocean due to the reduced ocean depth, the supercontinent palaeogeography predicts weak tides because the surrounding ocean is too large to host tidal resonances. Here we show, using an established numerical global tidal model and paleogeographic reconstructions, that the Cryogenian ocean hosted diminished tidal amplitudes and associated energy dissipation rates, reaching 10–50% of today’s rates, during the Snowball glaciations. We argue that the near-absence of Cryogenian tidal processes may have been one contributor to the prolonged glaciations if these were near-global. These results also constrain lunar distance and orbital evolution throughout the Cryogenian, and highlight that simulations of past oceans should include explicit tidally driven mixing processes.

Germanium/silica ratio and rare earth element composition of silica-filling in sheet cracks of the Doushantuo cap carbonates, South China: Constraining hydrothermal activity during the Marinoan snowball Earth glaciation

The rapid diversification of eukaryotes within 3 million years after the Marinoan global glaciation suggests the potential linkage between the snowball Earth event and biological evolution. To understand how the global glaciation and the biological evolution were linked, it is necessary to reconstruct the sequence of events with high temporal resolution that occurred during the deglaciation. In this study, we focus on silica-filling within sheet cracks. Sheet cracks abundantly occur in the basal part of the Marinoan cap carbonate (635 Ma), and are characterized by the interconnected horizontal and vertical cracks that are sequentially cemented by dolospars, silica and calcite. As the second stage of crack filling, the origin of silica-filling remains controversial. If the seawater origin is confirmed, the silica-filling might archive the signal of ocean chemistry in the aftermath of snowball Earth event. In this study, we measured the Germanium/Silica ratios (Ge/Si) and Rare Earth Elements (REE) compositions of silica-filling from the Doushantuo cap carbonate in the Yangtze Platform, South China. The silica-filling has low Ge/Si ratios (~1 μmol/mol), and displays a light REE (LREE) depleted REE pattern (normalized to PAAS) with positive Eu anomalies (Eu/Eu*). These results indicate the predominant seawater origin of silica with some contribution from hydrothermal fluid, suggesting the penecontemporaneous precipitation of silica-filling with respect to sheet crack formation. In order to quantify the contribution from hydrothermal fluid, we developed a binary mixing model by using Ge/Si ratio and Eu/Eu* as two independent proxies. The modeling results indicate mixing of ~5 vol.% hydrothermal fluid with contemporaneous seawater, or ~20 wt.% of Si in silica-filling from hydrothermal fluid. The modeling results also show that the synglacial hydrothermal flux might be one order of magnitude lower than the modern level, suggesting extremely weak hydrothermal activities during the Marinoan snowball glaciation.

Planktonic adaptive evolution to the sea surface temperature in the Neoproterozoic inferred from ancestral NDK of marine cyanobacteria

The optimum growth temperature of ancestral cyanobacteria inhabiting the sea surface in the Neoproterozoic was estimated based on the thermal stability of experimentally reconstructed ancestral NDK enzymes. Ancestral NDKs of cyanobacteria that diversified ∼1.7, ∼1.0, ∼0.9, ∼0.7, ∼0.6, and ∼0.5 billion years ago were reconstructed and analyzed, and the unfolding midpoint temperatures (Tms) ranged from ∼65°C to ∼70°C. Among the host of analyzed NDKs, the ancestors of marine, planktonic α-cyanobacteria diversified ≤ ∼1.0 Ga are highly likely to have inhabited marine environments during the Neoproterozoic, while ancestral cyanobacteria diversified ∼1.7 billion years ago were possibly marine but the habitat is less constrained compared to the others. According to the calibration curves derived from extant organisms, the obtained Tms of α-cyanobacteria diversified ≤ ∼1.0 Ga correspond to the range of optimum growth temperatures of around ∼33–48°C. The temperature range agrees well with the long-term sea temperature trend during Neoproterozoic suggested by δ18O and δ30Si records from marine cherts. Adaptation to the low temperature during the snowball glaciations in the late Neoproterozoic was not observed, implying that adaptation of optimum growth conditions to the episodic low temperature may not have been necessary. Therefore, ancestral marine plankton must have consistently adapted to the interglacial sea surface temperature in the Neoproterozoic, which was approximately 5–20°C higher than that is today. They may have survived the glaciations by acquiring cold tolerance and/or by suppressing growth rate.

The transition from a cyanobacterial to algal world and the emergence of animals.

The Neoproterozoic, 1000-541 million years (Myr) ago, saw the transition from a largely bacterial world to the emergence of multicellular grazers, suspension feeders and predators. This article explores the hypothesis that the first appearance of large, multicellular heterotrophs was fueled by an elevated supply of nutrients and carbon from the bottom of the food chain to higher trophic levels. A refined record of molecular fossils of algal sterols reveals that the transition from dominantly bacterial to eukaryotic primary production in open marine habitat occurred between 659 and 645 Myr ago, in the hot interlude between two Snowball Earth glaciations. This bacterial-eukaryotic transition reveals three characteristics: it was rapid on geological timescales, it followed an extreme environmental catastrophe and it was permanent - hallmarks of an ecological hysteresis that shifted Earth's oceans between two self-stabilizing steady states. More than 50 million years of Snowball glaciations and their hot aftermath may have purged old-world bacterial phytoplankton, providing empty but nutrient-rich ecospace for recolonization by larger algae and transforming the base of the food web. Elevated average and maximum particle sizes at the base of the food chain may have provided more efficient energy and nutrient transfer to higher trophic levels, fueling an arms race toward larger grazers, predators and prey, and the development of increasingly complex feeding and defense strategies.

Multiproxy geochemical and isotope stratigraphy records of a Neoproterozoic Oxygenation Event in the Ediacaran Sete Lagoas cap carbonate, Bambuí Group, Brazil

Abstract 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.

Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

Cryogenian (∼720-635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth's history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense chemical weathering on continents. However, direct geochemical evidence for the intense chemical weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)-indicative of an episode of intense chemical weathering-occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (<+0.40), suggesting moderate to low intensity of chemical weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of chemical weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO2 concentration was high. Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO2 and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition.

Dodging snowballs: Geochronology of the Gaskiers glaciation and the first appearance of the Ediacaran biota

Abstract The snowball Earth hypothesis predicts that low-latitude glaciation lasted millions of years while CO2 built up to critical levels to culminate in catastrophic deglaciation in a supergreenhouse climate. The Gaskiers Formation of eastern Newfoundland (Canada) has been attributed to a snowball glaciation event, but the lack of robust paleomagnetic data and precise geochronological constraints has precluded tests of the hypothesis. Here we present high-precision U-Pb zircon geochronology (chemical abrasion–isotope dilution–thermal ionization mass spectrometry) from eight tuffs from multiple distant stratigraphic sections that bracket glacial diamictites and the first appearance of large Ediacaran fossils. Including internal error, deposition of the Gaskiers diamictite on the Avalon Peninsula is constrained to have been between 580.90 ± 0.40 and 579.88 ± 0.44 Ma, and the Trinity diamictite on Bonavista Peninsula was deposited between 579.63 ± 0.15 and 579.24 ± 0.17 Ma. Assuming approximately synchronous deglaciation, these results imply a maximum duration for deposition of the Trinity diamictite of ≤340 k.y.; this is inconsistent with the multimillion year duration predicted by the snowball Earth hypothesis. Our geochronologic data also constrain the first appearance datum of Ediacaran fossils to &amp;lt;9.5 m.y. after the Gaskiers glaciation. Thus, despite existing paleomagnetic constraints that indicate that marine ice sheets extended to low to middle latitudes, it appears that Earth narrowly escaped a third Neoproterozoic snowball glaciation just prior to the late Ediacaran expansion of metazoan ecosystems.

Early sponges and toxic protists: possible sources of cryostane, an age diagnostic biomarker antedating Sturtian Snowball Earth.

The period 800-717million years (Ma) ago, in the lead-up to the Sturtian Snowball glaciation, saw an increase in the diversity of eukaryotic microfossils. To afford an independent and complementary view of this evolutionary period, this study presents the distribution of eukaryotic biomarkers from three pre-Sturtian successions across the supercontinent Rodinia: the ca. 780Ma Kanpa Formation of the Western Australian Officer Basin, the ca. 800-740Ma Visingsö Group of Sweden, and the 740Ma Chuar Group in Arizona, USA. The distribution of eukaryotic steranes is remarkably similar in the three successions but distinct from all other known younger and older sterane assemblages. Cholestane was the only conventional structure, while indigenous steranes alkylated in position C-24, such as ergostane, stigmastane, dinosterane and isopropylcholestane, and n-propylcholestane, were not observed. This sterane distribution appears to be age diagnostic for the pre-Sturtian Neoproterozoic. It attests to the distinct evolutionary state of pre-Snowball eukaryotes, pointing to a taxonomic disparity that was still lower than in the Ediacaran (635-541Ma). All three basins also show the presence of a new C28 sterane that was tentatively identified as 26-methylcholestane, here named cryostane. The only known extant organisms that can methylate sterols in the 26-position are demosponges. This assignment is plausible as molecular clocks place the appearance of the earliest animals into the pre-Sturtian Neoproterozoic. The unusual 26-methylsterol may have protected sponges, but also other eukaryotes, against their own membranolytic toxins. Some protists release lytic toxins to deter predators and kill eukaryotic prey. As conventional membrane sterols can be the site of attack for these toxins, sterols with unusual side-chain modification protect the cell. This interpretation of cryostane supports fossil evidence of predation in the Chuar Group and promotes hypotheses about the proliferation of eukaryophagy in the lead-up to the Cryogenian.

Snowball cooling after algal rise

The Earth underwent two snowball glaciation events between 720 and 635 million years ago. The preceding expansion of eukaryotic algae and a consequent rise in emissions of organic cloud condensation nuclei may have contributed to the dramatic cooling.