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Precambrian tropical glaciation is an enigma of Earth's climate. Overlooking fundamental difference of land/sea icelines, it was equated with a global frozen ocean, which is at odds with the sedimentary evidence of an active hydrological cycle, and its genesis via the runaway ice–albedo feedback conflicts with the mostly ice-free Proterozoic when its trigger threshold was well exceeded by the dimmer sun. In view of these shortfalls, I put forth two key hypotheses of the tropical glaciation: first, if seeded by mountain glaciers, the land ice would advance on sea level to be halted by above-freezing summer temperature, which thus abuts an open cozonal ocean; second, a tropical supercontinent would block the brighter tropical sun to cause the required cooling. To test these hypotheses, I formulate a minimal tropical/polar box model to examine the temperature response to a varying tropical land area and show that tropical glaciation is indeed plausible when the landmass is concentrated in the tropics despite uncertain model parameters. In addition, given the chronology of paleogeography, the model may explain the observed deep time climate to provide a unified account of the faint young Sun paradox, Precambrian tropical glaciations, and Phanerozoic glacio-epochs, reinforcing, therefore, the uniformitarian principle. [ABSTRACT FROM AUTHOR]

[Display omitted] Secular changes in atmospheric CO 2 and consequent global climate variations, are commonly attributed to global outgassing and the efficiency of silicate weathering, which may have been linked to mountain formation, land/arc distribution, and plant colonization through geological time. Although oxidative weathering has been shown to exert a significant role in the propagation of weathering fronts through the oxidation of Fe-bearing minerals, the influence of atmospheric O 2 concentration (p O 2) on silicate weathering, CO 2 consumption, and global climate has not been thoroughly evaluated. This study presents a numerical model aimed at estimating the effects of p O 2 on the climate, considering the influence of p O 2 on the regolith thickness and thus weathering duration of granitic domains. Our model simulations reveal that an increase in weathering efficiency, through deeper penetration of the oxidative weathering front in the granitic regolith, would independently introduce a steady-state climate cooling of up to ∼8 °C, in step with one-order of magnitude rise in p O 2. This temperature change may have repeatedly initiated the runaway ice-albedo feedback, leading to global glacial events (e.g., Neoproterozoic Snowball Earth). Increasing granitic weathering efficiency caused by a substantial p O 2 increase may also have contributed to the development of icehouse climate during the Phanerozoic. [ABSTRACT FROM AUTHOR]

The Bigganjarga Tillite on the Varanger Peninsula, northern Norway, was described by Hans H. Reusch in 1891 as one of the first evidence of a glaciation older than the Late Quaternary “real ice-age” in Europe. Since the late 1800s, considerable geoscientific research covering lithostratigraphy, sedimentology, paleontology and geochronology have been carried out in the Neoproterozoic and Early Paleozoic successions of East- Finnmark, and the Bigganjarga tillite (also referred to as the Reusch Moraine) is now recognized as a diamictite that correlates with global Marinoan glaciation in Cryogenian time. The Bigganjarga Tillite belongs to the Smalfjord Formation and marks the base of the Vestertana Group which rests on quartzite of the Veidnesbotn Formation with a significant unconformity. The site at Oaibáhčannjárga where Bigganjarga Tillite was first discovered became protected in 1967 and is today an international recognized Geoheritage from the Snowball Earth. New research studies are in progress at the site and the Bigganjarga Tillite is a prime example of how Geological Heritage is selected and recognized for its historic and potential future scientific values. [ABSTRACT FROM AUTHOR]

The Snowball Earth hypothesis predicts global ice cover; however, previous descriptions of Cryogenian (720-635 Ma) glacial deposits are limited to continental margins and shallow marine basins. The Tavakaiv (Tava) sandstone injectites and ridges in Colorado, USA, preserve a rare terrestrial record of Cryogenian low-latitude glaciation. Injectites, ridges, and chemically weathered crystalline rock display features characteristic of fluidization and pervasive deformation in a subglacial environment due to glacial loading, fluid overpressure, and repeated sand injection during meltwater events. In situ hematite U-Pb geochronology on hematite-quartz veins, which crosscut and are cut by Tava dikes, constrain sand injection at ~690-660 Ma. We attribute early Tava sand injection episodes to basal melting associated with rifting and geothermal heating, and later injections to meltwater generation during ~661 Ma Sturtian deglaciation. A modern analog is provided by the Ross Embayment of Antarctica, where rift-related faults border sediment-filled basins, overpressurized fluids circulate in confined aquifers below ice, and extensive preglacial topography is preserved. Field evidence and geochronology in Colorado further highlight that deep chemical weathering of Proterozoic bedrock and denudation associated with the Great Unconformity predate Cryogenian injection of fluidized sand, consistent with limited glacial erosion. [ABSTRACT FROM AUTHOR]

Neoproterozoic snowball Earth events lasted for multiple million years, experiencing many orbital cycles. Here we investigate whether the deglaciation of these events would be triggered more easily at certain orbital configurations than others, by using an atmosphere‐land model that considers meltpond formation on land ice. Results show that the threshold concentration of atmospheric CO2 (pCO2) required for deglaciation can vary from 6 to 10 × 104 ppmv under different orbital forcings. The threshold pCO2 decreases with the equatorial maximum monthly insolation (EMMI), which is affected most by the eccentricity and secondarily by obliquity. Therefore, we conclude that the snowball Earth deglaciation likely occurred when the eccentricity was high and obliquity was low. Compared to previous estimate that used present‐day orbital configuration which has a minimal eccentricity, the duration of snowball Earth events would likely be much shorter when the influence of orbital variations are considered. Plain Language Summary: This study explores how different orbital configurations might have influenced the termination of the Neoproterozoic snowball Earth events, during which Earth was covered by ice globally. Using a coupled atmosphere‐land model that is capable of simulating the formation of melt ponds on ice, we find that the atmospheric CO2 level needed to initiate the deglaciation varies with the Earth's orbital configurations. Specifically, the required CO2 levels are lower when the maximum monthly solar insolation received at the equator is higher, which is achieved when the Earth's orbit is more eccentric and the tilt of Earth's rotational axis is small. The results suggest that the duration of snowball Earth events could have been shorter when the influence of orbital forcing is considered. Key Points: Influence of orbital forcing on the snowball Earth deglaciation is tested using an atmosphere‐land model that considers meltpond on iceThe equatorial maximum monthly insolation is the most important factor that triggers the deglaciation of a snowball EarthThe needed CO2 to deglaciate a snowball Earth at the optimal orbital configuration is 40% lower than that under modern orbit [ABSTRACT FROM AUTHOR]

Earth may have been globally ice‐covered at least two times during the Cryogenian Period (720–635 Ma). Previous studies showed that clouds could strongly warm a hard snowball Earth and promote its deglaciation. However, the understanding of clouds was largely based on coarse‐resolution global simulations with parameterized convection and clouds, or high‐resolution cloud‐resolving simulations with explicit convection and clouds but limited to a small domain without the effects of large‐scale circulation. Here we present the first global non‐hydrostatic high‐resolution (30 km) simulation to investigate snowball clouds and their radiative effects. We show that spatial‐temporal cloud distributions exhibit clear meanders (patterns of curved lines rather than straight lines along the west‐east direction) and strong variabilities, which result from the tight interactions among the Inter‐Tropical Convergence Zone, Hadley cells, and baroclinic instability. The net cloud radiative effect is around 20 W m−2 in the tropics, consistent with previous global general circulation model simulations. Plain Language Summary: Earth may have been globally covered by snow and ice during the Cryogenian Period and is known as a hard snowball Earth, which is radically different from modern Earth. Using a global non‐hydrostatic high‐resolution model, within which vertical velocity and clouds are largely explicitly resolved, we find that the clouds on the hard snowball Earth show a more meandering pattern than modern Earth. Meanwhile, our results confirm the warming effect of snowball clouds, implying a lower CO2 concentration to terminate the global glaciation. Key Points: Snowball climate is simulated by a global non‐hydrostatic high‐resolution modelFine structures of clouds, Inter‐Tropical Convergence Zone, Hadley cells, and baroclinic eddies are clearly resolvedOur simulations confirm that clouds have a net warming effect on a hard snowball Earth [ABSTRACT FROM AUTHOR]

The temporal relationship between global glaciations and the Great Oxidation Event (GOE) suggests that climate change played an important role in Earth's oxygenation. The potential role of temperature is captured by the stratigraphic proximity between glacial deposits and sediments containing mass‐independent fractionation of sulfur isotopes (MIF‐S). We use a time‐dependent one‐dimensional photochemical model to investigate whether temperature changes associated with global glaciations can drive oscillations in atmospheric O2 levels and MIF‐S production across the GOE. We find that extreme climate change can cause atmospheric O2 to oscillate between pre (<10−6 times the present atmospheric level, PAL) and post‐GOE (>10−5 PAL) levels. Post‐glacial hot‐moist greenhouse climates lead to post‐GOE O2 levels because the abundant H2O vapor and oxidizing radicals drive the depletion of reduced species. This pattern is generally consistent with the MIF‐S signal observed in the sedimentary record, suggesting a link between global glaciations and O2 oscillations across the GOE. Plain Language Summary: The Great Oxidation Event was the most significant environmental and chemical transformation in Earth's history, marking the first time oxygen accumulated in the atmosphere around 2.4 billion years ago. Oxygen increased from below one millionth (low) to at least one‐thousandth of a percent (intermediate) of the present oxygen concentration during this event. However, measurements of geochemical oxygen proxies suggest that oxygen levels oscillated between low and intermediate levels before stabilizing after this event. The first rise of atmospheric oxygen occurred during a period of extreme climate variability indicated by the presence of glacial rock deposits around this time. In this study, we use a time‐dependent photochemical model to show that extreme temperature changes caused by global glaciations can drive oscillations in atmospheric oxygen levels across the Great Oxidation Event (GOE). Our results can help explain why atmospheric oxygen shows drastic changes across the GOE in a way that is consistent with the geochemical record. Key Points: Across the Great Oxidation Event (GOE), extreme climate change linked to global glaciations can drive oscillations in atmospheric O2 levels and mass‐independent fractionation of sulfur isotopes (MIF‐S) productionGlacial climates and hot‐moist greenhouse climates were likely characterized by pre‐GOE and post‐GOE O2 levels respectivelyTemperature changes associated with global glaciations can help explain the MIF‐S record across the GOE [ABSTRACT FROM AUTHOR]

Near the village of Duurwater Pos at the foot of the Fransfontein Ridge, north-western Namibia, an erratic megalith of basement monzogranite (Huab gneiss), 130 m long by 52 m wide, is perched on a pedestal of early Cryogenian (Sturtian) tillite. The pedestal had at least 134 m of palaeotopographic relief, plus the additional 41 m (tilt-corrected) height of the erratic itself. The tillite pedestal is inferred to be a hoodoo structure formed by differential erosion with shielding of the pedestal by the hard basement erratic. The erratic and its pedestal were preserved because of rapid marine inundation during Snowball Earth deglaciation, followed by onlap and burial by postglacial carbonate sediments of the middle Cryogenian Berg Aukas and Okonguarri Formations. As a glacial erratic, it is possibly the largest and oldest example known. [ABSTRACT FROM AUTHOR]

The Great Unconformity marks a major gap in the continental geological record, separating Precambrian basement from Phanerozoic sedimentary rocks. However, the timing, magnitude, spatial heterogeneity, and causes of the erosional event(s) and/or depositional hiatus that lead to its development are unknown. We present field relationships from the 1.07-Ga Pikes Peak batholith in Colorado that constrain the position of Cryogenian and Cambrian paleosurfaces below the Great Unconformity. Tavakaiv sandstone injectites with an age of ≥676 ± 26 Ma cut Pikes Peak granite. Injection of quartzose sediment in bulbous bodies indicates near-surface conditions during emplacement. Fractured, weathered wall rock around Tavakaiv bodies and intensely altered basement fragments within unweathered injectites imply still earlier regolith development. These observations provide evidence that the granite was exhumed and resided at the surface prior to sand injection, likely before the 717-Ma Sturtian glaciation for the climate appropriate for regolith formation over an extensive region of the paleolandscape. The 510-Ma Sawatch sandstone directly overlies Tavakaiv-injected Pikes granite and drapes over core stones in Pikes regolith, consistent with limited erosion between 717 and 510 Ma. Zircon (U-Th)/He dates for basement below the Great Unconformity are 975 to 46 Ma and are consistent with exhumation by 717 Ma. Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion. We propose that multiple Great Unconformities developed diachronously and represent regional tectonic features rather than a synchronous global phenomenon.

The snowball Earth (SBE) describes a state of the Earth's climate with global or near-global ice cover. The cap dolostone at the base of the Ediacaran successions serves as useful archives for studying environmental change during the Marinoan Snowball Earth deglaciation in Neoproterozoic. The characteristic compositions in dolomite provide critical information on continental weathering and coastal water mixing after glacial retreat. However, valid methods for pristine dolomite separation remain challenging. In this study, four selected cap dolostone samples from the base of the Ediacaran Lantian Formation were used for establishing a new 3-step leaching method, to remove the secondary calcite and other impurities before determination of δ26Mg and 87Sr/86Sr in dolomite. Non-destructive Raman, X-ray diffractometer (XRD) and scanning electron microscopy (SEM) were used to examine the distribution of dolomite and minor calcite/silicate in each sample. Micro-drill powders before each extraction procedure were examined in weight loss and mineralogical compositions, as well as the chemicals in the leaching solutions. Potential diagenetic artifacts were evaluated using Sr/Ca, Mn/Sr, 87Sr/86Sr and δ26Mg in solutions. By applying a simple two-end member mixing between the seawater and the silicate sources (R2 = 0.48, n = 23), the down-core variations of δ26Mg and 87Sr/86Sr in cap dolostone can be used to gain a better understand of the temporal weathering intensity changes, as well as the coastal oceanic mixing processes, after the Marinoan deglaciation. [ABSTRACT FROM AUTHOR]

On the southwest cape of the Congo craton, a subtropical carbonate bank the size of Greenland was heavily glaciated during two Cryogenian panglacial episodes spaced 10–20 Myr apart. In NW Namibia, the bank underwent crustal stretching with resultant Aegean Sea-type topography during the older and longer Sturtian glaciation (717–661 Ma). This is indicated by angular discordance between glacial and preglacial strata and diamictites sourced from all older units, including crystalline basement. In contrast, the bank was flat-topped and underwent broad thermal subsidence during Marinoan glaciation (646 ± 5–635 Ma), attested by stratal parallellism and diamictites sourced from ≤100 m stratigraphic depth. However, ≥2.0 km of relief existed on the Marinoan continental slope, where most glacial erosion and accumulation occurred. The average rates of Marinoan erosion (2.55–6.80 m/Myr, n = 190) and accumulation (2.65–7.07 m/Myr, n = 211) are indistinguishable, implying that the location in a continental promontory did not bias erosion over accumulation. The average accumulation rates for the Sturtian and Marinoan, scaled for different averaging times, including Marinoan uncertainty, are 3.95–4.93 m/Myr (n = 183) and 2.65–7.07 m/Myr (n = 190), respectively, suggesting that a Marinoan glacioeustatic coastal escarpment substituted for rift-related Sturtian basin-and-range topography. These slow rates, comparable to long-term pre-Quaternary accumulation rates on existing abyssal plains, reconcile glacial sedimentology with the feeble hydrologic cycle of snowball Earth. [ABSTRACT FROM AUTHOR]

The "Snowball Earths" were cataclysmic events during the late Neoproterozoic's Cryogenian period (720-635 Ma) in which most, if not all, of Earth's surface was covered in ice. Paleoenvironmental reconstructions of these events utilize isotopic systems, such as Δ17O and barium isotopes of barites. Other isotopic systems, such as zinc (Zn), can reflect seawater composition or environmental conditions (e.g., temperature changes) and biological productivity. We report here a multi-isotopic C, O, and Zn data set for carbonates deposited immediately after the Marinoan glaciation (635 Ma) from the Otavi Group in northern Namibia. In this study, we chemically separated calcite and non-calcitic carbonate phases, finding isotopically distinct carbon and oxygen isotopes. These could reflect changes in the source seawater composition and conditions during carbonate formation. Our key finding is largescale Zn isotopic variations over the oldest parts of the distal foreslope cap carbonate sections. The magnitude of variation is larger than any found throughout post-snowball cap carbonates to date, and in a far shorter sequence. This shows a heretofore undiscovered difficulty for Zn isotopic interpretations. The primary Zn sources are likely to be aeolian or alluvial, associated with the massive deglaciation related run-off from the thawing continent and a greater exposed surface for atmospheric aerosol entrainment. The samples with the lightest Zn isotopic compositions (δ66Zn < 0.3 ‰) potentially reflect hydrothermally sourced Zn dominating the carbonates' Zn budget. This finding is likely unique to the oldest carbonates, when the meltwater lid was thinnest and surface waters most prone to upwelling of hydrothermally dominated Snowball Earth brine. On the other hand, local variations could be related to bioproductivity affecting the Zn isotopic composition of the seawater. Similarly, fluctuations in sea-level could bring the depositional site below and above a redoxcline, causing isotopic variations. These variations in Zn isotope ratios preclude the estimation of a global Zn isotopic signature, potentially indicating localized resumption of export production. [ABSTRACT FROM AUTHOR]

The strongly 13C-depleted calcite in the Doushantuo cap dolostones (∼635 Ma) of South China is considered to form via thermochemical or biological oxidation of methane by sulfate in an environment with sulfate concentrations close to that of modern seawater (∼28 mM). Here we demonstrate that the coeval seawater sulfate levels were indeed low (∼1 to 3 mM) because all bulk δ34S values of pyrites are close to and higher than the coeval seawater, with only a few showing a wide in situ δ34S range from 5 to 62‰. Such low sulfate concentrations were initially consumed by sulfate reducing microorganisms to extremely low levels (<∼0.2 mM), and thus Mn-driven anaerobic oxidation of methane (AOM) by sulfate occurred in pore water to produce H 2 S, which was in turn oxidized by Mn oxides back to sulfate. Consequently, with increasing Mn reduction and decreasing sulfate concentrations in pore waters, the produced pyrite shows an increase of in situ δ34S values, and is in association with calcite with more negative δ13C values, and an inherited positive Ce anomaly from Mn oxides. Finally, the pyrite with the highest δ34S value may have co-precipitated with the 13C-depelted calcite (with a nadir of −58‰), and thus show similarly high Mn concentrations (∼20000 μg/g). This is the first report of sulfate-dependent AOM driven by Mn- or Fe- oxides from the entire Precambrian record. Such a previously unrecognized biogeochemical cycling during Marinoan global deglaciation may serve as the mechanism for major methane sink during the Proterozoic and Archean with low sulfate concentrations in the oceans. [ABSTRACT FROM AUTHOR]

Precambrian 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. Plain Language Summary: Between 717 and 635 million years ago, Earth experienced two dramatic global glacial events, known as "Snowball Earth" glaciations, during which ice covered the oceans all the way to the equator. Geoscientists are still seeking to fully understand what caused these extreme climate events and how life on Earth survived them. Although geochemists have a variety of tools to reconstruct the temperature of ancient oceans, these methods are difficult to apply in rocks this old because primary signals have been too altered. Instead, we looked for a key microscopic fingerprint ("guttulatic microfabric") of a type of calcium carbonate mineral ("ikaite") that only forms in cold‐water environments. Previous work had proposed that we might expect to find evidence of this cold‐water carbonate mineral associated with a specific type of sediment called "giant ooids." We found abundant evidence of guttulatic microfabric in sedimentary rocks containing giant ooids that formed in a low‐latitude shallow marine environment millions of years before the onset of global glaciation. Our observations suggest that Earth's climate was cold before the onset of global glaciation, which could mean that marine organisms were accustomed to cold conditions well before the Snowball glaciations. Key Points: Guttulatic microfabric is a characteristic fingerprint of ikaite, a mineral that forms only in cold‐water depositional environmentsWe report guttulatic microfabrics in grains and cements associated with giant ooids in the Tonian Beck Spring DolomiteOur findings demonstrate that global climate was cold millions of years before the onset of the Sturtian glaciation [ABSTRACT FROM AUTHOR]

Low temperature and acidic environments encompass natural milieus such as acid rock drainage in Antarctica and anthropogenic sites including drained sulfidic sediments in Scandinavia. The microorganisms inhabiting these environments include polyextremophiles that are both extreme acidophiles (defined as having an optimumgrowth pH <3), and eurypsychrophiles that grow at low temperatures down to approximately 4°C but have an optimum temperature for growth above 15°C. Eurypsychrophilic acidophiles have important roles in natural biogeochemical cycling on earth and potentially on other planetary bodies and moons along with biotechnological applications in, for instance, low-temperature metal dissolution from metal sulfides. Five low-temperature acidophiles are characterized, namely, Acidithiobacillus ferriphilus, Acidithiobacillus ferrivorans, Acidithiobacillus ferrooxidans, "Ferrovum myxofaciens," and Alicyclobacillus disulfidooxidans, and their characteristics are reviewed. Our understanding of characterized and environmental eurypsychrophilic acidophiles has been accelerated by the application of "omics" techniques that have aided in revealing adaptations to low pH and temperature that can be synergistic, while other adaptations are potentially antagonistic. The lack of known acidophiles that exclusively grow below 15°C may be due to the antagonistic nature of adaptations in this polyextremophile. In conclusion, this review summarizes the knowledge of eurypsychrophilic acidophiles and places the information in evolutionary, environmental, biotechnological, and exobiology perspectives. [ABSTRACT FROM AUTHOR]

The triple oxygen and hydrogen stable isotopic compositions of 24 glacial diamictite composites with depositional ages from 2.9 to 0.3 Ga are used to reconstruct the evolution of the continental crust and surface environments. The δ18O of the diamictite composites increases from the Archean to the Phanerozoic by ∼4 ‰ (3–5 ‰ in Mesoarchean to 13.5 ‰ in Neoproterozoic), a trend that is comparable to those seen in shales and granites, although the diamictites plot on the lower δ18O end of the range seen in shales. The Δ'17O 0.528 decreases from −0.04 to −0.15 ‰, overlapping with shales and granites. Nine zircon separates extracted from individual diamictites also show a ∼3 ‰ increase in δ18O from 3.75 to 4.5 ‰ to 7 ‰ between 2.9 and 0.6 Ga. The δ18O of diamictites and zircon separates negatively correlates with ɛ Nd(i) of the diamictites, suggesting that the longer the rocks reside in the upper continental crust (the lower the ɛ Nd), the more they are subjected to repeated weathering cycles, thus acquiring a higher δ18O and lower Δ'17O 0.528 during each cycle. On a triple O isotope plot (δ17O vs δ18O), diamictites define an array with a slope of 0.523, analogous to granites and shales. The δD values of nearly all diamictites overlap with the typical mantle and crustal ranges of −58 ± 12 ‰ and − 69 ± 24 ‰, respectively, but are on the lower end of coeval shale values. Water, contained primarily in sheet silicates, ranges from 2.85 wt% (pre-2.4 Ga) and 2.35 wt% (post-2.4 Ga), showing no temporal change, likely reflecting post-depositional reset. The youngest diamictites (0.3 Ga Dwyka Group) exhibit low δD values of −105 to −111 ‰, lower than the typical crustal values, potentially reflecting the near-polar position of South Africa at the time of their deposition, which is corroborated by the reconstructed δ18O value of −26 ‰ based on triple oxygen isotopic systematics. The chemical index of alteration (CIA) allows inverting measured δ18O and Δ'17O values of diamictite composites into a pure weathering product (wp) end member by subtracting values of coeval unweathered crust based on tzircon δ18O. This reveals that the δ18O wp also increases with time, suggesting that the increase in δ18O (decrease in Δ'17O) of the maturing crust is insufficient to fully explain the trends. Therefore, at least part of the increasing δ18O wp likely reflects increasing δ18O of the hydrosphere. Reconstructed surface temperatures around the time of diamictite deposition using δ18O wp and Δ'17O wp display a general decrease since the Archean. Given the diagenetic and post-diagenetic history experienced by the diamictites, the temporal and global trends should be interpreted with caution. Nonetheless, reconstructed δ18O water shows an increase from −21 ‰ at 2.9 Ga to −11 ‰ at 0.6 Ga. Thus, diamictites, like shales, broadly monitor the evolution of surface conditions on Earth from warmer to colder and from lower to higher δ18O water values of the hydrosphere. [Display omitted] [ABSTRACT FROM AUTHOR]

The deglaciation of the Neoproterozoic Snowball Earth occurred over a few thousands to several tens of thousands of years. This short interval was characterized by a rapid increase in global sea level, resulting in an abrupt deposition of the post-glacial cap carbonates overlying the glacial deposits. However, the transition from glacial diamictites to cap carbonates in different depositional environments displayed considerable variability, indicating a more complex deglacial process than previously thought. To elucidate the end of the Snowball Earth, a comprehensive facies analysis was performed on the Cryogenian Nantuo Formation (the Marinoan glacial deposits) in the Nanhua Basin, South China. The analysis revealed that the Nantuo Formation encompasses a spectrum of facies associations, including subglacial, proximal glaciomarine, distal glaciomarine, and non-glacial marine facies, with obvious differences observed in the deglacial sequences between deep and shallow marine environments. In the deep-water environments, which was the deglacial deposits underwent a transition from massive diamictite to turbidite sequences, succeeded by the deposition of fine-grained sandstone to siltstones, and finally capped by the deposition of cap carbonates. In contrast, the shallow-water environments showed a progression from sheared diamictites to massive diamictite, which then evolved into cap carbonates. These observed differences highlight the spatial heterogeneity inherent in the deglaciation process, with the deep-water facies deglaciating before the shallow-water facies, implying a sequential retreat of the ice sheet towards the terrestrial domain. The findings support the hypothesis that the Neoproterozoic Snowball Earth deglaciation was a progressive event, beginning offshore and gradually extending onshore, which induced changes in depositional patterns and ocean alkalinity. • The Neoproterozoic Marinoan deglaciation began offshore, extending inshore progressively. • Ice sheet retreat was complex, with spatial heterogeneity in marine environments. • Cap carbonates indicate post-glacial warming and reduced oceanic acidification. [ABSTRACT FROM AUTHOR]

The Great Unconformity, a profound gap in Earth's stratigraphic record often evident below the base of the Cambrian system, has remained among the most enigmatic field observations in Earth science for over a century. While long associated directly or indirectly with the occurrence of the earliest complex animal fossils, a conclusive explanation for the formation and global extent of the Great Unconformity has remained elusive. Here we show that the Great Unconformity is associated with a set of large global oxygen and hafnium isotope excursions in magmatic zircon that suggest a late Neoproterozoic crustal erosion and sediment subduction event of unprecedented scale. These excursions, the Great Unconformity, preservational irregularities in the terrestrial bolide impact record, and the first-order pattern of Phanerozoic sedimentation can together be explained by spatially heterogeneous Neoproterozoic glacial erosion totaling a global average of 3-5 vertical kilometers, along with the subsequent thermal and isostatic consequences of this erosion for global continental freeboard.

Low temperature and acidic environments encompass natural milieus such as acid rock drainage in Antarctica and anthropogenic sites including drained sulfidic sediments in Scandinavia. The microorganisms inhabiting these environments include polyextremophiles that are both extreme acidophiles (defined as having an optimum growth pH < 3), and eurypsychrophiles that grow at low temperatures down to approximately 4°C but have an optimum temperature for growth above 15°C. Eurypsychrophilic acidophiles have important roles in natural biogeochemical cycling on earth and potentially on other planetary bodies and moons along with biotechnological applications in, for instance, low-temperature metal dissolution from metal sulfides. Five low-temperature acidophiles are characterized, namely, Acidithiobacillus ferriphilus, Acidithiobacillus ferrivorans, Acidithiobacillus ferrooxidans, “Ferrovum myxofaciens,” and Alicyclobacillus disulfidooxidans, and their characteristics are reviewed. Our understanding of characterized and environmental eurypsychrophilic acidophiles has been accelerated by the application of “omics” techniques that have aided in revealing adaptations to low pH and temperature that can be synergistic, while other adaptations are potentially antagonistic. The lack of known acidophiles that exclusively grow below 15°C may be due to the antagonistic nature of adaptations in this polyextremophile. In conclusion, this review summarizes the knowledge of eurypsychrophilic acidophiles and places the information in evolutionary, environmental, biotechnological, and exobiology perspectives.

Ocean ventilation is an important regulator for atmospheric CO2 level (pCO2) by affecting the relative proportion of carbon stored in the atmosphere and deep ocean. Expansion of sea ice during glacial periods slows down ocean ventilation and its effect is expected to be the largest during the Neoproterozoic pre‐snowball stage. Here, our Community Earth System Model version 1.2.2 simulations demonstrate that averaged deep ocean ventilation age almost triples when the climate cools from a warm state with negligible sea ice to one in which the global sea‐ice coverage reaches ∼50% when pCO2 is lowered to 280 ppmv. Further cooling by reducing pCO2 from 280 to 70 ppmv increases the ventilation age from 1900 to 2300 years. This latter small increase in deep‐ocean ventilation age can reduce pCO2 by 48 ppmv, assuming Neoproterozoic organic production was comparable to present level. Therefore, the weakened ocean ventilation constitutes a significant positive feedback to the Late Neoproterozoic climate cooling. Plain Language Summary: Ocean ventilation is important for the carbon exchange between the atmosphere and the ocean. When the ventilation is slow, due to for example, sluggish ocean overturning circulation or sea‐ice coverage, more carbon tends to be stored in the deep ocean and atmospheric CO2 level (pCO2) decreases. During the Late Neoproterozoic (∼800–541 Ma), climate evolved toward an extremely cold state called snowball Earth due to the imbalance of CO2 sources and sinks; the sea ice extended toward the tropics. Here we show, using an Earth system model, that the ocean ventilation slowed down significantly as the climate cooled. Our quantitative estimate shows that if pCO2 is reduced from 280 ppmv to 70 ppmv, the slowdown of ocean ventilation will further draw down pCO2 by 48 ppmv. This is a strong positive feedback that could have promoted snowball Earth initiation. The slowdown of ocean ventilation not only affects pCO2 but also is expected to affect the ratio of carbon isotopes in the ocean. Our work indicates that the Late Neoproterozoic ocean biogeochemical cycle was critical for climate stability. Key Points: Ocean ventilation is estimated using Community Earth System Model version 1.2.2 for the Neoproterozoic pre‐snowball stage during which sea ice was extensiveCompared to an almost ice‐free warm climate, the deep ocean ventilation age would triple when sea‐ice edges extend to the tropicsWeakened ocean ventilation would induce a reduction of atmospheric CO2 level, promoting Neoproterozoic snowball Earth initiation [ABSTRACT FROM AUTHOR]

The timing of the first appearance of animals is of crucial importance for understanding the evolution of life on Earth. Although the fossil record places the earliest metazoans at 572–602 Ma, molecular clock studies suggest a far earlier origination, as far back as ~850 Ma. The difference in these dates would place the rise of animal life into a time period punctuated by multiple colossal, potentially global, glacial events. Although the two schools of thought debate the limitations of each other's methods, little time has been dedicated to how animal life might have survived if it did arise before or during these global glacial periods. The history of recent polar biota shows that organisms have found ways of persisting on and around the ice of the Antarctic continent throughout the Last Glacial Maximum (33–14 Ka), with some endemic species present before the breakup of Gondwana (180–23 Ma). Here we discuss the survival strategies and habitats of modern polar marine organisms in environments analogous to those that could have existed during Neoproterozoic glaciations. We discuss how, despite the apparent harshness of many ice covered, sub‐zero, Antarctic marine habitats, animal life thrives on, in and under the ice. Ice dominated systems and processes make some local environments more habitable through water circulation, oxygenation, terrigenous nutrient input and novel habitats. We consider how the physical conditions of Neoproterozoic glaciations would likely have dramatically impacted conditions for potential life in the shallows and erased any possible fossil evidence from the continental shelves. The recent glacial cycle has driven the evolution of Antarctica's unique fauna by acting as a "diversity pump," and the same could be true for the late Proterozoic and the evolution of animal life on Earth, and the existence of life elsewhere in the universe on icy worlds or moons. [ABSTRACT FROM AUTHOR]

It is not well understood how, in the immediate aftermath of the Sturtian Snowball Earth, marine sulfur cycling resulted in a global distribution of sedimentary pyrite with δ34S values higher than coeval seawater. Here, we analyze the quadruple sulfur isotope systematics of organic-bound sulfur (OS) from the lowermost post-Sturtian Datangpo Formation, South China, and identify two generations of OS formation, each sampling an isotopically distinct sulfate reservoir (δ34S ≈ 26‰ and 52–93‰) that differentially impacted its respective, co-occurring pyrite. Combining several lines of geochemical evidence, we argue that the first OS generation was the product of a sulfate-impoverished meltwater-influenced setting, with OS preservation being the result of resistance to acid hydrolysis. However, the second OS generation was sourced from H 2 S produced in sediments during early diagenesis via microbial reduction of a 34S-enriched sulfate pool derived from overlying euxinic or ferruginous seawater. This is the first ancient marine data set where all observed pyrite is more enriched in 34S than its associated OS. Our proposed origin may be applied to global superheavy pyrite (SHP) immediately after the Sturtian and is comparable to processes linked to freshwater-to-marine transitions during rising sea level in the wake of recent glaciation. [ABSTRACT FROM AUTHOR]

During more than a century since its original identification, the Gowganda Formation in Ontario (Canada) has gradually been reinterpreted from representing mainly subglacial tillites to secondary gravity flow and glaciomarine deposits. The main pieces of geological evidence advanced in favour of glaciation in recent articles are outsized clasts that have been interpreted as dropstones and patches of diamictites in a single small-sized area at Cobalt which is still interpreted as displaying subglacial basal tillites. The present research considers field evidence in the Gowganda Formation in the light of more recent work on gravity flows linked to tectonics. Detailed studies have demonstrated that the clasts which are interpreted to be dropstones rarely penetrate laminae and are commonly draped by sediments the appearance of which is similar to lonestones in gravity flows. The “subglacial area” at Cobalt displays evidence of tectonics and gravity flows, which can be traced from the underlying bedrock, and then further in the overlying sequence of diamictites and rhythmites. The sum of geological features displays appearances at odds with a primary glaciogenic origin, and there is no unequivocal evidence present of glaciation. The data indicate deposition by non-glaciogenic gravity flows, including cohesive debris flows for the more compact units, probably triggered by tectonic displacements.

Snowball Earth is a hypothesized state in the deep past of Earth in which the ocean was completely or nearly completely covered by sea ice, resulting from a runaway ice‐albedo feedback. Here, we address how the treatment of sea‐ice thermodynamics affects the initiation of a Snowball Earth in the global climate model ICON‐A run in an idealized slab‐ocean aquaplanet setup. Specifically, we study the impact of vertical resolution and brine pockets of ice by comparing the 3‐layer Winton and a 0‐layer Semtner scheme, and we investigate the impact of limiting ice thickness to 5 m. The internal heat storage of ice is increased by higher vertical resolution and brine pockets, which weakens surface melting and increases global albedo by allowing snow and ice to persist longer into the summer season. The internal heat storage weakens the melt‐ratchet effect, as is confirmed with offline simulations with the two ice schemes. The result is a substantially easier Snowball Earth initiation and an increase in the critical CO2 for Snowball initiation by 50%. Limiting ice thickness impedes Snowball initiation as the removal of excess ice leads to an artificial heat source. Yet, the impact is minor and critical CO2 is decreased by 5% only. The results show that while the sea‐ice thickness limit plays only a minor role, the internal heat storage of ice represents an important factor for Snowball initiation and needs to be taken into account when modeling Snowball Earth initiation. Plain Language Summary: Between 500 and 1000 million years ago, there were probably multiple occasions when the oceans of Earth were completely covered by ice. These "Snowball Earth" events are initiated by a growing area of sea ice, which reflects more of the incoming sunlight and thus cools the Earth, up to a point where climate becomes unstable and ice cover becomes global. In this work, we use a global climate model and investigate how the treatment of sea ice in a climate model influences the formation of a Snowball Earth. We find that representing the ice internal heat storage with higher vertical resolution and brine pockets makes it easier to reach a Snowball Earth, because it weakens melting of snow and ice, which brightens the surface and leads to more sunlight being reflected. Additionally, if sea ice is not allowed to grow thicker than 5 m, reaching a Snowball Earth is more difficult because the thin ice results in an artificial heat source that warms the atmosphere. Yet, the impact is small. Taken together, our results show that a high vertical resolution of sea ice is important for realistic investigations of the initiation of Snowball Earth. Key Points: Effect of sea‐ice thermodynamics on Snowball initiation studied in climate model with two ice schemesSnowball initiation strongly facilitated by energy storage in ice via enhanced ice‐albedo feedbackLimiting ice thickness leads to artificial heat flux, but impact on Snowball initiation is minor [ABSTRACT FROM AUTHOR]

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As the calcite crystals grew, micrometer-sized terra rossaderived mineral particles likely attached to the surfaces of growing calcite crystals, creating zones in the host calcite. Keywords: Calcite; cap carbonate; clay; dolomite; hematite; goethite; illite; kaolinite; nanoparticle; nucleation; phantom crystal; phyllosilicate; Raman spectroscopy; SEM; Snowball Earth; TEM; XRD EN Calcite cap carbonate clay dolomite hematite goethite illite kaolinite nanoparticle nucleation phantom crystal phyllosilicate Raman spectroscopy SEM Snowball Earth TEM XRD 1369 1377 9 07/05/22 20220701 NES 220701 Introduction A phantom crystal is a crystal embedded in another crystal of the same mineral species with visible outlines. Phyllosilicate nucleation on calcite and dolomite nucleation on phyllosilicates, as inferred from nanoscale mineralogical associations, suggest that carbonates can also provide surfaces for the growth of phyllosilicates, and this mutual enhancement may result in the formation of large-scale clay-carbonate successions in aqueous settings. [Extracted from the article]

The history of the nitrogen cycle on Earth is linked to the redox evolution of the surface environment. Many nitrogen cycle fluxes are microbially mediated, and the particular fluxes operating at any given time in an ecosystem depend on the presence, absence or abundance of oxygen. However, interpreting this relationship is complicated as several isotopic fractionations associated with N-cycling are not diagnostic of a particular redox state. Thus, linking nitrogen isotopic analyses with redox-sensitive proxies is essential when interpretating past environments. Specifically, we use concentrations of U, V and Mo, along with Fe-speciation, to augment and contextualize nitrogen isotopic measurements. As an example, we consider samples from the Neoproterozoic Cryogenian period to suggest that there was oxygenated water, with associated aerobic N cycle fluxes. This interpretation is based on positive δ15N values between 4 to 80/00, Fe-speciation data consistent with anoxic bottom water during the Snowball ocean and oxygenated after, and redox-sensitive trace metals indicative of oxic weathering and surface water. Typically, high δ15N values are interpreted to reflect enhanced denitrification. We propose potential causes including a post-Snowball freshwater melt lid that suppressed deep water ventilation and that denitrification occurred more rapidly at high temperatures after the Snowball. These interpretations are buttressed by combined N isotope and redox analyses. This approach is especially useful during times of dynamic redox in the ocean-atmosphere system to interpret biologic isotopic signals.

The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250-2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.

A series of dramatic oceanic and atmospheric events occurred in the immediate aftermath of the Marinoan “snowball Earth” meltdown ∼635 My ago. However, at the 10- to 100-ky timescale, the order, rate, duration, and causal-feedback relationships of these individual events remain nebulous. Nonetheless, rapid swings in regional marine sulfate concentrations are predicted to have occurred in the aftermath of a snowball Earth, due to the nonlinear responses of its two major controlling fluxes: oxidative weathering on the continents and pyrite burial in marine sediments. Here, through the application of multiple isotope systems on various carbon and sulfur compounds, we determined extremely 13C-depleted calcite cements in the basal Ediacaran in South China to be the result of microbial sulfate reduction coupled to anaerobic oxidation of methane, which indicates an interval of high sulfate concentrations in some part of the postmeltdown ocean. Regional chemostratigraphy places the 13C-depleted cements at the equivalent of the earliest Ediacaran 17O-depletion episode, thus confining the timing of this peak in sulfate concentrations within ∼50 ky since the onset of the deglaciation. The dearth of similarly 13C-depleted cements in other Proterozoic successions implies that the earliest Ediacaran peak in marine sulfate concentration is a regional and likely transient event. [ABSTRACT FROM AUTHOR]

The Neoproterozoic 'snowball Earth' hypothesis suggests that a runaway ice–albedo feedback led to two intense glaciations around 717–635 million years ago, and this global ice cover would have drastically impacted biogeochemical cycles. Testing the predictions of this hypothesis against the rock record is key to understanding Earth's surface evolution in the Neoproterozoic. A central tenet of the snowball Earth hypothesis is that extremely high atmospheric CO2 levels—supplied by volcanic degassing over millions of years—would be required to overcome a strong ice–albedo feedback and trigger deglaciation. This requires severely diminished continental weathering (and associated CO2 drawdown) during glaciation, and implies that carbonate minerals would not precipitate from syn‐glacial seawater due to a lack of alkalinity influxes into ice‐covered oceans. In this scenario, syn‐glacial seawater chemistry should instead be dominated by chemical exchange with the oceanic crust and volcanic systems, developing low pH and low Mg/Ca ratios. However, sedimentary rocks deposited during the Sturtian glaciation from the Adelaide Fold Belt—and contemporaneous successions globally—show evidence for syn‐sedimentary dolomite precipitation in glaciomarine environments. The dolomitic composition of these syn‐glacial sediments and post‐glacial 'cap carbonates' implies that carbonate precipitation and Mg cycling must have remained active during the ~50 million‐year Sturtian glaciation. These syn‐glacial carbonates highlight a gap in our understanding of continental weathering—and therefore, the carbon cycle—during snowball Earth. In light of these observations, a Precambrian global biogeochemical model (PreCOSCIOUS) was modified to explore scenarios of syn‐glacial chemical weathering, ocean chemistry and Sturtian carbonate mineralogy. Modelling results suggest that a small degree of chemical weathering during glaciation would have been capable of maintaining high seawater Mg/Ca ratios and carbonate precipitation throughout the Sturtian glaciation. This is consistent with a severe ice age during the Sturtian, but challenges predictions of biogeochemical cycling during the endmember 'hard snowball' models. A small degree of continental weathering might also help explain the extreme duration of the Sturtian glaciation, which appears to have been the longest ice age in Earth history. [ABSTRACT FROM AUTHOR]

The origin of the phenomenon known as the Great Unconformity has been a fundamental yet unresolved problem in the geosciences for over a century. Recent hypotheses advocate either global continental exhumation averaging 3 to 5 km during Cryogenian (717 to 635 Ma) snowball Earth glaciations or, alternatively, diachronous episodic exhumation throughout the Neoproterozoic (1,000 to 540 Ma) due to plate tectonic reorganization from supercontinent assembly and breakup. To test these hypotheses, the temporal patterns of Neoproterozoic thermal histories were evaluated for four North American locations using previously published medium- to low-temperature thermochronology and geologic information. We present inverse time-temperature simulations within a Bayesian modeling framework that record a consistent signal of relatively rapid, high-magnitude cooling of ~120 to 200 °C interpreted as erosional exhumation of upper crustal basement during the Cryogenian. These models imply widespread, synchronous cooling consistent with at least ~3 to 5 km of unroofing during snowball Earth glaciations, but also demonstrate that plate tectonic drivers, with the potential to cause both exhumation and burial, may have significantly influenced the thermal history in regions that were undergoing deformation concomitant with glaciation. In the cratonic interior, however, glaciation remains the only plausible mechanism that satisfies the required timing, magnitude, and broad spatial pattern of continental erosion revealed by our thermochronological inversions. To obtain a full picture of the extent and synchroneity of such erosional exhumation, studies on stable cratonic crust below the Great Unconformity must be repeated on all continents. [ABSTRACT FROM AUTHOR]

For tens of millions of years (Ma), the terrestrial habitats of Snowball Earth during the Cryogenian period (between 720 and 635 Ma before present–Neoproterozoic Era) were possibly dominated by global snow and ice cover up to the equatorial sublimative desert. The most recent time-calibrated phylogenies calibrated not only on plants but on a comprehensive set of eukaryotes indicate that within the Streptophyta, multicellular charophytes (Phragmoplastophyta) evolved in the Mesoproterozoic to the early Neoproterozoic. At the same time, Cryogenian is the time of the likely origin of the common ancestor of Zygnematophyceae and Embryophyta and later, also of the Zygnematophyceae–Embryophyta split. This common ancestor is proposed to be called Anydrophyta; here, we use anydrophytes. Based on the combination of published phylogenomic studies and estimated diversification time comparisons, we deem it highly likely that anydrophytes evolved in response to Cryogenian cooling. Also, later in the Cryogenian, secondary simplification of multicellular anydrophytes and loss of flagella resulted in Zygnematophyceae diversification as an adaptation to the extended cold glacial environment. We propose that the Marinoan geochemically documented expansion of first terrestrial flora has been represented not only by Chlorophyta but also by Streptophyta, including the anydrophytes, and later by Zygnematophyceae, thriving on glacial surfaces until today. It is possible that multicellular early Embryophyta survived in less abundant (possibly relatively warmer) refugia, relying more on mineral substrates, allowing the retention of flagella-based sexuality. The loss of flagella and sexual reproduction by conjugation evolved in Zygnematophyceae and zygomycetous fungi during the Cryogenian in a remarkably convergent way. Thus, we support the concept that the important basal cellular adaptations to terrestrial environments were exapted in streptophyte algae for terrestrialization and propose that this was stimulated by the adaptation to glacial habitats dominating the Cryogenian Snowball Earth. Including the glacial lifestyle when considering the rise of land plants increases the parsimony of connecting different ecological, phylogenetic, and physiological puzzles of the journey from aquatic algae to terrestrial floras. [ABSTRACT FROM AUTHOR]

When a large amount of CO2 is added to the atmosphere, both the mixing ratio and the surface pressure increase. This causes the troposphere to expand in two directions. The greenhouse effect from the increased mixing ratio causes the tropopause pressure to decrease, so the troposphere expands upward. The increased surface pressure causes the troposphere to expand downward. The first effect is radiative and well known, while the second is nonradiative and unexplored. Here, a method is presented to compare the effect of tropospheric expansion in each direction on the surface temperature of a Snowball Earth. A series of models, from a gray model to a spectral model with realistic Snowball parameters, are used to illustrate this concept. It is shown that near the deglaciation threshold, most of the increase in surface temperature that follows an increase in CO2 is due to the nonradiative downward tropospheric expansion at the surface, not the radiative upward expansion at the tropopause. The increased atmospheric mass due to the CO2, entirely apart from its radiative effect, causes an additional increase in surface temperature and therefore aids Snowball deglaciation. Plain Language Summary: The Earth was previously in a frozen state known as a Snowball Earth. There have been several explanations proposed for how the ice melted. All of these explanations require a large amount of carbon dioxide in the atmosphere, sometimes an unrealistically large amount. When a large amount of carbon dioxide is added to the atmosphere, the surface pressure increases. This enhances the greenhouse effect, which further warms the planet. But it also means that convection reaches higher into the atmosphere, which also warms the surface. This last effect has not been previously studied, and it means that the ice covering a Snowball Earth can melt with a lower amount of carbon dioxide than previously thought. Key Points: The amount of carbon dioxide required to deglaciate a Snowball Earth is large enough to significantly increase the surface pressureThe increase in surface pressure means that the troposphere expands, resulting in a warmer surfaceThis is a nonradiative effect separate from pressure broadening and collision‐induced absorption and also applies to non‐greenhouse gases [ABSTRACT FROM AUTHOR]

The Cryogenian Period (717–635 Ma), or 'Snowball Earth', was an unusually cool period of Earth history when glaciers extended to low latitudes. Past ideas on causes of this widespread glaciation include increased consumption of atmospheric carbon dioxide by silicate weathering due to continental drift into tropical paleolatitudes, or by voluminous, easily weathered volcanic tuffs. Alternatively, carbon sequestration from the atmosphere may have been intensified by advances in biomass on land or at sea. These hypotheses are tested with a new study of red siltstones of the Johnnys Creek Formation (785–717 Ma) in central Australia, where paleosols have long been recognised. Although these dolomitic red siltstones look like shales, they lack lamination. Instead, they have the massive bedding and grainsize distribution of dolomitic and calcareous loess, which precede tillites of the Areyonga Formation. Paleomagnetic studies indicate little drift from a paleolatitude of 26.2° during accumulation of the Johnnys Creek Formation. Nor does the Johnnys Creek Formation contain easily weathered volcanic ash, only local basalt flows. Paleoproductivity of the paleosols increases up section, as estimated in ppm soil CO2 from depth in paleosols to gypsic (By) and then calcic (Bk) horizons. Deepening and intensification of soil respiration reflects greater terrestrial carbon sequestration, and increased chemical weathering up section, and both would have drawn down atmospheric CO2. Comparable transition from gypsic to calcic soils in modern deserts reflects change from cyanobacterial-gypsic to fungal–algal calcic ecosystems. Snowball Earth glaciation may have been induced by evolutionary advances to eukaryotic and multicellular life on land, in the same way as Ordovician glaciation was induced by land plants, Permo-Carboniferous glaciation by trees, and Pleistocene glaciation by grasslands. Johnnys Creek Formation of central Australia has paleosols dated 785–717 Ma. Little drift from paleolatitude 26.2°, and no volcanic ash, only local basalt flows. Paleosol paleoproductivity and depth of weathering increased steadily up section. Glaciation may have been induced by eukaryotic and multicellular life on land. [ABSTRACT FROM AUTHOR]

Intense glaciation during the end of Cryogenian time (~635 million years ago) marks the coldest climate state in Earth history - a time when glacial deposits accumulated at low, tropical paleolatitudes. The leading idea to explain these deposits, the snowball Earth hypothesis, predicts globally frozen surface conditions and subfreezing temperatures, with global climate models placing surface temperatures in the tropics between -20 °C and -60 °C. However, precise paleosurface temperatures based upon geologic constraints have remained elusive and the global severity of the glaciation undetermined. Here we make new geologic observations of tropical periglacial, aeolian and fluvial sedimentary structures formed during the end-Cryogenian, Marinoan glaciation in South Australia; these observations allow us to constrain ancient surface temperatures. We find periglacial sand wedges and associated deformation suggest that ground temperatures were sufficiently warm to allow for ductile deformation of a sandy regolith. The wide range of deformation structures likely indicate the presence of a paleoactive layer that penetrated 2-4 m below the ground surface. These observations, paired with a model of ground temperature forced by solar insolation, constrain the local mean annual surface temperature to within a few degrees of freezing. This temperature constraint matches well with our observations of fluvial deposits, which require temperatures sufficiently warm for surface runoff. Although this estimate coincides with one of the coldest near sea-level tropical temperatures in Earth history, if these structures represent peak Marinaon glacial conditions, they do not support the persistent deep freeze of the snowball Earth hypothesis. Rather, surface temperatures near 0 °C allow for regions of seasonal surface melting, atmosphere-ocean coupling and possible tropical refugia for early metazoans. If instead these structures formed during glacial onset or deglaciation, then they have implications for the timescale and character for the transition into or out of a snowball state.

Intense glaciation during the end of Cryogenian time (~635 million years ago) marks the coldest climate state in Earth history - a time when glacial deposits accumulated at low, tropical paleolatitudes. The leading idea to explain these deposits, the snowball Earth hypothesis, predicts globally frozen surface conditions and subfreezing temperatures, with global climate models placing surface temperatures in the tropics between -20 °C and -60 °C. However, precise paleosurface temperatures based upon geologic constraints have remained elusive and the global severity of the glaciation undetermined. Here we make new geologic observations of tropical periglacial, aeolian and fluvial sedimentary structures formed during the end-Cryogenian, Marinoan glaciation in South Australia; these observations allow us to constrain ancient surface temperatures. We find periglacial sand wedges and associated deformation suggest that ground temperatures were sufficiently warm to allow for ductile deformation of a sandy regolith. The wide range of deformation structures likely indicate the presence of a paleoactive layer that penetrated 2-4 m below the ground surface. These observations, paired with a model of ground temperature forced by solar insolation, constrain the local mean annual surface temperature to within a few degrees of freezing. This temperature constraint matches well with our observations of fluvial deposits, which require temperatures sufficiently warm for surface runoff. Although this estimate coincides with one of the coldest near sea-level tropical temperatures in Earth history, if these structures represent peak Marinaon glacial conditions, they do not support the persistent deep freeze of the snowball Earth hypothesis. Rather, surface temperatures near 0 °C allow for regions of seasonal surface melting, atmosphere-ocean coupling and possible tropical refugia for early metazoans. If instead these structures formed during glacial onset or deglaciation, then they have implications for the timescale and character for the transition into or out of a snowball state.

The Adelaide Rift Complex in South Australia records the break-up of Rodinia at a time of great climatic and biological evolution. The Boucaut Volcanics within the Neoproterozoic Adelaide Rift Complex of the Adelaide Superbasin lie at the base of the Burra Group, marking the boundary between the Burra Group and underlying Callanna Group. Despite their significance as one of the few volcanic units within the rift complex, there has been no robust age determination published for the Boucaut Volcanics. We use U–Pb zircon LA-ICP-MS data to determine an age of 788 ± 6 Ma for the eruption of the bimodal Boucaut Volcanics. This has important implications for constraining the timing of stratigraphy within the Adelaide Superbasin. This also has far-reaching implications for plate tectonic reconstructions of Australia and Laurentia, and for correlating global isotope anomalies for the Neoproterozoic. New U–Pb zircon data provide a revised age for the Boucaut Volcanics of 788 ± 6 Ma. Whole-rock geochemistry data highlight the bimodality of the Boucaut Volcanics, with both mafic and felsic components present. Boucaut Volcanics potentially correlate with units in the southwest United States, which would support an AUSWUS plate tectonic configuration during the Neoproterozoic. [ABSTRACT FROM AUTHOR]

Planetary climate can be affected by the interaction of the host star spectral energy distribution with the wavelength-dependent reflectivity of ice and snow. In this study, we explored this effect with a one-dimensional (1-D), line-by-line, radiative transfer model to calculate broadband planetary albedos as input to a seasonally varying, 1-D energy balance climate model. A three-dimensional (3-D) general circulation model was also used to explore the atmosphere's response to changes in incoming stellar radiation, or instellation, and surface albedo. Using this hierarchy of models, we simulated planets covered by ocean, land, and water-ice of varying grain size, with incident radiation from stars of different spectral types. Terrestrial planets orbiting stars with higher near-UV radiation exhibited a stronger ice-albedo feedback. We found that ice extent was much greater on a planet orbiting an F-dwarf star than on a planet orbiting a G-dwarf star at an equivalent flux distance, and that ice-covered conditions occurred on an F-dwarf planet with only a 2% reduction in instellation relative to the present instellation on Earth, assuming fixed CO(2) (present atmospheric level on Earth). A similar planet orbiting the Sun at an equivalent flux distance required an 8% reduction in instellation, while a planet orbiting an M-dwarf star required an additional 19% reduction in instellation to become ice-covered, equivalent to 73% of the modern solar constant. The reduction in instellation must be larger for planets orbiting cooler stars due in large part to the stronger absorption of longer-wavelength radiation by icy surfaces on these planets in addition to stronger absorption by water vapor and CO(2) in their atmospheres, which provides increased downwelling longwave radiation. Lowering the IR and visible-band surface ice and snow albedos for an M-dwarf planet increased the planet's climate stability against changes in instellation and slowed the descent into global ice coverage. The surface ice-albedo feedback effect becomes less important at the outer edge of the habitable zone, where atmospheric CO(2) could be expected to be high such that it maintains clement conditions for surface liquid water. We showed that ∼3-10 bar of CO(2) will entirely mask the climatic effect of ice and snow, leaving the outer limits of the habitable zone unaffected by the spectral dependence of water ice and snow albedo. However, less CO(2) is needed to maintain open water for a planet orbiting an M-dwarf star than would be the case for hotter main-sequence stars.

There is now much geological evidence that the Earth was fully glaciated during several periods in the geological past (about 700 Myr ago) and attained a so-called Snowball Earth (SBE) state. Additional support for this idea has come from climate models of varying complexity that show transitions to SBE states and undergo hysteresis under changes in solar radiation. In this paper, we apply large-scale bifurcation analyses to a novel, fully-implicit Earth System Model of Intermediate Complexity (I-EMIC) to study SBE transitions. The I-EMIC contains a primitive equation ocean model, a model for atmospheric heat and moisture transport, a sea ice component and formulations for the adjustment of albedo over snow and ice. With the I-EMIC, high-dimensional branches of the SBE bifurcation diagram are obtained through parameter continuation. We are able to identify stable and unstable equilibria and uncover an intricate bifurcation structure associated with the ice-albedo feedback. Moreover, large-scale linear stability analyses are performed near major bifurcations, revealing the spatial nature of destabilizing perturbations. [ABSTRACT FROM AUTHOR]