Snowball Earth Events Research Articles

A theory of atmospheric oxygen.

Geological records of atmospheric oxygen suggest that pO2 was less than 0.001% of present atmospheric levels (PAL) during the Archean, increasing abruptly to a Proterozoic value between 0.1% and 10% PAL, and rising quickly to modern levels in the Phanerozoic. Using a simple model of the biogeochemical cycles of carbon, oxygen, sulfur, hydrogen, iron, and phosphorous, we demonstrate that there are three stable states for atmospheric oxygen, roughly corresponding to levels observed in the geological record. These stable states arise from a series of specific positive and negative feedbacks, requiring a large geochemical perturbation to the redox state to transition from one to another. In particular, we show that a very low oxygen level in the Archean (i.e., 10-7 PAL) is consistent with the presence of oxygenic photosynthesis and a robust organic carbon cycle. We show that the Snowball Earth glaciations, which immediately precede both transitions, provide an appropriate transient increase in atmospheric oxygen to drive the atmosphere either from its Archean state to its Proterozoic state, or from its Proterozoic state to its Phanerozoic state. This hypothesis provides a mechanistic explanation for the apparent synchronicity of the Proterozoic Snowball Earth events with both the Great Oxidation Event, and the Neoproterozoic oxidation.

Initiation of Snowball Earth with volcanic sulfur aerosol emissions

AbstractWe propose that the first Neoproterozoic Snowball Earth event, the Sturtian glaciation, was initiated by the injection of sulfate aerosols into the stratosphere. Geochronological data indicate that the Natkusiak magmatic assemblage of the Franklin large igneous province coincided with onset of the Sturtian glaciation. The Natkusiak was emplaced into an evaporite basin and entrained significant quantities of sulfur, which would have led to extensive SO2 and H2S outgassing in hot convective plumes. The largest of these plumes could have penetrated the tropopause, leading to stratospheric sulfate aerosol formation and an albedo increase sufficient to force a Snowball. Radiative forcing was maximized by the equatorial location of the Franklin and the cool Neoproterozoic background climate, which would have lowered the tropopause height, increasing the rate of stratospheric aerosol injection. Our results have implications for understanding Phanerozoic mass extinction events, exoplanet habitability, and aerosol perturbations to the present‐day climate.

Chronological constraints on the Paleoproterozoic Francevillian Group in Gabon

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

Maximum depositional age of the Neoproterozoic Jacobsville Sandstone, Michigan: Implications for the evolution of the Midcontinent Rift

A crucial constraint on the evolution of the ca. 1100 Ma Midcontinent Rift (MCR) in North America comes from the Jacobsville Sandstone, Bayfield Group, and other equivalent sedimentary rocks (JBE) that overlie the volcanics and sediments deposited in the MCR basin near Lake Superior. The MCR began extending ca. 1120 Ma and failed—ceased extending—at ca. 1096 Ma, although volcanism continued for ∼10 m.y. The JBE’s age is poorly constrained, with proposed ages ranging from ca. 1100 to ca. 542 Ma (i.e., youngest Precambrian). It has been proposed that the JBE were deposited shortly prior to or during the time when the MCR failed due to regional compression occurring ca. 1060 Ma as part of the Grenville orogeny (1300–980 Ma). However, the JBE are not conformable with the youngest rift-filling strata and differ compositionally from them. We present an analysis of 2050 new detrital-zircon ages showing that the JBE are younger than 959 ± 19 Ma. Thus, the JBE and the compression recorded in them that inverted the basin postdate the Grenville orogeny and are unrelated to the rift’s failure. The JBE may be significantly, perhaps ∼200–300 m.y., younger than the maximum age from zircons and may have been deposited shortly after a Snowball Earth event.

The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: II. Optical modeling

AbstractDuring the Snowball Earth events of the Neoproterozoic, tropical regions of the ocean could have developed a precipitated salt lag deposit left behind by sublimating sea ice. The major salt would have been hydrohalite, NaCl•2H2O. The crystals in such a deposit can be small and highly scattering, resulting in an allwave albedo similar to that of snow. The snow‐free sea ice from which such a crust could develop has a lower albedo, around 0.5, so the development of a crust would substantially increase the albedo of tropical regions on Snowball Earth. Hydrohalite crystals are much less absorptive than ice in the near‐infrared part of the solar spectrum, so their presence at the surface would increase the overall albedo as well as altering its spectral distribution. In this paper, we use laboratory measurements of the spectral albedo of a hydrohalite lag deposit, in combination with a radiative transfer model, to infer the inherent optical properties of hydrohalite as functions of wavelength. Using this result, we model mixtures of hydrohalite and ice representing both artificially created surfaces in the laboratory and surfaces relevant to Snowball Earth. The model is tested against sequences of laboratory measurements taken during the formation and the dissolution of a lag deposit of hydrohalite. We present a parameterization for the broadband albedo of cold, sublimating sea ice as it forms and evolves a hydrohalite crust, for use in climate models of Snowball Earth.

The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: 1. Laboratory measurements

AbstractThe ice‐albedo feedback mechanism likely contributed to global glaciation during the Snowball Earth events of the Neoproterozoic era (1 Ga to 544 Ma). This feedback results from the albedo contrast between sea ice and open ocean. Little is known about the optical properties of some of the possible surface types that may have been present, including sea ice that is both snow‐free and cold enough for salts to precipitate within brine inclusions. A proxy surface for such ice was grown in a freezer laboratory using the single salt NaCl and kept below the eutectic temperature (−21.2°C) of the NaCl‐H2O binary system. The resulting ice cover was composed of ice and precipitated hydrohalite crystals (NaCl · 2H2O). As the cold ice sublimated, a thin lag‐deposit of salt formed on the surface. To hasten its growth in the laboratory, the deposit was augmented by addition of a salt‐enriched surface crust. Measurements of the spectral albedo of this surface were carried out over 90 days as the hydrohalite crust thickened due to sublimation of ice, and subsequently over several hours as the crust warmed and dissolved, finally resulting in a surface with puddled liquid brine. The all‐wave solar albedo of the subeutectic crust is 0.93 (in contrast to 0.83 for fresh snow and 0.67 for melting bare sea ice). Incorporation of these processes into a climate model of Snowball Earth will result in a positive salt‐albedo feedback operating between −21°C and −36°C.

Manganese Deposits of Africa

Africa hosts just over 80 percent (%) of the world's known land-based ore resources of manganese metal and produced some 41.1% of the 18 million tonnes (Mt) of manganese metal in ores, that was mined during 2014. The deposits are mainly of sedimentary and supergene origin comprising four major types, namely banded iron formation (BIF)-hosted, black shale-hosted, oolitic and supergene/karst-hosted deposits. The BIF-hosted Kalahari Manganese Field (KMF) is by far the largest of these deposits holding some 4,200 Mt of manganese metal that represents about 77% of the world's known land-based resource. Other major deposits are present in the Francevillian of Gabon, represented by high-grade supergene manganese oxide ores derived from weathering of rhodochrosite-bearing black shale source rock, and manganese carbonates interbedded with black shale and greywackes associated with volcanicdominated orogenic belts in the Birimian of West Africa and Lukoshi Complex at Kisenge in the Democratic Republic of the Congo (DRC). High-grade supergene manganese oxide ores cap the manganese carbonate beds. All these major ore deposits formed in the interval 2.0-2.2 Ga. Additional deposits that contain only relatively small resources but that hold important scientific information about the Earth's history include the ~ 2.4 Ga BIF-hosted Rooinekke deposit of the Transvaal Supergroup, which formed prior to the ~ 2.32 Ga Great Oxidation Event (GOE), karst-hosted deposits of the Postmasburg area that formed along a 2.0 Ga palaeoweathering profile below red beds of the Gamagara/Mapedi succession in South Africa, the ~1.9 Ga Tolwe deposit that represents the oldest known example of oolitic manganese ore in the world and the BIF-hosted Otjosondu deposit in Namibia, related to the Neoproterozoic Sturtian Snowball Earth Event (an extremely cold period of time in geologic history when Earth was covered by a virtual global ice cap). The manganese carbonates present in most of these deposits are all highly enriched in light organic carbon isotopes indicating that they were derived from reduction of original manganese oxide precipitates by organic carbon during diagenesis. This implies that the water columns from which the sedimentary manganese beds originally precipitated, as well as the floors of the depositories, were oxygenated. Post Archaean Australian Shale (PAAS)-normalized cerium (Ce) anomalies in Rare Earth Element (REE) data, support the presence of oxic conditions in the basins. It is thus not surprising that all the major 2.0-2.2 Ga deposits formed after the GOE. In this regard the relatively small BIF-hosted Rooinekke manganese deposit is very significant in that it formed prior to the GOE suggesting that at least some free oxygen was present in ocean basins before that time. There also appears to have been a major tectonosedimentary control on the distribution of the major 2.0- 2.2 Ga deposits. All of them, including the KMF, apparently formed in back-arc basinal settings immediately prior to amalgamation of cratonic blocks along Eburnian orogenic belts and formation of the supercontinent Columbia.

A eukaryote assemblage intercalated with Marinoan glacial deposits in South Australia

Composite hematite–silica structures recovered from a siltstone bed in the Elatina Formation of South Australia include (1) sub-circular to whorl-shaped forms, (2) elongate to half-moon-shaped forms and (3) and lozenge-shaped forms locally linked into chains. They range from 200 to 500 µm in diameter and are interpreted as eukaryote tests. Evidence for internal etching of a calcite core of some tests indicates that at least some of the hematite–silica fabrics were acquired through replacement. Carbon isotope values of −20‰ δ 13 C are suggestive of precipitation by microbial activity, and imply a change in ambient fluid chemistry associated with a pH reduction. The tests occur within sandstone beds that were deposited on a tidally modulated braidplain during the Marinoan glaciation at the end of the Cryogenian. The quartz grains in the sandstone sample lack the typical textures (surface striae, internal fractures or irregular grain boundaries) expected for glacially transported material. Thus, on textural grounds we argue that the eukaryote tests represent a proglacial ecosystem during a late Cryogenian snowball Earth event. Supplementary material: Video files of digital X-ray tomographs (μCT) in the longitudinal and transverse planes are available at: https://doi.org/10.6084/m9.figshare.c.2209723 .

Variability, Instabilities, and Eddies in a Snowball Ocean

Abstract Oceanic variability and eddy dynamics during snowball Earth events, under a kilometer of ice and driven by a very weak geothermal heat flux, are studied using a high-resolution sector model centered at the equator, where previous studies have shown the ocean circulation to be most prominent. The solution is characterized by an energetic eddy field, equatorward-propagating zonal jets, and a strongly variable equatorial meridional overturning circulation (EMOC), on the order of tens of Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1), restricted to be very close to the equator. The ocean is well mixed vertically by convective mixing, and horizontal mixing rates by currents and eddies are similar to present-day values. There are two main opposite-sign zonal jets near the equator that are not eddy driven, together with multiple secondary eddy-driven jets off the equator. Barotropic stability analyses, the Lorenz energy cycle (LEC), and barotropic-to-baroclinic energy conversion rates together indicate that both baroclinic and barotropic instabilities serve as eddy-generating mechanisms. The LEC shows a dominant input into the mean available potential energy (APE) by geothermal heat flux and by surface ice melting and then transformation to eddy APE, to eddy kinetic energy, and finally to mean kinetic energy via eddy–jet interaction, similarly to the present-day atmosphere and unlike the present-day ocean. The EMOC variability is due to the interaction of warm plumes driven by geothermal heating that reach the ocean surface, leading to ice-melt events that change the stratification and, therefore, the EMOC. The results presented here may be relevant to the ocean dynamics of planetary ice-covered moons such as Europa and Enceladus.

A high-resolution chemostratigraphy of post-Marinoan Cap Carbonate using drill core samples in the Three Gorges area, South China

Cap Carbonates overlie the Marinoan Snowball Earth-related glacial diamictite, and possibly record the drastic surface environmental change and biological evolution after the Snowball Earth. We conducted on-land drilling from the Liantuo Formation, through the Nantuo, to the lower Doushantuo Formation in the Three Gorges area of South China to collect fresh, continuous samples in the Three Gorges area. We obtained high-resolution chemostratigraphies of δ13C and δ18O values of carbonates from the topmost part of the Nantuo Formation to the Cap Carbonate, in order to decode the detailed surface environmental change in the shallow marine setting. The δ13C chemostratigraphy possesses some unique characteristics: (1) stable δ13C values as a whole, but ubiquitous low δ13C anomalies through the Cap Carbonate, (2) increase of the δ13C values from −3 to +5‰ across the C2/C3 boundary, (3) no δ13C anomaly between the C1 and C2 boundary, and (4) presence of an anomalous high δ13C value (+2.3‰) and a faint positive correlation between δ13C and δ18O values in the C1 unit.Evidence of quite low δ13C anomalies (with a nadir of −41‰), ubiquitous negative δ13C anomalies through the Cap Carbonate, and a high δ13C anomaly accompanied with a faint positive correlation between δ13C and δ18O values in the C1 unit supports decomposition and formation of methane hydrate during Cap Carbonate formation. The drastic increase of δ13C values from the upper C2 to C3 units indicates enhancement of primary productivity and organic carbon burial, possibly due to high continental fluxes after the Snowball Earth event, evidenced by high Sr isotope values. The increase is restricted to the proximal side of the inner shelf in South China, and the timing of the increase of δ13C values of carbonates is earlier at Three Gorges area than any other area, suggesting that the enhancement of primary productivity started in the proximal environment because of higher continental influxes. The increase in oxygen contents of seawater due to the enhanced primary productivity possibly resulted in the emergence of multicellular animals soon after Cap Carbonate deposition.

Revealing the climate of snowball Earth from Δ17O systematics of hydrothermal rocks

The oxygen isotopic composition of hydrothermally altered rocks partly originates from the interacting fluid. We use the triple oxygen isotope composition ((17)O/(16)O, (18)O/(16)O) of Proterozoic rocks to reconstruct the (18)O/(16)O ratio of ancient meteoric waters. Some of these waters have originated from snowball Earth glaciers and thus give insight into the climate and hydrology of these critical intervals in Earth history. For a Paleoproterozoic [∼2.3-2.4 gigayears ago (Ga)] snowball Earth, δ(18)O = -43 ± 3‰ is estimated for pristine meteoric waters that precipitated at low paleo-latitudes (≤35°N). Today, such low (18)O/(16)O values are only observed in central Antarctica, where long distillation trajectories in combination with low condensation temperatures promote extreme (18)O depletion. For a Neoproterozoic (∼0.6-0.7 Ga) snowball Earth, higher meltwater δ(18)O estimates of -21 ± 3‰ imply less extreme climate conditions at similar paleo-latitudes (≤35°N). Both estimates are single snapshots of ancient water samples and may not represent peak snowball Earth conditions. We demonstrate how (17)O/(16)O measurements provide information beyond traditional (18)O/(16)O measurements, even though all fractionation processes are purely mass dependent.

The marine environments encompassing the Neoproterozoic glaciations: Evidence from C, Sr and Fe isotope ratios in the Hecla Hoek Supergroup in Svalbard

The Neoproterozoic Era records several important events in Earth history. The associations between BIF deposition, Snowball Earth events and the redox state of seawater are the key to explain the re-appearance of banded iron formations (BIF) during the Neoproterozoic. Unraveling ancient iron cycles provides important information about the linkage between the precipitation of BIF and the redox condition in the ocean, but current iron isotopic data are limited to sediments deposited only during the Sturtian glaciations and Ediacaran.We conducted a detailed geological survey of the Tonian to Ediacaran sedimentary succession in Nordaustlandet island, Svalbard. Our chemostratigraphies of δ13Ccarb from pre-Sturtian to post-Marinoan sedimentary successions in the Polarisbreen Group are consistent with those in coeval sediments.We first analyzed iron isotope ratios (δ56/54Fe) of individual pyrite grains with diverse shapes in carbonates, black shales, quartz arenites and diamictites in order to constrain the redox condition in seawater. We found large variations in iron isotope ratios; from ca. −2 to ca. +4‰. Positive δ56/54Fe values, over +1.0‰, are only preserved in euhedral and aggregated pyrite grains in black shales of the Backlundtoppen Formation and the MacDonaldryggen Member of the Elbobreen Formation. A partial oxidation of ferrous iron in seawater is necessary to explain such high δ56/54Fe values, which means that deep seawater was ferruginous (ferrous, iron-rich) before and after the Sturtian glaciation. This stands in opposition to the traditional idea that a glacial cover on an ocean is necessary for the accumulation of ferrous iron in Neoproterozoic seawater. This study presents the first evidence from iron isotopes that supports a deep ferruginous ocean around the Sturtian glaciation. If the deep ocean was intrinsically iron-rich, it is likely that Neoproterozoic BIFs formed by thermohaline circulation during the glaciation and by massive oxidation due to the expansion of oxic oceans after the glaciation.

The Snowball Earth events during the Proterozoic and formation of ore deposits

The Snowball Earth events during the Proterozoic and formation of ore deposits

Stable “Waterbelt” climates controlled by tropical ocean heat transport: A nonlinear coupled climate mechanism of relevance to Snowball Earth

AbstractOngoing controversy about Neoproterozoic Snowball Earth events motivates a theoretical study of stability and hysteresis properties of very cold climates. A coupled atmosphere‐ocean‐sea ice general circulation model (GCM) has four stable equilibria ranging from 0% to 100% ice cover, including a “Waterbelt” state with tropical sea ice. All four states are found at present‐day insolation and greenhouse gas levels and with two idealized ocean basin configurations. The Waterbelt is stabilized against albedo feedback by intense but narrow wind‐driven ocean overturning cells that deliver roughly 100 W m−2 heating to the ice edges. This requires three‐way feedback between winds, ocean circulation, and ice extent in which circulation is shifted equatorward, following the baroclinicity at the ice margins. The thermocline is much shallower and outcrops in the tropics. Sea ice is snow‐covered everywhere and has a minuscule seasonal cycle. The Waterbelt state spans a 46 W m−2 range in solar constant, has a significant hysteresis, and permits near‐freezing equatorial surface temperatures. Additional context is provided by a slab ocean GCM and a diffusive energy balance model, both with prescribed ocean heat transport (OHT). Unlike the fully coupled model, these support no more than one stable ice margin, the position of which is slaved to regions of rapid poleward decrease in OHT convergence. Wide ranges of different climates (including the stable Waterbelt) are found by varying the magnitude and spatial structure of OHT in both models. Some thermodynamic arguments for the sensitivity of climate, and ice extent to OHT are presented.

Geochemical characterization of the Marinoan “Cap Carbonate” of the Niari-Nyanga Basin (Central Africa)

Despite recent works on the Schisto-calcaire Group of the West-Congolian Supergroup in Central Africa, Neoproterozoic Cap Carbonate have not been well documented in the Niari-Nyanga basin that extends from Gabon to Angola. The couplet “Cap Carbonate (SCIa) and Upper Diamictite”, that is a marker of the “Snowball Earth event” was studied in 5 sections from the Niari-Nyanga basin and in one section from the external zone of the Mayombe folded belt (MFB). Our paper presents a high-resolution data set based on results from petrography, geochemistry, mineralogy and stable isotope studies performed on Cap Carbonate samples. The estimation of the illite crystallinity indexes indicates different degrees of post-sedimentary transformations evolving from deep diagenesis in the syncline basin to an epimetamorphism in the external zone of the MFB. Thin-sections of rocks sampled in this latter section display metamorphic features materialized by recrystallization of minerals in a S1 cleavage plane. Despite such an observation, all our samples clearly show preservation of primary petrographic structures such as biolaminations, fenestrae and peloids. Geochemical data leads us to highlight that the Cap Carbonate is composed by a variety of carbonate types. These results support the fact that the Cap Carbonate was derived from a combination of biological and chemical processes: organomineralization and chemical precipitation from supersaturated solutions, with a negligible impact of post-depositional transformations. For all the studied sections, stable isotope profiles display a drop in δ13C values, from about −2.6‰ to −4‰, associated with δ18O values ranging from −6‰ to −10‰. Moreover, the similarity of the isotopic profiles of the studied Cap Carbonate (from the Niari-Nyanga basin and the MFB) and the close fit to the expected Neoproterozoic δ13C and δ18O seawater values constitute a strong indication of a weak impact of a diagenetic to epimetamorphic evolution on the carbon isotopic signature of the SCIa Cap Carbonate. In the light of this data set, our isotopic records must be considered as a reliable tracer of the Marinoan Glaciation within the West-Congolian Supergroup.

Climate simulations of Neoproterozoic snowball Earth events: Similar critical carbon dioxide levels for the Sturtian and Marinoan glaciations

The Sturtian and Marinoan snowball Earth episodes initiated 720 and 650 million years ago, respectively, are among the most dramatic events in Earth's history. The ultimate causes of these events remain obscure, however, and there is still uncertainty about the critical levels of greenhouse gas concentrations at which the snowball transition occurs. Furthermore, earlier modelling results (with incomplete representations of important boundary conditions) provided conflicting indications for differences between the critical carbon dioxide concentrations for the Marinoan and the Sturtian, reporting either the earlier or the later epoch to be more susceptible to global glaciation. Both the absolute values of and possible differences between these glaciation thresholds have profound implications for scenarios of snowball initiations during the Neoproterozoic. Here, we present coupled climate simulations (using an ocean general circulation model with dynamic/thermodynamic sea ice coupled to a fast atmosphere) focusing on the differences between the Neoproterozoic glaciations. For the first time, our simulations use realistic boundary conditions in terms of changes in solar luminosity between the two epochs and the most recent continental reconstructions. In agreement with previous studies with models including ocean and sea-ice dynamics, we report low values for the critical carbon dioxide concentration during the Neoproterozoic. But in contrast to hints from earlier studies we find very similar values of 100–130 ppm for the snowball bifurcation point during the Sturtian and Marinoan. This highlights the importance of realistic boundary conditions for climate simulations of the Neoproterozoic glaciations.

Quaternary remagnetization of the Neoproterozoic limestone of Negash Synclinorium (Arabian–Nubian Shield, northern Ethiopia): With implications of no paleomagnetic testing for the proposed Snowball Earth events

Eighty-one paleomagnetic cores were collected from 10 locations across a black limestone unit within the core of Negash Synclinorium, northern Ethiopia in order to test the proposed Snowball Earth events for the diamictite unit of the Tambien Group. Cores were cut into two standard paleomagnetic specimens and were subjected to stepwise demagnetizations using both Thermal (TH) and alternating field (AF) techniques. Rock magnetic analyses on representative specimens were done and results revealed goethite, pyrrhotite, titano-magnetite, and titano-hematite to be the major magnetic materials carrying the magnetizations with PSD (pseudo single domain) grain size range. In most cases paleomagnetic directions are defined by a single component of magnetization, where a viscous component is present it is usually removed by heating to a temperature of ∼200°C or an AF of ∼10mT. The high stability component isolated above temperature of 200°C or AF of 15mT, defined straight line trajectories directed towards the origin and considered as the Characteristic Remanent Magnetization Direction (ChRM). The direction of magnetization of the ChRM is determined for samples with stable straight line segments by the best-fit line using the least square technique of Kirschvink (1980). In the cases of overlapping spectra and unblocking temperatures, direction of magnetization is determined by remagnetization circles of Halls (1976, 1978). When site mean ChRM directions are plotted on stereogram, their distribution is relatively clustered in geographic coordinates and the overall mean direction is Decg=358.5°, Incg=16.6° (α95=3.8°, K=162.8, N=10). After a structural restoration to the horizontal is made the directions disperse and fail the fold test of both McElhinny’s and McFadden’s tests and the mean direction is Decs=353.5°, Incs=8.8° (α95=18.9°, K=7.5, N=10). This is interpreted to result from a later remagnetization of the black limestone. All directions are normal in polarity and have mean unrestored paleomagnetic directions comparable to the Quaternary paleomagnetic directions. Virtual Geomagnetic poles (VGP) in the unrestored position is used to calculate overall mean VGP position resulting long=235.7°E, latg=84.5°N (A95=3.0°, N=10). Comparison of the obtained pole with the apparent polar wander path (APWP) curve for Africa of Besse and Courtillot (1991, 2003) and with the 2Ma reference pole of stable Africa (Kidane et al., 2003) is found to be consistent with remagnetizations during the Quaternary period. Hence supporting evidence for the proposed Snowball Earth event of the Sturtian glaciation in the Negash rocks could not, unfortunately, be obtained from paleomagnetism.

Investigating the Paleoproterozoic glaciations with 3-D climate modeling

It is generally assumed that the Earth's surface was warm during most of its early history but that significant cooling occurred between 2.45 and 2.22 Ga leading to the first global and cyclical glacial epoch. This onset of snowball Earth conditions was coeval with a large pulse of oxygenation that permanently oxygenated the atmosphere and shallow oceans (Great Oxygenation Event, GOE), though it is not known whether one influenced the other or if they were independent events. Hereafter we used a General Circulation climate Model (GCM) to estimate the partial pressures of atmospheric CO2 (pCO2) and CH4 (pCH4) required to account for the onset of snowball Earth conditions during the Paleoproterozoic. We show that Earth's surface can be maintained in an ice-free state under atmospheric CO2 concentrations lower than 2.6×10−2 bar without invoking the need of high CH4 concentrations. Assuming that the cooling of the Earth's surface is related to the collapse of atmospheric greenhouse gases, we tested the relevance of different scenarios including (i) the collapse of pCH4 in response to the GOE and (ii) the drawdown of pCO2 due to both a decrease in volcanic outgassing rate and an increase in global weathering efficiency. We show that the cyclical character of Paleoproterozoic glaciations is best explained by a long-lasted decrease of pCO2. To support this scenario, we examine how the long-term carbon cycle and the equilibrium pCO2 respond to the emplacement of large subaerial basaltic provinces (LIPs) and to a temporary shutdown of volcanism as supported by geologic data. We show that the sink of pCO2 through silicate weathering is limited by the absence of terrestrial higher plants. In such conditions, the equilibrium pCO2 remains high enough to preclude the onset of snowball conditions regardless the intensity of the pCH4 collapse. The combination of an increase in weathering efficiency and a decrease in volcanic outgassing rate can significantly reduce the equilibrium pCO2, thus favoring the onset of glaciations. Such a buffering of atmospheric CO2 cannot initiate global glaciations alone. Yet, it remains a required condition for the initiation of a snowball Earth event in response to the collapse of pCH4, possibly after 2.32 Ga.

Refugium for surface life on Snowball Earth in a nearly enclosed sea? A numerical solution for sea-glacier invasion through a narrow strait

Where photosynthetic eukaryotic organisms survived during the Snowball Earth events of the Neoproterozoic remains unclear. Our previous research tested whether a narrow arm of the ocean, similar to the modern Red Sea, could have been a refugium for photosynthetic eukaryotes during the Snowball Earth. Using an analytical ice-flow model, we demonstrated that a limited range of climate conditions could restrict sea-glacier flow sufficiently to allow an arm of the sea to remain partially free from sea-glacier penetration, a necessary condition for it to act as a refugium. Here we expand on the previous study, using a numerical ice-flow model, with the ability to capture additional physics, to calculate sea-glacier penetration, and to explore the effect of a channel with a narrow entrance. The climatic conditions are made self-consistent by linking sublimation rate to surface temperature. As expected, the narrow entrance allows parts of the nearly enclosed sea to remain safe from sea-glacier penetration for a wider range of climate conditions.

Correlation of Paleoproterozoic glaciations based on U–Pb zircon ages for tuff beds in the Transvaal and Huronian Supergroups

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