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Abstract

The behavior of the terrestrial glacial regime during the Neoproterozoic glaciations is still a matter of debate. Some papers claim that the glacial sequences cannot be explained with the snowball Earth scenario. Indeed, the near shutdown of the hydrological cycle simulated by climatic models, once the Earth is entirely glaciated, has been put in contrast with the need for active, wet-based continental ice sheets to produce the observed thick glacial deposits. A climate ice-sheet model is applied to the older extreme Neoproterozoic glaciation (around 750 Ma) with a realistic paleogeographic reconstruction of Rodinia. Our climate model shows that a small quantity of precipitation remains once the ocean is completely ice-covered, thanks to sublimation processes over the sea-ice at low latitudes acting as a water vapor source. After 10 ka of the ice-sheet model, the ice volume in the tropics is small and confined as separate ice caps on coastal areas where water vapor condenses. However, after 180 ka, large ice sheets can extend over most of the supercontinent Rodinia. Several areas of basal melting appear while ice sheets reach their ice-volume equilibrium state, at 400 ka, they are located either under the two single-domed ice sheets covering the Antarctica and the Laurentia cratons, or near the ice-sheet margins where fast flow occurs. Only the isolated and high-latitude cratons stay cold-based. Finally, among the simulated ice sheets, most have a dynamic behavior, in good agreement with the needs inferred by the preserved thick formations of diamictite, and share the features of the Antarctica present-day ice sheet. Therefore, our conclusion is that a global glaciation would not have hindered the formation of the typical glacial structures seen everywhere in the rock record of Neoproterozoic times.

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... Second, the magnitudes of both glacial and tectonic erosion are expected to be spatially heterogeneous. Fortunately, however, glacial and tectonic processes predict distinct spatial patterns of exhumationwith tectonic erosion focusing in tectonically active regions near cratonic margins and ice-sheet glacial erosion focusing in regions of wet-based ice-namely, in the models of Donnadieu et al. (33), broad regions of the low-latitude cratonic interiors away from ice divides, narrowing to a more "hit-or-miss" pattern at cratonic margins where basal slip is focused into only a few rapid outlet ice streams, as is observed at modern Greenland and Antarctic ice margins. Consequently, to resolve the relative contributions of all such climatic and tectonic drivers of erosion in the Neoproterozoic, not to mention their potential interactions, we require higher-resolution t-T paths from localities that can address the spatial pattern of Neoproterozoic exhumation at a global scale. ...
... For example, Cowton et al. (111) indicated that the modern Greenland ice sheet erosion rate is ∼2.2 to 7.4 km/My (from the ice margin to >50 km inland) during the deglacial phase, which is at least an order of magnitude higher than previously established ice-sheet erosion rate estimates (112) and places incision rates on par with empirical estimates of ∼1 to >10 km/My for temperate glaciers (113). The results of Neoproterozoic ice-sheet simulations demonstrate that only highlatitude Rodinian cratons (i.e., not Laurentia) would have been characterized by cold-based ice, with low-latitude interior basal ice temperatures near 0 • C and continental basal sliding displacement rates of ∼1 to >10 m/y (33). Furthermore, glacial incision is expected to increase with decreasing latitude (113) and the low-latitude position of Rodinia during the late Neoproterozoic favored increased continental weatherability and precipitation rates (114), thus creating a relationship where erosion would be maximized with lubricated basal ice increasing sliding-leading to more rapid erosion (115). ...
... In contrast, long-term glacial erosion will produce highmagnitude exhumation over areas of thousands of square kilometers, with ice-sheet margins experiencing either very little or extremely high incision due to fluctuating ice dynamics and runoff (33,108). The timing of cooling in our models is coincident with both rifting and glaciation in western North America. ...
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Significance The Great Unconformity involves a common gap of hundreds of millions to billions of years in the geologic record. The cause of this missing time has long eluded explanation, but recently two opposing hypotheses claim either a glacial or a plate tectonic origin in the Neoproterozoic. We provide thermochronologic evidence of rock cooling and multiple kilometers of exhumation in the Cryogenian Period in support of a glacial origin for erosion contributing to the composite basement nonconformity found across the North American interior. The broad synchronicity of this cooling signal at the continental scale can only be readily explained by glacial denudation.
... Secondly, the magnitude of both glacial and tectonic erosion are expected to be spatially heterogeneous. Fortunately, however, glacial and tectonic processes predict distinct spatial patterns of exhumation-with tectonic erosion focusing in tectonically active regions near cratonic margins, and ice-sheet glacial erosion focusing in regions of wet-based ice-namely, in the models of Donnadieu et al. [33], broad regions of the low-latitude cratonic interiors away from ice divides, narrowing to a more 'hit-or-miss' pattern at cratonic margins where basal slip is focused into only a few rapid outlet ice streams-as is observed at modern Greenland and Antarctic ice margins. Consequently, in order to resolve the relative contributions of all such climatic and tectonic drivers of erosion in the Neoproterozoic, not to mention their potential interactions, we require higher-resolution t-T paths from localities that can address the spatial pattern of Neoproterozoic exhumation at a global scale. ...
... For example, Cowton et al. [112] indicated that the modern Greenland ice sheet erosion rate is ⇠2.2-7.4 km Myr 1 (from the ice margin to > 50 km inland) during the deglacial phase, which is at least an order of magnitude higher than previously established ice sheet erosion rate estimates [113], and places incision rates on par with empirical estimates of ⇠1 to > 10 km Myr 1 for temperate glaciers [114]. The results of Neoproterozoic ice-sheet simulations demonstrate that only high-latitude Rodinian cratons (i.e., not Laurentia) would have been characterized by cold-based ice; with low-latitude interior basal ice temperatures near 0 C and continental basal sliding displacement rates of ⇠1 to > 10 m yr 1 [33]. Furthermore, glacial incision is expected to increase with decreasing latitude [114] and the low-latitude position of Rodinia during the late Neoproterozoic favored increased continental weatherability and precipitation rates [115], thus creating a relationship where erosion would be maximized with lubricated basal ice increasing sliding-leading to more rapid erosion [116]. ...
... In contrast, long-term glacial erosion will produce high-magnitude exhumation over areas of 1000s of km 2 , with ice sheet margins either experiencing very little or extremely high incision due to fluctuating ice dynamics and runo↵ [33,109]. The timing of cooling in our models is coincident with both rifting and glaciation in western North America. ...
Preprint
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 of more than 3–4 km during Cryogenian (717–635 Ma) snowball Earth glaciations, or alternatively, diachronous episodic exhumation throughout the Neoproterozoic (1000–540 Ma) due to plate tectonic reorganization from supercontinent Rodinia assembly and breakup. To test these hypotheses, the temporal pattern 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 modelling framework that record a consistent signal of relatively rapid, high magnitude cooling of ~120–200°C interpreted as erosional exhumation of upper crustal basement during the Cryogenian. These models imply widespread, synchronous cooling consistent with at least ~3–5 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.
... In many areas, geological observations imply that Sturtian glacial deposits accumulated during the active rifting of Rodinia (Fig. 5), whereas Marinoan ones accumulated during the drift stage of post-rift subsidence. If true, active rift flanks may have increased Sturtian paleotopography, resulting in more complete coverage of the continents by ice sheets (165,166), most importantly by glacial flow into the equatorial zone of sublimation. This would have raised the planetary albedo. ...
... The rates pertain to sea level (sea ice) and cannot be directly extrapolated to the elevated surfaces of grounded ice sheets. Early studies of ice-sheet development in atmospheric GCMs with Cryogenian paleogeography suggested that tropical ice sheets would thicken and achieve a dynamic steady state within a few 100 ky after the tropical ocean froze over (165,239). This implies that glacial erosion and sedimentation occur continuously during most of the time span of a Snowball cryochron. ...
... This implies that glacial erosion and sedimentation occur continuously during most of the time span of a Snowball cryochron. However, this conclusion was tentative because the simulated air circulation and precipitation patterns in the early studies (165,239) were not iteratively coupled to ice-sheet topographic development. New modeling with improved coupling will be described in the "Ice-sheet stability and extent under precession-like forcing and variable CO 2 " section. ...
Article
Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO 2 was 10 2 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO 2 rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The sub-glacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.
... Some land surface may not be covered by ice during a snowball Earth. For example, entire tropical continents may not be ice covered (Fig. 2B) in the very early stage of a snowball Earth (26), or in the late sage, depending on the greenhouse gas concentrations and orbital configurations (27); the coastal regions may be ice free even if entire continents are covered by ice sheets (Fig. 2A), like the Dry Valleys of the present-day Antarctica, where ice flow is restricted by nearby topography and by katabatic winds [e.g., (28)]. In a waterbelt Earth, which is much warmer than a snowball Earth, the tropical continents should be exposed more easily. ...
... The most comprehensive way of examining the exposure of land surface would be to simulate the ice sheet dynamics and climate at the same time by using a coupled GCM and ice sheet model. However, this will be extremely time-consuming, not only because it will take a very long time [a few hundred thousand of years; e.g., (26,37)] for the model to reach equilibrium but also a large amount of work has to be done manually to design surface topography in some local region. The latter part is needed to prevent the region from becoming ice covered due to ice flow, even though it is not able to grow ice locally because of either high temperature or too little precipitation. ...
Article
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In the equatorial regions on Earth today, the seasonal cycle of the monthly mean surface air temperature is <10°C. However, deep (>1 m) sand wedges were found near the paleoequator in the Marinoan glaciogenic deposits at ~635 million years ago, indicating a large seasonal cycle (probably >30°C). Through numerical simulations, we show that the equatorial seasonal cycle could reach >30°C at various continental locations if the oceans are completely frozen over, as would have been the case for a snowball Earth, or could reach ~20°C if the oceans are not completely frozen over, as would have been the case for a waterbelt Earth. These values are obtained at the maximum eccentricity of the Earth orbit, i.e., 0.0679, and will be approximately 10°C smaller if the present-day eccentricity is used. For these seasonal cycles, theoretical calculations show that the deep sand wedges form readily in a snowball Earth while hardly form in a waterbelt Earth.
... Continental glaciation extended to low paleolatitudes in three well-established Neoproterozoic intervals: the Sturtian (717-660 Ma), Marinoan (641-635 Ma), and Gaskiers (∼580 Ma)the first two envisioned as global "snowball" events (36,37) and the Gaskiers as an extensive, but not pan-glacial, event (38). While ice sheet thickness on a snowball Earth is imperfectly constrained and likely heterogeneous (0-6 km) (39)(40)(41), glaciation on all continents analogous to that currently found in Antarctica (∼2 km average thickness) would lower sea level by ∼787 m before isostatic adjustment. After isostatic and local gravitational adjustments, modeled freeboard for ice-covered Neoproterozoic continents is variable but positive, with global averages of 400-650 m for each glacial episode (39). ...
... The extent of ice-free ocean available to sustain hydrological cycling during such global glaciation is controversial (41,44). However, precipitation rates driven by sublimation alone appear sufficient for the development of localized wet-based ice streams with high basal sliding velocities and consequent erosive potential (40); evaporation from cryoconite ponds [a notable sink for solar radiation in a snowball state (45)] might further enhance hydrological cycling. Much of the characteristic field evidence for Neoproterozoic glaciation is unmistakably erosional, including striated pavements, striated and exotic clasts and dropstones, and preserved glacial diamictites (36,46,47). ...
Article
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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.
... The reasons why we are testing sensitivity to reductions in ice sheet thickness rather than increases are discussed below. The elevations of the ice-sheet surface obtained in Liu and Peltier (2010) are similar to, but most probably slightly higher than those obtained by Donnadieu et al. (2003;see their Fig. 5d) using an ice-sheet model forced by climate fields from an AGCM. ...
... This is because that the 100 % thickness case calculated by the UofT GSM coupled with an EBM is consistent with geological inference (Liu and Peltier 2013a, b); if the ice sheets are 50 % or more thinner than this control case, the sea level drop associated with their construction would be too small compared with observations (Hoffman et al. 2007). Further support for this argument is provided by the fact that the 100 % ice-sheet thickness adopted here is in fact close to that previously obtained by Donnadieu et al. (2003), in which the ice sheets were calculated for a hard snowball Earth state whose atmosphere should be much drier than the soft snowball Earth state (Yang et al. 2012a, b) assumed here. ...
Article
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The hard snowball Earth bifurcation point is determined by the level of atmospheric carbon dioxide concentration (pCO2) below which complete glaciation of the planet would occur. In previous studies, the bifurcation point was determined based on the assumption that the extent of continental glaciation could be neglected and the results thereby obtained suggested that very low values of pCO2 would be required (~100 ppmv). Here, we deduce the upper bound on the bifurcation point using the coupled atmosphere–ocean climate model of the NCAR that is referred to as the Community Climate System Model version 3 by assuming that the continents are fully covered by ice sheets prior to executing the transition into the hard snowball state. The thickness of the ice sheet is assumed to be that obtained by an ice-sheet model coupled to an energy balance model for a soft snowball Earth. We find that the hard snowball Earth bifurcation point is in the ranges of 600–630 and 300–320 ppmv for the 720 and 570 Ma continental configurations, respectively. These critical points are between 10 and 3 times higher than their respective values when ice sheets are completely neglected. We also find that when the ice sheets are thinner than those assumed above, the climate is colder and the bifurcation point is larger. The key process that causes the excess cooling when continental ice sheets are thin is shown to be associated with the fact that atmospheric heat transport from the adjacent oceans to the ice sheet-covered continents is enhanced in such conditions. Feedbacks from sea-ice expansion and reduced water vapor concentration further cool the oceanic regions strongly.
... A steady rate of tec- tonic extension, coupled with a highly non-steady rate of sedimentation, could produce the same ef- fect. In the snowball Earth hypothesis, there is a long period (100s of kyrs at least) following the initial ice-albedo runaway, when surface tempera- tures are coldest and continental ice sheets have not yet thickened enough to develop ice streams ( Donnadieu et al., 2003). Sedimentation rates dur- ing this period would be exceedingly low, but any topography that developed would be subject to ero- sion during the later, more dynamic glacial stage, when the bulk of the glacial record was likely de- posited. ...
... An active tectonic setting provides a greater variety of erodable ma- terial, and topographic relief favours glacial initi- ation and growth (DeConto et al., 2003). Coastal topography in the sub-tropics would be particularly favourable for ice sheet development on a snowball Earth because of the ablation of adjacent sea ice ( Donnadieu et al., 2003). After the ocean freezes over, it takes 100s kyrs for tropical ice sheets to develop and thicken to the point where ice streams develop that could supply the glacial deposits ob- served. ...
... cover-up of continental and oceanic surfaces by ice. This hypothesis seems to account for most field observations, and especially for the low paleolatitudes glacial deposits inferred from paleomagnetic results (see [Donnadieu et al., 2003] and [Pollard and Kasting, 2003]) as well as for the depletion in 13 C observed in the cap carbonates overlying the glacial deposits [Donnadieu et al., 2003; Hoffman et al., 1998a; Hoffman et al., 1998b]. Hoffman and colleagues interpret this negative shift in δ 13 C to be the consequence of prolonged low organic productivity during snowball events, high rates of carbonate sedimentation due to the high alkalinity and carbon fluxes into the ocean as a result of enhanced weathering during the " supergreenhouse " climate in the aftermath of the snowball glaciation . ...
... cover-up of continental and oceanic surfaces by ice. This hypothesis seems to account for most field observations, and especially for the low paleolatitudes glacial deposits inferred from paleomagnetic results (see [Donnadieu et al., 2003] and [Pollard and Kasting, 2003]) as well as for the depletion in 13 C observed in the cap carbonates overlying the glacial deposits [Donnadieu et al., 2003; Hoffman et al., 1998a; Hoffman et al., 1998b]. Hoffman and colleagues interpret this negative shift in δ 13 C to be the consequence of prolonged low organic productivity during snowball events, high rates of carbonate sedimentation due to the high alkalinity and carbon fluxes into the ocean as a result of enhanced weathering during the " supergreenhouse " climate in the aftermath of the snowball glaciation . ...
Article
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Whereas the snowball Earth hypothesis seems to account for most of the major fea-tures of the Neoproterozoic glacial records, the causes that drove the Earth into a snowball state remain largely open to debate. Most of the mechanisms leading to the initiation of a snowball Earth are based on the existence of the unusual preponder-ance of land masses in the tropics. However, the time of the youngest Neoprotero-zoic glaciation is characterised by a rather widely distributed geography from low-to-high latitudes. In the absence of reliable knowledge of Neoproterozoic topog-raphy, two series of coupled ocean-atmosphere climate model simulations were car-ried out with a Late Neoproterozoic paleogeography (580 Ma) and solar luminosity reduced by 6% relative to today, the first one with flat continents and the second one with mountain ranges that mimic the Pan-African Orogen occurring at this time. Those climatic simulations coupled to the long-term carbon cycle have allowed to better constrain the atmospheric pCO 2 and the associated climate by the time of the youngest late Proterozoic glaciation. The Pan-African Orogen runs result in a snow accumulation pattern compatible with a regional-scale glaciation more less exten-sive while the no relief runs do not succeed in initiating any glaciation. These results could give additional support to the inferences from many authors that some of the glacial deposits originally attributed to a snowball-like glaciation could in fact be the consequence of a more localised glaciation due to the important orogen occurring at the end of the Neoproterozoic.
... We use the Parallel Ice Sheet Model (PISM) forced by four equilibrated climate states from ECHAM5/MPI-OM (Table 1). Only two previous Neoproterozoic studies [Donnadieu et al., 2003;Pollard and Kasting, 2004] have forced an ice sheet model with GCM results, as opposed to with EBMs [e.g., Hyde et al., 2000;Crowley et al., 2001;Liu and Peltier, 2010], which are unable to calculate atmospheric heat and moisture transport in a dynamically consistent way. Pollard and Kasting [2004] considered glaciation in some states with open ocean and found mixed results. ...
... Our modeling framework may be biased toward ice sheet formation in a hard snowball because of small patches of seasonally open ocean in the SNOW simulation, which may lead to an overestimate of precipitation, and because the PDD parameterization may underestimate ablation in cold, hard snowball conditions [Pollard and Kasting, 2005]. Nevertheless, these limitations are likely quantitative, rather than qualitative, since both Donnadieu et al. [2003] and Pollard and Kasting [2005] also found large equatorial ice sheets in a hard snowball. ...
Article
A major goal of understanding Neoproterozoic glaciations and determining their effect on the evolution of life and Earth's atmosphere is establishing whether and how much open ocean there was during them. Geological evidence tells us that continental ice sheets had to flow into the ocean near the equator during these glaciations. Here we drive the PISM ice sheet model with output from four simulations of the ECHAM5/MPI-OM atmosphere-ocean general circulation model with successively narrower open ocean regions. We find that extensive equatorial ice sheets form on marine margins if sea ice extends to within about 20 degrees latitude of the equator or less (Jormungand-like and hard Snowball states), but do not form if there is more open ocean than this. Given uncertainty in topographical reconstruction and ice sheet ablation parameterizations, we perform extensive sensitivity tests to confirm the robustness of our main conclusions.
... [20] Based on simulations using an ice sheet model driven by output from the LMDz GCM, Donnadieu et al. [2003] argued that the hydrological cycle during a global glaciation would be sufficiently strong to produce the observed evidence of flowing continental glaciers. Measured by precipitation minus evaporation extrema, all models except FOAM simulate a hydrological cycle at least as strong as LMDz ( Figure 5). ...
... Measured by total global evaporation (latent heat), all models simulate a hydrological cycle stronger than LMDz (not shown). This suggests that the conclusions of Donnadieu et al. [2003] should be robust to changing the GCM driving the glacial model. ...
Article
[1] Atmospheric circulation in a Snowball Earth is critical for determining cloud behavior, heat export from the tropics, regions of bare ice, and sea glacier flow. These processes strongly affect Snowball Earth deglaciation and the ability of oases to support photosynthetic marine life throughout a Snowball Earth. Here we establish robust aspects of the Snowball Earth atmospheric circulation by running six general circulation models with consistent Snowball Earth boundary conditions. The models produce qualitatively similar patterns of atmospheric circulation and precipitation minus evaporation. The strength of the Snowball Hadley circulation is roughly double modern at low CO2 and greatly increases as CO2 is increased. We force a 1‒D axisymmetric sea glacier model with general circulation model (GCM) output and show that, neglecting zonal asymmetry, sea glaciers would limit ice thickness variations to (10%). Global mean ice thickness in the 1‒D sea glacier model is well‒approximated by a 0‒D ice thickness model with global mean surface temperature as the upper boundary condition. We then show that a thin‒ice Snowball solution is possible in the axysymmetric sea glacier model when forced by output from all the GCMs if we use ice optical properties that favor the thin‒ice solution. Finally, we examine Snowball oases for life using analytical models forced by the GCM output and find that conditions become more favorable for oases as the Snowball warms, so that the most critical time for the survival of life would be near the beginning of a Snowball Earth episode.
... 717.4-Ma volcanic rocks should only be taken as the minimum bound on the maximum age of Sturtian onset, meaning that onset could be as old or older than this constraint depending on whether the volcanic rocks are preglacial or synglacial. Additionally, it could have taken up to several 100 ka after the ocean froze over for continental ice sheets to thicken sufficiently to flow and produce glacial structures at their periphery (45,46), further delaying observable evidence of glaciation in the rock record from onset. ...
Article
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During the Cryogenian (720 to 635 Ma ago) Snowball Earth glaciations, ice extended to sea level near the equator. The cause of this catastrophic failure of Earth's thermostat has been unclear, but previous geochronology has suggested a rough coincidence of glacial onset with one of the largest magmatic episodes in the geological record, the Franklin large igneous province. U-Pb geochronology on zircon and baddeleyite from sills associated with the paleo-equatorial Franklin large igneous province in Arctic Canada record rapid emplacement between 719.86 ± 0.21 and 718.61 ± 0.30 Ma ago, 0.9 to 1.6 Ma before the onset of widespread glaciation. Geologic observations and (U-Th)/He dates on Franklin sills are compatible with major post-Franklin exhumation, possibly due to development of mafic volcanic highlands on windward equatorial Laurentia and increased global weatherability. After a transient magmatic CO2 flux, long-term carbon sequestration associated with increased weatherability could have nudged Earth over the threshold for runaway ice-albedo feedback.
... Unlike modern glaciations, the MAT and MAP records for Neoproterozoic cryochrons are poor. However, a quantity of precipitation remains in the form of snow during Neoproterozoic cryochrons due to sublimation process over the sea-ice at low latitudes acting as a water vapor source (Donnadieu et al., 2003), and a faster rotation rate of the Earth (daylength is 18 h) may increase predicted global MAP up to 1430 mm/yr (Taylor and McLennan, 1985;Condie, 1993). (Jenkins and Smith, 1999). ...
Article
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Three major late Neoproterozoic global cryochrons mark the coldest climate state in Earth history, with glaciers covering most, if not all, continents. However, the terrestrial paleosurface temperature differential between cryochron and nonglacial interlude is unknown. Time-equivalent Cryogenian and Ediacaran sedimentary successions from South China and Oman show multiple negative excursions of the corrected values of their chemical index of alteration (CIA corr). These excursions reflect climate cooling and correlate with the Sturtian, Marinoan and Gaskiers cryochrons. Based on the τ Na-MAT transfer function (sodium chemical depletion index-mean annual temperature), we estimate the temperature differential between Cryogenian cryochron and nonglacial interlude reached 20 ± 5.4 ℃ at around 30 • paleolatitude, and 12 ± 5.4 ℃ temperature difference between the Ediacaran cryochron and nonglacial interlude near the equator. The fluctuation of CIA corr and MAT trends within individual cryochrons likely indicate cold-warm climate cycling during the Cryogenian cryochron.
... It has been estimated that eustatic sea-level during the Snowball Earth events would have fallen between 250 and 1,500 m (Donnadieu, Fluteau, Ramstein, Ritz, & Besse, 2003;Hoffman, 2011). For comparison, during the last glacial maximum, the sea-level lowstand was roughly 120 m below current sea level (Denton et al., 2010). ...
Article
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[1] While cap dolostones are integral to the provocative Snowball Earth hypothesis, the current depositional model does not account for multiple geological observations. Here we propose a model that rationalises paleomagnetic, sequence‐stratigraphic and sedimentological data and supports rapid deglaciation with protracted cap dolostone precipitation. The Snowball Earth hypothesis posits that a runaway ice‐albedo can explain the climate paradox of Neoproterozoic glacial deposits occurring at low palaeolatitudes. This scenario invokes volcanic degassing to increase atmospheric greenhouse gases to a critical threshold that overcomes the albedo effect and brings the planet back from the ice‐covered state. Once this occurs, Earth should shift rapidly from a snowball to an extreme greenhouse. However, cap dolostone units overlying glacial sediments, typically interpreted as transgressive deposits, exhibit multiple magnetic reversals indicating they accumulated in >105 years. By reviewing modern post‐glacial systems, sequence stratigraphic concepts and principles of sedimentology, we suggest that cap dolostones are not restricted to the transgression but rather represent sediment starvation following a major landward shoreline migration associated with the demise of Snowball Earth. Thus, the duration in which cap dolostone accumulated is not directly coupled to the timescale of the Snowball Earth deglaciation.
... In an extension of that work, Hoffman et al. (2017) found the deglaciated areas in Benn et al. (2015) were able to occasionally reach above-freezing temperatures. In contrast, using different models, Donnadieu et al. (2003) and Rodehacke et al. (2013) were unable to produce deglaciated land in other Snowball Earth experiments, suggesting that the details of the ice sheet model matter and are one of the key caveats of our results. Our goal was to use a simple approach to identify the impact that ice sheet elevation could have on our results and find that very large ice sheets are a problem, but thinner ice sheets may not be. ...
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Plain Language Summary Studies examining the ability of Earth‐like planets to host life have often used conditions leading to “snowball” events, where sea ice extends all the way to the equator, as a limit to the range of habitable climates. This has been based on the assumption that snowball planets are always below freezing everywhere on their surfaces. As the chemical process by which CO2 is removed from the atmosphere and bound in surface rocks relies on warm temperatures and liquid water and therefore would not happen in globally cold conditions, this has also led to the conclusion that snowball events should be temporary, coming to an end when volcanoes release enough accumulated CO2 to warm the planet enough to melt the ice. We ran several thousand three‐dimensional computer simulations of Earth‐like climates and found that if there are inland areas of dark, bare ground under enough sunlight, those regions can be warm enough for liquid water and life without causing the sea ice to retreat. This suggests that snowball planets should not be excluded as inhospitable to life and that on some planets the burial of CO2 in surface rocks in these areas could balance volcanic emissions, resulting in permanent snowball conditions.
... The youngest population ages of zircons contained in Cretaceous-Tertiary kaolins, indicated Neoproterozoic ages. However, the Neoproterozoic is believed to have been characterised by periods with a global glaciated Earth, referred to as the Snowball Earth (Donnadieu et al., 2003(Donnadieu et al., , 2004. Hence, kaolins of the Douala Sub-Basin could not have been deposited during the Neoproterozoic. ...
Article
Detrital zircon geochronology is a reliable provenance tool used to trace known zircon age populations from their metamorphic or igneous source to their present location in sedimentary basins. This paper presents U/Pb LA-SF-ICP-MS data of detrital zircons in Cretaceous-Tertiary kaolins in the Douala Sub-Basin in order to determine their provenance. The minimum ages of parent rocks which kaolinised were determined using U-Pb LA-SFICP-MS dating of zircons in the kaolin deposits. Four main zircon populations were identified from radiogenic dating: the 1st between 550 and 650 Ma, the 2nd between 950 and 1050 Ma, the 3rd around 1600 Ma and the 4th between 2800 and 3200 Ma. These four zircon populations belong to the Proterozoic (Neo-, Meso- and Paleoproterozoic) and the Archean. The minimum ages of parent rocks which contained the primary minerals that were kaolinised, reflected by the youngest weighted averages of zircon populations varied between 588 ± 2 Ma and 612 ± 2 Ma, all belonging to the Ediacaran Period (Neoproterozoic), respectively. Ages of zircons in Cretaceous-Tertiary kaolins suggested that the zircons formed during two main tectonic events: the Eburnean orogeny, during which older zircons crystallised and the Pan-African orogeny, during which younger zircons crystallised. The main identified sources of these zircons are the Archean Ntem Complex, the Paleoproterozoic Nyong Group and the Neoproterozoic Yaounde Group.
... While it now seems that dynamic, wet-based glaciers capable of eroding and depositing large volumes of sediment would have been active several hundred thousand years after an initial snowball freezeover (Hoffman, 2000;Donnadieu et al., 2003), evidence for open water conditions from the Fiq Member glacials in the Oman Mountains on the Arabian Peninsula is more difficult to reconcile with a completely ice-covered ocean . The Fiq Member consists of a range of non-glacial to glaciomarine facies preserved in six transgressive-regressive cycles, several of which contain wave-generated ripples, demonstrating that open water conditions prevailed intermittently during this glaciation . ...
Book
The Neoproterozoic Era (1000–542 million years ago) is a geological period of dramatic climatic change and important evolutionary innovations. Repeated glaciations of unusual magnitude occurred throughout this tumultuous interval, and various eukaryotic clades independently achieved multicellularity, becoming more complex, abundant, and diverse at its termination. Animals made their first debut in the Neoproterozoic too. The intricate interaction among these geological and biological events is a centrepiece of Earth system history, and has been the focus of geobiological investigations in recent decades. The purpose of this volume is to present a sample of views and visions among some of the growing numbers of Neoproterozoic workers. The contributions represent a cross section of recent insights into the field of Neoproterozoic geobiology. Chapter One by Porter gives an up-- date review of Proterozoic heterotrophic eukaryotes, including fungi and various protists. Heterotrophs are key players in Phanerozoic ecosystems; indeed, most Phanerozoic paleontologists work on fossil heterotrophs. However, the fossil record of Proterozoic heterotrophs is extremely meagre.
... Sedimentological evidence for subglacial meltwater in a bedrock trough like the one at Omutirapo springs suggests that the trough was occupied by an ice stream, even if it is uncertain whether the trough was excavated by the ice stream or by an ancestral valley glacier (Bentley, 1987;Bamber et al., 2000;Bennett, 2003;Ottesen et al., 2005;Kessler et al., 2008). Ice streams develop in a coupled ice sheet-atmosphere general circulation model (Laboratoire de Glaciologie et de Géophysique de l'Environnement ice sheet model-Laboratoire de Météorologique Dynamique atmosphere general circulation model) prescribed with an ice-covered ocean (Donnadieu et al., 2003). This is not surprising because low accumulation rates of meteoric ice in a Snowball Earth (Abbot et al., 2013) result in relatively old and consequently warm basal ice, facilitating ice-stream development despite little frictional heat production within the generally slow-moving ice sheet (Rignot and Mouginot, 2012). ...
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Cryogenian synglacial deposits are regionally thin but locally thick, considering glacial duration, but the reasons for local thickening are poorly known. We studied three local thickenings of the Sturtian Chuos Formation in northern Namibia by measuring closely spaced columnar sections, not only of the synglacial deposits but also of the bounding pre- and post-glacial strata. This enabled incised paleovalleys filled by glacial debris to be distinguished from morainal buildups. In case 1, a U-shaped paleovalley, ~450 m deep by ~3.0 km wide, is incised into pre-glacial strata and 10% overfilled by ice-contact and subglacial meltwater deposits. In case 2, a wedge of glacial diamictite, ~220 m thick by 2.0 km wide, overlies a disconformity that is demonstrably not incised into underlying pre-glacial strata. The wedge, draped by a post-glacial cap carbonate and argillaceous strata, is erosionally truncated at its apex by Marinoan glacial deposits and their basal Ediacaran cap dolomite. The wedge was a positive topographic feature, either a terminal moraine or an erosional outlier of formerly more extensive glacial deposits. In case 3, a wedge of conglomerate, glacial diamictite, and subglacial lake deposits thickens to >2000 m where it abuts against granitoid basement rock uplifted along a border fault. Fault movement ceased before the Sturtian cap carbonate was deposited. The locus of maximum deposition shifted over time from proximal to distal with respect to the border fault, similar to Mesozoic half grabens developed above listric detachments imaged seismically on offshore North Atlantic margins.
... Unfortunately, such a model is still unavailable although both sophisticated AOGCMs and land ice sheet models do clearly exist. Nevertheless the very long timescale over which such a coupled structure would have to be integrated (∼ 100 kyr) associated with the formation of a snowball Earth (Donnadieu et al., 2003;Liu andPeltier, 2010, 2011) is not possible to contemplate on account of the computational resources that would be required. To adequately test the thin-sea-ice hypothesis, a sea-glacier model such as that developed in Tzipperman et al. (2012) may be required to replace the sea-ice module in the current AOGCMs as well as coupling to an explicit model of land ice-sheet evolution. ...
Article
We identify the "hard snowball" bifurcation point at which total sea ice cover of the oceans is expected by employing the comprehensive coupled climate model CCSM3 for two realistic Neoproterozoic continental configurations, namely a low-latitude configuration appropriate for the 720 Ma Sturtian glaciation and a higher southern latitude configuration more appropriate for the later 635 Ma Marinoan glaciation. We find that for the same total solar insolation (TSI) and atmospheric CO2 concentration (pCO2), the most recent continental configuration is characterized by colder climate than the 720 Ma continental configuration and enters the hard snowball state more easily on account of the following four factors: the low heat capacity of land in the south polar region, the higher albedo of the snow covered land compared to that of sea ice, the more negative net cloud forcing near the ice front in the Northern Hemisphere (NH), and more importantly, the more efficient sea ice transport towards the equator in the NH due to the absence of blockage by continents. Beside the paleogeography, we also find the optical depth of aerosol to have a significant influence on this important bifurcation point. When the high value (recommended by CCSM3 but demonstrated to be a significant overestimate) is employed, the critical values of pCO2, beyond which a hard snowball will be realized, are between 80–90 ppmv and 140–150 ppmv for the Sturtian and Marinoan continental configurations, respectively. However, if a lower value is employed that enables the model to approximately reproduce the present-day climate, then the critical values of pCO2 become 50–60 ppmv and 100–110 ppmv for the two continental configurations, respectively. All of these values are higher than previously obtained for the present-day geography (17–35 ppmv) using the same model, primarily due to the absence of vegetation, but are much lower than that obtained previously for the 635 Ma continental configuration using the ECHAM5/MPI-OM model in its standard configuration (∼500 ppmv). However, when the sea ice albedo in that model was reduced from 0.75 to a more appropriate value of 0.45, the critical pCO2 becomes ∼204 ppmv, closer to but still higher than the values obtained here. Our results are similar to those obtained with the present-day geography (70–100 ppmv) when the most recent version of the NCAR model, CCSM4, is employed.
... La complexité a toujours existé mais sa perception et sa prise en considération sont relativement récentes. Si certains outils ont été développés au cours du 19 ème siècle, ceux-ci étaient réduits à des composantes élémentaires et à des interactions linéaires entre ces dernières(Donnadieu et al., 2003). La prise en compte de l'instabilité, del'évolution, de l'ouverture et des fluctuations par exemple sont autant de facteurs qui fondent le recours croissant aux approches par système. ...
... At first it was thought that dynamic ice sheets were inconsistent with a snowball because the frozen ocean could not resupply them with snow. However, climate models suggest that net sublimation of subtropical sea-ice would supply ice sheets with up to centimeters of ice-equivalent annually, meaning that ice sheets would thicken sufficiently to generate dynamic ice streams within a few 10 5 year, a fraction of the estimated snowballs duration ( Donnadieu et al., 2003;Pollard and Kasting, 2004). Accordingly, the evidence for dynamic ice sheets is not incompatible with a frozen ocean. ...
... This grid point model includes a full seasonal cycle as well as a diurnal cycle. This 3D atmospheric model has been employed to investigate present and past climates [72,73,74,75,76]. Moreover, the LMDZ model has been validated by checking its capacity to simulate the current climate. ...
... This longer time span for cap carbonate deposition was also an outcome of the more extensive modeling in Le Hir et al. (2009). Accounting for the total amount of water released by the melting of the huge ice cap (200 Â 10 3 km 3 ; Donnadieu et al., 2003) and estimating the total flux of Mg 2þ and Ca 2þ released by weathering during the transgressive phase with a reactive-transport model, Le Hir et al. (2009) calculated that only 50 cm of pure dolostone would accumulate on the shelves in the first 10 ky, far below the estimated worldwide average thickness of 18.5 m (Hoffman et al., 2007). This slow rate of accumulation suggests that cap dolostone deposition may have spanned longer than 10 ky, supporting arguments for a longer time span of cap dolostone deposition based on the existence of multiple magnetic reversal events measured in various caps (Raub and Evans, 2006;Trindade et al., 2003). ...
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This article was originally published in Treatise on Geochemistry, Second Edition published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution's administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution's website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Donnadieu Y., Goddéris Y. and Le Hir G. (2014) Neoproterozoic Atmospheres and Glaciation. In: Holland H.D. and Turekian K.K. (eds.) Treatise on Geochemistry, Second Edition, vol. 6, pp. 217-229. Oxford: Elsevier.
... This grid point model includes a full seasonal cycle as well as a diurnal cycle. This 3D atmospheric model has been employed to investigate present and past climates [72,73,74,75,76]. Moreover, the LMDZ model has been validated by checking its capacity to simulate the current climate. ...
... In that time, the continent-fragments of the Earth were covered with icecaps that were up to four kilometers thick. These icecaps temporarily contained volumes of up to 350 million km 3 of frozen water [51] [52]. The sea was also most likely concealed by a mighty ice layer approximately one kilometer thick [53]. ...
Article
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Long periodic geodynamic processes with durations between 150 and 600 Million years appear to be in phase with similar galactic cycles, caused by the path of the solar system through the spiral arms of the Milky Way. This path is assumed by some authors to cause climate change due to cosmic ray fluctuations, affecting the cloud formation and the related albedo of the Earth, which periodically lead to glaciations every 150 Ma. With the glaciations, the sea level fluctuates accordingly. Subsequently, the varying sizes of shallow seas are causing periodic changes of the Moon’s tidal dissipation, which affects presumably other geodynamic processes on the Earth. The Moon may therefore synchronize directly or indirectly long periodic Phanerozoic cycles (sea level, orogeny, magmatism, sedimentation, etc.) with the Milky Way. As sea level fluctuations, orogeny, sedimentation and magmatism can be described as members of a geodynamic feedback system; no apparent reasons appear to be required to assign a cause of the cyclicity to agents outside of the galactic-climatically synchronized Earth-Moon system. However, recent observations of young volcanism on the near Earth terrestrial planets may require a new understanding. Magmatic/volcanic episodes on Venus, Mars and Mercury as well as on the Earth’s Moon are apparently contemporaneous thermal events accompanying increased magmatic/volcanic activities on the Earth, following a 300 myr cycle. Therefore, a collateral galactic thermal source within the Milky Way appears to be needed that only affects the interior of the planets without any recognizable direct effect on life and geology on the Earth. The search for such a source may lead to astrophysical questions, related to a spiral arm affected distribution of dark energy, dark matter or even specific neutrino sources. However, all possible astrophysical answers are outside of the author’s competence.
... This isotope conglomerate test supports the conclusion that the Trezona δ 13 C anomaly was recorded long before burial diagenesis could have occurred (Rose et al. 2012). Evidence for sub-glacial erosion of the carbonate platform by ice streams and ice-front instability within an overall deglacial sequence remains compatible with snowball Earth models (Donnadieu et al. 2003; Halverson et al. 2004). Our evidence suggests that the local deglaciation and instantaneous loss of gravitational attraction of the ice sheet on the nearby ocean caused a relative sea level fall, which may be comparable to Greenland during Pleistocene deglaciations. ...
Article
SUMMARY The Elatina Fm. records the younger Cryogenian ice age in the Adelaide Rift Complex (ARC) of South Australia, which has long-held the position as the type region for this low-latitude glaciation. Building upon a legacy of work, we document the pre- and syn-glacial sedimentary rocks to characterize the dynamics of the glaciation across the ARC. The Elatina Fm. records an array of well-preserved glacial facies at many different water depths across the basin, including ice contact tillites, fluvioglacial sandstones, dropstone intervals, tidal rhythmites with combined flow ripples, and turbidites. The underlying Yaltipena Fm. records the proglacial influx of sediment from encroaching land-based ice sheets. The onset of the glaciation is heralded by the major element ratios (Chemical Index of Alteration) of the pre-glacial facies across the platform that show a reduction in chemical weathering and a deterioration in climate towards the base of the Elatina Fm. The advancing ice sheets caused soft-sediment deformation of the beds below the glacial diamictite, including sub-glacial push structures, as well as sub-glacial erosion of the carbonate unit beneath. Measured stratigraphic sections across the basin show glacial erosion up to 130 m into the carbonate platform. However, δ13C measurements of carbonate clasts within the glacial diamictite units were used to assess provenance and relative timing of δ13C acquisition, and suggest that at least 500 m of erosion occurred somewhere in the basin. Detrital zircon provenance data from the Elatina Fm. suggest that glacial sediment may have been partially sourced from the cratons of Western Australia and that the Whyalla Sandstone, even if stratigraphically correlative, was not a sediment source. The remainder of the Elatina Fm. stratigraphy mostly records the deglaciation and can be divided into three facies: a slumped sandstone, dropstone diamictite, and currentreworked diamictite. The relative sea level fall within the upper Elatina Fm. requires that regional deglaciation occurred on the timescale of ice sheet – ocean gravitational interactions (instant) and/or isostatic rebound (~104 years). Structures previously interpreted as soft-sediment folds within the rhythmite facies that were used to constrain the low-latitude position of South Australia at the time of the Elatina glaciation are re-interpreted as stoss-depositional transverse ripples with superimposed oscillatory wave ripples. These combined-flow ripples across the ARC attest to open seas with significant fetch during the initial retreat of local glaciers. In addition, this interpretation no longer requires that the magnetization be syn-depositional, although we have no reason to believe that the low-latitude direction is a result of remagnetization, and positive reversal tests and tectonic fold tests are at least consistent with syndepositional magnetization. Together, these paired sedimentological and chemostratigraphic observations reveal the onset of the glaciation and advance of the ice sheet from land to create a heavily glaciated terrain that was incised down to at least the base of the pre-glacial Trezona Fm.
... Recent estimates of the mean ocean salinity in Snowball states lie somewhere between the present-day value of ;35 and two times this value (;70) [although see Knauth (2005)], based on the assumption that the ocean's Neoproterozoic salt content prior to the Snowball events was similar to present-day values and that the mean ocean water depth was about 2 km, about half of presentday values. This is based on an assumed 1-km sea level equivalent land ice cover (Donnadieu et al. 2003;Pollard and Kasting 2004) and 1-km ice cover over the ocean. We chose (somewhat arbitrarily) an initial salinity of 50. ...
Article
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Between similar to 750 and 635 million years ago, during the Neoproterozoic era, the earth experienced at least two significant, possibly global, glaciations, termed Snowball Earth. While many studies have focused on the dynamics and the role of the atmosphere and ice flow over the ocean in these events, only a few have investigated the related associated ocean circulation, and no study has examined the ocean circulation under a thick (similar to 1 km deep) sea ice cover, driven by geothermal heat flux. Here, a thick sea ice-flow model coupled to an ocean general circulation model is used to study the ocean circulation under Snowball Earth conditions. The ocean circulation is first investigated under a simplified zonal symmetry assumption, and (i) strong equatorial zonal jets and (ii) a strong meridional overturning cell are found, limited to an area very close to the equator. The authors derive an analytic approximation for the latitude-depth ocean dynamics and find that the extent of the meridional overturning circulation cell only depends on the horizontal eddy viscosity and (the change of the Coriolis parameter with latitude). The analytic approximation closely reproduces the numerical results. Three-dimensional ocean simulations, with reconstructed Neoproterozoic continental configuration, confirm the zonally symmetric dynamics and show additional boundary currents and strong upwelling and downwelling near the continents.
... Voraussetzung ist aber, dass beide einen entscheidenden Einfluss haben, was beim globalen Meeresspiegel und der Aus-eschweizerbart_xxx dinia (Li et al. 2008, Scotese 2004. In dieser Zeit waren die Kontinentfragmente der Erde mit Eisschichten bedeckt, die bis zu 4 km dick waren und kurzzeitig ein Volumen bis zu 350 Millionen km 3 umfassten (Hyde et al. 2000, Donnadieu et al. 2003. Das Meer wurde höchstwahrscheinlich von einer ca. 1 km mächtigen Eisschicht abgedeckt (Tziperman et al. 2012). ...
Article
German In der Sedimentfüllung des zentraleuropäischen Beckensystems, das im Wesentlichen aus einer Stapelung vom unterliegenden Variszischen Vorlandbecken, vom südlichen Permbecken mit dem Norddeutschen Becken als zentralem Teil, vom untergeordneten Niedersächsischen Becken und dem südlichen Nordsee-Becken besteht, wie auch in denen vieler anderer Sedimentbecken der Erde, verbergen sich Hunderte Millionen Jahre alte geodynamisch-klimatologische Ereignisse. Deren erfolgreiche wissenschaftliche Aufschlüsselung ist zu einem wesentlichen Teil den Kollegen zu verdanken, die sich mit der Suche nach Erdöl und Erdgas beschäftigen und beschäftigt haben. Nun hat die Erde in diesem langen Zeitraum – gebunden an ihr Sonnensystem – das Zentrum der Milchstraße ,,mehrfach“ umrundet, was Spuren hinterlassen haben könnte. Der Pfad des Sonnensystems durch die spiralförmigen Arme der im Orbit der Sonne mit geringerer Geschwindigkeit rotierenden Milchstraße führt u. a. zu einer Periodizität von ca. 150 Millionen Jahren, die von einigen Autoren mit langperiodischen Klimaänderungen korreliert wird. Glaziale Epochen lösen sich mit wärmeren Epochen des Erdklimas in vergleichbarer Dauer ab. Dies wird den Veränderungen der kosmischen Strahlung zugeordnet, deren Intensität innerhalb der Spiralarme höher als dazwischen ist und die, über eine intensivierte Wolkenbildung in niedriger Höhe (Nebelkammerprinzip) mit entsprechender Zunahme der Albedo der Erde, Temperaturreduktionen verursachen soll. Diese führen mit der Ansammlung ozeanischen Wassers als Eis auf Kontinenten (Vergletscherungen), unterbrochen von warmen ,,Treibhaus“-Perioden, zu kalten ,,Eishaus“-Bedingungen auf der Erde. Wegen der Vergletscherungen sinkt der Meeresspiegel, was eine Reduktion der Größe der Schelfgebiete verursacht, die Ablagerungsraum für Kohlenwasserstoff generierende und speichernde Sedimente sind. Dies wiederum verringert über die damit verbundene Gezeitenreibung die Wirkung des Mondes auf die Eigenrotation der Erde und davon beeinflusste Prozesse. Zyklische geodynamische Prozesse zwischen Kern-Mantel-Grenze und Erdoberfläche u. a. mit Perioden von ca. 600 und 300 Millionen Jahren müssen mit den galaktisch-periodischen Kräften interagieren (erzwungene Schwingung). In diesem Sinn werden solar-galaktische Periodizitäten und geodynamische Zyklizitäten analysiert und verglichen. Ähnlichkeiten zwischen geodynamischen Zyklen und dem Rhythmus der Milchstraße erscheinen offensichtlich in Perioden, Amplituden und Phasen und unterstützen die Annahme einer physikalisch synchronisierten Beziehung zwischen den beiden Prozesssystemen. Damit werden umgekehrt auch erdölgeologisch wichtige Ereignisse für die Analyse astronomischer Beobachtungen bedeutsam. English Within the sedimentary fill of the Central European Basin System (CEBS) as also within many other sedimentary basins of the earth hundred million years of past geodynamic climatological events are hidden. The CEBS essentially consists of a number of stacked basins, which include the underlying Variscan foreland basin, the Southern Permian Basin with the North German Basin as central part, the subordinate Lower Saxony Basin and the southerly North Sea Basin. Their successful scientific analysis is to be owed to an essential part to colleagues who are involved in the exploration of hydrocarbons. The earth has in this long time span, bound to the solar system, surrounded the centre of the Milky Way a number of times, what should have left tracks. The path of the solar system through the spiral arms of the Milky Way (resulting in a periodicity of about 150 million years) is assumed by some authors to cause climate change due to cosmic ray fluctuations. These control cloud formation at low heights (cloud chamber principle) with a corresponding temperature reducing increase of the Albedo of the earth. This leads to cold “icehouse” conditions on earth with the accumulation of oceanic water as ice on continents (glaciations), interrupted by warm “hothouse” periods of similar duration. Due to glaciations, the sea level drops accordingly, reducing the impact of sinks for tidal dissipation in shallow seas, the dominant area for the deposition of hydrocarbon generating and storing sediments, and alters the tidal forces of the moon on earth. Long periodic geodynamic processes of e.g.approximately 600 and 300 million years from the core-mantle boundary to the upper crust and ocean floor get most likely adjusted in course of the changing forces. In this sense, solar-galactic periodicity and geodynamic cycles are spectrally analysed and compared in the long-term over the range of hundreds of millions of years. Similarities between geodynamic cycles and the rhythm of the Milky Way appear obviously in periods and phases, supporting the assumption of a physical relationship between the two. Vice-versa also petroleum-geological important events may become significant for the analysis of astronomic observations.
... But it can redistribute tlie humidity over sea-ice and thus, change the hydrological cycle (Donnadieu et al., 2003). However, the climate system respoiises nonlinearly to linear change of t.he lieight of the ice-sheet (Romanova et al., 2004), which points t,o the cxistence of a t,hreshold, over which a runaway albedo feedback could be initiated. ...
... This is clearly somewhat unrealistic. Both on the basis of our own experience (Figures 1) and the analyses of others [e.g., Donnadieu et al., 2003], the shift of the climate from a relatively warm state to a (soft) snowball Earth state might take a few hundred thousand years (kyr). Likewise, as will be demonstrated below, sea level achieves a close to equilibrium distribution after a similar period of time. ...
Article
A preliminary theoretical estimate of the extent to which the ocean surface could have fallen with respect to the continents during the snowball Earth events of the Late Neoproterozoic is made by solving the Sea Level Equation for a spherically symmetric Maxwell Earth. For a 720 Ma (Sturtian) continental configuration, the ice sheet volume in a snowball state is ~750 m sea level equivalent, but ocean surface lowering (relative to the original surface) is ~525 m due to ocean floor rebounding. Because the land is depressed by ice sheets nonuniformly, the continental freeboard (which may be recorded in the sedimentary record) at the edge of the continents varies between 280 and 520 m. For the 570 Ma (Marinoan) continental configuration, ice volumes are ~1013 m in eustatic sea level equivalent in a "soft snowball" event and ~1047 m in a "hard snowball" event. For this more recent of the two major Neoproterozoic glaciations, the inferred freeboard generally ranges from 530 to 890 m with most probable values around 620 m. The thickness of the elastic lithosphere has more influence on the predicted freeboard values than does the viscosity of the mantle, but the influence is still small (~20 m). We therefore find that the expected continental freeboard during a snowball Earth event is broadly consistent with expectations (~500 m) based upon the inferences from Otavi Group sediments.
... The snowball hypothesis also predicts that grounded ice sheets enlarged toward dynamic steady state within a few 100 k.y. after the oceans froze over, fed by sublimation from the tropical sea-glacier (Donnadieu et al., 2003;Pollard and Kasting, 2004). This is supported by geological evidence for dynamic ice sheets on many continents (Hoffman and Li, 2009). ...
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Our detailed examination of the Ghaub Formation (possibly 635 Ma) on the distal foreslope of the Otavi carbonate platform is part of a regional study of the Congo paleocontinental margin in northwestern Namibia. Detrital carbonates of the Ghaub Formation disconformably overlie the Franni-aus Member of the Ombaatjie Formation, a coarsening-upward stack of carbonate turbidites and oolite-clast debris-flow breccias interpreted to be a glacioeustatic falling-stand wedge. Within the main Ghaub Formation, carbonate diamictites are interleaved with mesoscale, laminated to crosslaminated (climbing rippled) grainstones and mudstones, and conglomeratic carbonates. Amalgamation of diamictite units is observed where interleaved facies (grainstones/mudstones) are laterally discontinuous due to reactivation of erosion, followed by renewed deposition. The diamictite package is progradational overall and 80 m thick on average. It is overlain by the 5-15-m-thick Bethanis Member, which is unique in its lateral continuity, composite fining-upward trend, and distinctive interbedding of turbidite grainstones, argillaceous siltstones, climbing-rippled mudstones, and meter-scale stromatolite dropstones. Dropstones are ubiquitous within the finer-grained (Ghaub) lithofacies, and their presence, along with the facies context for subglacial and near grounding-line deposition, indicates a glacigenic origin for the Ghaub Formation, despite its subtropical paleolatitude and distal foreslope setting. We infer a glacial maximum represented by the sub-Ghaub disconformity, followed by the main Ghaub interval when an ice grounding line on the distal foreslope experienced abrupt step backs and readvances of limited magnitude, terminated by the Bethanis episode of unusually widespread iceberg calving and slope instability. The Bethanis Member is overlain conformably by the Keilberg Member of the Maieberg Formation. Reconstruction of the foreslope places the Ghaub grounding-line wedge >1.3 km vertically below the rim of the platform, implying an enormous base-level change upon deglaciation, when the platform was drowned below wave base for a period far exceeding the time scale for isostatic adjustment. The magnitude of base-level change supports the panglacial hypothesis that dynamic (thick) ice sheets existed simultaneously on virtually all continents. The snowball hypothesis that the oceans were also covered by glacial ice (sea-glacier) provides a simple explanation for the main Ghaub-to-Bethanis transition-terminal deglaciation was triggered by collapse of the sea-glacier.
... Recent estimates of the mean ocean salinity in Snowball states lie somewhere between the present-day value of ;35 and two times this value (;70) [although see Knauth (2005)], based on the assumption that the ocean's Neoproterozoic salt content prior to the Snowball events was similar to present-day values and that the mean ocean water depth was about 2 km, about half of presentday values. This is based on an assumed 1-km sea level equivalent land ice cover (Donnadieu et al. 2003;Pollard and Kasting 2004) and 1-km ice cover over the ocean. We chose (somewhat arbitrarily) an initial salinity of 50. ...
Article
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The dynamics of ocean circulation under Snowball conditions is still largely unexplored. Here we study oceanic circulation under a complete ice cover using the MIT oceanic general circulation model. We use idealized aqua-planet conditions with meridionally variable sea glacier depth and surface temperature, and spatially constant geothermal heating. We examine convection and meridional circulation developing due to brine rejection associated with ice production and freezing temperature variations, due to the dependence of freezing temperature on pressure and thus on the ice thickness. We show that variable freezing temperature and salinity have a crucial role on ocean circulation. These two factors may therefore have a significant effect on sea glacier dynamics as the heat flux at the bottom of the ice, and hence ice melting, is strongly affected by ocean circulation.
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The Great Unconformity has been recognized for more than a century, but only recently have its origins become a subject of debate. Hypotheses suggest global Snowball Earth glaciations and tectonic processes associated with the supercontinent Rodinia as drivers of widespread kilometer-scale erosion in the late Neoproterozoic. We present new integrated zircon and apatite (U-Th)/He and fission-track thermochronology from Precambrian basement samples of the central Canadian Shield in northern Manitoba to test these ideas. Bayesian inverse modeling indicates that 150–200 °C of cooling (>3 km of exhumation) occurred simultaneously with Cryogenian glaciations at ca. 690–650 Ma within interior North America. This estimate for the timing of unroofing is more precise than previous appraisals and does not align with any known tectonic or magmatic events (i.e., large igneous province eruptions) potentially associated with the supercontinent cycle that occurred during the late Proterozoic along the Laurentian margins. Based on these results and interpretations, the timing and magnitude of exhumation is best explained by glacial erosion, and further establishes the importance of multiple thermochronometers for resolving detailed deep-time thermal histories.
Article
On the southwest cape of 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. Average rates of Marinoan erosion (2.55−6.80 m Myr−1, n=190) and accumulation (2.65−7.07 m Myr−1, n=211) are indistinguishable, implying that location at a continental promontory did not bias erosion over accumulation. Average accumulation rates for the Sturtian and Marinoan, scaled for different averaging times including Marinoan uncertainty, are 3.95−4.93 m Myr−1 (n=183) and 2.65−7.07 m Myr−1 (n=190) respectively, suggesting that a Marinoan glacioeustatic coastal escarpment substituted for rift-related Sturtian basin-and-range topography. These slow rates, comparable to longterm pre-Quaternary acccumulation rates on existing abyssal plains, reconcile glacial sedimentology with the feeble hydrologic cycle of snowball Earth.
Article
Otavi Group is a 1.5−3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran−Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S−N crustal stretching (770−655 Ma), followed by post-rift thermal subsidence (655−600 Ma). Rifting under southern Swakop Group continued until 650−635 Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S−N plate divergence. Rift related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746 Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770−717 Ma; (2) glacial early Cryogenian/Sturtian, 717−661 Ma; (3) non-glacial middle Cryogenian, 661−646±5 Ma; (4) glacial late Cryogenian/Marinoan, 646±5−635 Ma; and (5) non-glacial early Ediacaran, 635−600±5 Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585 measured sections (totaling >190 km of strata) and >8,764 pairs of δ13C/δ18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5−9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3−8.8 m Myr−1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ≤1.6 km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9 m regional average thickness, n=140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ≤90 m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ≥0.6 km of paleobathymetric relief. Late Tonian shallow-neritic δ13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770−760 Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737−717 Ma) in Swakop Group. Devede Fm is isotopically heavy, +4−8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746 Ma volcanics and shows two negative excursions bridged by heavy δ13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic δ13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0−3.5‰. Early Ediacaran neritic δ13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (≤+11‰) with extreme point-to-point variability (≤10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform δ13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9±1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60−0.58 Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ≥1.85 km of megakarstic relief, and kmthick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
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More than 88% of the history of the Earth occurred in the Precambrian. The Precambrian began with the formation of the Earth 4.6 billion years ago (Ga) and ended 542 million years ago (International Stratigraphic Chart, www.stratigraphy.org). It is subdivided into two large eons: the Archean (between 4 and 2.5 Ga) and the Proterozoic (from 2.5 to 0.542 Ga). The International Commission on Stratigraphy is proposing to add an extra eon, the Hadean, covering the first 600 million years of the history of our planet. Notwithstanding, this eon is described as having an informal status since no pre-Archean rock has been observed today. In fact, the oldest rocks date back to 4 billion years ago (U/Pb dating on zircon crystals). These are the Acasta gneisses in the Slave Province of Canada. The two formal eons of the Precambrian are subdivided into eras. In particular, the Proterozoic contains three eras: Paleoproterozoic (2.5–1.6 Ga), Mesoproterozoic (1.6–1.0 Ga) and Neoproterozoic (1.0–0.542 Ga).
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The Shuram excursion represents the greatest negative carbon isotopic excursion in earth history, and provides an important chemostratigraphic marker horizon of global extent. The excursion is linked to the second great oxygenation event in earth history, an oxygen crisis that resulted in a transition from sulfidic oceans to a marine realm rich in sulfate. The Shuram excursion (560–550 Ma) is represented in Sonora, México by the Clemente oolite of the Clemente Formation. Ediacaran fossils (such as the Clemente biota of Unit 4 of the Clemente Formation) occur in rocks deposited below the excursion. The age of the Clemente Ediacaran biota thus falls between 550 and 560 Ma. In spite of the fact that the Sturtian glaciation apparently triggered the earliest known mass extinction on earth (the Tindir Mass Extinction), several lines of evidence suggest that the biosphere controlled the timing of and the onset of the Late Proterozoic glaciations, and that it also controlled the timing of the melting of the ice. Furthermore, it appears that the biosphere itself influenced the timing of the appearance of the Ediacaran biota. Whereas snowball earth events lurched suddenly from very cold (tillites) to very hot (cap carbonates) climate, the sequence going from the Gaskiers glacial event (c. 580 million years ago) to the Shuram was part of a wild climatic gyration where the earth went from hot (intense granite weathering at high latitudes) to cold (Gaskiers glaciation) to hot (Shuram event). The Shuram is the greatest negative carbon isotopic excursion in earth history, possibly because this is the moment in earth history when the burrowing animals assert themselves in a geochemical sense, and by remobilizing sea floor carbon, forestall a major glaciation.
Article
During Snowball Earth episodes of the Neoproterozoic and Paleoproterozoic, limited amounts of tropical open ocean (Jormungand), or tropical ocean with thin ice cover, would help to explain (1) vigorous glacial activity in low latitudes, (2) survival of photosynthetic life, and (3) deglacial recovery without excessive buildup of atmospheric CO2. Some previous models have suggested that tropical open ocean or thin-ice cover is possible; however, its viability in the presence of kilometer-thick sea glaciers flowing from higher latitudes has not been demonstrated conclusively. Here we describe a new method of asynchronously coupling a zonal sea-glacier model with a 3-D global climate model and apply it to Snowball Earth. Equilibrium curves of ice line versus CO2 are mapped out, as well as their dependence on ocean heat transport efficiency, sea-glacier flow, and other model parameters. No climate states with limited tropical open ocean or thin ice are found in any of our model runs, including those with sea glaciers. If this result is correct, then other refugia such as cryoconite pans would have been required for life to survive. However, the reasons for the differences between our results and others should first be resolved. It is suggested that small-scale convective dynamics, affecting fractional snow cover in low latitudes, may be a critical factor accounting for these differences.
Article
Geochemical, paleomagnetic, and geochronological data increasingly support the Snowball Earth hypothesis for Cryogenian glaciations. Yet, the fossil record reveals no clear-cut evolutionary bottleneck. Climate models and the modern cryobiosphere offer insights on this paradox. Recent modeling implies that Snowball continents never lacked ice-free areas. Wind-blown dust from these areas plus volcanic ash were trapped by snow on ice sheets and sea ice. At a Snowball onset, sea ice was too thin to flow and ablative ice was too cold for dust retention. After a few millenia, sea ice reached 100 s of meters in thickness and began to flow as a 'sea glacier' toward an equatorial ablation zone. At first, dust advected to the ablative surface was recycled by winds, but as the surface warmed with rising CO2 , dust aka cryoconite began to accumulate. As a sea glacier has no terminus, cryoconite saturated the surface. It absorbed solar radiation, supported cyanobacterial growth, and sank to an equilibrium depth forming holes and decameter-scale pans of meltwater. As meltwater production rose, drainages developed, connecting pans to moulins, where meltwater was flushed into the subglacial ocean. Flushing cleansed the surface, creating a stabilizing feedback. If the dust flux rose, cryoconite was removed; if the dust flux waned, cryoconite accumulated. In addition to cyanobacteria, modern cryoconite holes are inhabited by green algae, fungi, protists, and certain metazoans. On Snowball Earth, cryoconite pans provided stable interconnected habitats for eukaryotes tolerant of fresh to brackish cold water on an ablation surface 60 million km(2) in area. Flushing and burial of organic matter was a potential source of atmospheric oxygen. Dominance of green algae among Ediacaran eukaryotic primary producers is a possible legacy of Cryogenian cryoconite pans, but a schizohaline ocean-supraglacial freshwater and subglacial brine-may have exerted selective stress on early metazoans, or impeded their evolution.
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Over the past decade, a number of climate modeling studies have examined the possibility of simulating low-latitude glaciation using Neoproterozoic boundary conditions. Many of the studies undertaken have used the thermodynamic slab ocean, which includes the top 100 meters of the ocean. These models have successfully simulated Hard Snowball Earth conditions, while recent simulations using fully coupled atmosphere-ocean models have not been able simulated Hard Snowball Earth conditions. Moreover, ice-sheet models have been run offline using GCM fields to simulated ice-sheets on land under “Hard” and “Soft” Snowball Earth Conditions. However, until of the climate models include additional components of the climate system including geochemical, dynamic ocean, sea ice and ice-sheet components uncertainty will exist in our understanding of Neoproterozoic low-latitude glaciation.
Article
The aim of this article is to describe the state of the art of research on the Neoproterozoic atmospheres and glaciation. Using numerical modeling, the authors examine the potential for Earth to have been fully glaciated from the poles to the equator, evaluating the plausibility of a snowball Earth to have occurred. Physical processes and mechanisms suggested for the initiation of a global glaciation are explored and questions asked are as follows: (1) How can climate models that predict a totally ice-capped ocean be reconciled with paleontological evidence that establishes the persistence of photosynthetic activity throughout the snowball event? (2) How can the glacial sequences be explained with the Snowball Earth scenario? (3) What about the melting and the aftermath of the Snowball Earth solution? For each of these questions, 'modeling' solutions that are plausible but may not be definitive are presented, given the extreme severity of these events, which pushes the models to their physical limits. The models, nevertheless, provide important clues and quantitative constraints on the controversial question of whether these catastrophic events could have occurred in their most extreme form.
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Geologic evidence of tropical sea level glaciation in the Neoproterozoic is one of the cornerstones of the Snowball Earth hypothesis. However, it is not clear during what part of the Snowball Earth cycle that land-based glaciers or ice sheets could have grown: just before the collapse with tropical oceans still open, or after the collapse with oceans completely covered with sea ice. In the former state, the tropics may still have been too warm to allow flowing ice to reach sea level; in the latter, snowfall minus sublimation may have been too small to build significant ice. These possibilities are tested with a coupled global climate model and dynamic ice sheet model, with two continental configurations (~750 Ma, 540 Ma) and two CO2 levels bracketing the collapse to Snowball Earth (840, 420 ppmv). Prior to collapse large high- latitude ice sheets form at 750 Ma, but with flat continents, no low-latitude ice grows at 750 or 540 Ma. In the absence of reliable knowledge of Neoproterozoic topography, we apply a small-scale “test” profile in the ice sheet model, representing a coastal mountain range on which glaciers can be initiated and flow seaward. Prior to collapse, almost all low-latitude test glaciers fail to reach the coast at 750 Ma, but at 540 Ma many do reach the sea. After the collapse to full Snowball conditions, the hydrologic cycle is greatly reduced, but extensive kilometer-thick ice sheets form slowly on low-latitude continents within a few 100,000 years, both at 750 Ma and 540 Ma.
Our detailed examination of the Ghaub Formation (possibly 635 Ma) on the distal foreslope of the Otavi carbonate platform is part of a regional study of the Congo paleo continental margin in northwestern Namibia. Detrital carbonates of the Ghaub Formation disconformably overlie the Franni-aus Member of the Ombaatjie Formation, a coarsening-upward stack of carbonate turbidites and oolite-clast debris-flow breccias interpreted to be a glacioeustatic falling-stand wedge. Within the main Ghaub Formation, carbonate diamictites are interleaved with mesoscale, laminated to cross-laminated (climbing rippled) grainstones and mudstones, and conglomeratic carbonates. Amalgamation of diamictite units is observed where interleaved facies (grainstones/mudstones) are laterally discontinuous due to reactivation of erosion, followed by renewed deposition. The diamictite package is progradational overall and 80 m thick on average. It is overlain by the 5-15-m-thick Bethanis Member, which is unique in its lateral continuity, composite fining-upward trend, and distinctive interbedding of turbidite grainstones, argillaceous siltstones, climbing-rippled mudstones, and meter-scale stromatolite dropstones. Dropstones are ubiquitous within the finer-grained (Ghaub) lithofacies, and their presence, along with the facies context for subglacial and near grounding-line deposition, indicates a glacigenic origin for the Ghaub Formation, despite its subtropical paleolatitude and distal foreslope setting. We infer a glacial maximum represented by the sub-Ghaub disconformity, followed by the main Ghaub interval when an ice grounding line on the distal foreslope experienced abrupt step backs and readvances of limited magnitude, terminated by the Bethanis episode of unusually widespread iceberg calving and slope instability. The Bethanis Member is overlain conformably by the Keilberg Member of the Maieberg Formation. Reconstruction of the foreslope places the Ghaub grounding-line wedge >1.3 km vertically below the rim of the platform, implying an enormous base-level change upon deglaciation, when the platform was drowned below wave base for a period far exceeding the time scale for isostatic adjustment. The magnitude of base-level change supports the panglacial hypothesis that dynamic (thick) ice sheets existed simultaneously on virtually all continents. The snowball hypothesis that the oceans were also covered by glacial ice (seaglacier) provides a simple explanation for the main Ghaub-to-Bethanis transition-terminal deglaciation was triggered by collapse of the sea-glacier.
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SynonymsHard snowball; Ice-albedo instability; Makganyene glaciation; Marinoan glaciation; Pan-glacial; Runaway ice-albedo feedback; Slushball Earth; Sturtian glaciation; White EarthKeywordsAtmospheric oxygenation, CO2-hysteresis, energy-balance models, Fe-Mn-ore deposits, greenhouse effect, ice grounding-line, ice-sheet, paleomagnetism, planetary albedo, radiative forcing, geochemical carbon cycle, silicate-weathering feedback, volcanic-metamorphic outgassing, multicellular animalsDefinitionSnowball Earth refers to the planet’s appearance from space during parts of the Neoproterozoic and Paleoproterozoic eras, when geological evidence suggests that most continental areas and arguably the entire ocean were covered by dynamic ice-sheets, limiting air–sea gas exchange to cracks at ice grounding lines and shallow-water hydrothermal vents. Slushball Earth refers to an alternative scenario in which ice-sheets existed on all continents but the tropical ocean was lar ...
Chapter
Paleomagnetic data for Neoproterozoic glacial deposits in South Australia and elsewhere verify glaciomarine deposition near the paleoequator. Tidal rhythmites from such deposits in South Australia display symmetrical ripples indicating decades of continuous wave activity, and also record the annual oscillation of sea level. In low and moderate latitudes the annual oscillation of sea level results mostly from seasonal changes in heat content of the sea, indicating extensive and long-lived open seas in low latitudes during Neoproterozoic glaciations. Neoproterozoic periglacial sand wedges 3+ m deep, marking polygons 10-30 m across, are closely comparable to periglacial wedges in present high latitudes and imply large (∼40°C) seasonal changes of mean monthly temperature near the paleoequator. Periglacial wedges did not form at high elevations on the Pleistocene equator where temperatures were well below 0°C throughout the year and temperature fluctuations were mainly diurnal, which militates against diurnal fluctuations as the cause of the Neoproterozoic wedges. An extremely large (50%) octupole component of the geomagnetic field is required to make true moderate latitudes appear paleoequatorial, whereas Proterozoic paleomagnetic data suggest a maximum octupole component of ≤30%. A snowball or slushball Earth is difficult to reconcile with open seas and large seasonal temperature-changes in low paleolatitudes. A Proterozoic high obliquity (>54°) resulting from the Moon-producing single giant impact may explain glaciation and strong seasonality on the equator, but a mechanism is required to subsequently reduce the obliquity. At present the Neoproterozoic paleomagnetic and glacial records cannot be reconciled satisfactorily, demanding further wide-ranging research.
Article
Two discrete, mappable, glaciogenic formations occur within the Otavi Group, a 3±1-km-thick carbonate-dominated platform of late Neoproterozoic age, developed on the SW promontory of the Congo craton in northern Namibia and exposed in bordering late Ediacaran fold belts. Each is overlain abruptly by an expanded postglacial carbonate sequence, the younger of which begins with a globally-correlative transgressive dolopelarenite. The older Chuos glaciation (<746 Ma) occurred during a time of north-south crustal stretching. Debris derived from upturned older rocks collected in structural depressions. The younger Ghaub glaciation (635 Ma) occurred, after stretching ceased, on a thermally-subsiding marine platform and its distally-tapered foreslope. A continuous ice grounding-zone wedge (GZW) occurs on the distal foreslope, while the upper foreslope and outer platform are devoid of glacial debris and only small pockets of lodgement facies exist on the inner platform. Debris in the GZW is derived from a distinctive falling-stand wedge that is unique to the foreslope and from immediately older strata mined preferentially from the inner platform. The GZW rests on a smooth surface that includes a transverse steep-walled trough presumably cut by an ice-stream, within which is a towering doubly-crested moraine composed of composite, massive, carbonate diamictite. The surface suggests that the ice-sheet was grounded on the distal foreslope, implying a large fall in base level at a glacial maximum that predates the GZW. The glacial record ends with Fe-stained beds, rich in ice-rafted debris, that are notably absent from the moraine, upper foreslope and platform, which were apparently above sea-level at that time.
Article
We review most of the modelling studies performed to date to understand the initiation and melting of a Snowball Earth, as well as to describe the glacial environment during the glaciation itself. All the described scenarios explaining the onset of glaciation rely on a sufficient decrease in the concentrations of atmospheric greenhouse gases (GHGs), typically resulting from the equatorial palaeogeography of the late Proterozoic. It is still heavily debated whether or not the oceanic ice cover was thick during the glaciation itself. However, a consensus has arisen that the most climatically stable scenarios imply the existence of a globally frozen ocean, with a thick ice cover caused by the flowing of high-latitude sea-ice glaciers towards the equator. Depending on the characteristics of the ice, a thin ice layer may have persisted along the equator, but this numerical solution is rather fragile. During the snowball event itself, model results suggest the existence of wet-based continental glaciers. Some parts of the continents may have remained ice-free. From the modelling perspective, the most significant problem in the snowball hypothesis, particularly in its 'hard snowball' version (the most stable numerically), is the melting phase. With improved modelling, the CO 2 threshold required to melt the snowball is much higher than initially thought, significantly above 0.29 bar. Indeed, because of the very cold conditions prevailing at the surface of the Earth during the glacial event, the atmosphere becomes vertically isothermal, strongly limiting the efficiency of the greenhouse effect. This melting problem is further highlighted by geochemical modelling studies that show that weathering of the oceanic crust might be an active sink of CO 2 during the glacial event, limiting the rise in atmospheric CO 2. The solution might be found by considering the input of dark dust from catastrophic volcanic eruptions that would efficiently decrease the albedo of the ice. Finally, modelling studies also explore the aftermath of the glaciation. The world might have been drier than initially anticipated, resulting in the persistence of the supergreenhouse effect for at least one million years after the melting phase.
Article
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 condition 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 ranger of climate conditions.
Article
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We identify the "hard snowball" bifurcation point at which total sea-ice cover of the oceans is expected by employing the comprehensive coupled climate model CCSM3 (Community Climate System Model version 3) for two realistic Neoproterozoic continental configurations, namely a low-latitude configuration appropriate for the 720 Ma Sturtian glaciation and a higher southern latitude configuration reconstructed for 570 Ma but which has often been employed in the past to study the later 635 Ma Marinoan glaciation. Contrary to previous suggestions, we find that for the same total solar insolation (TSI) and atmospheric CO2 concentration (pCO2), the 570 Ma continental configuration is characterized by colder climate than the 720 Ma continental configuration and enters the hard snowball state more easily on account of the following three factors: the higher effective albedo of the snow-covered land compared to that of sea ice, the more negative net cloud forcing near the ice front in the Northern Hemisphere (NH), and, more importantly, the more efficient sea-ice transport towards the Equator in the NH due to the absence of blockage by continents. Beside the paleogeography, we also find the optical depth of aerosol to have a significant influence on this important bifurcation point. When the high value (recommended by CCSM3 but demonstrated to be a significant overestimate) is employed, the critical values of pCO2, beyond which a hard snowball will be realized, are between 80 and 90 ppmv (sea-ice fraction 55%) and between 140 and 150 ppmv (sea-ice fraction 50%) for the Sturtian and Marinoan continental configurations, respectively. However, if a lower value is employed that enables the model to approximately reproduce the present-day climate, then the critical values of pCO2 become 50-60 ppmv (sea-ice fraction 57%) and 100-110 ppmv (sea-ice fraction 48%) for the two continental configurations, respectively. All of these values are higher than previously obtained for the present-day geography (17-35 ppmv) using the same model, primarily due to the absence of vegetation, which increases the surface albedo, but are much lower than that obtained previously for the Marinoan continental configuration using the ECHAM5/MPI-OM model in its standard configuration (~500 ppmv). However, when the sea-ice albedo in that model was reduced from 0.75 to a more appropriate value of 0.45, the critical pCO2 becomes ~204 ppmv, closer to the values obtained here. Our results are similar to those obtained with the present-day geography (70-100 ppmv) when the most recent version of the NCAR model, CCSM4, was employed.
Article
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An enduring enigma of Neoproterozoic Earth history is the intimate association of glacial diamictites with typical warm-water carbonates. Among the many hypothesized explanations for this paleoclimatic dichotomy are high orbital obliquity, true polar wander, reduced solar luminosity, snowball albedo, CO2 drawdown, stagnant ocean overturn, and reinterpretation of diamictites as mega-impact ejecta. The Otavi carbonate platform on the Congo craton in Namibia contains two discrete intervals of diamictite and associated glaciomarine deposits, sandwiched by thick carbonates from which we have obtained detailed carbon-isotopic records. From subsidence analysis, we estimate maximum rates of shallow-water sediment accumulation. The magnitude and duration of isotopic variations permit critical assessment of the existing hypotheses.
Article
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A thick Neoproterozoic carbonate and glaciogenic succession of the southern Congo craton has yielded delta13C and 87Sr/86Sr records through the later Cryogenian (ca. 750 600 Ma) and earlier part of the Terminal Proterozoic (ca. 600 570 Ma). Sizeable negative delta13C excursions (to less than 50/00) occur above each of two glacial intervals and the 87Sr/86Sr values of marine carbonates shift from ˜0.7072 to ˜0.7079 at the upper glacial level. These geochemical constraints provide a Marinoan (younger Varanger) age for the upper glacial interval, previously regarded as a second phase of the Sturtian glaciation. The delta13C record from the Congo craton is therefore incompatible with recent global delta13C syntheses that have identified four or more separate ice ages during the Neoproterozoic. A cladistic analysis of geologic and geochemical characters of 12 Neoproterozoic glacial deposits identifies two distinct groups that are found in a consistent stratigraphic order whenever two glacial units occur within a single succession. We use delta13C and 87Sr/86Sr records from the Congo craton and other key successions to test the null hypothesis that there were only two global glaciations (Sturtian and Marinoan) during the Neoproterozoic. Placing the GSSP (global stratotype section and point) for the base of the Terminal Proterozoic within or just above a cap carbonate of the younger (Marinoan) glaciogenic succession would confine all known Neoproterozoic glaciations to the Cryogenian. The rapid shift in marine 87Sr/86Sr to more radiogenic values during the Marinoan glaciation is opposite that predicted by the snowball Earth scenario which calls for continental runoff to cease during glaciation, resulting in a shift to less radiogenic values.
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The Yudnamutana Subgroup is a thick succession of upper Proterozoic (Sturtian) glacigenic rocks preserved in the Yudnamutana trough in the northeastern part of the Adelaide "geosyncline'. In ascending order, the subgroup comprises the Fitton, Bolla Bollana, and Lyndhurst Formations. The name "Hamilton Creek Member', is proposed for a locally developed conglomeratic facies at the base of the Fitton Formation. These rocks are interpreted as the products of two glacial advances. Stratigraphic similarities exist between the Yudnamutana Sub-group and thinner successions to the west. These stratigraphic similarities probably reflect paleoclimatic changes. The twice-repeated sequence of diamictite followed by mudstone is attributed to two glacial advance retreat cycles. Local variations in thickness and stratigraphic succession are probably related to contemporaneous faulting. -from Authors
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The Fiq Member of the Ghadir Manqil Formation, Huqf Supergroup, Oman, is composed of ˜1.5 km of glacial rainout diamictites, mass-flow deposits, turbiditic sandstones, and slope and shelf sandstones and siltstones, overlain by a classic <10-m-thick transgressive cap dolostone. Paleocurrents, facies distribution, and subsurface imagery show that the Fiq Member was deposited in a series of extensional half-grabens and grabens. Facies associations represent proximal glaciomarine, distal glaciomarine, nonglacial sediment gravity flow, and nonglacial shallow-marine siliciclastic depositional environments and processes. The member is divided into seven units that can be correlated across the rift basin. Although the facies mosaic is complex, cycles of relative sea-level change, most likely driven by glacio-eustasy, show that glacial advance and retreat during the glacial epoch was strongly pulsed. Only the final (youngest) diamictite is overlain by a well-developed cap carbonate. It is unlikely that there was any prolonged and substantial shutdown of the hydrologic cycle during deposition of the Fiq Member. On the basis of sedimentological and stratigraphic data, the Neoproterozoic glacials of Arabia were more like the familiar oscillatory glaciations of the Pleistocene than those required by the snowball Earth hypothesis.
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The main features of the low-latitude Neoproterozoic glaciations remain the subject of controversial debates concerning possible triggers. Here we use an AGCM coupled with a slab ocean to test one of the earliest and simplest explanation for tropical glaciations: a higher obliquity of the Earth's rotation axis. We show that high obliquity may result in an extensive glaciation during the Sturtian episode (750 Ma), due to the location of continental masses in the tropical areas, but cannot produce a large glaciation in the case of mid to high latitude paleogeographies such as those thought to typify the Varangian-Vendian episodes (620–580 Ma).
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Thin (< 15 m) laterally persistent carbonate units cap glacial deposits in Neoproterozoic successions (1000-544 Ma) on almost every continent. Because these enigmatic carbonate units are typically isolated within siliciclastic successions, recurrently overlie successive glacigenic units, and define one of the most pronounced δ 13C excursions in the geologic record, they are interpreted to record a brief postglacial anomaly in ocean chemistry. In Australia, the Marinoan (Varanger equivalent ∼ 600 Ma) cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe, including fine-grained dolomitic mineralogy, lateral persistence at basinal scales, thin laminae, graded beds, intervals with abundant marine cements, crystal fans, and a distinctive negative δ 13C isotopic signature of up to -5‰ PDB. On the basis of these features, the Australian examples are interpreted here to be deeper-water deposits (below storm wave base) resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression. Detailed isotopic analysis of Australian cap dolostones indicates δ 13C values ranging from -1‰ to -5‰ and generally becoming more depleted in 13C upsection. Trace-element data indicate some diagenetic stabilization, but textural evidence and the presence of similar profiles in different basins argue against a pervasive recrystallization event. The range in δ 13C values between sections as well as more negative δ 13C values appear to correlate with greater paleobathymetry within basins. This implies either (1) only partial preservation of the complete oceanic variation of δ 13C or (2) precipitation of this peculiar facies owes its genesis to basinal-specific oceanographic processes such as proximity of a given section to a postglacial upwelling zone. In the later case, δ 13C values would not represent whole ocean values. Stratigraphic constraints and paleoenvironmental interpretations suggest that much of this excursion lies within the geologically brief period of postglacial transgression. This implies that the cause(s) of δ 13C variation likely operated on time scales significantly less than the residence time of carbon in the oceans (10 5 yr). The continent-wide and perhaps global nature of cap dolostones indicates that large volumes of carbonate were precipitated during the postglacial transgressive period. Such large-scale carbonate precipitation over the hypothesized short time interval of postglacial transgression must have caused (or have been the product of) profound changes in the carbon cycle and global climate at that time. Similar evidence for transgression and increased carbonate deposition in the Holocene are attributed to the changing basin shape and pH of the oceans (the Coral Reef Hypothesis of Berger 1982). Cap dolostones may record a Proterozoic equivalent of this process with the substitution of abiotic carbonate precipitation for skeletal precipitation by reef organisms.
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Regionally persistent, thin intervals of carbonate rock directly and ubiquitously overlie Proterozoic glacial deposits on almost every continent, and are commonly referred to as cap carbonates. Their unusual facies, stratigraphically abrupt basal and upper contacts, and strongly negative carbon isotopic signature (δ13C values between ∼0‰ and -5‰) suggest a chemical oceanographic origin, the details of which remain unresolved. Here we propose that these enigmatic deposits are related to the destabilization of gas hydrate in terrestrial permafrost following rapid postglacial warming and flooding of widely exposed continental shelves and interior basins. Supporting evidence for this hypothesis includes (1) the common occurrence within the cap carbonates of unusual fabrics, similar to those produced by cold methane seeps; (2) a distinctive time evolution for the carbon isotopic excursions indicative of a pulse addition of isotopically depleted carbon to the ocean-atmosphere system; and (3) agreement between mass-balance estimates of carbon released by hydrate destabilization and carbon buried in the cap carbonate. We infer that during times of low-latitude glaciation, characteristic of the Neoproterozoic, gas hydrates may have been in greater abundance than at any other time in Earth history.
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Paul F. Hoffman et al. ([1][1]) developed a modified “snowball Earth” hypothesis ([2][2]) to explain the association of Neoproterozoic low-latitude glaciation with the deposition of “cap carbonate” rocks bearing highly depleted carbon isotopic values (δ13C ≤ −5‰). According to Hoffman
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1] The Snowball Earth hypothesis explains the development of glaciation at low latitudes in the Neoproterozoic, as well as the associated iron formations and cap carbonates, in terms of a runaway ice-albedo feedback leading to a global glaciation followed by an extreme greenhouse climate. The initiation of a snowball glaciation is linked to a variety of unusual perturbations of the carbon cycle operating over different timescales, as evidenced by unusual patterns in the carbon isotopic composition of marine carbonate. Thus a theory for why multiple glaciations happened at this time, and not in the Phanerozoic nor earlier in the Proterozoic, requires a reexamination of the carbon cycle and the controls on climate stability. We propose that the concentration of continental area in the tropics was a critical boundary condition necessary for the onset of glaciation, both because the existence of substantial continental area at high latitudes may prevent atmospheric carbon dioxide from getting too low and because a tropical concentration of continental area may lead to more efficient burial of organic carbon through increased tropical river discharge. Efficient organic carbon burial sustained over tens of millions of years, required by the high carbon isotopic compositions of preglacial carbonate, may lead to the buildup of enormous quantities of methane, presumably in hydrate reservoirs. We examine how the slow release of this methane may explain the drop in d 13 C values immediately before the glaciation. Moreover, the accumulation of methane in the atmosphere coupled with the response of silicate weathering to the additional greenhouse forcing can lead to a climate with methane as the major greenhouse gas. This situation is unstable because methane is not buffered by a large ocean reservoir like carbon dioxide, and so the collapse of the methane source may provide a trigger for the onset of a runaway ice-albedo feedback. A simple model of the carbon cycle is used to explore the boundary conditions that would allow this to occur.
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Deep-sea sediment cores recovered from the Northeast Atlantic Ocean were examined in order to elucidate the influence of the Earth's orbital parameters on major ice rafting. Analyses of coarse-grained ice-rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal. Since 130,000 yr B.P., dropstone layers have been deposited each half period of a precessional cycle (11,000 ± 1000 yr). Ice rafting occurs during times of winter minimum/summer maximum insolation and summer minimum/winter maximum insolation. In the first case, high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter. In the second case, growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater. Both situations result in a southward penetration of polar water. Thus, the marine record of dropstones documents ice rafting not only during Weichselian stades but also during cold events within interstades. The regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes, not only in the region of the North Atlantic Ocean but also in remote areas such as the Pacific Ocean and the Antarctic.
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A thoroughly revised edition of the highly successful geology textbook that discusses all important new developments in the field. New features include an introductory chapter, a chapter on the Himalayas, plus updated material on early active continental margins, crustal evolution, greenstone belts in India and West Australia, and granulite-gneiss belts in India and the Limpopo. Includes an updated bibliography.
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A paleomagnetic investigation of Marinoan glacial and preglacial deposits in Australia was conducted to reevaluate Australia’s paleo- geographic position at the time of glaciation (ca. 610–575 Ma). The paleomagnetic results from the Elatina Formation of the central Flinders Ranges yield the first positive regional- scale fold test (significant at the 99% level), as well as at least three magnetic polarity inter- vals. Stratigraphic discontinuities typical of glacial successions prevent the application of a magnetic polarity stratigraphy to regional cor- relation, but the positive fold test and multiple reversals confirm the previous low paleolati- tude interpretation of these rocks (mean D = 214.9°, I = –14.7°, α95 = 12.7°, paleolatitude = 7.5°). The underlying preglacial Yaltipena For- mation also carries low magnetic inclinations (mean D = 204.0°, I = –16.4°, α95 = 11.0°, paleo- latitude = 8.4°), suggesting that Australia was located at low paleolatitude at the onset of glaciation. The number of magnetic polarity in- tervals present within the Elatina Formation and the Elatina’s lithostratigraphic relation- ship to other Marinoan glacial deposits suggest that glaciation persisted at low latitudes in Aus- tralia for a minimum of several hundreds of thousands to millions of years.
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Stratigraphic mapping of the Neoproterozoic glaciogenic Kingston Peak Formation (Death Valley, California) provides evidence for two temporally discrete extensional deformation episodes. These episodes are bracketed by the Sourdough Limestone and Noonday Dolomite, the facies characteristics and delta13C data (ranging between 2.15 and -2.560/00 and -1.88 and -4.860/00, respectively) of which make them equivalent to Sturtian and Varangian age cap carbonates, respectively. This constrains the two extensional episodes along the southwestern margin of Laurentia to ca. 700 Ma and ca. 600 Ma. These observations and data show that the field evidence for mid-Neoproterozoic breakup and the predictions from tectonic subsidence curves for a latest Neoproterozoic breakup are both correct. Thus, Neoproterozoic plate reconstructions must account for two discrete rift episodes separated by 100 m.y. or more. Confining rifting to within the Kingston Peak Formation thereby places the younger Proterozoic rocks of the southwestern Great Basin in the rift to drift tectonic phase.
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In order to simulate the Snowball Earth conditions that may have existed during the late Proterozoic we have conducted a series of GCM simulations using a simple 50-meter slab ocean, a reduced solar constant of 6% and varied CO2 concentrations. In this study, we vary the CO2 concentration from 100 to 3400-ppmv and use rotation rates corresponding to 18 and 24-hour day-lengths. We also examine the effects of increasing the poleward transport of heat by the oceans. Our results show that below a critical value of approximately 1700 ppmv of atmospheric CO2, sea-ice and sub-freezing temperatures occur from the poles to the Equator. A global mean annual two meter air temperature of 221°K is found for boundary conditions of 100 ppmv atmospheric CO2, 6% reduction in solar forcing and . a rotation rate corresponding to an 18 hour day. These results confirm those of earlier studies suggesting or implying that low-latitude glaciation occurred during the late Proterozoic. However, since the ocean is the critical factor for low-latitude glaciation, the results should be view cautiously because of the simple slab-ocean used in this study.
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The GISS GCM was used to determine if a diverse set of climate forcings, alone or in combination, could have initiated the low-latitude ice sheets of the Varanger (~600 Ma) glacial interval. The simulations use a realistic reconstruction of the paleocontinental distribution and test the following forcings, alone and in combination: 6% solar luminosity decrease, four atmospheric CO2 scenarios (1260, 315, 140, and 40 ppm), a 50% increase and a 50% decrease in ocean heat transports, and a change in obliquity to 60°. None of the forcings, individually, produced year-round snow accumulation on low-latitude continents, although the solar insolation decrease and 40ppmCO2 scenarios allowed snow and ice to accumulate at high and middle latitudes. Combining forcings further cools the climate: when solar luminosity, CO2, and ocean heat transports were all decreased, annual mean freezing and snow accumulation extended across tropical continents. No simulation would have initiated low-latitude glaciation without contemporaneous glaciation at higher latitudes, a finding that matches the distribution of glacial deposits but which argues against high obliquity as a cause of the Varanger ice age. Low-level clouds increased in most scenarios, as did surface albedo, while atmospheric water vapor amounts declined; all are positive feedbacks that drive temperatures lower. In the most severe scenario, global snow and ice cover increased to 68%, compared to 12% under modern conditions, and water vapor dropped by 90%. These results do not necessarily preclude a ``snowball'' Earth climate scenario for the Varanger glacial interval. However, either more severe forcings existed or radical changes occurred in the ocean/atmosphere system which are unaccounted for by the GCM. Also, as sea ice extent increased in these experiments, snow accumulation began to decline, because of an increasingly dry atmosphere. Under snowball Earth conditions, glaciation would be impossible, since the hydrological cycle would all but cease if the atmosphere's primary moisture source were cut off.
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Since the late 1980s, it has been hypothesized that the wide range of apparent argon ages seen within single K-feldspar samples might be due to a distribution of diffusion domain sizes within the mineral. To test and apply this idea, an analytical technique that combines conventional laboratory degassing experiments (resistance heating) with numerical inversion procedures has been developed to extract cooling history information from feldspars. A key part of the method involves careful control of temperature in the laboratory to constrain the diffusion parameters of the feldspar samples. In our study, we have K-feldspar data from single crystals that mimic the types of data seen in classic resistance heater fusion experiments. Our step-heating data are based on using a continuous argon-ion laser with no direct control on temperature. However, with only a single added free parameter in the model, we show that it is possible to analyze this data in the multi-domain style, and make some simple inferences on the nature of the cooling history of the Carion pluton in central Madagascar. The Carion granitic pluton in central Madagascar was intruded into warm continental crust following orogenic events related to the final amalgamation of Gondwana. U-Pb SHRIMP dating of the pluton yields an emplacement age of 532.1 ± 5.2 Ma followed by relatively slow cooling as constrained by 40Ar/39Ar ages on hornblende, biotite and K-feldspar. Four hornblende samples yielded a mean 40Ar/39Ar age of 512.7 ± 2.6 Ma. A biotite sample yielded an age of 478.9 ± 1.0 Ma and modeled K-feldspar ages show cooling from 350° C at 466 Ma to 100° C by 410 Ma. Collectively, the data suggest that the pluton cooled from 850° C at 532.1 ± 5.2 (U-Pb zircon) Ma to 500° C at 512.7 ± 2.6 Ma (40Ar/39Ar hornblende), or approximately 18 °C/Ma slowing to ∼4 °C/Ma between 512 Ma and 478 Ma and finally to about 3°C/Ma between 478 and 410 Ma.
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A 3-dimensional thermomechanical ice-sheet model is used to simulate the evolution of the geometry of Northern Hemisphere ice sheets through the Last Deglaciation. The ice-sheet model is forced by a time-evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the LMD5.3 atmospheric general circulation model (AGCM) at different periods of the Last Deglaciation (21, 15, 9, 6 and 0kyrBP). The AGCM is driven by insolation, atmospheric CO2 content, ice-sheet configuration and sea surface temperatures. Although our approach is able to produce the complete continental ice retreat, our simulated deglaciation presents a phase-lag with reconstructions based on observational evidences. This suggests that physical mechanisms related to climate forcing and/or ice-sheet internal dynamics are not properly represented. The influence of millennial-scale forcing, feedback mechanisms between ice-sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments. The rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature-mass balance relationships. Fennoscandia appears to be strongly sensitive to the small-scale ice-sheet instability. Both ice sheets are to some extent sensitive to an increased basal sliding.
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Stratigraphically discrete glacigenic dropstone intervals have been identified within six separate Neoproterozoic glaciomarine successions: the ca. 720 Ma Chuos Formation (southwestern Congo craton) and Surprise Diamictite (southwestern Laurentia), the ca. 600 Ma Ghaub Formation (Congo craton), Blässkrans Formation (northwestern Kalahari craton) and Wildrose Diamictite (southwestern Laurentia), and the post 595 Ma lower Southern Highland Group (northeast Laurentia). These dropstone intervals are interstratified with hemipelagic, dropstone-free lithologies that record periods devoid of glacial rainout. Such episodic deposition requires, at a most rudimentary level, temporally compatible environmental fluctuations to generate melting of ice. The Ghaub Diamictite is particularly revealing in this regard, given that, according to the snowball Earth hypothesis, it had to be deposited either during glacial maxima (totally frozen seas) or melt back (geologically instantaneous) owing to its low-latitude setting and association with carbonates that record highly depleted δ13C values. The consistency of facies features shared by all six glaciomarine successions leads us to speculate that the temporally discrete glacial- rainout events (including diamictites deposited in low latitudes) record a dynamic glacial environment and that such conditions were characteristic of Neoproterozoic glaciations in general. These observations indicate that Neoproterozoic seas were not totally frozen and that the hydrologic cycle was functioning.
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A fundamental question of earth history concerns the nature of the Late Proterozoic glaciogenic sequences that are known from almost all of the major cratonic areas, including North America, the Gondwana continents, and the Baltic Platform. A major controversy involves the probable latitude of formation for these deposits- were they formed at relatively high latitudes, as were those of the Permian and our modern glacial deposits, or were many of them formed much closer to the equator? Arguments supporting a low depositional latitude for many of these units have been discussed extensively for the past 30 years (e.g., Harland 1964), beginning with the field observations that some of the diamictites had a peculiar abundance of carbonate fragments, as if the ice had moved over carbonate platforms. Indeed, many of these units, such as the Rapitan Group of the Canadian Cordillera, are bounded above and below by thick carbonate sequences which, at least for the past 100 Ma, are only known to have been formed in the tropical belt within about 33° of the equator (Ziegler et al. 1984). Other anomalies include dropstones and varves in the carbonates, as well as evaporites (for a complete review, see Williams 1975). Either the earth was radically different during the late Precambrian glacial episode(s), or the major continental land masses spent an extraordinary amount of time traversing back and forth between the tropics and the poles.
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Recent reports that Jupiter's satellites Europa and Callisto may have ice-covered oceans have fueled speculation that they may be inhabited by living organisms. Gaidos et al. analyze the thermodynamic requirements for life on Europa by analogy with life on Earth today and during past "snowball" Earth episodes. They conclude that nearly all metabolic life-styles on the present Earth will be denied to organisms on Europa and indicate which types of organisms, if any, are most likely to be found there.
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The Eurasian climates of today, 10 million and 3O million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions. The resulting climates compare well with various palaeoclimate records. The retreat of the Paratethys-an epicontinental sea-shifts the central Asian climate from temperate to continental conditions, and plays as important a role as uplift of the Himalayan/Tibetan plateau in driving the Asian monsoon changes.
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Recent coupled energy balance/ice sheet modeling studies indicate that ice-covered continents can be simulated for the Late Precambrian with 6% solar constant reduction. We examine the ocean mixed layer response to such an ice sheet with the GENESIS 2 general circulation model and CO2 levels varying from 0.5-2.5 times present. The ocean ices over completely at 0.5-1.0X present levels, with the final phase of sea ice growth occurring within a single model year. A qualitatively different 2.5X CO2 solution is close to equilibrium and yields open water between ~25°N and ~25°S paleolatitude. The prevailing wind patterns suggest that an equatorial Pacific-type circulation may have developed in part of the Neoproterozoic ocean.
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Geologic evidence suggests that in the Late Neoproterozoic (∼600 Ma) almost all land masses were glaciated, with sea-level glaciation existing even at the equator. A recent modeling study has shown that it is possible to simulate an ice-covered Earth glaciation with a coupled climate/ice-sheet model. However, separate general circulation model experiments suggest that a second solution may exist with a substantial area of ice free ocean in the tropics. Although 0.1 to 0.3 of an atmosphere of CO2 (∼300 to 1000 X) is required for deglaciation of a “Snowball Earth,” the “exit” CO2 levels for an open water solution could be significantly less. In this paper we utilize a coupled climate/ice sheet model to demonstrate four points: (1) the open water solution can be simulated in the coupled model if the sea ice parameter is adjusted slightly; (2) a major reduction in ice volume from the open water/equatorial ice solution occurs at a CO2 level of about 4X present values—about two orders of magnitude less than required for exit from the “hard” snowball initial state; (3) additional CO2 increases are required to get fuller meltback of the ice; and (4) the open water solution exhibits hysteresis properties, such that climates with the same level of CO2 may evolve into either the snowball, open water, or a warmer world solution, with the trajectory depending on initial conditions. These results set useful targets for geochemical calculations of CO2 changes associated with the open-water solution.
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The rift-related Rapitan Group of the Mackenzie Mountains of northwestern Canada acquired several magnetizations due to pulses of hydrothermal activity. The first pulse, attributed to initiation of Rapitan rifting, produced a widespread overprint (P2) that may be reflected in the basal Mount Berg Formation. Two later pulses produced overprints similar to components found in an earlier study. Development of iron formation and hematite pigment in the overlying Sayunei Formation is attributed to the second pulse, represented by a paleopole (N = 10 sites; 334°E, 01°S; δp, δm = 4°, 9°) that coincides with poles of the Franklin igneous events of northern Canada. The Franklin episode, suggested on geological grounds to be coeval with Sayunei deposition, dates the Sayunei at ca. 725 Ma. This relation implies that rifting in Mackenzie Mountains could be related to rifting in northern Canada. A third pulse, reflected by a pole at 007°E, 16°N (N = 6 sites; δp, δm = 6°, 12°), is attributed to final rifting during deposition of the Shezal Formation at the top of the Rapitan. Overprints attributed to Sayunei and Shezal times indicate regional latitudes of 6 ± 4° and 4 ± 6° during the Sturtian glaciation. During Mount Berg time, the regional latitude could have exceeded 25°. All directions have been tilt corrected and some have been then rotated, based on comparisons with a P2 reference overprint.
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Negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia, combined with estimates of thermal subsidence history, suggest that biological productivity in the surface ocean collapsed for millions of years. This collapse can be explained by a global glaciation (that is, a snowball Earth), which ended abruptly when subaerial volcanic outgassing raised atmospheric carbon dioxide to about 350 times the modern level. The rapid termination would have resulted in a warming of the snowball Earth to extreme greenhouse conditions. The transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate rocks observed globally.
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A new thermomechanical three-dimensional model designed to simulate the evolution of the Antarctic ice sheet over long time periods is presented. This model incorporates the various types of ice flow found in Antarctica: relatively slow inland ice flow that is essentially due to ice deformation, fast ice flow in the regions with ice streams, and ice shelf flow. By coupling these three types of flow, it is possible to predict grounding line migration. Simulations covering four glacial-interglacial cycles have been conducted by forcing this model with a temperature record from Vostok and a sea level record from marine cores. Our findings indicate that grounding line migration induced by sea level changes is the primary factor governing the evolution of the Antarctic ice volume. On the other hand, the altitude of the ice sheet surface at Vostok is driven by accumulation rate variations. The amplitude of the altitude change does not exceed 150 m and is very similar for all the sites located on the Antarctic Plateau.
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