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Stephen G. Warren - One of the best experts on this subject based on the ideXlab platform.
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could promontories have restricted sea glacier penetration into marine embayments during Snowball Earth events
The Cryosphere, 2016Co-Authors: A. J. Campbell, Edwin D. Waddington, Betzalel Massarano, Stephen G. WarrenAbstract:Abstract. During the Neoproterozoic (∼ 1000–550 Ma), Earth experienced several climate excursions of extreme cold, often referred to as the Snowball Earth events. During these periods, thick flowing ice, referred to as sea glaciers, covered the entire planet's oceans. In addition, there is evidence that photosynthetic eukaryotic algae survived during these periods. With thick sea glaciers covering the oceans, it is uncertain where these organisms survived. One hypothesis is that these algae survived in marine embayments hydrologically connected to the global ocean, where the flow of sea glacier could be resisted. In order for an embayment to act as a refugium, the invading sea glacier must not completely penetrate the embayment. Recent studies have shown that straight-sided marine embayments could have prevented full sea-glacier penetration under a narrow range of climate conditions suitable for the Snowball Earth events. Here we test whether promontories, i.e., headlands emerging from a side shoreline, could further restrict sea-glacier flow. We use an ice-flow model, suitable for floating ice, to determine the flow of an invading sea glacier. We show that promontories can expand the range of climate conditions allowing refugia by resisting the flow of invading sea glaciers.
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the spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth 1 laboratory measurements
Journal of Geophysical Research, 2016Co-Authors: Bonnie Light, Regina C Carns, Stephen G. WarrenAbstract:The ice-albedo feedback mechanism likely contributed to global glaciation during the Snowball Earth events of the Neoproterozoic era (1 Ga to 544 Ma). This feedback results from the albedo contrast between sea ice and open ocean. Little is known about the optical properties of some of the possible surface types that may have been present, including sea ice that is both snow-free and cold enough for salts to precipitate within brine inclusions. A proxy surface for such ice was grown in a freezer laboratory using the single salt NaCl and kept below the eutectic temperature (−21.2°C) of the NaCl-H2O binary system. The resulting ice cover was composed of ice and precipitated hydrohalite crystals (NaCl · 2H2O). As the cold ice sublimated, a thin lag-deposit of salt formed on the surface. To hasten its growth in the laboratory, the deposit was augmented by addition of a salt-enriched surface crust. Measurements of the spectral albedo of this surface were carried out over 90 days as the hydrohalite crust thickened due to sublimation of ice, and subsequently over several hours as the crust warmed and dissolved, finally resulting in a surface with puddled liquid brine. The all-wave solar albedo of the subeutectic crust is 0.93 (in contrast to 0.83 for fresh snow and 0.67 for melting bare sea ice). Incorporation of these processes into a climate model of Snowball Earth will result in a positive salt-albedo feedback operating between −21°C and −36°C.
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salt precipitation in sea ice and its effect on albedo with application to Snowball Earth
Journal of Geophysical Research, 2015Co-Authors: Regina C Carns, Richard E Brandt, Stephen G. WarrenAbstract:During the initial freezing of the tropical ocean on Snowball Earth, the first ice to form would be sea ice, which contains salt within inclusions of liquid brine. At temperatures below −23°C, significant amounts of the salt begin to crystallize, with the most abundant salt being hydrohalite (NaCl·2H2O.) These crystals scatter light, increasing the ice albedo. In this paper we present field measurements of the albedo of cold sea ice and laboratory measurements of hydrohalite precipitation. Precipitation of salt within brine inclusions was observed on windswept bare ice of McMurdo Sound at the coast of Antarctica (78°S) in early austral spring. Salinity and temperature were measured in ice cores. Spectral albedo was measured on several occasions during September and October. The albedo showed a gradual increase with decreasing temperature, consistent with salt precipitation. Laboratory examination of thin sections from the ice cores showed that the precipitation process exhibits hysteresis, with hydrohalite precipitating over a range of temperatures between −28°C and −35°C but dissolving at about −23°C. The causes of the hysteresis were investigated in experiments on laboratory-grown sea ice with different solute mixtures. All mixtures showed hysteresis, suggesting that it may be an inherent property of hydrohalite precipitation within brine inclusions rather than being due to biological macromolecules or interactions between various salts in seawater. This article is protected by copyright. All rights reserved.
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Refugium for surface life on Snowball Earth in a nearly enclosed sea? A numerical solution for sea-glacier invasion through a narrow strait
Journal of Geophysical Research: Oceans, 2014Co-Authors: A. J. Campbell, Edwin D. Waddington, Stephen G. WarrenAbstract:Where photosynthetic eukaryotic organisms survived during the Snowball Earth events of the Neoproterozoic remains unclear. Our previous research tested whether a narrow arm of the ocean, similar to the modern Red Sea, could have been a refugium for photosynthetic eukaryotes during the Snowball Earth. Using an analytical ice-flow model, we demonstrated that a limited range of climate conditions could restrict sea-glacier flow sufficiently to allow an arm of the sea to remain partially free from sea-glacier penetration, a necessary condition for it to act as a refugium. Here we expand on the previous study, using a numerical ice-flow model, with the ability to capture additional physics, to calculate sea-glacier penetration, and to explore the effect of a channel with a narrow entrance. The climatic conditions are made self-consistent by linking sublimation rate to surface temperature. As expected, the narrow entrance allows parts of the nearly enclosed sea to remain safe from sea-glacier penetration for a wider range of climate conditions.
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effects of bubbles cracks and volcanic tephra on the spectral albedo of bare ice near the transantarctic mountains implications for sea glaciers on Snowball Earth
Journal of Geophysical Research, 2013Co-Authors: Ruzica Dadic, Richard E Brandt, Peter C Mullen, Martin Schneebeli, Stephen G. WarrenAbstract:[1] Spectral albedo was measured along a 6 km transect near the Allan Hills in East Antarctica. The transect traversed the sequence from new snow through old snow, firn, and white ice, to blue ice, showing a systematic progression of decreasing albedo at all wavelengths, as well as decreasing specific surface area (SSA) and increasing density. Broadband albedos under clear-sky range from 0.80 for snow to 0.57 for blue ice, and from 0.87 to 0.65 under cloud. Both air bubbles and cracks scatter sunlight; their contributions to SSA were determined by microcomputed tomography on core samples of the ice. Although albedo is governed primarily by the SSA (and secondarily by the shape) of bubbles or snow grains, albedo also correlates highly with porosity, which, as a proxy variable, would be easier for ice sheet models to predict than bubble sizes. Albedo parameterizations are therefore developed as a function of density for three broad wavelength bands commonly used in general circulation models: visible, near-infrared, and total solar. Relevance to Snowball Earth events derives from the likelihood that sublimation of equatorward-flowing sea glaciers during those events progressively exposed the same sequence of surface materials that we measured at Allan Hills, with our short 6 km transect representing a transect across many degrees of latitude on the Snowball ocean. At the equator of Snowball Earth, climate models predict thick ice, or thin ice, or open water, depending largely on their albedo parameterizations; our measured albedos appear to be within the range that favors ice hundreds of meters thick.
Aiko Voigt - One of the best experts on this subject based on the ideXlab platform.
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robust elements of Snowball Earth atmospheric circulation and oases for life
Journal of Geophysical Research, 2013Co-Authors: Dorian S Abbot, Mark Branson, Raymond T Pierrehumbert, Aiko Voigt, Dawei Li, David Pollard, Daniel D B KollAbstract:[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 O(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.
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Sea-ice dynamics strongly promote Snowball Earth initiation and destabilize tropical sea-ice margins
Climate of the Past, 2012Co-Authors: Aiko Voigt, D. S. AbbotAbstract:The Snowball Earth bifurcation, or runaway ice- albedo feedback, is defined for particular boundary condi- tions by a critical CO2 and a critical sea-ice cover (SI), both of which are essential for evaluating hypotheses related to Neoproterozoic glaciations. Previous work has shown that the Snowball Earth bifurcation, denoted as (CO2, SI) , dif- fers greatly among climate models. Here, we study the ef- fect of bare sea-ice albedo, sea-ice dynamics and ocean heat transport on (CO2, SI) in the atmosphere-ocean gen- eral circulation model ECHAM5/MPI-OM with Marinoan ( 635 Ma) continents and solar insolation (94 % of mod- ern). In its standard setup, ECHAM5/MPI-OM initiates a Snowball Earth much more easily than other climate mod- els at (CO2, SI) (500 ppm, 55 %). Replacing the model's standard bare sea-ice albedo of 0.75 by a much lower value of 0.45, we find (CO 2, SI) (204 ppm, 70 %). This is consis- tent with previous work and results from net evaporation and local melting near the sea-ice margin. When we additionally disable sea-ice dynamics, we find that the Snowball Earth bifurcation can be pushed even closer to the equator and occurs at a hundred times lower CO2: (CO2, SI) (2 ppm, 85 %). Therefore, the simulation of sea-ice dynamics in ECHAM5/MPI-OM is a dominant determinant of its high critical CO2 for Snowball initiation relative to other mod- els. Ocean heat transport has no effect on the critical sea-ice cover and only slightly decreases the critical CO2. For dis- abled sea-ice dynamics, the state with 85 % sea-ice cover is stabilized by the Jormungand mechanism and shares charac- teristics with the Jormungand climate states. However, there is no indication of the Jormungand bifurcation and hystere- sis in ECHAM5/MPI-OM. The state with 85 % sea-ice cover therefore is a soft Snowball state rather than a true Jor- mungand state. Overall, our results demonstrate that differ- ences in sea-ice dynamics schemes can be at least as impor- tant as differences in sea-ice albedo for causing the spread in climate models' estimates of the Snowball Earth bifurca- tion. A detailed understanding of Snowball Earth initiation therefore requires future research on sea-ice dynamics to de- termine which model's simulation is most realistic.
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Clouds and Snowball Earth deglaciation
Geophysical Research Letters, 2012Co-Authors: Dorian S Abbot, Mark Branson, Guillaume Le Hir, Raymond T Pierrehumbert, Aiko Voigt, David Pollard, Daniel D B KollAbstract:Neoproterozoic, and possibly Paleoproterozoic, glaciations represent the most extreme climate events in post-Hadean Earth, and may link closely with the evolution of the atmosphere and life. According to the Snowball Earth hypothesis, the entire ocean was covered with ice during these events for a few million years, during which time volcanic CO 2 increased enough to cause deglaciation. Geochemical proxy data and model calculations suggest that the maximum CO 2 was 0.01―0.1 by volume, but early climate modeling suggested that deglaciation was not possible at CO 2 = 0.2. We use results from six different general circulation models (GCMs) to show that clouds could warm a Snowball enough to reduce the CO 2 required for deglaciation by a factor of 10―100. Although more work is required to rigorously validate cloud schemes in Snowball-like conditions, our results suggest that Snowball deglaciation is consistent with observations.
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hadley cell dynamics in a virtually dry Snowball Earth atmosphere
Journal of the Atmospheric Sciences, 2012Co-Authors: Aiko Voigt, Isaac M Held, Jochem MarotzkeAbstract:The Hadley cell of a virtually dry Snowball Earth atmosphere under equinox insolation is studied in a comprehensive atmospheric general circulation model. In contrast to the Hadley cell of modern Earth, momentum transport by dry convection, which is modeled as vertical diffusion of momentum, is important in the upper branch of the Snowball Earth Hadley cell. In the zonal momentum balance, mean meridional advection of mean absolute vorticity is not only balanced by eddies but also by vertical diffusion of zonal momentum. Vertical diffusion also contributes to the meridional momentum balance by decelerating the Hadley cell through downgradient mixing of meridional momentum between its upper and lower branches. When vertical diffusion of momentum is suppressed in the upper branch, the Hadley cell strengthens by a factor of about 2. This is in line with the effect of vertical diffusion in the meridional momentum balance but incontrastwithitseffectinthezonalmomentumbalance.NeitheraxisymmetricHadleycelltheoriesbasedon angular momentum conservation nor eddy-permitting Hadley cell theories that neglect vertical diffusion of momentum are applicable to the Snowball Earth Hadley cell. Because the Snowball Earth Hadley cell is a particular realization of a dry Hadley cell, these results show that an appropriate description of dry Hadley cells should take into account vertical transport of momentum by dry convection.
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initiation of a marinoan Snowball Earth in a state of the art atmosphere ocean general circulation model
AGUFM, 2010Co-Authors: Aiko Voigt, Dorian S Abbot, Raymond T Pierrehumbert, Jochem MarotzkeAbstract:Abstract. We study the initiation of a Marinoan Snowball Earth (~635 million years before present) with the state-of-the-art atmosphere-ocean general circulation model ECHAM5/MPI-OM. This is the most sophisticated model ever applied to Snowball initiation. A comparison with a pre-industrial control climate shows that the change of surface boundary conditions from present-day to Marinoan, including a shift of continents to low latitudes, induces a global-mean cooling of 4.6 K. Two thirds of this cooling can be attributed to increased planetary albedo, the remaining one third to a weaker greenhouse effect. The Marinoan Snowball Earth bifurcation point for pre-industrial atmospheric carbon dioxide is between 95.5 and 96% of the present-day total solar irradiance (TSI), whereas a previous study with the same model found that it was between 91 and 94% for present-day surface boundary conditions. A Snowball Earth for TSI set to its Marinoan value (94% of the present-day TSI) is prevented by doubling carbon dioxide with respect to its pre-industrial level. A zero-dimensional energy balance model is used to predict the Snowball Earth bifurcation point from only the equilibrium global-mean ocean potential temperature for present-day TSI. We do not find stable states with sea-ice cover above 55%, and land conditions are such that glaciers could not grow with sea-ice cover of 55%. Therefore, none of our simulations qualifies as a "slushball" solution. While uncertainties in important processes and parameters such as clouds and sea-ice albedo suggest that the Snowball Earth bifurcation point differs between climate models, our results contradict previous findings that Snowball Earth initiation would require much stronger forcings.
Daniel P Schrag - One of the best experts on this subject based on the ideXlab platform.
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isotopically anomalous organic carbon in the aftermath of the marinoan Snowball Earth
Geobiology, 2020Co-Authors: Frasier L Liljestrand, Francis A Macdonald, Daniel P Schrag, Thomas A Laakso, D JohnstonAbstract:Throughout most of the sedimentary record, the marine carbon cycle is interpreted as being in isotopic steady state. This is most commonly inferred via isotopic reconstructions, where two export fluxes (organic carbon and carbonate) are offset by a constant isotopic fractionation of ~25 (termed eorg-carb ). Sedimentary deposits immediately overlying the Marinoan Snowball Earth diamictites, however, stray from this prediction. In stratigraphic sections from the Ol Formation (Mongolia) and Sheepbed Formation (Canada), we observe a temporary excursion where the organic matter has anomalously heavy δ13 C and is grossly decoupled from the carbonate δ13 C. This signal may reflect the unique biogeochemical conditions that persisted in the aftermath of Snowball Earth. For example, physical oceanographic modeling suggests that a strong density gradient caused the ocean to remain stratified for about 50,000 years after termination of the Marinoan Snowball event, during which time the surface ocean and continental weathering consumed the large atmospheric CO2 reservoir. Further, we now better understand how δ13 C records of carbonate can be post-depostionally altered and thus be misleading. In an attempt to explain the observed carbon isotope record, we developed a model that tracks the fluxes and isotopic values of carbon between the surface ocean, deep ocean, and atmosphere. By comparing the model output to the sedimentary data, stratification alone cannot generate the anomalous observed isotopic signal. Reproducing the heavy δ13 C in organic matter requires the progressively diminishing contribution of an additional anomalous source of organic matter. The exact source of this organic matter is unclear.
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dynamics of a Snowball Earth ocean
Nature, 2013Co-Authors: Yosef Ashkenazy, Francis A Macdonald, Daniel P Schrag, Hezi Gildor, Martin Losch, Eli TzipermanAbstract:Extensive glaciations, possibly even a globally ice-covered Snowball Earth, took place in the Neoproterozoic era, and here the possible ocean circulation at that time, under a kilometre of ice, is described. Active debate surrounds the possible existence of a global Snowball Earth during the Neoproterozoic era around 750 to 635 million years ago. But regardless of the global extent of the freeze, it is clear that extensive glaciations took place. Most research to date has focused on the atmospheric processes at work, largely ignoring the role of the oceans. Yosef Ashkenazy and colleagues now show that under an assumed one-kilometre layer of ice, and with a weak geothermal flux, a Snowball ocean would have featured vigorous mixing as well a strong equatorial overturning circulation, equatorial jets and extensive eddies. Coastal upwelling would likely have caused open water near continental margins. This work has implications for the survival of photosynthetic life during Snowball events and for the interpretation of current geological and geochemical observations. Geological evidence suggests that marine ice extended to the Equator at least twice during the Neoproterozoic era (about 750 to 635 million years ago)1,2, inspiring the Snowball Earth hypothesis that the Earth was globally ice-covered3,4. In a possible Snowball Earth climate, ocean circulation and mixing processes would have set the melting and freezing rates that determine ice thickness5,6, would have influenced the survival of photosynthetic life4,5,7,8,9, and may provide important constraints for the interpretation of geochemical and sedimentological observations4,10. Here we show that in a Snowball Earth, the ocean would have been well mixed and characterized by a dynamic circulation11, with vigorous equatorial meridional overturning circulation, zonal equatorial jets, a well developed eddy field, strong coastal upwelling and convective mixing. This is in contrast to the sluggish ocean often expected in a Snowball Earth scenario3 owing to the insulation of the ocean from atmospheric forcing by the thick ice cover. As a result of vigorous convective mixing, the ocean temperature, salinity and density were either uniform in the vertical direction or weakly stratified in a few locations. Our results are based on a model that couples ice flow and ocean circulation, and is driven by a weak geothermal heat flux under a global ice cover about a kilometre thick. Compared with the modern ocean, the Snowball Earth ocean had far larger vertical mixing rates, and comparable horizontal mixing by ocean eddies. The strong circulation and coastal upwelling resulted in melting rates near continents as much as ten times larger than previously estimated6,7. Although we cannot resolve the debate over the existence of global ice cover10,12,13, we discuss the implications for the nutrient supply of photosynthetic activity and for banded iron formations. Our insights and constraints on ocean dynamics may help resolve the Snowball Earth controversy when combined with future geochemical and geological observations.
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continental constriction and oceanic ice cover thickness in a Snowball Earth scenario
Journal of Geophysical Research, 2012Co-Authors: Eli Tziperman, Dorian S Abbot, David Pollard, Yosef Ashkenazy, Hezi Gildor, Christian Schoof, Daniel P SchragAbstract:[1] Ice flow over a Snowball ocean was shown in recent years to be capable of very effectively homogenizing ice thickness globally. Previous studies all used local or one-dimensional global (latitude-only) models, formulated in a way that is difficult to extend to two-dimensional global configuration. This paper uses a two-dimensional global ice flow model to study the effects of continental constriction on ice flow and ice thickness in a Snowball-Earth scenario using reconstructed Neoproterozoic landmass configuration. Numerical simulations and scaling arguments are used to show that various configurations of continents and marginal seas which are not represented by one dimensional models lead to large ice thickness variations, including narrow areas between sub-continents and marginal seas whose entrance is constricted. This study ignores many known important factors such as thermodynamic, optical effects, dust and dust transport, and is therefore meant as a process study focusing on one specific effect, rather than a realistic simulation of Neoproterozoic ice thickness. The model formulation developed here generalizes and extends previous results in several ways, including the introduction of corrections due to spherical coordinates and lateral geometry. This study is therefore a step toward coupling Snowball ice flow models to general circulation ocean and atmospheric models and allowing a more quantitative simulation of Neoproterozoic Snowball ice thickness.
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aftermath of a Snowball Earth
Geochemistry Geophysics Geosystems, 2003Co-Authors: John A Higgins, Daniel P SchragAbstract:[1] Using a simple 3-box model of the ocean-atmosphere system, we simulate the cycling of carbon and strontium in the aftermath of a global glaciation. Model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pCO2 conditions as well as ocean mixing, air-sea gas exchange, and biological productivity. The δ13C of the first carbonate precipitated after the glaciation depends on the pCO2, temperature, the saturation state of the surface ocean, and kinetic effects associated with mineral precipitation. With no biological productivity, the model produces δ13C values between +1‰ and −3‰, consistent with observations. This is in direct contradiction with arguments by Kennedy et al. [2001a], who suggest that the δ13C value of dissolved carbon in a Snowball ocean (and directly afterward) must be −5‰. Kennedy et al. assume the carbon isotope cycle is in steady state, which does not apply to a global glaciation, and also neglect any effect of high pCO2 on the carbonate chemistry of seawater. A major difference between our findings and the qualitative predictions of Hoffman et al. [1998] is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves. As a result, the ∼2‰ drop in the δ13C observed in the cap dolostone is unlikely to be the product of Rayleigh distillation of atmospheric CO2 via silicate weathering. Instead, we interpret the ∼2‰ drop in the δ13C values as indicative of an increase in sea surface temperature which lowers the fractionation between CO2 and carbonate. Kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important. Rayleigh distillation of atmospheric CO2 via silicate weathering is a viable explanation for the continued drop in the δ13C values in the limestone sequence above the cap dolostone, with biological productivity and carbonate weathering driving a slow increase in δ13C values once pCO2 levels decline. Our study also simulates the cycling of strontium in seawater. In contrast to the finding of Jacobsen and Kaufman [1999] and Kennedy et al. [2001a], model simulations show a drop in 87Sr/86Sr of less than 0.001 during 5 million years of global glaciation and an increase of less than 0.001 over the entire episode of silicate weathering. Our calculations emphasize the importance of considering the changes in seawater chemistry due to high pCO2 in evaluating the Snowball Earth hypothesis.
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the Snowball Earth hypothesis testing the limits of global change
Terra Nova, 2002Co-Authors: Paul F Hoffman, Daniel P SchragAbstract:The gradual discovery that late Neoproterozoic ice sheets extended to sea level near the equator poses a palaeoenvironmental conundrum. Was the Earth’s orbital obliquity > 60� (making the tropics colder than the poles) for 4.0 billion years following the lunar-forming impact, or did climate cool globally for some reason to the point at which runaway ice-albedo feedback created a ‘Snowball’ Earth? The high-obliquity hypothesis does not account for major features of the Neoproterozoic glacial record such as the abrupt onsets and terminations of discrete glacial events, their close association with large (> 10&) negative d 13 C shifts in seawater proxies, the deposition of strange carbonate layers (‘cap carbonates’) globally during post-glacial sea-level rise, and the return of large sedimentary iron formations, after a 1.1 billion year hiatus, exclusively during glacial events. A Snowball event, on the other hand, should begin and end abruptly, particularly at lower latitudes. It should last for millions of years, because outgassing must amass an intense greenhouse in order to overcome the ice albedo. A largely ice-covered ocean should become anoxic and reduced iron should be widely transported in solution and precipitated as iron formation wherever oxygenic photosynthesis occurred, or upon deglaciation. The intense greenhouse ensures a transient post-glacial regime of enhanced carbonate and silicate weathering, which should drive a flux of alkalinity that could quantitatively account for the world-wide occurrence of cap carbonates. The resulting high rates of carbonate sedimentation, coupled with the kinetic isotope effect of transferring the CO2 burden to the ocean, should drive down the d 13 C of seawater, as is observed. If cap carbonates are the ‘smoke’ of a Snowball Earth, what was the ‘gun’? In proposing the original Neoproterozoic Snowball Earth hypothesis, Joe Kirschvink postulated that an unusual preponderance of land masses in the middle and low latitudes, consistent with palaeomagnetic evidence, set the stage for Snowball events by raising the planetary albedo. Others had pointed out that silicate weathering would most likely be enhanced if many continents were in the tropics, resulting in lower atmospheric CO2 and a colder climate. Negative d 13 C shifts of 10–20& precede glaciation in many regions, giving rise to speculation that the climate was destabilized by a growing dependency on greenhouse methane, stemming ultimately from the same unusual continental distribution. Given the existing palaeomagnetic, geochemical and geological evidence for late Neoproterozoic climatic shocks without parallel in the Phanerozoic, it seems inevitable that the history of life was impacted, perhaps profoundly so.
Raymond T Pierrehumbert - One of the best experts on this subject based on the ideXlab platform.
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robust elements of Snowball Earth atmospheric circulation and oases for life
Journal of Geophysical Research, 2013Co-Authors: Dorian S Abbot, Mark Branson, Raymond T Pierrehumbert, Aiko Voigt, Dawei Li, David Pollard, Daniel D B KollAbstract:[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 O(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.
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Clouds and Snowball Earth deglaciation
Geophysical Research Letters, 2012Co-Authors: Dorian S Abbot, Mark Branson, Guillaume Le Hir, Raymond T Pierrehumbert, Aiko Voigt, David Pollard, Daniel D B KollAbstract:Neoproterozoic, and possibly Paleoproterozoic, glaciations represent the most extreme climate events in post-Hadean Earth, and may link closely with the evolution of the atmosphere and life. According to the Snowball Earth hypothesis, the entire ocean was covered with ice during these events for a few million years, during which time volcanic CO 2 increased enough to cause deglaciation. Geochemical proxy data and model calculations suggest that the maximum CO 2 was 0.01―0.1 by volume, but early climate modeling suggested that deglaciation was not possible at CO 2 = 0.2. We use results from six different general circulation models (GCMs) to show that clouds could warm a Snowball enough to reduce the CO 2 required for deglaciation by a factor of 10―100. Although more work is required to rigorously validate cloud schemes in Snowball-like conditions, our results suggest that Snowball deglaciation is consistent with observations.
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sea glacier flow and dust transport on Snowball Earth
Geophysical Research Letters, 2011Co-Authors: Raymond T PierrehumbertAbstract:[1] Accumulation of dust on the surface of ice in the tropics has been proposed as a possible mechanism for the termination of Neoproterozoic Snowball Earth episodes. This Mudball hypothesis relies on the assumption that sea glacier flow transports dust to the tropical ablation zone, leading to the accumulation of a dust moraine there and consequent lowering of tropical albedo. Here, we use a 1-D sea-glacier flow model to simulate the ice thickness and flow on a globally glaciated Earth, and study the dust transport associated with the ice flow. Dust is entirely confined to a meteoric ice layer which does not exchange water with the ocean. Dust falling onto the surface of this layer is carried downward with the ice flow in extratropical regions, carried equatorward by the global ice flow, and re-emerges on the top of the ice in the tropical ablation zone. The ice flow acts as a dust conveyor belt which converges dust to tropics, resulting in an amplification of effective tropical dust flux by a factor of 2 to 20 over the global average. This lends support to the Mudball hypothesis.
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initiation of a marinoan Snowball Earth in a state of the art atmosphere ocean general circulation model
AGUFM, 2010Co-Authors: Aiko Voigt, Dorian S Abbot, Raymond T Pierrehumbert, Jochem MarotzkeAbstract:Abstract. We study the initiation of a Marinoan Snowball Earth (~635 million years before present) with the state-of-the-art atmosphere-ocean general circulation model ECHAM5/MPI-OM. This is the most sophisticated model ever applied to Snowball initiation. A comparison with a pre-industrial control climate shows that the change of surface boundary conditions from present-day to Marinoan, including a shift of continents to low latitudes, induces a global-mean cooling of 4.6 K. Two thirds of this cooling can be attributed to increased planetary albedo, the remaining one third to a weaker greenhouse effect. The Marinoan Snowball Earth bifurcation point for pre-industrial atmospheric carbon dioxide is between 95.5 and 96% of the present-day total solar irradiance (TSI), whereas a previous study with the same model found that it was between 91 and 94% for present-day surface boundary conditions. A Snowball Earth for TSI set to its Marinoan value (94% of the present-day TSI) is prevented by doubling carbon dioxide with respect to its pre-industrial level. A zero-dimensional energy balance model is used to predict the Snowball Earth bifurcation point from only the equilibrium global-mean ocean potential temperature for present-day TSI. We do not find stable states with sea-ice cover above 55%, and land conditions are such that glaciers could not grow with sea-ice cover of 55%. Therefore, none of our simulations qualifies as a "slushball" solution. While uncertainties in important processes and parameters such as clouds and sea-ice albedo suggest that the Snowball Earth bifurcation point differs between climate models, our results contradict previous findings that Snowball Earth initiation would require much stronger forcings.
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mudball surface dust and Snowball Earth deglaciation
Journal of Geophysical Research, 2010Co-Authors: Dorian S Abbot, Raymond T PierrehumbertAbstract:[1] Recent modeling results have raised doubts about the ability to deglaciate from a global glaciation at atmospheric carbon dioxide levels that are realistic for a Neoproterozoic Snowball Earth. Here we argue that over the lifetime of a Snowball event, ice dynamics should lead to the development of a layer of continental and volcanic dust at the ice surface in the tropics that would significantly lower the tropical surface albedo and encourage deglaciation. This idea leads to the prediction that clay drapes found on top of Neoproterozoic glaciations should be thicker in tropical than extratropical regions. We test this idea by running the FOAM general circulation model (GCM) with an added tropical dust layer of different sizes and albedos and find that the tropical dust layer causes Snowball deglaciation at pCO2 = 0.01–0.1 bar in a reasonable regime of these parameters. We find similar, though more nuanced, results from a limited number of test cases using National Center for Atmospheric Research's CAM GCM.
Richard W Peltier - One of the best experts on this subject based on the ideXlab platform.
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influence of surface topography on the critical carbon dioxide level required for the formation of a modern Snowball Earth
Journal of Climate, 2018Co-Authors: Yonggang Liu, Richard W Peltier, Jun YangAbstract:AbstractThe influence of continental topography on the initiation of a global glaciation (i.e., Snowball Earth) is studied with both a fully coupled atmosphere–ocean general circulation model (AOGC...
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strong effects of tropical ice sheet coverage and thickness on the hard Snowball Earth bifurcation point
Climate Dynamics, 2017Co-Authors: Yonggang Liu, Richard W Peltier, Jun Yang, G Vettoretti, Yuwei WangAbstract: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.
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sea level variations during Snowball Earth formation 1 a preliminary analysis
Journal of Geophysical Research, 2013Co-Authors: Yonggang Liu, Richard W PeltierAbstract:[1] 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.
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Snowball Earth prevention by dissolved organic carbon remineralization
Nature, 2007Co-Authors: Richard W Peltier, Yonggang Liu, John W CrowleyAbstract:The 'Snowball Earth' hypothesis posits the occurrence of a sequence of glaciations in the Earth's history sufficiently deep that photosynthetic activity was essentially arrested. Because the time interval during which these events are believed to have occurred immediately preceded the Cambrian explosion of life, the issue as to whether such Snowball states actually developed has important implications for our understanding of evolutionary biology. Here we couple an explicit model of the Neoproterozoic carbon cycle to a model of the physical climate system. We show that the drawdown of atmospheric oxygen into the ocean, as surface temperatures decline, operates so as to increase the rate of remineralization of a massive pool of dissolved organic carbon. This leads directly to an increase of atmospheric carbon dioxide, enhanced greenhouse warming of the surface of the Earth, and the prevention of a Snowball state.
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co2 levels required for deglaciation of a near Snowball Earth
Geophysical Research Letters, 2001Co-Authors: Thomas J Crowley, William T Hyde, Richard W PeltierAbstract: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 CO 2 (-300 to 1000 X) is required for deglaciation of a Snowball Earth, the exit CO 2 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 CO, level of about 4X present values -about two orders of magnitude less than required for exit from the hard Snowball initial state; (3) additional CO 2 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 CO 2 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 CO 2 changes associated with the open-water solution.
