72 research outputs found
Orbitally forced ice sheet fluctuations during the Marinoan Snowball Earth glaciation
Two global glaciations occurred during the Neoproterozoic. Snowball Earth theory posits that these were terminated after millions of years of frigidity when initial warming from rising atmospheric CO2 concentrations was amplified by the reduction of ice cover and hence a reduction in planetary albedo. This scenario implies that most of the geological record of ice cover was deposited in a brief period of melt-back. However, deposits in low palaeo-latitudes show evidence of glacialâinterglacial cycles. Here we analyse the sedimentology and oxygen and sulphur isotopic signatures of Marinoan Snowball glaciation deposits from Svalbard, in the Norwegian High Arctic. The deposits preserve a record of oscillations in glacier extent and hydrologic conditions under uniformly high atmospheric CO2 concentrations. We use simulations from a coupled three-dimensional ice sheet and atmospheric general circulation model to show that such oscillations can be explained by orbital forcing in the late stages of a Snowball glaciation. The simulations suggest that while atmospheric CO2 concentrations were rising, but not yet at the threshold required for complete melt-back, the ice sheets would have been sensitive to orbital forcing. We conclude that a similar dynamic can potentially explain the complex successions observed at other localities
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Snowball Earth climate dynamics and Cryogenian geology-geobiology
Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and â„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 CO2 was 102 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 CO2 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 subglacial 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
La tempĂ©rature des ocĂ©ans de l'ArchĂ©en, modĂ©lisation des couples isotopiques â18O et â30Si et interactions climat-carbone
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Toward the snowball earth deglaciation...
International audienceThe current state of knowledge suggests that the Neoproterozoic snowball Earth is far from deglaciation even at 0.2 bars of CO2. Since understanding the termination of the fully ice-covered state is essential to sustain, or not, the snowball Earth theory, we used an Atmospheric General Climate Model (AGCM) to explore some key factors which could induce deglaciation. After testing the models' sensitivity to their parameterizations of clouds, CO2 and snow, we investigated the warming effect caused by a dusty surface, associated with ash release during a mega-volcanic eruption. We found that the snow aging process, its dirtiness and the ash deposition on the snow-free ice are key factors for deglaciation. Our modelling study suggests that, under a CO2 enriched atmosphere, a dusty snowball Earth could reach the deglaciation threshold
Scenario for the evolution of atmospheric pCO2 during a snowball Earth
International audienceThe snowball Earth theory, initially proposed by J.L. Kirschvink to explain the Neoproterozoic glacial episodes, suggests that the Earth was globally ice covered at 720 Ma (Sturtian episode) and 640 Ma (Marinoan episode). The reduction of the water cycle and the growth of large ice sheets led to a collapse of CO 2 consumption through continental weathering and biological carbon pumping. As a consequence, atmospheric CO 2 built up linearly to levels allowing escape from a snowball Earth. In this contribution, we question this assumed linear accumulation of CO 2 into the atmosphere. Using a numerical model of the carbon-alkalinity cycles, we suggest that during global glaciations, even a limited area of open waters (10 3 km 2) allows an effi cient atmospheric CO 2 diffusion into the ocean. This exchange implies that the CO 2 consumption through the low-temperature alteration of the oceanic crust persists throughout the glaciation. Furthermore, our model shows that rising CO 2 during the glaciation increases the effi ciency of this sink through the seawater acidifi cation. As a result, the atmospheric CO 2 evolution is asymptotic, limiting the growth rate of the atmospheric carbon reservoir. Even after the maximum estimated duration of the glaciation (30 m.y.), the atmospheric CO 2 is far from reaching the minimum deglaciation threshold (0.29 bar). Accounting for this previously neglected carbon sink, processes that decrease the CO 2 deglaciation threshold must be further explored
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