The Cryogenian period ( 50) encompasses the paired Neoproterozoic Snowball Earths and the brief nonglacial interlude ( Fig. P co 2, partial pressure of CO 2 wrt, with respect to. Qualitatively similar hysteresis is found in 3D general circulation models (GCMs). The ocean is predicted to undergo severe acidification and deacidification in response to the CO 2 hysteresis. The hysteresis loop predicts that Snowball glaciations were long-lived (b and c), began synchronously at low latitudes (a and b), and ended synchronously at all latitudes under extreme CO 2 radiative forcing (c and d). Intense silicate weathering and carbon burial lower atmospheric CO 2 (in 10 7 years) ( 164) to (e), the threshold for the reestablishment of a polar ice cap. Once the tropical ocean begins to open, ice-albedo feedback drives the ice line rapidly poleward (in ~2 ky) ( 327) to (d), where high CO 2 combined with low surface albedo creates a torrid greenhouse climate. With reduced sinks for carbon, normal volcanic outgassing drives atmospheric CO 2 higher over millions to tens of millions of years ( 73) until it reaches the deglaciation threshold (c). In response to lower forcing, ice line migrates equatorward to the ice-albedo instability threshold (a), whereupon the ice line advances uncontrollably to the equator (Eq) (b). Black arrows indicate nonequilibrium transitions. Yellow dots are stable climates possible with present-day forcing. Ice-line latitude as a function of solar or CO 2 radiative forcing in a one-dimensional (1D) (meridional) energy-balance model of the Budyko-Sellers type ( 3, 4), showing three stable branches (red, green, and blue solid lines) and the unstable regime (dashed line). The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance.
Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification.
The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Tropical ice sheets flow faster as CO 2 rises but lose mass and become sensitive to orbital changes. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. 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. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Geochemical data imply that CO 2 was 10 2 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Sadler, Eli Tziperman, Aiko Voigt, and Stephen G. Creveling, Yannick Donnadieu, Douglas H.
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