SNOWBALL EARTH: PROMISE AND PROBLEMS

Neoproterozoic glacial deposits occur world-wide and ice lines reached sea level in the tropics. Post-glacial ‘cap’ carbonates with ‘Lazarus’ (Archean-like) textures and low ¹³C contents are unique to Proterozoic ice ages, as are iron-oxide formations <1.8 Ga. The ‘snowball Earth’ hypothesis offers solutions to these enigmas through an interactive scenario involving the carbon cycle, ice-albedo feedback and plate tectonics. Climate models show that runaway ice-albedo feedback rapidly freezes the globe if atmospheric CO₂ drops to present levels with a 6% dimmer Sun. Oceans become anoxic, promoting dissolved iron; iron-oxide forms locally if oxygenic photosynthesis occurs under thin equatorial sea ice. Deglaciation requires >0.1 bar CO₂ for a planetary albedo ~0.6. Normal volcanic outgassing over millions of years will achieve this assuming negligible C sinks. Reverse ice-albedo feedback drives rapid deglaciation, leading to a transient ‘ultra-greenhouse’ (low albedo and high CO₂) aftermath. Intense carbonate and silicate weathering provide the alkalinity flux for ‘cap’ carbonates, and silicate weathering lowers atmospheric CO₂. Global ‘freeze-fry’ cycles occurred repetitively but only within two intervals of ~200 myr duration (2.45-2.25 and 0.78-0.58 Ga). Rare low-latitude disposition of the continents may be prerequisite.

Despite the appealing congruence of observations and theory, most geologists are wary of the hypothesis. Specialists on the glacial deposits argue that large volumes of mixtite imply dynamic, wet-based glaciers. Purported storm-wave structures indicate open water on the Adelaidean coast. Periglacial sand wedges in the same region imply strong equatorial seasonality, which suggests high orbital obliquity (incompatible with global glaciation because of hot polar summers). These observations may be reconciled with snowball Earth if ablative winds descending from the ice sheet keep coastal waters open, enhancing evaporation, snowfall and ice-sheet growth. Glacier surge cycles, rather than seasonality, may be responsible for temperature oscillations in periglacial soils manifested by sand wedges, obviating the need for high orbital obliquity.