Scenario for the evolution of atmospheric pCO₂ during a snowball Earth

The 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 CO₂ consumption through continental weathering and biological carbon pumping. As a consequence, atmospheric CO₂ built up linearly to levels allowing escape from a snowball Earth. In this contribution, we question this assumed linear accumulation of CO₂ 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 (10³ km²) allows an efficient atmospheric CO₂ diffusion into the ocean. This exchange implies that the CO₂ consumption through the low-temperature alteration of the oceanic crust persists throughout the glaciation. Furthermore, our model shows that rising CO₂ during the glaciation increases the efficiency of this sink through the seawater acidification. As a result, the atmospheric CO₂ 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 CO₂ is far from reaching the minimum deglaciation threshold (0.29 bar). Accounting for this previously neglected carbon sink, processes that decrease the CO₂ deglaciation threshold must be further explored.