Scenario for the evolution of atmospheric pCO2 during a snowball Earth

The snowball Earth theory, initially proposed by J.L. Kirschvinkto explain the Neoproterozoic glacial episodes, suggests thatthe Earth was globally ice covered at 720 Ma (Sturtian episode)and 640 Ma (Marinoan episode). The reduction of the water cycleand the growth of large ice sheets led to a collapse of CO₂consumption through continental weathering and biological carbonpumping. As a consequence, atmospheric CO₂ built up linearlyto levels allowing escape from a snowball Earth. In this contribution,we question this assumed linear accumulation of CO₂ into theatmosphere. Using a numerical model of the carbon-alkalinitycycles, we suggest that during global glaciations, even a limitedarea of open waters (10³ km²) allows an efficient atmosphericCO₂ diffusion into the ocean. This exchange implies that theCO₂ consumption through the low-temperature alteration of theoceanic crust persists throughout the glaciation. Furthermore,our model shows that rising CO₂ during the glaciation increasesthe efficiency of this sink through the seawater acidification.As a result, the atmospheric CO₂ evolution is asymptotic, limitingthe growth rate of the atmospheric carbon reservoir. Even afterthe maximum estimated duration of the glaciation (30 m.y.),the atmospheric CO₂ is far from reaching the minimum deglaciationthreshold (0.29 bar). Accounting for this previously neglectedcarbon sink, processes that decrease the CO₂ deglaciation thresholdmust be further explored.