FAINT YOUNG SUN AND THE CARBON CYCLE: IMPLICATION FOR THE PROTEROZOIC GLOBAL GLACIATIONS

Abstract

The Earth may have been globally ice-covered several times during the Proterozoic. While the Neoproterozoic and the Paleoproterozoic glaciations may have been 'snowball' Earth events, there is no evidence for such glaciation during the Phanerozoic. It might be hypothesized that a dimmer Sun earlier in Earth's history may have made the Earth more susceptible to global glaciation. In this paper, the roles of solar flux and soil biological activity in the carbon cycle and the climate during the Proterozoic are investigated using a simple carbon geochemical cycle model with a one-dimensional energy balance climate model. The results indicate, perhaps counterintuitively, that the Proterozoic Earth, with its dimmer Sun, was not more susceptible to 'snowball glaciation'. Metamorphic and volcanic CO2 fluxes accumulate in the atmosphere and ocean until such time that those inputs are balanced by silicate weathering followed by carbonate precipitation and net organic carbon burial. Because of the dependence of weathering rates on climatic conditions, changes in geologic CO2 inputs have a large influence on climatic conditions. In contrast, slow variation in solar flux has relatively little long-term impact on climate, because of large compensating changes in atmospheric CO2 level. A reduction in CO2 inputs lowers atmospheric CO2 level, which finally initiates global glaciation. The atmospheric CO2 level at the critical condition for a globally ice-covered state would have been high during the Proterozoic. However, roughly the same amount of CO2 flux reduction is required for both the Proterozoic and the Phanerozoic. This is essentially because the temperatures at the critical condition are very low, hence the silicate weathering rate (which should balance with a net CO2 input rate in a steady state) is also very low, regardless of the variation in solar flux. Furthermore, the effect of the lower solar flux on the CO2 input rate at the critical condition would have been largely canceled by a lower efficiency of the silicate weathering rate due to lower soil biological activity during the Proterozoic. As a result, CO2 flux conditions for initiating the global glaciation may be similar during both the Proterozoic and the Phanerozoic. Therefore, the explanation for the susceptibility of the Proterozoic Earth to 'snowball' conditions cannot hinge simply on the dimmer Sun; we must look to other differences in behaviors of the carbon cycle and the climate between these two ages.