CARBON CYCLE DURING THE NEOPROTEROZOIC: IMPLICATION FOR THE GLOBAL GLACIATIONS

Isotopic composition of seawater through the Neoproterozoic shows large oscillations with several negative carbon isotope excursions that might have related with glaciations (probably, snowball Earth events). It is considered that global glaciations may have been caused by increase in the burial rate of organic carbon as suggested by positive excursions of carbon isotopic composition just before the negative excursions. It is, however, unknown whether the amplitudes of positive excursions are really enough to cause global glaciations.

In this study, the carbon cycle during the Neoproterozoic is investigated by using a simple carbon isotope mass balance model combined with a one-dimensional energy balance climate model. Compiled data of carbon isotope during the Neoproterozoic by Hayes et al. [1999] is used here. Mass balance of carbon, carbon isotope, and alkalinity of the atmosphere-ocean system, dependencies of pCO2 and Ts on the silicate weathering rate, and dependency of pCO2 on the isotope fractionation due to photosynthesis are considered in the model. Variations of rates of organic carbon, carbonate precipitation, and weathering of carbonate and silicate (and so, pCO2 and Ts) are obtained from the model.

At the positive excursion, organic carbon burial rate increases, but carbonate precipitation rate and weathering rates decrease, suggesting cold climate. At the negative excursion, organic carbon burial rate decreases (almost to zero), but carbonate precipitation rate and weathering rates increase, suggesting warm climate. It is shown that, in order to cause the global glaciations, degassing rate of CO2 via volcanism should have been lowered to < 1/2-1/4 of the present level at the time of negative excursions of carbon isotope. Therefore, in addition to the enhanced organic carbon burial, volcanic (tectonic) activity may have been decreased greatly just before the Neoproterozoic glaciations.