Analysis of Carbon Cycle System During the Neoproterozoic: Implication for Snowball Earth Events

Abstract

Carbon cycle and climate change during the Neoproterozoic have not been understood well. In that period, global glaciations may have occurred several times. Carbon isotopic composition of seawater was generally heavy and fluctuated with very large amplitudes, suggesting unusual behaviors of the carbon cycle system. In this study, the carbon cycle and the climate change during the Neoproterozoic are analyzed quantitatively using an improved carbon isotope mass balance model with a one-dimensional energy balance climate model. According to the results, the organic carbon burial rate is generally high and the carbonate precipitation rate is generally low through the Neoproterozoic. However, before the glaciations, the organic carbon burial increases and the carbonate precipitation decreases. On the other hand, the organic carbon burial decreases and the carbonate precipitation increases after the glaciations. Because the carbonate precipitation rate should balance with the cation supply rate due to chemical weathering, the climate factors (i.e., T and pCO 2 ) also decrease before the glaciations and increase after the glaciations. The climate is, however, too warm to initiate glaciations when present values of the CO 2 degassing rate and the organic carbon weathering rate are assumed in the model. If the global glaciations were caused at the positive excursions of δ 13 C, the CO 2 degassing rate should have decreased to < 1/4-1/2 times the present rate at the same time. In order to reconstruct the carbon cycle and the climate change in further detail, we need to know variations of the tectonic forcing (e.g., volcanic activities) during the Neoproterozoic.