OCEANIC BORON CYCLE IMPLIES THE DIFFERENCE IN THE TWO NEOPROTEROZOIC SNOWBALL STATES

The two snowball events on the Neoproterozoic Earth (i.e., longer Sturtian and shorter Marinoan events) differed in duration while they occurred in a short period. One intuitive explanation for the difference could be that the atmospheric CO₂ level (pCO₂) necessary for the deglaciation of the Sturtian event is higher than that of the Marinoan event. However, it has been implied that pCO₂ after the Sturtian event is lower than after the Marinoan event by an ocean pH estimate via boron isotope. In this study, we developed a geochemical cycle model for carbon and boron to re-evaluate the boron isotope data of the previous paper and to approach the reason for the difference in the snowball duration. Assuming a hard snowball state (i.e., no continental weathering), the boron isotope ratio in the ocean (δ¹¹B_sw) increases with time during the snowball state. After the deglaciation, intensive continental weathering, which results from high pCO₂, decreases δ¹¹B_sw, and the decrease in δ¹¹B_sw tends to accompany undershooting. The extent of the undershooting increases with the duration of the snowball event. The undershooting in δ¹¹B_sw is consistent with δ¹¹B data of carbonate after the Marinoan event but not with the data after the Sturtian event. On the other hand, assuming a soft snowball state (i.e., small but non-zero continental weathering), the change in δ¹¹B_sw observed for the hard snowball case is suppressed and becomes more consistent with the boron isotope data after the Sturtian event. Thus, the boron isotope data from the Neoproterozoic would imply the difference in continental weathering (or other equivalents) during the two events. Also, the difference in continental weathering should have resulted in the difference in the duration of the snowball state. Therefore, the two snowball events on the Neoproterozoic Earth might have differed in continental weathering, resulting from their snowball states and causing the difference in the duration and the boron isotopes.