Paper No. 8-10
Presentation Time: 10:30 AM
DISSOLVED SILICA DRIVEN RAPID PRECIPITATION OF CAP CARBONATE DURING DEGLACIATION OF THE MARINOAN SNOWBALL EARTH
The Snowball Earth (SE) Event is one of the most prevalent hypotheses used to explain the evidence for globally distributed ice sheets at the end of Neoproterozoic. The SE contemplates a stable frozen Earth from the runaway ice-albedo effect. The SE event eventually ended with intensive deglaciation from the accumulated high pCO2 levels. We used a geochemical model that integrated the carbon, silica, calcium, and magnesium cycles to understand changes in sedimentary mineral depositions and ocean chemistry during and after a Snowball Earth event. Intense chemical weathering in response to the extremely high levels of pCO2 at the beginning of deglaciation delivered an immense amount of calcium, magnesium, and dissolved silica into the ocean resulting in a spike of dolomite, as "cap dolostone", and silica deposition. Previous models used CaCO3 to calculate the carbonate precipitation due to an insufficient understanding of dolomite precipitation criteria and rates. Our previous work demonstrated that dissolved silica could promote precipitation of disordered dolomite at room temperature, where Mg incorporation in Ca-Mg carbonates are positively correlated with dissolved silica levels. By incorporating the magnesium cycle and coupling dolomite precipitation with dissolved silica levels, we proposed a geochemical model to estimate the occurrence and duration of the post-glacial dolomite burial. This geochemical model also provided constraints on the relative intensity of silicate versus carbonate weathering based on the appearance of cap dolostone during deglaciation and the amplitude of pH change. These results are consistent with the geological observation of globally distributed cap dolostone and chert from the post-Marinoan period.