Coupled dolomite and silica precipitation from continental weathering during deglaciation of the Marinoan Snowball Earth

Abstract

Geological records show that multiple ice sheets have covered tropical areas toward the end of the Neoproterozoic. The Snowball Earth concept, one of the most prevalent hypotheses used to explain these records, contemplates a stable frozen Earth from the runaway ice-albedo effect. The Snowball Earth scenario eventually ended with intensive deglaciation due to extremely high pCO2 levels accumulated through millions of years. We used a geochemical model that integrated the carbon, silica, calcium, and magnesium cycles to understand changes in sedimentary mineral deposits 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 carbonate precipitation due to an insufficient understanding of dolomite precipitation criteria and rates. By incorporating the magnesium cycle and coupling dolomite precipitation with dissolved silica levels, we provide an explanation for the occurrence and duration of the postglacial dolomite burial. This model also provides 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.