Alkalinity, Snowball Glaciations, and the Great Oxidation Event

English Abstract:

The cause of the rapid rise in atmospheric oxygen at 2.4 Ga remains controversial. Using a model of biogeochemical cycling , we identify a new positive feedback that greatly increases the rate at which pO₂ rises across the GOE compared to simpler models. Increasing pO₂ strips ferrous iron from the oceans, reducing alkalinity and the precipitation of calcium carbonate. This is supplemented by sulfide oxidation, which also leads to a reduced flux of alkalinity. To compensate, pCO₂ rises, leading to additional silicate weathering and alkalinity production. This also increases the release of phosphate via apatite weathering, which must be balanced by increased organic carbon burial and further production of oxygen. We show that this positive feedback leads to multiple equilibria in pO₂. In an otherwise steady state, low-oxygen Archean with oxygenic photosynthesis, a transient increase in pCO₂ leads to increased oxygen production via the alkalinity feedback. If this pulse is sufficiently large, the ratio of oxygen to production to electron outgassing can pass the critical threshold without any change in mantle processes, and a permanent transition to high oxygen occurs. Otherwise, the pulse of oxygen is overwhelmed by reductants from the mantle, and pO₂ decays back to its Archean values. These dynamics may explain why the GOE appears to be associated with one of the Paleoproterozoic Snowball events.