The role of continents in modulating the whole Earth carbon and oxygen cycles

Abstract:

That Earth owes its oxygenated atmosphere to its presently unique claim on life is clear, but how its atmosphere evolved from an initially oxygen-free state remains unresolved. Here, we show that continents underwent a transformation from mafic to felsic compositions at about 2.7-2.5 Ga. We show that the generation of large amounts of felsic magmas requires hydrous parental magmas, which can only be generated by introduction of water into the mantle source of the hydrous parents. This suggests that this compositional shift in crust marked a switch from stagnant lid to a mobile lid (plate tectonics) convective regime. This compositional transformation resulted in a decrease in the oxidative weathering efficiency of the Earth’s surface, which allowed atmospheric O₂ to rise rapidly to a new steady state after ~2 Ga. Over the next billion years, steady accumulation of carbon deposited on continental shelves resulted in a gradual increase in the total CO₂ inputs into the ocean and atmosphere system as metamorphic and magmatically induced degassing of the growing continental carbon reservoir amplified that from the mantle. Because oxygen production is formed by net organic carbon burial, which scales with total CO₂ production, the increasing CO₂ inputs to the ocean and atmosphere results in increasing O₂ production. This causes slow build-up of atmospheric O₂ until a threshold is reached at which atmospheric O₂ levels run away, culminating in the oxygenated atmospheric conditions of the Phanerozoic. A two-step rise in atmospheric O₂ is thus a necessary consequence of plate tectonics, continent formation and the growth of a crustal carbon reservoir. A provocative prediction of this work is that atmospheric CO₂ concentrations may not have been as high in the Archean and Proterozoic as seems required to compensate for the faint young sun.