Mantle Cooling, Seafloor Serpentinization, and the Rise of Atmospheric O2

Abstract:

Atmospheric O₂ increased abruptly around 2.4 Ga in an episode called the Great Oxidation Event, or GOE. Cyanobacteria were producing oxygen well before this time, however (1), so the rise of O₂ appears to have been delayed by other factors, most likely a higher flux of hydrogen from Earth’s crust and interior. Changes in volcanic outgassing rates alone cannot be responsible, as the relatively constant C isotope record in marine carbonates would then imply that O₂ production should also have been greater (2). Catling and Claire (3,4) have proposed that secular oxidation of the continents was the key factor, leading to a decrease in reduced gases produced during metamorphism. This mechanism seems unlikely, though, because the amount of exposed continental area was small during the Archean (5) and because non-fossil sources of reduced metamorphic gases are also small. A more likely source of hydrogen on the Archean Earth was serpentinization of seafloor, which releases H₂ when ferrous iron is oxidized to magnetite. Today, serpentinization of seafloor is a minor H₂ source, or O₂ sink. The Archean Earth had a hotter mantle, however (6), which should have led to a higher degree of partial melting at the midocean ridges. This, in turn, should have created thick, ultramafic seafloor (7) which should have been prone to serpentinization and H₂ release. Gradual secular cooling of the mantle led to thinner, less mafic oceanic crust, thereby decreasing the source of H₂ and ultimately triggering the GOE. Thus, the rise of atmospheric O₂ on Earth and on Earth-like planets may be directly linked to the temperature of their interiors.

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