Triple oxygen isotope constraints on atmospheric O

Significance Constraining the abundance of molecular oxygen (O2) in Earth’s atmosphere over time is a problem of central importance for understanding the evolution of complex life. Here, we refine previous analyses of the rare oxygen isotope composition of sedimentary sulfates to develop improved estimates of atmospheric O2 during Earth’s mid-Proterozoic era. Previous analyses of these data had predicted O2 concentrations well below 1% present atmospheric level. Our new calculations suggest that this value is closer to a lower limit on atmospheric oxygen partial pressure unless the climate was warmed significantly by biogenic methane. The calculations also show that marine productivity cannot be reliably estimated from these data because of the slow rate of transfer of O2 across the air–sea interface.
Reconstructing the history of biological productivity and atmospheric oxygen partial pressure (pO2) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating pO2 and productivity during the Proterozoic. O-MIF, reported as Δ′17O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS2) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air–sea gas exchange. Previous analyses of these data concluded that pO2 at that time was