Triple oxygen isotope constraints on atmospheric O(2) and biological productivity during the mid-Proterozoic

Reconstructing the history of biological productivity and atmospheric oxygen partial pressure (pO(2)) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating pO(2) and productivity during the Proterozoic. O-MIF, reported as Δ′(17)O, 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 (FeS(2)) 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 pO(2) at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on pO(2) is essentially unconstrained by these data. Indeed, pO(2) levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that pCO(2) could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O(2)) between the atmosphere and surface ocean is controlled more by air–sea gas transfer rates than by biological productivity. Improved estimates of pCO(2) and/or improved proxies for Δ′(17)O of atmospheric O(2) would allow tighter constraints to be placed on mid-Proterozoic pO(2).