Abstract
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).