Triple oxygen isotope constraints on atmospheric O₂ and biological productivity during the mid-Proterozoic.

Abstract

Reconstructing the history of biological productivity and atmospheric oxygen partial pressure (<i>p</i>O₂) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating <i>p</i>O₂ 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 <i>p</i>O₂ at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on <i>p</i>O₂ is essentially unconstrained by these data. Indeed, <i>p</i>O₂ levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that <i>p</i>CO₂ 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 (O₂) between the atmosphere and surface ocean is controlled more by air-sea gas transfer rates than by biological productivity. Improved estimates of <i>p</i>CO₂ and/or improved proxies for Δ'¹⁷O of atmospheric O₂ would allow tighter constraints to be placed on mid-Proterozoic <i>p</i>O₂.