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

Abstract

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 <1% PAL (times the present atmospheric level). Our model shows that the upper limit on pO2 is essentially unconstrained by these data. Indeed, pO2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that pCO2 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 pCO2 and/or improved proxies for Δ′17O of atmospheric O2 would allow tighter constraints to be placed on mid-Proterozoic pO2.

Comparison between ozone column depths and methane lifetimes computed by one- and three-dimensional models at different atmospheric O 2 levels

“…The current (Kasting group) 1-D photochemical model is taken directly from Liu et al . [ 7 ]. It extends from the ground up to 100 km altitude in 1 km increments.…”

“…The treatment of lower boundary conditions for biogenic trace gases (CH 4 , N 2 O, CO, CH 3 Cl and H 2 ) is the same as in Liu et al . [ 7 ] and Segura et al . [ 3 ].…”

“…We used that strategy in a recent paper by Liu et al . [ 7 ]. That number is consistent with the revised lower estimate of ozone column depth based on observations.…”

“…Heterogeneous chemistry is not included in the chemical scheme. The treatment of lower boundary conditions for biogenic trace gases (CH 4 , N 2 O, CO, CH 3 Cl and H 2 ) is the same as in Liu et al [7] and Segura et al [3]. These trace gases are fixed at observed values for the modern atmosphere simulations.…”

Eukaryogenesis and oxygen in Earth history

“…Indeed, fossil and molecular clock evidence consistently suggest that the last eukaryote common ancestor (LECA) emerged hundreds of millions of years after Earth's initial oxygenation [16][17][18][19] . Furthermore, recent advances in geochemistry and Earth system modeling suggest that the oceans at the time of eukaryogenesis were predominantly anoxic at depth, with only weakly oxygenated surface waters (likely no more than 1-10% of modern atmospheric saturation) [20][21][22][23] . These revisions have fostered major modifications 7,[24][25][26] to the co-evolutionary narrative first synthesized by Sagan 1967 1 , inviting a reexamination of the redox conditions necessary for eukaryogenesis.…”