期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2021
卷号：118
期号：51
DOI：10.1073/pnas.2105074118
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Constraining the abundance of molecular oxygen (O
2) 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 O
2 during Earth’s mid-Proterozoic era. Previous analyses of these data had predicted O
2 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 O
2 across the air–sea interface.
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′
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 (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′
17O of atmospheric O
2 would allow tighter constraints to be placed on mid-Proterozoic
pO
2.
