New constraints on early oxygen levels

Reconstructing the protracted oxygenation of Earth’s atmosphere—from the time of the first photosynthesizing cyanobacteria perhaps as early as 3.5 billion years ago to the 21% concentration of today—is important for understanding the evolution of life on Earth and for evaluating oxygen concentration as a potential signature of life on other planets. Oxygen levels during the Proterozoic eon, 2.5–0.5 billion years ago, are particularly important. (See Physics Today, June 2018, page 16 .) Since atmospheric oxygen is toxic to anaerobes, the first appreciable rise in oxygen, around the start of the Proterozoic, had drastic consequences. And complex multicellular eukaryotes require a sufficiently high oxygen concentration. Proterozoic oxygen levels were not always on the climb, though, and they are poorly documented and hotly debated.

Eric Bellefroid of Yale University and colleagues report new evidence, based on cerium abundance in carbonates, that the oxygen concentration had dropped by 1.87 billion years ago. Unlike most other rare-earth elements, aqueous Ce will readily oxidize and precipitate out of marine waters. Of the Ce and other rare earths remaining in solution, some are captured during the formation of carbonate sediments. That makes Ce anomalies—variations in the ratio of Ce to other rare earths in sedimentary layers—a proxy for local oxygen conditions. To extract quantitative oxygenation estimates, the team first selected samples from northern Canada (see photo) that were well preserved and had formed at different water depths. Then, since marine oxygen levels would have varied with water depth, the researchers combined their Ce anomaly measurements with an ocean circulation model to account for vertical Ce transport. Their results suggest that dissolved oxygen was present only in a thin surface veneer of 50–100 m and that atmospheric oxygen concentrations were about 0.1% of the present level—below the current best estimates for what could support multicellular organisms. Applying their model to other reports of Ce levels suggests that oxygen levels remained low throughout most of the Proterozoic eon. (E. J. Bellefroid et al., Proc. Natl. Acad. Sci. USA, 2018, doi:10.1073/pnas.1806216115 ; photo courtesy of PNAS)