Chromium evidence for protracted oxygenation during the Paleoproterozoic

It has commonly been proposed that the development of complex life—e.g., aerobic eukaryotes—coincided with atmospheric oxygenation. To test this hypothesis, we measured chromium-based oxygen proxies in a >2400-m core from the Onega Basin (NW-Russia), deposited ~2.1–2.0 billion years ago—closely preceding the first eukaryote fossils. Fractionated chromium isotopes are documented throughout the section (max. 1.63±0.10‰ δ53Cr), suggesting higher and more stable oxygen levels during this interval (possibly >100 million years) than persisted for much of the billion years before or after. Significantly, unambiguous eukaryotic fossils have not been found in the Onega Basin or any contemporaneous successions. While this does not discount an increase in biological complexity during this oxic interval, our study calls into question the causative correlation between oxygenation and biological innovation. This work further highlights that while oxygenation is a necessary precursor for diverse eukaryote rich ecosystems, it need not drive eukaryogenesis or diversification.