Chromium evidence for protracted oxygenation during the Paleoproterozoic

It has commonly been proposed that the development of complex life is tied to increases in atmospheric oxygenation. However, there is a conspicuous gap in time between the oxygenation of the atmosphere 2.4 billion years ago (Ga) and the first widely-accepted fossil evidence for complex eukaryotic cells < 1.7Ga. At present the gap could either represent poor sampling, poor preservation, and/or difficulties in recognizing early eukaryote fossils, or it could be real and the evolution of complex cells was delayed due to relatively low and/or variable O₂ levels in the Paleoproterozoic. To assess the extent and stability of Paleoproterozoic O₂ levels, 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-a few hundred million years prior to the oldest definitive fossil evidence for eukaryotes. Fractionated chromium isotopes are documented throughout the section (max. 1.63 ± 0.10 ‰δ⁵³Cr), suggesting a long interval (possibly >100 million years) during which oxygen levels were higher and more stable than in the billion years before or after. This suggests that, if it is the case that complex cells did not evolve until after 1.7 Ga, then this delay was not due to O₂-limitation. Instead, it could reflect other limiting factors-ecological or environmental-or could indicate that it simply takes a long time-more than the tens to >100 million years recorded in Onega Basin sediments-for such biological innovations to evolve.