Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations ([O~2~]) around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine to calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in atmospheric _p_O~2~ at ~400 million years ago (Ma), and reveals a step-change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma.

Supplement to: Lu, Wanyi; Ridgwell, Andy; Thomas, Ellen; Hardisty, Dalton; Luo, Genming; Algeo, Thomas J; Saltzman, Matthew R; Gill, Benjamin C; Shen, Yanan; Ling, Hong-Fei; Edwards, Cole T; Whalen, Michael T; Zhou, Xiaoli; Gutchess, Kristina M; Jin, Li; Rickaby, Rosalind E M; Jenkyns, Hugh C; Lyons, Timothy W; Lenton, Timothy M; Kump, Lee R; Lu, Zunli (2018): Late inception of a resiliently oxygenated upper ocean. Science, eaar5372