Late inception of a resiliently oxygenated upper ocean

The rise of oxygen To understand the evolution of the biosphere, we need to know how much oxygen was present in Earth's atmosphere during most of the past 2.5 billion years. However, there are few proxies sensitive enough to quantify O 2 at the low levels present until slightly less than 1 billion years ago. Lu et al. measured iodine/calcium ratios in marine carbonates, which are a proxy for dissolved oxygen concentrations in the upper ocean. They found that a major, but temporary, rise in atmospheric O 2 occurred at around 400 million years ago and that O 2 levels underwent a step change to near-modern values around 200 million years ago. Science , this issue p. 174