Astrobiologists at Arizona State University and their colleagues have been working to constrain the abundance and distribution of dissolved oxygen in the Earth’s early oceans, prior to the rise of atmospheric oxygen about 2.4 billion years ago. Their analyses of 2.6- to 2.5-billion-year-old black shales from South Africa suggest that the production of oxygen in the surface ocean was vigorous at this time. Combined with studies conducted in Australia, they conclude that the productive regions along ocean margins during the late Archaean eon were sites of substantial O2 accumulation, at least 100 million years before it began to accumulate in the atmosphere. Their paper can be found in the current issue of Nature Geosciences.