The researchers investigate the balance between two mechanisms that remove carbon from the atmosphere and oceans over long timescales—weathering of terrestrial silicates and alteration of the ocean floor. We show that this balance should strongly influence atmospheric oxygen concentration, since it dictates the delivery rate of the ultimate limiting nutrient phosphorus to the ocean. Increasing solar luminosity and declining seafloor spreading rates over Proterozoic time are expected to have shifted the balance of carbon removal toward terrestrial weathering. This leads to the prediction of a gradually increasing oxygen concentration over Proterozoic time, with transient higher oxygen concentration in the late Neoproterozoic.