The Great Oxidation Event following the end of the Archaean eon (~2.4 Ga) was a profound turning point in the history of Earth and life, but the relative importance of various contributing factors remains an intriguing puzzle. Controls on methane flux to the atmosphere were of particular consequence; too much methane would have inhibited a persistent rise of O₂, but too little may have plunged Earth into severe and prolonged ice ages. Here, we document a shift in the weathering reactions controlling the ocean-bound flux of nickel—an essential micronutrient for the organisms that produced methane in Precambrian oceans—by applying Ni stable isotope analysis to Mesoarchaean and Palaeoproterozoic glacial sediments. Although Ni flux to the ocean dropped dramatically as Ni content of the continental crust decreased, the onset of sulfide weathering delivered a small, but vital, flux of Ni to the oceans, sustaining sufficient methane production to prevent a permanent icehouse, while allowing O₂ to rise.