Researchers have found ancient gases and fluids trapped in 1.4-billion-year-old halite crystals from northern Ontario, Canada. Their analyses directly constrain Mesoproterozoic (1.8 to 0.8 billion years ago) oxygen and carbon dioxide concentrations to 3.7% modern levels and 10 times preindustrial levels, respectively. The results show this was a period of equable climate and that atmospheric oxygen concentrations, at least transiently, surpassed the...
