An analysis of salt mineral sediments, or evaporites, from the oldest deposit of this type discovered to date provides a unique window on the atmospheric conditions of early Earth following the Great Oxidation Event 2.3 billion years ago. These findings also have implications for understanding the interaction between the global cycle of oxygen (O2) and carbon dioxide (CO2) and the climate during this era. Major changes in atmospheric and oceanic chemistry occurred in the Paleoproterozoic Era (2.5 to 1.6 billion years ago) following the Great Oxidation Event, when O2 first began to accumulate in Earth's atmosphere. Increasing oxidation (when oxygen combines with other compounds) dramatically changed the Earth's surface, but few quantitative measures describe this important transition, partly because geochemical data from time periods near the event are scarce. Now, core samples obtained from a two-billion-year-old evaporite deposit discovered at the Onega Basin in Karelia, Russia have extended the record of ancient seawater and surface chemistry in early Earth by almost a billion years. Clara Blättler and colleagues analyzed the cores, finding that two billion years ago, marine sulfate concentrations were around one-third as high as modern measures, providing 20% of the oxidizing capacity (the capacity to provide oxygen in a chemical reaction) of modern oceans. These results indicate large amounts of O2 reacted with sulfur and accumulated in the oceans as sulfates during this critical time of Earth's oxygenation, and further suggest that a sustained increase in net O2 production occurred following the Great Oxidation Event.