Major changes in atmospheric and ocean chemistry occurred in the Paleoproterozoic era (2.5 to 1.6 billion years ago). Increasing oxidation dramatically changed Earth s surface, but few quantitative constraints exist on this important transition. This study describes the sedimentology, mineralogy, and geochemistry of a 2-billion-year-old, ∼800-meterthick evaporite succession from the Onega Basin in Russian Karelia. The deposit consists of a basal unit dominated by halite (∼100 meters) followed by units dominated by anhydrite-magnesite (∼500 meters) and dolomite-magnesite (∼200 meters). The evaporite minerals robustly constrain marine sulfate concentrations to at least 10 millimoles per kilogram of water, representing an oxidant reservoir equivalent to more than 20% of the modern ocean-atmosphere oxidizing capacity. These results show that substantial amounts of surface oxidant accumulated during this critical transition in Earth s oxygenation. © 2017 The Authors, some rights reserved.