The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative ∆O values (down to -0.88‰) observed in sulfates, except for those from the terminal Cryogenian period. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO and O and the photosynthetic O flux at this time. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO (2-30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean and Phanerozoic primary production, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O over geologic time.

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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-meter-thick 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.