Triple oxygen isotope evidence for limited mid-Proterozoic primary productivity

The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today¹. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆¹⁷O) 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 ∆¹⁷O values (down to -0.88‰) observed in sulfates, except for those from the terminal Cryogenian period². 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 CO₂ and O₂ and the photosynthetic O₂ flux at this time³. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O₂ flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO₂ (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 O₂ was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean^4-6 and Phanerozoic primary production⁷, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O₂ over geologic time⁸.