Abstract

Independent models predicting the Phanerozoic (past 600 million years) history of atmospheric O₂ partial pressure (pO₂) indicate a marked rise to approximately 35% in the Permo-Carboniferous, around 300 million years before present, with the strong potential for altering the biogeochemical cycling of carbon by terrestrial ecosystems. This potential, however, would have been modified by the prevailing atmospheric pCO₂ value. Herein, we use a process-based terrestrial carbon cycle model forced with a late Carboniferous paleoclimate simulation to evaluate the effects of a rise from 21 to 35% pO₂ on terrestrial biosphere productivity and assess how this response is modified by current uncertainties in the prevailing pCO₂ value. Our results indicate that a rise in pO₂ from 21 to 35% during the Carboniferous reduced global terrestrial primary productivity by 20% and led to a 216-Gt (1 Gt = 10¹² kg) C reduction in the vegetation and soil carbon storage, in an atmosphere with pCO₂ = 0.03%. However, in an atmosphere with pCO₂ = 0.06%, the CO₂ fertilization effect is larger than the cost of photorespiration, and ecosystem productivity increases leading to the net sequestration of 117 Gt C into the vegetation and soil carbon reservoirs. In both cases, the effects result from the strong interaction between pO₂, pCO₂, and climate in the tropics. From this analysis, we deduce that a Permo-Carboniferous rise in pO₂ was unlikely to have exerted catastrophic effects on ecosystem productivity (with pCO₂ = 0.03%), and if pCO₂ levels at this time were >0.04%, the water-use efficiency of land plants may even have improved.