Abstract
Independent models predicting the Phanerozoic (past 600 million years) history of atmospheric O(2) partial pressure (pO(2)) 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 pCO(2) 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% pO(2) on terrestrial biosphere productivity and assess how this response is modified by current uncertainties in the prevailing pCO(2) value. Our results indicate that a rise in pO(2) from 21 to 35% during the Carboniferous reduced global terrestrial primary productivity by 20% and led to a 216-Gt (1 Gt = 10(12) kg) C reduction in the vegetation and soil carbon storage, in an atmosphere with pCO(2) = 0.03%. However, in an atmosphere with pCO(2) = 0.06%, the CO(2) 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 pO(2), pCO(2), and climate in the tropics. From this analysis, we deduce that a Permo-Carboniferous rise in pO(2) was unlikely to have exerted catastrophic effects on ecosystem productivity (with pCO(2) = 0.03%), and if pCO(2) levels at this time were >0.04%, the water-use efficiency of land plants may even have improved.