Impact of a Permo-Carboniferous high O-2 event on the terrestrial carbon cycle

Independent models predicting the Phanerozoic (past 600 million years) hist ory of atmospheric O-2 partial pressure (pO(2)) indicate a marked rise to a pproximately 35% in the Permo-Carboniferous. around 300 million years befor e present, with the strong potential for altering the biogeochemical cyclin g 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 Carbonifero us paleoclimate simulation to evaluate the effects of a rise from 21 to 35% pO(2) on terrestrial biosphere productivity and assess how this response i s modified by current uncertainties in the prevailing pCO(2) value. Our res ults 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, i n an atmosphere with pCO(2) = 0.03%. However, in an atmosphere with pCO(2) = 0.06%, the CO2 fertilization effect is larger than the cost of photorespi ration, and ecosystem productivity increases leading to the net sequestrati on of 117 Gt C into the vegetation and soil carbon reservoirs. In both case s, the effects result from the strong interaction between pO(2), pCO(2), an d climate in the tropics. From this analysis, we deduce that a Permo-Carbon iferous 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 imp roved.