Impact of a Permo-Carboniferous high O2 event on the terrestrial carbon cycle

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.

Figure 1

Global distribution of NPP (t C per ha⁻¹⋅yr⁻¹), vegetation biomass (kg C per m⁻²), and soil carbon concentrations (kg C per m⁻²) in the late Carboniferous for the atmospheric composition represented by the control case (21% O₂, 0.03% CO₂; a, d, and g, respectively), and the modeled effects of the Permo-Carboniferous pO₂ increase at two different pCO₂ levels, given as the difference between case 2 (35% O₂, 0.03% CO₂) and the control (b, e, and h) and between case 4 (35% O₂, 0.06% CO₂) and the control (c, f, and i). Note that maps b, c, and h as well as c, f, and i are difference maps and are plotted on different scales. Each individual map shows the three major landmasses of the land-sea mask used in the climate and vegetation modeling. These were Gondwana, dominating the Southern hemisphere; Eurasia, the largest Northern hemisphere landmass; and adjacent to this landmass, Kazakhstan.

Figure 2

Effects of the Permo-Carboniferous pO₂ rise at three different pCO₂ levels on the latitudinal gradient of terrestrial NPP during the late Carboniferous. The gradients represent the difference between the case 2 and the control simulation (21% O₂, 0.03% CO₂; dashed line), case 3 and the control simulation (dot-dash line), and case 4 and the control simulation (solid line).

Figure 3

Effects of the Permo-Carboniferous pO₂ rise at two different pCO₂ levels on annual (a) terrestrial NPP, (b) canopy transpiration (E_t), and (c) water-use efficiency (WUE, defined as NPP divided by E_t) during the late Carboniferous, analyzed on a site-by-site basis. Open symbols represent results from case 2 (35% O₂, 0.03% CO₂); solid symbols represent results from case 4 (35% O₂, 0.06% CO₂). All values are expressed as the differences between results from the specified atmospheric composition and the control (case 1).