Soil CO2 efflux simulations using Monte Carlo method and implications for recording paleo-atmospheric PCO2 in pedogenic gibbsite

a r t i c l e i n f o Monte Carlo simulation testing of the sensitivity of the Fickian diffusion model used for paleo-PCO 2 proxies indicates that assuming annual average soil respiration rate results in over-estimation of PCO 2. In all cases, the natural variability of the factors of the model, including soil porosity and inverse tortuosity and the δ 13 C value of the soil produced CO 2 were not able to account for discrepancies between measured δ 13 C values from a modern soil and modeled results using established assumptions. Assuming that there is negligible carbon isotope fractionation during the occlusion of CO 2 in pedogenic gibbsite, the flaw in the model appears to be the assumed value of soil respiration rate. If correct, the implication is that previous paleo-PCO 2 estimates from soil hydroxides may be too high by as much as a factor of ten. Increased concern about the link between changing atmospheric greenhouse gas concentrations and climate makes precise knowledge of atmospheric CO 2 concentrations throughout Earth's history useful for anticipating future extents and rates of change. The relationship between atmospheric CO 2 concentration and climate has been documented for times younger than 740 kya in the Dome C ice core record (Augustin et al., 2004), whose gas bubble CO 2 signal and ice O and H isotope pattern serve as credible recorders of coupled climate and atmospheric CO 2 change. The stable carbon isotope compositions of pedogenic oxyhydroxides and carbonates is two proxies that have been commonly used as records of atmospheric CO 2 on millennia to million year time scales (e. Use of these proxies gives insight into the potential range of climate response to changing atmospheric CO 2 concentration, because they occurred during periods of earth's history that were either much warmer or much cooler than today (Royer et al., 2007). One of the key assumptions used to assess the changing concentration of CO 2 in the atmosphere is the concentration of CO 2 in the soil, which is related to the rate of CO 2 efflux from the soil reservoir to the atmosphere. CO 2 efflux is the net transport of gas out of the earth's surface due to changes in atmospheric pressures and gradients caused by thermal and biological oxidation of carbon. The latter is collectively referred to as soil respiration which is mediated by the rate …