Refining a pedogenic-carbonate CO2 paleobarometer to quantify a middle Miocene greenhouse spike

Estimates of respired soil CO2 concentration in paleosols are needed to calculate past atmospheric CO2 levels from the difference between organic and carbonate carbon isotopic values in paleosols. A new compilation of CO2 concentrations late in the growing season in modern calcareous soils can be used to estimate respired soil CO2 by subtracting atmospheric CO2 concentrations. Furthermore, respired CO2 shows a significant relationship with depth to calcic horizon, and this relationship can be used to estimate respired CO2 concentrations in paleosols. These values, as well as estimates of δ13C of atmospheric CO2 from δ13C of soil organic matter, and of paleotemperature from Bw horizon chemical composition, refine the Cerling CO2 paleobarometer. These refinements are applied to quantify the middle Miocene greenhouse, which has been controversial because it was not detected by marine alkenone and boron paleobarometers. New estimates of middle Miocene atmospheric CO2 from paleosols in Railroad Canyon, Idaho, reveal levels at 16 Ma of 852 ± 86 ppmv. This is similar to predicted values by 2100, and 3 times pre-industrial values (PIL = 280 ppmv).