A new proxy for paleosol carbon dioxide concentrations for use in estimating past atmospheric carbon dioxide

Estimates of 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 157 CO₂ concentrations (C in ppmv) late in the growing season in 14 modern calcareous soils from Canada, USA, Israel, Azerbaijan and Russia shows a significant relationship (R² = 0.79, SE= ± 937 ppmv) with depth to calcic horizon (D in cm): C = 67.25D + 999.62. From this equation, former CO₂ concentrations in paleosols can be computed from depth to calcic horizon. These values, as well as estimates of δ¹³C of atmospheric CO₂ from δ¹³C of soil organic matter, and of paleotemperature from B horizon chemical composition, refine the Cerling CO₂ paleobarometer. These refinements are applied to quantify the middle Miocene greenhouse, which has been controversial because 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 823 ± 216 ppmv, equal to stomatal index estimates for the middle Miocene and to IPCC-predicted values for 2100, and more than three times pre-industrial values (280 ppmv).