Figure 5

Thermal and evolved gas analysis of two paleosol samples from the early Oligocene (33 Ma) Big Basin Member of the John Day Formation (adapted from ³¹). Dashed vertical line at 400 °C in both panels is from the thermal decomposition of organic carbon. H₂O—m/z 18, SO₂—m/z 64, CO₂—m/z 44. (A), Evolutions of CO₂, organic fragments (C₂H₂, C₂H₃, C₃H₃), and H₂O from the surface horizon of the Luca paleosol. Dashed trace is heat flow from differential scanning calorimetry (DSC) where exothermic reactions have a positive slope; (B) Evolutions of H₂O (blue trace), SO₂ (yellow trace) and CO₂ (red trace), from a thin Entisol (“Kskus” pedotype) stratigraphically below the Luca profile³⁰. Overlap of SO₂ and CO₂ peaks at 400 °C in B) suggests sulfur-bearing phases (possible sulfides) may also associated with organic carbon.