Using paleosols of the Picture Gorge Basalt to reconstruct the middle Miocene climatic optimum

The Picture Gorge Subgroup of the Columbia River Basalt preserves numerous, minimally altered, interfl ow paleosols that were formed during the middle Miocene climatic optimum. These paleosols are used to reconstruct a high-resolution paleoclimate record. Between 16.0–15.4 Ma ago, mean annual precipitation (MAP) was 500–900 mm/year and mean annual temperature (MAT) was 8–16°C, which is consistent with independent estimates of paleoprecipitation and paleotemperature from fossil plants and paleosols of the contemporaneous Mascall Formation. The record suggests cooling and aridifi cation similar to marine foraminiferal isotopic records. One possible cause for the middle Miocene climatic optimum is transient elevated atmospheric CO2 levels, though some recently compiled marine isotopic records indicate near-modern CO2 levels. This possibility is explored using an equilibrium model to simulate the formation of the dominant Picture Gorge paleosol type. Model results indicate elevated CO2 levels 2–3 times present atmospheric levels were necessary to form the observed mineral assemblage and mass-balance characteristics of the paleosols. This result is consistent with stomatal index studies of gingkos and laurels, which indicate CO2 levels 2–4 times present atmospheric levels.