PALEOSOL EVIDENCE FOR A SOIL-CO2 GLOBAL THERMOSTAT OVER THE PAST 300 MILLION YEARS (Invited Presentation)

CO₂ greenhouse spikes in deep time from large perturbations, such as flood basalt eruptions and asteroid impacts, were cooled by carbon-sequestration in grasslands expanding into deserts and tropical forests migrating toward the poles. This is revealed by rare appearances of deep-calcic paleosols such as Alfisols and Vertisols in long sequences of Aridisols, and also of extratropical Oxisols in long sequences of Alfisols. A lower limit for CO₂ was imposed by the spread of deserts (Aridisols) and ice caps (Gelisols) curbing plant productivity. This planetary thermostat is revealed by comparison of CO₂ paleobarometers from paleosols and from stomatal index of Ginkgo. For most of the past 300 million years CO₂ levels were between 400 and 600 ppm, but during life crises of the Permian-Triassic and Cretaceous-Tertiary and many other greenhouse crises, CO₂ levels soared to beyond 1500 ppm. Such data from paleosols of Utah over the past 300 million years reveal 1.1^oC increase in mean annual temperature and 110 mm increase in mean annual precipitation with doubling of CO₂. Too much CO₂ can be devastating for biodiversity: the pre-human amount of CO₂ causing extinction of 50% of marine invertebrate genera (LD₅₀) was 1355 ± 240 ppm. When CO₂ is above 1500 ppm, many animals are culled to redress the balance.