Middle Jurassic climate oscillations from paleosol records of the Sichuan Basin, SW China

Abstract

The Middle Jurassic is characterized by major plate tectonic changes, variable atmospheric CO2 concentrations (pCO2), and climate oscillations in both marine and terrestrial realms. However, the limited evidence from the terrestrial sedimentary records hampers a comprehensive understanding of global climate state in this period. Well-exposed strata of the Shaximiao Formation in the Sichuan Basin, Southwest China, preserve successions of paleosols that offer the possibility of evaluating the terrestrial paleoclimatic changes during the Middle Jurassic (Bajocian to Callovian). A total of 151 paleosol profiles belonging to four paleosol types (Entisol-like, Inceptisol-like, Aridisol-like and argillic Inceptisol-like paleosols) were identified and characterized. The quantitative paleoclimate reconstructions based on the bulk geochemistry of paleosol horizons and depths of carbonate nodules indicate a generally semiarid–subhumid climate alternating with arid–humid and cool/warm–temperate climates during this time interval. This dynamic climate terminated in a dry, subhumid, humid/perhumid and superhumid moisture regime characterized by steppe/wet or rain forest floral provinces. We suggest that these climatic fluctuations may have been related to global geodynamic (e.g., the fragmentation of Pangea, “megamonsoonal” circulation, and bolide impacts) and regional tectonic uplift (e.g., the uplift of the Longmenshan Mountains in the South China Block) during the Middle Jurassic. The pCO2 estimates suggest relatively low pCO2 levels within the range of ∼100 ppm–∼890 ppm during the Bajocian to Callovian. Our results coincide with the threshold for the initiation of glaciation and may help to explain the occurrence of cool intervals in the generally warm Jurassic. These pCO2 fluctuations, which coincide with paleotemperature variations in the low paleolatitudes of the Northern Hemisphere, are possibly attributable to global geological events (e.g., wildfire events, eustatic sea-level oscillations, and the transient development of glaciations) in the Middle Jurassic.