Early Jurassic climate and atmospheric CO 2 concentration in the Sichuan paleobasin, Southwest China

Abstract. Unlike marine archives, terrestrial sediments show more complicated and dynamic environment and climate. This work presents new results of climate-sensitive sediment observation and carbon-oxygen isotope analyses of lacustrine and pedogenic carbonates for the Early Jurassic Ziliujing Formation from the grand Sichuan paleobasin (GSB), Southwest China. Lithofacies analysis indicates calcisols were widespread in riverine and flood plain facies. Climate–sensitive sediments and carbon-oxygen isotopes with palynofloral assemblages manifest that an overall (semi-) arid climate dominated the GSB; and that it became drier through time, accompanied by occasional evaporites in the Toarcian. This climate pattern is similar with the arid climate in Colorado Plateau, western America, but distinct from the relatively warm-humid climate in North China and northern Gondwanaland in Southern Hemisphere. The estimated Early Jurassic atmospheric CO2 concentration (pCO2) from carbon isotopes of pedogenic carbonates shows a range of 980–2610 ppmV (~ 3.5–10 times the pre-industrial value) with a mean 1660 ppmV. Three phases of pCO2 (the Sinemurian 1500–2000 pmV, the Pliensbachian 1000–1500 ppmV, and the early Toarcian 1094–2610 ppmV) and two events of rapid falling pCO2 by ~ 1000–1300 ppmV are observed, illustrating the pCO2 perturbation in the Early Jurassic. The pattern and associated rapid falling events of pCO2 are compatible with the excursions of stable isotopes and seawater temperature from the coeval marine sediments, consistent with a positive feedback of climate to pCO2 through the Early Jurassic.