Reconstruction Of Paleoatmospheric CO₂ Concentration Based On Fossil Ginkgophytes From The Middle Jurassic Of The Northeastern Margin Of The Qaidam Basin

The problem of global climatic changes is widely concerned in the world today.The study of climate in geological history plays an important role in predicting the future environmental and climatic changes.Among them,the study of plant fossils and the reconstruction of paleoatmospheric CO₂ concentration is an important aspect of reconstruction of paleoclimate and paleoenvironment and global change research.In recent years,with the development of science and technology,a lot of important information have been accumulated in the study of macro morphology and cuticle microstructure of Ginkgophytes fossils.It has been widely used in reconstructing paleoatmospheric CO₂concentration.Ginkgophytes are especially suitable for reconstruction of Mesozoic paleo CO₂concentration.However,most of the studies currently on Ginkgophytes fossils are at the qualitative stage.In this paper,Six species of Ginkgophytes leaf fossils from Middle Jurassic in northeastern Qaidam Basin of Qinghai Province are identified.They are:Ginkgo longifolius(Phillips)Harris;Ginkgoites aganzhenensis Yang,Sun et Shen;Ginkgoites cf.robustus Sun;Ginkgoites cf.sibirica(Heer,1876)Seward;Baiera cf.furcata(Lindley et Hutton,1837)Braun and Stephenophyllum cf.solmsi(Seward,1919)Florin.For the first time,the methodologies of stomatal ratio method,SI-p CO₂revised curve,as well as photosynthetic leaf gas exchange model and carbon isotope model are used to estimate the paleoatmospheric CO₂concentration of the Middle Jurassic,and the paleoclimate condition in this stage was quantitatively determined.The stomatal ratio method and SI-p CO₂correction curve method(mean 1438.5ppmv and 986.35ppmv)are close to the average reconstructed results of other proxies,which could be represent the range of paleoatmospheric CO₂concentration in the Middle Jurassic.The results of photosynthetic gas exchange model(mean656ppmv)need to be further studied considering the differences among species and the effective values of various parameters.The carbon isotope model is may not suitable for the reconstruction of paleo CO₂concentration in the Middle Jurassic.The results show Ginkgo longifolius(Phillips)Harris could be a good candidate to reconstruct the paleoatmospheric CO₂concentration.The results of stomatal ratio method,modified curve method and photosynthetic leaf gas exchange model are close within the allowable error range,which are 1385ppmv,939ppmv and1095ppmv respectively.In addition,combined with paleotemperature,paleoflora,cuticle characteristics and fossil lithology analysis,the northeast margin of Qaidam Basin in the Middle Jurassic was in temperate climate.The climate of the study area was warm and humid in the early Middle Jurassic,and then changed to dry and hot in the late.