Ginkgophyte Fossils From The Middle Jurassic In Huating,Gansu Province And Paleoatmospheric CO₂ Reconstruction

There are numerous plant fossils in the Middle Jurassic in Huating,Gansu Province.Some plant fossils have been studied by previous researchers,but most of them were introduced as single genus,especially for the plant fossils of Ginkgophyte that lacked the systematic research.As“Relict plant”,Ginkgophyte contains only one species now,viz.Ginkgo biloba.But in the history of earth,Ginkgophyte was an important part of the plantae.Because of“hysteresis”in evolution,clear phylogenetic relationships and sensitivity of stomata to CO₂ concentration,Ginkgophyte was used to reconstruct the paleoatmospheric CO₂ concentration by many researcher.It is great significant to ascertain the changing rules of paleoatmospheric CO₂ concentration in the history of earth for understanding the future trend of atmospheric CO₂ concentration.In this paper,12 species of Gingoales and 5 species Czekanowskiales from the Middle Jurassic were used to carry out systematic morphological and anatomical studies.The width of lobes and stomatal parameters on the epidermis of Ginkgophyte fossils from the Middle Jurassic were analyzed.There is a certain correlation between the stomatal parameter and the lobe width,especially that of the stomatal index and lobe width shows an obvious negative correlation,but the interdependency between the stomatal density and the width of lobe is rather small.According to the analysis,it is more accurate that the hypostomatic lobes width>5 mm,which have large space between veins,are selected to reconstruct paleoatmospheric CO₂ concentration based on the stomatal index.Gingo/Gingoites and Phoenicopsis of which the lobes width>5 mm were used to reconstruct paleoatmospheric concentration of the middle Jurassic in Huating area based on the stomatal ratio.The paleostmospheric CO₂ concentration of the middle Jurassic was between 1037-1396 ppmv with an average value of 1228±116 ppmv based on the fossils of Gingo/Gingoites.And the paleoatmospheric CO₂ concentration by the Phoenicopsis fossils was 1243±75 ppmv,which is similar to the results from the fossils of Gingo/Gingoites,indicating that the Phoenicopsis with the larger lobe width was also a good material for the reconstruction of the paleoatmospheric CO₂concentration.Therefore,the paleoatmospheric CO₂ concentration in the middle Jurassic in Huating was estimated at 1228-1243 ppmv.The values of the paleoatmospheric CO₂ concentration which reconstructed by fossils from northwest China were gathered and combined with the results of this theme.It was concluded that the paleoatmospheric CO₂ concentration from the late Triassic to the middle Jurassic decreased at first and then increased in the northwestern China.According to the indications significance of the flora to the environment,it is assumed that the middle Jurassic of Huating has a warm and humid climate.