Study On Fossil Conifers From The Lower Cretaceous Dalazi Formation Of Jilin Province And Their Palaeoclimate Implications

The Cretaceous was one of the best examples of "greenhouse climate" in geological history.During this period,series of remarkable geological events happened and also resulted in global vegetation change.Accordingly,the studies on the Cretaceous fossil plants contribute to understanding the terrestrial vegetation,palaeoclimate and the change of atmospheric CO₂ concentration in this period.In this paper,fossil samples were collected from the Dalazi Formation in Zhixin basin and Luozigou basin of Jilin Province,which is one of the important strata of the late Early Cretaceous in northeastern China.Based on the field fossil collection,6 species of Cupressinocladus,a form genus of fossil conifers,were identified and described by morphological and anatomical studies.The cuticle structures of Cupressinocladus elegans?Chow?Chow and Cupressinocladus ovatus Cao were firstly reported in China,and a new species of Cupressinocladus yanbianensis sp.nov.was proposed by new materials.For another thing,the revisions of diagnosis were combed based on previous studies,and the change of diversity,spatial and temporal distribution of fossil records in China also have been reported here.The results indicate that Cupressinocladus originated in the Early Jurassic and then well developed from the Late Jurassic to Early Cretaceous,finally it declined rapidly and became extinct in the Late Cretaceous.Cupressinocladus is widely distributed in China,especially abundant in Zhejiang,Fujian,Shandong and northeastern China,and the distribution of Cupressinocladus in South China is more abundant than North China.Moreover,the evolution of Cupressinocladus in geohistory correlated with the changes of paleoclimate environment,which proves to be a generally proxy of paleoclimate.Furthermore,the present studies show that the northward extension of Cupressinocladus in the Early Cretaceous supported the northward shift of the boundary between the north-south China flora.The composition of the Dalazi Formation flora indicates that it belonged to the transition zone of the north-south flora and it was generally dominated by a dry and hot climate in this area during the late Early Cretaceous.In addition,this paper also conducted a preliminary discussion on the attribution of the Cupressinocladus and the importance to improve the classification accuracy.The atmospheric carbon dioxide concentration of the late Early Cretaceous has been reconstructed by using the stomata ratio method based on two extinct fossil conifer Cheirolepidiaceae species,Pseudofrenelopsis gansuensis and P.dalatzensis.The stomatal index was used to estimate the Palaeo-pCO₂.The average stomatal index value of the two species was 4.1 and the atmospheric CO₂ concentration was?1150 ppm by the mean of Recent standardization and Carboniferous standardization.It shows a palaeo-CO₂ concentration 3.9 times higher than the pre-industrial value?300 ppm?.During the interval of 104.8 ± 0.5Ma to 104.0±0,4Ma in Late Albian,the atmospheric CO₂ concentrations were possibally fluctuant,increasing slightly with a followed continuous decrease,and the highest value was 1240 ppmv in Luozigou basin.The higher atmospheric CO₂ concentration of Late Albian possibly linked to a‘greenhouse climate'time,which was caused by the production of world-wide pulses in oceanic crustal and the formation of large igneous provinces.