Multiple Reconstruction Of Miocene Paleoclimate Based On Plant Fossils In Tiantai,Eastern Zhejiang

Miocene,as an important transition period between the establishment of Antarctic ice sheet and the establishment of modern biological communities,is of great significance to the reconstruction of paleoclimate and paleoatmospheric CO₂concentration in this period.However,the reconstruction results of paleoatmospheric CO₂ concentration in Miocene are faced with the problems of low precision and low resolution.The reconstruction results of Geochemistry and palaeobotany are different from 100-500 ppmv.With the rapid development of modern science and technology,the interdisciplinary study of palaeobotany is imperative.With the emergence of new methods and models,it is necessary to update the reconstruction data of the flora which has been reconstructed with CO₂.Using the new model to test and update the results in order to obtain more accurate and more accurate CO₂ concentration data,which can provide valuable support materials for further interpretation of Miocene climate change and solving the remaining problems.As one of the better preserved plant communities in the Cenozoic,the flora in eastern Zhejiang mainly used the stomatal ratio method to reconstruct the paleoclimate in the past.Therefore,three plant fossil materials,Cunninghamia lanceolate,Quercus aff.Delavayi and Machilus tiantaiense,which have been proved to be more sensitive to atmospheric CO₂ concentration,are selected in this paper. Three kinds of plant fossils around Ma were further analyzed to provide more accurate data of paleoatmospheric CO₂ concentration in this area.Among them,the results of carbon isotope model reconstruction of three plant fossils are not ideal,which may be caused by the different assimilation rates between gymnosperms and angiosperms.The results of reconstruction of photosynthetic gas exchange model are very good.The result of Cunninghamia lanceolate is 385.81 ppmv,standard deviation is 8.245;the result of Quercus aff.Delavayi is 362.53 ppmv,standard deviation is 20.51;the result of Machilus tiantaiense is 368.57 ppmv,standard deviation is 10.632.The final result of the paleoatmospheric CO₂ concentration of the late Miocene 10 Ma in Tiantai,Zhejiang Province is 374.23 ppmv.The reconstruction resolution of the photosynthetic gas exchange model of the three plant fossils is much higher than that of other methods.By comparing the results of reconstruction of plant fossils in the same stratum and analyzing the advantages and disadvantages of various methods,the superiority of photosynthetic gas exchange model in principle and results is proved.The comparison of the data shows that the model is more efficient in considering the variables that affect the reconstruction results than the stomatal ratio method.More scientific calculation method also makes the calculation results of the model have higher resolution.In addition,the applicability and accuracy of the cell aspect ratio method were verified by using Chinese fir fossils and living Chinese fir collected from Wenxian County,Gansu,Xi'an,Yunnan,Kunming and Tiantai,Zhejiang Province.It was found that the cell aspect ratio only fit well with one climatic parameter of ancient mean temperature,but not with two climatic parameters of annual rainfall and relative humidity.Finally,the average annual temperature of about 10 Ma in late Miocene in Tiantai,Zhejiang Province is about 23°C.This value is slightly higher than the paleotemperature of late Miocene in eastern Zhejiang reconstructed by other methods.Therefore,this method has a certain research potential in the reconstruction of ancient mean temperature,but the parameters are relatively single,so it is necessary to further improve its formula and increase its rationality.