Betulaceae Fossils From The Pliocene Mangbang Formation In Tengchong,yunnan And Their Paleophytogeographic And Paleoenvironmental Significance

Yunnan is located on the southeastern edge of the Qinghai-Tibet Plateau,with complex topography and diverse climate types.It is one of the centers of plant diversity in our country and even in the East Asia.Tengchong is located on the west side of the Hengduan Mountains(Gaoligong Mountains).Due to the influence of South Asian monsoon and East Asian monsoon,this area forms the particular climate and vegetation types.Neogene strata are developed in this area,and plant fossils are abundant.This paper takes the Pliocene Betulaceae plant fossils in Tengchong,Yunnan as the research object,and conducts research on paleobotany systematics and paleoenvironmental changes,which has important scientific values for further understanding of the Neogene paleoclimate and paleoenvironmental evolution in western Yunnan.In this paper,three genera and six species of Betulaceae plant leaf fossils have been identified(including a new species),namely Alnus cf.cremastogyne,Betula praecylindrostachya sp.nov.,Betula mioluiminfera,Betula yunnanensis,Carpinus subcordata,and Carpinus miofangiana.According to the global fossil records of the genera Alnus,Betula and Carpinus,we carry out a study of their paleophytogeography,discussing the origin and spread of the three genera.It is inferred that the Alnus originated in the temperate region of East Asia in the Late Cretaceous,possibly in Japan or Northeastern China.The Alnus genus spread to Europe and the North America by different routes,reaching Europe and America during the Late Cretaceous,and spread to western Yunnan at the Eocene or earlier period.Betula may also have originated in Japan or Northeastern China,and the origin time may be between the Late Cretaceous and the Early Paleocene.Betula plants had grown in western Yunnan in the Oligocene or earlier.On the basis of fossil records,as well as the global distribution characteristics of extant species of Carpinus,we concluded that Carpinus originated in East Asia during the Paleocene,and was prosperous during the Miocene.Taking Betulaceae plant leaf fossils as the research materials,the Pliocene paleo-atmospheric CO₂ concentration in the Tengchong area of Yunnan was reconstructed by using the photosynthetic gas exchange model and the carbon isotope method for the first time.Accurate CO₂ concentration data in the late Pliocene can be obtained based on the comparison of the two methods and the stomatal ratio method.The average values of the reconstructed paleo-atmospheric CO₂concentrations by the stomatal ratio method,the photosynthetic gas exchange model and the carbon isotope model are 453 ppmv,466 ppmv,and 334 ppmv,respectively.The paleo-atmospheric CO₂concentration values recovered by the carbon isotope model and the photosynthetic gas exchange model are compared with the paleo-atmospheric CO₂ concentration values obtained by the stomatal ratio method.The reconstruction results of the three methods are basically the same,and the reconstruction results have strong credibility.In addition,we use the coexistence approach to quantitatively reconstruct the paleoclimate characteristics of the Tengchong area in Yunnan during the late Pliocene.The result shows that the mean annual precipitation is 1014-1478 mm,the annual actual evaporation is 695-938 mm,the mean annual temperature is 13.8-17.9?,the mean temperature of the warmest month is 18.4-27.1?,and the altitude is 700-2100 m.Therefore,we speculate the Pliocene climate in the Tengchong area is warm and humid.