Miocene Palynoflora From Zhejiang-Fujian Area,Southeast China And Its Palaeovegetational And Palaeoenvironmental Implications

During the late Cenozoic,frequent basaltic lava eruptions occurred along the coastal regions of Zhejiang-Fujian area,forming a set of strata consisting of basaltic rocks with sediments,which are mainly scattered in east Zhejiang Province(Shengxian Formation)and southeast Fujian Province(Fotan Formation)respectively.In recent decades,both megafossil plants and sporopollen from these sediments were used to determine the stratigraphic ages and analyze the palaeovegetation and palaeoenvironment,but the issue on the vegetational and environmental evolution of the fossil sites in a long time scale has still not been solved.On the basis of previous studies,we established or complemented the Miocene palynological sequences of Shengxian and Fotan Formation by pollen analysis.In the meanwhile,the infrageneric classification of Quercus and discrimination of evergreen and deciduous Liquidambar pollen were carried out through the analyses of pollen sculpture under scanning electron microscope(SEM).Then the above results were used to reconstruct the Miocene vegetational and environmental changes of east Zhejiang and southeast Fujian.The main research contents and conclusions are listed as follows:Based on the pollen analysis of 80 samples from 7 sections of Shengzhou-Tiantai-Ninghai area in Zhejiang and 118 samples from 7 sections of Zhangpu-Longhai area and Mingxi in Fujian,the palynological sequences of Shengxian Formation and Fotan Formation were established or complemented.The Miocene palynological sequence of the Shengxian Formation consists of the late Early-early Middle Miocene Quercus E.-Liquidambar-Carya assemblage,the late Middle-early Late Miocene Quercus E.-Pinus-Trapa assemblage and the late Late Miocene Quercus E.-Fagus-Artemisia assemblage;The Miocene palynological sequence of the Fotan Formation consists of the middle-late Early Miocene Quercus E.-Poaceae assemblage,the late Early-early Middle Miocene Quercus E.-Cyclobalanopsis-Hamamelidaceae assemblage and Quercus E.-Trapa assemblage,the late Middle-Late Miocene Hamamelidaceae-Celtis-Polypodiaceae assemblage and the subsequent three successive assemblages,i.e.,Hamamelidaceae-Symplocos assemblage,Symplocos-Liquidambar assemblage and Quercus E.-Liquidambar-Pinus assemblage.On the basis of the palynological sequence of the Fotan Formation,we recognized at least 3 wildfire events that caused severe vegetation damage as well as several low-intensity wildfires that disturbed the recovery of vegetation?this is the first report about the wildfire events as well as the subsequent history of vegetation succession in the study area.During the Miocene,the multiple basic lava eruptions probably were the main cause for the frequent wildfire events in our study area.In the meantime,the significant seasonal distribution of precipitation and the relatively high temperature,especially during the dry season,were also favorable conditions for the occurrence of wildfires.Besides,a series of hydrological and ecological changes occurred after burning as both soil erosion and weathering of rocks enhanced,and Concentricystes thrived due to the eutrophic water bodies.Based on the patterns of tectum ornamentation,84 Quercus pollen grains from Shengxian and Fotan palynoflorae were classified into different infrageneric groups,which are dominated by Quercus Group Cyclobalanopsis,then followed Q.Group Ilex,and with a small amount of Q.Group Protobalanus in both of the two palynoflorae,indicating that the Miocene vegetation in east Zhejiang and southeast Fujian were mountain forests with a certain altitudinal gradient.The occurrence of the pollen of alpine sclerophyllous oaks in Shengxian palynoflora indicates that during the Miocene there existed high mountains in east Zhejiang.And the composition of Q.Group Cyclobalanopsis of the two palynoflorae demonstrates that during the Miocene the vegetation of the lower elevation in east Zhejiang was similar to the typical subtropical evergreen broad-leaved forests,while that in southeast Fujian was the subtropical evergreen broad-leaved forests with distinct tropical features.In addition,the observational results of the above Quercus pollen by "single-grain techniques" support the discrimination between its evergreen and deciduous type using light microscope in our study.The tectum perforation of temperate and subtropical species(mainly deciduous)of extant Liquidambar is relatively bigger,while that of the tropical ones(mainly evergreen)is much smaller and even being totally filled.Based on this observation,32 Liquidambar pollen grains,which are from the Quercus E.-Liquidambar-Pinus assemblage of the Fotan Formation,were investigated using SEM and classified into 4 types based on the characteristics of the tectum perforation.Type ? and ? are characteristic of extreme tiny or filled tectum perforation,which are comparable to some of the extant evergreen species;type ? is characteristic of nonuniformly distributed perforation with larger diameter and irregular shape,which is comparable to some of the extant deciduous species;and type ? shares features with type ?except with a little larger diameter of perforation.Therefore,Liquidambar from the Quercus E.-Liquidambar-Pinus assemblage are comprised of both evergreen and deciduous species,and the statistics show that the former outnumbers the latter.Then the co-occurrence of Liquidambar(including evergreen and deciduous species)and Carya in the above assemblage demonstrates that the vegetation of southeast Fujian during the late Middle-Late Miocene had similar characteristics with modern south subtropical monsoon evergreen broad-leaved forests.Based on the above palynological sequences and megafossil plant records,together with the information deduced from the wildfire events and the composition of Quercus and Liquidambar,the evolutionary history of vegetation and environment during the Miocene in east Zhejiang and southeast Fujian were reconstructed.The results show that under the influence of the global climate variation,the changes of the ecological environment in the study area during the Miocene mainly manifested as the vertical migration of altitudinal belt of vegetation.Meanwhile,as the major driving factor for the evolution of East Asian climate since the very beginning of the Miocene,global ice volume/temperature changes controlled the wet/dry characteristics of the study area,having a great impact on the vegetation evolution-especially since the end of the Middle Miocene Climatic Optimum,significant stepwise global cooling not only led to the downward migration of altitudinal belt of vegetation,but also caused the reduction of water vapor,bringing about the appearance or increase of xerophytic plants.In addition,the frequent late Cenozoic volcanic activities also had a great impact on the vegetation and environment—in southeast Fujian,multiple basic lava eruptions caused frequent wildfires,which destructed the vegetation and disturbed the subsequent succession;while the appearance of abundant papillae on the cuticle of fossil Fokienia foliage from the Shengxian Formation was also regarded as its response to the poisonous atmosphere during the volcanic eruptions.