Early Cretaceous Plant Fossils And Paleoenvironment Of Zhongkouzi Basin In Beishan Area

The Cretaceous is consider as the typical?greenhouse climate?period.Many globally important geological events occurred during this period,resulting in major changes in the global environment,climate and flora.Therefore,studying the paleoenvironment and paleoclimate of this period is of great significance for understanding the evolution of the terrestrial environment as well as predicting future climate and environmental changes.The Cretaceous strata are relatively developed in northwest China and yielded abundant plant fossils.However,a few plant fossils have been studied and reported due to factors such as inconvenient transportation,inaccessible environment and poor preservation conditions.In the present paper,we systematically study plant fossils and fossil spores from the Early Cretaceous Chijinbao Formation in the Zhongkouzi Basin in the Beishan area for the first time.Geochemical,carbon and oxygen isotope and colorimetric analyses were also carried out,which led to the reconstruction of paleoclimate and paleoenvironment in the Zhongkouzi Basin during the Early Cretaceous,providing an important basis for the study of Cretaceous environmental changes in the Beishan region.In this study,the plant megafossils from the middle of the Chijinbao Formation,Zhongkouzi Basin,Beishan area were investigated.On the basis of detailed comparisons of gross morphological and microscopiccharacters,20 genera and 43 species of fossil plants were identified and described,including 95.4%of gymnosperms,2.3%of ferns and 2.3%of angiosperms,and 7 new species.This enriches the early Cretaceous plant fossils and their provenance,and also provides a reliable basis for discussing the classification and relationship of plant fossils.Meanwhile,the evolution and paleogeographic distribution characteristics of Dryopterites,Elatides,Ephedraceae,and angiosperms are discussed in depth.The characteristics of frond and in situ spores Dryopterites beishanensis sp.nov.was studied for the first time from northwest China.According to the fossil record of Dryopterites,we concluded that the genus Dryopterites most likely originated in East Asia in the Hauterivian-Aptian of the Early Cretaceous.The fossils of Ephedra from the Early Cretaceous in northwest China were studied in detail for the first time.A new genus Dichoephedra and a new species Dichoephedra beishanensis sp.nov.were established according to the different branching patterns of the upper and lower branches.It was concluded that the unusual changes in the branching morphology of the current specimen are important for the evolution of the branching morphology of Ephedra plants,which may represent a transition type from alternate to opposite branching.The occurrence of Dichoephedra may provide new clues to the connection between Gnetales and early angiosperms.Stephanofolium beishanensissp.nov.,a Menispermaceae fossil from the Early Cretaceous Chijinbao Formation in northwest China,was studied.It is suggested that Stephanofolium beishanensis may be a branch in the evolution of angiosperms,and may be the same ancestor as the extant Nymphaeaceae,which provides a new fossil evidence for Nymphaeaceae as a basal group of angiosperms.Three new species,Podozamites mazongshanensis sp.nov.,Cupressinocladus sumuensis sp.nov.and Cupressinocladus mazhongshanensis sp.nov.were established based on the macroscopic characteristics and partial microscopic structure.They enrich the fossil record of Early Cretaceous plants in northwest China.On the basis of the cuticular studies,Ginkgoites sibirica is used to reconstruct the paleo-CO₂concentration.The paleo-CO₂ was 548-1323 ppmv during the Hauterivian to Aptian based on the stomatal ratio method and the leaf gas-exchange model,which is consistent with the results of other studies.The comparison with other results shows that the photosynthetic gas exchange model can be used as an effective way to restore the paleoclimate change.The CO₂ variation trend during the Early Jurassic to the Early Cretaceous is reconstructed based on the leaf gas-exchange model.The results showed that the CO₂ concentration was high in the Early and Middle Jurassic,with a decreasing trend toward the Cretaceous,and then show a significant increasing during the late Aptian to latest Albian.Based on the variation of geochemical elements Cu,Ti,Mn,Sr,P content and Rb/K,w,V/Ni,Th/U,Sr/Ba,Sr/Cu,Sr/Ca,Fe/Mn,Rb/Sr,Mg/Sr,Mg/Ca,CIA values and the characteristics of chromaticity data sequences,the central part of the Chijinbao Formation in the Zhongkouzi Basin was recovered as a semi-saline saline-lacustrine depositional environment.From the old to the new,the overall climate shows a trend from warm and humid to long periods of aridity and hot to warm and humid again,experiencing a number of climate fluctuations of evolving wet and dry in between.The temperature shows a process of change from slowly decreasing to increasing and then decreasing.We established the variation sequence of carbon and oxygen isotope values in the central part of the Chijinbao Formation in the Zhongkouzi Basin,and recovered that its formation environment may be a saline lake phase exposure environment,with the temperature of the lake surface layer in summer ranging from 27.87-56.21℃.The temperature variation experienced a decreasing-increasing-decreasing-increasing-decreasing-increasing trend.56 genera and 84 species of sporopollen fossils were identified and described and three palynologic assemblages were divided based on the pattern of spore longitudinal distribution and the change of sporopollen abundance.The relative abundance of fern pollen is 28.5%,the angiosperm pollen contributed 63.5%of the total content,and the relative abundance of angiosperm is 9.0%.Based on the ecological data of the parent plants in each sporopollen assemblage,it is inferred that the vegetation types in the study area underwent atransition from semi-dry and semi-humid coniferous forest and shrub of southern subtropics to semi-arid coniferous forest and shrub of subtropics,and then to semi-humid coniferous forest of northern subtropics in a stratigraphically upward order.The geological age of Chijinbao Formation was determined to be Early Cretaceous（Hauterivian-Aptian）according to the special palynomorph assemblages with previous research results.Finally,according to the analysis of plant megafossils assemblages from the Chijinbao Formation,we reconstructed the flora of the middle Early Cretaceous in the study area and compared with the flora of the same period in China.The results indicate that the Early Cretaceous flora of the Zhongkouzi Basin in the Beishan region belongs to Early Cretaceous mixed type floristic provinces,which has the characteristics of both the Northern Floristic Province and Southern Floristic Province,and it is closer to the Northern Floristic Province according to the spore-pollen characters.Based on the plant macrofossil assemblages and sporepollen assemblages indicate an overall arid climate and locally humid climate in the study area during the Early Cretaceous.