Fossil Plants From The Lower Cretaceous Of Yumen, Gansu Province And Palaeoenvironmental Reconstruction

The Cretaceous, characterized by higher globally averaged surface temperature and sea level, is regarded as the typical longest greenhouse period. During this period, series of geological tectonic activities happened and resulted in numerous anomalous events, and the vegetation changed all over the world. Accordingly, the studies on the Early Cretaceous plant fossils are contributed to understanding the terrestrial ecosystem, palaeoenvironment and palaeoclimate in this stage.The Cretaceous strata, preserve abundant fossil plants and animals, are well developed in China. The fossils provide valuable materials to biostratigraphy researches and palaeoclimatic environment reconstruction. The present paper mainly focuses on the plant fossils collected from the Lower Cretaceous Chijinbao Formation, Xiagou Formation and Zhonggou Formation of Yumen, Gansu Province, and is purposed on the palaeoclimatic and palaeoenvironmental studies during the Early Cretaceous of northwest Gansu, NW China. Based on the systematic field fossil collection,22 genera and 32 species of fossil plants were identified and described indoors. The anatomical microstructures of some compressed plant fossils were investigated by making use of a light microscope (LM) and a scanning electron microscope (SEM), and the combination of the morphological and micromorphological studies improves the accuracy of identification and provides a reliable basis to the classification and affinity of fossil plants.Studies show that the fossil plant taxa from the Chijinbao Formation and Xiagou Formation are relatively monotonous, mainly comprised of plants lived under warm and humid environment, such as Sphenobaiera、Solenites、Pityocladus, Pityolepis, Cunninghamia and Podozamites. Some xerophytic plants such as Pagiophyllum and Cupressinocladus also occurred in the Xiagou Formation. While the fossil plant taxa of the Zhonggou Formation is relatively rich, mainly dominated by xerothermic scale-leaved conifers, but a certain amount of plants adapt to moist and warm conditions are occurred too, including coniferous Brachyphyllum, Pseudofrenelopsis, Pagiophyllum, Elatides, Athrotaxites, Podozamites, Cupressinocladus, Pityospermum and Carpolithus, ferns Equisetum, Cladophlebis, Sphenopteris; and cycads Dictyozamites and Ctenis, as well as the early possible angiosperm Beipiaooa. The variation of the Early Cretaceous vegetation of Yumen suggests a tendency of a hot and arid palaeoclimate from the early to late Early Cretaceous. The floral assemblage displays a strong affinity with the mixed-type floristic province, between the south floristic province and north floristic province of China. The vegetation is strongly influenced by the climatic fluctuation.Moreover, the uppermost part of the Zhonggou Formation was divided into six members based on the lithological characteristics. The fossil spores and pollens from 1-5 members were analyzed, and 26 species in 15 genera spores of ferns and bryophyte,37 species in 29 genera of gymnosperm pollens and 2 species in 2 genera of angiosperm pollens were identified. Three assemblages were recognized according to the spore-pollen characters. Both the plant macrofossil assemblages and spore-pollen assemblages of the uppermost of the Zhonggou Formation indicate an obviously dry and wet climate changes during the late Early Cretaceous in the area, while the fragmentarily occurrence of the hydromorphic taxa in the certain members suggests a locally wet habitat in a holistic arid climatic environment. In addition, the presence of Equisetum tubers and Athrotaxites multistage twigs implies that significant seasonal variation had already formed in Yumen by the late Early Cretaceous.On the basis of the cuticular studies, the species of Brachyphyllum and Pseudofrenelopsis from the Zhonggou Formation are used to reconstruct the palaeoatmospheric CO2 concentration in the late Early Cretaceous. Based on the stomatal ratios of fossil species and their nearest living equivalent species, the palaeoatmospheric CO2 concentration are about 530-1000ppmv during the late Early Cretaceous. The decrease trend of the palaeoatmospheric CO2 concentration is reflected, which may suggest a cooling trend of the palaeoclimate during the late Early Cretaceous in Yumen and provide information of the terrestrial plants response to the oceanic Aptian-Albian anoxic events (OAE1). The palaeoatmospheric CO2 concentration valued from our study is consistent with that of the previous studies deduced by Brachyphyllum species and GEOCARBⅡ. It shows that the species of Brachyphyllum and Pseudofrenelopsis (Cheirolepidiaceae) can be used as indictors of the palaeoatmospheric CO2 concentration, and the GEOCARB II model is available for the reconstruction of the palaeoatmospheric CO2 concentration during the Mesozoic.