Long-term arid and hot climate during early Mid-Cretaceous indicate by unvaried Cheirolepidiaceae-dominant palynoflora from the Liupanshan Basin, China

The Mid-Cretaceous was characterized by long-term and stabilized high temperatures confirmed by numerous palaeoclimatic information from marine sedimentary archives. However, few terrestrial records have yet been employed to verify the terrestrial humidity under this long-term hot climate. The Liupanshan Basin, located in central China during the Cretaceous contains thick Aptian-Albian sediments. Here, we reconstructed a long-term palaeovegetational record during late Aptian to Albian from the Huoshizhai Section and Sikouzi Section, Liupanshan Basin. Five palynological assemblages were defined from the bottom to top of these strata, and a total of 124 types of palynomorphs including 47 spore species and 77 gymnosperm pollen species were recognized in the palynological assemblages. Sporopollen analysis revealed that the spore-pollen spectra of Huoshizhai Section were dominated by Perinopollenites and Bisaccate pollen. In Sikouzi section, the Classopollis content dominate in all samples (average 63.8%), with a few other pollen, including conifer bisaccates types, Perinopollenites, Jiaohepollis, Ephedripites, Cycadopites and a few spores. The sporopollen assemblage reflected that the flora of the central Asia continent gradually changed from Taxodiacean-dominated to Cheirolepidiaceae-dominated in the late Aptian and the Albian, which further indicates that the central Asia continent palaeoclimate gradually changed from a wet and cold climate to a long-term dry and hot climate with small fluctuation. In addition, the Classopollis content also presented a slightly increased trend from late Aptian to late Albian, which probably indicated a gradually intensified hot and dry climate. The thermophilic floral changes and warmer/cooler ratio of the Sporomorph Ecogroup Model at the middle Madonghshan Formation also shows slightly decreased trends which correspond to the global short-term cooling events at the Aptian-Albian transition.