Late Miocene palynological records of vegetation and climate changes in the Otindag Dune field

The inland arid region of Asia is an environmental system closely related to the Asian monsoon region, and its formation and evolution are closely related to factors such as uplift of the Tibetan Plateau, disappearance of the Tethys Sea, and global cooling. Elucidating the aridification process and trends has notable theoretical and practical significance. The Otindag Dune field is located in the semiarid area north of Yinshan Mountain. Notably, studies on the history of Neogene aridification in the Otindag Dune field are rare. Based on pollen and grain size analysis of the BYNEN profile exposed in the northwestern part of the Otindag Dune field, a pollen-vegetation-climate change sequence was established from similar to 6.37-6.30 Ma. During the late Miocene, the pollen in the Otindag Dune field was dominated by herbaceous pollen originating from Artemisia, Amaranthaceae, and Fabaceae, indicating that the field has become dry since similar to 6.37 Ma and has undergone a process of increasing fluctuating drought in the later stage. The vegetation type changed from sparse forest grassland during similar to 6.37-6.35 Ma to temperate desert grassland for the interval of similar to 6.35-6.30 Ma. A higher Artemisia/Chenopodiaceae (A/C ratio) and tree pollen content indicated a relatively warm and humid climate in the period of similar to 6.37-6.35 Ma, while a lower A/C ratio and tree pollen content indicated a cold and dry climate for similar to 6.35-6.30 Ma. According to the coexistence approach (CA), the temperature and precipitation during the late Miocene were higher than those at present, indicating that the East Asian summer monsoon during the late Miocene was more intense than the current in the Otindag Dune field. By comparing the pollen records of the Otindag Dune field, Chinese Loess Plateau, and Tibetan Plateau, it can be concluded that tree cover was progressively reduced and drought-tolerant species increased as the intensity of the East Asian summer monsoon gradually waned since the late Miocene. On a tectonic scale, the climate of the Otindag Dune field has developed towards colder and drier conditions since the late Miocene, and the characteristics are widely comparable across the arid inland regions in Asia, possibly due to the dual effects of global cooling and Tibetan Plateau tectonic movement.