Miocene Sporopollen Assemblage From The Wulan Basin,Qinghai Province And Its Paleoclimate And Paleoelevation Significance

The uplift of the Tibetan Plateau has a significant impact on global climate changes and plant evolution.As the largest Cenozoic basin on the Tibetan Plateau,the Qaidam Basin is a natural laboratory for studying the uplift and climate evolution of the Tibetan Plateau.Based on the paleomagnetic chronology of the Wulan Basin in the northeastern margin of the Tibetan Plateau,the Miocene sporopollen flora of Youshashan Formation was studied to renew its paleovegetation,reconstruct its paleoclimate and paleoelevation,and discuss its paleoenvironmental significance and its response to global climate change and the uplift of the Tibetan Plateau.In this paper,172 sporopollen samples were collected from Youshashan Formation in the Wulan Basin.After experimental treatment,24 samples were rich in sporopollen,and 36 genera of 35 families were identified,among which gymnosperms had the highest pollen content(46.75%),angiosperms had the second(44.44%)and ferns had the lowest spore content(8.81%).According to the sporopollen assemblage characteristics of the Miocene Youshashan Formation in the Wulan Basin,four sporopollen zones are divided: Zone I(18～15.2 Ma): Aster-Picea-Artemisia-Ephedra;Zone II(15.2～12.5 Ma): Aster-Artemisia-Polygonaceae-Gramineae;Zone III(12.5～11.8Ma): Pinus-Aster-Picea-Ephedra;Zone IV(11.8～9.7 Ma): Picea-RosaceaePolygonaceae-Gramineae.According to the characteristics of the parent vegetation in the sporopollen assemblage zone,the Miocene paleovegetation evolution process and climate change process in the Wulan Basin were qualitatively restored.The climate types divided according to the sporopollen assemblage zone are: warm temperate semi-humid climate(Zone I)-warm temperate humid climate(Zone II)-warm temperate semi-arid climate(Zone III)-warm temperate semi-humid climate(Zone IV).Combined with the change of sporopollen content,it was found that Artemisia and Chenopodiaceae increased around 13 Ma,and ephedra increased around 12.4 Ma,which may lead to two shortterm droughts.Combined with the Coexistence Analysis method,the paleoclimate and paleoelevation of the Wulan Basin in 18～9.7 Ma were reconstructed quantitatively.The paleoclimate of the four sporopollen assemblage zones mainly experienced four stages:warm dry(Zone Ⅰ)-warm wet(Zone Ⅱ)-cold dry(Zone Ⅲ)-warm wet(Zone Ⅳ),which was quite different from the present climate,and the reason may be that it was different from the “Mid-Miocene Climatic Optimum”,“Mid-Miocene Climate Transition” and the uplift of the northern margin of the Tibetan Plateau.After temperature correction,it is found that the reconstructed paleoelevation is close to the present elevation after12.5 Ma,which indicates that tectonic movement occurred in the fault Zone around the Wulan basin after 12.5 Ma.The latest palynological TP/TPAP method was used to quantitatively reconstruct the Paleo mid-range of the mountains around the Wulan Basin from 12.5 to 9.7 Ma,and the paleoelevation of the Wulan Basin from 18 to 9.7 Ma was reconstructed according to the mean annual temperature recovered by the Coexistence Analysis.After 12.5～9.7Ma,the Wulan Basin may have experienced uplift,which may be related to the uplift of the northern margin of the Tibetan Plateau in the Late Miocene,mainly the rightlateral strike-slip fault activity of the Elashan Mountain and two small faults in the northern margin of Qaidam,and the strike-slip of the Wenquan fault may also have an impact on the structural changes of the Wulan Basin.