| The uplift of Tibetan Plateau not only has induced or strengthened the Asian monsoon circulation and aridification of the Asian interior,but also possibly has exerted deep impact both on the process of global cooling and the formation of the northern hemisphere ice sheet for it changed the Asian atmospheric circulation pattern since the Cenozoic.These important scientific problems,such as the time and amplitude of uplift of the Tibetan Plateau,the origin and development of the Asian monsoon and aridification of the Asian interior,has been receiveing attention by researchers.In addition,the Tibetan plateau and its adjacent regions,where live a large number of endemic species and its time of speciation and speciational evolution are related with the regional tectonic activity and climate change,is one of the hotspots of biodiversity in the world.Furthermore,researching on the biological evolution can also provide a unique perspective to understand the uplift of Tibetan Plateau and its environmental effects.The Longzhong Basin lies at the northeastern Tibetan Plateau,which is geomorphologically located at transition part of the Tibetan Plateau and the Chinese Loess Plateau,tectonically at the Tibetan plateau northeastward extends to the most front place,geographically at the junction of the Tibetan Plateau,the monsoon region and the northweast arid area.Therefore,this place is sensitive to climate change and tectonic response.The Tianshui basin,which is a sub-basin of the Longzhong basin,contains a widely distributed,continuous record of plants and mammalian fossil-rich sedimtens,recording the process of the uplift of Tibetan Plateau and its climate effects.Having lots of advantages such as hard and compact extine,anti-corrosion,large production,widespread and easy to save into fossils,sporopollen became one of the most important proxies to study paleoclimate and geomorphic evolution.Here we present the detailed palynological records of three sections(Yaodian,Lamashan and Huinigou)from the Tianshui Basin,reconstructing the vegetation sucession and climate change since the late Miocene,and explore its influence mechanism.Integrating with the achievements of Cenozoic paleoaltimetry reconstruction,the flora which have elevational significance and the species evolution researchs in the Tibetan plateau and its adjacent regions,we provide preliminary results on the process and amplitude of the uplift of Tibetan Plateau.The main results and conlusions are listed as follows:1.The evolution of vegetation and climate change since late Miocene in the Tianshui basin are in well comparison with those from adjacent regions.A temperate forest with a rather humid climate existed in the basin between 11.4 and 10.1 Ma,followed by a temperate open forest environment with a less humid climate between 10.1 and 7.4 Ma,then gave way to an open temperate forest–steppe environment with a relatively arid climate between 7.4 and 6.4 Ma,and arid steppe esisted in the interior basin between 3.6 and 3.2 Ma.After that,a subalpine coniferous forest developed in the basin between 3.2 and 2.6Ma with temperate cool and moist climate.2.The vegetation succession demonstrates that the aridification of the Asian interior occurred after ~7–8 Ma,which is confirmed by other evidence from Asia.Furthermore,the aridification trend on the northeastern Tibetan Plateau paralleled the global cooling of the late Miocene;the stepwise vegetation succession is consistent with the major uplift of the northeastern Tibetan Plateau during this time.These integrated environmental proxies indicate that the long-term global cooling and the Tibetan Plateau uplift caused the late Miocene aridification of the Asian interior.The late Pliocene humid climate attributed to this phenomenon may be that the strong uplift of NE Tibetan Plateau in the late Pliocene induced intense topography effects in the piedmont zone in Tianshui area,where generated orographic precipitation and hygrophilous subalpine coniferous forest was distributed,as well as the strengthen of the east Asian summer monsoon.3.The fine laminated sediments in the Lamashan formation formed in warm and humid climatic conditions.Using the method of Coexistence Analysis to reconstruct paleoclimate,seven climate indexes are calculated: the mean annual temperature 17.1(12.1–22.1)°C,the mean temperature of the warmest month 24.5(21.0–28.0)°C,the mean temperature of the coldest month 4.4(1.8–7.0)°C,the difference of temperature between the coldest and warmest months 19(13–25)°C,the mean annual precipitation 930(655–1204)mm,the maximum monthly precipitation 220(191–248)mm and the minimum monthly precipitation 13(7–19)mm.It suggests a warmer and wetter climate than today,similar to that of present-day Ziyang and Suining County,Sichuan Province,southwest China,with subtropical monsoon climate.Meanwhile,from the palynological and grain size evidences,we can speculate that the fine laminated sediment in Lamashan formation is varve(dark laminae forming in winter and spring,pale laminae forming in summer and fall),which can further confirm that a monsoon climate dominate the Tianshui Basin during the late Pliocene.4.Intergrated the Picea and Abies pollen distribution,we found that subalpine coniferous forest did not widely distributed in the plateau and its adjacent regions untill the late Pliocene,indicating the uplift of the Tibetan Plateau during that time.During the Quaternary,accompany with the further uplift of the Tibetan Plateau and the global cooling,subalpine coniferous forest existed in surrounding mountains of the Tibetan Plateau,and expanded downward area,meadow or desert appeared in internal region of the Tibetan Plateau.The studies from molecular phylogenetics and phylogeography indicated that the formation of endemic species and the differentiation time of its ancestry species in Tibetan plateau and its adjacent regions concentrated on the late Miocene and Pliocene,and intraspecific differentiations were accelerating during the Quaternary.The coincidence about the same differentiation time in diverse species may be implied that the evolution of plateau species was affected by the uplift of Tibetan Plateau during late Miocene-Pliocene. |
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