Several Cenozoic Plant Fossils From Nima, Xizang And Qumalai,Qinghai And Paleoenvironmental Significance

The climate patterns of China and the world have been affected by the uplift of the Qinghai–Tibet Plateau.The uplift of the Qinghai–Tibet Plateau resulted from the collision between the Indian and Eurasian Plates.Therefore,it is a natural region for the study of the collision orogeny,continental deformation,and the associated geodynamics.Plant fossils and organic collected from the Cenozoic sediments have recorded the uplift process and environmental changes of the Qinghai–Tibet Plateau in the Nima Basin,Xizang,and Qumalai,Qinghai.Organic carbon isotopes are a direct record of the biological processes of the earth and an important aspect of the evolution of the plant ecosystem.Plant fossils collected from the Cenozoic sediments of this plateau can also provide significant information about plant systematics and evolution.Furthermore,these plant fossils can help in understanding the paleoaltitude changes,formation mechanism,paleoclimatic evolution,and paleoenvironmental changes during the Cenozoic.This thesis systematically describes several Cenozoic plant and spore fossils from the high–altitude areas of Nima,Xizang,and Qumalai,Qinghai.In Total,five species belonging to four genera,four families?two species of pteridophytes and three species of angiosperm plant fossils?,and six palynomorphs are reported in this study.The plant and spore fossils were studied in detailed from the Nima Basin.The sediments from the Nima Basin were studied using trace element analysis,and the detailed carbon isotope compositions of the organic matter were analyzed.The anatomical features,the carbon isotope compositions of the organic matter,and the fillers in worm trails of two species were also studied in detail for the fossils from Qumalai.This is the first report on three genera?Equisetum cf.oppoistum,Equisetum sp.and Solidago sp.?and the identification of six palynomorphs?Osmundacidites,Foliopollenites qaidamensis,Camptotriletes plicatus,Fusidiporosporonites minutaestriatus,Diporisporites maximus,and Fungal spore?,which were collected from the Paleocene–Eocene Niubao Formation of the Nima Basin,Xizang.We selected Equisetum pretense as the nearest living relatives?NLRs?of Equisetum cf.oppoistum and Solidago virgaurea as the NLRs of Solidago sp.The paleoaltitude of this basin was reconstructed as 2,094–3,514 m using the Coexistence Approach during the Paleocene–Eocene.The Nima Basin and Lunpola Basin are closely similar in terms of age,sedimentary fill and developmental history.According to sporopollen fossils,the paleoaltitude of the Lunpola Basin was reconstructed as 2,300–2,600 m during the Paleocene–Eocene,which is consistent with the paleoaltitude of the Nima Basin.The current fossils were collected from an altitude of approximately 5,200 m,which suggests that the central Qinghai–Tibet Plateau has uplifted by 2,000–3,000 m since 40 million years ago.The Rhamnaceae and Fagaceae records are the first of these genera that have been collected from the Early Pleistocene Yeniugou Formation of Qumalai.The coprolite of the mature and immature oribatid has been found in Rhamnaceae and Fagaceae.The main species of Rhamnaceae and Fagaceae were seleceted as the NLRs of the current fossil woods,and the highest and lowest elevation of the distribution of their nearest living relatives were determined.The paleoaltitude of Qumalai is estimated to range between 749 and 3,549 m using the coexisting approach during the Early Pleistocene.The fossils were collected from a present-day altitude of approximately 4,500 m,which suggests that the northeastern Qinghai–Tibet Plateau has been uplifted approximately1,000–4,000 m over the past one million years.According to the trace element content and the ratio change in sections P1708,2017XD,and2017XD,the environmental change in the Nima Basin during the Paleocene–Eocene can be divided into nine ranges.This suggests that the basin experienced nine stages in the life cycle of the lakes:initial formation and expansion,development,shrinking,expansion,shrinking,expansion,shrinking,and expansion,and coexistence.The climate of the Nima Basin experienced fluctuations during the Paleocene–Eocene transition with oscillations between wet–a moderately dry–warm wet–a moderately dry–warm wet–a moderately dry–warm wet–a moderately dry–warm humid.The decrease in the organic carbon isotope values may be related to the Paleocene–Eocene Thermal Maximum?PETM?in the Nima Basin during the Paleocene–Eocene.However,the organic?¹³C carbon isotope values were distributed from-28.92‰to-23.66‰,indicating that the source of the organic matter in the Nima Basin was mainly the terrestrial C₃plant.The presence of Equisetum cf.oppositum implies that a significant seasonal variation already existed in the Nima Basin during the Paleocene–Eocene.Combining this information with the spore,plant,and animal fossil data from the Lunpola Basin suggests that the local environment in the Nima Basin was warm and wet during the Paleocene–Eocene.The paleoclimate of the Niubao formation of the Lunpola Basin was quantitatively reconstructed based on the Coexistence Approach of the NLRs of 22 fossil species from the flora.The paleoclimate of the Lunpola Basin during the Paleocene–Eocene was characterized by a mean annual temperature?MAT?range of15.8°C–17.3°C,a mean temperature of the warmest month?MWMT?range of 23°C–28°C,a mean temperature of the coldest month?MCMT?range of 2.4°C–5.4°C,a difference in temperature between the coldest and warmest month?DT?ranging between 23.3°C and 24.8°C,a mean annual precipitation?MAP?ranging between 1011.3 and 1653.5 mm,a mean maximum monthly precipitation?MMaP?ranging between 161.8 and 283.7 mm,and a mean minimum monthly precipitation?MMiP?ranging between 24.1 and 31.4 mm.The results demonstrate that the climate parameter of air temperature and precipitation in the Nima Basin during the Paleocene–Eocene were less than the present-day values.The results of the vulnerability index analysis for Rhamnaceae was 0.2<1,and that for the mesomorphy ratio analysis in the family was 75<88.5<200.These indicate that the current fossil woods bear a xeromorphic structure,suggesting that the paleoenvironment was mesophytic in Qumalai during the Early Pleistocene.The result of the vulnerability index analysis for Fagaceae was 1<2<2.5,and that for the mesomorphy ratio analysis in the family was 465>200.These indicate that the current fossil woods bear a neutral structure,suggesting that the paleoenvironment was humid in Qumalai during the Early Pleistocene.Based on the Energy Dispersive Spectrum?EDS?analysis,the paleoenvironment was humid and warm with more rainfall in Qumalai during the Early Pleistocene.In summary,there was a relatively warm and humid environment in Qumalai with more precipitation during the Early Pleistocene.The Ring Markedness Index in Rhamnaceae demonstrates that the plant grows in a slightly seasonal environment.The late wood percentage in the growth rings has a reducing trend based on linear fitting,which demonstrates that rainfall decreases in winter before plant growth.This suggests that the East Asian winter monsoon had an increasing trend during the Early Pleistocene.The carbon isotope compositions of organic matter were measured from several rings of Rhamnaceae and Fagaceae.Comparing the patterns of the carbon isotope values across the tree rings in deciduous and evergreen species with extant species suggests that Rhamnaceae and Fagaceae are deciduousness.Using the coexisting approach,the carbon isotope composition of the fossil wood suggests that the?¹³C_CO2values ranged from-7.5‰to-3.7‰in the northeastern Qinghai–Tibet Plateau during the Early Pleistocene.The average values of the?¹³C carbon isotope in the growth rings and the?¹³C_CO2carbon isotope of the atmosphere were also calculated.The linear fitting results suggest that the?¹³C carbon isotope values in the growth ring had a decreasing trend in Qumalai during the Early Pleistocene,and it suggests that the plants grew in a suitable environment.This may have been associated with the East Asian summer monsoon first decreasing and then increasing.The linear fitting results suggest that the?¹³C_CO2carbon isotope values in the growth ring had a decreasing trend in Qumalai during the Early Pleistocene.This may have caused the negative deviation of the carbon isotope between the end of Pliocene and Pleistocene.This negative deviation may have also been associated with the East Asian winter monsoon,which has a tendency to strengthen.This suggests that the East Asian winter monsoon had an increasing trend in Qumalai during the Early Pleistocene,which is consistent with the records from the South China Sea and Loess Plateau.The intensification of the East Asian winter monsoon is related to the Antarctic and Arctic ice expansion,global cooling,or uplift of the Qinghai–Tibet Plateau.The carbon isotope composition of the cellulose in the late wood of the current fossil wood suggests that in August,the average rainfall ranged between 1032 and 1078mm with an average temperature range of 0.73°C-1.36°C and an average relative humidity range of 121%-123%in Qumalai during the Early Pleistocene.