|Alternative Title||Several Cenozoic plant fossils from Nima, Xizang and Qumalai, Qinghai and paleoenvironmental significance|
|Place of Conferral||兰州|
|Keyword||青藏高原 新生代 植物化石 古海拔 古环境 东亚季风|
首次报道了西藏尼玛盆地古新统-始新统牛堡组的植物化石和孢子化石，植物化石鉴定为有节类对茎木贼（相似种）Equisetum cf. oppoistum、木贼（未定种）Equisetum sp.和被子植物一枝黄花属（未定种）Solidago sp.，孢子化石鉴定为紫萁孢属Osmundacidites、柴达木叶菌孢Foliopollenites qaidamensis、褶皱冠脊孢Camptotriletes plicatus、细肋纺锤双孔菌孢Fusidiporosporonites minutaestriatus、巨形双孔孢Diporisporites maximus、菌孢Fungal spore。选取现生草问荆Equisetum pratense作为Equisetum cf. oppoistum的现存最近亲缘种，选取现生毛果一枝黄花Solidago virgaurea作为Solidago sp.的现存最近亲缘种。运用共存分析法定量重建该盆地在古新世-始新世的海拔为2094~3514 m。尼玛盆地和伦坡拉盆地在形成时代、沉积物充填和演化历史上非常相似。根据孢粉化石重建伦坡拉盆地在古新世-始新世的海拔为2300~2600 m，与尼玛盆地在古新世-始新世的海拔较为一致。而当前化石采集地点的海拔为5200 m，显示青藏高原中部尼玛盆地自40 Ma以来隆升了2000~3000 m。
在青海曲麻莱下更新统野牛沟组第一次发现了木化石，鉴定为被子植物鼠李科Rhamnaceae和壳斗科Fagaceae。在Rhamnaceae和Fagaceae中发现了甲螨目成虫及幼虫的粪便化石。选取现生Rhamnaceae和现生Fagaceae的主要属种作为当前化石的现存最近亲缘种，并确定它们的现存最近亲缘种海拔分布的最高值和最低值。利用共存分析法定量重建了曲麻莱在早更新世的海拔为749~3549 m。而当前化石采集地点的海拔为4500 m，说明青藏高原东北部曲麻莱自1 Ma以来隆升了1000~4000 m。
根据实测剖面P1708、2017XD、2017XD中微量元素的含量及比值的变化，把尼玛盆地古新世-始新世的环境变化划分为九个环境变化波动区间带，说明尼玛盆地经历过开始形成-扩张发展-萎缩-扩张-萎缩-扩张-萎缩-再扩张-河湖共存阶段，对应的尼玛盆地古新世-始新世气候经历了湿润-温和偏干-温暖湿润-温和偏干-温和湿润-温和偏干-温暖湿润-温和偏干-温暖湿润的气候波动。尼玛盆地有机质碳同位素δ13C值的降低，推测与古新世-始新世全球的增温事件（PETM）有关。而有机质碳同位素δ13C值组成分布在-28.92~-23.66 ‰之间，表明尼玛盆地有机质来源主要为陆源C3植物。木贼属Equisetum cf. oppositum的发现意味着尼玛盆地在古新世-始新世已经存在显著的季节性变化。结合伦坡拉盆地的孢粉化石、植物化石和动物化石，认为尼玛盆地在古新世-始新世是一个温暖、潮湿的环境。选取伦坡拉盆地牛堡组植物群22个化石种的现存最近亲缘类群，利用共存分析法定量恢复了伦坡拉盆地在古新世-始新世的气候特征：年均温（MAT）为15.8~17.3 °C，最热月均温（MWMT）为23~28 °C，最冷月均温（MCMT）为2.4~5.4 °C，年较差（DT）为23.3~24.8 °C，年均降水量（MAP）为1011.3~1653.5 mm，最大月均降水量（MMaP）为161.8~283.7 mm，最小月均降水量（MMiP）为24.1~31.4 mm，与现代气候相比气温和降水量的气候参数普遍偏小。
通过测定木化石Rhamnaceae和Fagaceae中有机质的碳同位素δ13C值，并与现生植物的碳同位素δ13C值模式图进行对比，认为Rhamnaceae和Fagaceae均为落叶植物。利用共存分析法定量重建了曲麻莱在早更新世大气中CO2的碳同位素δ13CCO2值为-7.5~-3.7 ‰。计算了木化石Rhamnaceae年轮中碳同位素δ13C平均值，线性拟合结果显示年轮中碳同位素δ13C平均值有减小的趋势，说明植物生长的环境有变适宜的趋势，这可能与东亚夏季风先减弱后增强有关；计算了木化石Rhamnaceae记录的大气中CO2的碳同位素δ13CCO2平均值，线性拟合结果显示大气中CO2的浓度有减小的趋势，推论是由上新世末期–更新世碳同位素的负偏移引起的，而碳同位素的负偏移可能与东亚冬季风有增强的趋势有关。与南海和黄土高原中记录的东亚冬季风有增强的趋势有较明显的一致性，青藏高原东北部曲麻莱东亚冬季风的增强可能与两极冰川扩展、全球变冷或青藏高原的隆升有关。通过测定木化石晚材中纤维素的碳同位素δ13C值，定量重建了曲麻莱在早更新世期间8月份平均降水量为1032~1078 mm、平均温度为0.73~1.36 °C和平均相对湿度为121~123 %。
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 approximately 1,000–4,000 m over the past one million years.
According to the trace element content and the ratio change in sections P1708, 2017XD, and 2017XD, 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 δ13C 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 C3 plant. 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 of 15.8°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 δ13CCO2 values ranged from −7.5‰ to −3.7‰ in the northeastern Qinghai–Tibet Plateau during the Early Pleistocene. The average values of the δ13C carbon isotope in the growth rings and the δ13CCO2 carbon isotope of the atmosphere were also calculated. The linear fitting results suggest that the δ13C 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 δ13CCO2 carbon 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 1078 mm 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.
|杨国林. 西藏尼玛-青海曲麻莱新生代几种植物化石及古环境意义[D]. 兰州. 兰州大学,2018.|
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