内蒙古商都-化德盆地第三纪孢粉组合——兼论中国草原植被的形成

TERTIARY SPORO-POLLEN ASSEMBLAGES FROM THE SHANGDOUHUADE BASIN, INNER MONGOLIA--WITH DISCUSSION ON THE FORMATION OF STEPPE VEGETATION IN CHINA

本文研究了内蒙古商都-化德盆地第三纪孢粉,共建立6个组合。通过综合我国北方不同地区第三纪孢粉组合所反映的气候变化规律,并与地中海海相动物群和深海钻探氧同位素的研究结果比较,采用气候对比与组合对比相结合的方法,确定这些组合的时代为中晚渐新世—上新世,改正了以往研究者认为是晚始新世—渐新世的看法。 文中还在讨论商都-化德盆地第三纪植被演替的基础上,结合现有的孢粉资料,对中国草原植被的形成过程作了初步探讨。

The Tertiary lacustrine deposits are well-developed in the Shangdou-HuadeBasin, Inner Mongolia. Since 1972, enormous survey work on the basin has beencarried out by the Regional Geological Survey Team and the Regional HydrologicalTeam, with the geological ages of the deopsits assigned to the late Eocene--Oligocene according to their exploration. The present paper makes a detailedstudy from the palynological approach of the basin, where 6 palynological assem-blages are recognized and their geological ages are resettled. Prior to the discussion on the geological age of each assemblage is a study of theEarly Tertiary--Late Tertiary palynological boundary from several parts of NorthChina including Shache and the Kuche Basin of the Xinjiang Uygur AutonomousRegion,the Qaidam and Xining-Minhe Basins of Qinghai Province, the Weihe Basinof Shaanxi Province in Northwest China, and the coastal region of the BohaiSea in East China. All of the assemblages from those places reflect a ten-dency toward climatic deterioration commencing at the early Oligocene, andthen resuming from the early Miocene. This result is consistent with that revealedby the marine megafauna in the Mediterranean and the oxygen isotopic recordsfrom DSDP boreholes; that is to say, a major cooling step started from the earlyOligocene while the relatively warm, equitable conditions typified most of the earlyMiocene and culminated in ca. 17--15 Ma to become the climatic optimum of thewarm palaeotempertures. The authors consider it effective using the climatic fluctuation pattern reflect-ed by the oxygen isotopic studies to determine the Early Tertiary-Late Tertiaryboundary. A distinct cooling step is also recognized from our present Palynologi-cal Assemblages 1--2, which are represented by the decrease in tropical and sub-tropical elements, and the increase in land herb, conifer pollen including Cheno- poidipollis, Piceaepollenites,Abiespollenites, Tsugaepollenites, etc. The climatic con-ditions ameliorate from Assemblage 3. Both land herb and conifer pollen decline inAssemblage 3 and the conifer pollen grains reach their lowest level in Assemblages4 and 5, while Assemblage 4 contains a lot of the water herb Sporotrapoidites andAssemblage 5 many tropical and subtropical elements. Therefore, the authorssuggest that the Early Tertiary-Late Tertiary boundary should be placed betweenAssemblages 2 and 3, and the geological ages of Assemblages 1, 2 and 3 shouldbe assigned to the Middle Oligocene--Late Oligocene or the Late Oligocene, thelatest Oligocene and the Early Miocene respectively. Assemblages 4 and 5 are con-sidered to be corresponding to the ca. 17--15 Ma climatic optimum and Assemblage6 to be of the Pliocene owing to the large content of land herbs. After a general discussion on the evolution of vegetation in the Shangdou-Huade Basin, this paper also makes a study on the formation of the steppe vege-tation in China based on the present available p alynological data. In the authorstopinion the formation of the steppe vegetation is mainly controlled by two factors,the evolutionary course of the plant kingdom itself and the environmental changes,especially the climatic alteration on the earth. At the beginning of the Tertiary,the angiosperms were mostly in their original form and their living conditionwas rather even. Later on, with a series of major changes caused by orogenyand climatic fluctuations in geographic and ecological conditions, the plants gotmore chances to make their further development and evolution. Angiospermous herbs could be dated back before the Oligocene worldwide,but they were not thriving to form the steppe until the Oligocene. Originalsteppe vegetation made its first appearence in the latest Early Tertiary in China.Components of the vegetation then were rather monotonous with their distributionmostly isolated in other vegetational types or limited to the open groundbetween forests in Northwest China. Since its appearance in the latest Oligocene,the steppe vegetation experienced a series of extension and concentration becauseof climatic fluctu