摘要
环境是当今世界科学界最为关注的问题之一,重建古植被与古气候、预测未来环境变化已成为第四纪研究的热点。本文以内蒙古西乌珠穆沁旗地区的第四纪沉积物为研究对象,结合热释光测年与线性内插法得到的年龄数据,定性地分析了孢粉组合所反映的植被、气候演变规律。同时,论文借鉴前人所建立的地区孢粉-气候转换函数,通过适当的修正,将其应用于研究区剖面的化石孢粉资料分析,定量化估测了该地区的年平均气温和年降水量等4个气候参数。基于上述研究,综合定性分析与定量分析的结果,将西乌珠穆沁旗地区第四纪晚更新世晚期以来的植被、气候演替划分为七个阶段,通过纵向时段对比与横向区域对比,探讨了该地区晚更新世以来的环境演化规律。研究结果表明,定量重建的古气候环境基本符合我国东北部地区晚更新世以来植被气候演替的基本框架,定量估算的古气候参数值具有一定的可信性,较好地反映出西乌珠穆沁旗地区第四纪环境变化的基本特征。
Xiwuzhumuqinqi is situated in the typical region of Xilin Gol Grassland, which is one of the greatest grassland in the world, and there is beautiful grassland scenery in the area. Simultaneously, this area is the only region where collects all nine types of grassland in Inner Mongolia region, and also the first choice place to experience nomadic custom and culture for tourists all over the world. The study on characters of the sediments, vegetation succession and climate evolution during Quaternary in Xiwuzhumuqinqi area will not only promote the development of tourism in the region, but also contribute to the coordination of agriculture, animal husbandry, tourism and management of natural resources. And a scientific basis can be provided for managers to formulate a long-term policy in soil erosion, grassland desertification, salinization, and protection of biological resources by the study. Besides, revealing the laws of the environmental evolution in the region during Quaternary can be able to provide basic information for predicting the future trends of climate change, and it plays an important role in the analysis on the laws and reasons of climate change.
Therefore, taking the sediments during Quaternary from the area of Xiwuzhumuqinqi in Inner Mongolia Autonomous Region as the research object of this paper, we attempted to analyze the successive process of vegetation and climate qualitatively based on the results of the pollen analysis, and then we reconstructed the ancient climate in Xiwuzhumuqinqi area by using pollen-climate transfer function quantitatively. In the end, the paper explored and discussed the comprehensive laws of vegetation and climate evolution since the late Pleistocene by horizontal contrast, vertical contrast and integrating the results of qualitative analysis and quantitative analysis. During the process of research, the primary methods and techniques we used include the field observation records, GPS pointing device, digital camera, thermal luminescence dating, pollen analysis, linear interpolation and transfer function. In addition, stratigraphic profile diagram, pollen percentage diagram, histogram and corresponding tables of statistical data were used for comparison, analysis and discussion in this paper.
Through systematic spore-pollen analysis, the successive process of vegetation and climate reflected by the profile in the study area can be summarized as follows. Firstly, there were mainly cold-arid climate and desert vegetation composed by xerophile with cold tolerance in the region of section PⅠfrom 81.635±6.935ka B.P. to 12.085±1.025 ka B.P. Secondly, from 65.315±5.555ka B.P. ~ 35.860±3.050ka B.P. ~ 20.350±1.730ka B.P. ~ 8.240±0.700ka B.P. to 2.840±0.360ka B.P., the laws of vegetation succession in the region of PⅡprofile reflected as: "desert vegetation→grassland vegetation with coniferous forests→desert vegetation and grassland vegetation with coniferous forests→desert vegetation", and the corresponding climate changed from cold-arid to moderate-arid. Thirdly, as reflected in the PⅢprofile’s spore-pollen zones, the vegetation in the area had experienced the process of "grassland vegetation→grassland vegetation with coniferous forests→desert vegetation→grassland vegetation with coniferous forests→desertification--semi-desert vegetation→semi-desert vegetation" since 9.050±0.770ka B.P., and the corresponding climate evolution was "cold-wet→cold and drought→cold-wet→moderate→cool-wet ". Fourthly, the successive process of vegetation in the area of section PⅣhad been manifested as "desert vegetation→desert vegetation and grassland vegetation with coniferous forests→desert vegetation”, and the paleoclimate was mainly cold and arid. Fifthly, it can be found that the region with PⅤprofile was humid and warm-cool, and there was mainly grassland vegetation with coniferous forests in the area.
Based on the pollen-climate regression equation of northeastern part of Inner Mongolia region established by previous scholars, and through systematic analysis on the applicability and feasibility of the model, the mean annual temperature, annual rainfall and other climate parameters in Xiwuzhumuqinqi area had been reconstructed quantitatively. The result indicates that, since the late Pleistocene, the mean annual temperature and January temperature in the region are lower than that in recent 20 years, but the average temperature in July and annual precipitation are higher than the present values. The overall trend is that seasonal variation appears to be more and more obvious. Specifically, the difference of annual precipitation is within the scope of 88.824 ~ 99.645mm, and the differences of mean annual temperature, January temperature and July temperature are -5.4℃、-9.2℃and 10.3℃. According to the calculated factor values, the laws of climate change reflected in all profiles can be drawn out as follows: with the age of the sediments more and more new, the average annual temperature and January temperature remained basically unchanged; moreover, periodical fluctuations can be seen in the change of July temperature, and which overall showed a downward trend, but the specific changes in the laws during different period are dissimilar. Compared with other three climate parameters, the trend of annual precipitation varies from place to place, and the changes of precipitation reflected in all profiles do not have unity.
On the basis of the study above-mentioned and the integration of qualitative analysis and quantitative analysis results, the vegetation and climate succession in Xiwuzhumuqinqi area since the late Pleistocene can be divided into seven stages. In the first phase (82000-35000 a B.P.), there were desert vegetation and cold-arid climate, but the climate in this stage was more cold-wet than today; In the second phase (35000-21000 a BP), there was grassland vegetation with coniferous forest, reflecting cold-wet climate at that time; The third phase (21000-10000 a B.P.) is characterized by alternating desert vegetation and grassland vegetation with coniferous forests, and the climate was moderate-arid; The fourth phase (10000-8000 a B.P.): the grassland vegetation and cold-wet climate; The fifth phase (8000-4500 a B.P.): the vegetation was grassland with coniferous forests, and the climatic was still wet and cold; In the sixth stage (4500-2700 a B.P.), there were desert vegetation and grassland vegetation with coniferous forests, and climate fluctuations in this period mainly manifested as follows: cold and drought→cold-wet→moderate; The seventh stage (since 2700 a B.P.): the semi-desert vegetation was mainly composed by Cyperaceae and Pinus, and the climate was more humid compared with modern climate, the average temperature in July is basically the same as that in recent years. Studies have shown that the ancient vegetation and climate evolution reflected by the results of qualitative pollen analysis and quantitative paleoclimatic analysis could maintain the unity and complementarity, and the trend of climate change remained basically the same.
By comparing the characters of vegetation and climate change between Xiwuzhumuqinqi area and Song-Nen plain since the late Pleistocene, it can be seen that the evolution trends in the two regions is consistent and uniform in a certain degree. Whereas, due to their different geographical location, and influenced by the Ice Age climate in different degrees, they also show different characteristics in different successional stages. The ancient climate and environment reconstructed quantitatively in Xiwuzhumuqinqi area is in line with the basic framework of vegetation and climate succession in the northeast of China since the late Pleistocene. Simultaneously, the paleoclimatic parameter values calculated quantitatively has a certain degree of credibility, and they can provide a benchmark to the regional comparison and basic data to predict the future trend of climate change.
On account of lacking a large number of surface soil samples, we couldn’t establish the pollen-climate regression equation only for the area of Xiwuzhumuqinqi. Although the applicable scope of the model used in my research includes Xiwuzhumuqinqi area, but the region is wider than the study area. It is believed that the precision of transfer function correlates directly with the amount of topsoil samples and the accuracy of pollen analysis. Therefore, the precision of calculated paleoclimate factor values in the paper may not be so perfect, and to some extent, the paleoclimate environment reconstructed quantitatively may have slight errors compared with the real climate data, despite the satisfactory results we have achieved. But on the whole, a fairly good restoration of ancient vegetation and climate during the late Pleistocene in Xiwuzhumuqinqi area has been made, and it will contribute to the research on the quantitative reconstruction of paleoclimate in the north of China.
引文
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