晚新生代以来灵台黄土剖面元素碳记录与气候环境变化

【摘要】：Elemental Carbon (EC), produced by incomplete combustion of fuels, presenting a continuum of a series of components including irregular carbon and micrographit carbon, is widespread in sediments. The study of EC is good for reconstruction of paleoclimate and understanding of vegetation dynamic evolution, because it records fire history well. We applied chemical method, and obtained the EC and δ ~13C_ec data successfully from the loess of Lingtai Section in Gansu province. EC records provide a new substituted proxy for paleoclimatic study of loess.The content of EC varies from 0.01%. to 0.43%., and the average value is 0.1 %. The maximum value appeared in layer SO with a depth of 1.42m and age of 5900aB.P, suggesting an obvious arid event at that time. The peaks of EC content always appear in paleosol layers, suggesting the climatic pattern of abnormal aridity would survive in warm and humid environment. There are EC peaks both in loess layers and paleosol layers since L2, indicating that the climate system maybe instable. Through all the sequence, EC peaks point out the climate transition from warm and wet to arid.Since the late Cenozoic, the value of δ ~13C_ec varies from -35.71% to -12.32%., and the average value is -20.95%. According to the equation between δ ~13C_ec and the relative content of C3/ C4, we concluded that there had been both C3 and C4 plant since the late Cenozoic, and majored in C3 plant in many of the time in the Chinese Loess Plateau. The most negative peak of δ ~13C_ec appeared in the interim of S5 and L5, and there were several obvious peaks of light value in layer S3, which indicated that there had been instable forest for a short time in the Chinese Loess Plateau. The most positive peak of δ ~13C_ec appeared in layer L9, the top layer of sandy loess, when C4 plants had a proportion of 100%,which indicated that the climate would be extremely cold and arid. In the bottom layer of sandy loess L15, the average value of δ ~13C_ec was a little positive, but there was a rather negative peak in some point. Although L9 and L15 were accumulated during the cold episodes, the climatic conditions to form them would not in the same, and the climatic condition in layer L15 was not as cold and dry as in layer L9, but had the similar feature with other loess layer. The formation of sandy loess could be associated with an increased dust supply in response to quick uplift of the Tibetan Plateau.The EC record presents cyclic change on the loess-paleosol timescale. The average value of EC in the Loess of Lingtai Section of interglacial stage is higher than that of the glacial stage. The 8 13Cec curve shows the same change, that is, the average value of 8 13Cec is weight in loess layers and light in paleosol layers. Generally speaking, in the climate condition of warm and wet, paleosol layers develop, and the vegetation is abundant and there are a lot of C3 plants, and then the fuel is accumulative and fires easily occur. On contrary, when the climate is cold and dry, the loess deposits, and the vegetation is rare with a lot of C4 plants relatively, and fires is not intensive because of little fuel accumulation. On glacial-interglacial timescale, the EC record can show the biomass change.The EC record in the Loess of Lingtai Section has obvious feature periodically. The EC abundance curve showing a phased increase in 1.8MaB.P., 1.2MaB.R, 0.8MaB.R, 0.13MaB.R, 0.02Ma B.P., is concordant with the change of dust flux velocity in Lingtai Section, which indicates that the increase of winter monsoon and aridity would be the reason. All these periods coincide with the uplift periods of Tibetan Plateau, such as QingZang movement(including three phases of 3.6 MaB.P.,2.5 MaB.P.,1.7 MaB.R), KunLun-Yellow River Movement(l.l MaB.R, 0.8 MaB.R, 0.6 MaB.R), and GongHe Movement(0.14MaB.P.),which indicates that the uplift of Tibet Plateau maybe the cause of increasing winter monsoon in East Asia and aridity in inner Asian showing by EC records. A sudden more weight change in 2.6MaB.P. and a sudden lighten change in 0.64MaB.P. of the 8 13Cec value coincide with changes of the summer monsoon. Winter and summer monsoon variations recorded by EC abundance and 8 13Cec respectively show three evolution stages, which are 7.2-2.6MaB.R, 2.6-0.64MaB.R, 0.64-OMaB.R respectively. In 7.2-2.6MaB.P. and 0.64-OMaB.R, the negative correlation between EC abundance and 8 l3Cec indicates the winter and summer monsoon increasing simultaneously. In 2.6-0.64MaB.R, the positive correlation between EC abundance and 8 I3Cec indicates the winter and summer monsoon alternating mutually.Since the late Cenozoic, the EC abundance in the Loess of Lingtai Section increases by the time and increases more quickly by the time, suggesting that fires are influenced by climatic changes and by the human activities in later time.The spectral analysis of the EC abundance curve and the 8 13Cec curve shows that they have the same periodicities in Quaternary Period and Tertiary Period respectively, indicating that there are similar factors controlling their changes. But taking a short duration 0-0.42Ma for example, the spectral analysis of the EC abundance curve and the 8 13Cec curve shows that they have different periodicities, indicating that they are controlled by different climatic factors on short timescale. Analyzing the periodic variation of EC record, we think that it is not only related with the long period climatic change driven by orbital factors but also with sudden changes in climate. In addition, the dominant periodicities of EC records varied in different stage, so EC records have different significance as a climatic proxy on different timescale.