The Vegetation Evolvement And Climate Change During The S4Interglacial In Luochuan Area

Sedimentary sequence of loess and paleosol is one of the most important land sediments, which, as well as abyssal sediment and polar ice-core, is regarded as the most reliable information carrier for paleoenvironment evolution globally. Loess plateau has the thickest loess sedimentary bodies, with the fine profile of the exposed and complete continuous stratum, which is considered one of core areas for the study of loess of Quaternary period. Different researchers do the research in terms of change of paleoclimate and paleoenvironment from various environmental indicators-susceptibility, chroma, sporo-pollen, organic biomarkers, especially study in more detail the climate change in a big scale in Quaternary period. This article selects the fourth layers of the soil in the loess of Quaternary period to study. In the experiment, we measure and analyze the susceptibility and chroma of S4paleosol, extract and analyze the sporo-pollen. By doing this, we aim at using study mode with high-resolution to reveal that there are some subordinate geological events and.In terms of susceptibility, the S4susceptibility measured in Luochuan rises initially, and then falls, with a unimodal trend. Its minimum is38.20×10-8m3.kg-1, and the maximum is200.58×10-8m.kg-1, with the corresponding curve similar to the deep sea oxygen isotopes curve and the polar ice cores oxygen isotopes curve, both of which are about S4formation period. Through regression equation, we estimate that annual average temperature of S4period is9～19℃, and the annual rainfall is600～900mm, which is consistent with what we have studied. In terms of chroma, the curve we measured is proximately consistent with susceptibility curve, but with the bottom upward, which is considered to be related to the moving of chroma factors such as goethite influencing chroma, which happens in the process of formation of alfisol. Although there is a certain difference between the recovery of paleotemperature and paleorainfall based on chroma and the one of paleotemperature and paleorainfall based on susceptibility, it is within the scope of the error, so that we propose that paleoclimatic factors with many indicators are more reliable in the recovery of paleoclimate. In terms of sporo-pollen, through the laboratory analysis on the sporo-pollen samples,75samples provide rich sporo-pollen, with each sample containing more than100sporo-pollens, which is required statistically. Generally, in the75samples, the sporo-pollen of woody plants is identified to genus, and the one of herbaceous plant is identified to family, with126genus and family identified in total. Among them, the major families and genera are Meconopsis, Chenopodiaceae, grass family, urticaceae, Humulus, composite family, Ranunculaceae, Betulaceae, Pinus, Juglans, Pterocarya, oak, Ulmus, and Castanea.Through the analysis on susceptibility, chroma and sporo-pollen materials, we come to the following conclusions:(1) In the recovery of paleoclimatic factors, the susceptibility and chroma-red index are all practical, but different levels of defects exist. When the paleotemperature is above15℃, the recovery number of susceptibility to paleoclimatic factors is bigger; in alfisol, the recovery number of chroma-red index to paleoclimatic factors is bigger. So in the semi-quantitative recovery of paleoclimate, single indicator has defects more or less; while weighted average method with many indicators can reduce errors, providing more reliable information of paleoclimate.(2) In the recovery of paleoclimatic factors of S4paleosol of Luochuan profile using susceptibility and chroma-red index, we adopt the weighted average method with many indicators. We reach the conclusions that the annual average temperature of S4period is9～19℃, the annual rainfall is600～900mm, as well as during the hottest and moistest period of interglacial stage of S4, north boundary of subtropical zone had been over Qinling Mountains, reaching the middle area of loess plateau.(3) Sporo-pollen spectrum of S4paleosol in Luochuan area can be divided from bottom to top and from the old to the new into:Ⅰ zone、Ⅱ zone、Ⅲ zone、Ⅳ zone、 Ⅴ zone, and the age is about412-398.13ka BP,398.13-380.8ka BP、 380.8-374.33ka BP,374.33-360ka BP, corresponding to which is xeromorphism grassland, forest and grassland, forest, forest and grassland and xeromorphism grassland. Being attributed to vegetation reflecting directly the temperature and precipitation, High resolution analysis on sporo-pollen can reflect the change of small scale more in detail than magnetic susceptibility, contributing to deeper research on regional climate change.(4) The climate of S4paleosol period in Luochuan area can be divided into three stages, whose hot and humid degree is S4-2> S4-1> S4-3respectively. S4-1is located at the bottom of S4, corresponding to Ⅰ、Ⅱ zone of Sporo-pollen, where the climate turns hot and moist from being dry and cold, with rainfall and temperature similar to current loess plateau area, which belongs to xeromorphism grassland, forest and grassland climate, with such major plants as lamb’s-quarters, artemisia, pine and elm as well as temperature and rainfall increasing gradually. S4-1is at the middle of S4, corresponding to Ⅲ zone of Sporo-pollen, where the climate is warm, with annual average temperature of15～20℃as well as annual rainfall of about700～800mm, which belongs to the warm temperate zone-xeromorphism forest climate with such major plants as artemisia, lamb’s-quarters, humulus, betulaceae and pterocarya stenoptera. S4-3is at the top of S4, corresponding toⅣ、Ⅴ zone of Sporo-pollen, where the climate turns dry and cold from being hot and moist, which belongs to xeromorphism grassland, forest and grassland climate with plants group including betulaceae, juglan, pinus, humulus and grass family.