黄土高原西部地区末次冰期和全新世有机碳同位素变化与C3/C4植被类型转换研究

文章利用黄土高原西缘代表性的塬堡剖面有机碳同位素数据,估算了末次冰期以来地表植被中C₃/C₄植 物的相对丰度,指示出研究区域末次冰期几乎为纯粹的C₃植物,而全新世为C₃植物占优势的C₃和C₄混合植被类 型。温度是控制中国黄土高原C₄植物是否发生的关键性气候因素,末次冰期向全新世转化过程中存在的某“阈值 温度”控制了两种植被类型的存在。全新世土壤有机碳同位素偏正于末次冰期,符合前人研究得到的认识。末次 冰期间冰段(MIS3)至盛冰期,土壤有机碳同位素为偏正变化趋势,符合现代C₃植物本身随气候条件改变的碳同位 素组成变化。研究表明,利用黄土-古土壤有机碳同位素进行古气候变化研究,不能只将有机碳同位素简单的解 释为C₃/C₄植物相对丰度的变化,在单一植被类型下,还需要考虑植物本身碳同位素组成随气候条件的变化;另外, 研究还说明,我国黄土高原不同地区同时段土壤有机碳同位素值可以不同,其变化可以不具有相同的趋势,因此, 简单将有机碳同位素偏正归因于夏季风增强是值得商榷的。

VARIATION OF SOIL ORGANIC CARBON ISOTOPE AND C3/C4 VEGETATION TYPE TRANSITION IN THE WESTERN LOESS PLATEAU DURING THE LAST GLACIAL AND HOLOCENE PERIODS

About 650 samples have been measured for soil organic carbon isotopes (SOCI) at an interval of 4cm from the Yuanbao Loess Section. The measured part of the section is 25.72m long, located in the Linxia basin, Gansu Province of the west part of the Chinese Loess Plateau. The SOCI values are from -22.6‰ to -27.5‰ with a mean value of -25.4‰. The SOCI values during Holocene is generally more positive than those during the last glacial. According to the SOCI variation, the SOCI variations can be divided into four parts. The SOCI in the Holocene paleosol (S ₀) has the most positive values in the whole section, while in the Last glacial interstadial paleosol (L ₁S ₁) has the most negative values. The SOCI in the early and late Malan loess has values in the middle. Correlating the calculated relative abundance of C ₄ vegetation with loess-paleosol stratigraphy and magnetic proxy since the Last glacial, it can be deduced that the most positive SOCI values during the Holocene indicate the rise of the relative abundance of C ₄ plants, which agrees with the fact that the relative abundance of present C ₄ plants increases from the west part to the east part of the Chinese Loess Plateau along with the increase of temperature and precipitation. However, SOCI values during the relative warm and humid period of last glacial interstadials, i.e., MIS 3 or paleosol L ₁S ₁ formations, are more negative than during the cold and dry periods in the early and late last glacial (MIS 2 and MIS 4). This probably indicates that the SOCI during cold last glacial may not indicate the relative abundance of C ₄ plants, but the carbon isotopic change of pure single vegetation type under different climatic conditions. The pure vegetation type can be only deduced to be C ₃ vegetation according to the distribution of the SOCI values. Because previous study on the modern C ₃ plants in the north China does shows that the correlations of C ₃ plants carbon isotopic compositions not only with annual precipitation, but also with annual temperature are negative. It is obvious that the negative values of the SOCI during last glacial interstadial must be formed by the more negative of carbon isotopic composition of C ₃ plants developed under relatively warm and wet climatic condition. The pure C ₃ vegetation type during Last glacial in the west part of Chinese Loess plateau also agrees with the fact that the relative abundance of present C ₄ vegetation decreases from the east part to the west part of Chinese Loess Plateau. It is suggested that the temperature is the key factor to control the existence of C ₄ plant on the Chinese Loess Plateau, and there should have a “threshold temperature” to convert between the mixed C ₄/C ₃ plants with dominant C ₃ plant during the warm Holocene period and pure C ₃-type plant during the cold last glacial period. Therefore, it is quite complex to use organic carbon isotope value to indicate the summer monsoon or temperature. The more positive value of SOCI during the warm Holocene indicates relative abundance of C ₄ plant in the vegetation and thus relative warm-humid condition according to modern plant studies, while more positive value of SOCI during the cold glacial period indicates the pure C ₃-type plant responds to relative drier climate and thus weak summer monsoon. Therefore, the soil organic carbon isotope variation can not always be explained to indicate the changes of C ₃/C ₄ relative abundance because the organic carbon isotopic change of single type plant such as C ₃ plant must be took into account, especially when the vegetation is pure C ₃ or C ₄ plant. Our study indicates that there is no simple relationship between SOCI changes and summer monsoon variations.