黄土高原黄土稳定同位素与古季风研究进展

黄土高原黄土-古土壤完好地记录了过去环境信息,进入90年代以来,在利用碳、氧同位素作为气候替代指标研究古植被、降雨量、古环境温度及古季风演化方面取得了长足的进展,文章就这方面的主要研究成果作了综合分析与评述。

ADVANCES ON STUDIES OF STABLE ISOTOPE IN PALAEOSOL AND PALAEO MONSOON ON THE LOESS PLATEAU, CHINA

It has long been proved that the loess stratigraphy bears a variety of information on past global changes. The alternative loess palaeosol sequence is a well preserved materialized record of the glacial interglacial cycles, which is now conformed by various climatic indices such as grain size, stable isotope etc. obtained from loess profiles on the Loess Plateau, China. The great development on reconstruction is paleo vegetation, rainfall, paleo temperature and paleo monsoon etc. have been obtained according to carbon and oxygen stable isotope composition. The δ¹³C value in organic carbon in paleosol is between -146‰ and -220‰,reflecting that the paleo vegetation is mainly composed by C 4 and C 3 plant species which is gerbaceous; East Asia monsoon effect is dominant, climate tend warm, higher rainfall and humid, soilization is very strong during interglacial. The δ¹³C value in organic carbon in loess fall is between -220‰ and -249‰, reflecting that the paleo vegetation is mainly composed by CAM and locally C 3 plant species; winter monsoon effect is dominant and climate tend to dry and lower rainfall during glacial. The lower is the δ¹³C value in carbonate nodules, the larger is the rainfall, the stronger is the East Asia monsoon effect; the higher is the δ¹³C value, the lower is the rainfall, the stronger is the winter summer monsoon effect as carbon isotope almost do not produce fractionation in the process of precipitation of carbonate nodules. But, the higher is the δ 18 O value in carbonate nodules, the higher is the temperature; the lower is the δ 18 O value, the higher is the temperature as oxygen isotope produce fractionationin in the process of precipitation of carbonate nodules. The δ¹³C value in carbonate nodules is from -724‰ to -484‰, average at -579‰ and δ 18 O value from -758‰ to -897‰, average at -824‰,indicating that palaeo vegetation is mainly composed by the C 3 plant species, its environment is similar to the modern monsoon environment on the Loess Plateau; the highest annual surface temperature is 125℃ during last interglacial and the lowest annual surface temperature is 6℃ during last glacial. The δ 18 O value in inclusion water in carbonate nodules reveals that the δ 18 O value in meteoric water decreases and mean annual surface temperature decreases by 65℃ as from last interglacial to last glacial on the Loess Plateau. In δ 18 O-δ¹³C diagram, projection dot fall in near both monsoon line. The line(δ¹³C=-522δ 18 O-5154) represents southeast paleo monsoon and the line (δ¹³C=-227δ 18 O-2446) represents southwest paleo monsoon. Analysis suggests that rainfall represented by southeast monsoon line is similar to that represented by southwest monsoon line. The stage being lower rainfall is a period of southeast paleo monsoon and southwest paleo monsoon effects decrease and loess was strongly accumulated. It should be do that finding more actually relation between carbon isotopic composition and vegetation; improving time and regional resolution on climate change according to carbon and oxygen isotope; numerical converting of the proxy climatic indices of stable isotope; further studying oxygen and carbon isotope composition in modern soil on the Loess Plateau in order to improving the reliability of stable isotope in study on paleo environment.