Keywords: 
• △C /
• Carbon Isotope /
• CO₂content of the atmosphere

Abstract: It has long been known that CO₂, content of the atmosphere influences temperature by determining the amount of long-wave radiation trapped in the atmosphere. Therefore, Understanding the variation of atmospheric CO₂ during Quaternary is very important for studying the influence of atmospheric CO₂ on global climatic change. UP till now the information on variation of atmospheric CO₂ during Quaternary is mainly obtained by the composition analysis of ice core.Because the longeSt cores so far obtained (1400-3000m) cover only the last 100000-160000 years, the studying on variation, of atmospheric CO₂, during Quaternary is restricted.△C is a secondary carbonate in loess sediment with a characteristic thermo-decompositon interval(500600℃). The isotopic composition characteristics and geographical distributive feature of △C suggest that AC was formed under the condition of arid or semiarid, low temperatory, high evaporation and little vegetation. Under the condition loess CO₂is mainly from atmosphere and its carbon isotopic composition is veryclose to that of atmospheric CO₂.The relationship between CO₂ content of the atmosphere and its carbon isotopic composition in northwest of China is δ¹³C=5756/[CO₂]-22. 7Replacing δ¹³C of atmospheric CO₂, with that of △C, we preliminaryly estimate the CO₂ content of the paleo-atmosphere. The results show that the CO₂content of the atmosphere from E_1-1 to S₃ranges from 311. 1 × 10^-6 to 397. 0× 10^-6and the difference between estimated values and measured values decreases with the climatic evolution from wet-warm to dry-cold.