A THEORETICAL APPROACH TO P_(co_2) EVOLUTION IN THE ATMOSPHERE BASED ON ASSOCIATIONS OF SEDIMENTARY MINERALS

Based on the theory of pH evolution of sea water and the balance between the seawater and the atmosphere the authors discussed the problems about (i) the method ofcalculating P_(CO_2) in the ancient atmosphere with the associations of sedimentary miner-als; (ii) the evolution of P_(CO_2) values in the geologic history; (iii) the relations of thepH evolution of sea water with carbonate precipitations; and (iv) calculation of the pHlimit for some associations of sedimentary minerals and its corresponding P_(CO_2) valuesin the atmosphere. The authors pointed out that though carbonates had deposited little in the Archaean,the content of CO_2 gas in the Archaean atmosphere was very high and was gradually go-ing up to form a thick CO_2 atmosphere. Up to 2600 Ma ago, the P_(CO_2) had reached a gradeof 10- 50 atm. There was a general trend of evolution that from the early Proterozoicera to the present the depositional horizon of carbonate layers was gradually risingand finally surpassed the horizons of clay min     

根据沉积矿物的共生组合恢复大气CO_2气分压值的演化

本文依据海水pH值演化和海气平衡理论,讨论了根据地层中沉积矿物共生组合恢复大气CO_2气分压值的理论和方法,研究了海水pH值的演化与碳酸岩层沉淀的关系,计算了地层中一系列与碳酸盐矿物伴生的沉积矿物组合生成的pH值范围及其所对应的大气CO_2气分压值。指出太古代沉积碳酸岩层虽然很少,但大气CO_2气含量却很高,且逐渐增多,直至形成稠密CO_2气圈。26×10~2Ma前后,CO_2气分压曾达10—50atm。从早元古代至现代表现为碳酸岩层沉积层位逐渐升高。最后超越硫化物层和硅酸盐粘土矿物层的总趋势,与此相应大气CO_2气分压值逐渐降低,由稠密CO_2气圈降为现在的0.03％,按CO_2气分压值的范围和波动特点可将其历史演变划分为三个历史阶段。