内陆封闭湖泊自生碳酸盐氧同位素剖面的古气候意义  

PALAEOCLIMATIC IMPLICATIONS OF OXYGEN ISOTOPE PROFILES OF AUTHIGENIC CARBONATES FROM INLAND CLOSED LAKES

作　　者：卫克勤[1] 林瑞芬[1] 

机构地区：[1]中国科学院广州地球化学研究所

出　　处：《地球化学》1995年第3期215-224,共10页Geochimica

摘　　要：根据水体蒸发过程的同位素分馏方程，详细讨论了内陆封闭湖泊自生碳酸盐氧同位素剖面的古气候意义。计算结果表明，封闭湖泊水在蒸发过程中达到同位素组成稳定状态的时间小于二倍湖水平均滞留时间。因此，湖水同位素组成一般很快就达到与当时的气候环境相平衡的稳定状态，对气候环境的变化极其灵敏。湖泊自生碳酸盐的氧同位素组成则是湖泊汇水区气温、湿度和雨水同位素组成三者的函数。我国大部分地区夏季风盛行时期气候转暖，降水量增加，而雨水￣（１８）Ｏ含量反而下降，这为重建古气候环境和探讨夏季风变迁提供了新的同位素示踪手段。The evaporation from a terminal lake can be well described using the equations of Craig & Gordon and Gonfiantini. The δ-value of the lake water under steadystate conditions may be obtained on the simplified equation:δs=(l - h)δI + hδa+εtot, which indicates that the δ-value is mainly a function of h (relative humidity),δI and t (℃). The time interval to establish the steady-state conditions is rather short. The δ-value of the lake water is close to δs when t=2τ(residence time).The residence time of the Qinghai Lake is about 33.4a. Most points of the δ18O profiles represent the δ-value of the carbonates in equilibrium with the lake water under steady-state condition. The relative humidity is an important parameter but difficult to be measured.A method to estimate the average value of relative humidity is proposed based on the local evaporation line of meteoric water. The value h is about 21% for southern Xinjiang, where the climate is extremely dry, and about 48% for the centralpart of Tibet.The δ-values of the lake water and the authigenic carbonates at present time have been observed from three Chinese lakes. The results show that the temperature of crystallization of the authigenic calcite calculated on the fractionation equation is much higher than the local average temperature. It appears that the calcite precipitation takes place only after the melting of the ice cover of the winter period. The temperature effect on isotopic composition of meteoric water is the basis for palaeoclimatic reconstruction. The δ-value of precipitation is much lower in summer than in winter in the monsoon areas in China, due to the low concentration of heavy isotopes in warm-moist air masses brought in by the summer monsoon. It is shown that the ground surface temperature and relative humidity during the monsoon prevailing period (10 000-8500a B.P.) were higher than at present in the area around the Qinghai Lake. The summer monsoon has been weakening gradually and the clirnate getting colder and drier with higher δ

关 键 词：湖泊 沉积物 碳酸盐 氧同位素 古气候 

分 类 号：P578.6[天文地球—矿物学] P532[天文地球—地质学]