北京石花洞石笋500年来的δ￣(13)C记录与古气候变化及大气CO_2浓度变化的关系

INTERANNUALRESOLUTION δ13C RECORD OF STALAGMITES AS PROXY FOR THE CHANGES IN PRECIPITATION AND ATMOSPHERIC CO_2 IN SHIHUA CAVE,BEIJING

通过对采自北京石花洞石笋近５００年来分辨率为３～５年的δ１３Ｃ记录的研究，我们发现水动力条件的改变和近代大气ＣＯ２浓度的改变均对石笋的δ１３Ｃ值有影响，当气候湿润，降雨量增加时，洞穴包气带水的下渗速度加快，在石灰岩裂隙中滞留的时间缩短，ＣａＣＯ３的溶解量减少，造成渗出δ１３Ｃ水的值变轻。包气带水的补给速度增加，也可能使得溶液的ＣＯ２分压降低，造成石灰岩ＣａＣＯ３溶解减少。另外，洞内滴水和渗出水中ＣＯ２的逸出量与供水速度呈反比。滴水速度加快，ＣＯ２逸出量相对降低，沉淀出来的ＣａＣＯ３的δ１３Ｃ值偏轻。通过与δ１８Ｏ记录的对比，δ１３Ｃ记录也反映３０～４０年降雨量变化的周期，为这一地区夏风变化的规律提供佐证。近５０年来的δ１３Ｃ记录，反映了人类活动造成大气ＣＯ２浓度升高的现象。从１９４０年到现今，石笋的δ１３Ｃ值从－６．７‰降低到－８．１‰，降低速率每年为－０．０２８‰，直接反映了这一时期大气ＣＯ２的δ１３Ｃ值的变化。这一结果与其它研究相吻合。利用精细的、分辨率高达几年的δ１３Ｃ记录可以为岩溶作用与碳循环的研究提供有效手段。

Using a computercontrolled microsampling device, we have obtained a δ13C record from a stalagmite collected from Shihua Cave, Beijing, with a resolution of 3￣5 years. This 500year δ13C record exhibits apparent 30/40year cycles which correlate well with the δ18O record.The variation in δ13C on the 30/40year time scale is mainly influenced by the changes in the surface water supply rather than in C3 /C4 ratio. When precipitation increases inferred by the decreasing δ18O peaks, the δ13C of stalagmites can be decreased by the following reasons: (1)reduced residence time of seepage water in the overlying limestone leading to decreased amount of dissolved bedrock CaCO3; (2) reduced PCO2 in seepage water resetting less CaCO3 dissolution; and (3) increased drip rate of drip water in cave atmophere causing less CO2degassing from the solution. Therefore, highresolution δ13C recorded in speleothems may also provide information on the variability of precipitation. The changes in the δ13C record for the last 50 years reveal the effect of increased atmospheric CO2 concentration resulted from the consumption of fossil fuel on the δ13C of soil CO2. Since 1940, the δ13C values of the stalagmite was decreased from -6.7‰ to -81‰, with a rate of -0.028‰/yr, which is in excellent agreement with the previous studies. The imprint of such δ13C decreasing event may be useful for the study on the relationship of karst processes and carbon cycle in the future.