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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。
Through the study of δ13C records with resolution of 3 ~ 5 years collected from stalagmite in Beijing Shijia Cave in the past 500 years, we find that both the change of hydrodynamic conditions and the change of CO2 concentration in modern atmosphere have an impact on the δ13C value of stalagmite. When climate humid When the rainfall increases, the infiltration speed of the aeration water in the cave is accelerated, the retention time in the limestone fracture is shortened, and the dissolved amount of CaCO3 is reduced, resulting in a decrease of the exudation δ13C water value. Increasing the entrainment gas supply rate may also cause the CO2 partial pressure of the solution to decrease, resulting in the reduction of limestone dissolution of CaCO3. In addition, the amount of CO2 released from the drip and effluent water in the hole is inversely proportional to the rate of water supply. Dripping speed, CO2 release is relatively lower, the precipitation of CaCO3 δ13C value is too light. By comparing with δ18O record, the δ13C record also reflects the period of 30-40 years of rainfall change, which provides evidence for the change of summer wind in this area. The δ13C record in the past 50 years reflects the phenomenon of elevated atmospheric CO2 concentration caused by human activities. From 1940 to the present, the δ13C value of stalagmite decreased from -6.7 ‰ to -8.1 ‰, and the decreasing rate was -0.028 ‰ per year, which directly reflected the δ13C value of atmospheric CO2 during this period. This result is consistent with other studies. The use of fine δ13C records with resolution up to several years can provide an effective means for the study of karstification and carbon cycle.