高分辨率洞穴石笋中稳定同位素应用——北京元大都建立后对森林资源的破坏—δ^(13)C记录

Application of High-Resolution Carbon Isotope Record of a Stalagmite from the Shihua Cave, Beijing——δ^(13)C Record of Deforestation after the Establishment of the Grand Capital (Yuan Dadu) in 1272 A. D.

对采自北京西山石花洞内的石笋进行了δ^(18)O,δ^(13)C和Mg/Sr分析,获得分辨率为25a的北京地区3000a以来的古气候和古环境信息。以δ^(18)O和D_(Mg/Ca)作为气候变化信息,证实石笋δ^(13)C反映了古气候变化对植被的影响。当石笋δ^(13)C值增大时,表示C_3/C_4植物比值降低,气候变干热;当石笋δ^(13)C值减小时,表示C_3/C_4植物比值升高,气候变湿冷。同时,笔者也发现了北京元大都建立以来对森林资源的破坏。在650B P至360B P,从元大都建立到明代晚期北京外城的增筑,北京西山林木被砍伐用于建筑和烧制木炭,使得这一地区的植被遭受重大改变。C_3/C_4植物的比值大幅度降低,石笋的δ^(13)C值增大3‰。尽管在此之后的气候条件有利于C_3植物的发育,但这一地区的植被没有恢复到受人类破坏前的水平。通过与历史资料对比,证明石笋中的δ^(13)C记录的确能反映C_3/C_4植物比值的分配变化,从而可以间接地反演古气候和人类活动的影响。

The carbon isotopic composition of stalagmite reflects the δ13C of soil CO2 above the limestone cave, which varies as a function of the relative proportions of C3 and C4 plants grown at that locality. The C3 plants, which include trees, most shrubs and herbs, and cool season grasses, prefer wet and cool climatic conditions for growth. They has δ13C values between - 25‰ and -32‰ The C4 plants, which include maize, sorghum, prairie grasses of subtropic, and savanna grasses of Africa and South America, have δ13C values ranging from - 10‰ to - 14‰, and grow favorably under the conditions of higher minimum summer temperature and low soil moisture. Hence, at a given locality, the C3/C4 plant ratio is modulated by changes in climate, and can be recorded by the δ13C of speleothems. We have measured the δ18O, δ13C and Mg/Sr ratio of a stalagmite collected from the Shihua Cave in the Western Hills about 50 km southwest of Beijing. Using the δ18O and Mg/Sr records to infer climatic conditions for the past 3000 years, the authors have found that an increase in δ13C of the stalagmite occurred under warm and arid climatic conditions which favored the growth of higher proportions of C4 plants relative to C3. However, the changes in δ13C of the stalagmite after Beijing became the capital of China cannot be fully explained by the natural variation in vegetation. During the Yuan Dynasty, the construction of the Grand Capital (Yuan Dadu) in 1272 A. D. consumed trees from the Western Hills. The construction continued at least until the late Ming Dynasty in 1553 A. D. , and resulted in a serious deforestation and reduced the C3/C4 plant ratio in the region. The δ13C of the stalagmite during this period was elevated by 3‰ The distribution pattern of C3 and C4 plants has never returned to the level prior to the deforestation events even though the climatic conditions since 360 a B P have been in favor of the growth of C3 plants.