Abstract: The application of radiocarbon dating of speleothems is often limited by "dead carbon" contamination.In this paper we have established a new model to correct contamination of "dead carbon" and determine growth rate of a stalagmite which was collected from Shihua cave, Beijing.Based on the measurements of ¹⁴C activity on 11 samples from the stalagmite, we have found that 5 of the samples have little "dead carbon" contamination and four of the samples contain relatively constant contamination of "dead carbon".The two groups of the samples gives us two linear relationships between natural log of ¹⁴C activity and depth with the same slopes but different intercepts.These data results illustrate that the stalagmite has a relatively constant growth rate and no growth hiatus which are the assumptions of the model.Using the model we have reconstructed the chronology Of the stalagmite and obtained a mean growth rate of 0.042 mm/a.The results are in good agreement with the mean growth rate determined by the annual growth banding and the youngest age measured by ²¹⁰Pb method.

Key words: Carbon-14 dating, Lsotopic age dating, Limestone, Stalagmite growth rate

摘要： 由于受“死碳”的影响,年轻洞穴碳酸盐的¹⁴C测年始终受到限制,为此,建立了¹⁴C法测定石笋生长速率及校正“死碳”影响的理论模式,从而将石笋的测年时限和精度大大提高。这一理论成功地运用于北京石花洞年轻石笋的定年。利用¹⁴法与石笋生长光性年际纹层推算的平均生长速率十分吻合,表明石笋S312的生长速率为0.042mm/a.²¹⁰Pb法测定石笋顶部的年龄小于100a,与¹⁴结果的外推年龄基本相同。这一研究结果为下一步解释稳定同位素的古气候意义奠定了年代学基础。运用¹⁴C法测定石笋生长速率要求有足够的数据点进行线性拟合。在线性范围内,要求满足石笋生长速率均匀,生长连续,“死碳”影响程度一致的假设条件。从δ¹³C和¹⁴C强度的对比分析,判断“死碳”的影响可能与气候变化有关。初步认为,湿冷气候条件下有利于石灰岩的溶解,造成“死碳”的影响增加。

关键词: 碳－14年龄测定, 同位素年龄测定, 石灰岩, 石笋生长速率