环境因素对蜗牛Achatina fulica壳体碳酸盐δ^(13)C组成影响的实验研究

Experimental Study on the Effect of Environmental Factors on the Carbon Stable Isotope Composition of the Snail Achatina fulica Shell carbonate

陆生蜗牛化石稳定同位素组成是一种良好的古环境信息载体,常被用于古环境古气候的重建。由于缺少古环境因子对蜗牛壳体稳定同位素组成机理性的结论,因此越来越多的研究侧重于对现代蜗牛壳体的环境效应进行探讨。实验室蜗牛饲养实验则可以通过控制环境因素来确定其对壳体碳酸盐稳定同位素组成的影响程度,从而得到较为确切的结果。该文在前人的研究基础上,利用Achatina fulica进行实验室饲养实验。结果表明,在相同的温、湿度下,同种食物喂养的蜗牛壳体有非常稳定的分馏值,不同食物类型的结果有一定的差异,莴苣叶、玉米粉和饲料喂养的蜗牛壳体相对于食物的分馏值分别为16.70‰±0.2‰,10.57‰±0.2‰和10.65‰±0.2‰;在20~30℃实验条件下,壳体δ13C并不受环境温度的影响,主要受食物的影响,并得到两者之间的回归方程为δ13Cs=0.6665δ13Cv+6.2302(n=26);无机碳酸盐对Achatina fulica壳体文石δ13C值影响很少;根据端元组分分析方法的统计结果显示,食物是Achatina fulica壳体碳同位素组成的主要影响因素,贡献值约为80%±5%,除此以外,大气CO2的贡献值约为20%±5%。

The stable isotope composition of land snail shell fossil is correlated with the paleo-environmental information, and thereforecould be used to reconstruct the palaeo-environment. However, the debate continues on the mechanism how paleo-environment factorsaffect the stable isotope composition of snail shell carbonate. Recent studies focus more on the δ^13C value of the modern land snail shellcarbonate which could be applied to indicate the environmental and climatic information. The laboratory cultured experiment canisolate the contribution of each particular environmental factor to the δ^13C value of the snail shell carbonate. In this study, the culturedexperiments on the snail Achatina fulica were carried out to investigate the carbon isotope fractionation between snail shell carbonateand their diets,The results show that under constant temperature and humidity there are stable fractionation offsets among the shellcarbonates when the snails were fed by the same diet. The different types of diets caused the correspondingly different carbon isotope fractination：the average fractionation offsets between the shell carbonate and the diets of the lettuce, the corn and the forage are16.70‰±0.2‰, 10.57‰±0.2‰, 10.65‰±0.2‰, respectively. The experiment also confirms that the incubators temperature rangingfrom 20 to 30 oC does not affect thefractionation offset of13 C between the snail shell and its diet. An empirical equation between the δ^13Cvalues from the shell carbonate of Achatina fulica and the diets can be described as δ^13Cs=0.6665δ13Cv＋6.2302（n=26）. The inorganiccarbonate has negligible effect on the δ13C value of snail shell. According to the result of the end-member analysis, we concluded thatthe diet contributes dominantly to the carbon stable isotope composition of the shell carbonate of snail Achatina fulica with thecontribution being about 80%±5%, and the another one, i.e, from the atmospheric CO2, being 20%±5%.