A Research On The Carbon Stable Isotopic Composition In Modern Snail Shell Aragonite And Its Environmental Implications

The carbonate of snail shell acts as a good record of the environmental information, which was usually used to reconstruct the palaeo-environment. Recently, lots of studies on the ?13C of the modern land snail shells have been applied to explore the environmental and climatic implications. However, within the field sampling studies, it is difficult to isolate separately the contribution of the particular environmental factor to the ?13C of the shell.1. We find the mineral vertiate in intermediate layer in the Snail shell growth under SEM, showing a close packing of aragonite spheres, illustrate that in the process of growth the snail shell through the formation of the gel and retrogradation of crystallization process. XRD data showed that the snail shell forming aragonite structure in the early period, there is existing much amorphous vetirate, which truns out the result that the phase transfer of calcium carbonate minerals existing in the process of growth in the snail shell. The mineralogy of the snail shell orientation is due under the participation of organic matter, its experience in the process of forming the gel condensation for chondrules after aging crystallization process. In this paper, We summarize that the formation process of the snail shell has four stages as the formation of calcium carbonate gel, the formation of calcium carbonate spherulite, mineral phase transformation and the subsequent growth four stages. The theory still needs more experimental data support, says biology, mineral crystallography, geochemical and other related disciplines.2.Under laboratory condition, the cultured experiments on the snail Achatina fulica were carried out to conclude that the 13C fractionation between snail shell and their diets. There are very stable fractionation offsets between the shell aragonite when breeded the same diet, and the results of the different types of diets are rather different, says the average fractionation offsets between the shells and the diets of the lettuce, the corn and the forage were 16.70‰±0.2‰,10.57‰±0.2‰,10.65‰±0.2‰ respectively. carbon isotope fractionation between food and the snail shell with the diet of the C3 plants is close to many previous studies (in a relatively narrow range of 13.3‰-15.2‰), and the diet of C4 plants the corn and the forage turns out that the fraction is below this range. Under the condition of 20?30?, The laboratory experimental result also confirms that temperature of incubators ranging within 20?30? does not affect the fractionation offset of 13C between the snail shell and its diet. Inorganic carbonate does little effect to the ?13C of snail shell. CO2 in atmosphere contributes about 30%.to the snail shell carbon isotopic composition,the diet contributes almost 70% which make it the main factor. An empirical equation between the ?13C values from the shells of Achatina fulica and the diets can be derived as: ?13Cs=0.6665?13Cv+6.2301.3. When the moisture and the ?18O of water remains almost constant, the ?18O value of shell is mainly influenced by the environmental temperature, when the temperature arise 1?, the ?18O value of shell inceases 0.206%o, and the empirical equation is: 103ln? (Aragonite-H2O)=-2.7044*106T2+61.692.The relationship also indicates that the relationships between ?13C??18O of land snail shells and the temperature, moisture, rainfall, plant can be established via further laboratory investigation, and finally build up the quantitative indicator system.4. We measured carbon isotopic compositions of the 10 aragonite in both field collected 11 snail shells and laboratory cultured snail shells, to quantitatively explore the environmental and climatic implications of ?13C in the land snail shell aragonite. The snail Achatina fulica was cultured under stable incubator humidity. We found that the ? 13C fractionation between the cultured snail shell and its diet is 14.7‰±0.7‰. This result also confirms that limited amount of lime added to the diet and the slight temperature variation of incubators in the laboratory do not affect 13C fractionation between the snail shells and their diet. A total number of 265 modern land snail shells were collected from 18 sampling sites throughout the eastern China and ?13C values of these samples vary in a wider range from-18.60%o to-6.53‰. They are obviously deviated negatively from the previously reported worldwide range (-13.5‰-0.5‰) and north American (-13.2‰-0.0‰). The carbon isotopic compositions of modern snail shells in our sampling sites exhibit negative correlations with the average mean precipitation (AMP) (R=0.695; n=18; p<0.05) and the average mean temperature (AMT) (R=0.521;n=18;p<0.05) during the snail living season. Contrarily, the ?13C of field snail shells shows positive correlation with the elevation (R=0.688;n=18; p<0.05) and the latitude (R 24=0.586; n=18;p<0.05). We believe that the environmental factors impacting on 813C of snail shells via regulating and altering the local vegetation types or carbon isotopic compositions in an ecosystem. The precipitation and temperature during the snail living season are the fundamental factors that affect and may alter the distribution pattern of the vegetation with different photo-synthetic styles in nature. An empirical equation between the land snail shell 813C values and annual average precipitation (P) and temperature (T) in snail growing season can be derived as:?13 C snail shell=10.33-7.65 logT-3.89 logP (R=0.764, n=18,p<0.05).