Quantitative Research On The Relationship Between Carbon And Oxygen Isotope Composition With The Environment Factors

The carbonate of snail shell acts as a good record of the environmental information,which was usually used to reconstruct the palaeoenvironment and palaeoclimate.Recently,numbers of studies have investigated the relationships between the ?13C and ?18O of the modern land snail shells with the environmental and climatic factors.However,the stable isotopes of shells are influenced by complex natural environment conditions and some other factors,such as the physiological characteristics of snails,so it is difficult to isolate separately the contribution of the particular environmental factor to the stable isotope composition of the shell in the field sampling studies.Based on the field sampling of snails and the controlling experiment in lab,we obtained some conclusions as follows:1.The distribution of ?13Cshell values of individual shell samples in the study regions vary in a very wide range(-17.75‰?-6.6‰),deviating negatively from the ?13Cshell data range worldwide(-15.7‰?+1.7‰),which indicates that the land snail can adapt to different types of climate.The?13Cshell data from Baoji show approximately a Gaussian distribution pattern,suggesting that the average value is representative of the carbon stable isotopic composition of the whole region.The similarity of different species in Qixia Mountain could indicate that they have similar feeding habits,making them potential sources of isotopic information on the paleoecology of that location.With analysis of the shell-body isotope difference(?13Cshell-body)from 15 sampling areas,we found that the offset between shell and body tissue appeared to be influenced by the vegetation patterns in the natural environment.Furthermore,significant correlation between the ?13C of shells and the ?13C of soil organic carbon was obtained,so we conclude that the carbon isotope composition of soil organic residue is a comprehensive reflection of various plants' carbon isotopic composition and provide a new indicator for using the carbon isotope composition of wild snail shell to reflect local vegetation composition.2.More importantly,we identified significantly negative correlations between shell ?13C values and the mean monthly precipitation(MMP)and the mean monthly temperature(MMT),as well as positive correlations between that and the altitude and latitude,we suggest that the underlying principles for shell ?13C to be a climate indicator rest on the control of precipitation and temperature decrease along monsoon route from south to north and the associated relative abundance change in C4 plants which leads to an increase in plant ?13C.Therefore,climatic factors exert their impact indirectly on the shell ?13C values through altering the vegetation compositions in the nature,allowing us to conclude that snail shells recorded the evolution of East-Asian Summer Monsoon.An empirical equation relating shell ?13C and precipitation and temperature in this study region can be obtained as follows:?13Cshell=-0.047(MMT)-0.023(MMP)-9.147(R2=0.661)3.The relationship between ?18O of field snail shell from this study region and single climate factor(such as precipitation and temperature)was not found.But in this study,we used laboratorycultured land snail Achatina fulica to investigate the contribution of different environmental factors on the isotopecomposition of land snail shell and revealed that:(1)the environmental water is the dominant factor influencing the oxygen isotope composition of snail shells under the constant temperature condition,the regression equations between ?18O values of the sampled snail shells and the ?18O of environment water are y=0.5429x+1.4749(R2=0.9853,n=18)for snails fed lettuce and y=0.7821x+4.1556(R2=0.9651,n=18)for snails fed corn respectively;(2)temperature plays a negligible role on the isotope fractionation of snail shells when it varies within a small range;(3)the ?18O value of snail shell is not affected by inorganic carbonate,instead by the water contained in the food.