6 Conclusion

In this study, we systematically analyzed stable carbon and oxygen isotopes on cold-aridiphilous C. pulveratrix and sub-humidiphilous M. yantaiensis snail shell fossils from the Beiyao loess-paleosol section in southeastern Chinese Loess Plateau. Stable isotopes were measured on both the mixed multiple shells and the single shell along the growth band. The obtained δ13C and δ18O data were compared with Md and SUS from the same profile and deep-ocean δ18O curve to verify the reliability of snail shell stable isotopes for paleoclimate reconstruction. We reached the following conclusions:
1. δ18O of snail shells in strata can be used to indicate the intensity of summer monsoon rainfall. During MIS7 and MIS3 stages, the shell δ18O was more negative, indicating strong monsoonal rainfall, which showed a good correlation to Md, SUS, and deep-sea δ18O curve. Meanwhile, the shell δ13C can reflect the proportion of C4plants in snail’s food and ultimately trace the relative abundance of C4 plants in contemporary vegetation. The results showed that the relative abundance of C4 plants increased during the warm/humid MIS7 and MIS3.
2. The stable isotopes of C. pulveratrix and M. yantaiensis from the same horizon were largely different, reflecting differences in their eco-physiological habits. The δ18O of M. yantaiensis was significantly lower than that of C. pulveratrix , indicating that M. yantaiensis lived in warmer and more humid conditions than C. pulveratrix . The δ13C of M. yantaiensis was mostly higher than that of C. pulveratrix , suggesting thatM. yantaiensis ingested more C4 plants thanC. pulveratrix .
3. Intra-shell δ18O variations revealed that there was a significant difference in the climatic seasonality between glacial and interglacial periods. During the glacial periods (even-numbered MIS), the seasonal contrast was large, whereas the seasonal contrast was small during the interglacial periods (odd-numbered MIS). Stable isotope analyses of multiple shells of the same snail species within each sampling layer showed that intra-species isotope data were largely scattered during the glacial periods, indicative of highly unstable climates change at sub-millennial scale, whereas intra-species isotopic difference was relatively small during the interglacial periods, suggestive of a steady and uniform climatic condition within millennium.
4. During MIS3 and MIS7, there were evidences of human activities around the Beiyao site, but the corresponding climate background remained unclear. By analyzing whole-shell and intra-shell δ18O and faunal assemblage of the two species snails, we concluded that the climates were relatively warm and humid with a weak seasonality. This stable climatic condition was conducive to the regional expansion of prehistoric human activities.