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.