Figure 5. Intra-shell variations of δ18O and δ13C for the two species at various MIS stages.
5.4 Intra-shell variation of stable isotopes and climate seasonality
In this study, intra-shell stable isotope analyses were performed on both C. pulveratrix and M. yantaiensis snails at MIS3, MIS4, MIS6, and MIS7, respectively. The measured C. pulveratrixand M. yantaiensis snails were chosen from the same layer (10 cm) in each MIS stage. During MIS3 , the δ18O of C. pulveratrix and M. yantaiensis were among the most negative values of the four MIS stages, with averaged δ18O ​​of -9.5.‰ and -9.8‰, respectively. Moreover, the intra-shell variations in δ18O of the two snails were relatively small. For example, the δ18O of C. pulveratrix showed a variation magnitude of 5.9‰ whereas δ18O of M. yantaiensis only changed by 2.9‰ (Figure 5a, e). This suggested a weak seasonality during the warm/humid MIS3 stage. Padgett et al.(2019) also observed a steady trend of δ18O in land snail shell in warm and humid climate. In contrast, the magnitudes of intra-shell δ18O variations for C. pulveratrixand M. yantaiensis showed large increases, i.e., up to 10‰ and 4.9‰, respectively.
During MIS6, the average δ18O values of C. pulveratrix and M. yantaiensis became more positive and were around -3.6‰ and -5.7‰, respectively (Figure 5c, g). Meanwhile, the intra-shell δ18O of the two species exhibited largest variations during MIS6, i.e., a magnitude of 15.5‰ for C. pulveratrix and a magnitude of 12.1‰ for M. yantaiensis . These magnitudes were respectively 2.6 and 4 times of those for the same species during MIS3. It revealed extreme seasonal contrast during the cold/dry MIS6. It is worthy of mentioning that the intra-shell δ18O curve of C. pulveratrix displayed regular seasonal changes during MIS6 (Figure 5c). Judging from the sinusoidal cycles, the C. pulveratrix snail may have a life span of about two years. The snail possibly started to grow from the summer of the first year to the autumn of the second year. The highest δ18O values recorded in the shell growing in the spring and autumn seasons attained to ca +2‰ and the lowest δ18O recorded in the shell segments in summer was about -12‰. The large seasonal contrast was unlikely only attributed to temperature changes, which would be 56 °C offset if calculating by the carbonate oxygen isotope-temperature coefficient of 1‰ per 4 °C. Obviously, seasonal changes of rainfall largely contributed to the above fluctuation of δ18O of C. pulveratrix , that is, the negative values in shell δ18O being caused by rainfall amount effect in summer. An intra-shell δ18O study for the land snail collected from Ethiopia also revealed significant contribution of rainfall to the shape and amplitude of shell δ18O cycles (Leng et al. , 1998). Except for the shell lip part, the δ13C of C. pulveratrixshowed an overall opposite relationship with the shell δ18O (Figure 5c). When the δ18O was more negative in summer, the δ13C became more positive, implying the snail consumed increased amount of C4 plants in this season. In spring and autumn (at 30-45 mm from shell lip), more C3 plants were ingested by the snail. This seasonal change of C3/C4proportion in snail’s food diet is consistent with the seasonal distribution of C3 and C4 plants in natural vegetation (Sage et al. , 1999).
During MIS7, two individual shells for intra-shell isotope study were taken from the depth of 11.8 m, which happened to be within the period of strong prehistoric human activities (Du and Liu, 2014). Based on the previous discussions on δ18O of C. pulveratrixand M. yantaiensis , the climate was generally warm and humid during this time. The intra-shell δ18O variations forC. pulveratrix and M. yantaiensis were at amplitudes of 10.7‰ and 10.9‰, respectively. The variations were smaller than those during MIS6. This overall small seasonal contrast was conducive to regional spread of human activity.
In summary, the average amplitude of intra-shell δ18O variations for C. pulveratrix was ​​about 8.4‰ during the interglacial periods (i.e., MIS3 and MIS7), whereas it was 12.75‰ during the glacial periods (i.e., MIS4 and MIS6). In the same manor, the intra-shell δ18O of M. yantaiensis varied by 10.8‰ and 16.5‰, respectively, during the interglacial and glacial periods. Regardless of which species, the changing amplitude was 1.5 times larger during the glacial periods. Therefore, if the intra-shell variation of δ18O can be used to quantify the seasonal changes, the climatic seasonality during glacial periods would be about 1.5 times stronger than that during interglacial periods.
To explore the stable isotope differences among individual shells of each snail species from the same sampling horizon (10 cm layer), we analyzed δ13C and δ18O on C. pulveratrix from 7 layers and M. yantaiensis from 3 layers. The carbon and oxygen isotope data were shown in Table 3. Firstly, within the same MIS (i.e., MIS3 or MIS7), the δ18O of sub-humidiphilous species (M. yantaiensis ) showed little change, whereas the δ18O of cold-aridiphilous species (C. pulveratrix ) distributed much discretely. This may indicate that sub-humidiphilous species have a more strict requirement on climate conditions, i.e., only grow during the period of abundant rainfall, while cold-aridiphilous species had strong adaptability and can survive under large range of climate conditions. Secondly, for the cold-aridiphilous species, the shell δ18O changes during the even-numbered MIS (i.e., MIS2, MIS4, and MIS6) were larger than those during the odd-numbered MIS (i.e., MIS3 and MIS7). Since the snail shells collected each sampling layer may not strictly come from the same time year, the above phenomenon may indicate that the climates within the time-span of each sampling layer during glacial periods (even-numbered MIS) were very unstable, whereas the climates during interglacial periods (odd-numbered MIS) had relatively stable and uniform conditions within the time period of each sampling layer. Previous studies have shown that climate during the last glacial period was quite unstable, with climate oscillations at centennial to millennium scales (Ren et al. , 1996; Ding et al. , 1998). This is in accordance to the large intra-species variation of shell δ18O in each sampling layer.
Table 3 Statistics for intra-species δ18O and δ13C variations of two species at various MIS stages.