3 Materials and Methods

3.1 Snail shells pretreatment and sampling strategy
The entire shell was firstly cleaned with distilled water, and the soil particles attached to the shell surface were brushed using a toothbrush, and then the shell was placed in a drying oven and heated at 60 °C for 12 hours. The relatively large shells were chosen for sampling along the growth band. Firstly, weremoved the residual clay cements on the surface of shells using a dental drill, then cleaned the shells using a ultrasonic utility for multiple times, and finally dried the shells in an oven. The three dimension of each shell (i.e.,shell height, width and height of shell mouth) was measured using a ruler. For intra-shell sampling, we use a micro drill to take powders from the shell lip till apex at 1-2 mm interval along the growth direction of the snail (Figure 2). The drill bit was soaked in diluted hydrochloric acid solution after each sample to remove residual carbonate powder on it.
For the carbon and oxygen isotope analyses of the whole shell, about 10 shells were combined according to the availability of snail shells in each horizon. This can ensure the measured data to represent a general and average environment condition under which land snails lived. After the shell was cleaned and dried for the first time, it was broken into fragments. The clay cement attached to each shell fragment was physically removed, and then the fragments were further cleaned using an ultrasonic utility. After very clean shell fragments were obtained, we dried them in an oven at 60 °C. Finally, we ground them into powders and homogenized using a mortar and pestle.
3.2 Stable isotope analyses
The carbon and oxygen isotopic analyses of the snail shell powder were performed on the GasBench II multifunctional gas preparation system coupled with the Delta V Plus isotope ratio mass spectrometer (Thermo Fisher). A 100µg carbonate powder reacted with 100% H3PO4 at 72 °C for 1 hour. The generated CO2 passed through two NAFIONTM water traps to remove trace water and passed through a PoraPlot Q chromatography column at 45 °C to separate with other impurities. After that, the CO2 was introduced into the isotope ratio mass spectrometer to measure the carbon and oxygen isotope ratios. Both carbon and oxygen isotope data are reported relative to the VPDB. The standards used for data correction and calibration were GBW4416 (δ13CVPDB=1.61‰, δ18OVPDB=-11.59‰) and NBS19 (δ13CVPDB=1.95‰, δ18OVPDB =-2.20‰). The analytical precision of carbon and oxygen isotopes is 0.06‰ and 0.10‰, respectively. Detailed analytical method can be found in Wang et al. (2019).

4 Results

4.1 Carbon and oxygen isotopes of whole shell for two species land snails
The variation range of δ18OVPDB for cold-aridiphilous C. pulveratrix was -2.16‰ to -8.13‰, and the average value was -5.03‰. The maximum value of δ18OVPDB was at the depth of 1.1 m in the profile, which corresponds to MIS2, while the minimum value of δ18OVPDB was at the depth of 11.7m, which belongs to MIS7. The δ18OVPDBvalue for sub-humidiphilous M. yantaiensis ranged from -7.34‰ to -9.71‰, with an average of -8.43‰. The maximum δ18OVPDB value was at 4.6 m (MIS4) and the minimum at 11.6 m (MIS7).
The δ13CVPDB for C. pulveratrixranged from -3.17‰ to -6.62‰ with an average of -4.81‰. The maximum δ13C was at the depth of 10.1 m in the profile, which belongs to MIS6 whereas the minimum δ13C was at 6.6m, which corresponds to the MIS5. The range of δ13CVPDB for M. yantaiensis was between -3.05‰ and -5.03‰, and the average value was -3.95‰. The maximum δ13CVPDB for M. yantaiensisshowed at 3.4 m (MIS3) whereas the minimum δ13CVPDB occurred at 12 m (MIS7).
4.2 Carbon and oxygen isotope changes along the growth band of individual shell
In the MIS3 and MIS5, intra-shell δ18OVPDB variation for C. pulveratrix was from -12.3‰ to 0.2‰, and the variation of δ13CVPDB was between -6.9‰ and -4.9‰. In contrast, of the intra-shell variation of δ18OVPDB for M. yantaiensis was relatively small, i.e., from -10.1‰ to -5.9‰. The intra-shell δ13CVPDB ranged from -7.7‰ to -4.8‰ (Table 1).
During the MIS4 and MIS6 stages, the intra-shell δ18OVPDB and δ13CVPDB for C. pulveratrixvaried from -12.0‰ to -3.5‰ and from -7.8‰ to -2.6‰, respectively. The corresponding intra-shell variations for M. yanntaiensis were much larger, i.e., from -13.3‰ to -1.7‰ for δ18OVPDB and from -11.7 to -0.6‰ for δ13CVPDB (Table 1).
The cold-aridiphilous C. pulveratrix had a shell height of 1.1 to 1.5 cm, a shell lip height of 0.7 to 0.85 cm, and a shell lip width of 0.6 to 0.8 cm. In contrast, the sub-humidiphilous M. yantaiensishad shell height ranging from 0.55 to 0.95 cm, shell lip height ranging from 0.3 to 0.4 cm, and shell lip width ranging from 0.35 to 0.5 cm (Table 2). Obviously, the shell of C. pulveratrix was ​​significantly larger than that of M. yantaiensis . As a result, the intra-shell sampling number for C. pulveratrix was larger than that for M. yantaiensis .
Table 2 Statistics for Intra-shell δ18O and δ13C variations of two species at various MIS stages.