Figure 1. Location map of the study site (red star). The yellow shaded area is the distribution range of the Loess Plateau,edited from Kukla and An (1989).

2 Geological settings and sample collection

2.1 Geological settings

The loess-paleosol section is located at the Luoyang Beiyao archaeological site(34°42′24″N,112°28′46″E) on the southeast edge of the Chinese Loess Plateau (Figure 1). The Beiyao site lies on the third-grade loess accumulation terrace on the south bank of the Luo River in Luoyang. The terrace is about 20m higher than the modern river bed, and the loess section is 16.7m long from bottom to top. Grain size and magnetic susceptibility data combined with optical luminescence (OSL) and AMS 14C datings showed that the loess section has covered the last two glacial-interglacial cycles (Du et al., 2011). At present, the mean annual temperature and annual precipitation are 14.2℃ and 546 mm, respectively. The study area is located in a typical monsoonal region. Northerly wind prevails and climate is cold and dry in winter, while southerly wind dominates in summer with hot and rainy condition. A large number of stone artifacts were found in the Beiyao section at depth of 6.5~7.5m and 11~13m, indicating that there were prehistoric human activities.
The magnetic susceptibility and median particle size curves showed synchronous changes, and had a good correspondence with the marine oxygen-isotope stage (MIS) curve (Tang et al. , 2017). Therefore, in this study, we sub-divided the loess section to various oxygen isotope stages according to the grain size and magnetic susceptibility, referring to AMS 14C and OSL datings.

2.2 Collection of land snail fossils

During the sampling process at the Beiyao site, 1m × 1m × 10cm volume loess (or paleosol) was continuously excavated downward, and the snails in each horizon were collected by screening and washing using water and a 0.5 mm sieve. The identification and statistics of the snail fossils used in this study were completed by Yan Wu. Throughout the section, there were 1911 cold-aridiphilous Cathaica pulveratrix (C. pulveratrix ) and 241 sub-humidiphilous Metodontia yantaiensis(M. yantaiensis ) (Wu, 2011). When the fossil fragments were counted as Quaternary loess snail individuals, the calculation method developed by Puisségur (1976) was used to convert the fragments into snail fossil individuals and sum them as the total number of individuals. The conversion formula (Puisségur, 1976) is as followed:
Number of individuals = number of fragments/5 - number of fragments/5 × conversion factor
The conversion factor varies with the number of snail fossil fragments. When the number of snail fossil fragments is <50, 50-75, 75-100, and >100, the conversion factor is 10%, 20%, 33%, and 50%, respectively. Except for few fossils due to the strong pedogenesis at 6.8-7m, most of the section is rich in snail fossils. In this study, we used complete shell of land snails for stable isotope analysis. Totally, there are 577 C. pulveratrix shells from 59 horizons, and 97 M. yantaiensis shells from 15 horizons.