Abstract
Modern investigations have shown that oxygen and carbon isotopes of land snail shells are useful indicators of climate and vegetation in monsoonal region. However, stable isotope study on snail fossil shells in strata has been seldom done, and the reliability of those indicators needs further verification. Moreover, intra-shell stable isotope analysis of individual snail is rather scarce, and seasonal variation of the glacial-interglacial monsoonal climate remains unclear. In this context, we performed δ18O and δ13C analyses on fossil shells of cold-aridiphilous Cathaica pulveratrix and sub-humidiphilous Metodontia yantaiensis from the loess section over the last two glacial cycles at Beiyao site in southern Chinese Loess Plateau. The δ18O of fossil shells reflected monsoonal rainfall amount and more rainfall during MIS3 and MIS7. Meanwhile, the δ13C of fossil shells indicated relative abundance of C3/C4 plants and more C4 biomass during MIS3 and MIS7. The δ18O and δ13C of the two species from the same horizon are significantly different, reflecting differences in their growing season and/or physiological habits. Intra-shell variations of stable isotopes showed that climatic seasonality was relatively strong during the glacial periods whereas seasonality became weakened during the interglacials. Our findings provide an environmental background for explaining past human activities at the Beiyao site. The investigation of stone artifacts showed that ancient human activities were relatively strong during MIS3 and MIS7. During these stages, the warm and humid climate with smaller seasonal contrast was favorable for the regional expansion of human activities.

1 Introduction

Land snails are ideal materials for paleoclimate studies(Goodfriend,1992; Wang et al. , 2016; Wu et al., 2018). This is because they have advantages of being widely distributed, abundant and well preserved in strata. And they are relatively sensitive to climate changes. To date, researches on land snails include inferring the environmental conditions under which land snails survived through identifying faunal assemblage and living habit of each species (Gittenberger and Goodfriend., 1993; Wu et al. , 2008), and reconstructing the paleoclimates through analyzing stable isotopes of land snail shells (Goodfriend and Ellis, 2002; Liu et al. , 2006; Gu et al. , 2009; Colonese et al. , 2010; Rangarajan et al. , 2013; Yanes and Fernández-Lopez-de-Pablo, 2016; Prendergast et al. , 2016; Padgett et al. , 2019).
Theoretically, oxygen isotope in snail shell is determined by the oxygen isotope of snail body water and the temperature under which shell carbonate precipitates. Although body water oxygen isotopes of land snails are modified to different extents by evaporation due to differences in physiological habits of various species, it can still be generally used to track changes in precipitation oxygen isotopes (Zarrur et al. , 2011; Zhai et al. , 2019). Therefore, in the case of little temperature change, oxygen isotopes of land snail shells mainly reflect oxygen isotopes of rainfall (Prendergast et al. , 2016; Wang et al. , 2016; Milano et al. , 2018; Padgett et al. , 2019). The snail shell carbon isotope reflects the carbon isotope composition of food they intake, with a large proportion of dietary plants, e.g., organic food accounts for more than 70% of carbon sources of land snail shell (Xu et al. , 2010). In brief, the shell carbon isotope value can provide information on the relative abundance of C3/C4 plants in the food(Goodfriend and Ellis, 2002; Prendergast et al. , 2017).
Land snail fossils are abundant and widely distributed in the Asian monsoon region, especially in the Chinese Loess Plateau (Wu et al. , 1996;Wu et al. , 2002; Liu et al. , 2006; Gu et al. , 2009). However, researches on the stable isotopes of snail shells have mainly focused on studying modern land snails in different climatic regions (Liu et al. , 2006; Wang et al. , 2016; Bao et al. , 2018, 2019; Wang et al. , 2019; Zhai et al. , 2019). In contrast, stable isotope analyses of fossil snails in strata have been inadquently done, and only a few species of land snails were studied (Gu et al. , 2009; Huang et al. , 2012). In this context, it is necessary to perform stable isotope analyses on shell fossils of different land snail species from strata in the different regions and compare those data with other paleoclimatic proxy indicators to confirm their paleoenvironment and paleoclimate significances. Moreover, stable isotope analysis of individual shell along shell ontogeny has the potential to provide seasonal information (Leng et al. , 1998; Goodfriend and Ellis, 2002). However, the application of this type of research in paleoclimate is also less developed.
In this study, we systematically collected land snail fossils from loess-paleosol section over last two glacial-interglacial cycles at the Beiyao site in Luoyang, central China. Carbon and oxygen isotopes were measured on Cathaica pulveratrix (cold-aridiphilous) andMetodontia yantaiensis (sub-humidiphilous) land snails. We then compared these isotopic data with paleoclimate proxy indicators like grain size and magnetic susceptibility with attempt to reconstruct changes in climate and vegetation (C3/C4plants) in the study area. The Luoyang Beiyao site is an archaeological site with human activities in the Paleolithic Age. Recent studies have found some lithics in strata belonging to the late glacial period and the middle and late MIS7 stage (Du et al. , 2011; Du and Liu, 2014), indicating that there were human activities during those time periods. However, the climate and environmental context associated with the human activities is still unclear. This study will precisely analyze the environmental conditions for the human activities during the late glacial period and the middle and late of MIS7. At the same time, we also selected snail fossils during the typical periods of the glacials and interglacials, and analyzed intra-shell isotopic varation of each shell to obtain seasonal information during these periods, thereby helping us to understand changes in climatic seasonality from glacialto interglacial period.