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.