Elsevier

Chemical Geology

Volume 533, 5 February 2020, 119414
Chemical Geology

Clumped and stable isotopes of land snail shells on the Chinese Loess Plateau and their climatic implications

https://doi.org/10.1016/j.chemgeo.2019.119414Get rights and content

Abstract

We report stable (δ13Cshell, δ18Oshell) and clumped isotope (Δ47) compositions of modern and last glacial fossil snail shell carbonates from the Luochuan and Weinan sections on the central and southern Chinese Loess Plateau (CLP). Our study reveals that the average Δ47 temperature (T47) of modern snails is consistent with monitored temperatures during the snail growing season at the studied locations and is ~10 °C higher than that of fossil snails from glacial time. Moreover, the average δ13Cshell of modern snails is more depleted than that of fossils. We argue that the δ13Cshell cannot record changes in plant communities (i.e., the C3/C4 ratio) on the CLP and may mainly indicate arid conditions with depleted values reflecting reduced aridity. Additionally, the reconstructed snail body water δ18O (δ18Owater) of modern snails is more enriched than δ18O in modern growing season precipitation and δ18Owater of fossils. This contrast may be related to the high degree of evaporative enrichment of environmental water 18O in the body/ingested by modern snails under warm conditions. Therefore, we suggest that using δ18Oshell to directly reconstruct the oxygen isotopes of precipitation is difficult and that higher δ18Oshell and δ18Owater values probably indicate higher environmental temperature/stronger evaporative enrichment on glacial-interglacial timescales on the CLP.

Introduction

Widespread land snails are regarded as a valuable archive for studying environmental and climatic conditions because of their high sensitivity to temperature and humidity (Liu, 1985; Goodfriend, 1992; Wu et al., 2018). Moreover, their fossil remains are abundant in Quaternary deposits, such as the loess-paleosol sequences on the Chinese Loess Plateau (CLP), and they are considered typical ‘index animals’ in paleoenvironmental studies (Liu, 1985; Wu et al., 2018). Thus, snail faunal assemblages (Goodfriend, 1992; Rousseau and Wu, 1997; Wu et al., 2002, Wu et al., 2018) and isotopic compositions of snail shell carbonates (Goodfriend, 1992; Bonadonna and Leone, 1995; Abell and Plug, 2000; Goodfriend and Ellis, 2000; Balakrishnan et al., 2005a; Colonese et al., 2010, Colonese et al., 2013; Kehrwald et al., 2010; Yanes et al., 2011, Yanes et al., 2013a, Yanes et al., 2013b, Yanes et al., 2014, Yanes et al., 2017; Huang et al., 2012; Prendergast et al., 2016) have been widely used to decipher past climatic and environmental changes.

Many studies have suggested that the carbon isotope composition of land snail shell carbonate (δ13Cshell) is derived from three potential sources: diet, atmospheric CO2, and ingested carbonates (Goodfriend and Hood, 1983; Pigati et al., 2004; Quarta et al., 2007; Romaniello et al., 2008; Xu et al., 2011; Zhang et al., 2014). Laboratory studies indicated that the δ13Cshell reflects mostly the respired CO2 derived from ingested plants (Stott, 2002; Metref et al., 2003; Zhang et al., 2014). Thus, most published field investigations of modern/fossil land snails assume that they primarily consumed C3 and C4 plants in relation to their abundances in the surrounding environment, and δ13Cshell has therefore traditionally been used to deduce variations in C3/C4 plants in the landscape (Goodfriend and Ellis, 2000, Goodfriend and Ellis, 2002; Balakrishnan et al., 2005a, Balakrishnan et al., 2005b; Liu et al., 2007; Yanes et al., 2008, Yanes et al., 2009, Yanes et al., 2011, Yanes et al., 2013b, Yanes et al., 2014; Colonese et al., 2014; Prendergast et al., 2017). δ13Cshell can also be applied to infer changes in aridity in C3-dominated landscapes (Colonese et al., 2013; Prendergast et al., 2017). However, snails show a preferential use of C3 food when fed a mixed diet (Metref et al., 2003; Zhang et al., 2014). Moreover, radiocarbon dating work has indicated that carbon from ingested carbonates and atmospheric CO2 may also contribute to snail shells (Goodfriend and Stipp, 1983; Pigati et al., 2004; Quarta et al., 2007; Romaniello et al., 2008; Xu et al., 2011). These additions complicate the explanation of δ13Cshell from the perspective of climatic and environmental changes because the relative contributions from the three potential sources remain unclear.

Pioneering attempts showed that the oxygen isotope compositions of land snail shells (δ18Oshell) were inversely related to local relative humidity (Yapp, 1979). Goodfriend et al. (1989) and Baldini et al. (2007) found that the isotopic compositions of snail body water (δ18Obody water) and δ18Oshell were influenced by atmospheric water vapor. Other studies illustrated relationships between the oxygen isotope composition of meteoric water and those of land snail shells (Lécolle, 1985; Goodfriend et al., 1989; Zanchetta et al., 2005; Yanes et al., 2009, Yanes et al., 2017; Colonese et al., 2014; Prendergast et al., 2015). Zhang et al., 2018a, Zhang et al., 2018b suggested that food water should also be considered a water source, apart from rainwater. Thus, the flux balance model proposed that the δ18O value of snail shell carbonate represented the combined effects of the relative humidity, δ18O of ingested water (e.g., precipitation), and temperature at which the shell precipitated (Balakrishnan and Yapp, 2004). Overall, the explanation of the oxygen isotopes of land snail shells is more complex than that of the carbon isotopes due to the variable temperature, relative humidity and δ18O of water in the terrestrial environment.

The newly developed carbonate clumped isotope (expressed as Δ47) thermometry technique was proposed to quantitatively reconstruct the temperature of carbonate mineral growth under isotopic equilibrium conditions (Ghosh et al., 2006; Eiler, 2007). This technique is based on the abundance of 13Csingle bond18O bonds in the carbonate lattice relative to those expected for a random distribution of isotopes among all isotopologues and is independent of the δ18O of the waters from which carbonates grew (Ghosh et al., 2006, Eiler, 2007). Recently, published data on different carbonate minerals of biogenic/inorganic origin demonstrated that Δ47 generally follows a universal Δ47-temperature calibration (Bonifacie et al., 2017; Kelson et al., 2017). Therefore, we apply this thermometry to simultaneously obtain the shell formation temperature and the δ18Obody water of land snails in combination with δ18Oshell, both of which are indispensable for understanding the stable isotope composition of snail shells.

However, current studies showed discrepant results when converting snail shell clumped isotopes to land snail shell calcification temperature (T47) or the environmental temperature. For example, Zaarur et al. (2011) indicated that T47 values of snail shells are typically higher than either the mean annual or the snail activity season ambient temperatures. In contrast, Eagle et al. (2013) and Wang et al. (2016) revealed that T47 values for land snails are strongly correlated with environmental temperatures, although they are still higher than the local warm month mean temperatures for Cathaica sp. and Bradybaena sp. (Wang et al., 2016). Recent work by Zhai et al. (2019) and Guo et al. (2019) also reported higher than expected T47 versus either environmental temperatures or estimated snail activity temperatures. Furthermore, snail culturing experiments suggested that Δ47 of Acusta despecta land snails can be used to reconstruct the mean seasonal temperature of snail activity (Zhang et al., 2018a, Zhang et al., 2018b). In this context, the Δ47 values of modern and fossil snails deserve further investigation.

Therefore, we comprehensively studied the clumped and stable isotopes (Δ47, δ13Cshell, δ18Oshell, and δ18Obody water) of modern and last glacial fossil land snail shells from the Luochuan and Weinan sections on the central and southern CLP. The aims of this study are 1) to quantitatively reconstruct the temperature changes from the last glacial to modern time using the clumped isotope technique and 2) to further understand the climatic implications of δ13Cshell and δ18Oshell as well as δ18Obody water on the CLP on glacial-interglacial timescales.

Section snippets

Materials and methods

The Luochuan (35.7° N, 109.4° E) and Weinan (34.5° N, 109.6° E) sections are located in the center and on the southern edge of the Chinese Loess Plateau (CLP), respectively, and the linear distance between the two sections is ~150 km (Fig. 1). Approximately 15 kg of sediment was excavated at 10 cm intervals from both profiles since the last interglacial (S1 paleosol). The sediments of each sample were washed and sieved in the field on a 0.5 mm mesh sieve, and the fossil snail shells were picked

Results

Table 1 presents the stable isotope values and clumped isotope without acid fractionation correction (Δ47CDES90) results for fossil and modern shells analyzed in 2012 and 2017 at Caltech, as well as the calculated snail body water isotope and clumped isotope temperature (T47). The δ18Owater was calculated using the aragonite–water fractionation proposed by Kim et al. (2007), and the Δ47–T calibration line and the acid fractionation factor of 0.088 in Petersen et al. (2019), who reprocessed data

Clumped isotopes of snails

Previous studies reported that the T47 of modern snails is higher than the mean annual temperature and even higher than the growing season temperature (GST), and this phenomenon was attributed to snail eco-physiological adaptations (Zaarur et al., 2011; Wang et al., 2016; Zhai et al., 2019; Guo et al., 2019). In contrast, our results show that both the range and mean values of T47 for our modern snails are very close to the temperature from April to October when the monthly average T is >10 °C

Conclusions

We studied the clumped and stable isotopes (δ13Cshell, δ18Oshell, and δ18Owater) of modern land snail shells and fossil snail shells from the last glacial period at the Luochuan and Weinan sections on the CLP. Contrary to previous studies, our study shows that the clumped isotope temperature of snail shells is consistent with the snail growing season temperature at the studied locations and that it is ~10 °C lower for last glacial fossil snails than for their modern counterparts. Moreover, we

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We sincerely thank professor Peter Hale Molnar for comments on and revision of the manuscript, and we thank Dr. Linpei Huang for help in the identification of fossil snail species and Dr. Ryb Uri and Dr. Max Lloyd for help with data processing and discussion. This work was jointly supported by grants from the Training Program of the State Key Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences (QYZDY-SSW-DQC001 and ZDBS-SSW-DQC001), the MOST program (2016YFE0109500), the

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