Stable isotope and radiocarbon analyses of livestock from the Mongol Empire site of Avraga, Mongolia
Introduction
The archaeological site of Avraga in eastern Mongolia has produced evidence of a substantial number of wooden structures and an abundance of livestock remains (Kato and Shiraishi, 2005). This is unusual (though not unprecedented) in the Mongolian context where most archaeological sites consist of human burial structures or surface artifact scatters (Gunchinsuren, 2017; Wright, 2016; Wright et al., 2019; Seitsonen et al., 2018). Shiraishi, 2006, Shiraishi, 2009b has argued that the site was the location of Chinggis Khan's winter camp, or ordū. The evidence for this is indirect and includes historical, linguistic and chronological information. Here, we use a new series of radiocarbon determinations and modelling to clarify the chronological connection of Avraga with Chinggis Khan and the Mongol Empire. We also use multi-element stable isotope analysis of livestock bones and teeth from Avraga to examine animal husbandry and environmental isotopic variation. This provides a basis for re-assessing the interpretation of previously published human isotopic data that compared elite and non-elite sites in eastern Mongolia (Fenner et al., 2014).
The Avraga site is located in an open steppe environment along the Avraga River in east-central Mongolia (Fig. 1). A team from Niigata University and the Mongolian Academy of Science has investigated the archaeology of Avraga since 2001 under the New Century Project. Excavation and research is continuing but preliminary reports have been prepared (Kato and Shiraishi, 2005; Shiraishi, 2009a; Shiraishi and Tsogtbaatar, 2011, Shiraishi and Tsogtbaatar, 2015) as well as general overviews of results (Shiraishi, 2006, Shiraishi, 2009b). Avraga includes remains of wooden and packed earth buildings, twelve of which have been at least partially excavated (Shiraishi and Tsogtbaatar, 2015). Analyses of Avraga features and the artifact assemblage are ongoing.
Pastoral societies incorporate elaborate and intricate connections between people and animals. The needs of livestock shape the organisation, movements and customs of pastoralist communities while the people manage the structure and reproduction of the herd and of individual animals. All of this happens within environmental constraints, so changes in the environment evoke responses from both people and livestock. Archaeologists investigating ancient pastoralist societies thus must consider not only their human aspects but also livestock and environmental parameters. Stable isotope analysis of livestock remains provides direct evidence of livestock diet and movement, and can also reveal environmental conditions and variability. It is therefore increasingly used in archaeological analyses of pastoral groups worldwide (e.g., Chase et al., 2014; Dantas et al., 2014; Makarewicz et al., 2017; Ventresca Miller and Makarewicz, 2019). In Central Asia, most livestock stable isotope analyses have been undertaken to establish local isoscapes for comparison with human isotopic data (e.g., Hanks et al., 2018; Ananyevskaya et al., 2018; Svyatko et al., 2013).
Excavation at Avraga has yielded a large collection of livestock remains, from which we selected 55 bones of sheep (Ovis aris), goats (Capra hircus), cattle (Bos taurus), and horse (Equus ferus caballus) as well as one marmot skull (Marmota sp.). We performed stable isotope analyses of carbon and nitrogen in bone collagen, carbon and oxygen in bone and enamel bioapatite, and strontium in enamel bioapatite.
Stable isotope analysis has become a common archaeological technique, and overviews of the approach are available (Bentley, 2006; Lee-Thorp, 2008; Montgomery, 2010; Svyatko, 2016; Tykot, 2004). The most common approach involves analysis of carbon and nitrogen stable isotope ratios in bone collagen. Carbon isotope values from collagen (δ13Ccol) are derived from carbon isotope fractionation during photosynthesis in plants at the base of the food chain, with the two major photosynthetic pathways (termed C3 and C4) producing different δ13C values (Deines, 1980). Most grass species in the Avraga region use the C3 pathway, though the C4 species Cleistogenes squarrosa (hide seed grass) is known to be present (Shagdar and Yadamsuren, 2017). Crassulacean acid metabolism, a third pathway, produces intermediate values but is largely restricted to succulents such as cacti. Plant δ13C values can also be affected by environmental conditions, including aridity, temperature, and the vegetative canopy density. Collagen is a protein, and in mammals is preferentially created from dietary protein rather than other macronutrients, a process called protein routing. δ13Ccol values therefore largely reflect the protein portion of the diet (Fernandes et al., 2012). Nitrogen stable isotope ratios from collagen (δ15N) are entirely derived from dietary protein, and increase each time the protein is incorporated into animal tissue. δ15N thus is used as a trophic level indicator, although the δ15N values in plants at the base of the food chain are also affected by environmental conditions such as aridity and soil conditions as well as manuring (Fraser et al., 2011; Wu et al., 2018).
Carbon isotope values can also be measured in apatite, which is the inorganic, hard portion of bone (δ13CB) or tooth enamel (δ13CE). Apatite, being inorganic, is composed of carbon transported in blood bi‑carbonate formed from all dietary macronutrients (Tejada-Lara et al., 2018). Apatite carbon ratios therefore reflect the entire diet (Fernandes et al., 2012). While bone and enamel apatite have the same carbon source, bone may replace its carbon through remodelling during an individual's lifetime while tooth enamel, once formed, does not alter its chemical composition. Enamel also wears away as the tooth is used so archaeological specimens often do not include carbon from the entire period of enamel formation. Thus δ13CB and δ13CE values may represent different periods within an animal's lifetime and may differ in value due to dietary or environmental change over the animal's lifetime. Bone apatite is also more subject to post-depositional diagenetic alteration of its isotopic signature than is the denser and more crystalline apatite in enamel, although the cold and dry conditions in Mongolia are thought to be conducive to good preservation.
Oxygen isotope values in animal tissues are derived largely from water that was either directly consumed or incorporated within food. Large animals such as livestock get a high proportion of their water from drinking and thus are better monitors of environmental water than small animals such as mice that incorporate a high proportion of water from food. This is because water isotope ratios may be highly modified during incorporation into plant tissues (Hess et al., 2019). Oxygen isotope ratios in precipitation are controlled by a variety of climatic and hydrological conditions, including the source of water vapour, the number and intensity of rainouts, temperature (including seasonal variation) and elevation (Clark and Fritz, 1997). These conditions are often geographically fairly stable over archaeological timeframes, so archaeologists have used oxygen isotope data from bone (δ18OB) and enamel (δ18OE) apatite as geographic indicators when assessing movement over relatively long distances (Makarewicz et al., 2018; Price et al., 2017). Alternatively, oxygen isotope data from individuals in one location can be used to assess relatively large-scale environmental change over time.
Strontium substitutes for calcium in bone and enamel apatite, and is obtained from strontium in food and water. Strontium isotope ratios (87Sr/86Sr) are determined by the age and composition of geological formations that provide sediment on which food is grown and through which water flows (Bentley, 2006). They are thus strongly tied to geological formations that vary geographically and are used by archaeologists as geographic indicators. Strontium ratios are known to be very susceptible to diagenetic alteration in bone apatite, but are usually very well preserved in tooth enamel (Montgomery, 2010).
The rise, expansion and eventual collapse of the Mongol Empire was a transformative period in world history. Here we present a very brief history of aspects of the Empire relevant to the current project (Beckwith, 2009; Lane, 2009; Man, 2004; Pow, 2018; Fenner et al., 2014). The future Chinggis (or Genghis) Khan was born around 1162 CE in central Mongolia. Named Temujin, he was the son of a local leader but his family suffered hardship after the murder of his father. As an adult he established himself as a sort of local warlord, fighting battles and pursuing alliances, until finally overcoming his most powerful enemies and being named ruler of all the Mongols in 1206. Under the title Chinggis Khan, he then began a series of military conquests that would bring much of Central Asia—including parts of what is now northern China—under his control. Chinggis Khan died in 1227, but his son and successor Ogedei Khan (d. 1241) and later descendants continued to expand the Empire until it stretched from Poland and Hungary in the west to the Pacific Ocean in the east, and from the Arctic Circle in the north through China, Iran, and Iraq in the south. In 1271, Chinggis' grandson Kublai Khan defeated the Chinese Song Dynasty and established the Yuan Dynasty as a part of the Mongol Empire. Yuan Dynasty rule stretched from Siberia through the present-day territories of Mongolia (including Avraga), China, and the Koreas. The Yuan Dynasty lasted until 1368, when it was overthrown by Ming Dynasty forces. To the west, Mongol rule would continue to evolve and assimilate for centuries.
Section snippets
Material
Our objectives for the study were to clarify the chronological connection of Avraga with Chinggis Khan, to better understand livestock husbandry and environmental stable isotope variation at the site, and to apply this understanding to a consideration of human isotopic variation during the Mongol Empire in eastern Mongolia. We therefore selected for radiocarbon and stable isotope analyses a set of bones and teeth from the Avraga faunal osteological collection curated at the Mongolian Academy of
Radiocarbon dating
Collagen was extracted using an ultrafiltration procedure (Wood et al., 2014). Briefly, after mechanical grinding with a Dremel drill bit to remove the surface, bone was crushed, demineralized in HCl (0.5 M, overnight, 5 °C), soaked in NaOH (0.1 M, 30 min, room temperature) and HCl (0.5 M, 1 h, room temperature) before gelatinization (pH 3 water, 70 °C, 20 h), Ezee™ filtration and ultrafiltration (Vivaspin™ Turbo15 30 kDa). Collagen was combusted in a sealed tube with CuO wire and Ag foil and
Radiocarbon results
All twenty bone samples produced acceptable collagen and yielded valid radiocarbon determinations (Table 2). The 2σ calibrated date ranges fall within the period from the mid-twelfth century A.D. to the fifteenth century, which is consistent with other date information from Avraga (Kato and Shiraishi, 2005; Shiraishi, 2009a; Shiraishi and Tsogtbaatar, 2011, Shiraishi and Tsogtbaatar, 2015). Two groups of dates are seen: Platform 1, dating to the fourteenth century, and all other locations,
Radiocarbon chronology and Chinggis Khan
Shiraishi (2006) has argued that Avraga is the site of Chinggis Khan's “Great Ordū” – his winter camp when he was home in Mongolia. No artifacts have been recovered that directly tie Avraga to Chinggis Khan or indeed to any elite occupation so the argument is based on indirect evidence, which is briefly summarised here; see Shiraishi (2006) and Kato and Shiraishi (2005) for more detailed discussion. Mongolian pastoralists were highly mobile, and their traditional ger tents leave little
Conclusions
Twenty radiocarbon determinations firmly establish that most of the Avraga site was used during the thirteenth century—in fact, almost certainly during Chinggis Khan's lifetime. This provides additional chronological support for the proposal that Avraga was the location of Chinggis Khan's ordū. A large platform at the site was used well into the fourteenth century A.D. while there is evidence the rest of the site was abandoned.
Stable isotope analyses of faunal remains from Avraga have been used
Acknowledgements
We thank the Mongolian Government for permission to temporarily export and analyse the Avraga bone sample. Funding for this project was partially provided by an ANU RSHA Interdisciplinary/Cross-College Collaborative Research Grant. Hannah Davie kindly provided carbon and nitrogen isotope data from the Ikh Nart Nature Reserve project discussed in Davie et al. (2014). JNF thanks the Department of Anthropology at the University of Notre Dame for providing a collegial space for analysis and writing
Declaration of Competing Interest
None.
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