Population Mobility of Mersin Chal Residents During the Second Half of the First Millennium BC

Abstract
The Mersin Chal Cemetery lies on the southern bank of the Esperu River, southwest of Telajim village in Mehdishahr city, Semnan Province. It is situated on the southern slopes of the Eastern Alborz Mountains, between the desert regions and the Central Iranian Plateau on one side and northern and northeastern Iran on the other. It lies along the ancient east-west communication route, known as the Great Khorasan Road. Absolute chronology analysis indicates that the cemetery dates to the Achaemenid, Seleucid, and Parthian periods. This cemetery is highly significant, as it represents the first Achaemenid site excavated in the Semnan region. It highlights the Achaemenid Empire’s dynamics on a regional scale. Its importance is further underscored by its proximity to the Qumis civilization. In this regard, one aim of the third season of archaeological excavations at the Mersin Chal cemetery was to identify and investigate burial practices and to conduct biological anthropology studies. This study aimed to estimate age at death, determine sex, and identify indigenous and non-indigenous individuals buried in the cemetery through bioarchaeological research. To achieve these objectives, the following questions were posed: (1) What methods were used to estimate age and determine sex of individuals buried in this cemetery? (2) How can indigenous and non-indigenous individuals be distinguished? To address these questions, the study employed 87Sr/86Sr stable isotope analysis of tooth enamel samples from human remains in 12 of the 49 excavated graves at the Mersin Chal cemetery, along with SPSS statistical techniques. Analysis of stable strontium isotope ratios in adult male and female skeletons from the Mersin Chal cemetery suggests that an indigenous community most likely used the site during the latter half of the first millennium BC.
Keywords: Mersin Chal Cemetery, The Second Half of the First Millennium BC, Mobility, Tooth Enamel, Strontium Isotope.
 
Introduction
The strontium (Sr) isotope ratio is an important geochemical tracer employed across various disciplines, including archaeology, ecology, food authenticity studies, forensics, and more (Willmes et al., 2013: 763). In archaeology, Sr isotope analysis was pioneered by Erickson and has since become a widely adopted method for investigating patterns of human and animal mobility (Blank et al., 2018: 4). Strontium consists of four stable isotopes: 84Sr, 86Sr, 87Sr, and 88Sr; 87Sr originates from the decay of 87Rb (Dorado et al., 2012: 88). Analysis of strontium isotopes in archaeological materials such as teeth and bones is a valuable method for examining historical human mobility patterns (Willmes et al., 2018: 4; Bentley, 2006: 135-136; Gregoricka, 2021: 20). This technique relies on variations in strontium isotope ratios, which are shaped by local geology, because the 87Sr/86Sr ratio differs among rock and sediment types (Price, 2015: 78). Certain rocks, such as basalt, contain low levels of 87Sr, whereas others, like granite, are rich in it. In sedimentary rocks, the 87Sr concentration is closely linked to geological age, reflecting changes in the isotopic composition of seawater through time (Sołtysiak, 2020a: 116). Strontium isotope ratios vary with the bedrock type that forms the soil and also differ between individuals consuming marine or terrestrial resources (Katzenberg, 2008: 430). Through weathering, strontium from rocks is released and incorporated into the cycles of soil, vegetation, and animals. Humans, in turn, absorb strontium through their diet. Because strontium is chemically similar to calcium, it substitutes for calcium in biological apatite (bones and teeth), (Bentley, 2006: 141; Willmes et al., 2018: 4). Dental enamel is widely regarded as the ideal material for examining prehistoric human and animal mobility, owing to its greater resistance to diagenesis and contamination than bone (Blank et al., 2018: 5). “The strontium isotope ratio in tooth enamel reflects the bedrock geology of an individual’s birthplace. In contrast, the strontium isotope ratio in their bones, together with plant and animal remains and the surrounding bedrock, reveals the environment the individual experienced during adulthood” (Goodarzi et al., 2022: 61). This characteristic is useful for tracking mobility and changes in geological environment. “If an individual relocates to an area with different geological conditions or is buried in a new region, the isotopic composition of their dental enamel will not correspond to that of the new location” (Price, 2015: 78). This discrepancy enables the identification of individuals as migrants based on differing 87Sr/86Sr signatures in their teeth (Bentley et al., 2003: 474).
 
Discussion 
Mersin Chal Cemetery, located along the southeastern shore of the Caspian Sea, is recognized as “the first archaeological site from the Achaemenid period to be unearthed in this region of Iran. This discovery provides valuable insights into the Achaemenid Empire’s dynamics at the regional level” (Malekzadeh et al., 2023: 70). The Semnan region, historically known as Qumis or Hecatompylos, once served as the capital of the Parthians and was strategically situated along the Great Khorasan road of the Silk Road. This strategic location underscores its historical importance (Sharifi, 2019: 145). The East-West Road of Greater Khorasan served as a vital communication and trade route across the Iranian Plateau. It connected Central Asia to Khorasan and extended along the southern slopes of the Alborz Mountains, reaching Anatolia and Mesopotamia (Tahmasebi Zave & Iravani Ghadim, 2015: 77). Qumis now lies within Semnan Province, which makes the study of the Mersin Chal cemetery particularly significant because of its proximity to the Qumis civilization area. This underscores the importance of analyzing human bone data from the Mersin Chal cemetery to gain deeper insights into the population diversity, encompassing both indigenous and non-indigenous groups, of the communities that once inhabited this area. The location of this cemetery on the Iranian central plateau has long offered favorable environmental potential, supplying livelihood resources. Its strategic position along the communication route linking the East and West of the ancient world also played a crucial role. These advantageous conditions occasionally led to population movements and large-scale migrations into the region at certain times. In this study, displacement and mobility refer to the presence of non-local individuals within the community. “Being indigenous refers to individuals who have remained in a specific area for an extended period without significant migration during their lifetimes and have relied on native food resources” (Goodarzi et al., 2022: 53). Twelve skeletal remains with first molars were selected for strontium isotope analysis. This selection was because first molar enamel forms prenatally and generally retains a stable chemical composition over time (Price, 2015; Bentley, 2006; Hrnčíř & Laffoon, 2019: 3). Homogenization of sediment samples, chemical separation of strontium, and measurement of Sr isotope ratios were carried out at the Isotope Laboratory of Adam Mickiewicz University in Poznań. Strontium stable isotope analysis indicated that, although two samples displayed variations in their Sr ratios compared to the others, the evidence suggests that most of the samples were likely of local origin.
 
Conclusion 
In 2021, under the supervision of Mohammad Reza Nemati, three trenches were opened during the third season of archaeological excavations at the Mersin Chal cemetery. Trench C11, measuring 10 x 5 meters, contained four graves. Trench D11, measuring 10 x 10 meters, yielded 17 graves. Similarly, Trench E11, also measuring 10 x 10 meters, revealed 28 graves. In total, these three trenches revealed 49 burials, all dating to the first millennium BC. A total of 12 rectangular pit graves were selected for this study. These graves are comparable in construction, orientation, and grave goods. Each burial is individual, with the skeleton placed supine and oriented northeast-southwest. The remains are placed between vertical stone slabs, with two stones marking the grave, one at the head and the other at the feet. Recent radiocarbon (C14) analyses of human skeletons from the Mersin Chal cemetery have revealed that the cemetery remained in use during the late Achaemenid, Seleucid, and Parthian periods. The environmental potential and strategic position of the region have played a crucial role in maintaining cultural continuity there. Sr isotope analysis of dental enamel from human skeletal remains at the Mersin Chal cemetery indicates a distinct isotopic signature unique to the region, differing significantly from those of other areas. Sr isotope evidence revealed that, although two samples displayed variations in Sr ratios compared to the others, the cemetery was likely used predominantly by the local indigenous community during the second half of the first millennium BC. Moreover, the discovery of iron sickles in the graves might suggest agricultural activity in the region. Given the favorable climate and the biological resources available in the Telajim area, it is plausible that the inhabitants of the Mersin Chal met a portion of their subsistence needs using local resources. Consequently, large-scale movement to procure food supplies may not have been necessary. The similarity of burial practices and grave goods in this cemetery to those in the nearby Qumis area strongly highlights the indigenous culture and traditions of these people. There is no evidence of external influence on the burial methods or grave goods in the Mersin Chal cemetery. Overall, it can be concluded that most individuals buried in this cemetery likely experienced no significant displacement during their lifetimes and relied on a diet composed primarily of native, local resources.