As a small island state on the eastern edge of the Arabian Peninsula, the Kingdom of Bahrain has a long history as a strategic trading center connecting the Mediterranean, Mesopotamia, and India (Figure 1). Bahrain was home to the ancient Kingdom of Dilmun, which flourished from 2800 BC to 330 BC by engaging in maritime commerce and which is known for its many grave mounds, some of which are royal tombs with large stone chambers at their core (Crawford, 1998; Laursen, 2008). The following Tylos period (330 BC to AD 300) saw the rise of the Kingdom of Bahrain as a naval trade hub, which continued through to AD 629 (Laursen, 2008). The Hellenic world encountered the island of “Tylos” (i.e., the main island of Bahrain) in the Persian Gulf in 324 BC during the eastern campaigns of Alexander the Great (Crawford and Rice, 2002). Over this history of the island, the funerary customs of the Dilmun period continued into the Tylos period, but the identities of those buried in the tombs remain enigmatic.

Palmyra, a central Syrian city, functioned as a vital trading center from the late first century BC to the third century AD, linking the Mediterranean with India. Inscriptions from Palmyra (Seland, 2014) attest to a close relationship between Palmyra and Tylos. Tylos was a significant node in the Palmyrene trade network, which extended to the Indus River delta (Figure 1). A Palmyrene inscription from AD 131 names a Palmyrene satrap of Tylos, “Yarhai” (Gregoratti, 2018). Palmyrenes, who traveled as merchants and soldiers, left their cultural imprint in various locations, constructing tombs in the style of their homeland, such as those discovered on Khark Island in the Persian Gulf and a tombstone in Scotland, near Hadrian's Wall, that belonged to a local woman married to a Palmyrene man, adorned with reliefs and inscriptions in Palmyrene script (Colledge, 1976). These records suggest that Palmyrenes were integrated and influential in Tylos and also imply that Palmyrenes may have followed their own burial traditions in Bahrain. However, no clear evidence of Palmyrene presence in tombs of the Tylos period in Bahrain has been found, meaning that the origins and identities of the people interred in these tombs are uncertain.

In this study, we examined tombs of the Tylos period to ascertain whether evidence exists for Palmyrenes and other immigrant groups in the burial chambers of Bahrain. The investigation involved oxygen and carbon isotope analyses of dental enamel from buried human remains. Oxygen isotopic analysis of archaeological human and animal remains is a useful tool for reconstructing migration patterns, climate change, weaning ages, and the slaughtering ages of livestock. This method is based on the premise that oxygen in bodily tissues is derived primarily from regional drinking water and food sources (e.g., Sponheimer and Lee-Thorp, 1999; Bocherens, 2003; Metcalfe et al., 2011; Britton et al., 2015; Lazzerini et al., 2020; Eda et al., 2022). Many migration studies have used oxygen isotope compositions of animal tissues to trace the movements of species that undertake extensive migration, such as birds and sea turtles (e.g., Viljoen et al., 2016; Hobson, 2019), by establishing correlations between the isotopic composition of body tissues and ingested water. A comparison of the isotopic signature of the tissues of an organism with that of surface waters (precipitation, rivers, lakes, and seawater) can enable the habitat of the organism to be inferred.

For humans, the isotopic composition of dental enamel can be used as a marker for identifying individuals who migrated from their place of origin. Previous research has confirmed the correlation between the isotopic composition of human dental enamel and local tap water (Daux et al., 2008). The analysis of dental carbonates in the tooth enamel of skeletal remains is a common practice in archaeological studies (e.g., Lightfoot and O'Connell, 2016). Tooth enamel, which crystallizes after childhood and remains metabolically inert, preserves the isotopic signature acquired during early life into adulthood. Therefore, a discrepancy between the oxygen isotopic composition of the dental enamel of an individual and the isotopic profile of the water in the vicinity of the burial site strongly suggests that the individual was an immigrant from a region distinct from their location of burial. We applied this method to human remains from the Maqaba burial mounds cluster (MBM) in Bahrain to infer their origins and to give insights into the burial customs of the Tylos period.

The δ¹³C value of an organism is typically used to indicate the contributions of different types of cereals and marine resources to its diet. The isotopic compositions of plants that use the C₄ photosynthetic pathway, such as millet and maize, are higher (−12‰) than those of plants that use the C₃ photosynthetic pathway (−26‰) (O'Leary, 1981), as a result of differences in the efficiency of carbon fixation in C₃ and C₄ plants. Marine plants also commonly show higher δ¹³C values than terrestrial C₃ plants because of the difference of ~7‰ between dissolved inorganic carbon in seawater and atmospheric CO₂, as the diffusion of CO₂ is slower in oceans than in the atmosphere (Chisholm et al., 1982). Therefore, consumers of C₄ plants or marine resources generally show δ¹³C_C values that are higher than those of consumers of C₃ plants. The carbon isotopic composition of tooth enamel reflects the isotopic composition of the entire diet of an organism (Ambrose and Norr, 1993). The δ¹³C values of structural carbonate in enamel (δ¹³C_C) are higher than those of the diet, and humans with a C₃ diet show values of +13‰ enrichment, whereas humans with a mixed C₃-C₄ diet or a marine diet show values of +9.5‰ (Tykot et al., 2009).

The δ¹³C_C values of the human remains from MBM-1 (mean −11.1‰ ± 1.0‰) indicate a diet based on the C₃ plant ecosystem, with minimal dietary contribution from marine sources and C₄ plants such as millet (Figure 3). This result is consistent with the knowledge that C₄ plants were not widely cultivated in pre-modern Bahrain (Nesbitt, 1993) or the Near East (cf. Kubiak-Martens, 2015). Intriguingly, even though the studied individuals were buried in Bahrain, an oceanic island, the isotopic results reveal that they did not consume seafood during their lifetimes. The δ¹³C_C values of the buried individuals of the Tylos period in Bahrain overlap with those of the Bronze Age, Neo-Assyrian, and Islamic peoples of the Middle Euphrates in Syria (Tomczyk et al., 2016). The δ¹³C_C of the MBM-1 burials is reminiscent of the dietary habits in inland of the Near East.

The mean value of drinking water (δ¹⁸O_W) for the studied humans from MBM-1 was calculated to be −5.6‰ ± 2.5‰ via δ¹⁸O_C (Table 1 and Figure 4). The δ¹⁸O values differ with age, with the mean values of δ¹⁸Ow for adults and infants calculated as −6.7‰ ± 1.8‰ and −3.3‰ ± 2.0‰, respectively. However, it should be noted that δ¹⁸O_C of tooth enamel that forms during breastfeeding may differ from those of the compositions estimated from typical drinking water in residential regions. As breast milk is formed from the mother's body water, which has higher δ¹⁸O than drinking water, the milk has a higher isotopic composition compared with the mother's drinking water for both animals and humans (Britton et al., 2015; Pederzani and Britton, 2019). For instance, δ¹⁸O values of cow milk are higher than that of water on the same farm (Lin et al., 2003). A study of bone and enamel phosphate has revealed that deceased infants (< 3 years) excavated from a Medieval site in England had higher δ¹⁸O values than adult bones from the same site and that δ¹⁸O tended to decrease after the weaning age (Britton et al., 2015). This feature occurs not only with phosphate but also with tooth enamel carbonate, suggesting that teeth formed during breastfeeding have higher δ¹⁸O_C values than those formed after weaning. Moreover, breastfeeding also affects the δ¹⁸O values of tooth enamel in non-human mammals.

The enamel sample from the 06_F-0063 was obtained from a fragment of a mandibular molar. However, it could not be determined which molar it came from, M1, M2, or M3. Because the crown of a permanent first molar is completed early, at 2.5–3 years old (Schour and Massler, 1940), there may be remaining influence of breastfeeding on the M1. By contrast, M2 and M3 mineralizes at a later age, M2 completes between the ages of 7 and 8, and M3 completes between the ages of 12 and 16. It is uncertain which molar it was, making it difficult to determine whether or not the δ¹⁸O_C of the individual reflects breastfeeding. However, the sample from the 06_F-0063 has the lowest δ¹⁸O_C of all MBM-1 individuals, so it appears that he drank water with a lower δ¹⁸O than the other individuals, even if breastfeeding had an effect.

It is possible that breastfeeding may also influence the δ¹⁸O_C of the m1 of 02_F-0033 No. 1. On the other hand, among permanent teeth, incisors are 4–5 years old and canines are 6–7 years old when crown mineralization is complete. Thus, the I1 of the 02_F-0033 No. 1 would be expected to show evidence of the drinking water and diets after weaning. However, the M1 of the 02_F-0033 No. 1 had a lower δ¹⁸O_C than that of his I1 that formed after weaning, even though M1 is a tooth type with a higher δ¹⁸O_C expectation due to breastfeeding. This may indicate that the 02_F-0033 No. 1 was raised in higher latitudes with lower δ¹⁸O_W water when he was infant and had migrated to lower latitudes by 4 or 5 years old when I1 was forming.

In our study, one infant presented a δ¹⁸O_C (−3.9‰) value as low as those of the adults, whereas three infants displayed higher δ¹⁸O_C values (−2.5‰, −2.0‰, and −1.0‰). The data indicate that the δ¹⁸O values of the infants were influenced by breastfeeding, although it is possible that the uterine environment was a factor. Infants excavated from graves F-0028 and F-0033 were buried in the same coffins as the adults. There was a 0.9‰ difference between the δ¹⁸O_C value of the second deciduous incisor of the F-0028 infant (−2.0‰) and that of the incisor of the F-0028 adult male (−2.9‰). The δ¹⁸O_C value of the second deciduous incisor of the F-0033 infant (−2.5‰) was 0.7‰ to 1.4‰ higher than that of the first incisor of the F-0033 adult female (−3.2‰) and male (−3.9‰). The differences between adults and infants found in the same coffins were comparable with the differences reported in previous studies, i.e., 1.0‰ between deciduous tooth enamel phosphate and permanent first molars that form prior to weaning and other permanent teeth that form after weaning (White et al., 2004), and 1.2‰ between infant bone phosphate and adult bone phosphate (Britton et al., 2015). Correcting for the effects of breastfeeding, given these individuals buried in the same coffin are assumed to have had drinking water with δ¹⁸O particularly close to each other within the MBM-1.

Some adult human remains from MBM-1 show lower δ¹⁸O values than the local water range of −4.3‰ to +2.0‰. This result suggests that the adults drank water with different δ¹⁸O compared with Bahraini locals, during their childhood periods. Since dogs are assumed to have lived in Bahrain with minimal potential for transport from other regions, we considered the δ¹⁸O_W of Canis teeth from the MBM-1 tomb as indicators of local water sources, in addition to modern water samples. The δ¹⁸O_w values for the two Canis from MBM-1 were calculated as −3.7‰ and −5.1‰ (Table 1 and Figure 4). Of the two Canis bones analyzed, the δ¹⁸O_W of one fell slightly outside the range of modern Bahraini water, while the other was within this range. The fact that one sample is slightly outside the range may suggest that the δ¹⁸O range of drinking water in ancient Bahrain was slightly broader than what is observed in modern samples. However, this discrepancy could also result from uncertainties in calculating δ¹⁸O_W for canids, like human. The breastfeeding period for dogs is 3 weeks, whereas the mineralization of all permanent teeth is completed within 8 weeks of birth (Boy et al., 2016). However, the canine and second molar of Canis measured in this study are relatively slowly erupting teeth (Van den Broeck et al., 2023) and are thus likely to be slowly mineralized. Therefore, although it is likely that the teeth of these dogs directly reflect the δ¹⁸O_W of environmental water locally, they may also incorporate the remnant effects of breastfeeding. Even so, many adults from MBM-1 exhibit significantly lower δ¹⁸O_W values compared to these Canis bones, suggesting they grew up in regions with different δ¹⁸O water.

In the previous study, mean δ¹⁸O_C values of third molars from two Neolithic communities in the United Arab Emirates (UAE), situated along the Persian Gulf, have been reported as −2.2‰ ±0.5‰ (range −3.7‰ to −1.4‰) and −3.2‰ ± 0.6‰ (range −4.0‰ to −2.5‰) (Kutterer and Uerpmann, 2017; Figure 3). The Neolithic populations in UAE are considered as local populations based on their social background and strontium isotopic compositions, and therefore we suggest that they are an accurate representation of the local people born and raised in the Arabian Peninsula around the Persian Gulf. Transformation of the above δ¹⁸O_C values for the UAE gives δ¹⁸O_W values of −3.0‰ ± 0.8‰ and −4.7‰ ± 1.0‰, respectively. The δ¹⁸O_W range with the correcting error for the UAE populations fell within the δ¹⁸O mean ± 2SD range of UAE bottled water (−3.7‰ to +1.5‰). However, there is a significant difference in the δ¹⁸O_C values of the individuals between the MBM-1 and the Neolithic populations in UAE, as determined by t-test (p < 0.001). Therefore, it is possible that many of the adults in the MBM-1 were immigrants from regions located far from Bahrain and around the Persian Gulf.

Our results indicate that the immigrant individuals originated from inland or higher-latitude locations, where the oxygen isotopic compositions of water sources are lower than those in Bahrain. However, there is wide variability in oxygen isotopic composition among humans within a single population, meaning that determining the origin of an immigrant via δ¹⁸O analysis of skeletal remains is challenging (Lightfoot and O'Connell, 2016). Therefore, in this study we refrain from precisely pinpointing the origins of the immigrants interred at the MBM-1 based solely on the presented oxygen isotopic compositional data. However, we compare our data with the δ¹⁸O_W values associated with those of Syrian populations, particularly around Palmyra, because of our hypothesis. There was no significant difference in the δ¹⁸O_C values of individuals between the MBM-1 and Neo-Assyrian (p = 0.07) or Islamic (p = 0.66) populations in the east-central Syria (Tomczyk et al., 2016; Figure 3), by t-test. The δ¹⁸O_W values for the MBM-1 adults are similar to those measured for the Neo-Assyrian (−8.2‰ ± 3.6‰) and Islamic (−7.1‰ ± 3.7‰) populations. Moreover, δ¹⁸O_W values for the individuals from the MBM-1 are similar to those measured for modern precipitation across northern Syria to southeastern Turkey. The fact that Bahrain is an island, yet the people buried there ate little or no seafood according to our measured δ¹³C_C compositions, is also consistent with the assumption that they originated from inland Syria. Thus, it is reasonable to suggest that Palmyra was the homeland of the individuals interred within the mounds in Bahrain, as one of the candidates.

Measured oxygen and carbon isotopic compositions of structural carbonates in human dental enamel were used to ascertain the origins of individuals interred within MBM-1 in Bahrain during the Tylos period. The isotopic results of some of the adult individuals suggest that these were probably immigrants. Isotopic signatures in the dental remains of the adults hint at the elite entombed within the tombs of the Tylos period being likely dominated by foreign powers. This burial practice may reflect the important status of Bahrain as a flourishing nexus of commerce during this period. It is acknowledged that deducing origins solely from oxygen isotopic analysis of human remains has limitations and lacks the precision required for definitive identification of the homeland of migrants. Although our study indicates a higher latitude region, including Syria, as the origin of the immigrants, the isotopic data do not point to just from only one city, and it is possible that individuals found within a single cemetery might have originated from diverse cities and regions. Future investigations involving verification of burial goods or strontium isotope analysis etc. should be undertaken to identify the precise origins of those buried at the MBM cluster.