Defining local bioavailability

To establish whether non-local animals are present, it is imperative to determine the range of bioavailable strontium and sulphur in the vicinity of the site. Navan Fort is founded on Carboniferous limestone geology, covered by variable depths of glacial clays and sands17. Modern plant samples provide a useful means by which to determine the strontium biosphere range18. Snoeck et al.19 systematically analysed 88 plant samples from Northern Ireland to construct an initial strontium biosphere map. Only one sample location was relatively close to Navan Fort, on the Mid-Upper Ordovician Derryveeny formation, approximately 20 km southwest of the site. Analysis of shrubs and trees from this zone provided values between 0.7122 and 0.7128. Extrapolations from these data19 that relate to comparable lithologies surrounding the site suggest that values between 0.7099 and 0.7137 are likely at Navan Fort and its surrounding region (i.e. with an approximate radius of 20 km). As this local range involves extrapolation, seven plant samples were analysed from the immediate vicinity (within a 4 km radius) of the site (Fig. 5, Table 4). These results contrast with the expected bioavailable range defined by Snoeck et al.19. The results range from 0.7084 to 0.7096, which is consistent with expectation for the carboniferous limestone lithology on which the site is founded20 and this is thus defined as the local signature. However, there is no overlap with the previously published range19. As these samples from the immediate surroundings of the site are in close agreement they provide a coherent characterisation of the locality. This is not to say that the previously published map does not provide valid results, as it provides an important initial characterisation of Northern Ireland. Interpolations from comparable lithologies are necessary in mapping large areas such as this and these new data suggest that biosphere variation is likely to be more complex at a local scale, a feature that is becoming more commonly recognised in strontium isotope studies21. The published map may provide an indication of potential biosphere values in the broader region, but as only a single location was sampled in the vicinity of Navan Fort, further mapping is required.

Figure 5 Map showing the location of the extracted plant samples (map created by Kirsty Harding following information from GSNI). Full size image

Table 4 Sample details and results from the seven modern plants (lithological descriptions taken from the Geological Survey of Northern Ireland, see Fig. 2). Full size table

Defining the local sulphur isotope signal is more challenging. The nature of variation is arguably better understood for Ireland than mainland Britain, as a result of the study by Zazzo et al.15. This study on sheep wool (keratin) recorded δ34S values from 5.8‰ to 17.0‰, with a significant negative correlation between sulphur values and distance from the west coast and lower values being commonplace in the midlands or east coast. Data were used to generate an indicative contour map for sulphur values in sheep wool for the majority of Ireland, with the lowest expected values being in the eastern midlands and the east coast (7–8‰) and the highest values being on the exposed west coast of County Mayo (15‰). The degree to which this map is valid for comparison with bone collagen values requires consideration. The diet-consumer tissue offsets for keratin and bone collagen are similar22. Modern studies have shown this offset to be slight, only around 0.5‰13. As the offset is slight and collagen and keratin are comparable in terms of their diet-consumer spacing, the map should be applicable to collagen analysis. However, as with the vast majority of biosphere maps, contours rely on interpolations based on a limited number of sample sites and biosphere variation is therefore likely to be more complex. The closest sampling location was Ballyhaise, c. 60 km inland from Navan Fort, which provided a mean value of 6.9‰ (+/−0.26‰).

The research by Zazzo et al.15 indicates that Navan Fort is in an area characterised by low δ34S values in the range of 7–8‰. However, given the contrasting results between the published strontium biosphere map and local plant strontium values, the same plant samples were also subject to sulphur isotope analysis. It is possible that modern plant values will be affected by industrial SO 2 emissions23. However, reduced emissions in recent years and Ireland’s location on the western periphery of Europe with a predominantly southwesterly or westerly airflow should minimise any effect. Plant data are presented in Table 4. Sample values show very limited variation (6.4–8.7‰), excepting a single outlier (NAVP7 0.2‰). This aligns with the Zazzo map, suggesting low values are likely in animals raised in the vicinity of Navan Fort. The very low value (NAVP7) is markedly lower than any wool samples recorded by Zazzo et al.15. This suggests either that there are pockets in the local landscape that can produce even lower values, or that this sample from the observatory grounds has been subject to contamination by recent human activity and is not an accurate reflection of the prehistoric landscape baseline. Contamination is considered a possibility, as the observatory grounds are also quite close to a main road and Armagh town centre. Consequently, a relatively conservative local sulphur range of 6‰ to 9‰ can be suggested based on existing mapping data and new plant analyses, although the effects of modern contamination cannot be excluded.

Identifying non-local fauna

Only eight (22%) of the 35 animals have values that fall within the local strontium range of 0.7084 to 0.7096. Assessing taxonomic differences in the representation of non-local animals is problematic in an unbalanced sample, but based on strontium isotope analysis alone, it is clear that neither of the caprines were locally raised. Only six of the pigs and two of the cattle have strontium values consistent with a local origin. However, lithological zones that would be expected to produce strontium values such as this are relatively common in Ireland and analyses of archaeological and modern plant samples from various locations have produced values in this range19,24. In addition, this strontium range also overlaps with the value for rainfall and seawater (0.70918), so values could result from sea-spray effects in coastal areas. Therefore, these animals could certainly have been raised elsewhere. Seven of these animals have sulphur isotope values ranging from 13.1 to 16.6‰ (NAV 37 was not subject to sulphur isotope analysis). These values are far higher than the expected local range of 6–9‰. The sulphur mapping suggests that none of the animals were raised in the locality of Navan Fort, as the entire dataset sits outside of the local sulphur range.

Exploring origins

Strontium isotope values in the 35 animals from Navan Fort are exceptionally diverse. Although the strontium biosphere of the Republic of Ireland has only been partially mapped25 and mapping for Britain has not achieved optimal resolution (though it is improving rapidly26), it is clear that these animals have wide-ranging origins. Strontium values from 0.7071 to 0.7151 represent one of the largest ranges for any faunal dataset in the British Isles. Only Durrington Walls exhibits a wider range and this is in a far larger dataset of 138 animals14,27. Based on current published mapping this range encapsulated all strontium biosphere zones in the UK with the exception of very limited pockets of basalt lithology in the north of Ireland and highly radiogenic granitic lithology in northern Scotland19,20. While the wide-ranging values are certain to represent diverse origins, there are no clear clusters apparent within the data, whether using single or multiple isotope indices. This suggests that animals were not brought from distinct supply centres that supported the feasts, but rather that they were brought in small numbers from many locations.

According to the published strontium biosphere map19, all values in the dataset could be attained in Northern Ireland, but they are also consistent with origins throughout Ireland. Two pigs have highly radiogenic values (>0.714) that are well outside of the local range and that of the broader region19. Strontium values such as this are exceptionally rare in the northern Irish biosphere. Only four locations have produced plants in this range: two in the far south east of Ulster, one on the central northern coast and one near the western border of County Tyrone19. Of these, only the far south eastern location (County Down) could support the pig with the most radiogenic value (0.7151). None of these locations are close to Navan Fort (<50 km) and all represent relatively isolated pockets of radiogenic geology, uncharacteristic of a regional zone (they are therefore not presented as 0.714+ in the strontium biosphere map17). These animals must have been transported over distance to Navan Fort. They could have been raised in a restricted northerly area, principally feeding on resources from a constrained lithological zone. It is perhaps more likely that they derived from areas of older geology elsewhere, such as the metamorphic gneiss in limited areas of north western (Donegal), south eastern (Wexford) and central western (Galway) Ireland. Only one of the animals has a sulphur value (15.4‰). This is high and consistent with a coastal origin in western Ireland, making Donegal or Galway most likely.

The two lowest strontium values (0.7071) are the only two caprines that were analysed. A single pig also produced a similar result of 0.7072. These values are rare in the biosphere of Britain and Ireland. An origin in the tertiary basalts to the very north of Ireland (north and east of Lough Neagh) can be identified with confidence. The closest area to produce these values is south east of Lough Neagh (c. 30 km) but the Tertiary basalts dominate the north east of Northern Ireland and so origins cannot be further constrained using strontium alone. The caprines produced sulphur values of 15.6‰ and 16.5‰. These high values suggest a coastal origin in north Antrim (c. 100 km), as the closest basalt areas are in the zone that has produced the lowest sulphur values15. Sulphur isotope analysis of the pig failed to meet quality control criteria.

Strontium biosphere mapping outside of Ulster has thus far principally focused on the central eastern area (County Meath), indicating that values between 0.708 and 0.713 are likely in this region24,25. A limited study has also focused on the Carrowkeel area of County Sligo in western Ireland, providing values of 0.7097 to 0.711928. Ireland’s geology is dominated by carboniferous limestone (Fig. 6) that produces strontium biosphere values in the range of 0.7090 ± 0.0015 (2σ)24,29. Around half of the animals (12 pigs and 5 cattle) have values within this range and are very difficult to resolve geographically, as the limestone extends from the east to west coasts and from Lough Neagh in the north to County Waterford and Cork in the south. It is clear, however, that these animals came from several different regions of Ireland, as their sulphur values range from 13.1‰ to 16.9‰. Although this range is fairly restricted, it is not consistent with a single area of origin. In the wool dataset, the greatest range of mean sulphur values for individuals from the same location is 1.25‰15. Identifying likely origins is beyond the scope of the data at present, though the high sulphur values are suggestive of animals principally deriving from the west coast.

Figure 6 Geological map of Ireland showing the location of Navan Fort (map created by Kirsty Harding based on information from BGS/GSI/GSNI Bedrock Survey map). Full size image

The remaining nine pigs and four cattle have strontium values between 0.7108 and 0.7123. These values are also very difficult to resolve geographically. They do not represent a homogenous 87Sr/86Sr biosphere zone and have sulphur isotope values ranging between 13.9‰ and 17.1‰, clearly indicating varied origins. Biosphere and archaeological research suggest that the strontium values are likely in parts of southern Ulster and in County Meath, predominantly on areas of Silurian and Ordovician mudstones19,24,25. These lithologies are present in various pockets across Ireland (Fig. 6). They are less common in the west and therefore the cattle sample with the highest sulphur value in the dataset may have originated from the zone of Ordovician and Silurian metasediments on the coastal area at the south of County Mayo.

This is the first isotope study focusing on the origins of archaeological domestic animals in Ireland and the first sulphur isotope study on any archaeological remains in Ireland that the authors are aware of. Consequently, comparative data is scarce and equifinality remains an interpretative problem. Identifying areas of origin with confidence remains beyond the scope of current data in most instances. Sulphur isotope analysis is becoming ever more useful in faunal provenancing when used in multi-isotope studies14,30,31,32, but detailed mapping is required for potential to be fulfilled and the interpretation of these data should be refined in the future. It is clear that the 35 animals were raised in wide-ranging locations across Ireland. The best comparisons can be made with research on human enamel and cremated bone from the Mesolithic to the early medieval period in Ireland (Fig. 7). This comparative dataset comprises a total of 132 human and faunal strontium values from 29 sites (samples from Tara sites have been amalgamated as a single site), which are plotted against the Navan Fort fauna in Fig. 7. The absolute range of values is far higher at Navan Fort than any other site. Navan Fort also comprises the two most radiogenic values in the entire dataset and has less radiogenic values than all sites with the exception of Ballymacaldrack (located on Antrim basalts). This provides clear evidence for the movement of large numbers of livestock from various area of Ireland to Navan Fort. The dataset also provides supporting evidence for origins in some instances. For example, the three low values (<0.7075) are comparable to those from Ballymacaldrack, supporting the assertion that these individuals may derive from the north Antrim basalts. There is no comparative archaeological data to assist interpretation of the sulphur data and therefore modern mapping must be used. Sulphur isotope results were less variable than strontium in relative terms and the consistently high values suggest that animals were frequently brought from the west of Ireland, but that central and eastern areas supplied few animals for the feasts. However, mapping resolution and the lack of comparative data means this interpretation can only be tentatively made at present.

The catchment of the feasts at Navan Fort must have been substantial. Animals provide a useful proxy for human movement, particularly in prehistory, when the vast majority of livestock would have been raised at a household level, rather than by specialist producers33. Domestic animals may even provide a better isotopic proxy for human origins than humans themselves. Humans are likely to acquire their food from a wider catchment than domestic animals and their teeth develop more slowly meaning analyses may represent a blended signal relating to different locations and phases in the individual’s life. This issue is even more acute in the analysis of cremated bone, which represents a longer-term averaged signal. Animals’ teeth develop rapidly and analysis therefore provides a more time-limited snapshot for origins, although foddering and transhumance can still complicate signals.

Interpreting the carbon (δ13C) and nitrogen (δ13C) isotope data

The reconstruction of husbandry regimes through carbon and nitrogen isotope data requires the analysis of substantial numbers of animals and therefore only relatively fleeting comments can be made on this limited dataset.

The range of carbon and nitrogen values is not particularly wide but has marked outliers among the cattle and pigs. Wide-ranging values in pigs could result from variable foddering regimes in the same location. The spread of values in (herbivorous) cattle provides better evidence for different landscape origins, although local landscape variation and different foddering regimes cannot be discounted. Pigs generally have higher δ15N values than the cattle and sheep. This is a common pattern in carbon and nitrogen studies in temperate Europe34,35,36 and is consistent with pigs consuming some animal protein, potentially from meal scraps. However, the mean value for pigs is only 0.8‰ higher than that of cattle and therefore their diets are certain to be principally herbivorous. Even the two highest values of 7.6‰ are not consistent with a substantial input of animal protein. Higher values are commonplace in British pigs from the Neolithic14 to the medieval period, where nitrogen values above 12‰ have been attained37. The mean value of 6.3‰ is the same as for Iron Age pigs in the Pan-Irish study by Guiry et al.38, although pigs had a lower mean nitrogen value than cattle and caprines in this study. The low carbon values demonstrate that marine resources were not exploited for foddering the pigs. In spite of the wide-ranging origins of the pigs, it seems that feeding regimes show relatively limited variability and communities across Ireland were raising pigs on a largely herbivorous and entirely terrestrial diet, characteristic of a C3 ecosystem.

The low herbivore δ15N values (<7‰) indicate that none of the animals were raised in heavily manured terrain or on saltmarsh landscapes. By contrast the Guiry et al.38 study produced domestic herbivore values as high as 9.6‰ from Iron Age Ireland. The range of values is best explained by variable origins and underlying landscape variation in carbon and nitrogen isotope values. It is not possible to examine husbandry patterns relating to origins in such a limited dataset. There is no correlation between strontium and carbon (Pearson 0.31, sig. 0.875) or nitrogen (Pearson 0.115, sig. 0.553). Neither is there a correlation between sulphur and carbon (Pearson 0.142, sig. 0.471) or nitrogen (Pearson −0.52, sig. 0.793). However, it is noteworthy that the pig with the lowest strontium isotope value (0.7072), likely from the basalts of Antrim also had the lowest carbon (−22.9‰) and nitrogen (4.8‰) values in the dataset. This could relate to naturally low landscape values in this area (in which case low values would also be expected in the two caprines) or simply that this pig was raised on an entirely herbivorous diet in a landscape that had not been manured.

Concluding comments

This research on Early Iron Age fauna from Navan Fort is the first isotope study to explore patterns of mobility through the analysis of animal remains in Ireland. The research demonstrates the interpretative potential of a multi-isotope approach in an Irish context and also the value of animals as a proxy for human movement. However, it also highlights the need for more comprehensive biosphere mapping. Current biosphere maps can be used as an indicative guide, but analysis of plants should be integrated into projects to provide more detailed local ranges.

The results provide clear evidence that communities in Iron Age Ireland were very mobile and that livestock were also moved over considerable distances. People brought animals from across Ulster and beyond to Navan Fort and it is likely that the great prehistoric regional centres of Ireland acted as lynchpins in the landscape and centres for large-scale connectivity. The bringing of animals from great distances to Navan can be explained in two different ways. Documentary evidence indicates that cattle raiding was as endemic feature of the medieval Ireland with some raids taking place on an inter-provincial scale39. Navan Fort is one of the principal settings of the Ulster Cycle of legendary tales which has at its core a tale of cattle raiding40. Such raiding, however, was primarily concerned with cattle while pig was primarily the food of feasting, as indicated in such legends as The tale of Mac Da Thó’ pig41. Feasting, almost invariably associated with sacrifice, was a social necessity of early societies where the slaughter of a large domesticate necessitated the consumption of a large amount of meat in a short period of time11. The results of the analysis of the pig bones from Navan provides evidence for such occasional feasting at the site, with participants bringing their pigs, for sacrifice and consumption, from a wide catchment area. Recent analysis of pig bones from Durrington Walls has provided similar evidence at a large Late Neolithic site14.

Equifinality and limited mapping remains a hurdle to precise interpretation of the isotope data and, therefore, identifying areas of origin with confidence is difficult. However, the diverse strontium values demonstrate wide-ranging origins and the vast majority of animals (potentially 31 of 35 analysed) were not raised in the vicinity of the site and the sulphur data points to many animals coming from western Ireland and fewer from eastern and central areas. The feasts at Navan Fort were clearly not supported by just one or two distinct supply centres, but rather utilised livestock from a much wider catchment. This is likely to represent animals (predominantly pigs) raised at a household level and brought to Navan Fort for the feast. Although current mapping precludes the identification of origins for most samples, some assertions can be made. Two sheep and a pig with very low strontium values are highly likely to have been raised in the basalt area of northern Ulster. The very high values are more difficult to pin to a location with confidence, but areas of County Donegal, Galway, Down, Tyrone and Antrim provide the most plausible places of origin. This provides new, direct evidence for both the scale and volume of mobility in later prehistoric Ireland. It is clear that Navan Fort had a vast catchment and that the influence of the site was far-reaching.