Stable Isotopes and the Seasonality of the Oronsay Middens

Article excerpt

Research on six late Mesolithic shell middens on the small Hebridean island of Oransay address questions of permanent and seasonal occupation. Stable isotope analysis of human bones shows murine resources providing the majority of protein, supporting year-round occupation of Oronsay. One individual, however, demonstrated a mixed diet of marine and terrestrial protein, suggesting seasonal visits and different patterns of site occupation.


The small (c. 6 sq. km) island of Oransay is located off the southern tip of the Island of Colonsay, in the Inner Hebrides, off the West Coast of Scotland (FIGURE 1). There are five recently excavated Mesolithic shell middens on the island, which date to between c. 5200 and 6200 radiocarbon years BP (Switsur & Mellars 1987). Extensive excavation of the middens were carried out by Mellars from 1970 to 1979 (Mellars 1987), and the good preservation of faunal remains in the middens has provided much evidence about human subsistence during this period. However, the nature of this subsistence has been debated. Mellars & Wilkinson (1980) have argued from the faunal evidence that the island was occupied at several different seasons of the year by groups who could have been resident for most if not all of the year on Oronsay, and therefore heavily dependent on marine resources. Conversely, Mithen & Finlayson (1991) have argued that Oronsay was only occupied intermittently by people who spent most of their time elsewhere, and who subsisted on a combination of marine and terrestrial (particularly red deer) resources.

Human bone collagen stable isotope analysis can provide insights into the amount of marine protein versus the amount of terrestrial protein an individual has consumed over approximately the last 10 years of their lifetime (Schwarcz & Schoeninger 1991; Ambrose 1993). We undertook this analysis on some of the Mesolithic human bone material preserved in the Oronsay middens, to address this debate over subsistence.

Faunal remains

Palaeobotanical and land-snail evidence indicate that during the Mesolithic occupation the island supported substantial areas of woodland, which included birch, hazel, oak, elder and alder trees (Andrews et al. 1987). At this time the sea level was approximately 6 m higher (relative to the land) than at the present day, and the middens were located on the coast (Jardine 1987). The middens contained mostly food waste, particularly marine shells, fish remains, hazel nuts and bones of seals, otters, red deer and pig. Within and below the middens there is evidence of hearths and post holes, which imply some domestic activity at the midden sites (Mellars 1987). No domesticated mammals, such as cattle, sheep or goat were represented in the faunal assemblages (Grigson & Mellars 1987).

The best mammal faunal evidence comes from the Cnoc Coig midden, with much smaller assemblages from the other sites (Grigson & Mellars 1987: 246). The most abundant species in the Cnoc Coig midden were seals, particularly the Grey seal (Halichoerus grypus). There were 360 skeletal elements recovered, and the minimum number of individuals (MNI) represented was 9. Most parts of the skeleton were recovered, suggesting that the seals were butchered, and probably eaten, at the site. Three of the seals were very young pups, and must have been caught in September or October. The second most abundant species was the otter (Lutra lutra) (123 elements: MNI = 6). Again, as most of the skeleton was represented the otters were probably butchered at the site.

The third most represented species was red deer (Cervus elaphus) (70 elements: MNI = 7). For this species very few meat-bearing bones were represented; the majority of the remains consisted of broken antlers, lower limb and foot bones, which were the parts of the animal used to manufacture most of the bone and antler tools recovered from the middens. This suggests that the red deer were not butchered and consumed at the site, but instead selected parts of the animal were brought to the site primarily for tool making. …