Isotopic evidence for human mobility in late antique Bulla Regia (Tunisia)

Abstract

This paper represents the first isotopic study on late antique human mobility in North Africa, using the urban site of Bulla Regia in Tunisia as a case study. We also present the first values for bioavailable ⁸⁷Sr/⁸⁶Sr in northern Tunisia, analysing 63 plant and snail samples, as well as a simple method for the pre-processing of plants in the field to facilitate their export. Bulla Regia was a prominent Roman and late antique town situated on an important axis of transport and communication in North Africa and is therefore an ideal site to explore mobility in the region during this time period. Strontium (⁸⁷Sr/⁸⁶Sr) and oxygen (δ¹⁸O_Carb) isotopic analysis of 22 late antique individuals from a Christian church and cemetery identified at least seven or eight non-locals, while comparative analysis of five Roman individuals from a funerary enclosure on the same site classified all but one of them as potential locals. Most non-local individuals exhibit ⁸⁷Sr/⁸⁶Sr values that match various areas of northern Tunisia, which supports regional mobility rather than long-distance migration, although when combined with the oxygen results, inter-regional mobility from an area with a warmer climate may be hypothesised for some individuals. Examination of the spatial distribution of non-local individuals in their cemetery setting reveals that they were privileged individuals, thus they may reflect the mobility of wealthier town-dwellers in late antiquity, particularly perhaps along the Carthage-Hippo route.

Keywords: Isotopes; Late antiquity; Palaeomobility; Roman; Tunisia.

Fig. 1

Plan of Bulla Regia showing the location of the late antique church and cemetery and the Roman funerary enclosure (Bulla Regia Archaeological Project).

Fig. 2

Spatial representation of δ¹⁸O for modern annual mean precipitation in the region overlaid onto an elevation map (GTOPO30) with the location of Bulla Regia indicated. Map created by Helen Goodchild (University of York) using ArcGIS Pro 2.8. Precipitation data derives from Bowen, 2022, Bowen and Revenaugh, 2003 using data sourced from IAEA/WMO (2022).

Fig. 3

Location of sampling sites and underlying geology (geological map). Adapted from Schlüter, 2008. White zones indicate water, light blue represents Upper Jurassic limestones, clays and sandstones, while green corresponds to Lower Cretaceous sandstones, limestones, clays and sometimes terrestial intercalations. These zones have not been included in the map legend because they were not sampled.

Fig. 4

Visualization of ⁸⁷Sr/⁸⁶Sr values for differentially treated plants; notice the clear pairs formed by the values of the same plants Key: O: open fire & furnace; F: furnace; 1 = Grass (Cynoclon dactylon); 2 = Palmiet (Prionum serratum); 3 = Palm (Phoenix reclinata); 4 = Forest Fig (Ficus craterostoma); 5 = Fig (Ficus sycomorus); 6 = Sedge (Cyperus sp.).

Fig. 5

Scatterplot with marginal density distributions plotted for ⁸⁷Sr/⁸⁶Sr and δ¹⁸O_VSMOW(dw) values converted to drinking water for Bulla Regia individuals. Purple shading indicates the local ⁸⁷Sr/⁸⁶Sr baseline based on modern plants and snails. Yellow shading represents the estimated local δ¹⁸O_VSMOW(dw) calculated from modern precipitation values using Bowen (2022), Fig. 1.

Fig. 6

Plan of the church and cemetery showing the location of sampled skeletons: non-local signatures in red and local signatures in green (Bulla Regia Archaeological Project).