The 4,000-year-old barrows at Irthlingborough, Northamptonshire and Gayhurst, Buckinghamshire (both in England) are famous among Bronze Age archaeological sites for the unusual amounts of cattle remains associated with their central human burials. Archaeologists have been puzzled about their meaning since the sites were first excavated in the 1980s and 1990s. At Irthlingborough the cattle bones comprised mainly of skulls. Were they tokens brought from far away as gifts to the deceased? Or did these cows come from local herds? Jacqueline Towers, an archaeology PhD student at the University of Bradford, tried to solve the mystery by looking at the isotope composition of the cattle's teeth.
     Some chemical elements occur in heavier and lighter varieties, called isotopes. The proportion between heavy and light isotopes is closely related to environmental factors, such as geology or climate, depending on the element. Older rocks will be richer in the heavier strontium isotope and this feature is passed on to soils, plants and water. When animals eat locally grown plants and drink water from nearby springs, strontium atoms make their way into the body and accumulate in teeth and bones, where the heavy/light isotope ratio will reflect the local geology. "Isotope analysis is usually the only method available to determine the provenance of animal remains," says Towers. Or, rather, to exclude areas where the animals certainly did not come from.
     Towers and colleagues sampled several cattle teeth dug from the Irthlingborough and Gayhurst barrows and measured their strontium content at the NERC Isotope Geoscience Laboratory in Nottingham. The team found that most of the cattle grazed and lived near their final resting place. Some of the teeth from Irthlingborough had strontium ratios so similar that they might have come from the same herd. But the results published in the Journal of Archaeological Science are not entirely straightforward as Towers uncovered evidence that at least some of the animals recovered from both sites were not born locally.
     The strontium ratio found in the second molar of one animal from Irthlingborough is higher than the characteristic range of values for the local area. This means that this cow or bull was born in a region with older rocks - for example the Malvern Hills, Wales, the Lake District or even Scotland. But analyses from the third molar, which develops later, reveal lower strontium ratio values. "This animal probably travelled to the Irthlingborough neighbourhood during its first year, and spent several months there before it was slaughtered," explains Towers. For the Gayhurst barrow, Towers found a tooth with the lowest strontium ratio recovered during her research. A possible origin for this animal is an area rich in chalky rocks from the Cretaceous period, the closest one being about 25km southeast of Gayhurst.
     While strontium isotopes cannot be used to pinpoint an exact location, "they are useful to exclude Irthlingborough and Gayhurst as the original provenance of some of the cattle buried at the barrows," says Towers. "This means that the local inhabitants exchanged cattle and communicated with distant groups." The findings also suggest that the cattle at the barrows were not brought by non-local mourners as tokens to the deceased because the "animals spent at least a few months in the area before slaughter," says Towers.

Source: Planet Earth Online (27 October 2009)