We have used isotope analysis to investigate the places where the men buried at Palace Green spent their childhoods. We have looked at the isotopes of strontium, oxygen and lead. Each gives us slightly different information.
Isotopes are atoms of a chemical element which differ in mass. The proportions of different isotopes of a chemical element vary in nature. We measure the isotopes in tooth enamel because this forms during childhood and is not renewed or repaired in adult life. The isotope composition of the enamel is determined by diet and when those isotopes vary they can be used to investigate someone’s place of origin.
Strontium isotope ratios vary in different rocks (for reasons explained in more detail here.) The strontium from the rock is incorporated in the soil, taken up by plants and then the animals that eat them. Some strontium can also come from seafoods. Using maps of strontium isotope values in the modern environment (such as the one below from a paper by Jane Evans, Janet Montgomery and colleagues) we can indicate possible places of origin but the mapping is incomplete and can never account for mixing of foods grown on different rocks.
Oxygen isotopes in teeth derive primarily from drinking water. Drinking water in turn derives from local precipitation. The ratio of oxygen isotopes in precipitation varies with factors such as temperature, altitude and distance from the coast (see description here). The oxygen isotopes in enamel have to be related to those in drinking water using a calibration equation which introduces a large uncertainty. Oxygen isotopes in enamel can therefore be used to identify broad regions of origin.
There are multiple regions with similar strontium and oxygen isotope ratios so these isotope methods are primarily methods for excluding places of origin. When there is a specific hypothesis of origin they can be used to refute it or corroborate it.
Lead occurs naturally in trace amounts in foods and drinks, but it can also be introduced in larger quantities by human use of lead – for example, in water pipes, for pewter plates or in lead-glaze for pottery. The concentration of lead in tooth enamel tells us about the extent that people were exposed to anthropogenic lead. We know that in the UK levels above 0.8 parts per million do not occur with natural exposure. The lead isotopes also vary with exposure. Those with high lead levels have a narrow range of isotope ratios that match lead ores, while those with low exposure have very variable isotope ratios which depend on the local rocks.
In the seventeenth century Durham, and the rest of the north of England was supplied with lead from the Northern Pennines. Some of this lead was also exported to Scottish towns and cities. Lead from mines at Leadhills and Wanlockhead in Dumfriesshire was also used in Scotland but was not exported. Exposure to lead was generally higher in urban areas and among the wealthier in society.
Teeth were excavated for 13 of the skeletons and were in good condition. We analysed a second molar from each individual. The crown of this tooth forms at the ages of about 3-8 years.
In the skeletons from Palace Green we found that the strontium and oxygen isotope ratios were very variable. These men certainly did not spend their childhoods in the same place. The graph below compares their isotope ratios to a large number of other British samples and the observed environmental ranges from northern England and Scotland. The range for Scotland encompasses the whole of the range for northern England. Of the 13 skeletons we tested, 5 fall within the northern English and Scottish ranges, 5 fall within the Scottish range but outside the northern English range and three fall outside the Scottish range.
The lead concentrations and isotope ratios are also quite varied. The concentrations are all below 0.8 ppm so there is not conclusive evidence of exposure to lead artefacts. The isotope ratios are varied as would be expected for exposure dominated by natural sources, but those with higher concentrations are closer to the isotope ratios of Scottish ore sources. The lead measurements therefore favour the idea that these men were not from urban areas or the upper classes, and the lead isotopes favour a Scottish origin but are not conclusive.
Our results show that these men had varied origins and only a few could possibly have been from northern England. They are concordant with the varied origins of the Scottish army. As well as men from all over mainland Scotland, we know that there were ‘Dutchmen’ serving in the Scots army. At this time ‘Dutch’ could indicate anyone speaking what we would now call Flemish, Dutch or Low German – the area along the northern European coast from Belgium to Schleswig. This area is among those compatible with the strontium and oxygen isotope values.
In the second phase of research we are analysing more teeth from each man to investigate whether they moved during their childhoods. We will make a more detailed correlation of the isotope results with geographical locations and compare this to the historical records of the places where the army were recruited.
This work was a team effort. Part of it was the MSc dissertation of Laura Dodd. Geoff Nowell in Durham’s Department of Earth Sciences carried out the measurements of strontium and lead. Oxygen isotopes measurements were carried out by Christophe Lecuyer at the Laboratoire de Géologie de Lyon.
Author: Andrew Millard
Andrew’s first degree was in chemistry at Oxford, he subsequently moved into archaeology exploring the uranium-series dating of bone. In 1995 Andrew joined the department at Durham where his research broadened to cover the chemistry of bones and teeth and their application to archaeological problems. Andrew is also interested in Bayesian statistics and their application to the analysis of archaeological dating techniques. His previous research in isotopic analysis has included work on investigating weaning ages using nitrogen isotopes, looking at migration into and around Britain using strontium and oxygen isotopes and the migration of Crusaders.