Strontium isotope signals in cremated petrous portions as indicator for childhood origin

Abstract

Dental enamel is currently of high informative value in studies concerning childhood origin and human mobility because the strontium isotope ratio in human dental enamel is indicative of geographical origin. However, many prehistoric burials involve cremation and although strontium retains its original biological isotopic composition, even when exposed to very high temperatures, intact dental enamel is rarely preserved in cremated or burned human remains. When preserved, fragments of dental enamel may be difficult to recognize and identify. Finding a substitute material for strontium isotope analysis of burned human remains, reflecting childhood values, is hence of high priority. This is the first study comparing strontium isotope ratios from cremated and non-cremated petrous portions with enamel as indicator for childhood origin. We show how strontium isotope ratios in the otic capsule of the petrous portion of the inner ear are highly correlated with strontium isotope ratios in dental enamel from the same individual, whether inhumed or cremated. This implies that strontium isotope ratios in the petrous bone, which practically always survives cremation, are indicative of childhood origin for human skeletal remains. Hence, the petrous bone is ideal as a substitute material for strontium isotope analysis of burned human remains.

Figure 1. The petrous portion of the human temporal bone (pars petrosa) retains its morphology after cremation or similar intensive heat exposure, being one of the last bones of the body to burn.

Here, a typical cremated petrous portion as found in cremated human remains is shown.

Figure 2. The otic capsule surrounding the vestibulo-cochlear organs of the inner ear is the one of the densest bone tissues in the human body and does not remodel after the age of 2.

Here, the bone tissues in the inner ear are illustrated in a cross section of a petrous portion. The limit of the otic capsule is indicated by the arrow.

Figure 3. Bland and Altman plot showing the “line of equality”: the line on which the values would lie if Sr levels measured in premolar enamel (pm) exactly equaled the levels measured in the otic capsule of the petrous portion (pp).

The values cluster around the line of equality, thus no systematic bias is indicated. The horizontal and vertical lines indicate the upper limit for strontium levels in Denmark (0.711; the lower level is 0.708). The two individuals in the upper right quadrant are thus non-Danish, while the individuals in the lower quadrant can be assumed to be Danish. The individual in the upper left quadrant could be either, depending on whether the enamel or the petrosal value is used. The cremated individual from Rishøjen (K442/RH U2) is indicated by an arrow.

Figure 4. Bland and Altman plot showing the limits of agreement.

The difference between the enamel and the petrosal values (var) are plotted against the mean value of the enamel and petrosal values (st mean). Again, no bias is seen. The mean difference is 0.00007 (indicated by solid horizontal line) and +/−2SD (+/−0.001076) is indicated by the horizontal dotted lines. No values lie outside these limits, indicating that measuring either the premolar enamel or the petrosal Sr isotopic ratios will give the other value within 0.001075 (2SD).

Figure 5. Plot of Sr isotope ratios for the cremated individuals.

Error bars correspond to +/−2SD of the premolar enamel-petrous portion difference (0.001076) derived from the data in Figure 4. The individuals from the cremation graves KH HF, KH JE and OB QE can therefore be assumed to be non-local.