Strontium and oxygen isotopes as indicators of Longobards mobility in Italy: an investigation at Povegliano Veronese

For this study soil, tooth, and bone specimens of humans and animals were sampled for strontium isotope ratios (⁸⁷Sr/⁸⁶Sr), whereas teeth and bones of humans and animals were sampled for stable oxygen isotopes analysis δ(¹⁸O/¹⁶O). Sample collection, preparation and analysis was performed in accordance with relevant regulations for the treatment of ancient human remains. Permission to analyze the samples was granted by the local SABAP (Soprintendenza Archeologia, Belle Arti e Paesaggio per le province di Verona, Rovigo e Vicenza).

Because of the bad state of preservation of some of the human teeth, sampling for isotopic study could not be consistent. We decided to use the tooth that was mostly represented. Hence, we preferentially sampled canines, with second and third molars as possible substitutes. The enamel of the canines forms from 4 months to six-seven years of life; whereas the second molar forms from three to seven-eight years and the third molar from seven-eight to more than twelve years representing late childhood-adolescence. For dental enamel sampling, we collected for both analyses approximately 50 mg of enamel powder, with a microdrill mounting a diamond burr, from the lingual surface of the tooth. The extracted enamel came from the lower half of the crown in order to prevent alterations due to metabolism in early formation phases²³ and to reduce the noise in the data linked to random sampling.

For the analysis of bone, we took approximately 50 mg of cortical bone tissue, mostly from ribs²⁴. Several authors recommend to avoid analyzing bone for Sr isotope ratios because it is particularly prone to diagenetic processes^{12,25,26,27,28}. However, in order to test possible diagenesis, and to observe the difference between tooth value and bone value in the same individual, we run two bone analyses for strontium and sixteen analyses for oxygen.

In order to define the local range at Povegliano Veronese and to compare the data obtained from teeth and bones of humans and animals, we took soil samples from the burials of area H, namely: T 348, T 426, T 413, T 45 (Table 2).

In total, 39 individuals were selected from 35 burials at Povegliano Veronese, while a grand total of 8 animal samples came from a midden excavated near the main concentration of burials and dated to the phase of use of the cemetery (G. De Zuccato pers. comm.). We selected all animal species determined within the midden (using a MNI criterion, Table 2 SI); both domesticated and wild species were available for analyses, namely: one individual of Equus caballus, two individuals of Bos taurus, two of Sus domesticus, two of Ovis vel Capra end one of Cervus elaphus (Table 1).

The burials were selected according to position within the cemetery, dating of grave goods and typology of tomb structure¹⁷, we tried to keep a balance in the composition of the sample in accordance to sex and age at death of the individuals. In order to examine strontium and oxygen ratios in humans over time we chose 21 burials dated to phase 1, 9 burials of subsequent phases (2 and 3), and 4 burials with undetermined chronology. We further sampled individuals from 3 multiple burials, which despite not providing a date were considered worthy of investigation.

In particular, the strontium isotopic ratio was measured in 39 enamel samples and 2 human bones, 4 soils samples (from area H), 4 animal teeth (from one Equus caballus, one Bos taurus, one Sus domesticus and one of Ovis vel Capra respectively) and 2 animal bones (from one Equus caballus and one Sus domesticus). Oxygen isotopic ratios were measured in 13 human teeth, 16 human bones, and 5 animal teeth (one specimen of Equus caballus, two specimens of Bos Taurus, two of Sus domesticus, one specimen of Ovis vel Capra) and 7 animal bones (one specimen of Equus caballus, two specimens of Bos taurus, two of Sus domesticus, two of Ovis vel Capra end one of Cervus elaphus) (Table 1).

Strontium isotope ratio (⁸⁷Sr/⁸⁶Sr)

After cleaning the surface of each tooth by abrasion with a diamond burr, 20–30 mg of enamel powered were extracted and digested in 1 ml concentrated ultrapure HCl. The samples were then evaporated to dryness and redissolved in 2 ml 2 M ultrapure HCl.

For bone analysis, the samples were mechanically cleaned and authigenic carbonates removed with CH3COONH₄ buffer at pH 5 in an ultrasonic bath. Approximately 5 g of cortical bone were reduced to ashes in a furnace at 800 °C for 10 h and then the material was homogenized in an agate mortar and dissolved with 6 N ultrapure HCl. Once the bone dissolution completed, the samples were evaporated to dryness, redissolved in 2 ml 2 N ultrapure HCl.

Soil samples were leaching with 1 N CH₃COONH₄ at neutral pH to obtain the NH4-acetate extract that represents organically bound Sr²⁹. The extracts were processed for Sr isotopic analysis following the procedure of bone analysis.

Sr was separated from the matrix onto a preconditioned resin column with 2 mL of AG50W-X12 (200 − 400 mesh) following the procedure of Chao et al.³⁰.

Isotopic analyses were carried out at IGAG-CNR c/o Dipartimento di Scienze della Terra, as Sapienza, University of Rome using a FINNIGAN MAT 262RPQ multicollector mass spectrometer with W single filaments in static mode. Sr isotopic fractionations was corrected against ⁸⁶Sr/⁸⁸Sr = 0.1194. During the data acquisition, measured isotopic ratios of NBS 987 Sr standard, resulted as ⁸⁷Sr/⁸⁶Sr = 0.710285 ± 10 (2σ; n = 27). The within-run precision, expressed as 2se (standard errors), was better than 0.000012 for Sr. Total procedural blanks were below 2 ng.

Stable oxygen isotopes analysis

Stable oxygen isotopes δ(¹⁸O/¹⁶O)_ph analyses on the phosphate group of teeth and bones of human and animal bioapatite (ph) were carried out at the Stable Isotope Laboratory of the University of Parma.

To analyze the oxygen isotopic composition of the apatite phosphate group of bone and tooth of humans and animals we followed the protocol by Stephan³¹.

The sample treatments were the following: samples reacted with 2.5% NaOCl for 24 h to oxidize organic substances; then, the samples were reacted with 0.125 M of NaOH for 48 h to dissolve humic substances, 2 M HF for 24 h, 2 M KOH and buffered amine solution. The solutions were then warmed at 70 °C for 3 h and filtered to collect the precipitated crystals of Ag₃PO₄. The crystals were analyzed by means of TC/EA, thermal conversion-elemental unit on line with a mass spectrometer (IRMS).

According to IUPAC (International Union of Pure and Applied Chemistry), the isotope ratio ¹⁸O/¹⁶O is expressed as:

where δ¹⁸O_sample and δ¹⁸O_V-SMOV are the isotopic abundances in the sample in analysis and in the reference international standard V-SMOW (Vienna Standard Mean Oceanic Water), and ‰ = 1/1,000. The estimated analytical prediction uncertainty for ¹⁸δ is ≤ 0.35‰. Hereafter, for simplicity, we report δ¹⁸O in place of δ(¹⁸O/¹⁶O).

In order to relate the values δ¹⁸O_ph of the ({mathrm{P}mathrm{O}}_{4}^{3-}) anionic group of enamel and bone bioapatite to that, δ¹⁸O_w, of the drinking water, we used the following equations:

for humans:

δ¹⁸O_w = 1.847 δ¹⁸O_ph − 0.0384 Iacumin and Venturelli ²¹

for Capra:

δ¹⁸O_w = 1.14 δ¹⁸O_ph − 0.0274 Delgado Huertas et al.³²

for Ovis:

δ¹⁸O_w = 0.676 δ¹⁸O_ph − 0.0184 Delgado Huertas et al.³³

for Bos:

δ¹⁸O_w = 0.990 δ¹⁸O_ph − 0.0247 Delgado Huertas et al.³³

for Equus:

δ¹⁸O_w = 1.41 δ¹⁸O_ph − 0.0318 Delgado Huertas et al.³³

for Cervus:

δ¹⁸O_w = 0.885 δ¹⁸O_ph − 0.0227 D’Angela and Longinelli³⁴

δ¹⁸O_w = 1.16 δ¹⁸O_ph − 0.0264 Longinelli¹³

(It is noteworthy that the equations used for animals could not to be statistically different one from the other).

Source: Ecology - nature.com