A matter of months: High precision migration chronology of a Bronze Age female

Establishing the age at which prehistoric individuals move away from their childhood residential location holds crucial information about the socio dynamics and mobility patterns in ancient societies. We present a novel combination of strontium isotope analyses performed on the over 3000 year old “Skrydstrup Woman” from Denmark, for whom we compiled a highly detailed month-scale model of her migration timeline. When combined with physical anthropological analyses this timeline can be related to the chronological age at which the residential location changed. We conducted a series of high-resolution strontium isotope analyses of hard and soft human tissues and combined these with anthropological investigations including CT-scanning and 3D visualizations. The Skrydstrup Woman lived during a pan-European period characterized by technical innovation and great social transformations stimulated by long-distance connections; consequently she represents an important part of both Danish and European prehistory. Our multidisciplinary study involves complementary biochemical, biomolecular and microscopy analyses of her scalp hair. Our results reveal that the Skrydstrup Woman was between 17–18 years old when she died, and that she moved from her place of origin -outside present day Denmark- to the Skrydstrup area in Denmark 47 to 42 months before she died. Hence, she was between 13 to 14 years old when she migrated to and resided in the area around Skrydstrup for the rest of her life. From an archaeological standpoint, this one-time and one-way movement of an elite female during the possible “age of marriageability” might suggest that she migrated with the aim of establishing an alliance between chiefdoms. Consequently, this detailed multidisciplinary investigation provides a novel tool to reconstruct high resolution chronology of individual mobility with the perspective of studying complex patterns of social and economic interaction in prehistory.


The archaeological context of Skrydstrup's burial mound
The Skrydstrup Woman was unearthed in 1935 from a burial mound in southern Denmark (1). The mound from which she was recovered was part of a mound-group consisting of a total of eight burial mounds (2,3). The mound-group is located southwest of the village of Vojens in a geographical and geological liminal zone that divides the landscape in two by the East-Jutlandic maximum glaciation line of the last Ice Age (Weichsel). This end-moraine line stretches from south to north and divides the landscape: to the west, the land is flat and sandy, whereas to the east the landscape is dominated by hills and easily recognizable moraines. In consequence, the central trade/exchange route (known in Danish as the "Haervej") (4) ran parallel to this line, and hence also passed close to the Skrydstrup site.
Interesting for the archaeological context of the Skrydstrup Woman's burial is a contemporary settlement of longhouses nearby which were discovered in 1993. These longhouses are located about 600 meters to the east of the burial site (3) and consist of three 3-aisled structures, which according to the 14 C-dates, are contemporary with the Skrydstrup Woman's mound-group and therefore potentially point to some kind of relationship. One of the houses (House IV) exhibit an impressive total ground area of approximately 500 m 2 , making it the largest Early Bronze Age house excavated in Denmark to date. Another of the houses (House I) also stands out as 14 C-dates of the house overlap and are very similar to the 14 C-dates obtained for the Skrydstrup Woman. It might be possible, therefore, that the Skrydstrup Woman had some kind of knowledge about the existence (or even had a more direct relation) to this particular house (3).
In sum, the impressive size of the longhouse, combined with its proximity to the central trade route ("Haervej") provide evidence of this area's important role in interregional trading networks during the Nordic Early Bronze Age (most specifically in Periods II and III, cf. (5).

The Skrydstrup Woman
The Skrydstrup woman's remains represent the mound's primary grave and consist of skeletal as well as soft tissues including parts of her cheeks and chin, eyebrows, eyelids, eyelashes and her long hair (1, 6). Although only the hair remains of the latter, a large part of Skrydstrup Woman's skeleton is still intact; the teeth in particular are well preserved (1).
Her hair (which is of particular interest for our present study) measures in at over 60 cm, and is set in a highly complex hairstyle (1). In order for her locks to be set, her hair was first combed forward and downwards over a hair pad before being bound by a wool cord. The hair was then plaited across the forehead from temple to temple before finally being covered with a hairnet that seems to have been made from horse hair (6). The os coxae are present, although both right and left pubic symphyses were lost post mortem and the superior right ilium suffered some damage.
Although the right femur is present, the distal end is damaged. By contrast, the right tibia is present, but has a damaged proximal end. The right calcaneus and talus are present and so is the right navicular, lateral, intermediate and medial cuneiform as well as the first and second metacarpals.
The left femur is the best-preserved long bone of the Skrydstrup Woman. It is unfortunately mainly held together by the black glycerin coating mentioned in the main text. It has several fracture lines incurred post mortem. Due to post mortem damage prior to the glycerin coating, the distal one third of the bone is bent slightly in an anterior direction. The maximum length of the left femur was measured to be 469 mm, 0.6 mm less than the measurement from 1939 (1). The difference between the two measurements could be due to a measurement error or due to the shrinkage of the bone over the last seven decades.
There is an ongoing debate as to which method should be applied in estimating living stature from archaeological skeletal remains (7). The most widely used method within the osteoarchaeological literature is that by Trotter (8). Although generally considered the most accurate, the equations were developed on modern American samples (American females from various social backgrounds who died in the 1960s). Unfortunately, the correlation between long bone length and living stature is not always reliable within a living population, let alone between modern populations and ancestral groups. Trotter's methods (8)  Due to Denmark's Late Bronze Age cremation tradition as well as the country's particular soil conditions, there are very few skeletal remains to which the Skrydstrup find can be compared to.
One of these rare points of comparison is that of the female oak coffin burial from Borum Eshøj C whose left femur is slightly shorter (421mm) and is also more robust than that of the Skrydstrup Woman, and also exhibits marked muscle attachments. Despite the fragmentation, it is clear that the Skrydstrup Woman's long bones are both unusually long and slender at the same time. Initial measurements were taken of the long bones as well as the cranium, irrespective of their poor preservation. Even then, osteologists acknowledged that any measurements they took would not be representative of the true lenghts had the bones been intact. The robusticity index of the left femur was initially calculated to be 10.9 (1). Unfortunately, re-measurement was prevented by the glycerin.
No evidence of pathology was found, although the Skrydstrup Woman did exhibit a congenital anomaly insofar as she had a missing styloid of the third left metacarpal. The latter, however, would have had no impact on either her health or daily activities.
CT scanning was performed both of the skull and the postcranial bones. The head was CT scanned in 2011 using a Siemens Sensation 4 using 1 mm slice thickness, 1 mm slice increment and a smooth reconstruction algorithm. Post cranial bones were CT scanned in 2016 using a Siemens Somatom Definition with 0.6 mm slice thickness, 0.4 mm slice increment and a sharp reconstruction algorithm. Post-processing of the images was done using Mimics (Materialise) and 3D visualization was created using the segmentation methods described by Lynnerup (11) and Villa & Lynnerup (12).

Stable isotope analyses (δ 13 C and δ 15 N)
We conducted stable isotopes analyses of a 12 cm long sample of Skrydstrup Woman's scalp hair to investigate her dietary habits during roughly the final year of her life. The 12 cm long hair resolution incremental data on the Skrydstrup Woman's diet.
The following tables and figures show values for incremental measurements (δ 13 C and δ 15 N ratios of hair) recorded for the Skrydstrup Woman on instrument run ACQ625.

Microscopic analyses
Several scalp hair fibers from the Skrydstrup Woman were examined with a stereo microscope and then a transmitted light microscope as described in the main text. Remnant cuticular scales were found in places on all of the hair fibers. Only one of the examined fragments showed signs of possible pigment granules (Supp. Inf. Fig. 5 below).

C analyses
We performed 14 C analyses by AMS of the Skrydstrup Woman's scalp hair, which revealed an age of 3009 ± 27 BP (Lab. nr. AAR: 25433). This age corresponds to the Nordic Early Bronze Age Period III (1300 -1100 BC).
Prior to 14 C analysis, the hair sample was deeply pre-cleaned by submerging the hair in a mixture of acetone and ethanol (on a hotplate) and subsequently rinsed with demineralized water. The C:N ratio of 3.0-3.8 measured after these treatments indicate that the pre-cleaning procedure was successful as it corresponds well with C:N scalp hair values of modern humans.  contained a stone cist whose bottom was covered by wood (coffin remains) and which included animal hair, the badly-preserved remains of the deceased and a series of bronze artifacts which, when compared to the two spiral gold rings recovered on either side of the Skrydstrup Womans's skull (Fig. 3), are important and relevant for our current discussion about the Skrydstrup Woman's 15 origin. The placement in the grave, the material and the general form of the Melhøj spiral rings mirror those recovered from Skyrdstrup, however they are smaller in size. The Melhøj burial was richer than the one from Skrydstrup insofar as it included various bronze objects, including a tutulus belt disc, a tutulus, two bracelets, a necklace, a long pin, a small fibula, an ankle ring and a 28 cm-long dagger (21). The objects all place this burial in Nordic Bronze Age period III. Melhøj's organic material has been 14 C dated at 1220-980 BC (+/-1 std.), which imply that the Melhøj and Skrydstrup burials belong to the same period, namely the Nordic Bronze Age Period III, and were probably almost contemporary (21). Indeed, the presence of an embroidered border among the textile fragments from Melhøj, which must undoubtedly be interpreted as the ornamentation of the neckline of a blouse similar to the one recovered from the Skrydstrup burial (21), serves as further evidence for the similarity of the burials at Skrydstrup and Melhøj.
That being said, the gold spiral rings from the Skrydstrup burial also find close correlations in the gold wire industry which blossomed in the Urnfield and Tumulus cultures of Bohemia. Particularly in eastern Bohemia (Czech Republic) well over 400 gold objects are recorded which sum up to a total gold weight of approximately 5 kg (16). Most of these gold finds are wire objects, dominantly of the double wire variety (22), as the one found with the Skrydstrup Woman. As is the case with the find from Skrydstrup, the Bohemian gold wire rings exhibit round or sub-round cross-sections and are twisted into a series of loops, often made of doubled wire. However, the closest central European parallels to the Skrydstrup find are seldom recovered from funerary contexts; they are more often recovered from hoard finds.
While there are similarities between the Skrydstrup spiral rings and other contemporary finds from e.g. Bohemia, the discovery of similar material and a similar usage of the gold spirals at Melhøj in