The archaeological application of strontium isotope (⁸⁷Sr/⁸⁶Sr) analysis has majorly impacted the study of movement, mobility, interaction, and communication in prehistory (Slovak and Paytan, 2012; see Kristiansen, 2022). The basis of the archaeological understanding of mobility is now transferred from being object-based to being based on the research subjects themselves, that is, the remains of the humans and animals inhabiting the past. The uptake of ⁸⁷Sr/⁸⁶Sr into organic tissues (bone, teeth, etc.) is directly related to the local bioavailable strontium ratios. However, for ⁸⁷Sr/⁸⁶Sr analysis to provide meaningful information on migration, local ⁸⁷Sr/⁸⁶Sr “baselines,” that is, reference values, are needed for any geographical area. The understanding of the ⁸⁷Sr/⁸⁶Sr ratio variation in southern Scandinavia has significantly increased since the surge in strontium-related studies in the last few decades. Published baseline studies exist for Scania (e.g., Ladegaard-Pedersen et al., 2021; Boethius et al., 2022), western Sweden (e.g., Blank et al., 2018; Klassen et al., 2020), and Denmark (e.g., Frei et al., 2019). The ⁸⁷Sr/⁸⁶Sr strontium isotope mapping is cumulative, meaning that adding data increases the knowledge of local strontium isotopic variations and bioavailable strontium (e.g., Holt et al., 2021). The majority of ⁸⁷Sr/⁸⁶Sr in Scania is based primarily on samples from the southern and eastern parts. The northwest Scania, including the Bjäre Peninsula, is less sampled and thus less understood.

This article aims to fill this gap in the strontium isoscape map by presenting new baseline data from the Bjäre Peninsula and Halland. We contextualize the data by relating it to the baseline frames that exist for Scania and Halland. Like Ladegaard-Pedersen et al. (2021), we base the discussion on descriptive statistics rather than modeling approaches. We also incorporate earlier published ⁸⁷Sr/⁸⁶Sr reference values from Halland in our analysis (Klassen et al., 2020), presented in Supplementary Table S1. Finally, the aim is to apply this improved baseline archaeologically, with strontium isotope analyses on human and animal remains from recent excavations in Bjäre.

The Bjäre Peninsula constitutes the northwestern outpost of Scania, bordering the region of Halland to the north (Figure 1). Bjäre and parts of Halland lie on the Sveconorwegian orogen. Bjäre is covered by a granitoid to syenotoid migmatitic gneiss (1.7 Ga). This is also the case for Halland, with the addition of granite (1.7–1.0 Ga) and small spots of mainly amphibolite (1.7–0.9 Ga). The northernmost part of Halland consists of granitoid (1.6–1.5 Ga) and metamorphic bedrock, which extends northward above Lake Vänern. We also see areas of granite, syenitoid, and metamorphic equivalents (approximately 1.36–1.27 Ga) and gabbro, diorite, ultrabasic rock, dolerite (1.6–1.3 Ga), and metamorphic rocks. The intersection between Bjäre and Halland, that is, the bay interconnecting them, is Cretaceous sedimentary cover rock consisting of limestone, sandstone, and clay.

The similarity in bedrock across Halland and Bjäre and inland northern Scania indicates that most of these areas, disregarding any sea spray or marine effects, should hypothetically contain similar ⁸⁷Sr/⁸⁶Sr ratios. However, soil variation is often a more important consideration of bioavailable strontium (Wilhelmson, 2017, p. 199). The soil types are more heterogeneous in Bjäre, dominated by till with spots of exposed bedrock across the peninsula, while along the south coast and the “tip” of the peninsula are postglacial sand/gravel deposits (Figure 1). Halland, by comparison, is dominated by postglacial sand, with spots of fluvioglacial deposits, exposed bedrock, and clay/silt.

Leaves from two different plants from seven locations in Halland and three in Bjäre (Table 1) were sampled for the ⁸⁷Sr/⁸⁶Sr analysis. Three leaves were each collected from Lokal 18, Lokal 21, and Ängelbäcksstrand on Bjäre (Figure 1). We chose both coastal and inland locations to capture possible sea-spray effects or any marine variation (Figure 1). The preliminary strontium baseline is established on descriptive statistics of the isotopic results, as well as earlier published baseline ⁸⁷Sr/⁸⁶Sr values from Klassen et al. (2020). We have excluded samples from Klassen et al. (2020, p. 414), which we consider possibly unrepresentative of the local signal, such as modern minks, which probably were feralized from mink industries with an unknown food supply. Furthermore, we do not include humans and domestic animals, for example, pigs, sheep, and goats, that could hypothetically have a high degree of mobility (e.g., Meiri et al., 2017). This is a more conservative approach than the contextual one put forward by Klassen et al. (2020); however, we believe it is more robust in establishing the local ⁸⁷Sr/⁸⁶Sr baseline.

The ⁸⁷Sr/⁸⁶Sr results from the plants (Table 1) are summarized for each area as the sample mean (x) ± 2σ (Wilhelmson, 2017, p. 198), generating a preliminary single-proxy baseline for Bjäre of 0.7100 ± 0.0024 (2σ, n = 8) and Halland 0.7124 ± 0.0058 (2σ, n = 15). The ⁸⁷Sr/⁸⁶Sr ratios of plants show that Bjäre provided lower values than Halland. The ⁸⁷Sr/⁸⁶Sr variation is narrower in Bjäre (0.7082–0.7115) compared to Halland (0.7087–0.7177), which could be due to relatively fewer samples or a more homogeneous makeup of bioavailable strontium. The lower ⁸⁷Sr/⁸⁶Sr values in Bjäre and Halland, compared to, for example, northeast Scania, probably indicate a certain degree of sea-spray effect, as these regions are closer to seawater ⁸⁷Sr/⁸⁶Sr values of approximately 0.70918 (McArthur et al., 2001). Klassen et al. (2020) found that ⁸⁷Sr/⁸⁶Sr values from faunal samples were least radiogenic along the southern Halland coast (minimum 0.7087) and most radiogenic inland (maximum 0.7189). The lowest ⁸⁷Sr/⁸⁶Sr values in that study were from three rodent crania (0.7087, 0.7098, and 0.7099), collected from coastal sites in Halland, leading Klassen et al. (2020) to infer that sea spray must be affecting strontium ratios in western Sweden in general. In our plant data, we do not fully observe the sea-spray effect among the samples from the coastal settlements, especially Ängelbäcksstrand (Wil 20–22) and Skummeslöv (Wil 11–12). The latter is located on younger sedimentary rocks than in most of Bjäre and Halland (Figure 1), so this is somewhat surprising. The effect of sea spray on bioavailable strontium seems to vary locally, especially as geological soil formation is complex, due to the idiosyncratic ways the ice retreated during the Late Glacial (see Klassen et al., 2020, p. 416). At Skummeslöv, located on limestone (Figure 1), the soil type is similar to most of the gneissic and granitic Halland coast (postglacial sand/gravel). This may have an averaging effect on the bioavailable strontium in the whole area. The till from which the Bjäre samples were taken is similar to most of inland western Sweden in this area, so this may be one explanation for why the values are not as low as expected.

To contextualize our modern plant ⁸⁷Sr/⁸⁶Sr values from Halland, we turn to previous baseline samples as reported by Klassen et al. (2020), particularly those deriving from bone or teeth of smaller mammals (Supplementary Table S1). These values show a slightly lower ⁸⁷Sr/⁸⁶Sr baseline of 0.7111 ± 0.0041 (2σ, n = 9) than the plants. Integrating Klassen et al.'s faunal samples from Halland with our plants generates a multiproxy baseline for Halland of 0.7122 ± 0.0055 (2σ, n = 24). Klassen et al. (2020) provided one value from a hare, 0.7101, from Hallands Väderö, an islet outside the tip of Bjäre. This fits into the Bjäre baseline range proposed earlier (0.7100 ± 0.0024).

Like Bjäre, most of northern and middle Scania, down to the Ringsjö lakes and a few kilometers east of the city Hässleholm, is located on a granitic to syenotoid migmatitic gneiss. This area of Scania constitutes the largest part of “Area 1” as delineated by Ladegaard-Pedersen et al. (2021) in their division of Scania in terms of bioavailable strontium through a multiproxy approach, using plants, water, and soil leachates (see Figure 1). The strontium baseline for this area is 0.7184 ± 0.0061 (2σ, n = 16; Ladegaard-Pedersen et al., 2021). For the east of Scania, “Area 2,” covering the southern part of the previously mentioned gneiss bedrock to the eastern Scanian coastline, the calculated baseline was 0.7140 ± 0.0043 (2σ, n = 48; Ladegaard-Pedersen et al., 2021). Using these baselines, both areas are described with more radiogenic ⁸⁷Sr/⁸⁶Sr values than those we observe in Bjäre and Halland. The Bjäre ⁸⁷Sr/⁸⁶Sr values are most similar to southwest Scania, Ladegaard-Pedersen et al. (2021)'s “Area 3 + 4,” with an ⁸⁷Sr/⁸⁶Sr baseline of 0.7110 ± 0.0030 (2σ, n = 39). This is also comparable with many Danish baselines (Frei et al., 2019; Klassen et al., 2020), excluding the Bornholm region.

This short communication presents new strontium isotope data from the Bjäre Peninsula and Halland in southern Sweden. Based on plant data, we suggest 0.7100 ± 0.0024 (2σ, n = 8) as a specific ⁸⁷Sr/⁸⁶Sr baseline for Bjäre. The results indicate that the Bjäre Peninsula is affected by marine sea-spray effects, as the values are lower than values from inland localities with similar geological backgrounds. There is a greater ⁸⁷Sr/⁸⁶Sr variation in Halland, with generally higher ⁸⁷Sr/⁸⁶Sr values. Together with previously published faunal samples, we suggest a revised strontium isotope baseline for Halland to 0.7122 ± 0.0055 (2σ, n = 24). We applied these baselines to seven archaeological samples from human, cattle, and sheep/goat remains, dating to the Late Neolithic/Early Bronze Age, pre-Roman Iron Age, and the late Viking Age, retrieved from recent excavations in the southern part of Bjäre. The results from both human and animal samples are interesting for the discussion of mobility and interaction. Several samples yielded ⁸⁷Sr/⁸⁶Sr values indicative of provenience outside of Bjäre, which highlights the potential of studying mobility, communication, and interaction in this area through isotopic analysis. This research is incremental and can be added to the recent surge of studies, increasing the understanding of bioavailable ⁸⁷Sr/⁸⁶Sr in southern Scandinavia. Importantly, more studies refining and modifying our suggested ⁸⁷Sr/⁸⁶Sr baseline will surely follow in the future.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding authors.

Ethical approval was not required for the study involving humans in accordance with the local legislation and institutional requirements. Written informed consent to participate in this study was not required from the participants or the participants' legal guardians/next of kin in accordance with the national legislation and the institutional requirements. The manuscript presents research on animals that do not require ethical approval for their study.

SM: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing—original draft, Writing—review & editing. MS: Data curation, Investigation, Methodology, Writing—original draft, Writing—review & editing. AS: Project administration, Resources, Writing—review & editing. HW: Investigation, Writing—review & editing. ÅB: Investigation, Writing—review & editing. PW: Investigation, Writing—review & editing.