The application of 87 Sr/ 86 Sr in prehistoric mobility studies requires accurate strontium reference maps. These are often based from present-day surface waters. However, the use of agricultural lime in low to noncalcareous soils can substantially change the 87 Sr/ 86 Sr compositions of surface waters. Water unaffected by agriculture in western Denmark has an average 87 Sr/ 86 Sr ratio of 0.7124 as compared to an average of 0.7097 in water from nearby farmland. The 87 Sr/ 86 Sr ratio obtained from samples over 1.5 km along a stream, which originates in a forest and flows through lime-treated farmland, decreased from 0.7131 to 0.7099. Thus, 87 Sr/ 86 Sr-based mobility and provenance studies in regions with low to noncalcareous soils should be reassessed. For example, reinterpreting the iconic Bronze Age women at Egtved and Skrydstrup using values unaffected by agricultural lime indicates that it is most plausible that these individuals originated close to their burial sites and not far abroad as previously suggested.

The question of whether agricultural lime can affect the 87 Sr/ 86 Sr ratios of bioavailable strontium is of global importance, as sediments similar to the deposits studied here occur widespread and are commonly subjected to intensive farming and liming. If agricultural lime can significantly change the isotopic composition of bioavailable strontium, then it has serious implications for the development of 87 Sr/ 86 Sr baseline maps and for the use of 87 Sr/ 86 Sr in provenance and migration studies worldwide.

To test whether agricultural lime can influence the 87 Sr/ 86 Sr ratios of surface waters, we compare 87 Sr/ 86 Sr ratios and strontium concentrations measured in surface water samples from “pristine” areas, which never have been subject to agricultural lime, with data from neighboring farmland, where lime has been added to the soils. The use of surface water as a sample medium has the advantage that it allows for the determination of strontium concentration to aid in the interpretation of the strontium isotopic ratios. Lime is the main subject of this study, but it is evident that anthropogenic activities such as the use of fertilizers ( 10 ) and several physical factors including rainwater, dust, and marine sea spray may also influence the strontium content in surface water ( 11 ).

Considering the effect of chalk and limestone in northern Jutland, it was expected that the east-west shift across Jutland from the strongly calcareous soils in East Jutland to noncalcareous soils in West Jutland ( 17 , 18 ) would be visible in the 87 Sr/ 86 Sr map, with lower ratios in East Jutland than in West Jutland. However, this is apparently not the case according to the baseline map of Frei and Frei ( 15 ), which indicates that the 87 Sr/ 86 Sr ratios change very little across Jutland. A possible explanation for this lack of correlation between the 87 Sr/ 86 Sr ratio and the calcium carbonate content in the sediments could be agricultural lime, which for more than a century has been used for soil improvement throughout most of Denmark.

The 87 Sr/ 86 Sr map generated by Frei and Frei ( 15 ) show an isotopic homogeneity at odds with the geologically varied Pleistocene glacial deposits that constitute the sediment cover of most of Denmark ( 15 ). The only distinct geological feature shown in this map is a zone across northern Jutland with noticeable lower 87 Sr/ 86 Sr values (~0.708). This zone represents an area where glacial deposits directly overlie Upper Cretaceous and lower Paleocene chalk and limestone formations, both of which are rich in strontium carrying an 87 Sr/ 86 Sr ratio of ~0.708 ( 15 ).

On the basis of 192 samples of surface waters ( 15 ) and of data from animal remains ( 16 ), Frei et al. ( 5 ) calculated an 87 Sr/ 86 Sr baseline for Denmark (excluding the island of Bornholm) spanning from 0.708 to 0.711. The mean and 2 SDs of the dataset is 0.7096 ± 0.0015, and the full range is 0.7079 to 0.7128 ( 15 ). The baseline has since been used as a reference in several remarkable studies of prehistoric migration in Denmark, in particular, regarding female mobility during the Bronze Age. For example, Frei et al. ( 5 ) concluded that the iconic Egtved Girl excavated in Central Jutland spent her early years outside of Denmark, possibly in southern Germany. During the last 2 years of her life, she traveled back and forth between Denmark and a place outside of Denmark (likely here birthplace) before she died at Egtved at the age of 16 to 18. Another Bronze Age female from Jutland, the Skrydstrup Woman, appears to have first come to Denmark at the age of 12 to 13. Here, she died 4 years later and was buried in a mound at Skrydstrup ( 6 ).

In recent years, variations in strontium 87 Sr/ 86 Sr ratios have been applied to uncover migration patterns of prehistoric humans and animals with considerable success ( 1 – 6 ). The method is based on the observation that the 87 Sr/ 86 Sr ratios in soil substrates vary geographically, reflecting the lithology of the soil. Strontium is released from the substrate to groundwater and surface water and becomes bioavailable, and is taken up in plants and animals, with no change to the average 87 Sr/ 86 Sr ratio ( 4 , 7 – 9 ). Thus, the mobility of a prehistoric human can be estimated by comparing the 87 Sr/ 86 Sr value of the remains of an individual to a reference map showing the distribution of bioavailable strontium in the study area. The success of the method strongly depends on the accuracy of the reference map ( 10 – 14 ). However, investigations of prehistoric human migration are generally based on comparisons with modern-day isotopic maps, and a large variety of geological and biological sample materials have been analyzed to provide the necessary data ( 10 ), notably surface water and vegetation ( 10 , 15 ), which are two of the most widely used and probably most reliable materials.

RESULTS

The study area comprises the middle part of Jutland (Fig. 1A). The area is divided into three zones by two glaciogenic boundaries: (i) the Main Stationary Line (MSL), separating West and Central Jutland, marks the maximum extent of the Scandinavian ice sheet during the last Glaciation (22 ka ago); and (ii) the East Jutland Line (EJL), separating Central and East Jutland, marks the maximum extent of the later East Jutland advance (17 ka ago) (Fig. 1A) (17). The periglacial deposits of West Jutland consist of Weichselian outwash plains surrounding older Saale landscapes (Fig. 1B). Both of these units are essentially noncalcareous (see fig. S1). The deposits of Central and East Jutland consist predominantly of clayey tills. Whereas the near-surface deposits of the tills of Central Jutland are predominantly noncalcareous, the tills of East Jutland are strongly calcareous (fig. S1) (17–19).

Fig. 1 Simplified Quaternary geological maps of Denmark with sample localities. (A) Map showing the position of the MSL and the EJL, indicating the maximum extent of the Weichselian ice sheet in Denmark and the maximum advance of the East Jutland Till, respectively. The red rectangle marks the geographic location of the map in (B). (B) The MSL and EJL subdivide Jutland into three glaciogenic provinces: West Jutland (West), Central Jutland (Central), and East Jutland (East). The distribution is indicated in West Jutland of “Hill Islands” (Saale glacial deposits) and Weichsel outwash plains. Sample localities 1 to 20 are marked. Red dots denote samples obtained in West Jutland, blue dots denote those in Central Jutland, and green dots denote those in East Jutland. Maps are based on (17, 18). B.P., before the present.

A total of 84 samples of surface waters were analyzed for strontium concentration and isotopic composition (table S1). The sampling strategy was to follow streams from their sources in pristine areas to areas where agricultural lime has been applied. Where pristine streams are not available, lakes and ponds from pristine areas are compared to lakes and ponds from neighboring farmland areas. The identification of pristine and farmland areas was made a priori by overlaying maps and aerial photos of varying ages. A pond in a flat-lying area is considered to be pristine when there is a distance of at least 150 m to the nearest farmland. A stream is only regarded as “pristine” if it originates within the pristine area where it is sampled. Because streams generally occupy the lowest topographic points and tend to draw a wider catchment area than ponds, the buffer zone for pristine streams is set to 300 m. A full overview of localities and samples including geographic coordinates, sampling dates, distances between sampled pristine water bodies, and farmland and environmental affiliation is given in table S1. Two samples of stream water from locality 1 (K-13 and K-14) were collected closer to farmland than 300 m to examine the transition between pristine land and farmland. In table S1, they are classified as pristine.

The distance requirements for pristine ponds and streams are based primarily on Belgian studies of the effect of pesticides on farmland ponds (20, 21). These studies found that the levels of pesticides in ponds changed from a high degree of contamination at intensive land use with a 10-m buffer zone between pond and farmland to a very low degree of contaminations in more “pristine, natural” environments with a buffer zone of around 100 m (21). Overall, it was found that the load of a pesticide decreased to about a 20th when the buffer zone increased from 10 m to at least 100 m.

Denmark is a heavily cultivated country. About 57% of the total area is arable and is on a regular basis supplied with agricultural lime. Pristine areas in Denmark are therefore small and almost exclusively located in nature preserves or in old forests. When applying a buffer zone of 150 m, pristine areas (excluding coastal dune areas) make up about 5.6% of the total area of the country.

The samples obtained during the course of this study are distributed on 20 localities representing the three geological regions: (i) the noncalcareous West Jutland, (ii) the low to noncalcareous Central Jutland, and (iii) the calcareous East Jutland (Fig. 1B). This allows further examination of the effect of agricultural lime in relation to variations in the natural content of lime in the soil. In addition to surface water samples, a few samples of groundwater (well water) were analyzed. Maps and descriptions of most localities can be found in fig. S2.

Pristine samples versus farmland samples The 87Sr/86Sr ratios of all 84 samples of surface waters vary from 0.7080 to 0.7150, while the strontium concentrations vary from 0.001 to 0.400 mg/liter (table S1). Sixty samples were obtained from pristine areas unaffected by farming, while 24 samples were obtained from farmland. Basic statistical information of the dataset is given in table S2. The data from the noncalcareous West Jutland show a substantial difference between pristine and farmland samples (Fig. 2A). In the pristine samples, the 87Sr/86Sr values are high and variable (0.7104 to 0.7150), whereas in the farmland samples, the values are low and the range is narrower (0.7091 to 0.7115). By contrast, the strontium concentrations are low and relative variable in the pristine areas (0.002 to 0.038 mg/liter) and higher and less variable in the farmland areas (0.038 to 0.133 mg/liter). There is no overlap between the Sr concentration data from pristine and farmland areas (Fig. 2A). Fig. 2 87Sr/86Sr ratios and strontium concentrations for all surface water samples. Each sample is represented by two vertical bars, one for the 87Sr/86Sr ratio and one for the strontium concentration. The samples are subdivided into three main groups following the geological division of the study area: (A) Samples from the noncalcareous West Jutland, (B) samples from the low to noncalcareous Central Jutland, and (C) samples from the calcareous East Jutland. Within each of these geological zones, samples from pristine areas and samples from farmland areas are distinguished. (D) Box and whisker plots (25, 50, 75, and 100%) of 87Sr/86Sr ratios and strontium concentrations, with pristine samples from the low to noncalcareous West and Central Jutland added together, farmland samples from the low to noncalcareous West and Central Jutland added together, pristine samples from calcareous East Jutland, and the 87Sr/86Sr baseline for Denmark of Frei and Frei (5, 15). N = number of samples in each category. The results from Central Jutland resemble the results from West Jutland (Fig. 2, A and B). The 87Sr/86Sr ratios of the pristine samples exhibit a wide range of high values (0.7093 to 0.7136), whereas the farmland samples exhibit a narrower range of low values (0.7080 to 0.7095) (Fig. 2B). The Sr concentration data also show many similarities to the data from West Jutland. The strontium concentrations in the pristine samples are relatively low (0.001 to 0.084 mg/liter) but more variable than in West Jutland. The farmland samples have higher Sr concentrations, with a range from 0.045 to 0.146 mg/liter. The strontium distribution pattern of the calcareous East Jutland is markedly different from those of West and Central Jutland, in that there is no significant difference between the pristine and the farmland samples (Fig. 2C). The 87Sr/86Sr ratios are lower and with a narrower range (0.7085 to 0.7104 as compared to 0.7104 to 0.7150 for West Jutland), whereas the Sr concentrations are higher and more variable (0.057 to 0.338 mg/liter as compared to 0.002 to 0.038 mg/liter for West Jutland). Furthermore, it is apparent that strontium concentration is lower (max, 0.15 mg/liter) in the farmland water samples from West and Central Jutland supplied with agricultural lime than in the naturally calcareous water samples from East Jutland (max, 0.35 mg/liter). A plot of strontium concentrations versus 87Sr/86Sr ratios (Fig. 3) shows that there is no simple relationship between strontium concentration and isotopic composition of the pristine samples from the low to noncalcareous West and Central Jutland. The variability could reflect variable levels of rainwater dilution, different amounts of windblown dust, differing degrees of equilibrium between surface waters and soils, or variations in soil type. Several studies from northern Europe have shown that the strontium concentration in rainwater is very low, relative to the concentration in surface water (table S1) (15, 22, 23). Rainwater can thus have leverage on the strontium concentration in surface water but not on the strontium isotopic composition. Variations in grain size, lithology, and particularly in the cover of aeolian sand may explain why the pristine samples from the outwash plain at Kompedal Plantage (Fig. 1B, locality 1) show much smaller variation in strontium isotopic composition than the samples from the plain at Randbøl Hede (heathland) and Frederikshåb Plantage 60 km to the south (Fig. 1B, localities 4 and 5, and data in fig. S2, A and B). The southerly plain is overlain by a patchy cover of aeolian material, in contrast to the plain at Kompedal, where aeolian material is nearly absent. Fig. 3 Strontium concentration versus 87Sr/86Sr ratio for Danish surface water samples. Samples are subdivided into two groups following the geological subdivisions of the study area: (i) the noncalcareous West Jutland and low to noncalcareous Central Jutland and (ii) the calcareous East Jutland. The samples are classified as pristine or agricultural. Previously published samples (15), marked with gray dots are classified as agricultural. Shaded areas show the range in strontium concentrations (Sr conc.) and 87Sr/86Sr ratios for each group. Samples from Karup River are marked with internal yellow circles. Hypothetical mixing lines are shown for three end-members: (i) low 87Sr/86Sr, high Sr concentration waters derived from limestone; (ii) intermediate 87Sr/86Sr, low Sr concentration waters derived from quartz sand or dominated by rainwater; and (iii) high 87Sr/86Sr intermediate Sr concentration waters derived from quartz sand with significant input from felsic rocks. Data for samples from (15) obtained from the same geographical locations as those obtained in the course of this study (n = 4) are each marked with a colored dot corresponding to geographical location and sample classification. Analytical uncertainties in the y-axis values are smaller than the data symbols. Analytical uncertainties in the x-axis values are about the size of the data symbols for low-concentration samples and are smaller for high-concentration samples.

Pristine samples versus farmland samples at individual localities The differences between surface water from pristine areas and farmland areas are best detailed in the Vallerbæk stream–Karup River catchment area (Fig. 1B, locality 1, and Fig. 4). The Karup River is situated on the geologically homogeneous Karup outwash plain. The river is primarily sourced by groundwater, because of the coarse, highly permeable, sandy sediments. The Vallerbæk tributary originates in Kompedal Plantage (a 200-year-old extensively worked coniferous tree plantation with no soil improvement), where conditions are considered to be pristine, and runs in a shallow Holocene erosional valley (Fig. 4B). The uppermost sampling location (K-5) is a small pond. Between K-5 and K-3, the run is tiny and mostly dry. This part was not sampled. From K-3 to the farmland, the stream carries water, but the run is irregular and it frequently expands into swampy areas with no visible flow. At the beginning of the farmland at sampling location K-13, the flow is about 10 liters/s. After 1.5 km at sampling location K-4, the flow has increased to about 40 liters/s. The groundwater table in the plantation is 5 to 10 m below the forest floor, indicating that the water in this part of the stream is surface water with little interaction with groundwater. As the brook downstream erodes deeper into the outwash plain, the distance to the groundwater table decreases, such that in the farmland, the groundwater table and the water table of the stream are level, indicating that the stream water is primarily groundwater. Fig. 4 Variability in 87Sr/86Sr ratio and strontium concentration in the Vallerbæk stream-Karup River catchment area. (A) Strontium data along the main run of Karup River with location of the Vallerbæk tributary indicated. (B) Detailed map of Kompedal Plantage and Vallerbæk tributary with sample positions and strontium data indicated. (C) Strontium concentration and 87Sr/86Sr ratios for Vallerbæk stream and Karup River as a function of distance from the head of Vallerbæk stream. Maps are based on data from “Styrelsen for Dataforsyning og Effektivisering, skærmkortet, WMS-tjeneste.” The 87Sr/86Sr ratios of the surface water samples from within the tree plantation are high (>0.7130), whereas the Sr concentrations are low (<0.01 mg/liter) (Fig. 4C). Within 1.5 km downstream of the plantation, the 87Sr/86Sr ratio decreases to 0.7099, while the concentration increases to 0.066 mg/liter. Most of these changes occur over a distance of less than 500 m (Fig. 4C). For the next 57 km, the 87Sr/86Sr ratio remains nearly constant, while the Sr concentration increases gradually. The pristine groundwater in the plantation exhibits the same 87Sr/86Sr ratio as the surface water, but it has a much higher concentration (0.066 and 0.048 mg/liter as compared to <0.01 mg/liter for the surface water) (Fig. 4B). Localities 3, 4, and 5 (Gludsted, Sepstrup Sande, and Nørlund) are also centered around streams flowing from pristine forests to farmland areas (Fig. 1B and fig. S2, C, D, and F), but they are located in regions with more variable geology. The results from these localities follow the pattern observed in the Vallerbæk stream–Karup River system with 87Sr/86Sr ratios decreasing and concentrations increasing downstream, as the stream flows from pristine heathland and woodland into farmland. The difference in Sr isotopic composition between pristine heathland and woodland and farmland observed in streams is also apparent when comparing lakes and ponds, as illustrated by the samples from Bevtoft Plantage, which is situated on a noncalcareous outwash plain west of a major tunnel valley (Fig. 1B, locality 8, and fig. S2J). Two pristine samples give 87Sr/86Sr ratios of 0.7115 and 0.7141, as compared to a ratio of 0.7095 for a water sample taken in a pond on farmland just 800 m away (fig. S2J).

The effect of agricultural lime on the distribution of strontium isotopes in surface waters The most notable characteristic of the 87Sr/86Sr data from the low to noncalcareous West and Central Jutland is the distinct difference between the high 87Sr/86Sr ratios in samples from pristine areas and the low 87Sr/86Sr ratios in samples from farmland, and the reverse pattern in the strontium concentrations (Fig. 2, A and B). Geologically, there are no indications of changes in lithology in connection with the transition from pristine land to farmland, and it is highly unlikely that the shift is related to changes in geology. The only consistent environmental differences that may explain these changes appear to be differences in the degree of anthropogenic activity, from almost no activity in the pristine woodland and heathland to high activity in the farms. Without farming, the high 87Sr/86Sr ratios and the low strontium concentrations that characterize the upper pristine part of Vallerbæk stream would have been expected to have continued all the way to the mouth of Karup River at Skive (Fig. 4). Agricultural activities that could affect the compositions of strontium in surface waters include the spreading of agricultural lime, fertilizers, manure, animal feed, and pesticides. To analyze the relative effect of each of these anthropogenic components, we performed a quantitative analysis of the input and output of strontium in the Vallerbæk stream–Karup River catchment area. This catchment area was selected because it is located on a geologically homogeneous, noncalcareous, outwash plain, and because the hydrology of the Karup River system is well established (24). The quantities of lime, fertilizer, and other sources used in the calculations have been provided by farmers from the Vallerbæk area and by local agricultural consultants. Estimating the average strontium contribution from agricultural lime and the lime’s isotopic composition is relatively straightforward in the Vallerbæk catchment area. The lime applied in Jutland today consists of equal parts pure Upper Cretaceous chalk from northern Jutland and 2.5% magnesium chalk. The latter product is a mixture of Jutland chalk and Permian Magnesian Limestone imported from England. The average 87Sr/86Sr ratio of both products is 0.7079, while the strontium concentration varies from ~600 to 1055 mg/kg (table S3). These values are typical for northwest European chalk (25). Arable fields in the Vallerbæk catchment area are typically limed at a rate of 500 to 800 kg/ha per year of agricultural lime. The chalk is dissolved at a rate corresponding to the yearly supply. This means that about 720 g/ha per year of strontium with an 87Sr/86Sr ratio of 0.7079 is added to the farmland along Vallerbæk on the basis of agricultural lime alone. Estimating the contributions from fertilizers and animal feed is much more complex, as several types of fertilizers and animal feed are used, and these have highly different strontium contents. In table S3, the 87Sr/86Sr ratios and strontium concentrations are listed for the various fertilizers and animal feeds used in the Vallerbæk area. On the basis of these values, and of the amount used of each type of fertilizer and animal feed, a total strontium contribution from fertilizers and animal feed of about 128 g/ha per year has been calculated. The application of pesticides in Denmark is very low (~1 kg/ha per year) compared to the supply of agricultural lime (~500 to 800 kg/ha per year). As the strontium concentration in the most common pesticides is very low [~10 parts per million (ppm)] compared to that of agricultural lime, it is apparent that pesticides are of no importance for the strontium budget in Danish soils. Together, these numbers suggest that about 85 to 90% of the agricultural strontium added to the Vallerbæk catchment area every year come from agricultural lime, up to ~10% come from fertilizers, and up to ~10% come from manure (see calculations in table S4). About 20% of the strontium in the manure comes from nonlocal animal feed. These figures may change from year to year depending on the origin of the nonlocal food and on the components in the fertilizers. Because of the overwhelming importance of agricultural lime and for ease of terminology, all agricultural strontium sources are henceforth referred to as “agricultural lime.” Whereas lime has a significant influence on the 87Sr/86Sr ratios and strontium concentrations in surface waters from the low to noncalcareous deposits of West and Central Jutland, it has apparently little or no effect in East Jutland (Fig. 2, A to C). The reason for this difference is the high natural content of lime in the glacial deposits of East Jutland (18, 19), where percentages of calcium carbonate between 5 and 22 are normal. A hypothetical soil, 20 cm thick containing 10% CaCO 3 , would naturally hold about 320 metric tons/ha of calcium carbonate. The addition of 500 kg/ha of agricultural lime would increase this amount with about 0.16%, which would have minimal influence on both the strontium concentration and isotopic composition. Simple modeling detailing the relationship between the effect of agricultural liming and soil thickness, Sr content, and original Sr isotopic composition is presented in fig. S3. The data and data analysis presented above show that agricultural activity can have a strong impact on the 87Sr/86Sr ratio and strontium concentration in surface water in areas with low or noncalcareous soils. In Jutland, agricultural lime is by far the most important source of anthropogenic strontium. Fertilizers, animal feed, and other sources make up less than 15%. While the effect of agricultural lime seems not to have been investigated previously, the influence of fertilizers has been studied several times. Frei and Frei (15), in their investigation of strontium isotopes in Danish surface waters, found that the effect of fertilizers was negligible, but they used a study site with high calcium carbonate content in the soil. Other studies have found that fertilizers do have an effect (e.g., 10, 22, 26–31). However, environmental complexity combined with varied human activities has in most of these studies prevented a quantitative assessment of the contribution from fertilizers.