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Isotopic Baselines in the North Atlantic Region
T. Douglas Price, Karin Margarita Frei, and Elise Naumann

Journal of the North Atlantic, Special Volume 7 (2014): 103–136

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Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 103 Bioavailable Isotope Ratios Expected values for oxygen, strontium, and lead isotope ratios can be predicted based on the sources and location of rainfall (oxygen) or from the known geological units present in an area (strontium, lead). However there are often problems of fit between expected and observed. Geological isotope ratios of strontium and lead are not always a reliable indicator of locally available (bioavailable) values (Montgomery 2010, Price et al. 2002, Sillen et al. 1998). Differential erosion and solubility of the minerals in whole rock can contribute to significant differences between the geology and the bioavailable value. Diet also can have a substantial impact on the isotopic value of tooth enamel. Other anthropogenic or atmospheric factors such as dust or sea spray may alter the expected value. Oxygen isotope ratios, in addition to being subject to seasonal, annual, and long-term changes, exhibit substantial variation that is poorly understood. Oxygen isotopes fractionate along the path from rainfall to tooth enamel and exhibit values that range over several per mil in a local population. There are several issues that require further discussion with regard to the isotopic proveniencing of human remains. These include establishing isotopic levels in the local area, diet, identifying non-local individuals, diagenesis, and multiple isotope studies. Much of the discussion herein will focus on strontium isotopes since this system provides the foundation for human proveniencing in this study. Establishing Local Isotopic Levels In order to determine if “non-local” isotopic values are present in an area, it is essential to know what the local values are. This simple and obvious requirement is more easily stated than accomplished and is unfortunately not fulfilled in a number of published studies. Expected values for oxygen, strontium, and lead isotope ratios can usually be predicted based on the sources and location of rainfall or the known geological units present, respectively. However, these theoretical expectations are often not met in reality for a variety of reasons. The direct use of strontium isotope ratios from bedrock geology, for example, is confounded by several factors. Isotopic ratios in the local environment are composed of a mixture of strontium derived from both atmospheric sources and mineral weathering (e.g., Miller et al. 1993, Montgomery 2010). Biologically available strontium isotope ratios can differ substantially between bedrock and other environmental values. As Sillen et al. (1998: 2466) noted, “The large difference in strontium isotope composition between plant and available Sr on the one hand, and whole soil Sr on the other, suggest that potential applications of 87Sr/86Sr Isotopic Baselines in the North Atlantic Region T. Douglas Price1,*, Karin Margarita Frei2, and Elise Nauman3 Abstract - The isotopic proveniencing of human remains, using ratios of strontium, oxygen, and/or lead isotopes, has been employed in archaeology for more than two decades. The basic principles are essentially the same for the different elements and involve comparison of isotope ratios in human tooth enamel with local levels from the place of burial. Because isotopic ratios vary geographically, values in human teeth (place of birth) that differ from those of the local ratio (place of death) indicate movement and identify non-local individuals. However, there is often no easy answer to the question of where an individual came from because very distant and different places can have the same or similar isotopic ratios. To interpret the results, baseline values for isotopic ratios must be available from the place of discovery and also from potential places of origin. Estimates of isotopic ratios can be made for possible places of origin, either locations or regions, and bioavailable data can be collected to compare with human tooth enamel. Isotopic proveniencing cannot provide “proof” of a place of origin, only the possibility. This paper focuses on geographic variation in strontium and oxygen isotopes, specifically in terms of the bioavailable ratios present in the different parts of the North Atlantic study area. We provide a detailed overview of bioavailable isotope ratios. The discussion then moves to specific isotopic systems. The summary of strontium bioavailability is detailed from region to region, considering first the bedrock and surficial geology followed by an evaluation of bioavailable isotope ratios. We have also measured oxygen isotopes in human tooth enamel across the North Atlantic for comparison. Baseline oxygen isotope ratios are considered in a more general fashion in this paper because these vary at lower resolution across western Europe and a broader view is useful for understanding their distribution. We conclude with a synthesis of bioavailable isotope data for the North Atlantic. Viking Settlers of the North Atlantic: An Isotopic Approach Journal of the North Atlantic 1Laboratory for Archaeological Chemistry, University of Wisconsin-Madison, Madison, WI, USA. 2National Museum, Copenhagen, Denmark. 3Universitetet i Oslo, IAKH, Postboks 1019 Blindern, 0315 Oslo, Norway. *Corresponding author - tdprice@wisc.edu. 2015 Special Volume 7:103–136 Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 104 relationships should use biologically available strontium as a starting point, rather than substrate geology per se.” Thus, it is necessary to measure the baseline bioavailable levels of these isotopes in a study area (Price et al. 2002). The local isotopic signal can be measured in several ways: in human bone from the individuals whose teeth are analyzed, from other human bones at the site, from archaeological fauna at the site, or from modern flora or fauna in the vicinity. Several studies measuring bioavailable isotope ratios have been published (Evans et al., 2009, Maurer et al. 2012, Price et al. 2002). Various materials have been sampled including modern vegetation, soil, sediment leachates, and water. While there is no perfect proxy for bioavailable isotope ratios, we have for the most part used archaeological fauna in this study. Because bone is subject to contamination and diagenesis and the affect of diagenesis on the isotopic signal is uncertain, we generally do not use human bone from burials. We prefer to measure archaeological fauna, when available, to establish the bioavailable levels of strontium isotopes in the local environment (Price et al. 2002). Small wild mammals are a good choice because they have small home ranges, are unlikely to have been imported, and incorporate local strontium into their bone or teeth over months or a few years in most cases. Diagenesis of the bones of archaeological fauna is irrelevant for our purposes since the contaminant material is composed of local strontium. Samples are difficult to obtain, and measurements are expensive. As a consequence, baseline studies have rarely been done at an adequate level of intensity (cf., Knipper 2011). However, knowledge of baseline variation is the best way to distinguish local and non-local individuals. Determination of the local range of bioavailable isotope sources provides a means to determine where to draw the line to separate local from non-local individuals and is thus a very important step in isotopic proveniencing of human remains. Non-geological sources of strontium There are several atmospheric sources than can alter the normal geological 87Sr/86Sr values in an area: sea spray, rainwater, and atmospheric dust. In addition, the use of fertilizers on agricultural lands has the potential to change strontium isotope ratios, and their potential impact will be considered. It is also critical to remember diet and the sources of food consumed in the past when evaluating the local bioavailable isotope ratios. Thus, we discuss as well in this section the role of diet in determining isotope ratios. Sea spray and rainwater. Sea salt, produced by the oceans, makes a large contribution to atmospheric particles. There are numerous studies (e.g., Chadwick et al. 2001, Vitousek et al. 1999, Whipkey et al. 2000) that have reported substantial fractions of Sr in plants and soils in coastal areas from marine sources (through rainfall or sea-spray). Gosz and Moore (1989) analyzed precipitation from coastal and inland regions in the western US and found that precipitation nearer the ocean generally had a 87Sr/86Sr values similar to that of seawater. Seawater contributed 90% of the Sr to snowfall near the coast but only 10–30% some 300 km inland. Average residence time in the atmosphere for sea salt is 3 days (Berner and Berner 1987). Sea spray contains substantial amounts of sodium as well as calcium, strontium, and other elements (Junge 1972). The strontium isotope ratio of sea spray will be the same as seawater, 0.7092, and we expect that plants and animals taking in this strontium should exhibit values somewhere intermediate between the local terrestrial ratio and the sea. Measurement of marine aerosols in southern Sweden showed transport across the entire region, a distance of some 300 km, with a decrease in concentration downwind (Franzén 1990). A study in the Outer Hebrides (Montgomery and Evans 2006, Montgomery et al. 2003) found that although the island was underlain by radiogenic granites and gneisses (87Sr/86Sr of ~0.715), seawater dominates the bioavailable isotope ratio, which is less than 0.7105. Frei et al. (2009) report similar sea spray effects in sheep from the Shetland and Faroe Islands. Given the proximity to the sea of animal pastures, it is very likely that the soluble portion of Sr in the soils (and consequently the Sr in the respective wool samples) is dominated by sea-spray and rainwater derived from evaporated seawater. We have observed this effect in our own studies as well. A 87Sr/86Sr value of 0.703 has been reported for geological samples from Iceland’s basalts (Moorbath and Walker 1965, Sun and Jahn 1975, Wood et al. 1979). Yet Price and Gestsdottir (2006) measured higer ratios for modern fauna (0.705–0.706) and human enamel (0.706– 0.715). The values in modern fauna and Viking teeth are higher than the geological values for Iceland likely primarily due to sea spray. Other researchers have also documented the concentrations and distribution of sea spray in Iceland (Kettle and Turner 2007, Lovett 1978, Prospero et al. 1995). Arnórsson and Andrésdóttir (1995) reported that the high levels of chlorine and boron in the Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 105 natural waters of Iceland are mostly due to marine aerosols and sea spray. Rainwater in coastal areas is also a likely factor in introducing non-local 87Sr/86Sr values. Åberg (1995) reported a value of 0.706 from a farm reindeer on Iceland, where grass growing on volcanic soil had a 87Sr/86Sr ratio between 0.703 and 0.704. He suggested that rainwater, with a 87Sr/86Sr ratio of about 0.709, was responsible for the higher than expected value. Evans and Bullman (2009) found significant sea spray and rainwater effects in a study of migratory shorebirds in Iceland and Scotland. Raiber et al. (2009) demonstrated the clear effect of rainwater with a non-local 87Sr/86Sr value, changing groundwater ratios in southeastern Australia. In sum, we would expect that sea spray and rainwater in coastal areas will alter natural geological strontium isotope ratios in many parts of the North Atlantic and result in those values moving toward 0.7092. Nevertheless, there remain significant 87Sr/86Sr differences between geological regions that have utility for human provenience studies. Atmospheric dust. Atmospheric dust is another potential source of strontium in local sediments and the food chain. Varying effects have been reported in the literature. Aeolian sediments such as loess or volcanic ash are obvious and often substantial additions to the local surface geology. Chadwick et al. (2001) and Stewart et al. (2001) reported that wind-transported silt traveled 6000 km from China to Hawai’i and contributed substantially to the strontium in the soil. In the Sangre de Cristo Mountains of New Mexico, 50–75% of the strontium in local vegetation is derived from atmospheric deposition (Graustein and Armstrong 1983, Miller et al. 1993). Effects of atmospheric dust in Scandinavia and the North Atlantic are uncertain. Lupker et al. (2010) have reported atmospheric dust dating from 18th century in a Greenland ice core ranging from 0.709 to 0.720 87Sr/86Sr, and suggested that dust from both China and the Sahara had been deposited in the Greenland ice. If such dust were accumulating in sufficient amounts to change plant nutrients in the ice-free parts of Greenland, then we might expect to see some reduction in the local bioavailable values. Measured values, however, generally remain high, indicating that the amount of atmospheric dust deposited is likely quiet low. Fertilizer. In addition to sea spray and atmospheric dust, several authors have argued that some modern fertilizers also contain distinctively higher strontium isotope ratios and might shift differences between observed geological and bioavailable values. Böhlke and Horan (2000), for example, reported that soils with normal 87Sr/86Sr leachate values of ~0.708 are enriched by fertilizers with higher radiogenic ratios (~0.715) to 0.713–0.715 in coast areas of Maryland. Other studies, however, indicate that the effect of fertilizers is not a major factor. Vitoria et al. (2004) measured strontium isotope ratios in 27 different fertilizers from Spain and in all but a few cases found 87Sr/86Sr values below 0.709. Sattouf et al. (2007) measured in a series of phosphate fertilizers and report a wide range of values (0.703–0.709). Frei and Frei (2011) in a review of literature on the analysis of Danish surface waters and strontium concentrations in fertilizers concluded that fertilizer strontium contamination of Danish surface waters was minimal. In sum, the role of fertilizer in raising natural biogenic levels of 87Sr/86Sr does not appear significant in most areas of interest in this study . Diet. A certain proportion of the variability in isotope ratios used for human proveniencing is due to differences in diet, rather than place of birth. In fact, one of the early applications of strontium isotope ratios in archaeology (Sealy et al. 1991) focused on the use of 87Sr/86Sr as a dietary indicator in modern and archaeological bone. The application of 87Sr/86Sr in the study of modern populations, often suggested in forensic contexts (Beard and Johnson 2000), is probably not viable, at least in much of the developed world. Our foods today come long distances, often from other continents. For this reason, the isotope content of an individual’s foods today can be highly varied and produce an unreliable signal of place of origin. In the past, however, most foods would have been of local origin for the majority of the populations archaeologists study. The range of variation in isotopic ratios in tooth enamel for a particular location, however, will depend on the variation in the sources of isotopes among the places where food is obtained. In a homogeneous isotope environment, there should be relatively little variation in enamel isotope ratios among individuals. In a heterogeneous isotope environment, however, variation in enamel isotope ratios will be greater and will depend on the proportion of different isotope sources incorporated in the diet of the individual. It is useful at this point to return to the Iceland data for a simple example using strontium isotopes. It is essential to remember that there are 2 major sources of strontium in the human diet on Iceland: 1. Terrestrial (basalt 0.703 + sea spray 0.709 equals a baseline of 0.704–0.707), and 2. Marine (0.7092). Thus, the highest 87Sr/86Sr value available to the human population on Iceland is 0.7092, and any samples with values above that level must be nonJournal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 106 local. Human diet on Iceland during the Viking period was a mix of terrestrial and marine foods (e.g., Sveinbjörnsdóttir et al. 2010). Therefore, values in humans who lived in Iceland should range between 0.703 and 0.7092. Fifty-one of 83 individuals we tested had a measured ratio between 0.706 and 0.7092, indicating they spent their lives in Iceland. Thirty-two individuals we tested had ratios above 0.7092, and thus were apparently non-local, not born on Iceland. This example highlights the importance of knowing local bioavailable values and having some information on the composition of human diets in the past. In other parts of the world, there may be several different geological formations in close proximity to potential places of origin. In such cases, sampling of bioavailable sources should include these different terrains to insure that such differences are considered in the analysis. This information allows us to interpret the variation present in enamel ratios and to distinguish between local and non-local individuals in the sample set. Identifying Non-Local Individuals Some years ago, Price et al. (2002) suggested that the mean and standard deviation of strontium isotope ratios in a population be used to determine outliers to identify the non-local individuals in a set of samples. At the time, much of isotopic proveniencing was still experimental and there was no established procedure for distinguishing locals and non-locals. Since then, we have analyzed several thousand samples and often measured a large number of samples from single sites. Based on our experience with larger samples, we now would argue against using mean and standard deviation statistics in favor of common sense and the range of bioavailable values in an area (Price et al. 2010). The research in the North Atlantic was one of the primary reasons we have abandoned the statistical assignment of non-local individuals. Specifically, a series of samples from Iceland provide an important lesson regarding isotopic variability in human populations. Iceland is an exceptional place in many ways. It is one of the youngest landmasses on earth, spewed from the Mid-Atlantic Ridge as a large volcanic island over the last 20-25 million years. For this reason, the strontium isotope ratios on Iceland are very low and quite distinct from many other areas around the rim of the North Atlantic (Moorbath and Walker 1965, Sun and Jahn 1975, Wood et al. 2004). In order to determine the baseline bioavailable 87Sr/86Sr values on the island, we measured enamel from domestic animals, including modern sheep and archaeological cattle from different parts of Iceland. We were surprised to note that bioavailable strontium isotope ratios were significantly different from the basalt rock. Sheep tooth enamel from four locations around Iceland averaged 0.706, while two cows from northern Iceland provided values of 0.704. Sea-spray and rainfall are likely responsible for the higher than expected bioavailable value witnessed in modern fauna on Iceland. A number of researchers have documented the concentrations and distribution of sea spray in Iceland itself (e.g., Kettle and Turner 2007, Lovett 1978, Prospero et al. 1995). As noted above, rainwater will have the same 87Sr/86Sr value as the ocean from which it originates. The 87Sr/86Sr of sea spray and rainwater will be 0.7092, and plants and animals consuming this strontium will exhibit values somewhere intermediate between the basalt rock of Iceland and the sea. This is the pattern we observe in the fauna, flora, and humans on Iceland. Bioavailable Isotope Ratios in the North Atlantic In this context, we establish on a case-by-case basis the range of bioavailable strontium isotope values across the North Atlantic and discuss the criteria for each. A country by country review of the geology provides an indication of expected levels of bioavailable 87Sr/86Sr, while analysis of both modern and archaeological samples reveals some of the actual bioavailable ratios present. These data come from a variety of different materials and include both our own measurements and data published elsewhere. Oxygen isotope ratios in fresh water vary along several dimensions including latitude, elevation, distance from sources, amount of rainfall, and the nature of the freshwater source (e.g., well, stream, spring, reservoir). Oxygen isotope ratios also vary seasonally and over time with climatic change. Because of this variability and the absence of sufficient baseline information, modern oxygen isotope values are often used as background for archaeological studies. Oxygen isotopes will be reviewed herein for the North Atlantic region as a whole. Lead isotope background will be considered in a later publication. Strontium isotopes The geology of Scandinavia, Britain, and the other islands of the North Atlantic reflects a highly variable array of strontium isotope ratios. Generally speaking, some of the oldest rocks on the continent are to be found in Greenland, Norway, parts of Sweden, and the northern parts of Britain and Ireland. Also in general terms, with the exception of Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 107 Greenland, the oldest rocks and highest strontium isotope values are to be found in the more northerly parts of these areas. An important consideration in the isotope proveniencing of human remains is the geological variability in the study area. In the case of the North Atlantic, the geological context is almost ideal for such a study. Some of the biggest differences in reported strontium isotope ratios anywhere come from this region. We will focus on Greenland and Iceland and the potential homelands for these colonists. The most probable places of contact and origin for these settlers include the northern British Isles, Ireland, and the west coast of Norway. These areas in the North Atlantic generally have higher strontium isotope ratios than Iceland. Other areas of Norse homeland such as Denmark, Sweden, and northern Germany are regarded as less likely to have provided colonists to the uninhabited Atlantic islands, but may potentially have been homelands for some Viking settlers in the British Isles. We will not overlook this possibility, and will consider these areas as well in our isotopic analysis of more than 500 samples of Norse burials and archaeological fauna from settlements. The majority of our samples come from Scandinavia, Iceland, and Greenland. We also incorporate published data from the British Isles along with a few small studies we have been involved with in that region. In our discussion of strontium isotope variation below, we move from east to west, from the Norwegian Coast to the British Isles to Greenland. We give more attention to those areas where there is less information. There is a good bit of data available for bioavailable strontium isotopes in Denmark (Frei and Frei 2011, Frei and Price 2012) and England (e.g., Evans et al. 2010, 2012). Mention should perhaps be made here of a pan-European study of the distribution of 87Sr/86Sr for the commercial analysis of the origins of various foodstuffs (Vorkelius et al. 2010). This last study, while frequently cited, is based on limited sample points in many areas and often fails to capture the diversity or range of bioavailable 87Sr/86Sr values in a particular area. Denmark. Denmark is characterized by a relatively young (geologically) and homogenous “basement” geology. About 50% of the country is constructed of Late Cretaceous–Early Tertiary carbonate platforms, and the other 50% of marine clastic sediments, all covered by more or less thick sequences of diverse glaciogenic sediments deposited during the two last Ice Ages. The Quaternary glaciogenic sediments are composed, among other things, of various weathered Precambrian granitoids (gneiss and granite) from Norway and Sweden. Almost everywhere in Denmark, the glacial deposits are the source of strontium isotopes for plants, animals, and people. There is very little bedrock exposure anywhere in the country. Frei and Price (2012) presented strontium isotope ratios from samples of modern mice, snails, and archaeological fauna (Fig. 1). We compared these ratios with strontium isotope median values from human enamel samples from archaeological sites within Denmark. The fauna samples range from 87Sr/86Sr = 0.70717 to 0.71185 with an average of 0.70918. The humans (including non-locals) samples range from 87Sr/86Sr = 0.7086 to 0.7110, with an average of 0.7098. Frei and Frei (2011) measured 87Sr/86Sr in almost 200 samples of Danish surface water and found similar results. In both these data sets, we observed a small difference between the baseline values in the western (Jutland) and eastern (Funen, Zealand, and the southern islands) parts of Denmark. Therefore, we have proposed 2 slightly different baseline ranges for the bioavailable strontium isotopic values within Denmark. The western area has a 87Sr/86Sr range = 0.7079–0.7099, whereas the eastern portion of the country has a 87Sr/86Sr range = 0.7089–0.7108. Because of the overlap in values, however, it is not really possible to distinguish individuals from these 2 areas isotopically. Sweden. With the exception of the southwest corner of the country, Sweden’s geology is rather complex but generally can be divided into 3 main components: Precambrian crystalline rocks (which are part of the Baltic or Fennoscandian Shield, and include the oldest rocks found on the European continent), the remains of a younger sedimentary rock cover, and the Caledonides formation (Fredén 1994). The bedrock is covered in places by glacial moraine, but often is exposed intermittently to frequently on the surface, especially in the northern half of the country. The oldest rocks in Sweden are Archean (>2500 million years old), but they only occur to a limited extent in the northernmost part of Sweden. Most of the northern and central parts of Sweden consist of Precambrian rocks belonging to the Fennoscadian Shield, an ancient craton of mantle rock with generally high strontium isotope ratios. The Swedish Geological Service (SGU) has measured 87Sr/86Sr across the country and reports very high rock values from much of this region, generally greater than 0.722 (Sjogren et al. 2009). Further to the south, Phanerozoic sedimentary rocks rest upon the Precambrian shield area. They are less than 545 million years old and cover large parts of Skåne, the islands of Öland and Gotland, the Östgöta and Närke plains, Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 108 numbers per site are too small to provide much more information. Specifically, values generally range from 0.711 to 0.714 and probably reflect the local range in Bohuslän. A few higher values in the sample may well reflect non-local individuals buried in this region. We also have some additional data from the southern and eastern parts of Sweden as well, and, as discussed below, there are published data from northern Sweden and the Gulf of Bothnia region. These areas probably lie outside the homelands of the Viking settlers of the North Atlantic. Most of the Vikings in the Baltic region appear to have looked to the east in terms of expansion as large settlements appeared in the eastern Baltic and Russia (Boba 1967, Noonan 1991, Sawyer et al. 1982), where they have been viewed either as either founders of the state or peripheral pirates (Noonan 1991). A major set of 87Sr/86Sr data from the west coast includes 160 samples, 78 from fauna and 82 human (Sjögren et al. 2009). From the east coast and Gotland, we have ~40 samples, of which 8 are faunal (T.D. Price, unpubl. data). These data are summathe Västgöta mountains, the area around Lake Siljan in Dalarna, and areas along the Caledonian front in northern Sweden. The youngest rocks in Sweden are Tertiary rocks, formed about 55 million years ago. They occur in the most southerly and southwestern parts of Skåne. Quaternary deposits formed during and after the latest glaciation (when Sweden was completely covered by the inland ice sheet) partially covered the bedrock. The most common soil type in Sweden is till, covering about 75% of the landscape (SGU Soil Map of Sweden). Southernmost Sweden is a glaciated landscape much like the neighboring areas of Denmark, and expected strontium isotope ratios in this area should be similar as well. The west coast of Sweden was an area of known Viking settlement and a potential homeland for settlers of Iceland and Greenland. Studies along the west coast of Sweden provide some information on levels of 87Sr/86Sr in this region (Fig. 2). As part of a study of inland Neolithic sites in this area, Sjögren et al. (2009) measured a few samples of human enamel from sites in the coastal region. These samples exhibit substantial variation, although the sample Figure 1. Strontium isotope samples from fauna and human tooth enamel from Denmark and adjacent areas (Frei et al. 2012). Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 109 0.7174 ± 0.0078. A bar graph of the ranked 87Sr/86Sr values provides a look at the data (Fig. 3). The last two values stand out as significantly higher than the others and very likely identify non-local individuals. In sum, it is clear that the older rocks of the Fennoscandian Shield dominate most of Sweden and have high 87Sr/86Sr, above 0.711 with some higher than 0.735. These rocks are surficial and contribute to soil nutrients and hence to bioavailable strontium isotopes. Lower values around 0710–0.711 were found only in the southernmost part of the country in the province of Scania, on the island of Gotland in the Baltic, and in a few limited areas along the coasts. Norway. The Fennoscandian Shield covers Norway, most of Sweden, large parts of Finland, and the northwestern part of Russia. In Norway, there are some outcrops of Archaean rocks (very old—up to 3.5 billion years of age) exposed between younger metamorphic belts (Fig. 4). A special feature that characterizes Norway’s geology is the intense metamorphism/reworking that heavily altered the rocks there during the Caledonian orogeny, Today the region is composed primarily of crystallines and metamorphites. There are 3 major geological provinces in Norway. In the Oslo area and to the south down into Sweden lies the Southwestern Gneiss Province of 1700- to 900-Ma-old rocks. This southwestern gneiss province is divided in 2 parts by the Caledonides Province, formed during an ancient mountain-building episode in the Mesozoic, ca. 400 Ma ago. In the south and southwest where the Norwegian peninsula is wider, the southwestern gneiss is rather broad, but to the north only the coastal islands are composed of this gneiss. The Caledonides form the backbone of the entire country, stretching from southern Norway to the Arctic Circle, characterized by a rugged topography and peaks up to 2500 m in elevation. The Caledonides are made up of metasedimentary and metavolcanic rocks dating from 700–400 Ma ago. The Oslo Rift province runs through the Oslo region and the fjord, composed of younger rized in Figure 2. Two additional sites on this map come from Frei et al. (2009) and were measured on sheep wool from Dannås and Boserup. The Dannås sheep (0.716) was grazing on pastures with soils developed on very old Precambrian basement gneisses typical of Swedish bedrock, while the Boserup sheep (0.711) fed on soils developed in sedimentary rocks from a Late Triassic–Early Jurassic flooding event. We have 10 or more samples from several sites in eastern Sweden and the pattern of 87Sr/86Sr is similar at each (T.D. Price, unpubl. data). There is a high proportion of what appear to be local values showing a continuous range and then a few significantly higher values that very likely represent individuals from inland areas or much older terrains. The site of Birka near modern Stockholm was an important Viking center and the gateway to the east [(T.D. Price, unpubl. data). Much of the trade from Russia and the Arab world passed through Birka. We have sampled 10 individuals from the cemetery at Birka. These values range from 0.7103 to 0.7335, with a mean of Figure 2. Averaged strontium isotope ratios from human and archaeological fauna (blue) from southern and central Sweden. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 110 magmatic rocks of Permian age, 300–250 Ma ago. Neumann et al. (1988) have measured strontium isotope ratios in these old basaltic to granitic rocks. The volcanics, as expected, exhibit 87Sr/86Sr values in the range of 0.704–0.706, whereas the granitic rocks have very high values, ranging from 0.715 to 0.760 along with several higher values of 0.80, 0.90, and even 1.0. The valley floors and lower-lying areas of this largely mountainous region are buried under moraine and other deposits of glacial origin, often with lower strontium isotope values. The old age of some of these rocks suggests that 87Sr/86Sr values in Norway should be high. Values measured on granites and gneiss in southern Norway range from 0.7087 to 0.7185 and even 0.7519 and higher (Wilson et al. 1977). While bare rock with only thin ground cover is present in much of Norway, there are Quaternary deposits in local areas that provide sediments for soil development and some cultivation. Ground moraine and meltwater deposits are more common in the northeast and southeast of the country. Marine deposits occur in the region of the Oslo fjord and around the coastal zones of Bergen and Trondheim. Peat bogs are scattered across the landscape in lowlying areas. There are 2 published reports of strontium isotope ratios in biological materials from Norway. Frei et al. (2009) report a very low value of 0.7051 from sheep wool from Hemsedal in central Norway. The sheep is reported to have pastured on fields overlying predominantly Proterozoic mafic metamorphic rocks, which may explain such low values. Åberg et al. (1998) reported values ranging from 0.7077 to 0.7323 for 4 samples of Medieval human tooth enamel from different localities in southern Norway. We have measured a variety of archaeological and modern fauna and human remains from Norway to learn more about the bioavailable levels of strontium. Figure 5 is an outline map of Norway with a summary of our results. Values from sites with more than 1 sample are averaged unless they were highly divergent. Coverage is generally good along the west coast of Norway, but inadequate on the south coast. A number of samples are also available from the Oslo region and from some of the settled valleys in the interior. We measured 211 samples for strontium isotopes from Norway. This total includes 155 humans and 56 floral and faunal samples. The human data are discussed in Price and Nauman (in press, [this volume]). The floral and faunal remains include 1 horse, 1 cow, 5 plants, 2 snails, 15 beaver, 16 pigs, and 16 unknown species. The range of values for these samples is 0.7073–0.7254, with a mean of 0.7131 ± 0.0043. Several patterns emerge from the distribution of values. It is clear that a number of the lower values in Norway are found in the coastal areas where marine diets may have been prevalent and sea-spray effects were likely most pronounced. These lower values are particularly noticeable in the Oslo fjord area where the glacial moraine and marine sediments appear to contribute to values around 0.709–0.710. Bedrock 87Sr/86Sr in this area is also lower because of the old igneous province here. Inland areas in general show much higher average values. At the same time, on the west coast of Norway, at least, virtually all of the population lived along the sea. The general pattern of lower values in coastal areas is strong. However, there are also some higher values along the coast, and in several Figure 3. Bar graph of ranked 87Sr/86Sr values from human tooth enamel from Birka, Sweden. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 111 substantial amount of food from the sea. The beaver 87Sr/86Sr data are generally higher than other fauna, with 2 exceptions (probably from areas of marine deposits) found in eastern Norway. We can assume that the beaver reflect the local bioavailable 87Sr/86Sr for a specific location. Another sample of archaeological fauna comes from pig bones from medieval Bryggen (Bergen). These pigs were analyzed to obtain more bioavailable information for the Bryggen area. The mean and cases in close proximity to lower values. The question regarding the human data is whether the variation reflects local values, movement, or diet. The beavers should provide a good indication of local bioavailable values without the influence of long-distance movement or marine diets since beavers tend not to range far and primarily eat trees. The human remains can be non-representative if they are non-local individuals, i.e., moved to the place of burial from elsewhere, or if their diet included a Figure 4. Geological map of Norwegian bedrock (Geological Survey of Norway, NGU). Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 112 pigs ate fish or other marine foods, their terrestrial 87Sr/86Sr might well be dampened by the 0.7092 marine ratio. In sum, values above 0.709 are typical of Norway, and higher values are not unusual. At the same time, however, there are—as noted above—places, such as Hemsedal, with mafic bedrocks with modern fauna values as low as 0.7051 (Frei et al. 2009). The range of strontium isotope values within Norway standard deviation for the 16 pig bones was 0.7111 ± 0.002, with a range from 0.7079 to 0.7157. A graph of these values is shown in Figure 6. It is of course likely that some of the pigs had been transported to Bergen from elsewhere, which may account for the observed variation. The series of similar values around 0.710 may best represent the local terrestrial bioavailable signal around the Bergen area, but of course the diet of the pigs is also an issue. If the Figure 5. Strontium isotope ratios from faunal and floral sample s from Norway. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 113 is thus, unusually large. We will revisit the issue of distinguishing places of origin in Norway at the end of this section. Northern and Western Isles. The northern and western islands of Britain—the Orkneys, Shetlands, and Hebrides—have varying but generally older rocks and higher 87Sr/86Sr values. The following discussion includes both the geological and bioavailable sources of strontium isotopes. The Orkneys, a group of some 70 small islands just 16 km off the northeast coast of Scotland, are composed primarily of Palaeozoic (Devonian) sandstones and flagstones (Fig. 7). Evans et al. (2010) predicted bioavailable values of ~0.712–0.713 for these sandstones in Britain. We have also measured samples from the Orkneys. Four archaeological faunal teeth produced values between 0.7104 and 0.7106. Four samples of modern fauna and flora from the mainland and Rondalsay had values between 0.7094 and 07108. A single sample of modern barley, said to be from Orkney, had a high value of 0.7122. Montgomery et al. 2014 (this volume) reported an analysis of archaeological humans, fauna, and modern plants for the site of Westness on the Orkneys. Measurement of modern plant and grain samples produced values between ~0.709 and 0.710. A number of human burials from Orkney also document a range of 0.709–0.710 as the baseline value for these islands (Montgomery et al. 2014 [this volume], Toolis et al. 2008). As Montgomery (2010) points out, a combination of high amounts of rainfall, sea spray, fertilization practices using seaweed, and agricultural fields in marine sands along the coast can introduce sufficient strontium of marine origin into soils and plants to significantly dampen bioavailable and human ratios towards seawater strontium ratio. The Shetland Islands lie about 97 km north of the Orkneys and 360 km west of Bergen, Norway. The earliest evidence of settlement dates to the Mesolithic period, ca. 4300 BC (Noble et al. 2008). The Vikings arrived on Shetland during the late 8th and 9th centuries, and the island soon became a base for raids on England and Scotland. The Shetlands belonged to Norwegian and Danish kings until ca. AD 1470 (Barrett 2008). The geology of Shetland is complex with numerous faults and fold axes throughout a large and diverse range of bedrock—e.g., metasedimentary, metavolcanic, and metagranitoid of various ages. These islands are the northern outpost of the Caledonian orogeny and contain outcrops of Lewisian, Dalriadan, and Moine metamorphic rocks with similar histories to their equivalents on the Scottish mainland (Gillen 2003). Similarly as well, there are Old Red Sandstone deposits and granite intrusions. Glaciations entirely covered the islands and left deposits of moraine and outwash. These rocks types should have generally high strontium isotope ratios concomitant with their age and composition. For Figure 6. Bar graph of ranked 87Sr/86Sr values for the Bergen pigs. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 114 example, Jones et al. (2007) report basement rocks on the Shetlands at 0.735 and sources of steatite ranging from 0.710 to 0.722. Evans et al. (2012) reported 4 measurements of plants and soil extracts from coastal areas of Machair vegetation on the Shetlands. These values average 0.70937 ± 0.0001, reflecting the effects of sea spray and rainfall on local geological baseline values. Much of the human occupation appears to have focused on these same coastal areas and similar values should be expected for human tooth enamel. Frei et al. (2009) have reported 87Sr/86Sr from 5 samples of sheep wool and 4 samples of soil from several localities on the Shetlands ranging from 0.7095 to 0.7118; those soil leachates and wool samples show similar values. Some of these values may be biased by the location of sample sites near the coast and the reported use of seaweed for fodder for these animals. Figure 7. The geology of the Orkney Islands. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 115 Scotland. The geology of mainland Scotland is highly varied with 3 main sub-divisions: the Highlands and Islands are a diverse region to the north and west, the Central Lowlands comprise a rift valley primarily of Paleozoic formations, and the Southern Uplands are largely composed of Silurian deposits. The bedrock includes ancient Archean gneiss, metamorphic beds interspersed with granite intrusions created during the Caledonian mountainbuilding period, and the remains of substantial tertiary volcanoes. There is very little information on bioavailable strontium isotope ratios from this area. Pankhurst (1969) reported values for the Caledonian Basin Igneous Province in northeastern Scotland. Measurements of 87Sr/86Sr on whole rock varied considerably across this area, with the younger gabbro-syenites ranging from 0.703 to 0.712 and the older gneisses and corderites ranging from 0.719 to 0.743. The older rocks are predominant around the coastal region where Viking settlement might be expected. The densest settlements of Vikings occurred around Caithness, with Strathoykel as the southern frontier. Recent archaeological investigations are turning up numerous finds around Caithness, and at the site at the Udal in North Uist (Crawford 1996, Crawford and Switsur 1977). The majority of settlement in Scotland was in the Western Isles and the West Highland seaboard (Ritchie 1993). On mainland Scotland, Viking settlement tended to be mainly in narrow coastal areas of the southwest, the west, and the extreme north (Fig. 8). Degryse et al. (2010) reported rock and plant values associated with various rock types in Scotland (Table 1). Rock values exhibit a huge range of variation, between 0.7028 and 0.7288, with an average of 0.7106 ± 0.0079. Plants growing on these rocks range only from 0.7081 to 0.7129 with an average of 0.7097 ± 0.0011. No relationship between plant and rock values was found, likely due in part to the fact The introduction of marine strontium via rainfall, sea spray, and diet should also have the effect of moving values in human tooth enamel toward the 87Sr/86Sr value of 0.7092 for seawater. The Outer Hebrides, among the Western Isles of Scotland, were home to a number of Viking settlements. This area contains some of the oldest and youngest rocks in Scotland, from ancient Lewisian gneisses to 60-Ma volcanics. The Precambrian Lewisian gneisses represent the oldest rocks in Britain and date back to around 2.6 billion years ago. The ancient Lewisian gneisses also encompass metamorphic rocks such as quartzites, marbles, graphitic schists, and amphibolites, which are thought to have originally been sedimentary and volcanic rocks. These formations make up most of the Outer Hebrides. Montgomery et al. (2003) and Montgomery and Evans (2006) measured values averaging 0.709 from a Norse graveyard on the Isle of Lewis in the Hebrides. They concluded that one male, with an enamel value around 0.707, had probably migrated to Lewis from within the North Atlantic Tertiary Volcanic Province (e.g., the small Scottish islands of Skye, Mull, Canna, Eigg). Moorbath and Walker (1965) report whole rock 87Sr/86Sr values of 0.705–0.706 for the basic volcanic rocks of Skye. Pankhurst (1969) on the other hand, measured a dolerite sample from Skye with values ranging 0.7045–0.7053. The island of Skye in the Hebrides has been the focus of a detailed study of bioavailable 87Sr/86Sr (Evans et al. 2009). The geology of the island is unusual, dominated by the remains of a volcanic core that was active 70 Ma ago. The central mountains on the island are composed of the granitic Red Hill and the gabbroic Black Cuillin, the remains of unerupted magmas and magma chamber contents. The northern part of Skye is covered by lavas on top of older Jurassic deposits, which are exposed along the northeast coast of the island. In the south of Skye, the basement rocks on which the volcanoes formed are exposed include parts of the Lewisian Complex, an ancient gneissic formation from 2800 Ma ago, as well as sedimentary sequences from 1000 Ma ago and outcrops of Cambrian Limestone. Evans et al. (2009) measured some 44 samples of modern plants, water, snails, and bone from different parts of the island. The plant samples were most numerous and showed a wide range of values, 0.7050–0.7200, averaging 0.7108. The snails and bone showed a much smaller range of values, 0.7089–0.7101, with an average value of 0.7085 for the animal bone and 0.7094 for the snails. Evans et al. (2009) used this information and the distribution of values to construct a bioavailable strontium isotope map for the Isle of Skye. Table 1. Analytical results for strontium ratios of plants and rock in Scotland (Degryse et al. 2010). Plant Bedrock Rock 87Sr/86Sr Plant 87Sr/86Sr Bracken Granofelsic schist 0.7288 0.7103 Bracken Basaltic lava 0.7039 0.7081 Bracken Granodiorite 0.7084 0.7095 Bracken Granofelsic schist 0.7208 0.7129 Bracken Diorite 0.7052 0.7090 Bracken Metalimestone 0.7079 0.7090 Bracken Gabbro 0.7036 0.7091 Bracken Basaltic lava 0.7072 0.7092 Bracken Craignurite basalt 0.7061 0.7097 Bracken Basaltic lava 0.7095 0.7094 Heather Conglomerate 0.7152 0.7095 Average 0.7106 0.7097 St. dev. 0.0079 0.0011 Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 116 older, harder rocks with a generally higher relief in the northwest. Pleistocene glaciers left major changes on the landscape with extensive deposits of moraine, till, and other features associated with continental ice sheets. In Great Britain, soil leach values suggest labile 87Sr/86Sr variations among soils overlying sedimentary rocks from about 0.7073 on Cretaceous chalk to 0.7115 on Triassic sandstone (Budd et al. 2000). Soils formed on igneous and metamorphic rocks as well as rubidium-rich clay soils are likely to have far higher ratios. Scotland, the Northern Isles, and parts of County Antrim have varying but generally higher 87Sr/86Sr values (Evans et al. 2010). Evans et al. (2010) provided bioavailable strontium isotope data from modern plants across Britain, which show a general trend toward values ranging 0.707–0.712 in the south and east, 0.711–0.713 in the west, and 0.712–0.720 in the north. The result of their study is a first-approximation strontium isotope isobar map of Britain (Fig. 9). Montgomery et al. (2009) report a wide range of 87Sr/86Sr for mineralwater samples from across the UK, ranging between 0.7059 from Carboniferous volcanic rock sources and 0.7207 from Precambrian metamorphic rock in eastern Scotland. The waters from older rocks exhibit a more radiogenic signature than those from younger rocks. These studies provide a reasonably good introduction to strontium isotope ratios in Britain and Ireland, and for that reason, only a brief summary of UK strontium isotope ratios is presented here. A number of studies of archaeological human and faunal remains have been published over the last decade or so, providing additional useful information on variation in 87Sr/86Sr values in England (e.g., Buckberry et al 2014; Budd et al. 2000; Eckardt et al. 2009; Evans et al. 2006; Jay et al. 2013; Kendall et al. 2013; Montgomery et al. 2003, 2006, 2007a, 2007b, 2009, 2010). Budd et al. (2000) reported human enamel values from Anglo-Saxon England ranging from 0.708 to 0.712. Leach et al. (2009) determined values from 50 samples from a Roman cemetery at York. The distribution of values clearly shows a number of non-local individuals, while local individuals from York would appear to average approximately 0.7098 ± 0.0003. The large number of human proveniencing studies in Britain that the plants analyzed in that report were epiphytes and so obtained most of their nutrients and water from the air. Nevertheless, the information provides some indication of both geological and bioavailable 87Sr/86Sr values in a small part of western Scotland. The study also provides an object lesson in the importance of measuring bioavailable strontium isotope levels. England. Viking settlers in the British Isles came primarily from Norway and Denmark and settled in the northern half of England, the northern coasts of Scotland, the Isle of Man, and a variety of locations throughout Ireland (Fig. 8; Ritchie 1993). Viking settlement in England was concentrated in the regions of the East of England, the East Midlands, Yorkshire and the Umber, and the Northeast and Northwest. In Scotland, the Norse settled along the west and northern coasts and on the Northern and Western Isles. In Ireland, the Vikings have been found in Northern Ireland and around the modern cities of Dublin, Waterford, Wexford, Cork, and Limerick. The geology of England is mainly sedimentary. The age of the rocks is youngest in the southeast around London and becomes older in a northwesterly direction. In general terms, there are younger, softer, and lower-lying rocks in the southeast and Figure 8. The distribution of Viking settlement in the UK and Ireland. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 117 Figure 9. A first approximation strontium isotope map of Britain (Evans et al. 2010). Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 118 provides substantial information on isotopic variation in the UK. Isle of Man. The Isle of Man in the Irish Sea is 52 km north to south and 22 km at its widest point east to west. The highest point on the island is Snaefell, with a height of 620 m. Hills in the north and south are separated by a central valley. The complex bedrock geology of the island is made up of a variety of formations from a range of geological periods. A greatly condensed description of the geology of the island is presented in the following paragraphs, followed by a discussion of its strontium isotope landscape. The major bedrock groups are indicated in Figure 10 and described below. The primary literature for this summary of the geology comes from Ford et al. (2001), Kendall (1894), Lamplugh (1903), and Lewis (1894). The majority of the Isle of Man is made up of highly faulted, folded, and slightly metamorphosed sedimentary rocks collectively known as the Manx Group, composed largely of dark grey slates. This predominant feature comprises the island’s central ridge of slate and greywacke, deposited as sediments on the ocean floor during the Ordovician ~490–470 Ma ago. There is a belt of younger Silurian sandstones along the west coast (Dalby Group) and a small area of reddish Devonian sandstones to the north around Peel (Peel Sandstone). Limestones in the south of the island (Castletown Limestone) were formed in the Carboniferous period, some 330 Ma ago. The bedrock geology of the Isle of Man is largely visible only in natural cuts and sections and at higher elevations. Elsewhere glacial deposits from the Pleistocene cover the landscape. In addition, outwash materials left by meltwater from the glaciers and alluvial fans composed of sediment washed down from the mountains during summer thaws are common. The northern quarter of the island is composed of a deposit of glacial till (Deep Glacial Till), deeply burying underlying bedrock. Some of this material includes rock pushed by the ice from the mountains of Scotland and from the floor of the Irish Sea. In general, the glacial ice bulldozed the entire the island and generally homogenized its surface. The glacial deposits make up the agricultural soils in the cultivated areas of the island, whereas exposed bedrock appears at higher elevations. Marl and lime have been added to these soils in some areas to improve growing conditions, and seaweed is sometimes used as fertilizer in coastal areas. In sum, the geology of Manx is dominated by deposits of sedimentary rocks, largely from the Paleozoic period. A mantle of glacial till and outwash deposits covers most of the lower and mid-range elevations of the island. The boulder clay, sands, and clays of these deposits provide the nutrients for the plants, animals, and humans that have inhabited the island for thousands of years and exhibit a range of stron- Figure 10. Simplified geology of the Isle of Man and the location of archaeological tium isotope ratios. sites and baseline sample locations mentioned in the text. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 119 Some information is available on 87Sr/86Sr from whole-rock analyses. For example, Evans’ (1989) measurements on slates in Wales, similar to and contemporaneous with the Manx Group, yielded values ranging from 0.728 to 0.773, consistent with the high rubidium content of such rocks. Groundwater values, which should be closer to biological available Sr ratios, tend to be much lower. Shand et al. (2007, 2009) measured groundwater values in the same Welsh slates and found values that increased with depth. The highest water values (0.7115) were attributed to dissolution of high-strontium minerals whereas the lowest, surficial values (0.7093) matched that of current rainwater, which in turn was attributed to deposition from modern seawater. Montgomery et al. (2006) likewise measured 0.7139 in groundwater from an Ordovician slate aquifer in adjacent Cambria. Evans et al. (2010) mapped the bioavailable value for the Isle of Man as 0.711–0.712. Expected strontium isotope ratios for Carboniferous seawater, and the Castletown Limestone deposits, range from approximately 0.7075 to 0.7085 (Shields 2007, Vezier 1989). We have measured the bioavailable strontium isotope ratio at a number of locations on the island, using faunal remains and snail shells to measure the ratio and determine local baseline values. These data are presented in Table 2. The 16 87Sr/86Sr values show a relatively tight distribution with a mean of 0.7089, a standard deviation of ±0.0004, a minimum value of 0.7080, and a maximum value of 0.7095. These values help define the range of bioavailable strontium isotope ratios on the island. Empirically, we failed to find high values as reported by Evans et al. (2010), probably due to the fact that the Manx Group rocks are highly radiogenic because they have high Rb/Sr and thus relatively low Sr, in which case any marine influence will be disproportionately large. There may also be a contribution of lower 87Sr/86Sr values from carbonates in local limestone deposits. The impacts of sea spray and seafood, however, do not appear to have been significant in the case of the Isle of Man. Values for local terrestrial bioavailable strontium isotope ratios are substantially below the 0.7092 value of ocean water and many of the human enamel values are substantially higher than 0.7092. Ireland. The geology of Ireland is complex and consists, at the base, of the remains of ancient mountain ranges with heavily folded crystalline and metamorphic rocks (Holland 1981, Woodcock 2000). These rock formations are exposed as the hills and mountains of the north and the west of the island (Fig. 11). About 600 million years ago, Ireland lay under the ocean somewhere in the southern hemisphere. About 510 million years ago, the land began to form due to crustal movement, and the part of the earth’s crust that became Ireland migrated northwards towards its present location. The rocks, particularly in the north of the country, are of substantial age and likely to have quite high strontium isotope ratios. The island had a sizable landmass about 340 million years ago, but after tens of millions of years, much of the land was worn away and largely covered by the sea again (Woodcock 1994). Mud rich in the remains of sea-life was deposited on the floor of this sea and gradually formed carboniferous limestone, which covers a large section of Ireland today. With further crustal movement, the limestone cover was thrust upwards approximately 300 million years ago. Limestone deposits are found in limited areas, largely in the west and southwest of Ireland. These marine sediments will have radiogenic strontium isotope values closer to modern seawater, in which 87Sr/86Sr values are ~0.709 (Burke et al. 1982, McArthur et al. 2001, Veizer 1989). Jurassic clays were deposited on top of the Carboniferous limestone and lie beneath the subsequent Cretaceous limestone deposits and basalts that form the landscape today. The Cretaceous limestone has largely been eroded from most of Ireland, but survives in County Antrim because the area was covered by lava flows near the beginning of the Tertiary period (Mitchell 2004). The lavas known as the Antrim Basalts form a major part of northeast Ireland and most of County Antrim, visible from the River Bann east to the Antrim coast (Mitchell 2004). These Tertiary igneous volcanic rocks Table 2. Baseline samples for strontium and oxygen isotope ratios on the Isle of Man. Lab No Location Sample Material 87Sr/86Sr δ13C δ18O F4168 Close ny Chollagh Animal Bone 0.709509 -10.40 -3.98 F5208 Peel Castle Animal Bone 0.709262 F4174 Peel Castle Animal, cat? Bone 0.709072 -13.01 -3.63 F4174 Peel Castle Animal, cat? Bone 0.709072 -13.01 -3.63 F4175 Castle Rushen Animal Bone 0.709099 F5206 Castle Rushen Animal Bone 0.709092 F4175 Castle Rushen Animal Bone 0.709099 F4176 Castle Rushen Animal Bone 0.708705 -11.64 -4.65 F5207 Rushen Abbey Animal Bone 0.708726 F4176 Castle Rushen Animal Bone 0.708705 -11.64 -4.65 F6835 Port Douglas Snail Shell 0.708320 F6836 Ballaugh Snail Shell 0.709090 F6837 Knock-e-dooney Snail Shell 0.708320 F4171 Balladoole Animal Bone 0.709164 -8.95 -2.48 F5209 Balladoole Animal Bone 0.709171 F6838 Balladoole Snail Shell 0.708040 Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 120 are part of the British Tertiary Igneous Province made up of several extrusive plateau basalts which are relatively young and low in rubidium (Wallace et al. 1994). O’Connor (1988) reports strontium isotope ratios ranging from 0.7044 ± 0.0001 to 0.7062 ± 0.0009 for 50 rock samples from different components of the Antrim Basalts. Glacial moraines, deposited during the Pleistocene cover virtually all of Ireland, though are less obvious in Northern Ireland (McCabe 2007). Some of the glaciated lowlands of Ireland have moraine deposits over 30 m thick and form a landscape independent of the rock formations buried deeply beneath the ground (Clayton 1963, Geikie 1910). The material in the glacial moraine likely originated in part from the rocky structures of Ireland as the ice passed over the land surface and in part as detritus from the sea floor and Scandinavia transported by the ice. Thus, the bedrock geology of much of Ireland, including the Dublin region, is not a good guide to bioavailable strontium isotope ratios. A major Viking settlement is known from Dublin where the deeply buried bedrock consists primarily of marine basin facies and argillaceous and cherty limestone and shale that formed during the late Paleozoic (Geological Survey of Ireland 2009). Knudson et al. (2012) reported bioavailable values in 11 pig bones from Viking Age excavation in Dublin averaging 0.7094 ± 0003. Measurement of 11 samples of human tooth enamel from the same site produced an average value of 0.71017 ± 0.00074. Montgomery et al. (2014 [this volume]) measured values on several Viking Age skeletons from Dublin in the range of 0.709—0.720 and a bioavailable baseline in the same range as Knudson et al.’s (2012) findings. In general, these radiogenic strontium isotope data Figure 11. Geology of Ireland (Geological Survey of Ireland). Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 121 are consistent with other archaeological bone data from Ireland. We have measured archaeological human bone from Waterford on the southeastern coast of Ireland, with a mean 87Sr/86Sr of 0.7098 ± 0.0002 (n = 3), and from Tintern Abbey, also on the southeastern coast of Ireland, where the mean 87Sr/86Sr was also 0.7098 ± 0.0002 (n = 5). Similarly, archaeological human bone from Armagh in northeastern Ireland exhibited a mean 87Sr/86Sr = 0.7094 ± 0.0004 (n = 3). However, at Dunmisk in Northern Ireland, mean 87Sr/86Sr was somewhat higher at 0.7113 ± 0.0001 (n = 3). Faroe Islands. The Faroe Islands, part of the mid-Atlantic ridge, consist of a 3000-m-thick series of basaltic lavas of Paleocene age divided into 5 major series of flows. These basalts are covered by a thin layer of moraine and peat on the surface (Rasmussen and Noe-Nygaard 1970). Dark soils developed on these typical basaltic bedrocks from the Tertiary magmatic province of the North Atlantic realm have very low whole-rock 87Sr/86Sr values between 0.7020 and 0.7035 (Holm et al. 2001). However, Frei et al. (2009) have reported 87Sr/86Sr values from 4 sheep-wool samples and 1 soil sample from the small island of Koltur on the Faroes that fall in a narrow range from 0.7072 to 0.7087, with good correspondence between the soil and sheep wool, reflecting the influence of sea spray in the strontium isotope values. We have measured both human and archaeological fauna from the Faroes. The human tooth enamel comes from Medieval burials from 2 sites (Sandoy and Kirkjubør). The 12 human samples have virtually identical ranges and average 0.7094 ± 0.0006. Four samples of cattle bone averaged 0.7089 ± 0.0001. We also measured 4 tooth and bone samples from archaeological sheep and obtained a mean of 0.7090±0.0001. The slightly higher human values may be in part a consequence of marine foods in the diet along with a few immigrants in the sample. Sea spray must have played a large role in raising strontium isotope values from an expected range of 0.7020-0.7035 for mid-Atlantic basalts like the Faroes and Iceland. Iceland. Iceland is composed of some of the newest land on earth—basalts that continue to erupt from the Mid-Atlantic Ridge (Fig. 12). Strontium isotope ratios for Iceland, estimated from the age and composition of the basalt, suggest a value between 0.703 and 0.704. Measured ratios on geological formations at different locations in Iceland confirm this value as the best estimate for the island bedrock as a whole (e.g., Dickin 1997; Schilling 1973; Sun and Jahn 1979; Taylor et al. 1998; Wood et al. 1979a, b). However, strontium isotope ratios measured in enamel of modern sheep teeth originating from various locations in Iceland—Jadar, Heggstadanes (north), Bru ́, Biskupstungur (south), Ormarsstadir, Fellum (east) and Kjo ́afell, Kjo ́s (west)—range between 0.7059 and 0.7069 (Price and Gestsdóttir 2006) and are considerably higher than the reported geological values for Iceland. Archaeological cattle (2) and pig (1) from northern Iceland average 0.7042. Data for these fauna are presented in Table 3. We have also measured modern barley from Iceland and obtained a value of 0.7068. Juvenile redshank birds born on Iceland have an average 87Sr/86Sr of 0.7057 ± 0.008 (n = 5; Evans and Bullman 2009), also reflecting bioavailable strontium levels. In addition, Åberg (1995) reports a value of 0.706 from a reindeer on Iceland as intermediate between grass growing on volcanic soil with a 87Sr/86Sr value between 0.703 and 0.704, and seawater at 0.7092. We obtained δ13C ratios for the sheep tooth enamel from our baseline sample in order to check if the sheep were eating seaweed. Seaweed would have the value of seawater and might have raised their strontium isotope ratios and explained the differences between geological and bioavailable 87Sr/86Sr. Marie Balasse kindly carried out these measurements, and the data (Table 4) shows no evidence of marine food consumption. The farmers who kept these sheep stated that they were not fed seaweed and the sheep were all grazed in inland areas, which also reduces the likelihood of access to seaweed. Clearly, bioavailable strontium isotope ratios are higher than values reported for whole rock in Iceland. The reason for this offset likely relates to the effects of sea spray over large parts of the island. Ocean water has an of 0.7092 and sea spray, depositing minerals from the seawater, raised the bioavailable values in domestic animals and humans. Marine foods in human diets are another source of variability in 87Sr/86Sr in human tooth enamel, discussed in more detail in Price and Gestsdóttir (in press [this volume]). Table 3. Strontium isotope ratios on modern and archaeological fauna from Iceland. Species Material Context 87Sr/86Sr Sus Archaeological Enamel HRH 429 0.7044 Bos Archaeological Enamel HRH 003 0.7042 Bos Archaeological Enamel HRH 90 0.7042 Ovis Modern Enamel Bru, Biskupst, 0.7061 Sudurland Ovis Modern Enamel Kioafell, Kjos, 0.7064 Vesturland Ovis Modern Enamel Ormsstathir, Eithahr 0.7059 Ovis Modern Enamel Jaoar, Heggstaoanes, 0.7070 Nordurland Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 122 Greenland. Greenland has a very old geology, dominated by the crystalline rocks of the Precambrian shield, formed during a succession of Archaean and early Proterozoic orogenic events that stabilized as a part of the Laurentian shield about 1600 Ma ago. Subsequent geological developments mainly took place along the margins of the shield and involved the formation of major sedimentary basins largely in the northern parts of the island. Upper Palaeozoic and Mesozoic sedimentary basins developed along the continent–ocean margins in North, East, and West Greenland and are now preserved both onshore and offshore. During the Quaternary, Greenland was almost completely covered by ice sheets. The areas of the Eastern and Western Norse settlements on Greenland (black dots in Fig. 13) are dominated by Precambrian rocks constituting the Proterozoic and the Archaean craton (Kalsbeek 1997, Moorbath and Pankhurst 1976). The Western Settlement, near Nuuq, lies in an Archaean part of the craton, where some of the oldest rocks on Earth are found. The Eastern settlement is located on Proterozoic rocks of the Gardar province in southernmost Greenland, composed of Paleoproterozoic metamorphic intrusive and metamporphic rock sequences. Because of the varied geological terrains, there is of course substantial variation in whole-rock 87Sr/86Sr values and, due the antiquity of the rocks, these ratios are expected to be generally high. For example, Blaxlund et al. (1978) report a range of values from Gardar Province in southwest Greenland. Whole-rock granite samples from the region exhibit 87Sr/86Sr values ranging from 0.840 to 1.369. Hoppe et al. (2003) estimated Greenland values in the range between 0.725 and 0.755. Minimum values for the Disko Bay region were measured at greater than 0.725 (Kalsbeek and Taylor 1999). Thus, although there is substantial variation within the geological formations of Greenland, geological 87Sr/86Sr values are generally high. Because of the age of these rocks and the fascination they hold for geologists, numerous studies of strontium and lead isotopic ratios have been made Figure 12. Geology of Iceland. Table 4. Results of carbon and nitrogen isotope analyses of modern Icelandic sheep tooth enamel. Lab No. δ13C VPDB δ18O VPDB F 1153 A -13.474 -9.548 F 1154 A -14.524 -6.214 Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 123 in the last 30 years. Polat et al. (2003), for example, measured 87Sr/86Sr in 3.7–3.8 Ga pillow basalt in southwest Greenland and reported an extreme range of values from 0.715 to 0.906. The lower values in the range are likely the result of diagenesis by seawater. In terms of bioavailable values, Nelson et al. (1986) measured 87Sr/86Sr in both modern and archaeological reindeer from southwestern Greenland ranging from 0.737 to 0.758. Archaeological domestic sheep from the Western Settlement in Godthåbs Bay contained similar ratios (0.750-0.754; Nelson et al. 1986). For the determination of bioavailable isotope ratios, we took samples of faunal remains from various places in the Eastern and Western Settlement (Table 5, Fig. 14). A few samples are from farms from where we also have measured human skeletal samples (E29a Qassiarsuk, E64 Innoqqussaq). In addition, animal bones from the farms E60, E74, and E172 have been included. E60 and E172 are situated on the coast in the Igaliku fjord region, whereas E74 is situated inland in Vatnahverfi east of Igaliku fjord. E60 is a smaller farm with only 6 recorded ruins. Only smallscale test trenches have been dug here, and there are no dates from the site. E172 is a middle-sized farm consisting of 19 recorded ruins. Archaeological excavations as part of the Vatnahverfi-project show that the site has been populated from the Landnam to at least the late 1300s. The Vatnahverfi farm E74 was a small farm with only very few ruins recorded (Algreen Møller and Koch Madsen 2005, 2006). The strontium isotope ratios for these samples are shown in Figure 15. Wild species include arctic Figure13. Geological provinces of Greenland outside the ice sheet. The Eastern Settlement is located in the extreme south in Gardar Province; the Western Settlement is located in the Archaean craton near Nuuq on the west coast. Table 5. 87Sr/86Sr values of Greenland fauna samples. Lab No. Site No. Species Material 87Sr/86Sr F1853 E35 Cow Enamel 0.706532 F3900 E29 Cow Enamel 0.707373 F3901 W51 Caribou Enamel 0.761059 F3902 W51 Cow Enamel 0.715070 F5230 E60 Cow PH1 0.713800 F5231 E64 Cow PM 0.712691 F5232 E64 Cow PH1 0.713062 F5233 E74 Cow Enamel 0.715853 F5234 E74 Ptarmigan FEM 0.714931 F5235 E74 Ptarmigan TMT 0.713940 F5236 E74 Cow IN 0.711618 F5237 E74 Cow MTP 0.712040 F5238 E74 Cow TTH 0.712231 F5239 E74 Cow MO 0.713703 F5240 E74 Cow PM 0.712150 F5241 E74 Ptarmigan ULN 0.718776 F5242 E74 Cow MO 0.714511 F5243 E74 Cow TTH 0.712374 F5244 E172 Cow PM 0.713123 F5245 E172 Cow PM 0.712922 F5246 E172 Arctic Fox ULN 0.712565 F5247 E172 Arctic Fox MAN 0.712878 F5248 E172 Arctic Fox TIB 0.713147 F5249 E172 Caribou PM 0.721170 F5250 E172 Cow TIB 0.713294 F5251 E172 Cow PH3 0.712047 F5252 E172 Arctic Fox INN 0.712800 F5253 E172 Cow MO 0.713713 F5254 E172 Arctic Fox TRV 0.712163 F5255 E172 Arctic Fox INN 0.712101 F5256 E172 Caribou SCP 0.712711 F5257 E172 Cow MAN 0.713323 F5258 E172 Cow TTH 0.713482 F5259 E172 Cow MO 0.712583 F5260 E172 Cow IN 0.714534 F5261 E172 Cow MO 0.714627 F5262 E172 Cow IN 0.715075 F5950 GUS Arctic Hare CAL 0.752230 F5951 GUS Arctic Hare CAL 0.753207 F5952 GUS Arctic Hare VER 0.749556 F5953 W54 Arctic Hare MAN 0.749979 F5954 W54 Arctic Hare PEL 0.745319 F5955 W54 Arctic Hare TIB 0.754147 F5956 W51 Arctic Hare SCA 0.717470 F5957 W51 Arctic Hare PEL 0.711137 F5958 W51 Arctic Hare PEL 0.719655 F5959 W48 Arctic Hare MAN 0.757904 Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 124 fox, arctic hare, caribou, and ptarmigan. Hare and caribou show a dramatic range of variation from approximately 0.710 to values above 0.760, consistent with the known age of the rocks in the region. Arctic fox and ptarmigan do not show the extreme range of values seen in hare and caribou. These values vary between the Eastern and Western Settlement areas. Recall that the geology of the Western Settlement is older and should have higher 87Sr/86Sr values. There are 1 caribou and 10 arctic hare samples from the Western Settlement, and in general these exhibit very high strontium isotope ratios (Table 5). The single caribou has an 87Sr/86Sr value of 0.761. The arctic hare, with 3 exceptions, average around 0.750. The 3 distinctive exceptions are all below 0.720 and may be individuals that were feeding close to the coast and subject to a sea-spray effect. Examination of the values for fauna in a bar graph of samples from the Eastern Settlement (Fig. 15) provides further insight on the variation present in this area. Considering the faunal measurements from the 4 archaeological sites that we sampled, 3 groups of 87Sr/86Sr values are apparent: low, medium, and high. The majority of the values in fox and cattle fall in the middle range, between 0.711 and 0.716. There are 3 very low values between 0.706 and 0.707 that are likely non-local to the Eastern Settlement. There are no geological values below 0.711 reported from Greenland. These low values belong to domestic cattle, almost certainly imported from Iceland. The remaining cattle show a range of values from 0.71162 to 0.7159, consistent with the older rocks of the Greenland craton. There is one cow tooth from the Western Settlement in this group with a value of 0.7150. A higher value might have been anticipated for this cow given the sheep data reported by Nelson et al. (1986) as noted above, but this animal falls within the range of the Eastern Settlement cattle. Perhaps it was moved from the Eastern Settlement to the north. There are 2 very high values above 0.7165, one caribou and one ptarmigan, wild animals likely native to Greenland but perhaps from more inland areas with higher strontium isotope ratios. Based on Figure 14. Location of faunal samples from the Eastern Settlement, Greenland. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 125 the middle range of fauna values, it appears that we can expect bioavailable 87Sr/86Sr values in the Eastern Settlement area to be between 0.711 and 0.716, values which fit reasonably well with expectations based on geology. Oxygen baseline Baseline information on oxygen isotopes is rather limited in most areas. Althougth there are a number of measuring stations for δ18O in modern precipitation around the world, they are in fact rather far apart. Most projections of δ18O values are just that, projections or rough estimates. More importantly for archaeological studies, modern rainfall isotope ratios are not a reliable proxy for past values. There is very little information avaliable on oxygen isotope ratios in the past. Since these ratios are a proxy for atmospheric temperature and since climate change has characterized both the near and distant past, it is essential that records of δ18O distribution Figure 15. 87Sr/86Sr values from archaeological fauna on Greenland. Blue and red lines indicate the Eastern and Western settlements respectively. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 126 through time be established. Such information is not yet available in most areas. There is some useful information regarding oxygen isotopes in the North Atlantic. Generally speaking, variation in oxygen isotopes is pronounced in northern latitudes, making the ratio useful as a proveniencing tool in this region. In addition, oxygen isotopes have played an important role in the study of ice cores from Greenland and the reconstruction of past climate (Dansgaard et al. 1969, Langway 2008). An important early study of phosphate oxygen isotope ratios in human teeth was done using human and faunal remains from the Viking period on Greenland (Fricke et al. 1995). Figure 16 depicts largescale variation in phosphate oxygen isotope ratios in modern precipitation across the North Atlantic. Values range from -18.0‰ along the west coast of Greenland, to approximately -7.0‰ in Iceland, to between -8.0‰ and -6.0‰ in northwest Europe. Fricke et al. (1995) measured δ18Op in human tooth enamel from a series of sites in Greenland and a comparative site in Denmark. Their interests in oxygen isotopes were largely as a proxy for climate change, rather than human proveniencing. Nevertheless, the values they measured provide additional information on oxygen isotope levels in Greenland (Fig. 17). The range of values from Greenland is quite high, and some of the more positive values from Thjodhilde’s Church likely represent samples from migrant individuals from Iceland or Scandinavia. δ18Oen PBD values from the Eastern and Western Settlements (non-Inuit sites) range from approximately -8.0‰ to -4.0‰. Values from Risby in Denmark range from approximately -6.0‰ to -3.4‰. These values compare well with mean values from our data from Greenland (-7.7‰ ± 1.88) and Denmark (-4.3‰ ± 0.74), respectively. Lecolle (1985) measured the oxygen isotope composition of modern land snail shells as a proxy indicator for precipitation and mapped δ18O values across parts of western Europe, Scandinavia, and Iceland (Fig. 18). This map indicates values in Iceland between -8.0‰ and -6.0‰, higher values in Greenland, and a range of values across western Europe, with little variation in Denmark and substantial variation with latitude in Norway. A more detailed view of oxygen isotopic differences across northwestern Europe appears as Figure 19 (Hughes et al. 2014). Values on this map range from -5.0‰ to -11.0‰ and document substantial variation from west to east across Ireland and the Figure 16. Oxygen isotope ratios in the western North Atlantic (after Fricke et al. 1995). Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 127 British Isles, Scandinavia, and Western Europe. There are pronounced differences in this homeland region of the Norse. Values range from -4.0‰ in the Outer Hebrides to -8.0‰ across parts of England and Scotland. Of particular import for our study is the variation along the coast of Norway where values in precipitation range from -6.0‰ in the south to greater than -10.0‰ to the north. It is important to note that most of the southwest coast of Norway, from Stavanger to Trondheim has predicted values between -7‰ and -8‰ for annual precipitation. These values are easily resolvable with present instrumentation. This variation means that theoretically it should be possible to distinguish different areas of the Norwegian coast as homelands for the migrants using oxygen isotopes. Unfortunately the similar ranges of δ18O values in southern Norway and the north of Scotland and Ireland means that this ratio will not be useful in distinguishing these 2 regions. Another more recent map of oxygen isotope distribution across Europe (Bowen 2012) provides a somewhat different picture (Fig. 20). Details of this mapping procedure to produce “isoscapes” is provided in Bowen and Revenaugh (2003). In this depiction, variation in δ18O across northwestern Europe is less pronounced, and differences range from -8.0‰ to -15.0‰ from southern England to Figure 17. Oxygen isotope ratios from Viking Greenland and Denmark (Fricke et al. 1995). Figure 18. Oxygen isotope ratios from Western Europe ( L e c o l l e 1985). Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 128 northern Norway. Again, however, substantial similarities between the northern British Isles and southern Norway are observed. Somewhat more detailed distribution maps of modern δ18O are available for a few countries. Figure 21 shows the isoscape for average annual precipitation for Sweden, with a range from slightly less than -8.0‰ to -14.0‰ in the northern parts of the country (Burgman et al.1987). A similar map is also available for the UK and Ireland (Toolis et al. 2008), documenting a range from -5.0‰ in the west to -8.0‰ in the eastern part of the country (Fig. 22). At the same time it is important to remember that these maps of geographic variation in oxygen isotope values are models, estimates of the distribution of δ18O across space, and based on modern precipitation or ground water measurements. There have been few detailed, systematic attempts to map δ18O and most maps/models are based on very few data points. There are substantial differences for the same areas among the various models. It is not realistic to assume that archaeological materials will consistently fit modern models. In addition, there are significant other problems with oxygen isotopes and proveniencing. In addition to maps of modern δ18O distribution across Europe, there are also several bioarchaeological studies of past human remains. For example, Chenery et al. (2011) report oxygen isotope ratios of -6‰ to -8‰ from burials at Catterick, a small Roman town and minor fort north of York. Eckhardt et al. (2009) in a study of Roman burials from Winchester in the south of England suggest a range of δ18Op in the entire UK between -8.7‰ and -4.7‰. Values from the Winchester enamel cover most of that range. Lamb et al. (2012) found oxygen isotope ratios averaging -7.3‰ ± 1.5 for a medieval graveyard in southeastern Scotland. Evans et al. (2012) summarized strontium and oxygen isotope variation in archaeological human tooth enamel excavated in Britain. The strontium isotope ratios range between 0.7078 and 0.7165 (excluding individuals thought to be of non-British origin). The oxygen isotope data is normally distributed with a mean of approximately -7.1‰ ± 0.5. Two sub-populations have been identified that provide different baseline averages for human enamel values: -7.5‰ ± 1.8 (2 s.d.) from the eastern side of Britain where there are lower rainfall levels, and -5.8‰ ± 1.8. from the western part of Britain where rainfall levels are higher. Fig. 19. Isoscape map of mean annual δ18O values for precipitation in western Europe (Hughes et al. 2014). Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 129 Knudson et al. (2012) in their study of Viking burials in Dublin reported δ18O values between -7‰ and -17‰ with an average of -7.2‰ from a sample of 12 individuals. Montgomery et al. (2014 [this volume]) reported δ18O for Dublin as averaging around -8.0‰, based on the analysis of several Viking Age skeletons. Similar samples from the Westness cemetery on Orkney exhibit δ18O values between -10.0‰ and -7.0‰. Darling et al. (2003) reported surfacewater values in the Shetlands averaging -6.1‰. Average values for δ18O from several studies in northwestern Europe document jslightly lower values across the region. In Scandinavia, oxygen isotopes in the enamel of the local inhabitants of the Viking Age cemetery at Trelleborg in Denmark averaged -4.54‰ ± 0.5 (Price et al. 2011). δ18O from Figure 20. Average annual δ18O in precipitation in modern Europe (Bowen 2012). Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 130 20 burials at Fräslegården in Falbygden region of central Sweden (Sjögren et al. 2009) have a mean value of -4.4‰ ± 0.90. Fourteen samples from the medieval cemetery at Bryggen, Norway, produce a mean δ18O of -4.27‰ ± 0.7. Ireland exhibits somewhat different values. We have summarized a substantial series of measurements in Table 6 to provide average and standard deviation for oxygen isotope values from archaeological humans in 6 areas for comparison. We have attempted to screen these samples for local individuals and remove non-locals (indicated by the condition column). There is surprisingly little variation among these areas in mean δ18O, with the exception of Greenland and Dublin having distinctly higher values than Denmark, Norway, Iceland, and the Faroes. Such data suggest that while oxygen may provide information on geographic origins in some cases (e.g., Greenland vs. Iceland or Norway), this ratio will not be useful in many other situations. Oxygen remains a very uncertain measure of geographic variation for several reasons. There are a number of potential sources of variation. For example, the consumption of 18O-enriched breast milk affect the oxygen isotope values in enamel and bone that formed before and during weaning (e.g., Knudson 2009, Wright and Schwarcz 1998) Variation within a population is quite high, often more than Figure 21. Average annual δ18O in precipitation in modern Sweden (Burgman et al. 1987). Figure 22. Oxygen isotope ratios in UK water (Toolis et al. 2008). Journal of the North Atlantic T.D. Price, K.M. Frei, and E. Nauman 2015 Special Volume 7 131 between geographic regions. Estimates of oxygen isotope ratios based on measurement of modern rainfall or surface water usually fail to correlate with prehistoric samples. As noted in (Price, in press [this volume]), there are also the concerns that oxygen has been measured in both carbonate and phosphate components of human skeletal tissue and that different standards have been used (e.g., SMOV, PDB). δ18O values for carbonate or phosphate oxygen using different standards are comparable though calculation (Chenery et al. 2011). Furthermore, what are presumed to be basic principles of oxygen isotope variation do not seem to be followed in some baseline samples. For example, we measured oxygen isotope ratios in archaeological beaver from a series of sites along the west coast of Norway. A plot of δ18O vs. the UTM north coordinates for the samples is shown in Figure 23. There is an observable increase in the δ18O values with latitude north. These data contradict the basic principle of oxygen isotope variation that ratios should become more negative inland with elevation and north or south toward colder air and the polar regions. The beaver data show a clearly negative correlation between oxygen isotope ratios and northern latitude, ratios become more positive as the location of the samples moves north. Conclusions Knowledge of baseline isotopic values is essential for the investigation of past human mobility. Fortunately there is a good bit of data now available to permit the assessment of these values. Based ultimately on the geology of the region, a basic principle relates older rocks to higher strontium isotope ratios. Very old rocks dominate parts of the North Atlantic region, particularly in Norway, Sweden, Scotland, Northern Ireland, and Greenland. Very young rocks are found in Iceland and the Faroes as part of the expansion of the mid-Atlantic ridge and the volcanic eruptions that define that phenomenon. Between these extremes there is a common isotope range between ~0709 and 0.711 that is found in many parts of northwest Europe, particularly in areas dominated by glacial and periglacial deposits and coastal regions where marine foods and sea spray have altered the geological background. Because Iceland has a distinctive geology of very young rock with very low strontium isotope ratios, the presence of non-local individuals from older terrains should be quite obvious. In a similar fashion, individuals from Iceland who move to Greenland would also have very distinctive strontium isotope value among the higher 87Sr/86Sr values there. Greenland has 2 areas of Norse settlement with distinctive 87Sr/86Sr in each area. For this reason, movement between the Eastern and Western Settlement should also be visible in the isotope data. On the other hand, the distinction between individuals in originating in Norway, northern Britain, and Greenland using 87Sr/86Sr may be a difficult undertaking given the generally higher 87Sr/86Sr values in these areas. Oxygen isotopes may provide some resolution of this difficulty particularly in separating individuals from Greenland with more negative δ18O, from individuals from Norway and northern Britain who should have less-negative values. On the other hand, as we have noted, oxygen isotopes Table 6. Average values for oxygen isotope ratios in selected countries of Northwest Europe. Place Condition n Mean sd Denmark less than 0.711 71 -4.3 0.7 Norway -- 15 -4.4 1.2 Faroe Islands -- 11 -3.4 0.7 Iceland less than 0.709 10 -4.7 1.1 Greenland >0.709 35 -7.7 1.9 Dublin -- 12 -7.2 1.0 Figure 23. Scatterplot of δ18O vs. UTM coordinate north for archaeological beaver from Norway. Journal of the North Atlantic T.D. Price, K.M. Frei, and E. 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