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Conclusions and Reflections
T. Douglas Price

Journal of the North Atlantic, Special Volume 7: 186–192

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Journal of the North Atlantic T.D. Price 2018 Special Volume 7 186 Baseline Briefing This section is intended to provide a final overview of isotopic baselines around the North Atlantic for the purpose of evaluating the origins of the settlers of this region. This discussion begins with strontium isotopes. Knowledge of baseline isotopic values is essential for the investigation of past human mobility. Fortunately, considerable data are now available. A basic principle relates older rocks to higher strontium isotope ratios in human skeletons. Very old rocks dominate parts of the North Atlantic region, particularly in Norway, Sweden, Scotland, Northern Ireland, and Greenland. Very young rocks with low 87Sr/86Sr are found in Iceland and the Faroes as part of volcanic activity at the divergent plate boundary of the mid-Atlantic ridge. There are intermediate values of strontium isotope ratios between ~0.709 and ~0.711 that are found in many parts of northwest Europe, including parts of southern Scandinavia and the North European Plain, particularly in areas dominated by glacial and periglacial deposits and coastal regions where marine diet and sea spray have altered the relations between human values and geology. Because Iceland has a distinctive geology of very young rocks with very low strontium isotope ratios, detecting the presence of non-local individuals from older terrains is quite feasible. In a similar fashion, individuals from Iceland who moved to Greenland would also have very distinctive strontium isotope values among the higher 87Sr/86Sr values common there. The 2 areas of Norse settlement on Greenland have distinctive 87Sr/86Sr values. For this reason, movement between the Eastern and Western Settlements should be visible in the isotope data. On the other hand, a 87Sr/86Sr distinction among individuals originating in Norway, northern Britain, and Greenland is difficult given the generally similar 87Sr/86Sr values in these areas. In sum, there is good and bad news for strontium isotopic studies in the North Atlantic. The good news is that there are distinct differences between Iceland and the rest of the North Atlantic. Local strontium isotope ratios in Iceland fall almost exclusively between 0.706 and 0.7092. Local values from Norway, northern Britain, Ireland, and Greenland are greater than 0.7092. The good news is also that oxygen may provide some information on origins and that lead isotopes have the potential to resolve some of these differences, for example, between Greenland and Norway. The bad news is that we cannot at present distinguish individuals coming from northern Britain, Ireland, and Norway, so that these potential homelands for the settlers of Greenland and Iceland cannot be segregated isotopically. The bad news is also that there are numerous places with strontium isotope values between 0.709 and 0.710 that cannot be separated using 87Sr/86Sr alone. At the same time, the new information that has been obtained through isotopic investigations is remarkable, and summarized in the following pages. Oxygen isotopes may provide some resolution of this difficulty, particularly in separating Greenland individuals, who have more-negative δ18O, from individuals native to southern Norway and Northern Britain, who should have less-negative values. On the other hand, oxygen isotopes exhibit significant variability and do not always follow expectations. Oxygen isotopes are considered in more detail in the next section. Isotopic Results Strontium isotope ratios are very useful for identifying place of origin for the inhabitants of Iceland and Greenland. Although there are limitations because of geologies of similar antiquity across parts of the homeland area, there are still Conclusions and Reflections T. Douglas Price* Abstract - This article summarizes what has been learned from the isotopic investigations of the settlement of the North Atlantic. In addition, I consider what questions remain and where future research may take us. I begin with a brief synthesis of the interpretation of human mobility in the North Atlantic region with a specific focus on the issue of local vs. non-local individuals on Iceland and Greenland. The results of the isotopic analyses provide new insight on the settlement of the North Atlantic. It is also possible to compare the results from Iceland and Greenland to examine similarities and differences in the process of colonization. The article concludes with a review of the research questions raised in the introduction to this volume, some of the answers that have been found, and some of the questions that remain. Viking Settlers of the North Atlantic: An Isotopic Approach Journal of the North Atlantic *Laboratory for Archaeological Chemistry, University of Wisconsin-Madison, Madison, WI 53706, USA; tdprice@wisc.edu. 2018 Special Volume 7:186–192 Journal of the North Atlantic T.D. Price 2018 Special Volume 7 187 meaningful differences. Average 87Sr/86Sr values from the major areas of interest reveal these differences (Table 1). These averages include a high proportion of non-local individuals in the case of Iceland and Greenland. This variation related to migration is reflected in part in the standard deviations, which are highest for Iceland and lowest for Norway, where most of the inhabitants are native to the region. The mean values for Norway and Greenland are very similar but for very different reasons. The Greenland samples consist of a number of low values for non-local Icelanders and some high values for local Greenlanders. The Norwegians are largely local and reflect values from ~0.709 primarily in coastal areas to higher values largely inland. While there are a few lower values, these likely represent migrants to Norway and are not applicable for determining a local range for Norway. The range of 87Sr/86Sr values from Norway has important implications for the identification of the first settlers of the North Atlantic. 87Sr/86Sr values for human enamel from Iceland, for example, fall between 0.7056 and 0.709. Local Icelandic human values probably do not reach 0.7092, the highest isotope source available, because it would require a fully marine diet, which is not observed in the 13C/12C bone collagen data. Thus, strontium isotope ratios from Iceland and Norway rarely overlap. Greenland is another story. Because of the ancient geology of Greenland, strontium isotope ratios are generally higher. Bioavailable 87Sr/86Sr values from archaeological fauna from the Eastern Settlement fall for the most part between 0.711 and 0.716. Human values at the Eastern Settlement vary considerably due to diet and the presence of a high proportion of native Icelanders among the settlers. However, values at the site of Narsarsuaq, a later settlement with few or no native Icelanders, fall largely between 0.710 and 0.720, which likely best reflects the local human 87Sr/86Sr values in the Eastern Settlement. This variation in values is very similar to that recorded for Norway. Most of the fauna measured in the Western Settlement had extremely high values between 0.750 and 0.760. A few animals had lower values between 0.710 and 0.720, likely due to home ranges near the coast. The human samples from the Western Settlement came from a single site, Sandnes. The average 87Sr/86Sr value for the 11 samples was 0.717 ± 0.066, with a min–max = 0.7101–0.7314. These values likely vary widely due to the role of marine resources in the diet. Although there are 3 individuals from Sandnes with 87Sr/86Sr values greater than 0.720, it does not appear possible to distinguish samples below 0.720 at the Western Settlement from either the Eastern Settlement or Norway on the basis of strontium isotopes alone. Values above 0.720 are rare or absent in the Eastern Settlement and Norway. It is also useful to compare oxygen isotope ratios of the tooth enamel between areas. We have substantial data from Norway, pagan Iceland, and Greenland, and these data are plotted against the corresponding 87Sr/86Sr values in Figure 1, one of the more important graphics from our study. Although there is some overlap between the samples from Greenland, Iceland, and Norway, there is a very clear segregation of individuals. It is important to remember that, especially in the case of the Greenland and Iceland datasets, there are a number of non-local individuals included in the plot. The majority of the Iceland samples fall below 0.709 87Sr/86Sr and above -7.0‰ in δ18O. Most of the Norwegian samples fall above 0.7095 and above -8.0‰. The Greenland samples are largely below -6.0‰ with a very wide range of 87Sr/86Sr. The difference between the Eastern and Western Settlements in Greenland can be discerned in the more negative oxygen ratios and higher 87Sr/86Sr among the Western Settlement samples. Table 1. Descriptive statistics for 87Sr/86Sr values from Norway, Iceland, and Greenland. Country Count Mean s.d. Min Max Norway 154 0.7124 0.003 0.7075 0.7317 Iceland 127 0.7087 0.030 0.7056 0.7257 Greenland 51 0.7125 0.005 0.7069 0.7314 Figure 1. Scatterplot of δ18O vs. 87Sr/86Sr of tooth enamel for Norwegian (yellow), Greenland (green), and pagan Icelandic (blue) skeletons. The darker green dots represent the Eastern Settlement, the light green indicate data from the Western Settlement. Journal of the North Atlantic T.D. Price 2018 Special Volume 7 188 As noted in Price et al. (2015 [this volume]), the 78 δ18OPDB enamel values from Norway have a mean of -4.9‰ ± -1.3 with a minimum of -7.7‰ and a maximum of -1.4‰. Converting these values to precipitation δ18OSMOW results in a mean of -7.7‰ with a minimum of -12.1‰ and a maximum of -1.8‰. While the mean value fits well with the region of central and southern Norway, the value of -12.1‰ is found only in Greenland, northern Norway, Sweden, and Finland, and -1.8‰ is out of range for the European continent (Bowen and Revenaugh 2003). In addition to the 3 larger groups that can be identified in the plot in Figure 1, there are several other observations that can be made. There are a number of Greenlanders among the Icelanders, suggesting that these Greenlanders were born on Iceland. The 3 Greenland samples with strontium isotope ratios between 0.709 and 0.710 and oxygen isotope ratios more positive than would fit with the Greenland local range almost certainly represent individuals from Norway or the northern British Isles. A large number of the non-local Icelanders have strontium and oxygen values that fit closely with the Norwegian samples and likely confirm their place of origin somewhere in that country. There are other individuals from Iceland with extreme strontium and oxygen isotope ratios that do not appear to fall within the Norwegian baseline. There are 2 Norwegians with oxygen isotope ratios close to -8.0‰ that might well represent individuals born on Greenland who later moved to Norway where they were buried. There is also a single Norwegian with a 87Sr/86Sr value below 0.708 who falls in the middle of the Iceland group and could very well have been born on Iceland and died in Norway. It would seem that the Greenland individuals with δ18O values below -7.0‰ are locally born because these values are only very rarely observed in Iceland or Norway (2 examples). On the other hand, the Greenlanders with oxygen isotope values between -5.0‰ and -8.0‰ and strontium isotope ratios above 0.7092 could also be from Norway. It is impossible to confirm these suggestions at the present, but ongoing work with lead isotopes may provide further resolution. It is also important to consider the northern British Isles as a potential homeland for the Norse settlers of Greenland and Iceland. As noted elsewhere in this volume, Eckhardt et al. (2009) suggested a range of δ18O in precipitation (δ18Op ) for the entire UK between -8.7‰ and -4.7‰. Evans et al. (2012) summarized the oxygen isotope data for Britain and identified 2 areas—the eastern side of Britain where values average approximately -7.5‰ ± 1.8 (2 sd) and the western regions where rainfall levels are higher and δ18Op averages -5.8‰ ±1.8. The distribution of δ18Op values (Darling 2004, Toolis 2008) indicates that values between -5.0‰ and -8.0‰ should be expected for the northern portions of Britain and Ireland. These areas also have generally high 87Sr/86Sr values and unfortunately cannot be distinguished from Norwegian sources. Thus strontium and oxygen isotope ratios cannot resolve these 2 areas. It is our hope that ongoing work with lead isotopes from these areas will provide new insights. A plot of δ13C vs. 87Sr/86Sr for the Norwegians, Greenlanders, and pagan Icelanders (Fig. 2) is informative with regard to differences in diet. The 3 sets of values tend to cluster separately, but at the same time there is a significant overlap. In general terms, there is some negative correlation between δ13C and 87Sr/86Sr in which more negative carbon isotope ratios are found with more positive strontium isotope values. This relationship reflects the impact of marine foods, sea spray, and rainfall along the coasts and a higher proportion of terrestrial foods inland. The pagan Icelanders generally had a more terrestrial diet than the Norwegians or Greenlanders, with the majority of δ13C values between -14.0‰ and -17.0‰. The Greenlanders tended to have the most-marine diets (highest δ13C values). A number of the Greenland burials have δ13C and 87Sr/86Sr values that fall among the Icelanders and Norwegians (or northern Britain and Ireland), reflecting their place of origin. There is also overlap among the Norwegians and Icelanders, reflecting birth origins as well. The Iceland and Norwegian burials among the Greenlanders reflect the homelands of the original settlers. In addition, Figure 2. Scatterplot of δ13C vs. 87Sr/86Sr for Norwegian (yellow), Greenland (green), and pagan Icelandic (blue) graves. Journal of the North Atlantic T.D. Price 2018 Special Volume 7 189 some of the Greenlanders and Icelanders found among the Norwegians were very likely inhabitants of those places who traveled to Norway later in life and were buried there. The present data shows that mobility in both directions across the North Atlantic was quite pervasive and not uncommon. Questions Migration is a large and complex issue in the social sciences. There are many factors involved. Questions arise concerning conditions at home, motivations, methods, segments of the population involved, timing and duration, and reception and return, among others. Within archaeology, these questions have been limited by a methodology in which artifacts were used as a proxy for people when investigating past human movement. The development of isotopic and molecular methods offers a means for directly assessing past human behavior and activity from human skeletal remains and rigorously assessing more detailed aspects of ancient mobility such as the identification of new arrivals, the rate of movement, the timing and duration of migration, the population segments involved, and on occasion, the places of origin. In the Introduction for this volume, a series of research questions, repeated below, were raised regarding the colonization of the North Atlantic. Following the questions, I provide some responses that have derived from the isotopic investigations. 1. Where did the colonists come from? 2. Did migration to Iceland and Greenland continue after the initial period of colonization? 3. Are there gender, age, and/or status differences among the migrants? 4. What are major questions, concerns, or problems about the isotopic data? Strontium? Oxygen? Lead? Carbon? Nitrogen? 5. Where should isotopic and genetic research focus in the future? Origins of the colonists One of the essential facts regarding the isotopic proveniencing of human remains is that the technique is very good for distinguishing local and non-local individuals, but identifying place of origin is much more difficult. This limitation arises because different geographic locations can have similar geological formations or sediments that cannot be distinguished isotopically. In the case of the settlement of the North Atlantic, we are unable to distinguish large parts of Norway and Sweden from the northern British Isles and the north of Ireland. Both regions, Scandinavia and northern Britain/Ireland, have almost identical geologies composed of ancient Archean gneiss and metamorphic beds interspersed with granite intrusions created during the Caledonian mountain building period. 87Sr/86Sr values in these geological conditions are usually greater than 0.715. Bioavailable values are normally lower. Both of these areas are likely homelands for the settlers of the North Atlantic. Both the genetic evidence and some historical documents report a number of individuals, often female, as coming from northern Britain and Ireland. At the same time, it is very clear from the 87Sr/86Sr analyses that the early inhabitants of Greenland and Iceland came from several different places. Both the diverse 87Sr/86Sr values and some differences in oxygen isotope ratios implicate multiple places of origin. It is also possible to identify the Icelandic colonists of Greenland among the skeletal remains in the Eastern and Western Settlements. Early Christian graveyards in the Eastern Settlement are often dominated by Iceland-born inhabitants. In addition, there are some burials whose combined strontium and oxygen values point to Norway or the northern British Isles or Ireland as the place of origin. The isotopes also provide indications that some of the settlers of the Western Settlement came from the Eastern Settlement on Greenland. Finally, isotope ratios permit us to suggest that several of the individuals analyzed from Norway were in fact born on Iceland or Greenland, emphasizing the mobility among the North Atlantic colonies and the Norwegian homeland. Duration of migration It is clear from the comparison of 87Sr/86Sr values and radiocarbon dates for some 37 individuals from Iceland that the arrival of new colonists there continued through the pagan period. At the same time, it is also evident that the migration was largely over by the beginning of the early Christian period, as there are virtually no non-local strontium isotope ratios among the burials from this period. We do not have similar data from Greenland because the colonization took place largely in the Christian period. We also do not have a large number of 14C dates to provide a chronology of the colonization. However, archaeological and historical evidence suggests that Greenland was almost continuously settled until the later years of the colony when environmental conditions had deteriorated (Masson- Delmotte et al. 2012). The expansion of population to the Western Settlement documents the continuing colonization of the island prior to abandonment. Journal of the North Atlantic T.D. Price 2018 Special Volume 7 190 Gender, age, status Age differences in migration in Iceland were not discernible because our sample was largely adult individuals and the age at arrival on Iceland was not related to the age at death and burial. Sex differences were distinctly notable, however, with a much higher incidence of migration (2X) among the females. The exact reasons for this difference are unknown and suggest that there may have been an imbalance in numbers between the sexes for some period of time on Greenland. Status differences are more difficult to infer on Iceland, given the absence of artifacts in many of the graves. In addition, there is the very real possibility of a bias in the pagan graves that have been excavated over the years and that provided the samples for our study. Although the initial aim of the isotope project was to look at the movement of people into Iceland, the results of the analysis has proven to be invaluable when studying the pattern of settlement and burial practices in Iceland. First of all, rather than the settlement process having been relatively quick, spanning only the first 60 years of the settlement, the data suggests that large-scale movement of people into the country continued towards the end of the 10th century. Secondly, when one compares the results of the strontium analysis with the demography of the pagan burials, they seem to indicate that these burials are most likely a high-status subgroup, comprised to a large extent of males and immigrants. We may be witnessing different burial traditions in which not all individuals in society are interred in a way readily visible to archaeology. Perhaps as Vésteinsson (2016 [this volume]) argues, the disparity in burial visibility marks a distinction between the late-9th-century founding population and an upper class with foreign roots and a Norse identity. Religious status, as distinct from social status, certainly revealed a major difference in migration on Iceland. Following the arrival of Christianity ca. AD 1000, migration ceases almost completely, and there is only 1 likely migrant identifiable from the 44 individuals in 2 Christian cemeteries that were analyzed. It is, of course, unlikely that the arrival of Christianity played a role in the cessation of migration. Little information on status is available from the graves on Greenland because of the Christian tradition of simple burial. Differences between males and females in isotopic values is clear, with females having both higher average and greater variance for 87Sr/86Sr values. These differences suggest that females may have originally arrived in smaller numbers than males on Greenland and from more varied places of origin. Some males and fewer females came to Greenland from Iceland. Questions about isotopes In general terms, isotopic methods for human proveniencing and determination of paleodiet are well established, reliable, and useful. There are numerous examples of the application of these methods in various parts of the world (e.g., Ambrose et al. 2003; Copeland et al. 2011; Harrison and Katzenberg 2003; Montgomery et al. 2000; Price et al. 1994, 2010; Slovak and Paytan 2011; Tykot 2004). At the same time, the development of these methods is ongoing, and new information on sources of variation and analytical procedures continues to appear. Strontium isotopes are generally well understood using a deep background in the geological literatures. Problems remain because of the widespread occurrence of 87Sr/86Sr values between 0.709 and 0.710 that appear with many types of marine, fluvial, and glacial sedimentary deposits. Such widespread similar values often make the identification of place of origin a difficult undertaking. In this context, the use of other isotopic systems is essential in order to bypass this ambiguity. Oxygen isotopes remain rather contentious. There is a significant variation at the individual and regional level for oxygen isotopes that reduces the accurate characterization of locally born individuals. There is also significant variation in δ18O on a seasonal, annual, and long-term basis. Because oxygen isotope ratios ultimately reflect atmospheric temperatures, past climate changes impact these values and our understanding of changes in these ratios over time for specific areas is not good. At the same time, similar oxygen isotope ratios are found over a large part of the modern temperate regions of the world. δ18OSMOw values for mean annual precipitation between -6‰ and -10‰ are common from North America to West Africa to Europe (IAEA/WMO 2006). Such homogeneity is not conducive to distinguishing places of origin. Lead isotopes offer another avenue for investigating questions of human provenience. Limitations are due largely to a lack of information on variation in lead isotopes and insufficient understanding of the variation in values found in humans (Albarède et al. 2012). These isotopes originate in limited geological sources for lead that have been isotopically characterized for a number of places across northern Europe and the North Atlantic. However, there is currently relatively little information available on the variation in lead isotopes in tooth enamel. There are 4 principle isotopes of lead, which can be used to calculate at least six different Journal of the North Atlantic T.D. Price 2018 Special Volume 7 191 old geological terrains such as Norway, northern Britain, and Ireland. 3. The colonization of Iceland continued throughout the pagan period. 4. Women in the sample from Iceland were twice as likely to be migrants as men. 5. There was little or no migration to Iceland during the early Christian period, after AD 1000. 6. Human diet on Iceland varied from largely terrestrial to largely marine. 7. Human diet on Iceland was determined, at least in part, by the location of settlement, i.e., coastal marine vs. inland terrestrial. 8. A number of the settlers of Greenland came from Iceland, and at least some brought their cows with them. 9. The Eastern and Western Settlements on Greenland can be isotopically distinguished with a combination of strontium and oxygen isotopes. 10. Diets on Greenland are more marine than on Iceland and more marine at the Western Settlement than the Eastern one. Inuit diets are heavily marine in composition. 11. Some of the first locally born children on Greenland can be identified at Innoqquasaq (E64). 12. Several of the individuals buried in the Western Settlement probably came from the Eastern Settlement. 13. One or 2 individuals from the Western Settlement were buried in the Eastern Settlement. 14. Migration is a two-way street. Several Greenlanders were buried on Iceland. 15. In addition, a few Greenlanders and Icelanders were buried in Norway. 16. Mobility appears to have been commonplace in the Viking period. 17. Strontium isotope ratios in tooth enamel, sometimes in combination with oxygen isotopes, provide a powerful tool for identifying non-local individuals among human burials. 18. The comparison of collagen and apatite carbon isotope ratios offers additional insights into past human diet. 19. Lead isotopes have significant potential for human proveniencing in the North Atlantic region. 20. Isotopic proveniencing has provided exceptional new information regarding the peopling of the North Atlantic. Literature Cited Albarède, F., A.-M. Desaulty, and J. Blichert-Toft. 2012. A geological perspective on the use of Pb isotopes in archaeometry. Archaeometry 54:853–867. Ambrose, S.H., J.E. Buikstra, and H.W. Krueger. 2003. Status and gender differences in diet at Mound 72, Cahokia, revealed by isotopic analysis of bone. Journal of Anthropological Archaeology 22:217–226. ratios of interest, and it is not always clear which of these may be most useful in human proveniencing. Our project is currently involved in the analysis of lead isotopes across the North Atlantic, and we hope to publish this study in the near future when it has been completed. Future research As always, more research needs to be done. Further characterization of baseline isotope values across northern Europe is essential to the success of proveniencing past humans in this region. Such studies are just beginning in many areas, and the creation of a substantial database is a major investment of time and resources. As noted, there is much more to be done, especially in terms of oxygen and lead isotopes, to understand variation in these systems. More work on Viking Age burials in northern Britain and Ireland is needed in order to document the variation of isotope values in human remains and to characterize more places of origin. The development of additional isotopic systems for proveniencing would be most helpful but seems unlikely since these isotope ratios have to be both geographically variable and deposited in human tooth enamel. There is also the potential for refining methods of analysis to look at periods within the lives of the individuals we investigate through microscopic sampling of teeth (e.g., Richards et al. 2008, Slovak and Paytan 2011, Wright 2013), the analysis of dentine in conjunction with enamel (e.g., Beaumont et al. 2013), and the selection of bones or specific bone components (Bell et al. 2001, Cox and Sealy 1997) with different turnover rates in the human skeleton. Ancient DNA studies of these human remains where protein is preserved will also provide extraordinary information on the relationships and probable origins of some of the inhabitants of the North Atlantic islands. Conclusions A set of conclusions regarding the peopling of the North Atlantic based on the isotopic data presented in this volume can be enumerated. These results provide insight into the characteristics of the migrating populations, the behavior of individuals involved in the process of colonization, and the utility of isotopic proveniencing: 1. A large proportion (39%) of the pagan burials on Iceland were non-local. 2. The early colonists of Iceland came from a number of different places, primarily areas with Journal of the North Atlantic T.D. Price 2018 Special Volume 7 192 Price, T.D., K.M. Frei, and E. Naumann. 2015. Isotopic Baselines in the North Atlantic Region. Journal of the North Atlantic Special Volume 7:103–136. Richards, M., K. Harvati, V. Grimes, C. Smith, T. Smith, J.-J. Hublin, P. Karkanas, and E. Panagopoulou. 2008. Strontium isotope evidence of Neanderthal mobility at the site of Lakonis, Greece, using laserablation PIMMS. Journal of Archaeological Science 35:1251–1256. Slovak, N.M., and A. Paytan. 2011. Applications of Sr isotopes in archaeology. Advances in Isotope Geochemistry 5:743–768. Toolis, R. 2008. 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Booth, J.A. Evans, A. Lamb, and G. Müldner. 2009. Oxygen and strontium isotope evidence for mobility in Roman Winchester. Journal of Archaeological Science 36:2816–2825. Evans, J.A., C.A. Chenery, and J. Montgomery. 2012. A summary of strontium and oxygen isotope variation in archaeological human tooth enamel excavated from Britain. Journal of Analytical Atomic Spectrometry 27:754–760. Harrison, R.G., and M.A. Katzenberg. 2003. Paleodiet studies using stable carbon isotopes from bone apatite and collagen: Examples from southern Ontario and San Nicolas Island, California. Journal of Anthropological Archaeology 22:227–244. I n t e r n a t i o n a l A t o m i c E n e rg y A g e n c y / Wo r l d Meteorological Organization (IAEA/WMO). 2006. Global Network of Isotopes in Precipitation: The GNIP Database. Accessible online at http://www.iaea. org/water. Accessed 4 May 2015. Masson-Delmotte, V., et al. 2012. Greenland climate change: From the past to the future. 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