| Summary: | This paper examines 2 potential sources of the radiocarbon offset between human and terrestrial mammal (horse) bones recovered from Norse (~AD 8701000) pagan graves in Mvatnssveit, north Iceland. These are the marine and freshwater 14C reservoir effects that may be incorporated into human bones from dietary sources. The size of the marine 14C reservoir effect (MRE) during the Norse period was investigated by measurement of multiple paired samples (terrestrial mammal and marine mollusk shell) at 2 archaeological sites in Mvatnssveit and 1 site on the north Icelandic coast. These produced 3 new R values for the north coast of Iceland, indicating a R of 106 10 14C yr at AD 868985, and of 144 28 14C yr at AD 12801400. These values are statistically comparable and give an overall weighted mean R of 111 10 14C yr. The freshwater reservoir effect was similarly quantified using freshwater fish bones from a site in Mvatnssveit. These show an offset of between 1285 and 1830 14C yr, where the fish are depleted in 14C relative to the terrestrial mammals. This is attributed to the input of geothermally derived CO2 into the groundwater and subsequently into Lake Mvatn. We conclude the following: i) some of the Norse inhabitants of Mvatnssveit incorporated non-terrestrial resources into their diet that may be identified from the stable isotope composition of their bone collagen; ii) the MRE off the north Icelandic coast during the Norse period fits a spatial gradient of wider North Atlantic MRE values with increasing values to the northwest; and iii) it is important to consider the effect that geothermal activity could have on the 14C activity of samples influenced by groundwater at Icelandic archaeological sites.
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