Reservoirs and radiocarbon: C-14 dating problems in Myvatnssveit, northern Iceland

This paper examines 2 potential sources of the radiocarbon offset between human and terrestrial mammal (horse) bones recovered from Norse (similar to AD 870-1000) pagan graves in Myvatnssveit, north Iceland. These are the marine and freshwater C-14 reservoir effects that may be incorporated into human bones from dietary sources. The size of the marine C-14 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 Myvatnssveit and 1 site on the north Icelandic coast. These produced 3 new Delta R values for the north coast of Iceland, indicating a Delta R of 106 +/- 10 C-14 yr at AD 868-985, and of 144 +/- 28 C-14 yr at AD 1280-1400. These values are statistically comparable and give an overall weighted mean Delta R of 111 +/- 10 C-14 yr. The freshwater reservoir effect was similarly quantified using freshwater fish bones from a site in Myvatnssveit. These show an offset of between 1285 and 1830 C-14 yr, where the fish are depleted in C-14 relative to the terrestrial mammals. This is attributed to the input of geothermally derived CO2 into the groundwater and subsequently into Lake Myvatn. We conclude the following: i) some of the Norse inhabitants of Myvatnssveit 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 C-14 activity of samples influenced by groundwater at Icelandic archaeological sites.