Reservoirs and Radiocarbon:¹⁴C Dating Problems in Mývatnssveit, Northern Iceland

Abstract

ABSTRACT. This paper examines 2 potential sources of the radiocarbon offset between human and terrestrial mammal (horse) bones recovered from Norse (~AD 870-1000) pagan graves in Myvatnssveit, north Iceland. These are the marine and freshwater l4 C reservoir effects that may be incorporated into human bones from dietary sources. The size of the marine 14 C 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 AR values for the north coast of Iceland, indicating a AR of 106 ± 10 14 C yr at AD 868-985, and of 144 ± 28 l4 C yr at AD 1280-1400. These values are statistically comparable and give an overall weighted mean AR of 111 ± 10 l4 C 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 l4 C yr, where the fish are depleted in 14 C relative to the terrestrial mammals. This is attributed to the input of geothermally derived C0 2 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 ,4 C activity of samples influenced by groundwater at Icelandic archaeological sites.

Temporal and Spatial Variations in Freshwater14C Reservoir Effects: Lake Mývatn, Northern Iceland

“… 15 N values for all samples are between 1.0‰ and 5.9‰ for cow bone and between 0.1‰ and 4.6‰ for pig bone, with some evidence for a relationship between isotopic value and 14 C offset. Inclusion of the previous pig bone  13 C and 14 C offset from Ascough et al (2007) in this data set strengthens the relationship between these variables significantly ( Figure 3A and 3B).…”

“…In addition, we obtained samples of bone from Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta) located in a single context at Hofstaðir. These were from 2 articulated fish of each species and were analyzed in order to verify that the apparent age range of fish from Viking deposits was commensurate with the previous assessment of 1500-1900 14 C yr (Ascough et al 2007). Although no direct paired terrestrial samples were measured for these samples, the earliest midden deposits at this site are located immediately above a tephra ash layer from the Veiðivötn system, dated to about AD 940 (Sigurgeirsson et al 2002;McGovern et al 2006).…”

“…Geyh et al 1998). In Mývatn, the range in FRE values calculated from Arctic char bones in a single, constrained archaeological deposit was 1500-1900 14 C yr (Ascough et al 2007) and suggests that spatial variation of at least 400 14 C yr was in operation at cal AD 868-968. To investigate the potential for FRE variation, we examined the 14 C activity within modern samples from the lake and surrounding environment, comparing calculated FRE values for the Viking and modern periods.…”

“…Icelandic terrestrial vegetation uses the Calvin-Benson (C 3 ) photosynthetic pathway, where C 3 plants have a typical  13 C of 20‰ to 30‰ (Ehrlinger et al 1997(Ehrlinger et al , 2002. The  13 C values of archaeological cattle bones from Mývatnssveit range from 20.9‰ to 21.7‰ (Ascough et al 2007), typical of terrestrial mammals in C 3 -dominated environments, given a trophic level fractionation of 0-2‰ (e.g. DeNiro and Epstein 1978;Post 2002).…”

An Icelandic freshwater radiocarbon reservoir effect: Implications for lacustrine 14C chronologies

“…Lake Mývatn is almost entirely fed from groundwater springs, and a substantial FRE has been attributed to input of 14 C-depleted carbon within the spring waters (Ascough et al, 2007;2010). This FRE is present in archaeological samples from the period 868-968 cal AD, and appears to vary by over 1500 14 C years during a calibrated calendar period of ~100 years Ascough et al, 2010 (Figure 1), and water inputs are almost exclusively from cold (c.5ºC) and warm (geothermal, ~20ºC) springs along the eastern shore (Ólafsson 1979).…”