A cross-check of radiocarbon dates from Stone Age sites i Northern Sweden Ekholm, Therese http://kulturarvsdata.se/raa/fornvannen/html/2015_227 Fornvännen 2015(110):4 s. 227-240 Ingår i samla.raa.se Art. Ekholm 227-240_Layout 1 2015-11-26 14:42 Sida 227 A cross-check of radiocarbon dates from Stone Age sites in Northern Sweden By Therese Ekholm Ekholm, T., 2015. A cross-check of radiocarbon dates from Stone Age sites in Nor - thern Sweden. Fornvännen 110. Stockholm. This study compares the results from radiocarbon measurements performed on charcoal and burnt bone from the same contexts in order to assess the reliability of the two materials for dating. The study deals with seven Mesolithic sites in Norr- land and Dalecarlia, an area where datable organic material is difficult to find. The bone samples have been chosen for the study and the charcoal samples are mainly from previous work. The study shows that both materials are suitable for dating as long as they are sampled correctly and with knowledge of the errors that can occur. Therese Ekholm, Department of Archaeology and Ancient History, Uppsala University. Box 626, SE–751 26 Uppsala [email protected] In the late 1990s a new method made it possible thin vegetation, prehistoric sites are often found to carbon date burnt bones (Lanting & Brindley right beneath the top soil, which complicates the 1998). This has changed our chronology and under- interpretation of the stratigraphy. The assemblages standing of the arrival of both humans and ani- often represent palimpsests of material from seve- mals to Norrland, the northern three quarters of ral periods making the relation of dates, features Sweden. It has been shown that bone is more reli- and artefacts difficult. Furthermore, due to the able for dating archaeological sites than wood decomposition of organic materials in the acid because of its negligible intrinsic age (Lanting et soil, artefacts made of wood and unburnt bones al. 2001, p. 249; Hedman 2009, p. 5), which is the are rarely found in sunken features (Borg et al. age of the sampled organism. Much radiocarbon- 1994, p. 97 f; Björdal 1999, p. 120 f; Christensson dated wood from long-lived tree species suffers 1999, p. 172 f) where burnt bones do survive from the Old Wood Effect. A similar problem is (Lanting & Brindley 1998, p. 7). For decades the curation or storage age, when objects have been only method available for dating these sites was kept in use or circulation for long periods (Brock radiocarbon dating of charcoal, especially the & Dee 2013, p. 41). In addition burnt bones found sites excavated during the expansion of hydro- in a sunken feature are more likely to represent electric power in the 1940s through 70s (Iregren human action and connect people’s activities to a & Ekman 1984). Most sites are chronologically particular site, while charcoal can result natural- mixed, and so even at sites with stratigraphy such ly from, for example, a forest fire at any time dur- as Rappasundet, Döudden/Varghalsen, Garase- ing a site’s formation process. let (Knutsson 1993, p. 25; Bergman 1995a, p. 143 One of the problems of dating prehistoric sites ff; 111 ff) it is often difficult to distinguish which in Norrland is how to argue for an association feature belongs to which period. At all of these between radiocarbon dates and artefacts. Due to inland sites it is therefore important to do a care- Fornvännen 110 (2015) Art. Ekholm 227-240_Layout 1 2015-11-26 14:42 Sida 228 228 Therese Ekholm ful contextual analysis of the provenance of arte- sen et al. 2013. In 2001 radiocarbon dates obtai- facts and samples for radiocarbon analyses. ned from burnt bones were compared to measure- This study deals with two problems of the ments on charcoal from the same context. In some chronology which can be elucidated by systematic cases the charcoal looked older, which was ex- radiocarbon dating of burnt bones. plained by reference to a higher intrinsic age of the sample (Lanting et al. 2001). 1. Due to the difficulty of identifying the The famous Early Bronze Age oak-coffin grave sources of charcoal found at a prehistoric from Egtved in Denmark contained both an in- site, it is uncertain what event is dated. humation and a cremation in a bark bucket. Ol - This study investigates if there is any sen et al. (2013) have compared the radiocarbon difference in the dates of burnt bone and date of the jawbone from the cremated individ- charcoal taken from the same feature, ual, a five-year-old child, with the dendro-chrono- and if the charcoal therefore is connected logical date (1370 BC) of the log coffin. Rather to the activity in which the feature was unusually, they converted the dendro date back- involved. wards into an uncalibrated BP date by means of 2. The intrinsic age of the charcoal sample the IntCal09 radiocarbon curve (Reimer et al. is often unknown, which may be one rea- 2009) and got 3054±16 BP. Two combined sam- son why dates obtained from charcoal ples from the child’s jawbone yielded a date in often seem misleading. This study inves- 3127±20 BP. The difference between the coffin tigates if there is a systematic error here and the bone was 73±26 radiocarbon years, the that can be controlled for by using bone bone being older. dates as a baseline. Olsen et al. suggest that the child was cremat- ed in connection with the funeral of the Egtved Bones are however not unproblematic. Bones ly- woman, because remains of the funeral pyre were ing in the soil can become contaminated by ab- found in the bark bucket. No such remains, they sorbing bicarbonate from the surrounding sedi- argue, would have been there if the deposition of ments and the results may thus show a later date. the child’s remains represented reburial of a long- This does not affect bones burnt at more than dead individual (Olsen et al. 2013, p. 31 f). The 600 °C, provided that the bones are burnt before charcoal from the cremation found with the burnt they end up in the soil (Lanting & Brindley 1998, bones were not radiocarbon dated, as these re- pp. 2, 6; Olsen et al. 2008, p. 792). Bones from mains were assumed to produce identical re sults. animals that live in the sea or feed on animals from This was because of the intrinsic age of the wood marine habitats may yield dates exceeding their used as fuel which, according to the authors, was actual age, due to the reservoir effect. It is therefore transferred to the bones by means of CO2. To important to choose terrestrial over marine animals prove this, the radiocarbon date of the burnt bone for analysis, as well as animals with a terrestrial diet was instead compared to the dendrochronology over those feeding on marine sources (Lanting & on the oak coffin. The results showed that the burnt van der Plicht 1998, p. 151; Lanting et al. 2001, p. bone was older than the coffin after the reservoir 249 f; Hedman 2009, p. 5). This does not, however, effect was excluded. This outcome was interpreted affect burnt bones as much as it does unburnt bo- to mean that the burnt bone had been contami- nes. Because structural carbonate, which is used nated by the intrinsic age of the wood used as fuel when dating burnt bones, originates from the whole for the cremation (Olsen et al. 2013, p. 33). diet (Lanting & Brindley 1998, pp. 6). This comparison of radiocarbon dating and dendrochronology is not unproblematic as the Previous studies authors assumed that the two deaths were con- Studies comparing radiocarbon dates from burnt temporaneous. The bucket with the cremated bones and other burnt materials have been under - material could have been unearthed intact from taken several times and with different results, such its first burial place and placed in the oak coffin as in Lanting et al. 2001, Olsen et al. 2008 and Ol - as a secondary burial event. The cremated bone Fornvännen 110 (2015) Art. Ekholm 227-240_Layout 1 2015-11-26 14:42 Sida 229 A cross-check of radiocarbon dates from Stone Age sites 229 is, furthermore, an unreliable sample because it Contract excavations for the E4 highway north derives from the spongy part of the jaw bone. It of Uppsala in 2002–2004 occasioned a compara- is not advisable to use spongy bone tissue for tive study (Guinard 2006) of radiocarbon dates radiocarbon dating as its cavities can trap other for a number of Stone Age contexts including the materials such as radiocarbon date charcoal and sites Stormossen, Bålmyren and Postboda 2, and roots (Borg et al. 1994, p. 97). Solid bones are hazelnut shells, seal bones as well as charcoal. The more suitable for dating. Since all the bones in aim was to study whether any systematic pattern the bucket derive from a child and children’s in the reservoir effect could be discerned. It was bones are overall very spongy, the samples were assumed that the seals would have acquired a not suitable to use as a foundation to build argu- reservoir. The dates of the bones from four con- ments concerning the reliability of radiocarbon texts at Stormossen were, as expected, consis- dating. tently too early. The radiocarbon measurements In a 2008 study, Olsen et al. compare radio- of the seal bones from the Mesolithic sites Bål - carbon dates on burnt bones with radiocarbon myren and Postboda 2, however, gave later dates dates on other materials from the same contexts, than the hazelnut shells from the same contexts, such as charcoal and pitch, and also with dendro- which Michel Guinard does not discuss further.