243 Comments on Sveinbjörnsdóttir Et Al. (2004) And

RADIOCARBON, Vol 48, Nr 2, 2006, p 243–252 © 2006 by the Arizona Board of Regents on behalf of the University of Arizona COMMENTS ON SVEINBJÖRNSDÓTTIR ET AL. (2004) AND THE SETTLEMENT OF ICELAND Ingrid U Olsson Department of Physics, University of Uppsala, Box 530, SE-751 21 Uppsala, Sweden. Email: [email protected]. ABSTRACT. Radiocarbon dates on samples aimed to date the settlement of Iceland are given together with comments by the laboratory, since many of the results and descriptions given by Sveinbjörnsdóttir et al. (2004) in Radiocarbon, together with new results, are in error. The intention of this paper is to present correct dates and further relevant information regarding samples used earlier and to discuss possible complications inherent in the method of Sveinbjörnsdóttir et al. (2004). Examples are given of how critical the collection, treatment, and interpretation of samples may be. An age difference between birch charcoal and grains for a site is expected due to various reasons. If the difference amounts up to ~100 yr, as reported by Sveinb- jörnsdóttir et al. (2004), it must only to a small degree be due to biological age. Reference to an excavation report, details regarding stratigraphy, and discussions of the risk for displacement and contamination are missing in their paper. A final eval- uation of the time for settlement should not be done until more research is completed and other possible or earlier suggested or even dated sites are discussed. A summary is given of the research on the island and volcanic effects on the 14C activity of the atmospheric CO2, especially over Iceland. INTRODUCTION The Settlement Layer The authors (Sveinbjörnsdóttir et al. 2004) briefly relate, with references, the opinion that the settlement layer may be of almost the same age as a tephra layer deriving from the Vatnaöldur eruption. Sample Quality Origin Discussed by Various Authors The risk for erroneous results because of driftwood, shipped wood, biological age, and time difference between death and subsequent use must be considered (Sveinbjörnsdóttir et al. 2004:389; Ols- son 1981, 1990:216, 1997). Because of inconsistent results from the Stockholm Laboratory (see Vil- hjálmsson 1991:109), on a few samples from Papey, a separation into species was suggested, and the 2 new results, from 1971, on charred birch were 980 ± 100 BP (St-3604) and 935 ± 100 BP (St- 3605). A sample sent to the Uppsala Conventional Laboratory was separated into species by Eric Åberg, Uppsala, and birch charcoal was dated as U-4014 to 1090 ± 80 BP (Olsson 1974:313). Other aspects were briefly discussed by Olsson (1981, 1997) and Simonsen (1983). Larix, Pinus, and Picea are suspect when driftwood is used in hearths and as building material, although driftwood has often been used for shore displacement studies (e.g. on Spitsbergen). Birch and willow easily sink in water, but when frozen into ice, these species may be transported over long distances. Errors due to biological age can often be avoided by the choice of twigs or certain tree rings, pref- erably counted from the bark when visible, but such precautions were mostly not possible for the Reykjavík samples dated in Uppsala. Even if Icelandic birch trees may reach an age of 100 yr, the mean age of samples recovered at excavations will not be 100 yr but only a few decades. Branches and the lower parts of the stem naturally can yield dates that are too old, if the tree was killed by tephra from a volcanic eruption. Such wood may be burnt and collected for dating. There is no guarantee that small samples have no or negligible biological ages. Different possibilities for age differences between charcoal and grains must be considered. Small samples such as small charcoal pieces and grains may be displaced by biological activity, e.g. worms, upfreezing, and mechanical displacements, for example at frost cracks and cavities after 243 244 I U Olsson roots. Reused constructions, burnt discarded tools, and mistakes at excavations cannot always be excluded. Different Fractions Considered in Uppsala If possible, wood and charcoal samples are scraped to remove contaminated surface layers. The laboratory procedure then involves a treatment to remove any contaminating carbonate by leaching with an acid and, for the organic samples, a step to separate the fraction soluble in a basic solution (SOL) from the insoluble fraction (INS). Careful washings between the steps are included. The SOL fraction may be humic acid and may derive from other layers. An estimate of the content of the different fractions is made. Sometimes more than 1 SOL extraction is necessary; sometimes both fractions are measured to judge the risk for contamination. The fraction to be dated is always slightly acidified shortly before combustion to avoid CO2 adsorption during the treatment with the basic solution or during storage. This was demonstrated to the technicians in connection with the dating of the Gardsendi, Heimaey samples (Olsson et al. 1967:458). Peat (0.1-m-thick layer) collected in 1964 and described by Kjartansson (1966) contained small pieces of Salix, determined by Thorleifur Einarsson. These samples were in good condition and dated to 5110 ± 200 BP (U-521). The peat was dated after 2 separate pretreatments to 5310 ± 170 and 5760 ± 120 (U-519 and -539), respectively. A third time, we deliberately added HCl without stirring and did not burn the sample until some months later. The result was 4120 ± 90 BP (U-517), indicating the necessity of a final proper HCl treatment. The statistical spread for the 3 other dates may indicate a risk for peat samples like these to be dated older than wood because of some contamination in nature. Kjartansson expressed con- cern about whether the wood was stems or roots, but writes “stems” in Figure 3. If the samples are roots, the date would be younger than that of the peat. Thus, context, choice of sample, treatment, and fraction should be stated. The same activity for 2 fractions does not guarantee freedom from contamination. The bark derives from several years and should appear older than the outermost tree rings. Thus, it is removed before the chemical treatment. No 13C normalization was performed for some samples dated in other laboratories. The present author prefers discussing uncalibrated results in the actual tables because of the author’s investiga- tions indicating a lower activity over Iceland, at least for certain periods of time, compared to the activity over the continents where wood for calibration is growing (see below). REMARKS ON TEXT AND TABLES Text Sveinbjörnsdóttir et al. (2004:389) refer to Vilhjálmsson (1991) and write that he criticized various researchers for misinterpreting 14C dates. Indeed, Vilhjálmsson (1991) has numerous errors in his tables and questionable statements in the text. Olsson (1992) criticized Vilhjálmsson and included these opinions in a lecture given in Reykjavík in 1990—a survey intended to be popular and read by anybody interested in Iceland. Due to unlucky circumstances, the manuscript was not published until 1997. There and in Olsson (1992) lists from the Uppsala laboratory are given. Missing 13C Normalization Sveinbjörnsdóttir et al. (2004: Figure 1) includes 2 results from Thórarinsson (1977:669): St-4891 as 1190 ± 90 BP and St-5704 as 1150 ± 80 BP, respectively. Since the activities overlap, it would be natural to calculate a mean value and to calibrate that date. The result is given (Sveinbjörnsdóttir et al. 2004:387) as AD 845 ± 45. The mimeographed list from Stockholm indicates that no 13C normal- Comments on a Recent Paper on the Settlement of Iceland 245 ization is done, checked, or confirmed by correspondence. The age should thus be discussed as if it was about 25 14C yr younger or still younger, since many Icelandic birch charcoal samples have δ13C values hovering around –26.8‰ (Sveinbjörnsdóttir et al. 2004) or deviating more from –25‰. Results Given in Sveinbjörnsdóttir et al. (2004) from Non-Archaeological Sites The value given on birch wood 1155 ± 75 BP (U-2809) from Lágafell (immediately below the Land- nám tephra layer) was discussed together with 12 other samples from the Markarfljót area by Haraldsson (1981:34–8). The wood was reported by the authors as a bog sample. Five peat samples (Hallsdóttir 1987) collected from a hillside bog, S of Mosfell farm (64°08′N, 20°36′W), were dated by Håkansson (1977:432). Two samples derive from the peat above, and 2 samples derive from peat below the layer with the main part of the Landnám tephra layer dated on plant remains; all layers are 1 cm thick. The lowermost sample, Lu-1170, was dated to 1290 ± 50 BP, although the date is given as 1170 ± 50 by Sveinbjörnsdóttir et al. (2004: Figure 1), who also calibrated that erroneous value. They also refer to Theodórsson (1993) who gave the value AD 835 ± 20 for the ash layer after wig- gle-matching. Olsson (1990:215) preferred to calculate a mean and concluded that the ash layer derived from a time with intercepts on the calibration curve shortly after AD 800 and in the range of AD 880 to 890. Earlier, Hallsdóttir (1987:23–5) presented another calibration. The section from Mosfell was considered suitable because of the high rate of peat growth. The samples got a “mild pretreatment with NaOH and HCl” (Håkansson 1977). There is a marked step in the δ13C values of the 2 lower samples (Lu-1170 and Lu-1169), from about –22‰ to that with the ash, dated to 1180 ± 50 BP (Lu-1168), –25.3‰. The higher layers had values of –25.5 and –26.0, respectively. Archaeological Sites in Reykjavík and on Vestmannaeyjar (Heimaey) Laboratory dates are given by Olsson (1997) as tables with comments and according to the archae- ologists Nordahl (1988) and Hermanns-Audardóttir (1989) responsible for the 2 respective exten- sive excavations.

Load more