Radiocarbon (¹⁴C) Dating

Radiocarbon dating

Radiocarbon (¹⁴C) dating was developed some 60 years ago by Willard Libby from the University of Chicago. Its power to determine the absolute age of objects containing the cosmic-ray produced radioisotope ¹⁴C (half-life = 5730 ± 40 years) was realised early on, and was awarded with the 1960 Noble Prize in Chemistry. Although the basic principle of ¹⁴C dating is simple, there are a variety of methodological subtleties which have to be well understood in order to arrive at reliable dates. Libby himself expressed the situation well in his Noble Lecture: “Radiocarbon dating is something like the discipline of surgery – cleanliness, care, seriousness, and practice.” Since that early time, the ¹⁴C method has experienced a number of important refinements, both in technical terms (transition from beta decay counting to atom counting with accelerator mass spectrometry), and in tracing the distribution of ¹⁴C on Earth through time with ever better refinement (Reimer et al. 2004). It is important to note that ¹⁴C dating relies on a calibration, because any radiogenic 14N, the decay product of ¹⁴C, is overwhelmed by the ocean of 14N in our environment and cannot be measured. Thus ¹⁴C dating is not an absolute dating method in the strict sense of the word (Szabo et al. 1998) as K-Ar and U-Th dating are. However, for the Holocene a very reliable calibration has been established by using dendro-dated tree rings (Reimer et al. 2004). 

Acclerator Mass spectrometry (AMS)

Some 30 years ago, the advent of AMS opened a new era for ¹⁴C dating. Compared to classical beta-counting, the reduction in sample size by approximately a factor of thousand (milligrams instead of grams) allowed one to pursue ¹⁴C-dating with unprecedented detail. In 1996, a new AMS facility called VERA (Vienna Envrionmental Research Facility) came into operation at the Vienna University. (Fig. 1).

Fi. 1 Schematic layout of the VERA facility in its present stage (2008). The isotopes measured for a ¹⁴C determination are indicated with red labels.

From the onset VERA was planned to be a general-purpose AMS facility capable for measuring ultra-low levels of long-lived radioisotopes over a large mass range (Vockenhuber et al. 2003, Steier et al. 2004a, Kutschera 2005). However,  like in almost all AMS facilities, ¹⁴C clearly dominates the field. It was therfore quite natural that VERA would join the SCIEM 2000 project. During the running period of SCIEM 2000, VERA has established itself as one of the top ¹⁴C AMS laboratories in the world (Steier et al. 2004b, Wild et al. 2005). Close collaboration with other leading AMS laboratories, e.g. at the University of Oxford, were pursued for quality cheks in important experiments related to the Second Millennium B.C.

Radiocarbon dating for SCIEM 2000

In connection with SCIEM 2000, ¹⁴C dating provides absolute time information on objects of interest, principally independent on archaeological or historical time scales. As such it is an important check on the interpretation of archaeological information linked to the only other absolute time frame, the Historical Chronology of Ancient Egypt (Kitchen 2003, Kraus 2006). SCIEM 2000 has offered an unprecedented opportunity to compare ¹⁴C results with the vast archaeological evidence for the Second Millennium B.C. in the Eastern Mediterranean. It opened up a unique interplay between methods in sciences and the humanities.

One of the aims in ¹⁴C dating within the SCIEM 2000 project was to perform measurements at archaeological sites with well-established stratigraphic sequences of phases. Provided that short-lived organic samples exist (e.g. seeds, grass), which can be unambiguously assigned to respective phases of an excavation site, the Bayesian method of statistical analysis (Buck et al. 1991, Bronk Ramsey 1995) allows one to substantially reduce the uncertainty of calibrated ¹⁴C dates. Ideally, the information from the stratigraphy tells one which is the relative chronological order of samples (e.g. samples from deeper layers must be older). Bayesian mathematic allows one then to incorporate this “prior” information mathematically correct in the calibration procedure (Weninger et al. 2006). Concentrating on short-lived material is important to avoid unknown intrinsic age-offsets of the materials (e.g. “old wood” effect). The ideal conditions, both from the point of few of archaeology and of ¹⁴Csample materials, are rarely met. Therefore, a lot of effort was spent in SCIEM 2000 to  find conditions which minimize sources of  errors. Although an overall consistent picture of the synchronisation of civilisations in the second Millennium BC has not been reached, we are confident that SCIEM 2000 has laid the foundation to eventually solve the chronological problems of this important time period in the development of human civilisations.

Results

Approximately 350 high-precision ¹⁴C AMS data were measured for ~25 sites from the East Mediterranean, contributing information for absolute chronology at the respective sites. Some of the ¹⁴C results are in an advanced state of analysis such as the one in Aegina Kolonna (Wild et al. 2008a), and the one in Tell el-Daba (Kutschera et al. 2008). It is well known that “wiggles” in the ¹⁴C calibration curve do not allow to translate uncalibrated ¹⁴C ages into equally precise absolute calendar dates (Guilderson et al. 2004). In order to arrive at more precise dates for the second millennium BC, in particular the critical date of the volcanic eruption at Thera, the relative time sequence of samples derived from the stratigraphy of the site to be dated is incorporated using the Bayesian approach. This, then, allows to substantially reduce the uncertainty of the calibrated dates. In rare cases such as Tell el-Daba there is a link to the Historical Chronology of Ancient Egypt. It is fair to say that the agreement of ¹⁴C dates with the archaeological evidence linked to the historical chronology of Egypt is not satisfactory. The main ‘bone of contention’ is the volcanic eruption date of Santorini (Thera) in the middle of the Second Millennium BC, where a currently irreconcilable offset of 100 to 150 years between ¹⁴C dating and the archaeological assessment persists (Fig. 2), with older ages derived from the ¹⁴C measurements (Friedrich et al. 2006, Manning et al. 2006, Bruins et al. 2008). A commentary on this situation can be found by Balter (2006).

Fig. 2. Comparison of the radiocarbon-based and the archaeological (conventional) chronology for the Aegean Late Bronze Age according to Manning el al. (2006). Whereas the ¹⁴C results point to an eruption date between 1650 and 1600 BC, the conventional chronology put this event after the beginning of the New Kingdom in Egypt (1530 BC, Bietak and Höflmayer 2007).

A similar offset is also observed for the entire first half of the second Millennium BC at the Tell el-Daba site in the Nile delta. Despite great efforts on both sides – sciences and humanities - a solution of this conflict has not yet been reached. On the one hand, the fact that ¹⁴C dating requires a calibration to correct for ¹⁴C variations in the atmosphere opened the door to speculations that regional and temporal effects in the East Mediterranean may have caused unknown deviations from the master calibration curve. On the other hand, ¹⁴C results of historically dated Egyptian artefacts (e.g. papyri) and other objects from historical Egyptian sites seem to be in agreement with the Egyptian chronology (Wild et al. 2008b), particularly for the time period after the beginning of the New Kingdom (1530 B.C.).

References

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