9 July 2010, by Sara Coelho
Egyptologists can recite the list of Egyptian pharaohs by heart, but they have found it difficult to pin the sequence of their reigns to a precise calendar. Now, scientists have perfected a radiocarbon dating method to put exact dates on ancient Egyptian history.
Historical methods to estimate the chronology of Egyptian rulers are based on the study of inscriptions, old documents and artefacts. This provides an accurate sequence of events, but very few absolute dates.
In a few cases it's possible to match a known historical event to a precise date, using astronomical observations registered in ancient texts. But scholarly work alone is not bullet-proof and the academic community has been debating these chronologies for decades.
The breakthrough, published in Science, helps egyptologists to calibrate their theories with radiocarbon dating, which is based on carbon-14 isotopes. Carbon atoms come in several varieties, or isotopes, and some are lighter than others.
'For the first time, radiocarbon dating has become precise enough to constrain the history of ancient Egypt to very specific dates.'
Professor Christopher Bronk Ramsey, University of Oxford
The heavy carbon-14 is radioactive and with time decays into the stable nitrogen-14. So by measuring the amount of carbon-14 that is left in charcoal, plant material, or even clothes, scientists can gauge how old they are.
Up to now, 'radiocarbon was seen as too imprecise to date material from ancient Egypt,' says Professor Christopher Bronk Ramsey, who leads the radiocarbon dating team at the University of Oxford.
Bronk Ramsey and his international colleagues were not satisfied with this shortcoming and embarked on a three-year project to overcome it. To do this, they analysed ancient Egyptian plant samples as well as baskets, textiles and other artefacts directly associated with a specific reign.
The team stayed clear of charcoal and wooden materials, which are often used for radiocarbon studies. Ancient Egyptians sometimes used wood that had been collected years before and recycling old planks and boards from building to building was not uncommon. 'We excluded these kind of sample to avoid error,' explains Bronk Ramsey.
The team then analysed the radiocarbon dates using statistical analysis software devised by Bronk Ramsey, known as OxCal. The results allowed the researchers to assign absolute dates to the reigns of Egyptian pharaohs.
They arrived at dates for events in the Old Kingdom that are earlier than some archaeologists thought. Djoser, for example, acceded the throne between 2691 and 2625 BCE, about 50 to 100 years earlier than some recent estimates.
'For the first time, radiocarbon dating has become precise enough to constrain the history of ancient Egypt to very specific dates,' says Bronk Ramsey.
The findings support the general consensus within the archaeological community, but they also help to overrule some chronologies that could not have been dismissed based only on historical research.
'The results provide independent and scientific corroboration to a century of scholarly work by archaeologists and historians,' adds Bronk Ramsey.