Category Archives: Dating Methods

Electron Spin Resonance Dating

Electron spin resonance (ESR) measures the number of trapped electrons accumulated, since the time of burial, in the flaws of dental enamel’s crystalline structure. At sites containing human and animal teeth, ESR can be used to determine how long the teeth have been in the ground, but finding teeth at an archaeological site is unusual, so this dating method is not as common as thermoluminescence or radiocarbon dating.

ESR has been used to estimate the age of many archaeological finds, such as WLH3 (‘Mungo Man’), Homo floresiensis (‘The Hobbit’), and much older human ancestors like Homo ergaster and Homo neanderthalis. It has also been used to date many megafaunal finds.

teeth

Luminescence Dating

There are two types of luminescence dating: thermoluminescence (TL) and optically stimulated luminescence (OSL). Both measure the number of trapped electrons that have accumulated in quartz (sand) grains since the last time they were exposed to sunlight.

In order to release these trapped electrons, the quartz grains must be subject to intense heat (TL) or intense light (OSL). Luminescence dating is often used to determine age-estimates of sediment in stratigraphy where artefacts have been located.

The graph below shows the progress of luminescence dating in Australia since the 1950s. (Note the similarity with radiocarbon dates.)

Luminescence dates in Australia
Luminescence dates in Australia

Where radiocarbon dating has a ‘barrier’ of about 40,000 years, luminescence can be used to date material millions of years old. Many older sites in Australia are being dated using a combination of luminescence and radiocarbon, to ensure the accuracy of dates.

While luminescence is an accurate method of dating the sand grains in sediment, there are a number of pitfalls associated with using this dating method to determine the age of sites. For a start, 3000 sand grains are required, and the sample will likely contain a mixture of earlier and later grains, especially if it is taken from less than 20cm below the ground surface, or within 20cm of bedrock. Contamination from earlier or later layers can sometimes misrepresent the correct age of the sediment, hence the age-estimates for the associated artefacts may be inaccurate.

Some sites that were originally dated using luminescence have since been re-examined and found to be older or younger than initially thought. A good example of this type of error occurred in 1996 when archaeologists excavated the Jinmium rockshelter and preliminary findings suggested artefacts as old as 116,000 years. Later analysis debunked this finding to reveal a much younger site of between 10,000 and 20,000 years. For more on this, see the Jinmium entry.

References:

Burke, H. & Smith, C. (2004) The Archaeologist’s Field Handbook. Crows Nest: Allen & Unwin.

Hiscock, P. (2008) The Archaeology of Ancient Australia. New York: Routlege.

Mulvaney, J. & Kamminga, J. (1999) Prehistory of Australia. Crows Nest: Allen & Unwin.

Radiocarbon Dating

Radiocarbon dating is used to determine the age of historic and prehistoric sites all over the world. To do this, archaeologists need uncontaminated organic samples – that is, samples that are taken straight from the ground and not touched by anyone on site before going to the lab. Most of the time, these samples are charcoal. Other organic materials that can be dated using radiocarbon are leather, shell, plant material (e.g. seeds and pollen), eggshell, fish and insect remains, paper and parchment, wood, hair, bone and ice cores.

Radiocarbon dating measures the rate of decay in the unstable, radioactive isotope ¹⁴C to determine the age of organic matter. Because the ¹⁴C isotope is unstable, calibrating precise dates is sometimes difficult, especially with older sites. There is what’s known as a ‘radiocarbon barrier’, which means that this dating method can only produce reliable dates for sites younger than about 40,000 years. If archaeologists find a site they believe to be older than 40,000 years, they must compare radiocarbon dates with different dating methods to ensure an accurate result (see luminescence dating). Comparisons are often conducted with younger sites anyway, just to ensure the accuracy of the data.

Since the first radiocarbon dates were announced in the mid-20th Century, our understanding of Australia’s antiquity has altered dramatically. As each year passes, earlier dates are being published, extending Australia’s occupation further into the past. The graph below shows the progress of radiocarbon dating in Australia since the 1950s.

Radiocarbon dates in Australia
Radiocarbon dates in Australia

Radiocarbon dating is a highly complex procedure. In order to determine the age of a site, the radiocarbon years need to be converted to solar years, and then an age range is calculated. For example, if a radiocarbon result is 5,000 years, the solar years would be about 5,750, and the age estimate for that site might be calculated at 5,700-5,800 years old. Some archaeologists prefer more accurate calibrations, but many round the totals to the nearest 50 or 100 years, meaning that sites up to 26,000 years old are often given age estimates such as 8,400 ± 100.

Sites older than 26,000 years present greater issues for radiocarbon dating. For the period before 26,000 years, it is much harder to assign accurate dates because the distance between radiocarbon and solar years becomes increasingly difficult to calculate. For older sites, age estimates are usually stated as minimums and maximums, e.g. 42,000-46,000 years old.

Today, the generally accepted baseline for people arriving in Australia sits at 50,000 years ago. But who knows – future findings may challenge this figure and suggest an even earlier arrival time.

References:

Burke, H. & Smith, C. (2004) The Archaeologist’s Field Handbook. Crows Nest: Allen & Unwin.

Hiscock, P. (2008) The Archaeology of Ancient Australia. New York: Routlege.

Mulvaney, J. & Kamminga, J. (1999) Prehistory of Australia. Crows Nest: Allen & Unwin.