- BBC - History - Ancient History in depth: The Story of Carbon Dating
- Radiocarbon Dating and Archaeology
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- How has radiocarbon dating changed archaeology?
Unless something was obviously attributable to a specific year -- say a dated coin or known piece of artwork -- then whoever discovered it had to do quite a bit of guesstimating to get a proper age for the item. The excavator might employ relative dating, using objects located stratigraphically read: But by using these imprecise methods, archeologists were often way off.
BBC - History - Ancient History in depth: The Story of Carbon Dating
Fortunately, Willard Libby, a scientist who would later win the Nobel Prize in Chemistry, developed the process known as radiocarbon dating in the late s. It's still the most commonly used method today.
In a nutshell, it works like this: After an organism dies, it stops absorbing carbon , so the radioactive isotope starts to decay and is not replenished. Archaeologists can then measure the amount of carbon compared to the stable isotope carbon and determine how old an item is. The sample-context relationship is not always straightforward. Date of a sample pre-dates the context it is found.
Radiocarbon Dating and Archaeology
Some samples, like wood, already ceased interacting with the biosphere and have an apparent age at death and linking them to the age of the deposits around the sample would not be wholly accurate. There are also cases when the association between the sample and the deposit is not apparent or easily understood. Great care must be exercised when linking an event with the context and the context with the sample to be processed by radiocarbon dating.
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An archaeologist must also make sure that only the useful series of samples are collected and processed for carbon dating and not every organic material found in the excavation site. It is important that the radiocarbon scientists and archaeologists agree on the sampling strategy before starting the excavation so time, effort, and resources will not be wasted and meaningful result will be produced after the carbon dating process.
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It must be stressed that archaeologists need to interact with radiocarbon laboratories first before excavation due to several factors. Laboratories have limitations in terms of the samples they can process for radiocarbon dating. Some labs, for example, do not date carbonates. Laboratories must also be consulted as to the required amount of sample that they ideally like to process as well as their preference with certain samples for carbon dating. Other labs accept waterlogged wood while others prefer them dry at submission.
Contaminants must not be introduced to the samples during collection and storing. Hydrocarbons, glue, biocides, polyethylene glycol or polyvinyl acetate PVA must not come in contact with samples for radiocarbon dating. Other potential contaminants include paper, cardboard, cotton wool, string and cigarette ash. Samples must be stored in packaging materials that will protect them during transport and even during prolonged storage. Labels attached to the packaging materials must not fade or rub off easily.
How has radiocarbon dating changed archaeology?
Glass containers can be used when storing radiocarbon dating samples, but they are susceptible to breakage and can be impractical when dealing with large samples. Aluminum containers with screw caps are safe, but it is still best to consult the radiocarbon laboratory for the best containers of carbon dating samples.
It is recommended that archaeologists, or any client in general, ask the laboratory if results have systematic or random errors. They should also ask details about the calibration used for conversion of BP years to calendar years.
Radioactive atoms decay into stable atoms by a simple mathematical process. Half of the available atoms will change in a given period of time, known as the half-life. For instance, if atoms in the year had a half-life of ten years, then in there would be left. In , there would be left, and in there would be left. By counting how many carbon atoms in any object with carbon in it, we can work out how old the object is - or how long ago it died. So we only have to know two things, the half-life of carbon and how many carbon atoms the object had before it died.
The half-life of carbon is 5, years. However knowing how many carbon atoms something had before it died can only be guessed at. The assumption is that the proportion of carbon in any living organism is constant. It can be deduced then that today's readings would be the same as those many years ago. When a particular fossil was alive, it had the same amount of carbon as the same living organism today.