Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Heronumber0 on 03/08/2007 18:38:04
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An ancient city was found underwater off the coast of India in 2002 (off the Bay of Cambay, West India). I have given a similar picture of a Harrappan city below.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi180.photobucket.com%2Falbums%2Fx236%2FHeronumber0%2Fann_oldcity_harappan.jpg&hash=dcfa17f6722e3e04ac746b867fa999bd)
Why is it so important?
And carbon dating would have presumably used for human/animal tissue. How does pottery and other material get dated, given the 9500 year age estimate?
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Carbon dating will work up to an age of about 50,000 years. Anything that contains material from a plant is datable; this includes people and animals because they eat plant matter.
Plants pick up C14 from the atmosphere in the course of photosynthesising (soaking up CO2 and turning it into sugar and wood). When the plant dies it ceases to collect more C14, which is only in the atmosphere. The amount of C14 remaining in the sample is therefore proportional to the time elapsed since it "died".
Chris
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The primary means of dating pottery (where one cannot determine from the style of the pottery, or the context) is a technique known as thermoluminescence.
Essentially, many crystalline materials (such as ceramics) when they are exposed to radiation (e.g. from radioactive decay of naturally occurring uranium, or potassium (K-40), or other similar radioactive isotope) with cause electrons to become energised, and then trapped in the crystal structure. When the material is heated (or exposed to strong sunlight), it will release the energy bound in these electrons, and this can be measured by the small flash of light that is given off by the electron.
When pottery is first fired, it is assumed that all of the bound up energy in the clay is released, so any energy accumulated within the crystals from this process must have happened since the time when the pottery was fired. If you then heat the pottery again, you can then see how much light is given off, and so one can estimate how long it was since the pottery was last heated.
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Thanks for the answers guys. A couple of questions come to mind:
1. I thought carbon dating had an error level of + or - 5000 years. Does that mean that the 9500 year old figure can be inaccurate so that it becomes either 4500 years old minimum or 14500 years old maximum?
2. Will history have to be rewritten because this city marked a 'colony' that is older than Chinese or Egyptian civilisations?
3. Do you believe that ancient civilisations contacted each other and entered into trade agreements, cultural exchange trips etc... IMHO that seems likely.
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I think you may have over-estimated the error margin quite significantly; it's not that big. However, as I'm not an expert on radiocarbon dating I've written to a friend in Australia who is a world leader and asked him for clarification.
Chris
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Chris - You're right of course, I think I have grossly overestimated the margin of error. It looks likely to have a confidence margin in the range of + or - 200 years by analogy to this article which dated wood I think:
http://www.flmnh.ufl.edu/natsci/vertpaleo/aucilla12_1/radio99.htm (http://www.flmnh.ufl.edu/natsci/vertpaleo/aucilla12_1/radio99.htm)
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accuracy must depend on how old a sample you are using. It seems that carbon dating can be used to about 60,000 years, so ofcourse the closer you get to that 60,000 year limit, the less the accuracy; although there are other parameters that can alter the accuracy as well.
There is a period known as the Younger Dryas that is from about 12,900-11,500 BP, where carbon dating is particularly unreliable.
It seems that at around 10,000 years before present (BP), we need to place about 1,500 years correction on the raw data, but from what I can gather (but I am not sure if my interpretation of what I am reading does actually mean this) that with the correct calibrations, one might get down to 70 years accuracy in 10,000 years. Ofcourse, the calibration itself may be subject to problems where local environments don't match the environment where the calibration was derived, or where the material you are sampling was substantially recycled.
Maybe when Chris' friend comes back, we might get a more definitive answer.