32402f06 - Introduction to Archaeology Class 6 Absolute...

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Introduction to Archaeology: Class 6 Absolute dating: More physics tricks and dating in historical archaeology Copyright Bruce Owen 2002 More physics-based dating methods Potassium-argon dating Like radiocarbon, this is a radiometric method Depends on the decay of radioactive isotopes This is absolutely regular, not affected by temperature, humidity, etc. Potassium in nature is partially 40 K, which decays to 40 Ar (stable, inert Argon gas) Potassium is a very common constituent of many rocks In molten rock, the argon gas is free to diffuse away So when the rock solidifies into crystals, it starts off with no argon The 40 K decays, producing 40 Ar that is trapped in the crystal So the ratio of trapped 40 Ar to remaining 40 K increases over time, allowing calculation of the time since the crystal solidified Unfortunately for archaeologists, people have rarely lived in solidifying molten lava But if a site was covered by a lava or volcanic ash fall, that would give a date of the site’s destruction Or if an artifact is found in the sediments above one basalt flow but below another, they will bracket the artifact But 40 K has a very long half-life (1.3 billion years), so it takes a very long time for enough 40 Ar to build up for a reliable measurement So this method is good for old samples, starting at about 1 million years old, up to beyond the age of the earth’s crust (lunar rocks at over 5 billion years old) Geochron lab will not date samples expected to be under 2 million years old While Thomas says it can be used up to just 200,000 years ago He is probably thinking of some very experimental attempts to stretch the method to younger samples Currently not very useful to archaeologists other than those working with pre- Homo sapiens sites Cost Geochron lab: $400-$600 per sample, depending on preparation Under 5 million years expected age has 50% surcharge for extra care in handling, since contamination is more of a problem with such tiny amounts of argon Argon-argon A clever modification, but not widely mentioned Obsidian hydration dating A wonderful tool in that date is tied to a specific human behavior we might be interested in: flaking stone to make tools review the method; Thomas's description is OK The hydrated “rim” is also sometimes called the “rind”
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Introduction to Archaeology F 2002 / Owen: Absolute dating: More physics tricks and historical dating p. 2 Thomas says obsidian “one day may rival ceramics as archaeology’s most useful artifact for controlling time” Not a chance In many places obsidian is relatively rare “dates” are pretty approximate the uncertainty is often not even mentioned, because it is hard to estimate results are usually not even converted to dates in calendar years, but are given as rim thicknesses in microns with one or more estimates of the relevant rate (years per micron) typical examples: 220 ±44 years/micron 458 ±92 years/micron the reader can calculate calendar ages, but the writer does not imply that these are
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32402f06 - Introduction to Archaeology Class 6 Absolute...

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