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Lecture 14

Lecture 14 Part 2 – Absolute Dating

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Anthropology 2229F/G
Christopher Ellis

Lecture 14 Part 2 – Absolute Dating  Radiocarbon Dating o Invented by Wilfred Libby o C-14 most commonly used dating technique in archaeology worldwide o Based on natural radioactive decay of isotopes over time (radiometric)  Unstable; break down into other forms  Decay at a constant rate over time o Carbon-14 changes to Nitrogen-14 o Unaffected by certain variables: not effected by temperature, climate, moisture, etc. o Rate of decay is measured in half-life: time it takes for half of the radioactive isotope to decay o Radiocarbon has a short half life: 5730 years  5,568 years (Libby half life, not accurate) o Can only be used to date things that are less than 40-50,000 years o How is carbon-14 formed?  Created when cosmic rays come into contact with nitrogen-14 in the upper atmosphere which forms carbon-14  Over time, it reaches an equilibrium (equal amounts being created and decaying)  Carbon-14 in a constant ratio in the atmosphere; reacts with oxygen to created carbon dioxide (very tiny percentage of all CO2(0.00000001%)  Ratio of C-14:C-12 is constant, in specific proportion  Carbon taken into the food chain by plants; later taken in by animals who eat plants and animals that eat plants o When an animal is alive, there is an equilibrium of C-14 in the body,
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