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- Time is measure on an ordinal scale. It measures greater or less than relationships, but not how much or > or <.
- Time is registered as a sequence of events w/out a calendric date but w/older than/younger than relationships stipulated.
- 1. stratighraphy (principle of super position)
- 2. seriation (gives order but you need other data to tell direction time is going)
Time is measured on an interval scale. Bothe greather than and less than relationships are specified AND how much greater than or less than. It is registered as a sequence of events, each of which can be referred to a calendrical date. Absolute dating is synonymous with chronometric dating. The date approximate calendar or solar years but are not always an exact calendar date
- The result is tha time is measured w/respect to some scale (days, years). The methods for absolute dating are many and varied:
- a. dendrochronology (tree-rings)
- b. radiocarbon (C14) - most often used
- c. potassium-argon (K/Ar)
- d. thermoluminescence
- e. obsidian hydration (depending on how its done it can be relative or absolute)
- developed by Willard F. Libby in the late 1940s. Libby worked on the Manhattan project and won a Nobel prize for his effort.
- - Neutrons creted when cosmic rays enter Earth's upper atmosphere
- - Nitrogen 14 atom absorbs a neutron and becomes Carbon14
- - Carbon14 combines with oxygen to form carbon dioxide
- - Carbon dioxide is taken in by plants, and fixed into tissue (also dissolves in water).
- - When plants and animals die they stop taking in CO2 and thus "start the clock"
- - Carbon 14 is radioactive and unstable and thus decays back into N14
- - C14 has a half-life of 5730 +/- 40 years
Assumption was that there was a constant supply on N14 and cosmic radiation; therefore a C14 year is equal to a calander year. However, C14 production rate has not been constant. Differences in solar wind, Earth's magentic field, sun spots, industrial revolution, and nuclear testing in the 1960's changed C14 production. Using dendrochronology, coral reef deposition, glacial ice cores, physicists can correct or calibrate radio carbon dates (b.p.) to approximte calendar dates (b.c.-a.d.)
What can Radiocarbon Date?
any organic material that contains carbon; must be cautious with bone to only use the collagen portion and to know the diet of the animal; and with wood you need to take the outer layer (age of tree death).
Conventional radiocarbon dating
dates range from 150-40,000 years ago (6-7.5 half-lives); need 10+ grams of material; counts beta particle emissions
Accelerator Mass Spectrometry radiocarbon dating
dates range from 150-60,000 years ago; only need <1 gram of material; much more accurate because it counts the actual atoms.
- - based on the principle of radioactive decay
- - used to date volcanic rock
- - counts decay of Ar40 to K40
- - rock dating 4-5 billion years to 100,000 years old
- - notably uded to date early hominid sites in East Africa
- Method for dating baked prehistoric materials such as clay vesels. Physicists have also developed methods of dating soil and rocks
- - materials from which pottery are made store energy by trapping electrons
- - stored energy can be released by heating the pottery, at which time visible light rays, known as thermoluminescene, are emitted
- - alpha particles then begin to be absorbed by the material again and the thermoluminescence potential increases until the ceramic is reheated in the laboratory (destructive).
- measures the last time a flake was removed from the surface of an obsidian object. it can be used as both relative and absolute dating method.
- - obsidian is volcanically formed glass, which contains between 0.1% and 0.3% water nturally
- - after formation, obsidian will absorb water to a maximum of 3.5%
- - it absorbs water inward to form hydration layer that is visible under magnification
- - the thicker the hydration layer (n microns thick), the older the object is
- - can be used to detect tool recycling or stratigraphic mixing
- Hydration Rate is controlled by:
- 1. Temperature
- 2. Humidity
- 3. Chemical makeup of obsidian flow
As an absolute dating method-
in certain parts of the world the hydration rate has been determined using independant methods such as radiocarbon dating or dendrochronology (e.g., California, Oregon, New Zealand)