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Lecture 14

# Anthropology 2229F/G Lecture Notes - Lecture 14: Standard Deviation

Department
Anthropology
Course Code
ANTH 2229F/G
Professor
Christopher Ellis
Lecture
14

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Lecture 14 Part 3 Radiocarbon Dating
6. Isotopic Fractionation
Assumes plants gain carbon randomly from the
atmosphere
Don’t favour C-12 to C-14
Plants can be biased for/against certain isotopes of
carbon
C-14 molecules move more quickly and are taken in
preferentially by plants
o E.g. corn (maize), millet: take in more
radiocarbon than is in the atmosphere
Still probably the most reliable technique that we have today
We can control for biases
Recent fluctuations of the Earth’s magnetic fields,
cosmic rays and glaciation can be corrected for
Date objects of known ages to see what the difference is
and correct the date by taking it into account
Farther back in time we go, the radiocarbon dates are
underestimating the age by about 2000 years
o But differences are systematic, can fix the date
based on the systematic deviation
Called correction factors
o Calibrated radiocarbon dates: date corrected to
calendrical dates
Because of mixing problems in the ocean, we don’t get a
very accurate date, but we can correct for it
Industrial fossil fuels and atomic bombs standardize
what the levels would have been before
o Must be reported as BP if the date is not calibrated
o Stands for before present
o Present starts at AD 1950
o Always accompanied with a +/- figures; varies greatly
Range for error, it is a statistical estimate/average standard
deviation (sigma)
68% chance it is within the first standard deviation (1 sigma),
95% chance it is within the second standard deviations (2
sigma), etc.
Statistical estimates, not absolute probability they
could be wrong
Always get multiple dates from different objects; average out the
dates to be more precise by reducing the standard deviation
Can relate objects…
o Easiest to do if they overlap at one standard deviation