Transcript Carbon Dating

CARBON DATING
Determining the actual age of fossils
Radiometric dating
• Radioactive isotopes decay at a predictable rate
• The amount of time it takes for half of a sample of
the isotope to decay is called its half-life
• The half life is a function of the nuclear stability of
the atoms
• A more stable atom will have a longer half-life,
while one that is particularly unstable will have a
shorter half-life
Half-lives of some important isotopes
Uranium-238
Uranium-235
Lead-206
Lead-207
Currently
Accepted HalfLife Values
4.5 billion years
704 million years
Thorium-232
Rubidium-87
Potassium-40
Lead-208
Strontium-87
Argon-40
14.0 billion years
48.8 billion years
1.25 billion years
Samarium-147
Neodymium-143
106 billion years
Parent Isotope
Stable Daughter
Product
How to determine age
• As time passes, the number of atoms of the radioactive
isotope will diminish, while the number of atoms of the
stable product of the decay will increase
• The proportion of the two is a mathematical function of the
age
• In simple terms, a rock that contains both Uranium-238
and Lead-206 probably once had a higher content of
U238, which then decayed into the Pb206
• The more Pb206 in proportion to U238, the older the rock
The actual formula for calculating age
I was told that there would be no math
• The equation from the previous slide isn’t really all that
difficult, but it’s difficult enough.
• There are also difficulties to be overcome that have
nothing to do with the math. For example, the equation is
useless if you don’t have rocks that still contain both the
original isotope and the stable product of decay
• Maybe your rock never had Uranium-238
• Maybe the stable product of decay was a gas that
escaped (as it does in the decay of Carbon-14)
Choosing an Isotope for Dating
• No, not that kind of dating
• You need an isotope that is actually part of the rock or the
fossil you are trying to date
• You need an isotope that decays at a rate that is
appropriate for the age of rock that you have
• Older rocks can only be dated using isotopes with long
half-lives, where younger fossils need to be dated with
shorter half-life isotopes. U238 has a half-live of 4.5
billion years. It can’t give you an accurate age of a fossil
a few thousand years old
Why Carbon?
• Carbon-14 is a radioactive isotope of carbon. It
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comprises about 1.1% of all carbon atoms on earth
Carbon-14 has a half-life of 5770 years
Carbon-14 decays into Nitrogen-14 (which will escape)
All living things contain carbon, so rocks containing fossils
will contain carbon (of which 1.1% will be C-14)
Through the process of feeding and metabolism,
organisms will reach equilibrium with their environment.
In other words, living organisms will maintain a content of
about 1.1% carbon-14
Once they die, they will no longer maintain that
equilibrium, and C-14 will start turning into N-14
Carbon Dating
• So, if an organism was 1.1% carbon-14 at the moment of
death, and the half-life of carbon-14 is 5770 years, a fossil
5770 years old would be .55% carbon 14
• After another 5770 years the C-14 content would be half
of that, and so on . . .
• http://www.youtube.com/watch?v=31-P9pcPStg
• Using the data table on the next slide, construct a graph
of Carbon-14 content vs. Time
Radioactive Decay rate of Carbon 14
half lives
Time (years)
% Carbon 14
0
0
1.1
1
5770
0.55
2
11500
0.275
3
17300
0.1375
4
23100
0.0688
5
28900
0.0344
6
34600
0.0172
7
40400
0.0086
8
46200
0.0043
9
51900
0.0021
10
57700
0.0011
11
63500
0.0005
12
69200
0.0003
13
75000
0.0001
Video
• http://www.youtube.com/watch?v=2io5opwhQMQ
• http://www.youtube.com/watch?v=ErgdpG_N9vQ
• http://www.youtube.com/watch?v=udkQwW6aLik
• http://www.youtube.com/watch?v=w5369-OobM4
Resources:
• http://pubs.usgs.gov/gip/geotime/radiometric.html