1. LESSON 10: Radiometric Dating (ABSOLUTE DATING)

2. What are Isotopes? Same number of PROTONS Different number of NEUTRONS
C12
C14

3. Why are some Isotopes ‘unstable’?
Elements are composed of one or more naturally occurring isotopes, which are normally stable. Some elements, however, have unstable (radioactive) isotopes
Unstable atoms are radioactive because their nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves.
Radioactive isotopes decay so slow that a fraction still remains since they were created (examples: uranium, potassium)
About 339 isotopes occur naturally on Earth, of which about 79% are stable

4. What Isotopes are used for Dating Fossils?
Technique
Range (in yrs) Daughter Mol Half life
What they are Used for:
lead 210
Pb210
1 – 150
Bismuth 210
22.3 year
lake and ocean sediments, glacial ice
carbon 14
C14
1 – 60,000
Nitrogen 14
5,730 yrs
previously living things
Uranium 235
U235
1 – 400,000
Lead 207
704 million yrs
bone, teeth, coral, shells, eggs
Potassium 40
K40
10,000 – 3 billion
Argon 40
1.25 billion
minerals, igneous rocks
Uranium 238
U238
1 million billion
Lead 206
4.5 billion yrs
Rubidium 87
60 million – 4.5 billion
Strontium 87
48.8 billion yrs

5. What is Radiometric Decay?
Naturally-occurring radioactive materials break down into other materials at known rates.
Radioactive parent elements decay to stable daughter elements.
Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.
Each radioactive isotope has its own unique half-life
time it takes for half of the radioactive element to decay

6. RADIOACTIVE DECAY GRAPH

7. What is Carbon Dating?
Method that uses the naturally occurring 14C isotope to determine the age of carbonaceous materials up to about 70,000 years old.
One of the frequent uses of the technique is to date organic remains from archaeological sites. Plants fix atmospheric carbon during photosynthesis, so the level of C14 in living plants and animals equals the level of C14 in the atmosphere.

8. How does Carbon Dating Work?
Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide.
Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle.
All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion).
At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14.
The change in the Carbon 14 to Carbon 12 ratio is the basis for dating.
The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old.

9. How does Radiometric Dating work?
Carbon-14
50%
5,730 years
Carbon-14
25%
11460 years
Carbon -14
12.5%
17190 years
Carbon-14
100%
0 years
1000 particles
500 particles
250 particles
125 particles

10. What is the Oldest Dated Rock?
Currently, the oldest known rocks in the world come from the Acasta area of Canada's Northwest Territories.
They have been dated using the isotope U-Pb.
The oldest rocks dated around 3900 million years old or roughly 3.9 billion years. (why not 4.5 billion years?)

11. Let’s Practice Carbon-14 half-life is 5,732 years
A fossil has 50% carbon-14 and 50% Nitrogen-14
How old is the fossil?
5732 years old
A fossil has 25% carbon-14 and 75% Nitrogen-14
11464 years old

12. Let’s Practice Some More:
An ancient Eocene fossil fish Priscacara liops from Green River formation in Utah was found in a sedimentary layer of rock that contained 50 parent molecules (C14 ) and 950 daughter molecules (N14 ). How old is this fossil?

13. ANSWER: C14 Decays at a rate of ~ 5700 yrs per half life.
There were a total of 1000 atoms originally (50 C N14 )
After 1 half-life there would have been 500, 2nd 250, 3rd 125, 4th 62.5, 5th 31.25, etc.
The number of parent atoms falls between the 4th and 5th half-life, so….
The Fossil is approximately 22,800 years old
Can you explain how I got this answer?